Cyclin-dependent kinase inhibitors, p21(waf1/cip1) and p27(kip1), are expressed site- and hair cycle-dependently in rat hair follicles.

In order to investigate the role of cyclin-dependent kinase (CDK) inhibitors in hair growth, we analyzed the expressions of p21(waf1/cip1) and p27(kip1) during the synchronized hair cycle of rat coat. The mRNAs of both p21(waf1/cip1) and p27(kip1) were detected in anagen hair follicles by reverse transcription and polymerase chain reaction and their localization was clearly demonstrated in the upper half portion of the hair bulb and the cortex by in situ hybridization. The dermal tissue containing hair follicles was then excised from the anterior dorsal skin of the 5-12-week-old rats at 0.5 week intervals and the expressions of p21(waf1/cip1) and p27(kip1) were analyzed by northern blot hybridization. The mRNA of both CDK inhibitors was expressed at relatively high levels during anagen than during telogen, a fact which correlated with the mRNA expression levels of hair differentiation markers, type I hair keratin (Ha3) and high sulfur protein B2. These results imply that CDK inhibitors, p21(waf1/cip1) and p27(kip1), are involved in the differentiation of follicular epithelial cells.