Ciprofibrate stimulates protein kinase C-dependent phosphorylation of an 85 kDa protein in rat Fao hepatic derived cells

The effect of ciprofibrate on early events of signal transduction was previ ously studied in Fao cells. Protein kinase C (PKC) assays performed on perm eabilized cells showed a more than two-fold increase in PKC activity in cel ls treated for 24 h with 500 mu M ciprofibrate. To show the subsequent effe ct of this increase on protein phosphorylation, the in vitro phosphorylatio n on particulate fractions obtained from Fao cells was studied. Among sever al modifications, the phosphorylation of protein(s) with an apparent molecu lar mass of 85 kDa was investigated. This modification appeared in the firs t 24 h of treatment with 500 mu M ciprofibrate. It was shown to occur on Se r/Thr residue(s). It was calcium but not calmodulin-dependent. The phosphor ylation level of this/these protein(s) was reduced with kinase inhibitors a nd especially with 300 nM GF-109203X, a specific inhibitor of PKC. All thes e results suggest that the phosphorylation of the 85 kDa protein(s) is due to a PKC or to another Ser/Thr kinase activated via a PKC pathway. A possib le biochemical candidate for 85 kDa protein seems to be the beta isoform of phosphatidylinositol 3-kinase regulatory subunit. (C) 2000 Societe francai se de biochimie et biologie moleculaire / Editions scientifiques et medical es Elsevier SAS.