Induction of p16^ink4a and p19^ARF by TGFβ1 contributes to growth arrest and senescence response in mouse keratinocytes

TGFβ1 acts as a potent negative regulator of the cell cycle and tumor suppressor in part through induction of cyclin dependent kinase inhibitors p15^ink4b, p21, and p57. We previously showed that primary mouse epidermal keratinocytes (MEK) expressing a v-ras^Ha oncogene undergo hyperproliferation followed by growth arrest and senescence that was dependent on TGFβ1 signaling and associated with increased levels of p16 ^ink4a and p19^ARF. Here we show that the induction of both p16^ink4a and p19^ARF in v-ras^Ha expressing keratinocytes is dependent on TGFβ1 signaling, as TGFβ1 treatment or Smad3 overexpression induces both p16^ink4a and p19^ARF protein and mRNA, while Smad3 depletion or Smad7 overexpression blocks induction. Genetic ablation of the cdkn2a (ink4a/arf) locus reduced sensitivity to TGFβ1 mediated cell cycle arrest and induction of senescence suggesting that alteration of TGFβ1 responses may be an additional pathway impacted by the inactivation of cdkn2a locus during tumor development.