Regulation of microRNA Expression by STAT1 Transcription Factors - Relevance for Melanoma Development

The type-II-cytokine IFN-γ (interferon gamma) is not only a pivotal player in innate immune responses but also assumes functions in controlling tumor cell growth by orchestrating cellular responses against neoplastic cells. It predominantly triggers cellular responses through the Janus Kinase (Jak)/ Signal Transducer and Activator of Transcription 1 (STAT1) pathway leading to STAT1 binding to the promoter region of target genes. As key regulators of mRNA and protein expression levels, microRNAs (small non-coding RNAs) take part in fine-tuning complex biological processes such as cell proliferation, neoplastic transformation, apoptosis, immune surveillance and differentiation. MiR-29, one of the most interesting miRNA families in humans to date, consists of three mature members miR-29a, miR-29b and miR-29c, which are encoded in two genetic clusters. In this PhD thesis, the miR-29 primary cluster pri-29a~b-1 was shown to be IFN-γ-induced and STAT1-dependently up-regulated in melanoma cell lines. Furthermore, expression levels of mature miR-29a and miR-29b were elevated in cell lines and in primary melanoma patient samples while the pri-29b-2~c cluster was almost undetectable in cell lines. Moreover, tumor-suppressing properties of miR-29 family members have been detected: inhibition of melanoma cell proliferation could be induced by miR-29a, which down-regulated CDK6 (cyclin-dependent kinase 6), an important player in cell cycle G1/S transition. Also, knockdown of CDK6 resulted in reduced proliferation of melanoma cells, suggesting that miR-29-mediated growth inhibitory effects may be brought about by CDK6-downregulation. These findings identify the pri-29a~b-1 cluster as a novel IFN-γ-regulated gene. Furthermore, a potential novel signaling pathway was identified: IFN-γ  Jaks  P-STAT1  miR-29  CDK6, which opens up new connections between miRNAs, interferon signaling and malignant melanoma, possibly clearing the way to novel concepts for new treatment options in the future.