STAT3 activation accompanies keratinocyte differentiation.

The signal transducers and activators of transcription (STAT) family of transcription factors has been demonstrated to play key roles in a variety of cell types under conditions that promote differentiation or cell cycle exit. We report our studies of primary murine keratinocytes in which we demonstrate activation of STAT3 during growth arrest and differentiation. In adherent cells, STAT3-specific DNA binding activity was detected in quiescent cultures, down-regulated upon mitogenic stimulation, and found to reaccumulate as cells reentered quiescence. Suspension culturing of proliferating keratinocytes, which induces differentiation, also resulted in induction of STAT3 activity. Furthermore, induction of STAT3 after suspension culturing did not occur in MK cells, an immortalized murine keratinocyte cell line that does not undergo differentiation. Because STAT3 activation in these cells corresponded tightly with the growth status, we examined whether there was a relationship between the cell cycle machinery and STAT3 activation by inhibiting p27kip1 accumulation, which is observed during growth arrest, with antisense oligonucleotides and by using keratinocytes lacking functional p27kip1. In both cases, there was a loss of STAT3 activation and a concomitant delay in terminal cell cycle withdrawal and in the expression of the differentiation specific marker, keratin 1. Thus, in addition to controlling transcription mediated through E2F, our data demonstrate that alterations in the cell cycle machinery are required for appropriate up-regulation of STAT3 activity that occurs during keratinocyte differentiation.