Suppressive Effect of Transforming Growth Factor-β on the Phosphorylation of Endogenous Substrates by Conventional and Novel Protein Kinase C in Primary Cultured Mouse Epidermal Cells

Abstract

The effect of transforming growth factor-β (TGF-β) on the endogenous protein phosphorylation caused by phorbol 12-myristate 13-acetate (PMA), a potent activator of protein kinase C (PKC), was examined in primary cultured mouse epidermal cells. PMA markedly stimulates phosphorylation of endogenous proteins, i.e. KP-1 and KP-2, through Ca 2+-dependent conventional PKC (cPKC), and KP-10 through Ca 2+-independent novel PKC (nPKC) in intact epidermal cells. TGF-β strongly suppressed the PMA-stimulated phosphorylation of these three proteins. Rate of dephosphorylation of these phosphorylated proteins was not affected by TGF-β. Treatment of epidermal cells with TGF-β decreased cPKC activity both in cytosolic and particulate fractions, but not nPKC activity. These results indicate that TGF-β suppresses cPKC- and nPKC-mediated endogenous protein phosphorylation in intact epidermal cells, but the mechanisms of suppression are different.