INVOLVEMENT OF THE SP3 TRANSCRIPTION FACTOR IN INDUCTION OF P2(CIP1 WAF1) IN KERATINOCYTE DIFFERENTIATION/

The cyclin-dependent kinase inhibitor p21 is induced in several in vit ro terminal differentiation systems as well as in differentiating tiss ues in vivo. To determine the mechanism responsible for p21 induction during differentiation of mouse primary keratinocytes, we performed a deletion analysis of the p21 promoter. The minimal region of the p21 p romoter required for its induction in keratinocyte differentiation con sists of a contiguous stretch of 78 base pairs, which contains a GC-ri ch region as well as the TATA box. We determined that transcription fa ctors Spl and Sp3, present in primary keratinocyte nuclear extracts, b ind the GC region concomitantly. Expression studies established that b oth Spl and Sp3 activate the p21 promoter, but showed that only Sp3 ov erexpression enhances promoter inducibility during differentiation. Fu rthermore, disruption of the GC-rich region dramatically decreases tra nscription factor binding as well as promoter activity and inducibilit y upon differentiation. The overexpression of either Spl or Sp3 restor es the basal activity of the disrupted promoter, but only Sp3 can rest ore its inducibility. These findings show that both Spl and Sp3 can co ntribute to the basal activity of the p21 promoter, and establish Sp3 as a specific transcription factor involved in the induction of p21 pr omoter during keratinocyte differentiation.