The potential use of protein kinase D inhibitors for prevention/treatment of epidermal tumors

Abstract

The serine/threonine kinase protein kinase D (PKD) has been proposed to be a pro-proliferative, anti-differentiative signal in epidermal keratinocytes. Indeed, the phorbol ester tumor promoter, 12-O-tetradecanoylphorbol 13-acetate (TPA) induces biphasic PKD activation, which mirrors the biphasic response of initial differentiation followed by proliferation and tumor promotion seen in TPA-treated keratinocytes in vitro and epidermis in vivo. Our objective was to test the idea that PKD's pro-proliferative and/or anti-differentiative effects in keratinocytes contribute to TPA-induced tumorigenesis. Using western analysis and assays of keratinocyte proliferation and differentiation, we investigated the effect of inhibitors of PKD on keratinocyte function. We found that overexpression of a constitutively active PKD mutant increased, and of a dominant-negative PKD mutant decreased, keratinocyte proliferation. A recently described selective PKD inhibitor showed low potency to inhibit keratinocyte proliferation or PKD activation. Therefore, we tested the ability of known only relatively selective PKD inhibitors on keratinocyte function and protein kinase activation. H89 {N-[2-(p-bromocinnamylamino) ethyl]-5-isoquinoline-sulfonamide}, a reported inhibitor of PKD and cAMP-dependent protein kinase, enhanced the effect of a differentiating agent on a marker of keratinocyte differentiation. Another reported non-selective PKD inhibitor, resveratrol stimulated differentiation and inhibited proliferation. The protein kinase C/PKD inhibitor Gö6976 blocked the increase in proliferation (as measured by DNA specific activity) induced by chronic TPA without affecting the initial TPA-elicited differentiation. Our results support the idea that relatively selective PKD inhibitors, such as Gö6976, H89 and resveratrol, might be useful for preventing/treating epidermal tumorigenesis without affecting keratinocyte differentiation.