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

Abstract

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.

M6A‐induced transcription factor IRF5 contributes to the progression of cervical cancer by upregulating PPP6C

“…Beyond that, IRF5 has been proven to improve PPP6C transcription through directly binding to the PPP6C promoter region, implying that PPP6C was a new IRF5‐target gene. As a serine–threonine phosphatase, PPP6C has been confirmed to promote the tumorigenesis and development of multiple human cancers 32–34 . Consistent with the former report, 27 we found that PPP6C content was clearly elevated in CC samples and cell lines.…”

“…As a serine-threonine phosphatase, PPP6C has been confirmed to promote the tumorigenesis and development of multiple human cancers. [32][33][34] Consistent with the former report, 27 we found that PPP6C content was clearly elevated in CC samples and cell lines. Apart from that, IRF5 absence-mediated CC cell growth and glycolysis inhibition were partly overturned by PPP6C upregulation.…”

Innate immune checkpoint inhibitor resistance is associated with melanoma sub-types exhibiting invasive and de-differentiated gene expression signatures

“…One of the most established factors that controls neural-crest migration, and melanocyte differentiation, is the expression level of microphthalmia-associated transcription factor ( MITF ). In melanoma cell biology, MITF is a central player, controlling aspects of phenotypic switching ( 68 ), and is one of several critical transcriptional regulators involved in melanocyte development, upregulating a set of genes to drive melanocytic differentiation ( 3 ). All melanomas exhibit at least two distinct MITF transcriptional cell states, which are represented by melanomas with high levels of MITF , versus melanomas with low levels of MITF ( 50 ).…”

“…The incidence of melanoma has increased continuously over the past 30 years ( 1 ) and cutaneous malignant melanoma accounts for 73% of skin cancer-related death ( 2 ). Although targeted therapies and immune checkpoint inhibitors (ICI) have become available to treat cutaneous melanoma ( 3 ), unfortunately, half of all the patients respond poorly to currently available targeted therapies and only 20–40% of melanoma patients respond well to anti-PD1 ICI monotherapies. Treatment with a combination of these drugs can result in approximately 60% response rates being achieved ( 4 ), but resistance to therapy and disease recurrence is very common, despite recent therapeutic advances provided by immunotherapy and targeted drugs.…”

The Impact of Mutational Hotspots on Cancer Survival

“…Recent studies have also shown the functional impact of PPP2R1A-179 in UCEC ( n = 26) [ 27 , 31 ] and CIC-215 in glioblastomas [ 32 ]. Other less frequent but known hotspots include VHL-158 in KIRC ( n = 7) [ 33 ], PPP6C-264 in SKCM ( n = 7) [ 34 ], and SF3B1-625 in UVM ( n = 13) [ 35 ]. The hotspot RUNX1-96 in BRCA, which is formed by frameshifts and lacks death events, has not been thoroughly studied.…”