MicroRNA-340-mediated degradation of microphthalmia-associated transcription factor mRNA is inhibited by the coding region determinant-binding protein.

Alternative cleavage and polyadenylation generate multiple transcript variants of mRNA isoforms with different length of 3'-untranslated region (UTR). Alternative cleavage and polyadenylation enable differential post-transcriptional regulation of transcripts via the availability of different cis-acting elements in 3'-UTRs. Microphthalmia-associated transcription factor (MITF) is a master regulator of melanocyte development and melanogenesis. It has also been implicated in melanoma development. Here we show that melanoma cells favor the expression of MITF mRNA with shorter 3'-UTR. This isoform of mRNA is regulated by microRNA, miR-340. miR-340 interacts with two of its target sites on the 3'-UTR of MITF mRNA, causing mRNA degradation and decreased expression and activity of MITF. On the other hand, the RNA-binding protein coding region determinant-binding protein, shown to be highly expressed in melanoma, directly binds to the 3'-UTR of MITF mRNA and prevents the binding of miR-340 to its target sites, resulting in stabilization of the MITF transcript and elevated expression and transcriptional activity of MITF. This interplay between RNA-binding protein and miRNA describes the important mechanism of regulation of MITF in melanocytes and malignant melanomas.