Suppression of PP2A is critical for protection of melanoma cells upon endoplasmic reticulum stress

The role of protein phosphatase 2A (PP2A) in the unfolded protein response (UPR) of plants

Protein phosphatase 2A (PP2A) is a key regulator of plant growth and development, but its role in the endoplasmic reticulum (ER) stress response remains elusive. In this study, we investigated the function of PP2A under ER stress using loss-of-function mutants of ROOTS CURL of NAPHTHYLPHTHALAMIC ACID1 (RCN1), a regulatory A1 subunit isoform of Arabidopsis PP2A. RCN1 mutants (rcn1-1 and rcn1-2) exhibited reduced sensitivity to tunicamycin (TM), an inhibitor of N-linked glycosylation and inducer of unfolded protein response (UPR) gene expression, resulting in less severe effects compared to wild-type plants (Ws-2 and Col-0). TM negatively impacted PP2A activity in Col-0 plants but did not significantly affect rcn1-2 plants. Additionally, TM treatment did not influence the transcription levels of the PP2AA1(RCN1), 2, and 3 genes in Col-0 plants. Cantharidin, a PP2A inhibitor, exacerbated growth defects in rcn1 plants and alleviated TM-induced growth inhibition in Ws-2 and Col-0 plants. Furthermore, cantharidin treatment mitigated TM hypersensitivity in ire1a&b and bzip28&60 mutants. These findings suggest that PP2A activity is essential for an efficient UPR in Arabidopsis.

MicroRNA-30c-2-3p regulates ER stress and induces apoptosis in ovarian cancer cells underlying ER stress

Ovarian cancer is a common gynecologic cancer with a high rate of recurrence, drug resistance, and mortality, thereby necessitating novel molecular target therapies. Ovarian cancer as a solid tumor has constantly been challenged by endoplasmic reticulum stress (ERS). Currently, XBP1 as a therapeutic target in solid tumors plays a key role in adaptation to ERS. Single-stranded RNAs usually modulate posttranscriptional of the gene activity. miR-30c-2-3p has been demonstrated to inhibit the expression of XBP1. Here, we evaluated the effect of miR-30c-2-3p on controlling XBP1-CHOP-BIM and its apoptotic effects on ovarian cancer cell lines during ERS. The ER stress was assessed using Thioflavin T staining in OVCAR3 and SKOV3 cells. The expression of ER stress genes was measured by QRT-PCR. The protein levels of XBP1(s), BIP/GRP78, CHOP, and BIM were evaluated using Western blotting. Cell viability and apoptosis in STF-083010 and Tunicamycin (Tm) co-treated cells were evaluated using BrdU, MTT, Annexin V-FITC/PI staining, and caspase-12 and -3 activities assays. We found that miR-30c-2-3p significantly decreased the folding capacity of ER, leading to ERS intensification (P<0.05). Additionally, the Western blot analysis showed the modest up-regulation of CHOP and BIM with pro-apoptotic activity and down-regulation of the BIP protein. Furthermore, mimic miR-30c-2-3p transfection not only decreased cell proliferation but also induced cell death in ovarian cancer cells in response to the Tm-treatment. Our results indicated that the apoptotic pathway was induced possibly through activation of caspases -12 and -3 and elevation of the Bax/Bcl-2 ratio. Overall, the present paper adds new evidence to the possible treatment of miR-30c-2-3p via impeding the XBP1 transcription in ovarian cancer cells provoking apoptotic pathways by XBP1/CHOP/BIM mediators.

Tau-induced upregulation of C/EBPβ-TRPC1-SOCE signaling aggravates tauopathies: A vicious cycle in Alzheimer neurodegeneration

Intracellular accumulating of the hyperphosphorylated tau plays a pivotal role in neurodegeneration of Alzheimer disease (AD), but the mechanisms underlying the gradually aggravated tau hyperphosphorylation remain elusive. Here, we show that increasing intracellular tau could upregulate mRNA and protein levels of TRPC1 (transient receptor potential channel 1) with an activated store‐operated calcium entry (SOCE), an increased intraneuronal steady‐state [Ca²⁺]_i, an enhanced endoplasmic reticulum (ER) stress, an imbalanced protein kinases and phosphatase, and an aggravated tauopathy. Furthermore, overexpressing TRPC1 induced ER stress, kinases‐phosphatase imbalance, tau hyperphosphorylation and cognitive deficits in cultured neurons and mice, while pharmacological inhibiting or knockout TRPC1 attenuated the hTau‐induced deregulations in SOCE, ER homeostasis, kinases‐phosphatase balance, and tau phosphorylation level with improved synaptic and cognitive functions. Finally, an increased CCAAT‐enhancer‐binding protein (C/EBPβ) activity was observed in hTau‐overexpressing cells and the hippocampus of the AD patients, while downregulating C/EBPβ by siRNA abolished the hTau‐induced TRPC1 upregulation. These data reveal that increasing intracellular tau can upregulate C/EBPβ‐TRPC1‐SOCE signaling and thus disrupt phosphorylating system, which together aggravates tau pathologies leading to a chronic neurodegeneration.

Mechanism of inositol-requiring enzyme 1-alpha inhibition in endoplasmic reticulum stress and apoptosis in ovarian cancer cells

IRE1α endonuclease is a key regulator of endoplasmic reticulum (ER) stress that controls cell survival/apoptosis in cancers. Inhibition of IRE1α endonuclease leads to decreased splice XBP1 which decreases cell proliferation and increases cell death in cancer cells. Therefore, this study investigated the effects and mechanism of STF-083010 (an IRE1α inhibitor) on the cell growth/apoptosis of ovarian malignant cells via the XBP1-CHOP-Bim pathway following the induction of ER stress (ERS). ERS in OVCAR3 and SKOV3 cells was measured using Thioflavin T staining. The expression of ER stress response genes was evaluated by QRT-PCR. The levels of XBP1(s), PERK, phospho-PERK, p-PP2A, ATF4, BIP/GRP78, CHOP, and Bim proteins were evaluated using western blotting. Cell viability and apoptosis in STF-083010 and Tunicamycin (Tm) co-treated cells were assessed using BrdU, MTT, Annexin V-FITC/PI staining, and caspases-12 and -3 activity assays. The results showed increased XBP1, CHOP, and ATF-4 mRNA expression levels as well as high protein aggregation in STF-083010 and Tm co-treated cells. The IRE1α inhibitor down-regulated sXBP1 and BIP proteins, while XBP-1, p-PERK, ATF-4, CHOP, and Bim proteins were up-regulated. STF-083010 reduced cell proliferation and induced apoptosis through the activation of caspases-12 and -3 and Bax/Bcl-2 protein expression. In summary, the present data revealed the effects of STF-083010 in ER stress and apoptosis as well as signaling via XBP1/CHOP/Bim mediators. Thus, STF-083010 is proposed as a new target for the control of ERS in ovarian cancer cells.

Endoplasmic reticulum stress induces liver cells apoptosis after brain death by suppressing the phosphorylation of protein phosphatase 2A

The present study aimed to investigate whether brain death (BD) induces the activation of endoplasmic reticulum stress (ERS) and protein phosphatase 2A (PP2A), and reveal the possible association with BD-induced liver cell apoptosis. A total of 30 healthy adult male Sprague-Dawley rats were randomized into three groups: Sham-operated group (S), BD group and 4-phenylbutyric acid group (BD + 4-PBA), with 10 rats in each group. All rats were anesthetized. The model of BD was established by inflating a balloon catheter that was placed into the extradural space after anesthesia. 4-PBA was administered via an intraperitoneal injection when the BD model was established. Anesthesia of the S group of rats was maintained for 6 h. Liver tissues were harvested after 6 h of BD. HE staining was used to evaluate the damage of liver. Terminal deoxynucleotidyl transferase-mediated 2′-deoxyuridine 5′-triphosphate nick-end labeling staining was used to observe the apoptosis of liver cells. Activation of ERS and PP2A was examined by western blotting and immunohistochemical staining. We reported that the apoptosis of liver cells after BD was significantly promoted than in the S group. Activation of ERS and PP2A was induced in the BD group when compared with S group. Phosphorylation of PP2A was suppressed in BD group. Application of 4-PBA decreased the activation of ERS and apoptosis rate compared with the BD group. In addition, activation of PP2A in the BD + 4-PBA group was decreased due to the reduction of PP2A phosphorylation compared with the BD group, but the levels were higher than in the S group. (P<0.05). In summary, our results indicated that BD induced ERS, then activated PP2A by suppressing the phosphorylation of PP2A, resulting in the apoptosis of liver cells.

The cyclin B2/CDK1 complex inhibits separase activity in mouse oocyte meiosis I

Chromosome segregation is driven by separase, activity of which is inhibited by binding to securin and cyclin B1/CDK1. In meiosis, premature separase activity will induce aneuploidy or abolish chromosome segregation owing to the untimely destruction of cohesin. Recently, we have proved that cyclin B2 can compensate for cyclin B1 in CDK1 activation for the oocyte meiosis G2/M transition. In the present study, we identify an interaction between cyclin B2/CDK1 and separase in mouse oocytes. We find that cyclin B2 degradation is required for separase activation during the metaphase I-anaphase I transition because the presence of stable cyclin B2 leads to failure of homologous chromosome separation and to metaphase I arrest, especially in the simultaneous absence of securin and cyclin B1. Moreover, non-phosphorylatable separase rescues the separation of homologous chromosomes in stable cyclin B2-arrested cyclin B1-null oocytes. Our results indicate that cyclin B2/CDK1 is also responsible for separase inhibition via inhibitory phosphorylation to regulate chromosome separation in oocyte meiosis, which may not occur in other cell types.

Novel roles of ER stress in repressing neural activity and seizures through Mdm2- and p53-dependent protein translation

Seizures can induce endoplasmic reticulum (ER) stress, and sustained ER stress contributes to neuronal death after epileptic seizures. Despite the recent debate on whether inhibiting ER stress can reduce neuronal death after seizures, whether and how ER stress impacts neural activity and seizures remain unclear. In this study, we discovered that the acute ER stress response functions to repress neural activity through a protein translation-dependent mechanism. We found that inducing ER stress promotes the expression and distribution of murine double minute-2 (Mdm2) in the nucleus, leading to ubiquitination and down-regulation of the tumor suppressor p53. Reduction of p53 subsequently maintains protein translation, before the onset of translational repression seen during the latter phase of the ER stress response. Disruption of Mdm2 in an Mdm2 conditional knockdown (cKD) mouse model impairs ER stress-induced p53 down-regulation, protein translation, and reduction of neural activity and seizure severity. Importantly, these defects in Mdm2 cKD mice were restored by both pharmacological and genetic inhibition of p53 to mimic the inactivation of p53 seen during ER stress. Altogether, our study uncovered a novel mechanism by which neurons respond to acute ER stress. Further, this mechanism plays a beneficial role in reducing neural activity and seizure severity. These findings caution against inhibition of ER stress as a neuroprotective strategy for seizures, epilepsies, and other pathological conditions associated with excessive neural activity.

One-third of epilepsy patients respond poorly to current anti-epileptic drugs. Thus, there is an urgent need to characterize cellular behavior during seizures, and the corresponding molecular mechanisms in order to develop better therapies. Seizures are known to induce ER stress but how the ER stress response functions to modulate seizure activity is unknown. Our study provides evidence to demonstrate a novel and beneficial role for the ER stress response in reducing neural activity and seizure severity. Mechanistically, we found that these beneficial effects are mediated by elevated protein translation, which is triggered by the activation of Mdm2-p53 signaling, during the early ER stress response. Our findings suggest that therapeutic attempts to reduce ER stress in epilepsies may result in worsening seizure activity and therefore caution against inhibition of ER stress as a neuroprotective strategy for epilepsies.

Ceramide and sphingosine 1-phosphate in adipose dysfunction

The increased adipose tissue mass of obese individuals enhances the risk of metabolic syndrome, type 2 diabetes and cardiovascular diseases. During pathological expansion of adipose tissue, multiple molecular controls of lipid storage, adipocyte turn-over and endocrine secretion are perturbed and abnormal lipid metabolism results in a distinct lipid profile. There is a role for ceramides and sphingosine 1-phosphate (S1P) in inducing adipose dysfunction. For instance, the alteration of ceramide biosynthesis, through the de-regulation of key enzymes, results in aberrant formation of ceramides (e.g. C_16:0 and C_18:0) which block insulin signaling and promote adipose inflammation. Furthermore, S1P can induce defective adipose tissue phenotypes by promoting chronic inflammation and inhibiting adipogenesis. These abnormal changes are discussed in the context of possible therapeutic approaches to re-establish normal adipose function and to, thereby, increase insulin sensitivity in type 2 diabetes. Such novel approaches include blockade of ceramide biosynthesis using inhibitors of sphingomyelinase or dihydroceramide desaturase and by antagonism of S1P receptors, such as S1P₂.

Dual functions for OVAAL in initiation of RAF/MEK/ERK prosurvival signals and evasion of p27-mediated cellular senescence

Here, we report that the long noncoding RNA (lncRNA) ovarian adenocarcinoma-amplified lncRNA (OVAAL) is a mediator of cancer cell resistance, counteracting the effects of apoptosis-inducing agents acting through both the extrinsic and intrinsic pathways. Building upon previous reports associating OVAAL amplification with ovarian and endometrial cancers, we now show that OVAAL overexpression occurs during the pathogenesis of colorectal cancer and melanoma. Mechanistically, our findings also establish that OVAAL expression more generally contributes a prosurvival role to cancer cells under steady-state conditions. OVAAL accomplishes these actions utilizing distinct functional modalities: one promoting activation of RAF/MEK/ERK signaling and the other blocking cell entry into senescence. Our study demonstrates that expression of a single OVAAL in cancer cells drives two distinct but coordinated actions contributing to cancer pathology.

Long noncoding RNAs (lncRNAs) function through a diverse array of mechanisms that are not presently fully understood. Here, we sought to find lncRNAs differentially regulated in cancer cells resistant to either TNF-related apoptosis-inducing ligand (TRAIL) or the Mcl-1 inhibitor UMI-77, agents that act through the extrinsic and intrinsic apoptotic pathways, respectively. This work identified a commonly up-regulated lncRNA, ovarian adenocarcinoma-amplified lncRNA (OVAAL), that conferred apoptotic resistance in multiple cancer types. Analysis of clinical samples revealed OVAAL expression was significantly increased in colorectal cancers and melanoma in comparison to the corresponding normal tissues. Functional investigations showed that OVAAL depletion significantly inhibited cancer cell proliferation and retarded tumor xenograft growth. Mechanically, OVAAL physically interacted with serine/threonine-protein kinase 3 (STK3), which, in turn, enhanced the binding between STK3 and Raf-1. The ternary complex OVAAL/STK3/Raf-1 enhanced the activation of the RAF protooncogene serine/threonine-protein kinase (RAF)/mitogen-activated protein kinase kinase 1 (MEK)/ERK signaling cascade, thus promoting c-Myc–mediated cell proliferation and Mcl-1–mediated cell survival. On the other hand, depletion of OVAAL triggered cellular senescence through polypyrimidine tract-binding protein 1 (PTBP1)–mediated p27 expression, which was regulated by competitive binding between OVAAL and p27 mRNA to PTBP1. Additionally, c-Myc was demonstrated to drive OVAAL transcription, indicating a positive feedback loop between c-Myc and OVAAL in controlling tumor growth. Taken together, these results reveal that OVAAL contributes to the survival of cancer cells through dual mechanisms controlling RAF/MEK/ERK signaling and p27-mediated cell senescence.

RIP1 protects melanoma cells from apoptosis induced by BRAF/MEK inhibitors

Many recent studies have uncovered the necessary role for the receptor-interacting protein kinase 1 (RIP1) in regulating apoptosis and necrosis that cells undergo in response to various cellular stresses. However, the functional significance of RIP1 in promoting cancer cells survival remains poorly understood. Here, we report that RIP1 was upregulated and contributed to both intrinsic and acquired resistance of melanoma cells to BRAF/MEK inhibitors through activation of NF-κB. Strikingly, Snail1-mediated suppression of CYLD played a crucial role in promoting RIP1 expression upon ERK activation, particularly, in melanoma cells with acquired resistance to BRAF inhibitors. In addition, RIP1 kinase activity was not required for melanoma cells to survive BRAF/MEK inhibition as RIP1 mediated NF-κB activation through its intermediate domain. Collectively, our findings reveal that targeting RIP1 in combination with BRAF/MEK inhibitors is a potential approach in the treatment of the disease.

ACTN4 regulates the stability of RIPK1 in melanoma

The actin crosslinking protein α-actinin-4 (ACTN4) is emerging as an important contributor to the pathogenesis of cancer. This has largely been attributed to its role in regulating cytoskeleton organization and its involvement in transcriptional regulation of gene expression. Here we report a novel function of ACTN4 as a scaffold necessary for stabilization of receptor-interacting protein kinase 1 (RIPK1) that we have recently found to be an oncogenic driver in melanoma. ACTN4 bound to RIPK1 and cellular inhibitor of apoptosis protein 1 (cIAP1) with its actin-binding domain at the N-terminus and the CaM-like domain at the C-terminus, respectively. This facilitated the physical association between RIPK1 and cIAP1 and was critical for stabilization of RIPK1 that in turn activated NF-κB. Functional investigations showed that silencing of ACTN4 suppressed melanoma cell proliferation and retarded melanoma xenograft growth. In contrast, overexpression of ACTN4 promoted melanocyte and melanoma cell proliferation and moreover, prompted melanocyte anchorage-independent growth. Of note, the expression of ACTN4 was transcriptionally activated by NF-κB. Taken together, our findings identify ACTN4 as an oncogenic regulator through driving a feedforward signaling axis of ACTN4-RIPK1-NF-κB, with potential implications for targeting ACTN4 in the treatment of melanoma.

Dysregulation of the MEK/ERK/MNK1 signalling cascade by middle T antigen of the trichoydsplasia spinulosa polyomavirus

BACKGROUND

Trichodysplasia spinulosa (TS) is a disfiguring folliculocentric cutaneous disease caused by infection with the trichodysplasia spinulosa polyomavirus (TSPyV). The TSPyV genome contains splice variants encoding the middle tumour (mT) antigen, although the potential role for TSPyV mT antigen in disease development remains unknown.

OBJECTIVE

The current study was designed to investigate the mechanistic properties of TSPyV mT antigen, which may further our understanding of TS pathogenesis and provide insight into potential therapies.

METHODS

A lentiviral packaging system was used to create an inducible cell line expressing TSPyV mT antigen. Proteins were extracted, separated by SDS-PAGE and subjected to Western blot analysis. Co-immunoprecipitation experiments and mutational analyses were also performed to evaluate protein-protein interactions of mT antigen.

RESULTS

We describe a novel mechanism of action for mT antigen that involves hyperactivation of MEK, ERK and MNK1. Our findings suggest that dysregulation of these key signalling molecules depends upon TSPyV mT antigen interaction with protein phosphatase 2A (PP2A) via intact Zn binding motifs.

CONCLUSION

Given that PP2A interaction and MEK/ERK/MNK1 phosphorylation are associated with high levels of cell proliferation and inflammation, our findings provide new evidence that TSPyV mT antigen may contribute to the pro-proliferative conditions that lead to TS development.

MAP4K4 is a novel MAPK/ERK pathway regulator required for lung adenocarcinoma maintenance

About 76% of patients with lung adenocarcinoma harbor activating mutations in the receptor tyrosine kinase (RTK)/RAS/RAF pathways, leading to aberrant activation of the mitogen-activated protein kinase (MAPK) pathways particularly the MAPK/ERK pathway. However, many lung adenocarcinomas lacking these genomic mutations also display significant MAPK pathway activation, suggesting that additional MAPK pathway alterations remain undetected. This study has identified serine/threonine kinase mitogen-activated protein 4 kinase 4 (MAP4K4) as a novel positive regulator of MAPK/ERK signaling in lung adenocarcinoma. The results showed that MAP4K4 was drastically elevated in lung adenocarcinoma independently of KRAS or EGFR mutation status. Knockdown of MAP4K4 inhibited proliferation, anchorage-independent growth and migration of lung adenocarcinoma cells, and also inhibited human lung adenocarcinoma xenograft growth and metastasis. Mechanistically, we found that MAP4K4 activated ERK through inhibiting protein phosphatase 2 activity. Our results further showed that downregulation of MAP4K4 prevented ERK reactivation in EGFR inhibitor erlotinib-treated lung adenocarcinoma cells. Together, our findings identify MAP4K4 as a novel MAPK/ERK pathway regulator in lung adenocarcinoma that is required for lung adenocarcinoma maintenance.

PPP2R5B, a regulatory subunit of PP2A, contributes to adipocyte insulin resistance

Insulin resistance is associated with deregulation of insulin signaling owing to the chronic exposure of insulin (hyperinsulinemia) to the tissues. Phosphorylation and dephosphorylation events in insulin signaling pathway play an essential role in signal transduction and glucose uptake. Amongst all, Akt protein is considered to be central to the overall insulin signaling proteins. In glucose responsive tissues like adipose and muscles, activation of Akt is responsible for triggering GLUT4 translocation and glucose transport. Several phosphatases such as PTEN, PP2A have been reported to be involved in dephosphorylation and inactivation of Akt protein. We have identified increased PP2A activity during state of chronic hyperinsulinemia exposure along-with development of adipocyte insulin resistance. This increased phosphatase activity leads activation of cAMP/PKA axis, which in turn increased cAMP levels in insulin resistant (IR) adipocytes. Okadaic acid, an inhibitor of PP2A restored and increased insulin stimulated glucose uptake in insulin resistant (IR) and insulin sensitive (IS) adipocytes respectively. In IS adipocyte, chemical activation of PP2A through MG132 and FTY720 showed decreased insulin sensitivity corroborated with decreased Akt phosphorylation and glucose uptake. We also observed an increased expression of PP2A-B (regulatory) subunit in IR adipocytes. We found PPP2R5B, a regulatory subunit of PP2A is responsible for the dephosphorylation and inactivation of Akt protein. Increased expression of PPP2R5B was also confirmed in white adipose tissue of high fat diet induced IR mice model. Overexpression and suppression strategies confirmed the role of PPP2R5B in regulating insulin signaling. Thus, we conclude that PPP2R5B, a B subunit of PP2A is a negative regulator of Akt phosphorylation contributing partly to the chronic hyperinsulinemia induced insulin resistance in adipocytes.

Noxa upregulation by oncogenic activation of MEK/ERK through CREB promotes autophagy in human melanoma cells

Reduction in the expression of the anti-survival BH3-only proteins PUMA and Bim is associated with the pathogenesis of melanoma. However, we have found that the expression of the other BH3-only protein Noxa is commonly upregulated in melanoma cells, and that this is driven by oncogenic activation of MEK/ERK. Immunohistochemistry studies showed that Noxa was expressed at higher levels in melanomas than nevi. Moreover, the expression of Noxa was increased in metastatic compared to primary melanomas, and in thick primaries compared to thin primaries. Inhibition of oncogenic BRAF^V600E or MEK downregulated Noxa, whereas activation of MEK/ERK caused its upregulation. In addition, introduction of BRAF^V600E increased Noxa expression in melanocytes. Upregulation of Noxa was due to a transcriptional increase mediated by cAMP responsive element binding protein, activation of which was also increased by MEK/ERK signaling in melanoma cells. Significantly, Noxa appeared necessary for constitutive activation of autophagy, albeit at low levels, by MEK/ERK in melanoma cells. Furthermore, it was required for autophagy activation that delayed apoptosis in melanoma cells undergoing nutrient deprivation. These results reveal that oncogenic activation of MEK/ERK drives Noxa expression to promote autophagy, and suggest that Noxa has an indirect anti-apoptosis role in melanoma cells under nutrient starvation conditions.

RIP1 Kinase Is an Oncogenic Driver in Melanoma

Although many studies have uncovered an important role for the receptor-binding protein kinase RIP1 in controlling cell death signaling, its possible contributions to cancer pathogenesis have been little explored. Here, we report that RIP1 functions as an oncogenic driver in human melanoma. Although RIP1 was commonly upregulated in melanoma, RIP1 silencing inhibited melanoma cell proliferation in vitro and retarded the growth of melanoma xenografts in vivo. Conversely, while inducing apoptosis in a small proportion of melanoma cells, RIP1 overexpression enhanced proliferation in the remaining cells. Mechanistic investigations revealed that the proliferative effects of RIP1 overexpression were mediated by NF-κB activation. Strikingly, ectopic expression of RIP1 enhanced the proliferation of primary melanocytes, triggering their anchorage-independent cell growth in an NF-κB-dependent manner. We identified DNA copy-number gain and constitutive ubiquitination by a TNFα autocrine loop mechanism as two mechanisms of RIP1 upregulation in human melanomas. Collectively, our findings define RIP1 as an oncogenic driver in melanoma, with potential implications for targeting its NF-κB-dependent activation mechanism as a novel approach to treat this disease.

RIPK1 regulates survival of human melanoma cells upon endoplasmic reticulum stress through autophagy

Although RIPK1 (receptor [TNFRSF]-interacting protein kinase 1) is emerging as a critical determinant of cell fate in response to cellular stress resulting from activation of death receptors and DNA damage, its potential role in cell response to endoplasmic reticulum (ER) stress remains undefined. Here we report that RIPK1 functions as an important prosurvival mechanism in melanoma cells undergoing pharmacological ER stress induced by tunicamycin (TM) or thapsigargin (TG) through activation of autophagy. While treatment with TM or TG upregulated RIPK1 and triggered autophagy in melanoma cells, knockdown of RIPK1 inhibited autophagy and rendered the cells sensitive to killing by TM or TG, recapitulating the effect of inhibition of autophagy. Consistently, overexpression of RIPK1 enhanced induction of autophagy and conferred resistance of melanoma cells to TM- or TG-induced cell death. Activation of MAPK8/JNK1 or MAPK9/JNK2, which phosphorylated BCL2L11/BIM leading to its dissociation from BECN1/Beclin 1, was involved in TM- or TG-induced, RIPK1-mediated activation of autophagy; whereas, activation of the transcription factor HSF1 (heat shock factor protein 1) downstream of the ERN1/IRE1-XBP1 axis of the unfolded protein response was responsible for the increase in RIPK1 in melanoma cells undergoing pharmacological ER stress. Collectively, these results identify upregulation of RIPK1 as an important resistance mechanism of melanoma cells to TM- or TG-induced ER stress by protecting against cell death through activation of autophagy, and suggest that targeting the autophagy-activating mechanism of RIPK1 may be a useful strategy to enhance sensitivity of melanoma cells to therapeutic agents that induce ER stress.

Induction of endoplasmic reticulum stress as a strategy for melanoma therapy: is there a future?

Melanoma cells employ several survival strategies, including induction of the unfolded protein response, which mediates resistance to endoplasmic reticulum (ER) stress-induced apoptosis. Activation of oncogenes specifically suppresses ER stress-induced apoptosis, while upregulation of ER chaperone proteins and antiapoptotic BCL-2 family members increases the protein folding capacity of the cell and the threshold for the induction of ER stress-induced apoptosis, respectively. Modulation of unfolded protein response signaling, inhibition of the protein folding machinery and/or active induction of ER stress may thus represent potential strategies for the therapeutic management of melanoma. To this aim, the present article focuses on the current understanding of how melanoma cells avoid or overcome ER stress-induced apoptosis, as well as therapeutic strategies through which to harness ER stress for therapeutic benefit.

Tristetraprolin and its role in regulation of airway inflammation

Chronic inflammatory diseases, such as asthma and chronic obstructive pulmonary disease (COPD), are clinically and socioeconomically important diseases globally. Currently the mainstay of anti-inflammatory therapy in respiratory diseases is corticosteroids. Although corticosteroids have proven clinical efficacy in asthma, many asthmatic inflammatory conditions (e.g., infection, exacerbation, and severe asthma) are not responsive to corticosteroids. Moreover, despite an understanding that COPD progression is driven by inflammation, we currently do not have effective anti-inflammatory strategies to combat this disease. Hence, alternative anti-inflammatory strategies are required. p38 mitogen-activated protein kinase (MAPK) has emerged as an important signaling molecule driving airway inflammation, and pharmacological inhibitors against p38 MAPK may provide potential therapies for chronic respiratory disease. In this review, we discuss some of the recent in vitro and in vivo studies targeting p38 MAPK, but suggest that p38 MAPK inhibitors may prove less effective than originally considered because they may block anti-inflammatory molecules along with proinflammatory responses. We propose that an alternative strategy may be to target an anti-inflammatory molecule farther downstream of p38 MAPK, i.e., tristetraprolin (TTP). TTP is an mRNA-destabilizing, RNA-binding protein that enhances the decay of mRNAs, including those encoding proteins implicated in chronic respiratory diseases. We suggest that understanding the molecular mechanism of TTP expression and its temporal regulation will guide future development of novel anti-inflammatory pharmacotherapeutic approaches to combat respiratory disease.

Involvement of vacuolar H(+)-ATPase in killing of human melanoma cells by the sphingosine kinase analogue FTY720

Targeting the sphingosine 1-phosphate (S1P)/S1P receptor (S1PR) signalling axis is emerging as a promising strategy in the treatment of cancer. However, the effect of such an approach on survival of human melanoma cells remains less understood. Here, we show that the sphingosine analogue FTY720 that functionally antagonises S1PRs kills human melanoma cells through a mechanism involving the vacuolar H(+) -ATPase activity. Moreover, we demonstrate that FTY720-triggered cell death is characterized by features of necrosis and is not dependent on receptor-interacting protein kinase 1 or lysosome cathepsins, nor was it associated with the activation of protein phosphatase 2A. Instead, it is mediated by increased production of reactive oxygen species and is antagonized by activation of autophagy. Collectively, these results suggest that FTY720 and its analogues are promising candidates for further development as new therapeutic agents in the treatment of melanoma.

67-kDa laminin receptor-dependent protein phosphatase 2A (PP2A) activation elicits melanoma-specific antitumor activity overcoming drug resistance

The Ras/Raf/MEK/ERK pathway has been identified as a major, druggable regulator of melanoma. Mutational activation of BRAF is the most prevalent genetic alteration in human melanoma, resulting in constitutive melanoma hyperproliferation. A selective BRAF inhibitor showed remarkable clinical activity in patients with mutated BRAF. Unfortunately, most patients acquire resistance to the BRAF inhibitor, highlighting the urgent need for new melanoma treatment strategies. Green tea polyphenol (-)-epigallocatechin-3-O-gallate (EGCG) inhibits cell proliferation independently of BRAF inhibitor sensitivity, suggesting that increased understanding of the anti-melanoma activity of EGCG may provide a novel therapeutic target. Here, by performing functional genetic screening, we identified protein phosphatase 2A (PP2A) as a critical factor in the suppression of melanoma cell proliferation. We demonstrated that tumor-overexpressed 67-kDa laminin receptor (67LR) activates PP2A through adenylate cyclase/cAMP pathway eliciting inhibitions of oncoproteins and activation of tumor suppressor Merlin. Activating 67LR/PP2A pathway leading to melanoma-specific mTOR inhibition shows strong synergy with the BRAF inhibitor PLX4720 in the drug-resistant melanoma. Moreover, SET, a potent inhibitor of PP2A, is overexpressed on malignant melanoma. Silencing of SET enhances 67LR/PP2A signaling. Collectively, activation of 67LR/PP2A signaling may thus be a novel rational strategy for melanoma-specific treatment.

Mastl is required for timely activation of APC/C in meiosis I and Cdk1 reactivation in meiosis II

The Greatwall kinase orthologue Mastl regulates timely activation of APC/C to allow meiosis I exit and suppresses PP2A activity and thereby allows the rapid rise of Cdk1 activity that is necessary for meiosis II entry in mouse oocytes.

In mitosis, the Greatwall kinase (called microtubule-associated serine/threonine kinase like [Mastl] in mammals) is essential for prometaphase entry or progression by suppressing protein phosphatase 2A (PP2A) activity. PP2A suppression in turn leads to high levels of Cdk1 substrate phosphorylation. We have used a mouse model with an oocyte-specific deletion of Mastl to show that Mastl-null oocytes resume meiosis I and reach metaphase I normally but that the onset and completion of anaphase I are delayed. Moreover, after the completion of meiosis I, Mastl-null oocytes failed to enter meiosis II (MII) because they reassembled a nuclear structure containing decondensed chromatin. Our results show that Mastl is required for the timely activation of anaphase-promoting complex/cyclosome to allow meiosis I exit and for the rapid rise of Cdk1 activity that is needed for the entry into MII in mouse oocytes.

Progesterone production is affected by unfolded protein response (UPR) signaling during the luteal phase in mice

AIMS

We examined whether the three unfolded protein response (UPR) signaling pathways, which are activated in response to endoplasmic reticulum (ER)-stress, are involved in progesterone production in the luteal cells of the corpus luteum (CL) during the mouse estrous cycle.

MAIN METHODS

The luteal phase of C57BL/6 female mice (8 weeks old) was divided into two stages: the functional stage (16, 24, and 48 h) and the regression stage (72 and 96 h). Western blotting and reverse transcription (RT)-PCR were performed to analyze UPR protein/gene expression levels in each stage. We investigated whether ER stress affects the progesterone production by using Tm (0.5 μg/g BW) or TUDCA (0.5 μg/g BW) through intra-peritoneal injection.

KEY FINDINGS

Our results indicate that expressions of Grp78/Bip, p-eIF2α/ATF4, p50ATF6, and p-IRE1/sXBP1 induced by UPR activation were predominantly maintained in functional and early regression stages of the CL. Furthermore, the expression of p-JNK, CHOP, and cleaved caspase3 as ER-stress mediated apoptotic factors increased during the regression stage. Cleaved caspase3 levels increased in the late-regression stage after p-JNK and CHOP expression in the early-regression stage. Additionally, although progesterone secretion and levels of steroidogenic enzymes decreased following intra-peritoneal injection of Tunicamycin, an ER stress inducer, the expression of Grp78/Bip, p50ATF6, and CHOP dramatically increased.

SIGNIFICANCE

These results suggest that the UPR signaling pathways activated in response to ER stress may play important roles in the regulation of the CL function. Furthermore, our findings enhance the understanding of the basic mechanisms affecting the CL life span.

Proteome-wide search for PP2A substrates in fission yeast

PP2A (protein phosphatase 2A) is a major phosphatase in eukaryotic cells that plays an essential role in many processes. PP2A mutations in Schizosaccharomyces pombe result in defects of cell cycle control, cytokinesis and morphogenesis. Which PP2A substrates are responsible for these changes is not known. In this work, we searched for PP2A substrates in S. pombe using two approaches, 2D-DIGE analysis of PP2A complex mutants and identification of PP2A interacting proteins. In both cases, we used MS to identify proteins of interest. In the DIGE experiment, we compared proteomes of wild-type S. pombe, deletion of pta2, the phosphoactivator of the PP2A catalytic subunit, and pab1-4, a mutant of B-type PP2A regulatory subunit. A total of 1742 protein spots were reproducibly resolved by 2D-DIGE and 51 spots demonstrated significant changes between PP2A mutants and the wild-type control. MS analysis of these spots identified 27 proteins that include key regulators of glycerol synthesis, carbon metabolism, amino acid biosyntesis, vitamin production, and protein folding. Importantly, we independently identified a subset of these proteins as PP2A binding partners by affinity precipitation, suggesting they may be direct targets of PP2A. We have validated our approach by demonstrating that phosphorylation of Gpd1, a key enzyme in glycerol biogenesis, is regulated by PP2A and that ability of cells to respond to osmotic stress by synthesizing glycerol is compromised in the PP2A mutants. Our work contributes to a better understanding of PP2A function and identifies potential PP2A substrates.

Insulin induces drug resistance in melanoma through activation of the PI3K/Akt pathway

Introduction

There is currently no curative treatment for melanoma once the disease spreads beyond the original site. Although activation of the PI3K/Akt pathway resulting from genetic mutations and epigenetic deregulation of its major regulators is known to cause resistance of melanoma to therapeutic agents, including the conventional chemotherapeutic drug dacarbazine and the Food and Drug Administration-approved mutant BRAF inhibitors vemurafenib and dabrafenib, the role of extracellular stimuli of the pathway, such as insulin, in drug resistance of melanoma remains less understood.

Objective

To investigate the effect of insulin on the response of melanoma cells to dacarbazine, and in particular, the effect of insulin on the response of melanoma cells carrying the BRAF^V600E mutation to mutant BRAF inhibitors. An additional aim was to define the role of the PI3K/Akt pathway in the insulin-triggered drug resistance.

Methods

The effect of insulin on cytotoxicity induced by dacarbazine or the mutant BRAF inhibitor PLX4720 was tested by pre-incubation of melanoma cells with insulin. Cytotoxicity was determined by the MTS assay. The role of the PI3K/Akt pathway in the insulin-triggered drug resistance was examined using the PI3K inhibitor LY294002 and the PI3K and mammalian target of rapamycin dual inhibitor BEZ-235. Activation of the PI3K/Akt pathway was monitored by Western blot analysis of phosphorylated levels of Akt.

Results

Recombinant insulin attenuated dacarbazine-induced cytotoxicity in both wild-type BRAF and BRAF^V600E melanoma cells, whereas it also reduced killing of BRAF^V600E melanoma cells by PLX4720. Nevertheless, the protective effect of insulin was abolished by the PI3K and mTOR dual inhibitor BEZ-235 or the PI3K inhibitor LY294002.

Conclusion

Insulin attenuates the therapeutic efficacy of dacarbazine and PLX4720 in melanoma cells, which is mediated by activation of the PI3K/Akt pathway and can be overcome by PI3K inhibitors.

Sustained IRE1 and ATF6 signaling is important for survival of melanoma cells undergoing ER stress

Apoptosis triggered by endoplasmic reticulum (ER) stress is associated with rapid attenuation of the IRE1α and ATF6 pathways but persistent activation of the PERK branch of the unfolded protein response (UPR) in cells. However, melanoma cells are largely resistant to ER stress-induced apoptosis, suggesting that the kinetics and durations of activation of the UPR pathways are deregulated in melanoma cells undergoing ER stress. We show here that the IRE1α and ATF6 pathways are sustained along with the PERK signaling in melanoma cells subjected to pharmacological ER stress, and that this is, at least in part, due to increased activation of the MEK/ERK pathway. In contrast to an initial increase followed by rapid reduction in activation of IRE1α and ATF6 signaling in control cells that were relatively sensitive to ER stress-induced apoptosis, activation of IRE1α and ATF6 by the pharmacological ER stress inducer tunicamycin (TM) or thapsigargin (TG) persisted in melanoma cells. On the other hand, the increase in PERK signaling lasted similarly in both types of cells. Sustained activation of IRE1α and ATF6 signaling played an important role in protecting melanoma cells from ER stress-induced apoptosis, as interruption of IRE1α or ATF6 rendered melanoma cells sensitive to apoptosis induced by TM or TG. Inhibition of MEK partially blocked IRE1α and ATF6 activation, suggesting that MEK/ERK signaling contributed to sustained activation of IRE1α and ATF6. Taken together, these results identify sustained activation of the IRE1α and ATF6 pathways of the UPR driven by the MEK/ERK pathway as an important protective mechanism against ER stress-induced apoptosis in melanoma cells.

Oncogenic suppression of PHLPP1 in human melanoma

Akt is constitutively activated in up to 70% of human melanomas and has an important role in the pathogenesis of the disease. However, little is known about protein phosphatases that dephosphorylate and thereby inactivate it in melanoma cells. Here we report that suppression of pleckstrin homology domain and leucine-rich repeat Ser/Thr protein phosphatase 1 (PHLPP1) by DNA methylation promotes Akt activation and has an oncogenic role in melanoma. While it is commonly downregulated, overexpression of PHLPP1 reduces Akt activation and inhibits melanoma cell proliferation in vitro, and retards melanoma growth in a xenograft model. In contrast, knockdown of PHLPP1 increases Akt activation, enhances melanoma cell and melanocyte proliferation, and results in anchorage-independent growth of melanocytes. Suppression of PHLPP1 involves blockade of binding of the transcription factor Sp1 to the PHLPP1 promoter. Collectively, these results suggest that suppression of PHLPP1 by DNA methylation contributes to melanoma development and progression.

Oncogenic activation of MEK/ERK primes melanoma cells for adaptation to endoplasmic reticulum stress

Cancer cells commonly undergo chronic endoplasmic reticulum (ER) stress, to which the cells have to adapt for survival and proliferation. We report here that in melanoma cells intrinsic activation of the ER stress response/unfolded protein response (UPR) is, at least in part, caused by increased outputs of protein synthesis driven by oncogenic activation of mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (MEK/ERK) and promotes proliferation and protects against apoptosis induced by acute ER stress. Inhibition of oncogenic BRAF(V600E) or MEK-attenuated activation of inositol-requiring enzyme 1 (IRE1) and activating transcription factor 6 (ATF6) signaling of the UPR in melanoma cells. This was associated with decreased phosphorylation of eukaryotic initiation factor 4E (eIF4E) and nascent protein synthesis and was recapitulated by knockdown of eIF4E. In line with this, introduction of BRAF(V600E) into melanocytes led to increases in eIF4E phosphorylation and protein production and triggered activation of the UPR. Similar to knockdown of glucose-regulated protein 78 (GRP78), inhibition of XBP1 decelerated melanoma cell proliferation and enhanced apoptosis induced by the pharmacological ER stress inducers tunicamycin and thapasigargin. Collectively, these results reveal that potentiation of adaptation to chronic ER stress is another mechanism by which oncogenic activation of the MEK/ERK pathway promotes the pathogenesis of melanoma.

cIAP-2 protects cardiac fibroblasts from oxidative damage: an obligate regulatory role for ERK1/2 MAPK and NF-κB

Cardiac fibroblasts are resistant to several pro-apoptotic factors that prevail in the diseased myocardium. Resistance to death signals may, in the short-term, enable these cells to play a central role in tissue repair following myocyte loss but, in the long-term, facilitate their persistence in the infarct scar, resulting in disproportionate stromal growth and pump dysfunction. Surprisingly, the molecular basis of apoptosis resistance in cardiac fibroblasts remains unclear. We explored the recruitment of anti-apoptotic mechanisms in cardiac fibroblasts subjected to oxidative stress, a major component of ischemia-reperfusion injury and heart failure. Cardiac fibroblasts exposed to H2O2 expressed enhanced levels of anti-apoptotic cIAP-2 mRNA and protein, revealed by real time PCR and western blot analysis, respectively. Pulmonary fibroblasts did not express cIAP-2 and were more susceptible than cardiac fibroblasts to H2O2. cIAP-2 knockdown by RNA interference promoted apoptosis in H2O2-treated cardiac fibroblasts. Electrophoretic mobility shift assay showed NF-κB activation in cells under oxidative stress. NF-κB inhibition in H2O2-treated cells resulted in significant attenuation of cIAP-2 mRNA and protein expression and apoptosis, indicating involvement of NF-κB in cell survival via regulation of cIAP-2. Further, pCMV promoter-driven constitutive expression of cIAP-2 reduced viability loss in NF-κB-inhibited cardiac fibroblasts exposed to oxidative stress. H2O2 also caused ERK1/2 activation, which, upon inhibition, prevented IκBα degradation and nuclear translocation of NF-κB. Moreover, ERK1/2 inhibition attenuated H2O2-induced cIAP-2 expression and compromised viability in H2O2-treated cardiac fibroblasts. We propose for the first time that ERK1/2-dependent activation of NF-κB and consequent induction of cIAP-2 protects cardiac fibroblasts from oxidative damage.