Role for Protein Kinase C-α in Keratinocyte Growth Arrest

Multiple protein kinase C (PKC) isoforms have been associated with the epidermal keratinocyte (KC) granular layer differentiation program. Here we show PKC alpha membrane localization and substrate phosphorylation in the first suprabasal KCs of normal human epidermis, suggesting activation in vivo in the lower spinous layers where terminal differentiation-associated growth arrest occurs. To determine if PKC alpha is sufficient for KC growth arrest, we expressed a constitutively active PKC alpha (PKC alpha Delta 22-28) in normal human KCs and observed growth arrest and accumulation of cells in G1. PKC alpha Delta 22-28 inhibited DNA synthesis through the induction of the cyclin-dependent kinase inhibitor p21. Furthermore, downregulation of PKC alpha in an in vitro organotypic epidermis resulted in increased basal and suprabasal proliferation marker expression, decreased differentiation, and reduced epidermal stratification. Together these results indicate that PKC alpha activation is both necessary and sufficient to trigger irreversible growth arrest during human KC differentiation.