PPP6C negatively regulates oncogenic ERK signaling through dephosphorylation of MEK

Cell Senescence and the Genetics of Melanoma Development

Cutaneous malignant melanoma is an aggressive skin cancer with an approximate lifetime risk of 1 in 38 in the UK. While exposure to ultraviolet radiation is a key environmental risk factor for melanoma, up to ~10% of patients report a family history of melanoma, and ~1% have a strong family history. The understanding of causal mutations in melanoma has been critical to the development of novel targeted therapies that have contributed to improved outcomes for late-stage patients. Here, we review current knowledge of the genes affected by familial melanoma mutations and their partial overlap with driver genes commonly mutated in sporadic melanoma development. One theme linking a set of susceptibility loci/genes is the regulation of skin pigmentation and suntanning. The largest functional set of susceptibility variants, typically with high penetrance, includes CDKN2A, RB1, and telomerase reverse transcriptase (TERT) mutations, associated with attenuation of cell senescence. We discuss the mechanisms of action of these gene sets in the biology and progression of nevi and melanoma.

Melanoma genomics - will we go beyond BRAF in clinics?

In the era of next-generation sequencing, the genetic background of cancer, including melanoma, appears to be thoroughly established. However, evaluating the oncogene BRAF mutation in codon V600 is still the only companion diagnostic genomic test commonly implemented in clinics for molecularly targeted treatment of advanced melanoma. Are we wasting the collected genomic data? Will we implement our current genomic knowledge of melanoma in clinics soon? This question is rather urgent because new therapeutic targets and biomarkers are needed to implement more personalized, patient-tailored therapy in clinics. Here, we provide an update on the molecular background of melanoma, including a description of four already established molecular subtypes: BRAF+, NRAS+, NF1+, and triple WT, as well as relatively new NGS-derived melanoma genes such as PREX2, ERBB4, PPP6C, FBXW7, PIK3CA, and IDH1. We also present a comparison of genomic profiles obtained in recent years with a focus on the most common melanoma genes. Finally, we propose our melanoma gene panel consisting of 22 genes that, in our opinion, are "must-have" genes in both melanoma-specific genomic tests and pan-cancer tests established to improve the treatment of melanoma further.

Spatial maps in piriform cortex during olfactory navigation

Odors are a fundamental part of the sensory environment used by animals to inform behaviors such as foraging and navigation1,2. Primary olfactory (piriform) cortex is thought to be dedicated to encoding odor identity3-8. Here, using neural ensemble recordings in freely moving rats performing a novel odor-cued spatial choice task, we show that posterior piriform cortex neurons also carry a robust spatial map of the environment. Piriform spatial maps were stable across behavioral contexts independent of olfactory drive or reward availability, and the accuracy of spatial information carried by individual neurons depended on the strength of their functional coupling to the hippocampal theta rhythm. Ensembles of piriform neurons concurrently represented odor identity as well as spatial locations of animals, forming an "olfactory-place map". Our results reveal a previously unknown function for piriform cortex in spatial cognition and suggest that it is well-suited to form odor-place associations and guide olfactory cued spatial navigation.

SHANK3 depletion leads to ERK signalling overdose and cell death in KRAS-mutant cancers

The KRAS oncogene drives many common and highly fatal malignancies. These include pancreatic, lung, and colorectal cancer, where various activating KRAS mutations have made the development of KRAS inhibitors difficult. Here we identify the scaffold protein SH3 and multiple ankyrin repeat domain 3 (SHANK3) as a RAS interactor that binds active KRAS, including mutant forms, competes with RAF and limits oncogenic KRAS downstream signalling, maintaining mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) activity at an optimal level. SHANK3 depletion breaches this threshold, triggering MAPK/ERK signalling hyperactivation and MAPK/ERK-dependent cell death in KRAS-mutant cancers. Targeting this vulnerability through RNA interference or nanobody-mediated disruption of the SHANK3-KRAS interaction constrains tumour growth in vivo in female mice. Thus, inhibition of SHANK3-KRAS interaction represents an alternative strategy for selective killing of KRAS-mutant cancer cells through excessive signalling.

The protein phosphatase PP6 promotes RIPK1-dependent PANoptosis

BACKGROUND

The innate immune system serves as the first line of host defense. Transforming growth factor-β-activated kinase 1 (TAK1) is a key regulator of innate immunity, cell survival, and cellular homeostasis. Because of its importance in immunity, several pathogens have evolved to carry TAK1 inhibitors. In response, hosts have evolved to sense TAK1 inhibition and induce robust lytic cell death, PANoptosis, mediated by the RIPK1-PANoptosome. PANoptosis is a unique innate immune inflammatory lytic cell death pathway initiated by an innate immune sensor and driven by caspases and RIPKs. While PANoptosis can be beneficial to clear pathogens, excess activation is linked to pathology. Therefore, understanding the molecular mechanisms regulating TAK1 inhibitor (TAK1i)-induced PANoptosis is central to our understanding of RIPK1 in health and disease.

RESULTS

In this study, by analyzing results from a cell death-based CRISPR screen, we identified protein phosphatase 6 (PP6) holoenzyme components as regulators of TAK1i-induced PANoptosis. Loss of the PP6 enzymatic component, PPP6C, significantly reduced TAK1i-induced PANoptosis. Additionally, the PP6 regulatory subunits PPP6R1, PPP6R2, and PPP6R3 had redundant roles in regulating TAK1i-induced PANoptosis, and their combined depletion was required to block TAK1i-induced cell death. Mechanistically, PPP6C and its regulatory subunits promoted the pro-death S166 auto-phosphorylation of RIPK1 and led to a reduction in the pro-survival S321 phosphorylation.

CONCLUSIONS

Overall, our findings demonstrate a key requirement for the phosphatase PP6 complex in the activation of TAK1i-induced, RIPK1-dependent PANoptosis, suggesting this complex could be therapeutically targeted in inflammatory conditions.

Protein phosphatase 6 activates NF-κB to confer sensitivity to MAPK pathway inhibitors in KRAS- and BRAF-mutant cancer cells

The Ras-mitogen-activated protein kinase (MAPK) pathway is a major target for cancer treatment. To better understand the genetic pathways that modulate cancer cell sensitivity to MAPK pathway inhibitors, we performed a CRISPR knockout screen with MAPK pathway inhibitors on a colorectal cancer (CRC) cell line carrying mutant KRAS. Genetic deletion of the catalytic subunit of protein phosphatase 6 (PP6), encoded by PPP6C, rendered KRAS- and BRAF-mutant CRC and BRAF-mutant melanoma cells more resistant to these inhibitors. In the absence of MAPK pathway inhibition, PPP6C deletion in CRC cells decreased cell proliferation in two-dimensional (2D) adherent cultures but accelerated the growth of tumor spheroids in 3D culture and tumor xenografts in vivo. PPP6C deletion enhanced the activation of nuclear factor κB (NF-κB) signaling in CRC and melanoma cells and circumvented the cell cycle arrest and decreased cyclin D1 abundance induced by MAPK pathway blockade in CRC cells. Inhibiting NF-κB activity by genetic and pharmacological means restored the sensitivity of PPP6C-deficient cells to MAPK pathway inhibition in CRC and melanoma cells in vitro and in CRC cells in vivo. Furthermore, a R264 point mutation in PPP6C conferred loss of function in CRC cells, phenocopying the enhanced NF-κB activation and resistance to MAPK pathway inhibition observed for PPP6C deletion. These findings demonstrate that PP6 constrains the growth of KRAS- and BRAF-mutant cancer cells, implicates the PP6-NF-κB axis as a modulator of MAPK pathway output, and presents a rationale for cotargeting the NF-κB pathway in PPP6C-mutant cancer cells.

Depletion of Ppp6c in hematopoietic and vascular endothelial cells causes embryonic lethality and decreased hematopoietic potential

Protein phosphatase 6 (PP6) is a serine/threonine (Ser/Thr) protein phosphatase, and its catalytic subunit is Ppp6c. PP6 forms the PP2A subfamily with PP2A and PP4. The diverse phenotypes observed following small interfering RNA (siRNA)-based knockdown of Ppp6c in cultured mammalian cells suggest that PP6 plays roles in cell growth and DNA repair. There is also evidence that PP6 regulates nuclear factor kappa B (NF-κB) signaling and mitogen-activated protein kinases and inactivates transforming growth factor-β-activated kinase 1 (TAK1). Loss of Ppp6c causes several abnormalities, including those of T cell and regulatory T cell function, neurogenesis, oogenesis, and spermatogenesis. PP2A has been reported to play an important role in erythropoiesis. However, the roles of PP6 in other hematopoietic cells have not been investigated. We generated Ppp6cfl/fl;Tie2-Cre (Ppp6cTKO) mice, in which Ppp6c was specifically deleted in hematopoietic and vascular endothelial cells. Ppp6cTKO mice displayed embryonic lethality. Ppp6c deficiency increased the number of dead cells and decreased the percentages of erythroid and monocytic cells during fetal hematopoiesis. By contrast, the number of Lin-Sca-1+c-Kit+ cells, which give rise to all hematopoietic cells, was slightly increased, but their colony-forming cell activity was markedly decreased. Ppp6c deficiency also increased phosphorylation of extracellular signal-regulated kinase 1/2 and c-Jun amino (N)-terminal kinase in fetal liver hematopoietic cells.

Oncogenic K-RasG12V cannot overcome proliferation failure caused by loss of Ppp6c in mouse embryonic fibroblasts

Protein phosphatase 6 is a Ser/Thr protein phosphatase and its catalytic subunit is Ppp6c. Ppp6c is thought to be indispensable for proper growth of normal cells. On the other hand, loss of Ppp6c accelerates growth of oncogenic Ras-expressing cells. Although it has been studied in multiple contexts, the role(s) of Ppp6c in cell proliferation remains controversial. It is unclear how oncogenic K-Ras overcomes cell proliferation failure induced by Ppp6c deficiency; therefore, in this study, we attempted to shed light on how oncogenic K-Ras modulates tumor cell growth. Contrary to our expectations, loss of Ppp6c decreased proliferation, anchorage-independent growth in soft agar, and tumor formation of oncogenic Ras-expressing mouse embryonic fibroblasts (MEFs). These findings show that oncogenic K-RasG12V cannot overcome proliferation failure caused by loss of Ppp6c in MEFs.

Drug dependence in cancer is exploitable by optimally constructed treatment holidays

Cancers with acquired resistance to targeted therapy can become simultaneously dependent on the presence of the targeted therapy drug for survival, suggesting that intermittent therapy may slow resistance. However, relatively little is known about which tumours are likely to become dependent and how to schedule intermittent therapy optimally. Here we characterized drug dependence across a panel of over 75 MAPK-inhibitor-resistant BRAFV600E mutant melanoma models at the population and single-clone levels. Melanocytic differentiated models exhibited a much greater tendency to give rise to drug-dependent progeny than their dedifferentiated counterparts. Mechanistically, acquired loss of microphthalmia-associated transcription factor in differentiated melanoma models drives ERK-JunB-p21 signalling to enforce drug dependence. We identified the optimal scheduling of 'drug holidays' using simple mathematical models that we validated across short and long timescales. Without detailed knowledge of tumour characteristics, we found that a simple adaptive therapy protocol can produce near-optimal outcomes using only measurements of total population size. Finally, a spatial agent-based model showed that optimal schedules derived from exponentially growing cells in culture remain nearly optimal in the context of tumour cell turnover and limited environmental carrying capacity. These findings may guide the implementation of improved evolution-inspired treatment strategies for drug-dependent cancers.

Protein phosphatase 6 regulates trametinib sensitivity, a mitogen-activated protein kinase kinase (MEK) inhibitor, by regulating MEK1/2-ERK1/2 signaling in canine melanoma cells

Melanoma is a highly aggressive and metastatic cancer occurring in both humans and dogs. Canine melanoma accounts for a significant proportion of neoplastic diseases in dogs, and despite standard treatments, overall survival rates remain low. Protein phosphatase 6 (PP6), an evolutionarily conserved serine/threonine protein phosphatase, regulates various biological processes. Additionally, the loss of PP6 function reportedly leads to the development of melanoma in humans. However, there are no reports regarding the role of PP6 in canine cancer cells. We, therefore, conducted a study investigating the role of PP6 in canine melanoma by using four canine melanoma cell lines: CMec1, CMM, KMeC and LMeC. PP6 knockdown increased phosphorylation levels of mitogen-activated protein kinase kinase 1/2 (MEK1/2) and extracellular signal-regulated kinase 1/2 (ERK1/2) but not Akt. Furthermore, PP6 knockdown decreased sensitivity to trametinib, a MEK inhibitor, but did not alter sensitivity to Akt inhibitor. These findings suggest that PP6 may function as a tumor suppressor in canine melanoma and modulate the response to trametinib treatment. Understanding the role of PP6 in canine melanoma could lead to the development of more effective treatment strategies for this aggressive disease.

Inducible Protein Degradation as a Strategy to Identify Phosphoprotein Phosphatase 6 Substrates in RAS-Mutant Colorectal Cancer Cells

Protein phosphorylation is an essential regulatory mechanism that controls most cellular processes, including cell cycle progression, cell division, and response to extracellular stimuli, among many others, and is deregulated in many diseases. Protein phosphorylation is coordinated by the opposing activities of protein kinases and protein phosphatases. In eukaryotic cells, most serine/threonine phosphorylation sites are dephosphorylated by members of the Phosphoprotein Phosphatase (PPP) family. However, we only know for a few phosphorylation sites which specific PPP dephosphorylates them. Although natural compounds such as calyculin A and okadaic acid inhibit PPPs at low nanomolar concentrations, no selective chemical PPP inhibitors exist. Here, we demonstrate the utility of endogenous tagging of genomic loci with an auxin-inducible degron (AID) as a strategy to investigate specific PPP signaling. Using Protein Phosphatase 6 (PP6) as an example, we demonstrate how rapidly inducible protein degradation can be employed to identify dephosphorylation sites and elucidate PP6 biology. Using genome editing, we introduce AID-tags into each allele of the PP6 catalytic subunit (PP6c) in DLD-1 cells expressing the auxin receptor Tir1. Upon rapid auxin-induced degradation of PP6c, we perform quantitative mass spectrometry-based proteomics and phosphoproteomics to identify PP6 substrates in mitosis. PP6 is an essential enzyme with conserved roles in mitosis and growth signaling. Consistently, we identify candidate PP6c-dependent dephosphorylation sites on proteins implicated in coordinating the mitotic cell cycle, cytoskeleton, gene expression, and mitogen-activated protein kinase (MAPK) and Hippo signaling. Finally, we demonstrate that PP6c opposes the activation of large tumor suppressor 1 (LATS1) by dephosphorylating Threonine 35 (T35) on Mps One Binder (MOB1), thereby blocking the interaction of MOB1 and LATS1. Our analyses highlight the utility of combining genome engineering, inducible degradation, and multiplexed phosphoproteomics to investigate signaling by individual PPPs on a global level, which is currently limited by the lack of tools for specific interrogation.

Molecular Modeling Unveils the Effective Interaction of B-RAF Inhibitors with Rare B-RAF Insertion Variants

The Food and Drug Administration (FDA) has approved MAPK inhibitors as a treatment for melanoma patients carrying a mutation in codon V600 of the BRAF gene exclusively. However, BRAF mutations outside the V600 codon may occur in a small percentage of melanomas. Although these rare variants may cause B-RAF activation, their predictive response to B-RAF inhibitor treatments is still poorly understood. We exploited an integrated approach for mutation detection, tumor evolution tracking, and assessment of response to treatment in a metastatic melanoma patient carrying the rare p.T599dup B-RAF mutation. He was addressed to Dabrafenib/Trametinib targeted therapy, showing an initial dramatic response. In parallel, in-silico ligand-based homology modeling was set up and performed on this and an additional B-RAF rare variant (p.A598_T599insV) to unveil and justify the success of the B-RAF inhibitory activity of Dabrafenib, showing that it could adeptly bind both these variants in a similar manner to how it binds and inhibits the V600E mutant. These findings open up the possibility of broadening the spectrum of BRAF inhibitor-sensitive mutations beyond mutations at codon V600, suggesting that B-RAF V600 WT melanomas should undergo more specific investigations before ruling out the possibility of targeted therapy.

Tackling FGFR3-driven bladder cancer with a promising synergistic FGFR/HDAC targeted therapy

Bladder cancer (BC) is one of the most prevalent malignancies worldwide and FGFR3 alterations are particularly common in BC. Despite approval of erdafitinib, durable responses for FGFR inhibitors are still uncommon and most patients relapse to metastatic disease. Given the necessity to discover more efficient therapies for BC, herein, we sought to explore promising synergistic combinations for BC with FGFR3 fusions. Our studies confirmed the synergy between FGFR and HDAC inhibitors in vitro and demonstrated its benefits in vivo. Mechanistic studies revealed that quisinostat can downregulate FGFR3 expression by suppressing FGFR3 translation. Additionally, quisinostat can also sensitize BC cells to erdafitinib by downregulating HDGF. Furthermore, the synergy was also confirmed in BC cells with FGFR3 S249C. This study discovers a new avenue for treatment of FGFR3-driven BC and uncovers new mechanistic insights. These preclinical studies pave the way for a direct translation of this combination to early phase clinical trials.

Inducible protein degradation as a strategy to identify Phosphoprotein Phosphatase 6 substrates in RAS-mutant colorectal cancer cells

Protein phosphorylation is an essential regulatory mechanism that controls most cellular processes, including cell cycle progression, cell division, and response to extracellular stimuli, among many others, and is deregulated in many diseases. Protein phosphorylation is coordinated by the opposing activities of protein kinases and protein phosphatases. In eukaryotic cells, most serine/threonine phosphorylation sites are dephosphorylated by members of the Phosphoprotein Phosphatase (PPP) family. However, we only know for a few phosphorylation sites which specific PPP dephosphorylates them. Although natural compounds such as calyculin A and okadaic acid inhibit PPPs at low nanomolar concentrations, no selective chemical PPP inhibitors exist. Here, we demonstrate the utility of endogenous tagging of genomic loci with an auxin-inducible degron (AID) as a strategy to investigate specific PPP signaling. Using Protein Phosphatase 6 (PP6) as an example, we demonstrate how rapidly inducible protein degradation can be employed to identify dephosphorylation SITES and elucidate PP6 biology. Using genome editing, we introduce AID-tags into each allele of the PP6 catalytic subunit (PP6c) in DLD-1 cells expressing the auxin receptor Tir1. Upon rapid auxin-induced degradation of PP6c, we perform quantitative mass spectrometry-based proteomics and phosphoproteomics to identify PP6 substrates in mitosis. PP6 is an essential enzyme with conserved roles in mitosis and growth signaling. Consistently, we identify candidate PP6c-dependent phosphorylation sites on proteins implicated in coordinating the mitotic cell cycle, cytoskeleton, gene expression, and mitogen-activated protein kinase (MAPK) and Hippo signaling. Finally, we demonstrate that PP6c opposes the activation of large tumor suppressor 1 (LATS1) by dephosphorylating Threonine 35 (T35) on Mps One Binder (MOB1), thereby blocking the interaction of MOB1 and LATS1. Our analyses highlight the utility of combining genome engineering, inducible degradation, and multiplexed phosphoproteomics to investigate signaling by individual PPPs on a global level, which is currently limited by the lack of tools for specific interrogation.

A Comprehensive Analysis of Cutaneous Melanoma Patients in Greece Based on Multi-Omic Data

Cutaneous melanoma (CM) is the most aggressive type of skin cancer, and it is characterised by high mutational load and heterogeneity. In this study, we aimed to analyse the genomic and transcriptomic profile of primary melanomas from forty-six Formalin-Fixed, Paraffin-Embedded (FFPE) tissues from Greek patients. Molecular analysis for both germline and somatic variations was performed in genomic DNA from peripheral blood and melanoma samples, respectively, exploiting whole exome and targeted sequencing, and transcriptomic analysis. Detailed clinicopathological data were also included in our analyses and previously reported associations with specific mutations were recognised. Most analysed samples (43/46) were found to harbour at least one clinically actionable somatic variant. A subset of samples was profiled at the transcriptomic level, and it was shown that specific melanoma phenotypic states could be inferred from bulk RNA isolated from FFPE primary melanoma tissue. Integrative bioinformatics analyses, including variant prioritisation, differential gene expression analysis, and functional and gene set enrichment analysis by group and per sample, were conducted and molecular circuits that are implicated in melanoma cell programmes were highlighted. Integration of mutational and transcriptomic data in CM characterisation could shed light on genes and pathways that support the maintenance of phenotypic states encrypted into heterogeneous primary tumours.

Elaiophylin triggers paraptosis and preferentially kills ovarian cancer drug-resistant cells by inducing MAPK hyperactivation

Finely tuned mitogen-activated protein kinase (MAPK) signaling is important for cancer cell survival. Perturbations that push cells out of the MAPK fitness zone result in cell death. Previously, in a screen of the North China Pharmaceutical Group Corporation's pure compound library of microbial origin, we identified elaiophylin as an autophagy inhibitor. Here, we demonstrated a new role for elaiophylin in inducing excessive endoplasmic reticulum (ER) stress, ER-derived cytoplasmic vacuolization, and consequent paraptosis by hyperactivating the MAPK pathway in multiple cancer cells. Genome-wide CRISPR/Cas9 knockout library screening identified SHP2, an upstream intermediary of the MAPK pathway, as a critical target in elaiophylin-induced paraptosis. The cellular thermal shift assay (CETSA) and surface plasmon resonance (SPR) assay further confirmed the direct binding between the SHP2 and elaiophylin. Inhibition of the SHP2/SOS1/MAPK pathway through SHP2 knockdown or pharmacological inhibitors distinctly attenuated elaiophylin-induced paraptosis and autophagy inhibition. Interestingly, elaiophylin markedly increased the already-elevated MAPK levels and preferentially killed drug-resistant cells with enhanced basal MAPK levels. Elaiophylin overcame drug resistance by triggering paraptosis in multiple tumor-bearing mouse models resistant to platinum, taxane, or PARPi, suggesting that elaiophylin might offer a reasonable therapeutic strategy for refractory ovarian cancer.

Rab40c regulates focal adhesions and PP6 activity by controlling ANKRD28 ubiquitylation

Rab40c is a SOCS box-containing protein which binds Cullin5 to form a ubiquitin E3 ligase complex (Rab40c/CRL5) to regulate protein ubiquitylation. However, the exact functions of Rab40c remain to be determined, and what proteins are the targets of Rab40c-Cullin5-mediated ubiquitylation in mammalian cells are unknown. Here we showed that in migrating MDA-MB-231 cells Rab40c regulates focal adhesion's number, size, and distribution. Mechanistically, we found that Rab40c binds the protein phosphatase 6 (PP6) complex and ubiquitylates one of its subunits, ankyrin repeat domain 28 (ANKRD28), thus leading to its lysosomal degradation. Furthermore, we identified that phosphorylation of FAK and MOB1 is decreased in Rab40c knock-out cells, which may contribute to focal adhesion site regulation by Rab40c. Thus, we propose a model where Rab40c/CRL5 regulates ANKRD28 ubiquitylation and degradation, leading to a decrease in PP6 activity, which ultimately affects FAK and Hippo pathway signaling to alter focal adhesion dynamics.

PP6 deficiency in mice with KRAS mutation and Trp53 loss promotes early death by PDAC with cachexia-like features

To examine effects of PP6 gene (Ppp6c) deficiency on pancreatic tumor development, we developed pancreas-specific, tamoxifen-inducible Cre-mediated KP (KRAS(G12D) plus Trp53-deficient) mice (cKP mice) and crossed them with Ppp6cflox / flox mice. cKP mice with the homozygous Ppp6c deletion developed pancreatic tumors, became emaciated and required euthanasia within 150 days of mutation induction, phenotypes that were not seen in heterozygous or wild-type (WT) mice. At 30 days, a comparative analysis of genes commonly altered in homozygous versus WT Ppp6c cKP mice revealed enhanced activation of Erk and NFκB pathways in homozygotes. By 80 days, the number and size of tumors and number of precancerous lesions had significantly increased in the pancreas of Ppp6c homozygous relative to heterozygous or WT cKP mice. Ppp6c-/- tumors were pathologically diagnosed as pancreatic ductal adenocarcinoma (PDAC) undergoing the epithelial-mesenchymal transition (EMT), and cancer cells had invaded surrounding tissues in three out of six cases. Transcriptome and metabolome analyses indicated an enhanced cancer-specific glycolytic metabolism in Ppp6c-deficient cKP mice and the increased expression of inflammatory cytokines. Individual Ppp6c-/- cKP mice showed weight loss, decreased skeletal muscle and adipose tissue, and increased circulating tumor necrosis factor (TNF)-α and IL-6 levels, suggestive of systemic inflammation. Overall, Ppp6c deficiency in the presence of K-ras mutations and Trp53 gene deficiency promoted pancreatic tumorigenesis with generalized cachexia and early death. This study provided the first evidence that Ppp6c suppresses mouse pancreatic carcinogenesis and supports the use of Ppp6c-deficient cKP mice as a model for developing treatments for cachexia associated with pancreatic cancer.

PPP6C, a serine-threonine phosphatase, regulates melanocyte differentiation and contributes to melanoma tumorigenesis through modulation of MITF activity

It is critical to understand the molecular mechanisms governing the regulation of MITF, a lineage specific transcription factor in melanocytes and an oncogene in melanoma. We identified PPP6C, a serine/threonine phosphatase, as a key regulator of MITF in melanoma. PPP6C is the only recurrently mutated serine/threonine phosphatase across all human cancers identified in sequencing studies and the recurrent R264C mutation occurs exclusively in melanoma. Using a zebrafish developmental model system, we demonstrate that PPP6C expression disrupts melanocyte differentiation. Melanocyte disruption was rescued by engineering phosphomimetic mutations at serine residues on MITF. We developed an in vivo MITF promoter assay in zebrafish and studied the effects of PPP6C(R264C) on regulating MITF promoter activity. Expression of PPP6C(R264C) cooperated with oncogenic NRAS(Q61K) to accelerate melanoma initiation in zebrafish, consistent with a gain of function alteration. Using a human melanoma cell line, we examined the requirement for PPP6C in proliferation and MITF expression. We show that genetic inactivation of PPP6C increases MITF and target gene expression, decreases sensitivity to BRAF inhibition, and increases phosphorylated MITF in a BRAF(V600E) mutant melanoma cell line. Our data suggests that PPP6C may be a relevant drug target in melanoma and proposes a mechanism for its action.

Arginine Depletion in Human Cancers

Arginine is encoded by six different codons. Base pair changes in any of these codons can have a broad spectrum of effects including substitutions to twelve different amino acids, eighteen synonymous changes, and two stop codons. Four amino acids (histidine, cysteine, glutamine, and tryptophan) account for over 75% of amino acid substitutions of arginine. This suggests that a mutational bias, or "purifying selection", mechanism is at work. This bias appears to be driven by C > T and G > A transitions in four of the six arginine codons, a signature that is universal and independent of cancer tissue of origin or histology. Here, we provide a review of the available literature and reanalyze publicly available data from the Catalogue of Somatic Mutations in Cancer (COSMIC). Our analysis identifies several genes with an arginine substitution bias. These include known factors such as IDH1, as well as previously unreported genes, including four cancer driver genes (FGFR3, PPP6C, MAX, GNAQ). We propose that base pair substitution bias and amino acid physiology both play a role in purifying selection. This model may explain the documented arginine substitution bias in cancers.

Regulation of TLR4 signaling through the TRAF6/sNASP axis by reversible phosphorylation mediated by CK2 and PP4

Recognition of invading pathogens by Toll-like receptors (TLRs) activates innate immunity through signaling pathways that involved multiple protein kinases and phosphatases. We previously demonstrated that somatic nuclear autoantigenic sperm protein (sNASP) binds to TNF receptor-associated factor 6 (TRAF6) in the resting state. Upon TLR4 activation, a signaling complex consisting of TRAF6, sNASP, interleukin (IL)-1 receptor-associated kinase 4, and casein kinase 2 (CK2) is formed. CK2 then phosphorylates sNASP to release phospho-sNASP (p-sNASP) from TRAF6, initiating downstream signaling pathways. Here, we showed that protein phosphatase 4 (PP4) is the specific sNASP phosphatase that negatively regulates TLR4-induced TRAF6 activation and its downstream signaling pathway. Mechanistically, PP4 is directly recruited by phosphorylated sNASP to dephosphorylate p-sNASP to terminate TRAF6 activation. Ectopic expression of PP4 specifically inhibited sNASP-dependent proinflammatory cytokine production and downstream signaling following bacterial lipopolysaccharide (LPS) treatment, whereas silencing PP4 had the opposite effect. Primary macrophages and mice infected with recombinant adenovirus carrying a gene encoding PP4 (Ad-PP4) showed significant reduction in IL-6 and TNF-α production. Survival of Ad-PP4-infected mice was markedly increased due to a better ability to clear bacteria in a sepsis model. These results indicate that the serine/threonine phosphatase PP4 functions as a negative regulator of innate immunity by regulating the binding of sNASP to TRAF6.

Current Perspectives and Novel Strategies of NRAS-Mutant Melanoma

Melanoma is the deadliest cutaneous cancer. Activating mutations in NRAS are found in 20% of melanomas. NRAS-mutant melanoma is more aggressive and, therefore, has poorer outcomes, compared to non-NRAS-mutant melanoma. Despite promising preclinical data, to date immune checkpoint inhibitors remain the standard of care for locally advanced unresectable or metastatic NRAS melanoma. Data for efficacy of immunotherapy for NRAS melanoma mainly come from retrospective cohorts with divergent conclusions. MEK inhibitors have been the most developed targeted therapy approach. Although associated with an increase in progression-free survival, MEK inhibitors do not provide any benefit in terms of overall survival. Combination strategies with PI3K-AKT-mTOR pathway and CDK4/6 inhibitors seem to increase MEK inhibitors' benefit. Nevertheless, results from clinical trials are still prelaminar. A greater comprehension of the biology and intracellular interactions of NRAS-mutant melanoma will outline novel impactful strategies which could improve prognosis of these subgroup of patients.