Apigenin inhibition of involucrin gene expression is associated with a specific reduction in phosphorylation of Protein Kinase C δ Tyr³¹¹

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Apigenin is a plant-derived flavanoid that has significant promise as a skin cancer chemopreventive agent. In the present study, we examine the mechanism whereby apigenin regulates normal human keratinocyte differentiation. Expression of involucrin (hINV), a marker of keratinocyte differentiation, is increased by differentiating agents via a PKCδ, Ras, MEKK1, MEK3 cascade that increases AP1 factor level and AP1 factor binding to DNA elements in the hINV promoter. We show that apigenin inhibits this response. Apigenin suppresses the 12-O-tetradeconylphorbol-13-acetate (TPA)-dependent increase in AP1 factor expression and binding to the hINV promoter, and the increase in hINV promoter activity. Apigenin also inhibits the increase in promoter activity observed following overexpression of PKCδ, constitutively-active Ras, or MEKK1. The suppression of PKCδ activity is associated with reduced phosphorylation of PKCδ

-Y311. The physiological importance of this phosphorylation event was confirmed by showing that the PKCδ phosphorylation-defective mutant, PKCδ-Y311F, is less able to increase hINV promoter activity. Activation of hINV promoter activity by the green tea polyphenol, (-)-epigallocathecin-3-gallate, is also inhibited by apigenin, suggesting that the two chemopreventive agents can produce opposing actions in keratinocytes. Additional studies show that the apigenin-dependent suppression of differentiation is associated with reduced cell proliferation but that there is no evidence of apoptosis.