Trim17-mediated ubiquitination and degradation of Mcl-1 initiate apoptosis in neurons

The coffee diterpene kahweol enhances sensitivity to sorafenib in human renal carcinoma Caki cells through down-regulation of Mcl-1 and c-FLIP expression

Sorafenib is approved for the treatment of hepatocellular carcinoma (HCC) and advanced renal cell carcinoma (RCC). However, low tumor response and side effects have been widely reported. Therefore, to improve the efficacy of sorafenib, we investigated whether combined treatment with sorafenib and kahweol, the coffee-specific diterpene, has a synergistic effect on apoptotic cell death. Combined treatment with sorafenib and kahweol markedly induced caspase-mediated apoptosis in renal carcinoma Caki cells. Combined treatment with sorafenib and kahweol induced down-regulation of Mcl-1 and c-FLIP expression. We found down-regulation of Mcl-1 and c-FLIP expression was modulated by the ubiquitin-proteasome pathway. Ectopic expression of Mcl-1 inhibited sorafenib plus kahweol-induced apoptosis. Interestingly, combined treatment with sorafenib and kahweol induced apoptotic cell death in c-FLIP overexpressed cells. In addition, combined treatment with sorafenib and kahweol markedly induced apoptosis in human lung carcinoma (A549) and breast carcinoma (MDA-MB-361) cells, but not in human normal mesangial cells and human skin fibroblast cells (HSF). Collectively, our study demonstrates that combined treatment with sorafenib and kahweol induces apoptotic cell death through down-regulation of Mcl-1 expression.

The Cdk5-Mcl-1 axis promotes mitochondrial dysfunction and neurodegeneration in a model of Alzheimer's disease

Cdk5 deregulation is highly neurotoxic in Alzheimer's disease (AD). We identified Mcl-1 as a direct Cdk5 substrate using an innovative chemical screen in mouse brain lysates. Our data demonstrate that Mcl-1 levels determine the threshold for cellular damage in response to neurotoxic insults. Mcl-1 is a disease-specific target of Cdk5, which associates with Cdk5 under basal conditions, but is not regulated by it. Neurotoxic insults hyperactivate Cdk5 causing Mcl-1 phosphorylation at T92. This phosphorylation event triggers Mcl-1 ubiquitylation, which directly correlates with mitochondrial dysfunction. Consequently, ectopic expression of phosphorylation-dead T92A-Mcl-1 fully prevents mitochondrial damage and subsequent cell death triggered by neurotoxic treatments in neuronal cells and primary cortical neurons. Notably, enhancing Mcl-1 levels offers comparable neuroprotection to that observed upon Cdk5 depletion, suggesting that Mcl-1 degradation by direct phosphorylation is a key mechanism by which Cdk5 promotes neurotoxicity in AD. The clinical significance of the Mcl-1-Cdk5 axis was investigated in human AD clinical specimens, revealing an inverse correlation between Mcl-1 levels and disease severity. These results emphasize the potential of Mcl-1 upregulation as an attractive therapeutic strategy for delaying or preventing neurodegeneration in AD.

HDAC1 Upregulation by NANOG Promotes Multidrug Resistance and a Stem-like Phenotype in Immune Edited Tumor Cells

Cancer immunoediting drives the adaptation of tumor cells to host immune surveillance. Immunoediting driven by antigen (Ag)-specific T cells enriches NANOG expression in tumor cells, resulting in a stem-like phenotype and immune resistance. Here, we identify HDAC1 as a key mediator of the NANOG-associated phenotype. NANOG upregulated HDAC1 through promoter occupancy, thereby decreasing histone H3 acetylation on K14 and K27. NANOG-dependent, HDAC1-driven epigenetic silencing of cell-cycle inhibitors CDKN2D and CDKN1B induced stem-like features. Silencing of TRIM17 and NOXA induced immune and drug resistance in tumor cells by increasing antiapoptotic MCL1. Importantly, HDAC inhibition synergized with Ag-specific adoptive T-cell therapy to control immune refractory cancers. Our results reveal that NANOG influences the epigenetic state of tumor cells via HDAC1, and they encourage a rational application of epigenetic modulators and immunotherapy in treatment of NANOG⁺ refractory cancer types. Cancer Res; 77(18); 5039-53. ©2017 AACR.

Thioridazine enhances sensitivity to carboplatin in human head and neck cancer cells through downregulation of c-FLIP and Mcl-1 expression

Carboplatin is a less toxic analog of cisplatin, but carboplatin also has side effects, including bone marrow suppression. Therefore, to improve the capacity of the anticancer activity of carboplatin, we investigated whether combined treatment with carboplatin and thioridazine, which has antipsychotic and anticancer activities, has a synergistic effect on apoptosis. Combined treatment with carboplatin and thioridazine markedly induced caspase-mediated apoptosis in head and neck squamous cell carcinoma (AMC-HN4) cells. Combined treatment with carboplatin and thioridazine induced downregulation of Mcl-1 and c-FLIP expression. Ectopic expression of Mcl-1 and c-FLIP inhibited carboplatin plus thioridazine-induced apoptosis. We found that augmentation of proteasome activity had a critical role in downregulation of Mcl-1 and c-FLIP expression at the post-translational level in carboplatin plus thioridazine-treated cells. Furthermore, carboplatin plus thioridazine induced upregulation of the expression of proteasome subunit alpha 5 (PSMA5) through mitochondrial reactive oxygen species (ROS)-dependent nuclear factor E2-related factor 2 (Nrf2) activation. In addition, combined treatment with carboplatin and thioridazine markedly induced apoptosis in human breast carcinoma (MDA-MB231) and glioma (U87MG) cells, but not in human normal mesangial cells and normal human umbilical vein cells (EA.hy926). Collectively, our study demonstrates that combined treatment with carboplatin and thioridazine induces apoptosis through proteasomal degradation of Mcl-1 and c-FLIP by upregulation of Nrf2-dependent PSMA5 expression.

TRIM17 contributes to autophagy of midbodies while actively sparing other targets from degradation

TRIM proteins contribute to selective autophagy, a process whereby cells target specific cargo for autophagic degradation. In a previously reported screen, TRIM17 acted as a prominent inhibitor of bulk autophagy, unlike the majority of TRIMs, which had positive roles. Nevertheless, TRIM17 showed biochemical hallmarks of autophagy-inducing TRIMs. To explain this paradox, here, we investigated how TRIM17 inhibits selective autophagic degradation of a subset of targets while promoting degradation of others. We traced the inhibitory function of TRIM17 to its actions on the anti-autophagy protein Mcl-1, which associates with and inactivates Beclin 1. TRIM17 expression stabilized Mcl-1-Beclin-1 complexes. Despite its ability to inhibit certain types of selective autophagy, TRIM17 promoted the removal of midbodies, remnants of the cell division machinery that are known autophagy targets. The selective loss of anti-autophagy Mcl-1 from TRIM17-Beclin-1 complexes at midbodies correlated with the ability of TRIM17 to promote midbody removal. This study further expands the roles of TRIMs in regulating selective autophagy by showing that a single TRIM can, depending upon a target, either positively or negatively regulate autophagy.

BAG3-mediated Mcl-1 stabilization contributes to drug resistance via interaction with USP9X in ovarian cancer

Paclitaxel in combination with carboplatin improves survival among patients with susceptible ovarian cancers, but no strategy has been established against resistant ovarian cancers. BAG3 (Bcl-2-associated athanogene 3) is one of six BAG family proteins, which are involved in such cellular processes as proliferation, migration and apoptosis. In addition, expression of BAG3 with Mcl-1, a Bcl-2 family protein, reportedly associates with resistance to chemotherapy. Our aim in this study was to evaluate the functional role of BAG3 and Mcl-1 in ovarian cancer chemoresistance and explore possible new targets for treatment. We found that combined expression of BAG3 and Mcl-1 was significantly associated with a poor prognosis in ovarian cancer patients. In vitro, BAG3 knockdown in ES2 clear ovarian cancer cells significantly increased the efficacy of paclitaxel in combination with the Mcl-1 antagonist MIM1, with or without the Bcl-2 family antagonist ABT737. Moreover, BAG3 was found to positively regulate Mcl-1 levels by binding to and inhibiting USP9X. Our data show that BAG3 and Mcl-1 are key mediators of resistance to chemotherapy in ovarian cancer. In BAG3 knockdown ES2 clear ovarian cancer cells, combination with ABT737 and MIM1 enhanced the efficacy of paclitaxel. These results suggest that inhibiting BAG3 in addition to anti-apoptotic Bcl-2 family proteins may be a useful therapeutic strategy for the treatment of chemoresistant ovarian cancers.

Myeloid cell leukemia 1 (Mcl(-1)) protects against 1-methyl-4-phenylpyridinium ion (MPP+) induced apoptosis in Parkinson's disease

The myeloid cell leukemia 1 (Mcl(-1)) is an anti-apoptotic member of the Bcl-2 family, which plays an essential role in protecting cells against apoptosis. The expression pattern and potential roles of Mcl(-1) in Parkinson's diseases (PD) are still unknown. In this study, our results indicated that 1-methyl-4-phenylpyridinium (MPP+) treatment augmented the expression of Mcl(-1) at both messenger RNA (mRNA) and protein levels in a dose-dependent manner in SH-SY5Y cells. Moreover, we observed increased phosphorylation of Elk-1at Ser383 as well as nuclear translocation of Elk-1 in exposure to MPP+ treatment. Importantly, the elevated expression of Mcl(-1) induced by MPP+ was abolished by knockdown of Elk-1. It was also found that inhibition of Mcl(-1) by small RNA transfection exacerbates MPP + -induced LDH release after 48 h incubation. In addition, Hoechst 33,258 nuclear staining results demonstrated that silence of Mcl(-1) induced a significant increase in apoptosis in cells when compared with the control condition. Mechanistically, the levels of cleaved Caspase3 and PARP were elevated in MPP+ treated cells, which was exacerbated by knockdown of Mcl(-1). These findings suggest that Mcl(-1) might be a potential therapeutic target for PD treatment.

Regulation of Bim in Health and Disease

The BH3-only Bim protein is a major determinant for initiating the intrinsic apoptotic pathway under both physiological and pathophysiological conditions. Tight regulation of its expression and activity at the transcriptional, translational and post-translational levels together with the induction of alternatively spliced isoforms with different pro-apoptotic potential, ensure timely activation of Bim. Under physiological conditions, Bim is essential for shaping immune responses where its absence promotes autoimmunity, while too early Bim induction eliminates cytotoxic T cells prematurely, resulting in chronic inflammation and tumor progression. Enhanced Bim induction in neurons causes neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's diseases. Moreover, type I diabetes is promoted by genetically predisposed elevation of Bim in β-cells. On the contrary, cancer cells have developed mechanisms that suppress Bim expression necessary for tumor progression and metastasis. This review focuses on the intricate network regulating Bim activity and its involvement in physiological and pathophysiological processes.

Oncostatin M-dependent Mcl-1 induction mediated by JAK1/2-STAT1/3 and CREB contributes to bioenergetic improvements and protective effects against mitochondrial dysfunction in cortical neurons

Oncostatin M (OSM), a cytokine in the interleukin-6 (IL-6) family, has been proposed to play a protective role in the central nervous system, such as attenuation of excitotoxicity induced by N-methyl-D-aspartate (NMDA) and glutamate. However, the potential neuroprotective effects of OSM against mitochondrial dysfunction have never been reported. In the present study, we tested the hypothesis that OSM may confer neuronal resistance against 3-nitropropionic acid (3-NP), a plant toxin that irreversibly inhibits the complex II of the mitochondrial electron transport chain, and characterized the underlying molecular mechanisms. We found that OSM preconditioning dose- and time-dependently protected cortical neurons against 3-NP toxicity. OSM stimulated expression of myeloid cell leukemia-1 (Mcl-1), an anti-apoptotic Bcl-2 family member expressed in differentiating myeloid cells, that required prior phosphorylation of Janus kinase-1 (JAK1), JAK2, extracellular signal-regulated kinase-1/2 (ERK1/2), signal transducer and activator of transcription-3 (STAT3), STAT1, and cAMP-response element-binding protein (CREB). Pharmacological inhibitors of JAK1, JAK2, ERK1/2, STAT3, STAT1, and CREB as well as the siRNA targeting at STAT3 and Mcl-1 all abolished OSM-dependent 3-NP resistance. Finally, OSM-dependent Mcl-1 induction contributed to the enhancements of mitochondrial bioenergetics including increases in spare respiratory capacity and ATP production. In conclusion, our findings indicated that OSM induces Mcl-1 expression via activation of ERK1/2, JAK1/2, STAT1/3, and CREB; furthermore, OSM-mediated Mcl-1 induction contributes to bioenergetic improvements and neuroprotective effects against 3-NP toxicity in cortical neurons. OSM may thus serve as a novel neuroprotective agent against mitochondrial dysfunction commonly associated with pathogenic mechanisms underlying neurodegeneration.

Regulation of the DNA damage response by ubiquitin conjugation

In response to DNA damage, cells activate a highly conserved and complex kinase-based signaling network, commonly referred to as the DNA damage response (DDR), to safeguard genomic integrity. The DDR consists of a set of tightly regulated events, including detection of DNA damage, accumulation of DNA repair factors at the site of damage, and finally physical repair of the lesion. Upon overwhelming damage the DDR provokes detrimental cellular actions by involving the apoptotic machinery and inducing a coordinated demise of the damaged cells (DNA damage-induced apoptosis, DDIA). These diverse actions involve transcriptional activation of several genes that govern the DDR. Moreover, recent observations highlighted the role of ubiquitylation in orchestrating the DDR, providing a dynamic cellular regulatory circuit helping to guarantee genomic stability and cellular homeostasis (Popovic et al., 2014). One of the hallmarks of human cancer is genomic instability (Hanahan and Weinberg, 2011). Not surprisingly, deregulation of the DDR can lead to human diseases, including cancer, and can induce resistance to genotoxic anti-cancer therapy (Lord and Ashworth, 2012). Here, we summarize the role of ubiquitin-signaling in the DDR with special emphasis on its role in cancer and highlight the therapeutic value of the ubiquitin-conjugation machinery as a target in anti-cancer treatment strategy.

STAT3 promotes survival of mutant photoreceptors in inherited photoreceptor degeneration models

Inherited photoreceptor degenerations (IPDs), a group of incurable progressive blinding diseases, are caused by mutations in more than 200 genes, but little is known about the molecular pathogenesis of photoreceptor (PR) death. Increased retinal expression of STAT3 has been observed in response to many retinal insults, including IPDs, but the role of this increase in PR death is unknown. Here, we show that the expression of Stat3 is increased in PRs of the Tg(RHO P347S) and Prph2(rds) (/+) mouse models of IPD and is activated by tyrosine phosphorylation. PR-specific deletion of Stat3 substantially accelerated PR degeneration in both mutant strains. In contrast, increased PR-specific expression of ROSA26 (R26) alleles encoding either WT STAT3 (Stat3(wt)) or the gain-of-function variant STAT3(C) (Stat3(C)) improved PR survival in both models. Moreover, PR signaling in Tg(RHO P347S) mice carrying either a R26-Stat3(wt) or R26-Stat3(C) allele demonstrated increased a-wave amplitude of the scotopic electroretinogram. Phosphorylation of STAT3 at tyrosine 705 was required for the prosurvival effect because an R26-Stat3(Y705F) allele was not protective. The prosurvival role of enhanced Stat3 activity was validated using recombinant adenoassociated virus (rAAV) vector-mediated PR Stat3 expression in Tg(RHO P347S) mice. Our findings (i) establish that the increase in endogenous PR Stat3 expression is a protective response in IPDs, (ii) suggest that therapeutic augmentation of PR Stat3 expression has potential as a common neuroprotective therapy for these disorders, and (iii) indicate that prosurvival molecules whose expression is increased in mutant PRs may have promise as novel therapies for IPDs.

Anti-apoptotic BCL-2 family proteins in acute neural injury

Cells under stress activate cell survival and cell death signaling pathways. Cell death signaling frequently converges on mitochondria, a process that is controlled by the activities of pro- and anti-apoptotic B-cell lymphoma 2 (BCL-2) proteins. In this review, we summarize current knowledge on the control of neuronal survival, development and injury by anti-apoptotic BCL-2 family proteins. We discuss overlapping and differential effects of the individual family members BCL-2, BCL-extra long (BCL-X_L), myeloid cell leukemia 1 (MCL-1), and BCL2-like 2 (BCL-W) in the control of survival during development and pathophysiological processes such as trophic factor withdrawal, ischemic injury, excitotoxicity, oxidative stress and energy stress. Finally we discuss recent evidence that several anti-apoptotic BCL-2 proteins influence mitochondrial bioenergetics and control neuronal Ca²⁺ homeostasis independent of their classical role in cell death signaling.

Control of neuronal apoptosis by reciprocal regulation of NFATc3 and Trim17

Neuronal apoptosis induced by survival factor deprivation is strongly regulated at the transcriptional level. Notably, the nuclear factor of activated T cell (NFAT) transcription factors have an important role in the control of the survival/death fate of neurons. However, the mechanisms that regulate NFAT activity in response to apoptotic stimuli and the target genes that mediate their effect on neuronal apoptosis are mostly unknown. In a previous study, we identified Trim17 as a crucial E3 ubiquitin ligase that is necessary and sufficient for neuronal apoptosis. Here, we show that Trim17 binds preferentially SUMOylated forms of NFATc3. Nonetheless, Trim17 does not promote the ubiquitination/degradation of NFATc3. NFAT transcription factors are regulated by calcium/calcineurin-dependent nuclear-cytoplasmic shuttling. Interestingly, Trim17 reduced by twofold the calcium-mediated nuclear localization of NFATc3 and, consistent with this, halved NFATc3 activity, as estimated by luciferase assays and by measurement of target gene expression. Trim17 also inhibited NFATc4 nuclear translocation and activity. NFATc4 is known to induce the expression of survival factors and, as expected, overexpression of NFATc4 protected cerebellar granule neurons from serum/KCl deprivation-induced apoptosis. Inhibition of NFATc4 by Trim17 may thus partially mediate the proapoptotic effect of Trim17. In contrast, overexpression of NFATc3 aggravated neuronal death, whereas knockdown of NFATc3 protected neurons from apoptosis. This proapoptotic effect of NFATc3 might be due to a feedback loop in which NFATc3, but not NFATc4, induces the transcription of the proapoptotic gene Trim17. Indeed, we found that overexpression or silencing of NFATc3, respectively, increased or decreased Trim17 levels, whereas NFATc4 had no significant effect on Trim17 expression. Moreover, we showed that NFATc3 binds to the promoter of the Trim17 gene together with c-Jun. Therefore, our results describe a novel mechanism regulating NFAT transcription factors beyond the calcium/calcineurin-dependent pathway and provide a possible explanation for the opposite effects of NFATc3 and NFATc4 on neuronal apoptosis.

Inhibition of protein phosphatase 2A (PP2A) prevents Mcl-1 protein dephosphorylation at the Thr-163/Ser-159 phosphodegron, dramatically reducing expression in Mcl-1-amplified lymphoma cells

Abundant, sustained expression of prosurvival Mcl-1 is an important determinant of viability and drug resistance in cancer cells. The Mcl-1 protein contains PEST sequences (enriched in proline, glutamic acid, serine, and threonine) and is normally subject to rapid turnover via multiple different pathways. One of these pathways involves a phosphodegron in the PEST region, where Thr-163 phosphorylation primes for Ser-159 phosphorylation by glycogen synthase kinase-3. Turnover via this phosphodegron-targeted pathway is reduced in Mcl-1-overexpressing BL41-3 Burkitt lymphoma and other cancer cells; turnover is further slowed in the presence of phorbol ester-induced ERK activation, resulting in Mcl-1 stabilization and an exacerbation of chemoresistance. The present studies focused on Mcl-1 dephosphorylation, which was also found to profoundly influence turnover. Exposure of BL41-3 cells to an inhibitor of protein phosphatase 2A (PP2A), okadaic acid, resulted in a rapid increase in phosphorylation at Thr-163 and Ser-159, along with a precipitous decrease in Mcl-1 expression. The decline in Mcl-1 expression preceded the appearance of cell death markers and was not slowed in the presence of phorbol ester. Upon exposure to calyculin A, which also potently inhibits PP2A, versus tautomycin, which does not, only the former increased Thr-163/Ser-159 phosphorylation and decreased Mcl-1 expression. Mcl-1 co-immunoprecipitated with PP2A upon transfection into CHO cells, and PP2A/Aα knockdown recapitulated the increase in Mcl-1 phosphorylation and decrease in expression. In sum, inhibition of PP2A prevents Mcl-1 dephosphorylation and results in rapid loss of this prosurvival protein in chemoresistant cancer cells.

Mcl-1 ubiquitination: unique regulation of an essential survival protein

Mcl-1 is an anti-apoptotic protein of the Bcl-2 family that is essential for the survival of multiple cell lineages and that is highly amplified in human cancer. Under physiological conditions, Mcl-1 expression is tightly regulated at multiple levels, involving transcriptional, post-transcriptional and post-translational processes. Ubiquitination of Mcl-1, that targets it for proteasomal degradation, allows for rapid elimination of the protein and triggering of cell death, in response to various cellular events. In the last decade, a number of studies have elucidated different pathways controlling Mcl-1 ubiquitination and degradation. Four different E3 ubiquitin-ligases (e.g., Mule, SCF^β-TrCP, SCF^Fbw7 and Trim17) and one deubiquitinase (e.g., USP9X), that respectively mediate and oppose Mcl-1 ubiquitination, have been formerly identified. The interaction between Mule and Mcl-1 can be modulated by other Bcl-2 family proteins, while recognition of Mcl-1 by the other E3 ubiquitin-ligases and deubiquitinase is influenced by phosphorylation of specific residues in Mcl-1. The protein kinases and E3 ubiquitin-ligases that are involved in the regulation of Mcl-1 stability vary depending on the cellular context, highlighting the complexity and pivotal role of Mcl-1 regulation. In this review, we attempt to recapitulate progress in understanding Mcl-1 regulation by the ubiquitin-proteasome system.

Ubiquitin at the crossroad of cell death and survival

Ubiquitination is crucial for cellular processes, such as protein degradation, apoptosis, autophagy, and cell cycle progression. Dysregulation of the ubiquitination network accounts for the development of numerous diseases, including cancer. Thus, targeting ubiquitination is a promising strategy in cancer therapy. Both apoptosis and autophagy are involved in tumorigenesis and response to cancer therapy. Although both are categorized as types of cell death, autophagy is generally considered to have protective functions, including protecting cells from apoptosis under certain cellular stress conditions. This review highlights recent advances in understanding the regulation of apoptosis and autophagy by ubiquitination.

Parkin-dependent degradation of the F-box protein Fbw7β promotes neuronal survival in response to oxidative stress by stabilizing Mcl-1

Parkinson's disease (PD) is characterized by progressive loss of midbrain dopaminergic neurons resulting in motor dysfunction. While most PD is sporadic in nature, a significant subset can be linked to either dominant or recessive germ line mutations. PARK2, encoding the ubiquitin ligase parkin, is the most frequently mutated gene in hereditary Parkinson's disease. Here, we present evidence for a neuronal ubiquitin ligase cascade involving parkin and the multisubunit ubiquitin ligase SCF(Fbw7β). Specifically, parkin targets the SCF substrate adapter Fbw7β for proteasomal degradation. Furthermore, we show that the physiological role of parkin-mediated regulation of Fbw7β levels is the stabilization of the mitochondrial prosurvival factor Mcl-1, an SCF(Fbw7β) target in neurons. We show that neurons depleted of parkin become acutely sensitive to oxidative stress due to an inability to maintain adequate levels of Mcl-1. Therefore, loss of parkin function through biallelic mutation of PARK2 may lead to death of dopaminergic neurons through unregulated SCF(Fbw7β)-mediated ubiquitylation-dependent proteolysis of Mcl-1.

Evading apoptosis in cancer

Carcinogenesis is a mechanistically complex and variable process with a plethora of underlying genetic causes. Cancer development comprises a multitude of steps that occur progressively starting with initial driver mutations leading to tumorigenesis and, ultimately, metastasis. During these transitions, cancer cells accumulate a series of genetic alterations that confer on the cells an unwarranted survival and proliferative advantage. During the course of development, however, cancer cells also encounter a physiologically ubiquitous cellular program that aims to eliminate damaged or abnormal cells: apoptosis. Thus, it is essential that cancer cells acquire instruments to circumvent programmed cell death. Here we discuss emerging evidence indicating how cancer cells adopt various strategies to override apoptosis, including amplifying the antiapoptotic machinery, downregulating the proapoptotic program, or both.