Cross talk among calcineurin, Sp1/Sp3, and NFAT in control of p21(WAF1/CIP1) expression in keratinocyte differentiation

Calcium functions as a trigger for the switch between epithelia I cel I gro wth and differentiation. We report here that the calcium/calmodulin-depende nt phosphatase calcineurin is involved in this process. Treatment of primar y mouse keratinocytes with cyclosporin A, an inhibitor of calcineurin activ ity, suppresses the expression of terminal differentiation markers and of p 21(WAF1/Cip1) and p27(KIP1), two cyclin-dependent kinase inhibitors that ar e usually induced with differentiation. in parallel with down-modulation of the endogenous genes, suppression of calcineurin function blocks induction of the promoters for the p21(WAF1/Cip1) and loricrin differentiation marke r genes, whereas activity of these promoters is enhanced by calcineurin ove rexpression. The calcineurin-responsive region of the p21 promoter maps to a 78-bp Sp1/Sp3-binding sequence next to the TATA box, and calcineurin indu ces activity of the p21 promoter through Sp1/Sp3-dependent transcription. W e find that the endogenous NFAT-1 and -2 transcription factors, major downs tream targets of calcineurin, associate with Sp1 in keratinocytes in a calc ineurin-dependent manner, and calcineurin up-regulates Sp1/Sp3-dependent tr anscription and p21 promoter activity in synergism with NFAT1/2. Thus, our study reveals an important role for calcineurin in control of keratinocyte differentiation and p21 expression, and points to a so-far-unsuspected inte rconnection among this phosphatase, NFATs, and Sp1/Sp3-dependent transcript ion.