Cyclin-dependent kinase 1 expression is inhibited by p16(INK4a) at the post-transcriptional level through the microRNA pathway.

The p16(INK4a) protein regulates cell cycle progression mainly by inhibiting the activity of G1-phase cyclin-dependent kinases (CDKs) 4 and 6, the subsequent retinoblastoma protein (pRb) phosphorylation and E2F transcription factor release. The p16(INK4a) protein can also repress the activity of other transcription factors, such as c-myc, nuclear factor-kappaB and c-Jun/AP1. Here, we report that, in two p16(-/-), pRb(WT) and p53(WT) cell lines (MCF7 and U87), p16(INK4a) overexpression induces a dramatic decrease in CDK1 protein expression. In response to p16(INK4a), the decreased rate of CDK1 protein synthesis, its unchanged protein half-life, unreduced CDK1 mRNA steady-state levels and mRNA half-life allow us to hypothesize that p16(INK4a) could regulate CDK1 expression at the post-transcriptional level. This CDK1 downregulation is mediated by the 3'-untranslated region (3'UTR) of CDK1 mRNA as shown by translational inhibition in luciferase assays and is associated with a modified expression balance of microRNAs (miRNAs) that potentially regulate CDK1, analyzed by TaqMan Human microRNA Array. The p16(INK4a)-induced expression of two miRNAs (miR-410 and miR-650 chosen as an example) in MCF7 cells is confirmed by individual reverse transcription-qPCR. Furthermore, we show the interaction of miR-410 or miR-650 with CDK1-3'UTR by luciferase assays. Endogenous CDK1 expression decreases upon both miRNA overexpression and increases with their simultaneous inhibition. The induction of miR-410, but not miR-650 could be related to the pRb/E2F pathway. These results demonstrate the post-transcriptional inhibition of CDK1 by p16(INK4a). We suggest that p16(INK4a) may regulate gene expression by modifying the functional equilibrium of transcription factors and consequently the expression balance of miRNAs.