Star-PAP Controls Oncogene Expression Through Primary Mirna 3′-End Formation to Regulate Cellular Proliferation and Tumour Formation

Star -PAP is a non -canonical poly(A) polymerase that is down regulated in breast cancer. While Star -PAP down regulation impairs target mRNA polyadenylation, paradoxically, we see up regulation of a large number of oncogenes on Star -PAP knockdown. Using two breast cancer cells (MCF7 with high Star -PAP, and MDA-MB-231 with negligible Star -PAP level), we discover that Star -PAP negatively regulates oncogene expression and subsequently cellular proliferation. This regulation is compromised with Star -PAP mutant of 3 ' -end processing function (serine 6 to alanine, S6A phospho-mutation). Concomitantly, xenograft mice model using MDA-MB-231 cells reveals a reduction in the tumour formation on ectopic Star -PAP expression that is ameliorated by S6A mutation. We find that Star -PAP control of target oncogene expression is independent of Star -PAP -mediated alternative polyadenylation or target mRNA 3 ' -end formation. We demonstrate that Star -PAP regulates target oncogenes through cellular miRNAs (miR-421, miR-335, miR-424, miR-543, miR-205, miR-34a, and miR-26a) that are down regulated in breast cancer. Analysis of various steps in miRNA biogenesis pathway reveals that Star -PAP regulates 3 ' -end formation and synthesis of primary miRNA (host) transcripts that is dependent on S6 phosphorylation thus controlling mature miRNA generation. Using mimics and inhibitors of two target miRNAs (miR-421 and miR-424) after Star -PAP depletion in MCF7 or ectopic expression in MDA-MB-231 cells, we demonstrate that Star -PAP controls oncogene expression and cellular proliferation through targeting miRNAs that regulates tumour formation. Our study establishes a novel mechanism of oncogene expression independent of alternative polyadenylation through Star -PAP -mediated miRNA host transcript polyadenylation that regulates breast cancer progression.