Itch/AIP4-independent proteasomal degradation of cFLIP induced by the histone deacetylase inhibitor SAHA sensitizes breast tumour cells to TRAIL

Triptolide-mediated downregulation of FLIPS in hepatoma cells occurs at the post-transcriptional level independently of proteasome-mediated pathways

Cellular c-FLIP prevents apoptosis mediated by death receptor through inhibiting activation of caspase-8. Therefore, when c-FLIP is downregulated or eliminated, caspase-8 activation is promoted, and death receptor ligand-induced apoptosis is activated. It was reported that triptolide (TPL) sensitized tumor cells to TNF-α-induced apoptosis by blocking TNF-α-induced activation of NF-κB and transcription of c-IAP1 and c-IAP2. However, the effect of TPL on basal c-FLIP expression was not understood. In this study, we found that the combination of TNF-α and TPL accelerated apoptosis in human hepatocellular carcinoma cells and TNF-α-induced elevated as well as basal level of FLIP_S protein were downregulated by TPL. Additionally, we demonstrated that the basal level of FLIP_S in Huh7 cells was continuously downregulated following the incubation of TPL and downregulated more when dosage of TPL for treatment was increased. Subsequently, we showed that TPL reduced FLIP_S level in a transcription- and degradation-independent mechanism. Our findings suggest that TPL induces loss of FLIP_S at the post-transcriptional level independently of proteasome-mediated pathway, an additional mechanism of TPL sensitizing cancer cells to TNF-α-induced apoptosis.

Glucocorticoid modulatory element-binding protein 1 (GMEB1) interacts with the de-ubiquitinase USP40 to stabilize CFLARL and inhibit apoptosis in human non-small cell lung cancer cells

Background

GMEB1 was originally identified via its interaction with GMEB2, which binds to the promoter region of the tyrosine aminotransferase (TAT) gene and modulates transactivation of the glucocorticoid receptor gene. In the cytosol, GMEB1 interacts with and inhibits CASP8, but the molecular mechanism is currently unknown.

Methods

Human non-small cell lung cancer cells and 293FT cells were used to investigate the function of GMEB1/USP40/CFLAR_L complex by WB, GST Pull-Down Assay, Immunoprecipitation, Immunofluorescence and Flow cytometry analysis. A549 cells overexpressing green fluorescent protein and GMEB1 shRNA were used for tumor xenograft using female athymic nu/nu 4-week-old mice.

Results

We found GMEB1 interacted with CFLAR_L (also known as c-FLIP_L) in the cytosol and promoted its stability. USP40 targeted CFLAR_L for K48-linked de-ubiquitination. GMEB1 promoted the binding of USP40 to CFLAR_L. USP40 knockdown did not increase CFLAR_L protein level despite GMEB1 overexpression, suggesting GMEB1 promotes CFLAR_L stability via USP40. Additionally, GMEB1 inhibited the activation of pro-caspase 8 and apoptosis in non-small cell lung cancer (NSCLC) cell via CFLAR_L stabilization. Also, GMEB1 inhibited the formation of DISC upon TRAIL activation. CFLAR_L enhanced the binding of GMEB1 and CASP8. Downregulation of GMEB1 inhibited A549 xenograft tumor growth in vivo.

Conclusions

Our findings show the de-ubiquitinase USP40 regulates the ubiquitination and degradation of CFLAR_L; and GMEB1 acts as a bridge protein for USP40 and CFLAR_L. Mechanistically, we found GMEB1 inhibits the activation of CASP8 by modulating ubiquitination and degradation of CFLAR_L. These findings suggest a novel strategy to induce apoptosis through CFLAR_L targeting in human NSCLC cells.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1182-3) contains supplementary material, which is available to authorized users.

cFLIP critically modulates apoptotic resistance in epithelial-to-mesenchymal transition

Epithelial cancers (carcinomas) comprise the top four causes of cancer-related deaths in the United States. While overall survival has been steadily improving, therapy-resistant disease continues to present a major therapeutic challenge. Carcinomas often exploit the normal developmental program, epithelial-to-mesenchymal transition (EMT), to gain a mesenchymal phenotype associated with increased invasiveness and resistance to apoptosis. We have previously shown that an isoxazole-based small molecule, ML327, partially reverses TGF-β-induced EMT in an immortalized mouse mammary epithelial cell line. Herein, we demonstrate that ML327 reverses much of the EMT gene expression program in cultured carcinoma cell lines. The reversal of EMT sensitizes these cancer cells to the apoptosis-inducing ligand TRAIL. This sensitization is independent of E-cadherin expression and rather relies on the downregulation of a major anti-apoptotic protein, cFLIP_S. Loss of cFLIP_S is sufficient to overcome resistance to TRAIL and exogenous overexpression of cFLIP_S restores resistance to TRAIL-induced apoptosis despite EMT reversal with ML327. In summary, we have utilized an isoxazole-based small molecule that partially reverses EMT in carcinoma cells to demonstrate that cFLIP_S critically regulates the apoptosis resistance phenotype associated with EMT.

Molecular mechanisms of apoptosis and roles in cancer development and treatment

Programmed cell death (PCD) or apoptosis is a mechanism which is crucial for all multicellular organisms to control cell proliferation and maintain tissue homeostasis as well as eliminate harmful or unnecessary cells from an organism. Defects in the physiological mechanisms of apoptosis may contribute to different human diseases like cancer. Identification of the mechanisms of apoptosis and its effector proteins as well as the genes responsible for apoptosis has provided a new opportunity to discover and develop novel agents that can increase the sensitivity of cancer cells to undergo apoptosis or reset their apoptotic threshold. These novel targeted therapies include those targeting anti-apoptotic Bcl-2 family members, p53, the extrinsic pathway, FLICE-inhibitory protein (c-FLIP), inhibitor of apoptosis (IAP) proteins, and the caspases. In recent years a number of these novel agents have been assessed in preclinical and clinical trials. In this review, we introduce some of the key regulatory molecules that control the apoptotic pathways, extrinsic and intrinsic death receptors, discuss how defects in apoptotic pathways contribute to cancer, and list several agents being developed to target apoptosis.

FLIP the Switch: Regulation of Apoptosis and Necroptosis by cFLIP

cFLIP (cellular FLICE-like inhibitory protein) is structurally related to caspase-8 but lacks proteolytic activity due to multiple amino acid substitutions of catalytically important residues. cFLIP protein is evolutionarily conserved and expressed as three functionally different isoforms in humans (cFLIP_L, cFLIP_S, and cFLIP_R). cFLIP controls not only the classical death receptor-mediated extrinsic apoptosis pathway, but also the non-conventional pattern recognition receptor-dependent apoptotic pathway. In addition, cFLIP regulates the formation of the death receptor-independent apoptotic platform named the ripoptosome. Moreover, recent studies have revealed that cFLIP is also involved in a non-apoptotic cell death pathway known as programmed necrosis or necroptosis. These functions of cFLIP are strictly controlled in an isoform-, concentration- and tissue-specific manner, and the ubiquitin-proteasome system plays an important role in regulating the stability of cFLIP. In this review, we summarize the current scientific findings from biochemical analyses, cell biological studies, mathematical modeling, and gene-manipulated mice models to illustrate the critical role of cFLIP as a switch to determine the destiny of cells among survival, apoptosis, and necroptosis.

Correlation between Polymorphism of TRAIL Gene and Condition of Intervertebral Disc Degeneration

Background

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been suggested to be related with the pathogenesis and progression of osteoarticular degenerations. This study therefore aimed to investigate the relationship between the polymorphism of the TRAIL gene and the pathogenesis and severity of intervertebral disc degeneration (IDD) via detection of serum TRAIL expression levels.

Material/Methods

A total of 100 IDD patients in our hospital were recruited in the experimental group, while another cohort of 100 healthy individuals was employed as the control group. Blood samples collected from all people were quantified for TRAIL level using enzyme-linked immunosorbent assay (ELISA), in addition to allele and genotype frequency analysis via fluorescent PCR for TRAIL gene.

Results

At loci 1525 and 1529 in 3′-untranslated region (UTR) of 5^th exon of TRAIL gene, 3 different genotypes were identified: experimental group had higher frequency of 1525CG/1595CC, 1525G and 1595C alleles, compared to the control group (p<0.05). Patients under Schneiderman grade IV had significantly higher allele frequency compared to those at grade II or III. Serum TRAIL level was also higher in the experimental group compared to the control group, and in grade IV patients compared to grade II or III patients (p<0.05).

Conclusions

The G/C mutation at loci 1525/1595 of TRAIL gene may induce the progression of IDD, as the down-regulation of TRAIL can aggravate the severity of the disease.

Targeting extrinsic apoptosis in cancer: Challenges and opportunities

Apoptosis is a form of programmed cell death that plays a critical role in the regulation of various physiological and pathophysiological processes. Since apoptosis is typically disturbed in human cancers, therapeutic targeting of apoptosis represents a promising avenue for the development of novel therapeutic approaches. This strategy is particularly relevant, since many currently used anticancer therapies utilize apoptosis signaling pathways to exert their antitumor activities. A better understanding of these signaling networks and their deregulation in human cancers is anticipated to open new perspectives for the development of apoptosis-targeted therapies for the treatment of cancer.

Targeting the Anti-Apoptotic Protein c-FLIP for Cancer Therapy

Cellular FLICE (FADD-like IL-1beta-converting enzyme)-inhibitory protein (c-FLIP) is a major resistance factor and critical anti-apoptotic regulator that inhibits tumor necrosis factor-alpha (TNF-alpha), Fas-L, and TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis as well as chemotherapy-triggered apoptosis in malignant cells. c-FLIP is expressed as long (c-FLIP(L)), short (c-FLIP(S)), and c-FLIP(R) splice variants in human cells. c-FLIP binds to FADD and/or caspase-8 or -10 in a ligand-dependent and-independent fashion, which in turn prevents death-inducing signaling complex (DISC) formation and subsequent activation of the caspase cascade. Moreover, c-FLIP(L) and c-FLIP(S) are known to have multifunctional roles in various signaling pathways, as well as activating and/or upregulating several cytoprotective signaling molecules. Upregulation of c-FLIP has been found in various tumor types, and its downregulation has been shown to restore apoptosis triggered by cytokines and various chemotherapeutic agents. Hence, c-FLIP is an important target for cancer therapy. For example, small interfering RNAs (siRNAs) that specifically knockdown the expression of c-FLIP(L) in diverse human cancer cell lines augmented TRAIL-induced DISC recruitment and increased the efficacy of chemotherapeutic agents, thereby enhancing effector caspase stimulation and apoptosis. Moreover, small molecules causing degradation of c-FLIP as well as decreasing mRNA and protein levels of c-FLIP(L) and c-FLIP(S) splice variants have been found, and efforts are underway to develop other c-FLIP-targeted cancer therapies. This review focuses on (1) the functional role of c-FLIP splice variants in preventing apoptosis and inducing cytokine and drug resistance; (2) the molecular mechanisms that regulate c-FLIP expression; and (3) strategies to inhibit c-FLIP expression and function.

Combination of Vorinostat and caspase-8 inhibition exhibits high anti-tumoral activity on endometrial cancer cells

Histone deacetylase inhibitors such as Vorinostat display anti-neoplastic activity against a variety of solid tumors. Here, we have investigated the anti-tumoral activity of Vorinostat on endometrial cancer cells. We have found that Vorinostat caused cell growth arrest, loss of clonogenic growth and apoptosis of endometrial cancer cells. Vorinostat-induced the activation of caspase-8 and -9, the initiators caspases of the extrinsic and the intrinsic apoptotic pathways, respectively. Next, we investigated the role of the extrinsic pathway in apoptosis triggered by Vorinostat. We found that Vorinostat caused a dramatic decrease of FLIP mRNA and protein levels. However, overexpression of the long from of FLIP did not block Vorinostat-induced apoptosis. To further investigate the role of extrinsic apoptotic pathway in Vorinostat-induced apoptosis, we performed an shRNA-mediated knock-down of caspase-8. Surprisingly, downregulation of caspase-8 alone caused a marked decrease in clonogenic ability and reduced the growth of endometrial cancer xenografts in vivo, revealing that targeting caspase-8 may be an attractive target for anticancer therapy on endometrial tumors. Furthermore, combination of caspase-8 inhibition and Vorinostat treatment caused an enhancement of apoptotic cell death and a further decrease of clonogenic growth of endometrial cancer cells. More importantly, combination of Vorinostat and caspase-8 inhibition caused a nearly complete inhibition of tumor xenograft growth. Finally, we demonstrate that cell death triggered by Vorinostat alone or in combination with caspase-8 shRNAs was inhibited by the anti-apoptotic protein Bcl-XL. Our results suggest that combinatory therapies using Vorinostat treatment and caspase-8 inhibition can be an effective treatment for endometrial carcinomas.

Intrinsic and extrinsic apoptotic pathway signaling as determinants of histone deacetylase inhibitor antitumor activity

Histone deacetylase inhibitors (HDACi) can elicit a range of biological responses that impede the growth and/or survival of tumor cells. Depending on the physiological context, HDACi can induce apoptosis via two well-defined apoptotic pathways; the intrinsic/mitochondrial pathway and the death receptor (DR)/extrinsic pathway. A number of groups have demonstrated that overexpression of prosurvival Bcl-2 family members significantly reduces HDACi-mediated tumor cell death and therapeutic efficacy in preclinical models. In many cases, HDACi activate the intrinsic pathway via upregulation of a number of proapoptotic BH3-only Bcl-2 family genes including Bim, Bid, and Bmf. Additionally, HDACi can engage the extrinsic pathway through upregulation of DR expression, reductions in c-FLIP, and upregulation of ligands such as TRAIL. Overall, it appears that activation of the intrinsic apoptotic pathway is the predominant mechanism of HDACi-induced tumor cell death; however, the DR pathway may also be engaged, either to amplify the apoptotic signal through the intrinsic pathway or to directly induce cell death.

Novel phosphorylation and ubiquitination sites regulate reactive oxygen species-dependent degradation of anti-apoptotic c-FLIP protein

The cytosolic protein c-FLIP (cellular Fas-associated death domain-like interleukin 1β-converting enzyme inhibitory protein) is an inhibitor of death receptor-mediated apoptosis that is up-regulated in a variety of cancers, contributing to apoptosis resistance. Several compounds found to restore sensitivity of cancer cells to TRAIL, a TNF family death ligand with promising therapeutic potential, act by targeting c-FLIP ubiquitination and degradation by the proteasome. The generation of reactive oxygen species (ROS) has been implicated in c-FLIP protein degradation. However, the mechanism by which ROS post-transcriptionally regulate c-FLIP protein levels is not well understood. We show here that treatment of prostate cancer PPC-1 cells with the superoxide generators menadione, paraquat, or buthionine sulfoximine down-regulates c-FLIP long (c-FLIP(L)) protein levels, which is prevented by the proteasome inhibitor MG132. Furthermore, pretreatment of PPC-1 cells with a ROS scavenger prevented ubiquitination and loss of c-FLIP(L) protein induced by menadione or paraquat. We identified lysine 167 as a novel ubiquitination site of c-FLIP(L) important for ROS-dependent degradation. We also identified threonine 166 as a novel phosphorylation site and demonstrate that Thr-166 phosphorylation is required for ROS-induced Lys-167 ubiquitination. The mutation of either Thr-166 or Lys-167 was sufficient to stabilize c-FLIP protein levels in PPC-1, HEK293T, and HeLa cancer cells treated with menadione or paraquat. Accordingly, expression of c-FLIP T166A or K167R mutants protected cells from ROS-mediated sensitization to TRAIL-induced cell death. Our findings reveal novel ROS-dependent post-translational modifications of the c-FLIP protein that regulate its stability, thus impacting sensitivity of cancer cells to TRAIL.

Histone lysine trimethylation or acetylation can be modulated by phytoestrogen, estrogen or anti-HDAC in breast cancer cell lines

AIM

The isoflavones genistein, daidzein and equol (daidzein metabolite) have been reported to interact with epigenetic modifications, specifically hypermethylation of tumor suppressor genes. The objective of this study was to analyze and understand the mechanisms by which phytoestrogens act on chromatin in breast cancer cell lines.

MATERIALS & METHODS

Two breast cancer cell lines, MCF-7 and MDA-MB 231, were treated with genistein (18.5 µM), daidzein (78.5 µM), equol (12.8 µM), 17β-estradiol (10 nM) and suberoylanilide hydroxamic acid (1 µM) for 48 h. A control with untreated cells was performed. 17β-estradiol and an anti-HDAC were used to compare their actions with phytoestrogens. The chromatin immunoprecipitation coupled with quantitative PCR was used to follow soy phytoestrogen effects on H3 and H4 histones on H3K27me3, H3K9me3, H3K4me3, H4K8ac and H3K4ac marks, and we selected six genes (EZH2, BRCA1, ERα, ERβ, SRC3 and P300) for analysis.

RESULTS

Soy phytoestrogens induced a decrease in trimethylated marks and an increase in acetylating marks studied at six selected genes.

CONCLUSION

We demonstrated that soy phytoestrogens tend to modify transcription through the demethylation and acetylation of histones in breast cancer cell lines.

Down-regulation of cellular FLICE-inhibitory protein (Long Form) contributes to apoptosis induced by Hsp90 inhibition in human lung cancer cells

Background

Cellular FLICE-Inhibitory Protein (long form, c-FLIP_L) is a critical negative regulator of death receptor-mediated apoptosis. Overexpression of c-FLIP_L has been reported in many cancer cell lines and is associated with chemoresistance. In contrast, down-regulation of c-FLIP may drive cancer cells into cellular apoptosis. This study aims to demonstrate that inhibition of the heat shock protein 90 (Hsp90) either by inhibitors geldanamycin/17-N-Allylamino-17-demethoxygeldanamycin (GA/17-AAG) or siRNA technique in human lung cancer cells induces c-FLIP_L degradation and cellular apoptosis through C-terminus of Hsp70-interacting protein (CHIP)-mediated mechanisms.

Methods

Calu-1 and H157 cell lines (including H157-c-FLIP_L overexpressing c-FLIP_L and control cell H157-lacZ) were treated with 17-AAG and the cell lysates were prepared to detect the given proteins by Western Blot and the cell survival was assayed by SRB assay. CHIP and Hsp90 α/β proteins were knocked down by siRNA technique. CHIP and c-FLIP_L plasmids were transfected into cells and immunoprecipitation experiments were performed to testify the interactions between c-FLIP_L, CHIP and Hsp90.

Results

c-FLIP_L down-regulation induced by 17-AAG can be reversed with the proteasome inhibitor MG132, which suggested that c-FLIP_L degradation is mediated by a ubiquitin-proteasome system. Inhibition of Hsp90α/β reduced c-FLIP_L level, whereas knocking down CHIP expression with siRNA technique inhibited c-FLIP_L degradation. Furthermore, c-FLIP_L and CHIP were co-precipitated in the IP complexes. In addition, overexpression of c-FLIP_L can rescue cancer cells from apoptosis. When 17-AAG was combined with an anti-cancer agent celecoxib(CCB), c-FLIP_L level declined further and there was a higher degree of caspase activation.

Conclusion

We have elucidated c-FLIP_L degradation contributes to apoptosis induced by Hsp90 inhibition, suggesting c-FLIP and Hsp90 may be the promising combined targets in human lung cancer treatment.

Targeting c-FLICE-like inhibitory protein (CFLAR) in cancer

INTRODUCTION

Evasion of apoptosis (programmed cell death) is one of the characteristic hallmarks of human cancers and may be caused by aberrant expression of antiapoptotic proteins. Among those is c-FLICE-like inhibitory protein (c-FLIP), a protein that not only blocks apoptosis signaling but also regulates additional cell death pathways.

AREAS COVERED

Because c-FLIP is regulated both at the transcriptional and posttranscriptional level by various mechanisms and is a short-lived protein with a rapid turnover, the regulation of c-FLIP expression represents a versatile tool to modulate cell death signaling pathways. Because c-FLIP is aberrantly expressed in various cancers, it represents a promising target for therapeutic intervention.

EXPERT OPINION

Therefore, insights into the molecular events that regulate c-FLIP expression and activity in human cancers will provide the basis for the development of new strategies to target c-FLIP expression in human cancers.

Breast cancer proteome takes more than two to tango on TRAIL: beat them at their own game

Breast carcinogenesis is a multidimensional disease that has resisted drug-related solutions to date because of heterogeneity, disorganized spatiotemporal behavior of signal transduction cascades, cell cycle checkpoints, cell transition, plasticity, and impaired pro-apoptotic response. These synchronized oncogenic events, including protein-protein interaction, transcriptional-regulatory, and signaling networks, trigger genomic and transcriptional disturbances in TRAIL-mediated signaling network neighborhoods. Therefore, tumor cells often acquire the ability to escape death by suppressing cell death pathways that normally function to eliminate damaged and harmful cells. This review describes the TRAIL-mediated cell death signaling pathways, the interactions between these pathways, and the ways in which these pathways are deregulated in breast cancer.

Elevation of c-FLIP in castrate-resistant prostate cancer antagonizes therapeutic response to androgen receptor-targeted therapy

PURPOSE

To characterize the importance of cellular Fas-associated death domain (FADD)-like interleukin 1β-converting enzyme (FLICE) inhibitory protein (c-FLIP), a key regulator of caspase-8 (FLICE)-promoted apoptosis, in modulating the response of prostate cancer cells to androgen receptor (AR)-targeted therapy.

EXPERIMENTAL DESIGN

c-FLIP expression was characterized by immunohistochemical analysis of prostatectomy tissue. The functional importance of c-FLIP to survival and modulating response to bicalutamide was studied by molecular and pharmacologic interventions.

RESULTS

c-FLIP expression was increased in high-grade prostatic intraepithelial neoplasia and prostate cancer tissue relative to normal prostate epithelium (P < 0.001). Maximal c-FLIP expression was detected in castrate-resistant prostate cancer (CRPC; P < 0.001). In vitro, silencing of c-FLIP induced spontaneous apoptosis and increased 22Rv1 and LNCaP cell sensitivity to bicalutamide, determined by flow cytometry, PARP cleavage, and caspase activity assays. The histone deacetylase inhibitors (HDACi), droxinostat and SAHA, also downregulated c-FLIP expression, induced caspase-8- and caspase-3/7-mediated apoptosis, and increased apoptosis in bicalutamide-treated cells. Conversely, the elevated expression of c-FLIP detected in the CRPC cell line VCaP underpinned their insensitivity to bicalutamide and SAHA in vitro. However, knockdown of c-FLIP induced spontaneous apoptosis in VCaP cells, indicating its relevance to cell survival and therapeutic resistance.

CONCLUSION

c-FLIP reduces the efficacy of AR-targeted therapy and maintains the viability of prostate cancer cells. A combination of HDACi with androgen deprivation therapy may be effective in early-stage disease, using c-FLIP expression as a predictive biomarker of sensitivity. Direct targeting of c-FLIP, however, may be relevant to enhance the response of existing and novel therapeutics in CRPC.

Histone deacetylase inhibitor-mediated sensitization to TRAIL-induced apoptosis in childhood malignancies is not associated with upregulation of TRAIL receptor expression, but with potentiated caspase-8 activation

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has great potential for the treatment of cancer because it targets tumor cells while sparing normal cells. Several cancers, however, fail to respond to TRAIL's antineoplastic effects. These resistant tumors require cotreatment with sensitizing agents in order for TRAIL to exert anticancer activity. Histone deacetylase inhibitors (HDACi) have been recognized as potent TRAIL sensitizers. In searching for the determinants of TRAIL responsiveness, HDACi-mediated TRAIL sensitization has been predominantly attributed to TRAIL receptor upregulation. This explanation, however, has been challenged by a few studies. The aim of the present study was to explore the relevance of TRAIL receptor expression for HDACi-mediated TRAIL sensitization in childhood tumors, i.e., in medulloblastoma, Ewing's sarcoma and osteosarcoma. In previous studies, we had shown that TRAIL and HDACi were synergistic in inducing apoptosis in medulloblastoma and Ewing's sarcoma. In the present study, we demonstrate that HDACi cooperated with TRAIL in eliciting cell death in osteosarcoma. However, HDACi treatment did not alter or even reduced cell surface expression of TRAIL receptors in the three childhood tumors. In gaining insight into the apoptotic pathway involved in TRAIL sensitization, HDACi were found to potentiate TRAIL-induced caspase-8 activation. Taken together, our findings suggest that HDACi-mediated TRAIL sensitization is not the result of TRAIL receptor upregulation, but the result of a receptor-proximal event in childhood tumor cells.