A MAPK/miR-29 Axis Suppresses Melanoma by Targeting MAFG and MYBL2.

Olga Vera,Ariana Angarita,Nicol Mecozzi,Ilah Bok,Neel Jasani,Florian A Karreth,Koji Nakamura,Xiaonan Xu,Kaizhen Wang,Kenneth Y Tsai

doi:10.3390/cancers13061408

Abstract

Simple SummaryThe miR-29 family is subjected to complex regulation by tumor suppressors and oncogenes and has tumor suppressive potential in several cancers. We demonstrate that, in melanoma, oncogenic BRAF paradoxically induces miR-29 in concert with p53, thereby forming a barrier to melanoma progression. This barrier is overcome by reduced expression of miR-29, likely via diminished p53 activity. We further identify the transcription factors MAFG and MYBL2 as targets of miR-29 and show that their repression is detrimental for melanoma cells. Targeting MAFG- and MYBL2-regulated processes may therefore represent a promising therapeutic strategy to treat miR-29-low melanoma.The miR-29 family of microRNAs is encoded by two clusters, miR-29b1~a and miR-29b2~c, and is regulated by several oncogenic and tumor suppressive stimuli. While in vitro evidence suggests a tumor suppressor role for miR-29 in melanoma, the mechanisms underlying its deregulation and contribution to melanomagenesis have remained elusive. Using various in vitro systems, we show that oncogenic MAPK signaling paradoxically stimulates transcription of pri-miR-29b1~a and pri-miR-29b2~c, the latter in a p53-dependent manner. Expression analyses in melanocytes, melanoma cells, nevi, and primary melanoma revealed that pri-miR-29b2~c levels decrease during melanoma progression. Inactivation of miR-29 in vivo with a miRNA sponge in a rapid melanoma mouse model resulted in accelerated tumor development and decreased overall survival, verifying tumor suppressive potential of miR-29 in melanoma. Through integrated RNA sequencing, target prediction, and functional assays, we identified the transcription factors MAFG and MYBL2 as bona fide miR-29 targets in melanoma. Our findings suggest that attenuation of miR-29b2~c expression promotes melanoma development, at least in part, by derepressing MAFG and MYBL2.

Highlights

While genetic and genomic alterations are established drivers of melanoma formation, aberrant control of gene expression is emerging as a major contributor to melanoma progression. microRNAs bind to the 3 UTRs of target mRNAs to negatively regulate gene expression [1]
Given the regulation of miR-29 by several cancer-associated pathways, we first examined if BRAFV600E modulates miR-29 levels in mouse embryonic fibroblasts (MEFs) carrying a Cre-inducible endogenous BrafV600E allele (LSL-BrafV600E) [18]
The previously reported increased expression of mature miR-29a in response to p53 activity is likely explained by the inability of the mature miRNA qRT-PCR assay to distinguish between miR-29a and miR-29c, which only differ in one nucleotide, as has been suggested previously [16]

Summary

Introduction

While genetic and genomic alterations are established drivers of melanoma formation, aberrant control of gene expression is emerging as a major contributor to melanoma progression. microRNAs (miRNAs) bind to the 3 UTRs of target mRNAs to negatively regulate gene expression [1]. Various miRNAs have been reported to regulate the biology of melanoma [2,3] including miR-29a, for which melanoma suppressive potential has recently been proposed based on in vitro analyses [4]. MiR-29 is considered a tumor suppressor miRNA given its ability to repress genes involved in proliferation and cell survival, such as AKT3 [7,8], DNMT3A/B [9], MCL1 [10], and CDK6 [11], and its frequent downregulation in cancer [5], including leukemia [12], ovarian [13], or breast. MiR-29 has emerged as a major regulatory hub that integrates signaling from potent oncogenes and tumor suppressors. The regulation of miR-29 and its function in melanoma biology are poorly understood

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