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Ubiquitylation in immune disorders and cancer: from molecular mechanisms to therapeutic implications
(2012)
Conjugation of ubiquitin to proteins (ubiquitylation) has emerged to be one of the most crucial post-translational modifications controlling virtually all cellular processes. What was once regarded as a mere signal for protein degradation has turned out to be a major regulator of molecular signalling networks. Deregulation of ubiquitin signalling is closely associated with various human pathologies. Here, we summarize the current knowledge of ubiquitin signalling in immune deficiencies and cancer as well as the available therapeutic strategies targeting the ubiquitin system in combating these pathogenic conditions.
An abdominal aortic aneurysm (AAA) is a pathological widening of the aortic wall characterized by loss of smooth muscle cells (SMCs), extracellular matrix degradation, and local inflammation. This condition is often asymptomatic until rupture occurs, leading to high morbidity and mortality rates. Diagnosis is mostly accidental and the only currently available treatment option remains surgical intervention. Circular RNAs (circRNAs) represent a novel class of regulatory non-coding RNAs that originate from backsplicing. Their highly stable loop structure, combined with a remarkable enrichment in body fluids, make circRNAs promising disease biomarkers. We investigated the contribution of circRNAs to AAA pathogenesis and their potential application to improve AAA diagnostics. Gene expression analysis revealed the presence of deregulated circular transcripts stemming from AAA-relevant gene loci. Among these, the circRNA to the Ataxia Telangiectasia Mutated gene (cATM) was upregulated in human AAA specimens, in AAA-derived SMCs, and serum samples collected from aneurysm patients. In primary aortic SMCs, cATM increased upon angiotensin II and doxorubicin stimulation, while its silencing triggered apoptosis. Higher cATM levels made AAA-derived SMCs less vulnerable to oxidative stress, compared with control SMCs. These data suggest that cATM contributes to elicit an adaptive oxidative-stress response in SMCs and provides a reliable AAA disease signature.
Simple Summary:
CDK9, in combination with Cyclin T1, is one of the major regulators of RNA Polymerase II mediated productive transcription of critical genes in any cell. The activity of CDK9 is significantly up-regulated in a wide variety of cancer entities, to aid in the overexpression of genes responsible for the regulation of functions, which are beneficial to the cancer cells, like proliferation, survival, cell cycle regulation, DNA damage repair and metastasis. Enhanced CDK9 activity, therefore, leads to poorer prognosis in many cancer types, offering the rationale to target it using small-molecule inhibitors. Several, increasingly specific inhibitors, have been developed, some of which are presently in clinical trials. Other approaches being tested involve combining inhibitors against CDK9 activity with those against CDK9’s upstream regulators like BRD4, SEC and HSP90; or downstream effectors like cMYC and MCL-1. The inhibition of CDK9’s activity holds the potential to be a highly effective anti-cancer therapeutic.
Abstract:
Cyclin Dependent Kinase 9 (CDK9) is one of the most important transcription regulatory members of the CDK family. In conjunction with its main cyclin partner—Cyclin T1, it forms the Positive Transcription Elongation Factor b (P-TEFb) whose primary function in eukaryotic cells is to mediate the positive transcription elongation of nascent mRNA strands, by phosphorylating the S2 residues of the YSPTSPS tandem repeats at the C-terminus domain (CTD) of RNA Polymerase II (RNAP II). To aid in this process, P-TEFb also simultaneously phosphorylates and inactivates a number of negative transcription regulators like 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole (DRB) Sensitivity-Inducing Factor (DSIF) and Negative Elongation Factor (NELF). Significantly enhanced activity of CDK9 is observed in multiple cancer types, which is universally associated with significantly shortened Overall Survival (OS) of the patients. In these cancer types, CDK9 regulates a plethora of cellular functions including proliferation, survival, cell cycle regulation, DNA damage repair and metastasis. Due to the extremely critical role of CDK9 in cancer cells, inhibiting its functions has been the subject of intense research, resulting the development of multiple, increasingly specific small-molecule inhibitors, some of which are presently in clinical trials. The search for newer generation CDK9 inhibitors with higher specificity and lower potential toxicities and suitable combination therapies continues. In fact, the Phase I clinical trials of the latest, highly specific CDK9 inhibitor BAY1251152, against different solid tumors have shown good anti-tumor and on-target activities and pharmacokinetics, combined with manageable safety profile while the phase I and II clinical trials of another inhibitor AT-7519 have been undertaken or are undergoing. To enhance the effectiveness and target diversity and reduce potential drug-resistance, the future of CDK9 inhibition would likely involve combining CDK9 inhibitors with inhibitors like those against BRD4, SEC, MYC, MCL-1 and HSP90.
The ability to escape apoptosis or programmed cell death is a hallmark of human cancers, for example pancreatic cancer. This can promote tumorigenesis, since too little cell death by apoptosis disturbs tissue homeostasis. Additionally, defective apoptosis signaling is the underlying cause of failure to respond to current treatment approaches, since therapy-mediated antitumor activity requires the intactness of apoptosis signaling pathways in cancer cells. Thus, the elucidation of defects in the regulation of apoptosis in pancreatic carcinoma can result in the identification of novel targets for therapeutic interference and for exploitation for cancer drug discovery. Keywords: apoptosis; pancreatic cancer; TRAIL; IAPs; mitochondria
Apoptosis is deregulated in most, if not all, cancers, including hematological malignancies. Smac mimetics that antagonize Inhibitor of Apoptosis (IAP) proteins have so far largely been investigated in acute myeloid leukemia (AML) cell lines; however, little is yet known on the therapeutic potential of Smac mimetics in primary AML samples. In this study, we therefore investigated the antileukemic activity of the Smac mimetic BV6 in diagnostic samples of 67 adult AML patients and correlated the response to clinical, cytogenetic and molecular markers and gene expression profiles. Treatment with cytarabine (ara-C) was used as a standard chemotherapeutic agent. Interestingly, about half (51%) of primary AML samples are sensitive to BV6 and 21% intermediate responsive, while 28% are resistant. Notably, 69% of ara-C-resistant samples show a good to fair response to BV6. Furthermore, combination treatment with ara-C and BV6 exerts additive effects in most samples. Whole-genome gene expression profiling identifies cell death, TNFR1 and NF-κB signaling among the top pathways that are activated by BV6 in BV6-sensitive, but not in BV6-resistant cases. Furthermore, sensitivity of primary AML blasts to BV6 correlates with significantly elevated expression levels of TNF and lower levels of XIAP in diagnostic samples, as well as with NPM1 mutation. In a large set of primary AML samples, these data provide novel insights into factors regulating Smac mimetic response in AML and have important implications for the development of Smac mimetic-based therapies and related diagnostics in AML.
Evasion of apoptosis, for example, by inhibitor of apoptosis (IAP) proteins, contributes to treatment resistance and poor outcome in acute myeloid leukemia (AML). Here we identify a novel synergistic interaction between the small-molecule second mitochondria-derived activator of caspases (Smac) mimetic BV6, which antagonizes X-linked IAP, cellular IAP (cIAP)1 and cIAP2, and the demethylating agents 5-azacytidine or 5-aza-2′-deoxycytidine (DAC) to induce cell death in AML cells, including apoptosis-resistant cells. Calculation of combination index (CI) confirms that this drug combination is highly synergistic (CI 0.02–0.4). In contrast, BV6 and DAC at equimolar concentrations do not cause synergistic toxicity against normal peripheral blood lymphocytes, pointing to some tumor cell selectivity. Molecular studies reveal that BV6 and DAC cooperate to trigger the activation of caspases, mitochondrial perturbations and DNA fragmentation, consistent with apoptotic cell death. However, the broad-range caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD.fmk) fails to protect against BV6/DAC-induced cell death and even significantly increases the percentage of Annexin-V/propidium iodide double-positive cells. Importantly, BV6/DAC-induced cell death in the presence of zVAD.fmk is significantly reduced by pharmacological inhibition of key components of necroptosis signaling, that is, receptor-interacting protein (RIP) 1 using necrostatin-1 or mixed lineage kinase domain-like protein (MLKL) using necrosulfonamide. This indicates a switch from BV6/DAC-induced cell death from apoptosis to necroptosis upon caspase inhibition. Thus, BV6 cooperates with demethylating agents to induce cell death in AML cells and circumvents apoptosis resistance via a switch to necroptosis as an alternative mode of cell death. The identification of a novel synergism of BV6 and demethylating agents has important implications for the development of new treatment strategies for AML.
In eukaryotes, double-stranded (ds) RNA induces sequence-specific inhibition of gene expression referred to as RNA interference (RNAi). We exploited RNAi to define the role of HER2/neu in the neoplastic proliferation of human breast cancer cells. We transfected SK-BR-3, BT-474, MCF-7, and MDA-MB-468 breast cancer cells with short interfering RNA (siRNA) targeted against human HER2/neu and analyzed the specific inhibition of HER2/neu expression by Northern and Western blots. Transfection with HER2/neu-specific siRNA resulted in a sequence-specific decrease in HER2/neu mRNA and protein levels. Moreover, transfection with HER2/neu siRNA caused cell cycle arrest at G0/G1 in the breast cancer cell lines SKBR-3 and BT-474, consistent with a powerful RNA silencing effect. siRNA treatment resulted in an antiproliferative and apoptotic response in cells overexpressing HER2/neu, but had no influence in cells with almost no expression of HER2/neu proteins like MDA-MB-468 cells. These data indicate that HER2/neu function is essential for the proliferation of HER2/neuoverexpressing breast cancer cells. Our observations suggest that siRNA targeted against human HER2/neu may be valuable tools as anti proliferative agents that display activity against neoplastic cells at very low doses.
Shikonin reduces growth of docetaxel-resistant prostate cancer cells mainly through necroptosis
(2021)
Simple Summary: Prostate carcinoma (PCa) is the most common tumor in men with an increasing age-associated risk. Several therapy strategies, one of which is docetaxel (DX) chemotherapy, have been established. However, due to the development of therapy resistance, in which chemotherapy no longer effectively combats the cancer, advanced, metastasized PCa with a poor prognosis may become manifested and therapy inevitably fails. Thus, new treatment options are urgently needed. Shikonin (SHI), from Traditional Chinese Medicine, has revealed promising antitumor activity in several tumor entities. In the current study, the impact of SHI on four therapy-sensitive and four respective DX-resistant PCa cell lines was determined. SHI induced growth inhibition mainly by necroptosis, a type of cell death, in all the tested therapy-sensitive, but more importantly, DX-resistant PCa cell lines. Corresponding molecular alterations contributing to growth inhibition after SHI exposure were found. SHI could, therefore, be a promising additive in treating advanced PCa.
Abstract: The prognosis for advanced prostate carcinoma (PCa) remains poor due to development of therapy resistance, and new treatment options are needed. Shikonin (SHI) from Traditional Chinese Medicine has induced antitumor effects in diverse tumor entities, but data related to PCa are scarce. Therefore, the parental (=sensitive) and docetaxel (DX)-resistant PCa cell lines, PC3, DU145, LNCaP, and 22Rv1 were exposed to SHI [0.1–1.5 μM], and tumor cell growth, proliferation, cell cycling, cell death (apoptosis, necrosis, and necroptosis), and metabolic activity were evaluated. Correspondingly, the expression of regulating proteins was assessed. Exposure to SHI time- and dose-dependently inhibited tumor cell growth and proliferation in parental and DX-resistant PCa cells, accompanied by cell cycle arrest in the G2/M or S phase and modulation of cell cycle regulating proteins. SHI induced apoptosis and more dominantly necroptosis in both parental and DX-resistant PCa cells. This was shown by enhanced pRIP1 and pRIP3 expression and returned growth if applying the necroptosis inhibitor necrostatin-1. No SHI-induced alteration in metabolic activity of the PCa cells was detected. The significant antitumor effects induced by SHI to parental and DX-resistant PCa cells make the addition of SHI to standard therapy a promising treatment strategy for patients with advanced PCa.
Signaling via the intrinsic (mitochondrial) pathway of apoptosis represents one of the critical signal transduction cascades that control the regulation of cell death. This pathway is typically altered in human cancers, thereby providing a suitable target for therapeutic intervention. Members of the Bcl-2 family of proteins as well as cell survival signaling cascades such as the PI3K/Akt/mTOR pathway are involved in the regulation of mitochondria-mediated apoptosis. Therefore, further insights into the molecular mechanisms that form the basis for the control of mitochondria-mediated apoptosis will likely open new perspectives to bypass evasion of apoptosis and treatment resistance in human cancers.
Linear Ubiquitin chain Assembly Complex (LUBAC) is an E3 ligase complex that generates linear ubiquitin chains and is important for tumour necrosis factor (TNF) signaling activation. Mice lacking Sharpin, a critical subunit of LUBAC, spontaneously develop inflammatory lesions in the skin and other organs. Here we show that TNF receptor 1 (TNFR1)-associated death domain (TRADD)-dependent TNFR1 signaling in epidermal keratinocytes drives skin inflammation in Sharpin-deficient mice. Epidermis-restricted ablation of Fas-associated protein with death domain (FADD) combined with receptor-interacting protein kinase 3 (RIPK3) deficiency fully prevented skin inflammation, while single RIPK3 deficiency only delayed and partly ameliorated lesion development in Sharpin-deficient mice, showing that inflammation is primarily driven by TRADD- and FADD-dependent keratinocyte apoptosis while necroptosis plays a minor role. At the cellular level, Sharpin deficiency sensitized primary murine keratinocytes, human keratinocytes, and mouse embryonic fibroblasts to TNF-induced apoptosis. Depletion of FADD or TRADD in Sharpin-deficient HaCaT cells suppressed TNF-induced apoptosis, indicating the importance of FADD and TRADD in Sharpin-dependent anti-apoptosis signaling in keratinocytes.
Regulation of the antiapoptotic protein cFLIP by the glucocorticoid Dexamethasone in ALL cells
(2018)
We recently reported that the Smac mimetic BV6 and glucocorticoids, e.g. Dexamethasone (Dexa), synergize to induce cell death in acute lymphoblastic leukemia (ALL) in vitro and in vivo. Here, we discover that this synergism involves Dexa-stimulated downregulation of cellular FLICE-like inhibitory protein (cFLIP) in ALL cells. Dexa rapidly decreases cFLIPL protein levels, which is further enhanced by addition of BV6. While attenuating the activation of non-canonical nuclear factor-kappaB (NF-κB) signaling by BV6, Dexa suppresses cFLIPL protein but not mRNA levels pointing to a transcription-independent downregulation of cFLIPL by Dexa. Analysis of protein degradation pathways indicates that Dexa causes cFLIPL depletion independently of proteasomal, lysosomal or caspase pathways, as inhibitors of the proteasome, lysosomal enzymes or caspases all failed to protect from Dexa-mediated loss of cFLIPL protein. Also, Dexa alone or in combination with BV6 does not affect overall activity of the proteasome. Importantly, overexpression of cFLIPL to an extent that is no longer subject to Dexa-imposed downregulation rescues Dexa/BV6-mediated cell death. Vice versa, knockdown of cFLIP increases BV6-mediated cell death, thus mimicking the effect of Dexa. Altogether, these data demonstrate that Dexa-mediated downregulation of cFLIPL protein promotes Dexa/BV6-mediated cell death, thereby providing novel insights into the synergistic antitumor activity of this combination treatment.
Cell death and survival programs are controlled by the cellular redox state, which is typically dysregulated during oncogenesis. A recent study reports that the inhibition of antioxidant defenses resulting from glutathione depletion can prime acute lymphoblastic leukemia cells for death induced by Smac mimetics.
Polo-like kinase 1 inhibition sensitizes neuroblastoma cells for vinca alkaloid-induced apoptosis
(2015)
High polo-like kinase 1 (PLK1) expression has been linked to poor outcome in neuroblastoma (NB), indicating that it represents a relevant therapeutic target in this malignancy. Here, we identify a synergistic induction of apoptosis by the PLK1 inhibitor BI 2536 and vinca alkaloids in NB cells. Synergistic drug interaction of BI 2536 together with vincristine (VCR), vinblastine (VBL) or vinorelbine (VNR) is confirmed by calculation of combination index (CI). Also, BI 2536 and VCR act in concert to reduce long-term clonogenic survival. Importantly, BI 2536 significantly enhances the antitumor activity of VCR in an in vivo model of NB. Mechanistically, BI 2536/VCR co-treatment triggers prolonged mitotic arrest, which is necessary for BI 2536/VCR-mediated apoptosis, since pharmacological inhibition of mitotic arrest by the CDK1 inhibitor RO-3306 significantly reduces cell death. Prolonged mitotic arrest leads to phosphorylation-mediated inactivation of BCL-2 and BCL-XL as well as downregulation of MCL-1, since inhibition of mitotic arrest by RO-3306 also prevents phosphorylation of BCL-2 and BCL-XL and MCL-1 downregulation. This inactivation of antiapoptotic BCL-2 proteins promotes activation of BAX and BAK, cleavage of caspase-9 and -3 and caspase-dependent apoptosis. Engagement of the mitochondrial pathway of apoptosis is critically required for BI 2536/VCR-induced apoptosis, since ectopic expression of a non-degradable MCL-1 phospho-mutant, BCL-2 overexpression or BAK knockdown significantly reduce BI 2536/VCR-mediated apoptosis. Thus, PLK1 inhibitors may open new perspectives for chemosensitization of NB.
Photodynamic treatment of oral squamous cell carcinoma cells with low curcumin concentrations
(2017)
Objective: Curcumin is known for its anti-oxidative, anti-inflammatory and anti-tumorigenic qualities at concentrations ranging from 3.7µg/ml to 55µg/ml. Therefore it is pre-destined for tumour therapy. Due to high oral doses that have to be administered and the low bioavailability of curcumin new therapy concepts have to be developed. One of these therapy concepts is the combination of low curcumin concentrations and UVA or visible light. Aim of our study was to investigate the influence of this treatment regime on oral squamous cell carcinoma cells.
Materials and Methods: A human oral squamous cell carcinoma cell line (HN) was pre-incubated with low curcumin concentrations (0.01µg/ml to 1µg/ml). Thereafter cell cultures were either left un-irradiated or were irradiated either with 1J/cm2 UVA or for 5min with visible light. Quantitative analysis of proliferation, membrane integrity, oxidative potential and DNA fragmentation were done.
Results: It could be shown that low curcumin concentrations neither influenced proliferation, nor cell morphology, nor cell integrity nor apoptosis. When combining these curcumin concentrations with UVA or visible light irradiation cell proliferation as well as development of reactive oxygen species was reduced whereas DNA fragmentation was increased. Concentration as well as light entity specific effects could be observed.
Conclusions: The present findings substantiate the potential of the combination of low curcumin concentrations and light as a new therapeutic concept to increase the efficacy of curcumin in the treatment of cancer of the oral mucosa.
Simple Summary:
Pharmacological activation of tumor suppressor p53 is a promising therapeutic strategy for a range of hematologic and solid cancers. Whether p53 activation augments or suppresses anti-tumor innate immunity is less understood. Here we show that treatment of differentiating human macrophages with a p53 activator idasanutlin suppresses their inflammatory responses to activators of toll-like receptors (TLR) -4 and -7/8. This is accompanied by reduced expression of TLR7, TLR8, as well as TLR4 co-receptor CD14. These data help evaluating the possibilities of combining p53-targeting and immunostimulatory anti-cancer therapies.
Abstract:
The transcription factor p53 has well-recognized roles in regulating cell cycle, DNA damage repair, cell death, and metabolism. It is an important tumor suppressor and pharmacological activation of p53 by interrupting its interaction with the ubiquitin E3 ligase mouse double minute 2 homolog (MDM2) is actively explored for anti-tumor therapies. In immune cells, p53 modulates inflammatory responses, but the impact of p53 on macrophages remains incompletely understood. In this study, we used the MDM2 antagonist idasanutlin (RG7388) to investigate the responses of primary human macrophages to pharmacological p53 activation. Idasanutlin induced a robust p53-dependent transcriptional signature in macrophages, including several pro-apoptotic genes. However, idasanutlin did not generally sensitize macrophages to apoptosis, except for an enhanced response to a Fas-stimulating antibody. In fully differentiated macrophages, idasanutlin did not affect pro-inflammatory gene expression induced by toll-like receptor 4 (TLR4), TLR3, and TLR7/8 agonists, but inhibited interleukin-4-induced macrophage polarization. However, when present during monocyte to macrophage differentiation, idasanutlin attenuated inflammatory responses towards activation of TLR4 and TLR7/8 by low doses of lipopolysaccharide or resiquimod (R848). This was accompanied by a reduced expression of CD14, TLR7, and TLR8 in macrophages differentiated in the presence of idasanutlin. Our data suggest anti-inflammatory effects of pharmacological p53 activation in differentiating human macrophages.
Novel insights into the synergistic interaction of Bortezomib and TRAIL: tBid provides the link
(2011)
The proteasome inhibitor Bortezomib has been identified as a potent enhancer of TRAIL-induced apoptosis in several human cancers. However, the identification of the underlying molecular mechanisms of this synergistic cell death induction has been ongoing over the last years. A recent study identifies a new mechanism of action for the synergism of TRAIL and Bortezomib.
Colorectal cancer (CRC) is one of the most frequently diagnosed tumor in humans and one of the most common causes of cancer-related death worldwide. The pathogenesis of CRC follows a multistage process which together with somatic gene mutations is mainly attributed to the dysregulation of signaling pathways critically involved in the maintenance of homeostasis of epithelial integrity in the intestine. A growing number of studies has highlighted the critical impact of members of the tripartite motif (TRIM) protein family on most types of human malignancies including CRC. In accordance, abundant expression of many TRIM proteins has been observed in CRC tissues and is frequently correlating with poor survival of patients. Notably, some TRIM members can act as tumor suppressors depending on the context and the type of cancer which has been assessed. Mechanistically, most cancer-related TRIMs have a critical impact on cell cycle control, apoptosis, epithelial–mesenchymal transition (EMT), metastasis, and inflammation mainly through directly interfering with diverse oncogenic signaling pathways. In addition, some recent publications have emphasized the emerging role of some TRIM members to act as transcription factors and RNA-stabilizing factors thus adding a further level of complexity to the pleiotropic biological activities of TRIM proteins. The current review focuses on oncogenic signaling processes targeted by different TRIMs and their particular role in the development of CRC. A better understanding of the crosstalk of TRIMs with these signaling pathways relevant for CRC development is an important prerequisite for the validation of TRIM proteins as novel biomarkers and as potential targets of future therapies for CRC.
Hepatic cells are sensitive to internal and external signals. Ethanol is one of the oldest and most widely used drugs in the world. The focus on the mechanistic engine of the alcohol-induced injury has been in the liver, which is responsible for the pathways of alcohol metabolism. Ethanol undergoes a phase I type of reaction, mainly catalyzed by the cytoplasmic enzyme, alcohol dehydrogenase (ADH), and by the microsomal ethanol-oxidizing system (MEOS). Reactive oxygen species (ROS) generated by cytochrome (CYP) 2E1 activity and MEOS contribute to ethanol-induced toxicity. We aimed to: (1) Describe the cellular, pathophysiological and clinical effects of alcohol misuse on the liver; (2) Select the biomarkers and analytical methods utilized by the clinical laboratory to assess alcohol exposure; (3) Provide therapeutic ideas to prevent/reduce alcohol-induced liver injury; (4) Provide up-to-date knowledge regarding the Corona virus and its affect on the liver; (5) Link rare diseases with alcohol consumption. The current review contributes to risk identification of patients with alcoholic, as well as non-alcoholic, liver disease and metabolic syndrome. Additional prevalence of ethnic, genetic, and viral vulnerabilities are presented.
Efferocytosis is critical for tissue homeostasis, as its deregulation is associated with several autoimmune pathologies. While engulfing apoptotic cells, phagocytes activate transcription factors, such as peroxisome proliferator-activated receptors (PPAR) or liver X receptors (LXR) that orchestrate metabolic, phagocytic, and inflammatory responses towards the ingested material. Coordination of these transcription factors in efferocytotic human macrophages is not fully understood. In this study, we evaluated the transcriptional profile of macrophages following the uptake of apoptotic Jurkat T cells using RNA-seq analysis. Results indicated upregulation of PPAR and LXR pathways but downregulation of sterol regulatory element-binding proteins (SREBP) target genes. Pharmacological inhibition and RNA interference pointed to LXR and PPARδ as relevant transcriptional regulators, while PPARγ did not substantially contribute to gene regulation. Mechanistically, lysosomal digestion and lysosomal acid lipase (LIPA) were required for PPAR and LXR activation, while PPARδ activation also demanded an active lysosomal phospholipase A2 (PLA2G15). Pharmacological interference with LXR signaling attenuated ABCA1-dependent cholesterol efflux from efferocytotic macrophages, but suppression of inflammatory responses following efferocytosis occurred independently of LXR and PPARδ. These data provide mechanistic details on LXR and PPARδ activation in efferocytotic human macrophages.
The deregulation of Polo-like kinase 1 is inversely linked to the prognosis of patients with diverse human tumors. Targeting Polo-like kinase 1 has been widely considered as one of the most promising strategies for molecular anticancer therapy. While the preclinical results are encouraging, the clinical outcomes are rather less inspiring by showing limited anticancer activity. It is thus of importance to identify molecules and mechanisms responsible for the sensitivity of Polo-like kinase 1 inhibition. We have recently shown that p21Cip1/CDKN1A is involved in the regulation of mitosis and its loss prolongs the mitotic duration accompanied by defects in chromosome segregation and cytokinesis in various tumor cells. In the present study, we demonstrate that p21 affects the efficacy of Polo-like kinase 1 inhibitors, especially Poloxin, a specific inhibitor of the unique Polo-box domain. Intriguingly, upon treatment with Polo-like kinase 1 inhibitors, p21 is increased in the cytoplasm, associated with anti-apoptosis, DNA repair and cell survival. By contrast, deficiency of p21 renders tumor cells more susceptible to Polo-like kinase 1 inhibition by showing a pronounced mitotic arrest, DNA damage and apoptosis. Furthermore, long-term treatment with Plk1 inhibitors induced fiercely the senescent state of tumor cells with functional p21. We suggest that the p21 status may be a useful biomarker for predicting the efficacy of Plk1 inhibition.
Carcinogenesis is a multistep process. Besides somatic mutations in tumor cells, stroma-associated immunity is a major regulator of tumor growth. Tumor cells produce and secrete diverse mediators to create a local microenvironment that supports their own survival and growth. It is becoming apparent that iron acquisition, storage, and release in tumor cells is different from healthy counterparts. It is also appreciated that macrophages in the tumor microenvironment acquire a tumor-supportive, anti-inflammatory phenotype that promotes tumor cell proliferation, angiogenesis, and metastasis. Apparently, this behavior is attributed, at least in part, to the ability of macrophages to support tumor cells with iron. Polarization of macrophages by apoptotic tumor cells shifts the profile of genes involved in iron metabolism from an iron sequestering to an iron-release phenotype. Iron release from macrophages is supposed to be facilitated by ferroportin. However, lipid mediators such as sphingosine-1-phosphate, released form apoptotic tumor cells, upregulate lipocalin-2 (Lcn-2) in macrophages. This protein is known to bind siderophore-complexed iron and thus, may participate in iron transport in the tumor microenvironment. We describe how macrophages handle iron in the tumor microenvironment, discuss the relevance of an iron-release macrophage phenotype for tumor progression, and propose a new role for Lcn-2 in tumor-associated macrophages.
Impact of Polo-like kinase 1 inhibitors on human adipose tissue-derived mesenchymal stem cells
(2016)
Polo-like kinase 1 (Plk1) has been established as one of the most promising targets for molecular anticancer intervention. In fact, various Plk1 inhibitors have been identified and characterized. While the data derived from the bench are prospective, the clinical outcomes are less encouraging by showing modest efficacy. One of the explanations for this discrepancy could be unintendedly targeting of non-malignant cells by Plk1 inhibitors. In this work, we have addressed the effect of Plk1 inhibition in adipose tissue-derived mesenchymal stem cells (ASCs). We show that both visceral and subcutaneous ASCs display monopolar spindles, reduced viability and strong apoptosis induction upon treatment with BI 2536 and BI 6727, the Plk1 kinase domain inhibitors, and with Poloxin, the regulatory Polo-box domain inhibitor. While Poloxin triggers quickly apoptosis, BI 2536 and BI 6727 result in mitotic arrest in ASCs. Importantly, survived ASCs exhibit DNA damage and a pronounced senescent phenotype. In addition, Plk1 inhibition impairs ASCs’ motility and homing ability. These results show that Plk1 inhibitors target slowly proliferating ASCs, an important population of anti-inflammation and immune modulation. The toxic effects on primary cells like ASCs could be partially responsible for the reported moderate antitumor activity in patients treated with Plk1 inhibitors.
We previously reported that aberrant HH pathway activation confers a poor prognosis in rhabdomyosarcoma (RMS). Searching for new treatment strategies we therefore targeted HH signaling. Here, we identify a novel synthetic lethality of concomitant inhibition of HH and PI3K/AKT/mTOR pathways in RMS by GLI1/2 inhibitor GANT61 and PI3K/mTOR inhibitor PI103. Synergistic drug interaction is confirmed by calculation of combination index (CI < 0.2). Similarly, genetic silencing of GLI1/2 significantly increases PI103-induced apoptosis. GANT61 and PI103 also synergize to induce apoptosis in cultured primary RMS cells emphasizing the clinical relevance of this combination. Importantly, GANT61/PI103 cotreatment suppresses clonogenic survival, three-dimensional sphere formation and tumor growth in an in vivo model of RMS. Mechanistic studies reveal that GANT61 and PI103 cooperate to trigger caspase-dependent apoptosis via the mitochondrial pathway, as demonstrated by several lines of evidence. First, GANT61/PI103 cotreatment increases mRNA and protein expression of NOXA and BMF, which is required for apoptosis, since knockdown of NOXA or BMF significantly reduces GANT61/PI103-induced apoptosis. Second, GANT61/PI103 cotreatment triggers BAK/BAX activation, which contributes to GANT61/PI103-mediated apoptosis, since knockdown of BAK provides protection. Third, ectopic expression of BCL-2 or non-degradable phospho-mutant MCL-1 significantly rescue GANT61/PI103-triggered apoptosis. Fourth, GANT61/PI103 cotreatment initiate activation of the caspase cascade via apoptosome-mediated cleavage of the initiator caspase-9, as indicated by changes in the cleavage pattern of caspases (e.g. accumulation of the caspase-9 p35 cleavage fragment) upon addition of the caspase inhibitor zVAD.fmk. Thus, combined GLI1/2 and PI3K/mTOR inhibition represents a promising novel approach for synergistic apoptosis induction and tumor growth reduction with implications for new treatment strategies in RMS.
Apoptosis represents one of the most important forms of cell death in higher organisms and is typically dysregulated in human cancers, including pediatric tumors. This implies that ineffective engagement of cell death programs can contribute to tumor formation as well as tumor progression. In addition, the majority of cytotoxic therapeutic principles rely on the activation of cell death signaling pathways in cancer cells. Blockade of signaling networks that lead to cell death can therefore confer treatment resistance. A variety of genetic and epigenetic events as well as dysfunctional regulation of signaling networks have been identified as underlying causes of cell death resistance in childhood malignancies. Apoptosis pathways can be therapeutically exploited by enhancing proapoptotic signals or by neutralizing antiapoptotic programs. The challenge in the coming years will be to successfully transfer this knowledge into the development of innovative treatment approaches for children with cancer.
Curcumin—a rhizomal phytochemical from the plant Curcuma longa—is well known to inhibit cell proliferation and to induce apoptosis in a broad range of cell lines. In previous studies we showed that combining low curcumin concentrations and subsequent ultraviolet A radiation (UVA) or VIS irradiation induced anti-proliferative and pro-apoptotic effects. There is still debate whether curcumin induces apoptosis via the extrinsic or the intrinsic pathway. To address this question, we investigated in three epithelial cell lines (HaCaT, A431, A549) whether the death receptors CD95, tumor necrosis factor (TNF)-receptor I and II are involved in apoptosis induced by light and curcumin. Cells were incubated with 0.25–0.5 µg/mL curcumin followed by irradiation with 1 J/cm2 UVA. This treatment was combined with inhibitors specific for distinct membrane-bound death receptors. After 24 h apoptosis induction was monitored by quantitative determination of cytoplasmic histone-associated-DNA-fragments. Validation of our test system showed that apoptosis induced by CH11 and TNF-α could be completely inhibited by their respective antagonists. Interestingly, apoptosis induced by curcumin/light treatment was reversed by none of the herein examined death receptor antagonists. These results indicate a mechanism of action independent from classical death receptors speaking for intrinsic activation of apoptosis. It could be speculated that a shift in cellular redox balance might prompt the pro-apoptotic processes
EFP1 is an ER stress-induced glycoprotein which interacts with the pro-apoptotic protein Par-4
(2009)
We have isolated the rat ortholog of EFP1 (EF-hand binding protein 1) as a novel interaction partner of the pro-apoptotic protein Par-4 (prostate apoptosis response-4). Rat EFP1 contains two thioredoxin domains, the COOH-terminal one harboring a CGFC motif, and has a similar protein domain structure as members of the protein disulfide isomerase (PDI) family. In REF52.2 and CHO cells, EFP1 colocalized with the endoplasmic reticulum (ER) marker PDI. Furthermore, EFP1 possesses catalytic activity as demonstrated by an insulin disulfide reduction assay. Western blot analysis revealed two EFP1 protein bands of approximately 136 and 155 kDa, representing different glycosylation states of the protein. Complex formation between EFP1 and Par-4 was confirmed in vitro and in vivo by co-immunoprecipitation, dot blot overlay and pull-down experiments. In CHO cells, coexpression of EFP1 and Par-4 resulted in enhanced Par-4-mediated apoptosis, which required the catalytic activity of EFP1. Interestingly, EFP1 was specifically upregulated in NIH3T3 cells after induction of ER stress by thapsigargin, tunicamycin, and brefeldin A, but not by agents that induce oxidative stress or ER-independent apoptosis. Furthermore, we could show that the induction of apoptosis by Ca2+ stress-inducing agents was significantly decreased after siRNA oligonucleotide-mediated knockdown of Par-4. Our data suggest that EFP1 might represent a cell-protective enzyme that could play an important role in the decision between survival and initiation of Par-4-mediated apoptosis.
Recently, the conserved intracellular digestion mechanism ‘autophagy’ has been considered to be involved in early tumorigenesis and its blockade proposed as an alternative treatment approach. However, there is an ongoing debate about whether blocking autophagy has positive or negative effects in tumor cells. Since there is only poor data about the clinico-pathological relevance of autophagy in gliomas in vivo, we first established a cell culture based platform for the in vivo detection of the autophago-lysosomal components. We then investigated key autophagosomal (LC3B, p62, BAG3, Beclin1) and lysosomal (CTSB, LAMP2) molecules in 350 gliomas using immunohistochemistry, immunofluorescence, immunoblotting and qPCR. Autophagy was induced pharmacologically or by altering oxygen and nutrient levels. Our results show that autophagy is enhanced in astrocytomas as compared to normal CNS tissue, but largely independent from the WHO grade and patient survival. A strong upregulation of LC3B, p62, LAMP2 and CTSB was detected in perinecrotic areas in glioblastomas suggesting micro-environmental changes as a driver of autophagy induction in gliomas. Furthermore, glucose restriction induced autophagy in a concentration-dependent manner while hypoxia or amino acid starvation had considerably lesser effects. Apoptosis and autophagy were separately induced in glioma cells both in vitro and in vivo. In conclusion, our findings indicate that autophagy in gliomas is rather driven by micro-environmental changes than by primary glioma-intrinsic features thus challenging the concept of exploitation of the autophago-lysosomal network (ALN) as a treatment approach in gliomas.
Smac mimetics antagonize IAP proteins, which are highly expressed in several cancers. Recent reports indicate that Smac mimetics trigger a broad cytokine response and synergize with immune modulators to induce cell death. Here, we identify a differential requirement of TRAIL or TNFα as mediators of IFNα/Smac mimetic-induced cell death depending on the cellular context. Subtoxic concentrations of Smac mimetics cooperate with IFNα to induce cell death in various solid tumor cell lines in a highly synergistic manner as determined by combination index. Mechanistic studies show that IFNα/BV6 cotreatment promotes the formation of a caspase-8-activating complex together with the adaptor protein FADD and RIP1. Assembly of this RIP1/FADD/caspase-8 complex represents a critical event, since RIP1 silencing inhibits IFNα/BV6-induced cell death. Strikingly, pharmacological inhibition of paracrine/autocrine TNFα signaling by the TNFα scavenger Enbrel rescues HT-29 colon carcinoma cells, but not A172 glioblastoma cells from IFNα/BV6-induced cell death. By comparison, A172 cells are significantly protected against IFNα/BV6 treatment by blockage of TRAIL signaling through genetic silencing of TRAIL or its cognate receptor TRAIL receptor 2 (DR5). Despite this differential requirement of TNFα and TRAIL signaling, mRNA and protein expression is increased by IFNα/BV6 cotreatment in both cell lines. Interestingly, A172 cells turn out to be resistant to exogenously added recombinant TNFα even in the presence of BV6, whereas they display a high sensitivity towards TRAIL/BV6. In contrast, BV6 efficiently sensitizes HT-29 cells to TNFα while TRAIL only had limited efficacy. This demonstrates that a differential sensitivity towards TRAIL or TNFα determines the dependency on either death receptor ligand for IFNα/Smac mimetic-induced cell death. Thus, by concomitant stimulation of both death receptor systems IFNα/Smac mimetic combination treatment is an effective strategy to induce cell death in TNFα- or TRAIL-responsive cancers.
Betulinic acid is a natural product with a range of biological effects, for example potent antitumor activity. This anticancer property is linked to its ability to induce apoptotic cell death in cancer cells by triggering the mitochondrial pathway of apoptosis. In contrast to the cytotoxicity of betulinic acid against a variety of cancer types, normal cells and tissue are relatively resistant to betulinic acid, pointing to a therapeutic window. Compounds that exert a direct action on mitochondria present promising experimental cancer therapeutics, since they may trigger cell death under circumstances in which standard chemotherapeutics fail. Thus, mitochondrion-targeted agents such as betulinic acid hold great promise as a novel therapeutic strategy in the treatment of human cancers.
Keywords: apoptosis, cancer, betulinic acid, mitochondria
Keywords: AIF, apoptosis inducing factor; Apaf-1, Apoptotic protease activating factor-1; BA, betulinic acid; DIABLO, direct IAP Binding protein with Low PI; HtrA2, high temperature requirement protein A; IAPs, Inhibitor of Apoptosis Proteins; MOMP, mitochondrial outer membrane permeabilization; ROS, reactive oxygen species; PARP, Poly (ADP-ribose) Polymerase; Smac, second mitochondria-derived activator of caspase; TNF, tumor necrosis factor; TRAIL, tumor necrosis factor-related apoptosis-inducing ligand; zVAD.fmk, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone
Macrophages ingesting apoptotic cells attenuate inflammatory responses, such as reactive oxygen species (ROS) generation. In atherosclerosis, ongoing inflammation and accumulation of apoptotic/necrotic material are observed, suggesting defects of phagocytes in recognizing or responding to dying cells. Modified lipoproteins such as oxidized LDL (oxLDL) are known to promote inflammation and to interfere with apoptotic cell clearance. Here, we studied the impact of cells exposed to oxLDL on their ability to interfere with the oxidative burst in phagocytes. In contrast to apoptotic cells, cells dying in response to or in the presence of oxLDL failed to suppress ROS generation despite efficiently being taken up by phagocytes. In addition, apoptotic cells, but not oxLDL-treated cells, inhibited phosphorylation of extracellular signal-regulated kinase, which is important for NADPH oxidase activation. oxLDL treatment did not interfere with activation of the antiinflammatory transcriptional regulator peroxisome proliferator-activated receptor gamma by apoptotic cells. Moreover, cells exposed to oxLDL failed to suppress lipopolysaccharide- induced proinflammatory cytokine expression, whereas apoptotic cells attenuated these phagocyte responses. Thus, the presence of oxLDL during cell death impaired the ability of apoptotic cells to act antiinflammatory with regard to oxidative burst inhibition and cytokine expression in phagocytes.
BAG3, a multifunctional HSP70 co-chaperone and anti-apoptotic protein that interacts with the ATPase domain of HSP70 through its C-terminal BAG domain plays a key physiological role in cellular proteostasis. The HSP70/BAG3 complex determines the levels of a large number of selective client proteins by regulating their turnover via the two major protein degradation pathways, i.e. proteasomal degradation and macroautophagy. On the one hand, BAG3 competes with BAG1 for binding to HSP70, thereby preventing the proteasomal degradation of its client proteins. By functionally interacting with HSP70 and LC3, BAG3 also delivers polyubiquitinated proteins to the autophagy pathway. BAG3 exerts a number of key physiological functions, including an involvement in cellular stress responses, proteostasis, cell death regulation, development, and cytoskeletal dynamics. Conversely, aberrant BAG3 function/expression has pathophysiological relevance correlated to cardiomyopathies, neurodegeneration, and cancer. Evidence obtained in recent years underscores the fact that BAG3 drives several key hallmarks of cancer, including cell adhesion, metastasis, angiogenesis, enhanced autophagic activity, and apoptosis inhibition. This review provides a state-of-the-art overview on the role of BAG3 in stress and therapy resistance of cancer, with a particular focus on BAG3-dependent modulation of apoptotic signaling and autophagic/lysosomal activity.
Cisplatin, which induces DNA damage, is standard chemotherapy for advanced bladder cancer (BCa). However, efficacy is limited due to resistance development. Since artesunate (ART), a derivative of artemisinin originating from Traditional Chinese Medicine, has been shown to exhibit anti-tumor activity, and to inhibit DNA damage repair, the impact of artesunate on cisplatin-resistant BCa was evaluated. Cisplatin-sensitive (parental) and cisplatin-resistant BCa cells, RT4, RT112, T24, and TCCSup, were treated with ART (1–100 µM). Cell growth, proliferation, and cell cycle phases were investigated, as were apoptosis, necrosis, ferroptosis, autophagy, metabolic activity, and protein expression. Exposure to ART induced a time- and dose-dependent significant inhibition of tumor cell growth and proliferation of parental and cisplatin-resistant BCa cells. This inhibition was accompanied by a G0/G1 phase arrest and modulation of cell cycle regulating proteins. ART induced apoptos is by enhancing DNA damage, especially in the resistant cells. ART did not induce ferroptosis, but led to a disturbance of mitochondrial respiration and ATP generation. This impairment correlated with autophagy accompanied by a decrease in LC3B-I and an increase in LC3B-II. Since ART significantly inhibits proliferative and metabolic aspects of cisplatin-sensitive and cisplatin-resistant BCa cells, it may hold potential in treating advanced and therapy-resistant BCa.
The interaction of macrophages with apoptotic cells is required for efficient resolution of inflammation. While apoptotic cell removal prevents inflammation due to secondary necrosis, it also alters the macrophage phenotype to hinder further inflammatory reactions. The interaction between apoptotic cells and macrophages is often studied by chemical or biological induction of apoptosis, which may introduce artifacts by affecting the macrophages as well and/or triggering unrelated signaling pathways. Here, we set up a pure cell death system in which NIH 3T3 cells expressing dimerizable Caspase-8 were co-cultured with peritoneal macrophages in a transwell system. Phenotype changes in macrophages induced by apoptotic cells were evaluated by RNA sequencing, which revealed an unexpectedly dominant impact on macrophage proliferation. This was confirmed in functional assays with primary peritoneal macrophages and IC-21 macrophages. Moreover, inhibition of apoptosis during Zymosan-induced peritonitis in mice decreased mRNA levels of cell cycle mediators in peritoneal macrophages. Proliferation of macrophages in response to apoptotic cells may be important to increase macrophage numbers in order to allow efficient clearance and resolution of inflammation.
To search for novel strategies to enhance the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis pathways in glioblastoma, we used the B-cell lymphoma 2/Bcl2-like 2-inhibitor ABT-737. Here we report that ABT-737 and TRAIL cooperate to induce apoptosis in several glioblastoma cell lines in a highly synergistic manner (combination index <0.1). Interestingly, the concerted action of ABT-737 and TRAIL to trigger the accumulation of truncated Bid (tBid) at mitochondrial membranes is identified as a key underlying mechanism. ABT-737 and TRAIL cooperate to cleave BH3-interacting domain death agonist (Bid) into its active fragment tBid, leading to increased accumulation of tBid at mitochondrial membranes. Coinciding with tBid accumulation, the activation of Bcl2-associated X protein (Bax), loss of mitochondrial membrane potential, release of cytochrome-c and second mitochondria-derived activator of caspase (Smac) into the cytosol and caspase activation are strongly increased in cotreated cells. Of note, knockdown of Bid significantly decreases ABT-737- and TRAIL-mediated Bax activation and apoptosis. Also, caspase-3 silencing reduces ABT-737- and TRAIL-induced Bid cleavage and apoptosis, indicating that a caspase-3-driven, mitochondrial feedback loop contributes to Bid processing. Importantly, ABT-737 profoundly enhances TRAIL-triggered apoptosis in primary cultured glioblastoma cells derived from tumor material, underlining the clinical relevance. Also, ABT-737 acts in concert with TRAIL to suppress tumor growth in an in vivo glioblastoma model. In conclusion, the rational combination of ABT-737 and TRAIL cooperates to trigger tBid mitochondrial accumulation and apoptosis. This approach presents a promising strategy for targeting the apoptosis pathways in glioblastoma, which warrants further investigation.
Apigenin (4′,5,7-trihydroxyflavone) (Api) is an important component of the human diet, being distributed in a wide number of fruits, vegetables and herbs with the most important sources being represented by chamomile, celery, celeriac and parsley. This study was designed for a comprehensive evaluation of Api as an antiproliferative, proapoptotic, antiangiogenic and immunomodulatory phytocompound. In the set experimental conditions, Api presents antiproliferative activity against the A375 human melanoma cell line, a G2/M arrest of the cell cycle and cytotoxic events as revealed by the lactate dehydrogenase release. Caspase 3 activity was inversely proportional to the Api tested doses, namely 30 μM and 60 μM. Phenomena of early apoptosis, late apoptosis and necrosis following incubation with Api were detected by Annexin V-PI double staining. The flavone interfered with the mitochondrial respiration by modulating both glycolytic and mitochondrial pathways for ATP production. The metabolic activity of human dendritic cells (DCs) under LPS-activation was clearly attenuated by stimulation with high concentrations of Api. Il-6 and IL-10 secretion was almost completely blocked while TNF alpha secretion was reduced by about 60%. Api elicited antiangiogenic properties in a dose-dependent manner. Both concentrations of Api influenced tumour cell growth and migration, inducing a limited tumour area inside the application ring, associated with a low number of capillaries.
Leukotrienes constitute a group of bioactive lipids generated by the 5-lipoxygenase (5-LO) pathway. An increasing body of evidence supports an acute role for 5-LO products already during the earliest stages of pancreatic, prostate, and colorectal carcinogenesis. Several pieces of experimental data form the basis for this hypothesis and suggest a correlation between 5-LO expression and tumor cell viability. First, several independent studies documented an overexpression of 5-LO in primary tumor cells as well as in established cancer cell lines. Second, addition of 5-LO products to cultured tumor cells also led to increased cell proliferation and activation of anti-apoptotic signaling pathways. 5-LO antisense technology approaches demonstrated impaired tumor cell growth due to reduction of 5-LO expression. Lastly, pharmacological inhibition of 5-LO potently suppressed tumor cell growth by inducing cell cycle arrest and triggering cell death via the intrinsic apoptotic pathway. However, the documented strong cytotoxic off-target effects of 5-LO inhibitors, in combination with the relatively high concentrations of 5-LO products needed to achieve mitogenic effects in cell culture assays, raise concern over the assignment of the cause, and question the relationship between 5-LO products and tumorigenesis. Keywords: leukotriene, apoptosis, cell proliferation, mitogenic effects, cytotoxicity