Dephosphorylation of activated protein kinase C contributes to downregulation by bryostatin

We show that bryostatin 1 (Bryo) rapidly produces an inactive, incompetent 76-kDa form of protein kinase C-α (PKC-α) in the LLC-MK₂ line of renal epithelial cells. Bryo, like phorbol 12-myristate 13-acetate (PMA), acutely activated PKC, as indicated by autophosphorylation and translocation of PKC-α the predominant PMA-sensitive isoform expressed by the cells. Bryo concomitantly increased the ³²P labeling of 80-kDa PKC-α by autophosphorylation and produced a 76-kDa form of PKC-α that lacked detectable ³²P. The 76-kDa form was in the particulate rather than the cytosolic fraction, which suggests that it was produced from activated kinase. Alkaline phosphatase treatment of immunoprecipitated PKC-α converted the 80-kDa form to 76 kDa, but it had no effect on the mobility of the 76- kDa form, suggesting that it was not phosphorylated. Pulse-chase labeling of PKC-α with [³⁵S]Met/Cys indicated that there is a precursor-product relationship between the 80- and 76-kDa forms, respectively. Inhibition of protein synthesis had no effect on the production of 76-kDa PKC-α by Bryo. PMA also produced 76-kDa PKC-α but was less potent and efficacious than Bryo. Bryo produced a more rapid loss of 80-kDa PKC-α protein and total Ca²⁺ and phospholipid-dependent PKC activity than PMA. The 76-kDa form is inactive and incompetent because it lacked detectable ³²P under conditions that strongly autophosphorylated the 80-kDa form. We suggest that dephosphorylation predisposes PKC to proteolysis, and greater production of the 76-kDa form explains the more efficient downregulation of the kinase by Bryo vs. PMA.