JunB suppresses cell proliferation by transcriptional activation of p16(INK4a) expression

A role for the transcription factor JunB in proliferation control was inves tigated in genetically modified mouse fibroblasts. Increased JunB expressio n induced high levels of the cyclin-dependent kinase inhibitor p16(INK4a), leading to premature senescence in primary cells and reduced proliferation in 3T3 cells, whereas lack of JunB expression results in decreased p16 leve ls. Furthermore, JunB-mediated p16 induction in 3T3 cells completely abolis hed cyclin D-associated kinase activity, resulting in reduced pRb hyperphos phorylation and G(1)-phase extension. Moreover, three API-like binding site s were identified in the p16 promoter through which JunB directly activates p16 transcription. Elevated JunB expression in 3T3 tells also inhibited Ra s- and Src-mediated transformation and tumour growth in vivo. The suppressi ve effect of JunB on cell proliferation was shown to be dependent on p16 si nce it did not occur in INK4a(-/-) fibroblasts that lack both p16 and p19(A RF). These results demonstrate that p16 is a direct transcriptional target gene of JunB and identify JunB as a negative regulator of cell proliferatio n.