Stable induction of c-jun mRNA expression in normal human keratinocytes by agents that induce predifferentiation growth arrest.

A variety of agents can induce predifferentiation growth arrest (PGA) in human keratinocytes; these include transforming growth factor beta 1 (TGF-beta 1) and razoxane. We evaluated the ability of these and other agents to induce the expression of a variety of transcription factor genes including c-fos, c-myc, junB, and c-jun. The results show that both TGF-beta 1 and razoxane induce maximal c-jun mRNA expression 4 days after initiation of treatment concurrent with the development of PGA. In contrast, no detectable induction of c-fos, c-myc, or junB was observed. Keratinocytes maintained in the presence of TGF-beta 1 for an additional 3 days continued to show high levels of c-jun mRNA, indicating stable induction. Razoxane treatment also induces PGA and high c-jun mRNA levels for 4 days, but thereafter a decay of c-jun expression occurs. Run-off transcription experiments comparing rapidly growing cells with cells treated with TGF-beta 1 for 4 days demonstrated a significant increase in transcriptional activity of the c-jun gene. This result indicates that the increase in c-jun gene expression is due in part to a change in transcriptional regulation of c-jun. The stable induction of c-jun mRNA in keratinocytes at the PGA state is unique because the induction of this gene is usually transient. The finding that c-fos is not coinduced suggests that c-Jun homodimers or other AP-1 heterodimers may be formed at the PGA state to facilitate the stable induction of c-jun mRNA. This experimental system should therefore serve as a model system to study the molecular mechanisms for the stable control of c-jun gene expression and the control of AP-1-dependent gene expression during the process of keratinocyte differentiation.