DIFFERENTIATION OF MOUSE KERATINOCYTES IS ACCOMPANIED BY PKC-DEPENDENT CHANGES IN AP-1 PROTEINS

The conversion of cultured basal keratinocytes to the spinous and gran ular cell phenotypes seen in the skin can be stimulated by raising the levels of extracellular calcium, Here me show that AP-1 DNA binding a ctivity is very low in primary cultures of basal keratinocytes, but th at this activity is induced 24-48 h after increasing the concentration of extracellular calcium from 0.05 to 0.12 mM. As such, the induction of AP-1 DNA binding activity correlates with events occurring during the terminal stages of keratinocyte differentiation, Calcium-induced A P-1 DNA binding complexes consist of Pra-1, Fra-2, c-Jun, JunB and Jun D and are independent of c-Fos, since the induction of DNA binding act ivity and the composition of the AP-1 binding complexes are identical in differentiating keratinocytes derived from c-fos null and wild type mice, The formation of calcium-induced AP-1 binding complexes is regu lated by protein kinase C (PKC) and requires a functional PKC alpha is ozyme, as determined through pharmacological down-modulation of specif ic PKC isozymes in differentiating keratinocytes, Moreover, PKC activa tion is required for the increased expression of Fra-2, JunB and JunD in the nucleus of differentiating cells in vitro. This observation pro vides a link between the obligate activation of PKC during keratinocyt e differentiation and the nuclear response required to alter gene expr ession, In vivo expression patterns suggest that the predominant AP-1 heterodimer in the granular layer consists of Fra-2 and JunB while a J unD and Fra-1 complex predominates the spinous layer of mouse epidermi s, These findings suggest distinct functions for different AP-1 protei ns in the regulation of events related to keratinocyte maturation.