Regulation of the Src-PP2A interaction in tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-induced apoptosis

Regulation of programmed death ligand 1 (PD-L1) expression by TNF-related apoptosis-inducing ligand (TRAIL) in triple-negative breast cancer cells

Triple-negative breast cancer (TNBC) is an aggressive form of breast cancer that lacks targeted therapies. Previous studies have shown that TNBC cells are highly sensitive to tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), making it a promising agent for treating TNBC. However, the development of TRAIL resistance limits its further clinical development, and the underlying mechanisms are not fully understood. In this study, we report the role of PD-L1 in TRAIL resistance. Specifically, we found that TRAIL treatment increases PD-L1 expression in TRAIL-sensitive cells and that basal PD-L1 expression is increased in acquired TRAIL-resistant cells. Mechanistically, we found that increased PD-L1 expression was accompanied by increased extracellular signal-regulated kinase (ERK) activation. Using both genetic and pharmacological approaches, we showed that knockdown of ERK by siRNA or inhibition of ERK activation by the mitogen-activated protein kinase kinase inhibitor U0126 decreased PD-L1 expression and increased TRAIL-induced cell death. Furthermore, we found that knockout or knockdown of PD-L1 enhances TRAIL-induced apoptosis, suggesting that PD-L1-mediated TRAIL resistance is independent of its ability to evade immune suppression. Therefore, this study identifies a noncanonical mechanism by which PD-L1 promotes TRAIL resistance, which can be potentially exploited for immune checkpoint therapy.

Targeting PP2A for cancer therapeutic modulation

Protein phosphatases play essential roles as negative regulators of kinases and signaling cascades involved in cytoskeletal organization. Protein phosphatase 2A (PP2A) is highly conserved and is the predominant serine/threonine phosphatase in the nervous system, constituting more than 70% of all neuronal phosphatases. PP2A is involved in diverse regulatory functions, including cell cycle progression, apoptosis, and DNA repair. Although PP2A has historically been identified as a tumor suppressor, inhibition of PP2A has paradoxically demonstrated potential as a therapeutic target for various cancers. LB100, a water-soluble, small-molecule competitive inhibitor of PP2A, has shown particular promise as a chemo- and radio-sensitizing agent. Preclinical success has led to a profusion of clinical trials on LB100 adjuvant therapies, including a phase I trial in extensive-stage small-cell lung cancer, a phase I/II trial in myelodysplastic syndrome, a phase II trial in recurrent glioblastoma, and a completed phase I trial assessing the safety of LB100 and docetaxel in various relapsed solid tumors. Herein, we review the development of LB100, the role of PP2A in cancer biology, and recent advances in targeting PP2A inhibition in immunotherapy.

Breast Cancer Metastasis

Owing to increased awareness of the importance of mammogram and advances in surgical technology, survival rate of patients with primary breast cancer has dramatically increased. Despite all these advances in breast cancer treatment, there are no currently available treatments for this disease once it metastasizes to distant organs including bones, lungs, brain, and liver. This is mainly attributed to the complexity of metastatic process. Recent advances in technology enabled cancer biologists to dissect each step of the metastatic process, and this led to discovery of major players and molecules in this process. In this section, we will discuss recent discovery and advances in the field of breast cancer metastasis research.

Inorganic Phosphate (Pi) Signaling in Endothelial Cells: A Molecular Basis for Generation of Endothelial Microvesicles in Uraemic Cardiovascular Disease

Hyperphosphataemia increases cardiovascular mortality in patients with kidney disease. Direct effects of high inorganic phosphate (Pi) concentrations have previously been demonstrated on endothelial cells (ECs), including generation of procoagulant endothelial microvesicles (MVs). However, no mechanism directly sensing elevated intracellular Pi has ever been described in mammalian cells. Here, we investigated the hypothesis that direct inhibition by Pi of the phosphoprotein phosphatase PP2A fulfils this sensing role in ECs, culminating in cytoskeleton disruption and MV generation. ECs were treated with control (1 mM [Pi]) vs. high (2.5 mM [Pi]), a condition that drives actin stress fibre depletion and MV generation demonstrated by confocal microscopy of F-actin and NanoSight Nanoparticle tracking, respectively. Immuno-blotting demonstrated that high Pi increased p-Src, p-PP2A-C and p-DAPK-1 and decreased p-TPM-3. Pi at 100 μM directly inhibited PP2A catalytic activity. Inhibition of PP2A enhanced inhibitory phosphorylation of DAPK-1, leading to hypophosphorylation of Tropomyosin-3 at S284 and MV generation. p-Src is known to perform inhibitory phosphorylation on DAPK-1 but also on PP2A-C. However, PP2A-C can itself dephosphorylate (and therefore inhibit) p-Src. The direct inhibition of PP2A-C by Pi is, therefore, amplified by the feedback loop between PP2A-C and p-Src, resulting in further PP2A-C inhibition. These data demonstrated that PP2A/Src acts as a potent sensor and amplifier of Pi signals which can further signal through DAPK-1/Tropomyosin-3 to generate cytoskeleton disruption and generation of potentially pathological MVs.

Akkermansia muciniphila Aspartic Protease Amuc_1434* Inhibits Human Colorectal Cancer LS174T Cell Viability via TRAIL-Mediated Apoptosis Pathway

Mucin2 (Muc2) is the main component of the intestinal mucosal layer and is highly expressed in mucous colorectal cancer. Previous studies conducted by our lab found that the recombinant protein Amuc_1434 (expressed in Escherichia coli prokaryote cell system, hereinafter termed Amuc_1434*), derived from Akkermansia muciniphila, can degrade Muc2. Thus, the main objective of this study was to explore the effects of Amuc_1434* on LS174T in colorectal cancer cells expressing Muc2. Results from this study demonstrated that Amuc_1434* inhibited the proliferation of LS174T cells, which was related to its ability to degrade Muc2. Amuc_1434* also blocked the G0/G1 phase of the cell cycle of LS174T cells and upregulated the expression of tumor protein 53 (p53), which is a cell cycle-related protein. In addition, Amuc_1434* promoted apoptosis of LS174T cells and increased mitochondrial ROS levels in LS174T cells. The mitochondrial membrane potential of LS174T cells was also downregulated by Amuc_1434*. Amuc_1434* can activate the death receptor pathway and mitochondrial pathway of apoptosis by upregulating tumor-necrosis-factor-related apoptosis-inducing ligand (TRAIL). In conclusion, our study was the first to demonstrate that the protein Amuc_1434* derived from Akkermansia muciniphila suppresses LS174T cell viability via TRAIL-mediated apoptosis pathway.

Targeting PP2A inhibits the growth of triple-negative breast cancer cells

Triple-negative breast cancer (TNBC) does not respond to widely used targeted/endocrine therapies because of the absence of progesterone and estrogen receptors and HER2 amplification. It has been shown that the majority of TNBC cells are highly sensitive to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis, but the development of TRAIL resistance limits its efficacy. We previously found that protein phosphatase 2A (PP2A) plays an important role in TRAIL resistance. In this study, we evaluated the effects of PP2A inhibition on cell death in TRAIL-resistant TNBC cells. We found that the PP2A inhibitor LB-100 effectively inhibits the growth of a panel of TNBC cell lines including lines that are intrinsically resistant to TRAIL. Using two TRAIL-resistant cell lines generated from TRAIL-sensitive parental cells (MDA231 and SUM159), we found that both TRAIL-sensitive and -resistant cell lines are equally sensitive to LB-100. We also found that LB-100 sensitizes TNBC cells to clinically used chemotherapeutical agents, including paclitaxel and cisplatin. Importantly, we found that LB-100 effectively inhibits the growth of MDA468 tumors in mice in vivo without apparent toxicity. Collectively, these data suggest that pharmacological inhibition of PP2A activity could be a novel therapeutic strategy for treating patients with TNBC in a clinical setting.

Hypermethylation of DcR1, DcR2, DR4, DR5 gene promoters and clinical significance in tongue carcinoma

OBJECTIVE

Tongue squamous cell carcinoma (TSCC) is one of the most common malignancies in the oral cavity, and its incidence and mortality have been constantly increasing these years. A large number of tumor suppressor genes are involved in the development of the TSCC and it has been reported that the aberrant hypermethylation of tumor suppressor genes may play a key role in the process of the TSCC. In this study, we sought to analyze the association of methylation of DcR1, DcR2, DR4 and DR5 gene promoters and clinical significance in the TSCC to evaluate association between methylation of DcR1, DcR2, DR4 and DR5 gene and Clinical Significance in tongue squamous cell carcinoma.

METHODS

Methylation-specific PCR(MSP) was used to analyze the methylation of the promoters of TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) receptors in 45 TSCC cases. Real-Time PCR was used to detect the expression of the DcR1, DcR2, DR4 and DR5 gene.

RESULTS

All the four genes (DcR1, DcR2, DR4 and DR5) showed different methylation of promoters in TSCC, while methylation of these promoters in paired adjacent normal tissues were almost undetectable. Patients with high methylation index were diagnosed at younger age when compared with the ones with low methylation index. DcR1 and DR4 hypermethylation was correlated significantly with patients' TNM stage.

CONCLUSIONS

Methylation of DcR1, DcR2,DR4 and DR5 promoters are found in TSCC and may associate with its occurrence and development. Taking the reversibility of methylation into account，methylation is a potential targeted therapy of TSCC.

Developing TRAIL/TRAIL death receptor-based cancer therapies

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF superfamily that can initiate the apoptosis pathway by binding to its associated death receptors DR4 and DR5. The activation of the TRAIL pathway in inducing tumor-selective apoptosis leads to the development of TRAIL-based cancer therapies, which include recombinant forms of TRAIL, TRAIL receptor agonists, and other therapeutic agents. Importantly, TRAIL, DR4, and DR5 can all be induced by synthetic and natural agents that activate the TRAIL apoptosis pathway in cancer cells. Thus, understanding the regulation of the TRAIL apoptosis pathway can aid in the development of TRAIL-based therapies for the treatment of human cancer.

TRAIL induces pro-apoptotic crosstalk between the TRAIL-receptor signaling pathway and TrkAIII in SH-SY5Y cells, unveiling a potential therapeutic "Achilles heel" for the TrkAIII oncoprotein in neuroblastoma

TrkAIII expression in neuroblastoma (NB) associates with advanced stage disease, worse prognosis, post therapeutic relapse, and in NB models TrkAIII exhibits oncogenic activity and promotes chemotherapeutic-resistance. Here, we report a potential therapeutic “Achilles heel” for the TrkAIII oncoprotein in a SH-SY5Y NB model that is characterised by one-way TRAIL-induced, pro-apoptotic crosstalk between the TRAIL receptor signaling pathway and TrkAIII that results in the delayed induction of apoptosis. In TrkAIII SH-SY5Y cells, blocked in the intrinsic apoptosis pathway by elevated constitutive Bcl-2, Bcl-xL and Mcl-1 expression, TRAIL induced delayed caspase-dependent apoptosis via the extrinsic pathway and completely abrogated tumourigenic capacity in vitro. This effect was initiated by TRAIL-induced SHP-dependent c-Src activation, the induction of TrkAIII/SHP-1/c-Src complexing leading to SHP-mediated TrkAIII de-phosphorylation, subsequent induction of complexing between de-phosphorylated TrkAIII and cFLIP associated with a time-dependent increase the caspase-8 to cFLIP ratio at activated death receptors, resulting in delayed caspase cleavage and caspase-dependent apoptosis. We also confirm rate-limiting roles for c-FLIP and Mcl-1 in regulating the sensitivity of TrkAIII SH-SY5Y cells to TRAIL-induced apoptosis via the extrinsic and intrinsic pathways, respectively. Our study unveils a novel mechanism for the TRAIL-induced apoptosis of TrkAIII expressing NB cells that depends upon SHP/Src-mediated crosstalk between the TRAIL-receptor signaling pathway and TrkAIII, and supports a novel potential pro-apoptotic therapeutic use for TRAIL in TrkAIII expressing NB.

PP2A as the Main Node of Therapeutic Strategies and Resistance Reversal in Triple-Negative Breast Cancer

Triple negative breast cancer (TNBC), is defined as a type of tumor lacking the expression of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2). The ER, PR and HER2 are usually the molecular therapeutic targets for breast cancers, but they are ineffective for TNBC because of their negative expressions, so chemotherapy is currently the main treatment strategy in TNBC. However, drug resistance remains a major impediment to TNBC chemotherapeutic treatment. Recently, the protein phosphatase 2A (PP2A) has been found to regulate the phosphorylation of some substrates involved in the relevant target of TNBC, such as cell cycle control, DNA damage responses, epidermal growth factor receptor, immune modulation and cell death resistance, which may be the effective therapeutic strategies or influence drug sensitivity to TNBCs. Furthermore, PP2A has also been found that could induce ER re-expression in ER-negative breast cancer cells, and which suggests PP2A could promote the sensitivity of tamoxifen to TNBCs as a resistance reversal agent. In this review, we will summarize the potential therapeutic value of PP2A as the main node in developing targeting agents, disrupting resistance or restoring drug sensitivity in TNBC.

RACK1 stabilises the activity of PP2A to regulate the transformed phenotype in mammary epithelial cells

Conflicting reports implicate the scaffolding protein RACK1 in the progression of breast cancer. RACK1 has been identified as a key regulator downstream of growth factor and adhesion signalling and as a direct binding partner of PP2A. Our objective was to further characterise the interaction between PP2A and RACK1 and to advance our understanding of this complex in breast cancer cells. We examined how the PP2A holoenzyme is assembled on the RACK1 scaffold in MCF-7 cells. We used immobilized peptide arrays representing the entire PP2A-catalytic subunit to identify candidate amino acids on the C subunit of PP2A that might be involved in binding of RACK1. We identified the RACK1 interaction sites on PP2A. Stable cell lines expressing PP2A with FR69/70AA, R214A and Y218F substitutions were generated and it was confirmed that the RACK1/PP2A interaction is essential to stabilise PP2A activity. We used Real-Time Cell Analysis and a series of assays to demonstrate that disruption of the RACK1/PP2A complex also reduces the adhesion, proliferation, migration and invasion of breast cancer cells and plays a role in maintenance of the cancer phenotype. This work has significantly advanced our understanding of the RACK1/PP2A complex and suggests a pro-carcinogenic role for the RACK1/PP2A interaction. This work suggests that approaches to target the RACK1/PP2A complex are a viable option to regulate PP2A activity and identifies a novel potential therapeutic target in the treatment of breast cancer.

Caspase-8 tyrosine-380 phosphorylation inhibits CD95 DISC function by preventing procaspase-8 maturation and cycling within the complex

Caspase-8 is a key initiator of apoptotic cell death where it functions as the apical protease in death receptor-mediated apoptosis triggered via the death-inducing signalling complex (DISC). However, the observation that caspase-8 is upregulated in many common tumour types led to the discovery of alternative non-apoptotic, pro-survival functions, many of which are contingent on phosphorylation of a tyrosine residue (Y380) found in the linker region between the two catalytic domains of the enzyme. Furthermore, Src-mediated Y380 phosphorylation leads to increased resistance to CD95-induced apoptosis; however, the mechanism underlying this impaired response to extrinsic apoptotic stimuli has not been identified. Consequently, we have employed a number of model systems to further dissect this protective mechanism. First, using an in vitro DISC model together with recombinant procaspase-8 variants, we show that Y380 phosphorylation inhibits procaspase-8 activation at the CD95 DISC, thereby preventing downstream activation of the caspase cascade. Second, we validated this finding in a cellular context using transfected neuroblastoma cell lines deficient in caspase-8. Reconstitution of these lines with phosphomimetic-caspase-8 results in increased resistance to CD95-mediated apoptosis and enhanced cell migration. When the in vitro DISC is assembled in the presence of cell lysate, caspase-8 Y380 phosphorylation attenuates DISC activity by inhibiting procaspase-8 autoproteolytic activity but not recruitment or homodimerization of caspase-8 within the complex. Once incorporated into the DISC, phosphorylated caspase-8 is unable to be released from the complex; this inhibits further cycling and release of active catalytic subunits into the cytoplasm, thus resulting in increased apoptotic resistance. Taken together, our novel findings expand our understanding of the key mechanisms underlying the anti-apoptotic functions of caspase-8 which may act as a critical block to existing antitumour therapies. Importantly, reversal or inhibition of caspase-8 phosphorylation may prove a valuable avenue to explore for sensitization of resistant tumours to extrinsic apoptotic stimuli.

p21-activated Kinases (PAKs) Mediate the Phosphorylation of PREX2 Protein to Initiate Feedback Inhibition of Rac1 GTPase

Phosphatidylinositol 3,4,5-trisphosphate (PIP3)-dependent Rac exchanger 2 (PREX2) is a guanine nucleotide exchange factor (GEF) for the Ras-related C3 botulinum toxin substrate 1 (Rac1) GTPase, facilitating the exchange of GDP for GTP on Rac1. GTP-bound Rac1 then activates its downstream effectors, including p21-activated kinases (PAKs). PREX2 and Rac1 are frequently mutated in cancer and have key roles within the insulin-signaling pathway. Rac1 can be inactivated by multiple mechanisms; however, negative regulation by insulin is not well understood. Here, we show that in response to being activated after insulin stimulation, Rac1 initiates its own inactivation by decreasing PREX2 GEF activity. Following PREX2-mediated activation of Rac1 by the second messengers PIP3 or Gβγ, we found that PREX2 was phosphorylated through a PAK-dependent mechanism. PAK-mediated phosphorylation of PREX2 reduced GEF activity toward Rac1 by inhibiting PREX2 binding to PIP3 and Gβγ. Cell fractionation experiments also revealed that phosphorylation prevented PREX2 from localizing to the cellular membrane. Furthermore, the onset of insulin-induced phosphorylation of PREX2 was delayed compared with AKT. Altogether, we propose that second messengers activate the Rac1 signal, which sets in motion a cascade whereby PAKs phosphorylate and negatively regulate PREX2 to decrease Rac1 activation. This type of regulation would allow for transient activation of the PREX2-Rac1 signal and may be relevant in multiple physiological processes, including diseases such as diabetes and cancer when insulin signaling is chronically activated.

Surviving at a Distance: Organ-Specific Metastasis

The clinical manifestation of metastasis in a vital organ is the final stage of cancer progression and the main culprit of cancer-related mortality. Once established, metastasis is devastating, but only a small proportion of the cancer cells that leave a tumor succeed at infiltrating, surviving, and ultimately overtaking a distant organ. The bottlenecks that challenge cancer cells in newly invaded microenvironments are organ-specific and consequently demand distinct mechanisms for metastatic colonization. We review the metastatic traits that allow cancer cells to colonize distinct organ sites.

The role of Cullin3-mediated ubiquitination of the catalytic subunit of PP2A in TRAIL signaling

Protein phosphatase 2A (PP2A) is the major serine-threonine phosphatase that regulates a number of cell signaling pathways. PP2A activity is controlled partially through protein degradation; however, the underlying mechanism is not fully understood. Here we show that PP2A/C, a catalytic subunit of PP2A, is degraded by the Cullin3 (Cul3) ligase-mediated ubiquitin-proteasome pathway. In response to death receptor signaling by tumor-necrosis factor-related apoptosis-inducing ligand (TRAIL), PP2A/C, caspase-8 and Cul3, a subunit of the cullin family of E3 ligases, are recruited into the death-inducing signaling complex (DISC) where the Cul3 ligase targets PP2A/C for ubiquitination and subsequent degradation. Functionally, knockdown of PP2A/C expression by siRNA or pharmacological inhibition of PP2A activity increases TRAIL-induced apoptosis. In cancer cells that have developed acquired TRAIL resistance, PP2A phosphatase activity is increased, and PP2A/C protein is resistant to TRAIL-induced degradation. Thus, this work identifies a new mechanism by which PP2A/C is regulated by Cul3 ligase-mediated degradation in response to death receptor signaling and suggests that inhibition of PP2A/C degradation may contribute to resistance of cancer cells to death receptor-induced apoptosis.

LB100, a small molecule inhibitor of PP2A with potent chemo- and radio-sensitizing potential

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase that plays a significant role in mitotic progression and cellular responses to DNA damage. While traditionally viewed as a tumor suppressor, inhibition of PP2A has recently come to attention as a novel therapeutic means of driving senescent cancer cells into mitosis and promoting cell death via mitotic catastrophe. These findings have been corroborated in numerous studies utilizing naturally produced compounds that selectively inhibit PP2A. To overcome the known human toxicities associated with these compounds, a water-soluble small molecule inhibitor, LB100, was recently developed to competitively inhibit the PP2A protein. This review summarizes the pre-clinical studies to date that have demonstrated the anti-cancer activity of LB100 via its chemo- and radio-sensitizing properties. These studies demonstrate the tremendous therapeutic potential of LB100 in a variety of cancer types. The results of an ongoing phase 1 trial are eagerly anticipated.

Selective activators of protein phosphatase 5 target the auto-inhibitory mechanism

This paper describes the identification of compounds, which stimulate the activity of the protein phosphatase PPH-5 and addresses the influence of the identified compounds on the enzymatic properties and the potential mechanism of their action.

Protein phosphatase 5 (PP5) is an evolutionary conserved serine/threonine phosphatase. Its dephosphorylation activity modulates a diverse set of cellular factors including protein kinases and the microtubule-associated tau protein involved in neurodegenerative disorders. It is auto-regulated by its heat-shock protein (Hsp90)-interacting tetratricopeptide repeat (TPR) domain and its C-terminal α-helix. In the present study, we report the identification of five specific PP5 activators [PP5 small-molecule activators (P5SAs)] that enhance the phosphatase activity up to 8-fold. The compounds are allosteric modulators accelerating efficiently the turnover rate of PP5, but do barely affect substrate binding or the interaction between PP5 and the chaperone Hsp90. Enzymatic studies imply that the compounds bind to the phosphatase domain of PP5. For the most promising compound crystallographic comparisons of the apo PP5 and the PP5–P5SA-2 complex indicate a relaxation of the auto-inhibited state of PP5. Residual electron density and mutation analyses in PP5 suggest activator binding to a pocket in the phosphatase/TPR domain interface, which may exert regulatory functions. These compounds thus may expose regulatory mechanisms in the PP5 enzyme and serve to develop optimized activators based on these scaffolds.

Hepatocellular carcinoma: targeting of oncogenic signaling networks in TRAIL resistant cancer cells

Apoptotic response in hepatocellular carcinoma (HCC) cells is impaired because of interconnectivity of proteins into complexes and signaling networks that are highly divergent in time and space. TNF-related apoptosis-inducing ligand (TRAIL) has emerged as an attractive anticancer agent reported to selectively induce apoptosis in cancer cells. Although diametrically opposed roles of TRAIL are reported both as an inducer of apoptosis and regulator of metastasis, overwhelmingly accumulating experimental evidence highlighting apoptosis inducing activity of TRAIL is directing TRAIL into clinical trials. Insights from TRAIL mediated signaling in HCC research are catalyzing new lines of study that should not only explain molecular mechanisms of disease but also highlight emerging paradigms in restoration of TRAIL mediated apoptosis in resistant cancer cells. It is becoming progressively more understandable that phytochemicals derived from edible plants have shown potential in modelling their interactions with their target proteins. Rapidly accumulating in vitro and in-vivo evidence indicates that phytonutrients have anticancer activity in rodent models of hepatocellular carcinoma. In this review we bring to limelight how phytonutrients restore apoptosis in hepatocellular carcinoma cells by rebalancing pro-apoptotic and anti-apoptotic proteins. Evidence has started to emerge, that reveals how phytonutrients target pharmacologically intractable proteins to suppress cancer. Target-based small-molecule discovery has entered into the mainstream research in the pharmaceutical industry and a better comprehension of the genetics of patients will be essential for identification of responders and non-responders.