Progression from actinic keratosis to cutaneous squamous cell carcinoma involves miR-204 silencing and a switch towards STAT3 activation

Squamous cell carcinoma is the second most common skin cancer and frequently progress from an intraepithelial actinic keratosis. The role of microRNAs during the progression from actinic keratosis to cutaneous squamous cell carcinoma (cSCC) remains to be elicited. By using an Agilent microRNA expression microarray we found the expression of miR-204 to be markedly downregulated in cSCC when compared to actinic keratoses. DNA methylation of the TRPM3 promoter region upstream of miR-204-5p was identified as one of the repressive mechanisms that accounts for miR-204 silencing in cSCC. Functional studies on HaCaT cells revealed that this microRNA downregulates the Signal Transducer and Activator of Transcription 3 (STAT3) pathway and favours the MAPK signaling pathway, likely acting through PTPN11, a tyrosine phosphatase that is a direct miR-204 target. We found that activated STAT3, as detected by pY705-STAT3 immunofluorescence, is retained in the membrane and cytoplasm compartment in AK, whereas cSCC displayed STAT3 in the nuclei. Taken together, our data indicates that MiR-204 may act as a “rheostat” that controls the signaling towards the MAPK pathway or the STAT3 pathway. 4 samples were analyzed: HaCaT tansduced with control shRNA (n=2) and HaCaT transduced with shRNA against miR-204 (n=2)