The bile acid analog fusidic acid can replace phosphatidylserine in the activation of protein kinase C by 12-O-tetradecanoylphorbol 13-acetate in vitro

Abstract

Protein kinase C (PKC) is a Ca²⁺- and phospholipid-dependent protein kinase which binds and is activated by tumor promoters such as the phorbol ester 12-O-tetra-decanoylphorbol-13-acetate (TPA). PKC can be activated in vitro by phosphatidylserine (PS) plus either TPA or Ca²⁺. We report here that the bile acid analog fusidic acid can replace the requirement for PS in the activation of PKC by TPA. In addition, fusidic acid can enhance the activation of PKC by Ca²⁺ and PS as well as by TPA and PS. Fusidic acid is an excellent model compound for in vitro studies of the direct effects of bile acids on PKC activity because, unlike many bile acids, it is completely soluble in standard PKC assay mixtures, obviating the exposure of the enzyme and lipid micelles to organic solvents. The colonic mucosa is exposed to millimolar concentrations of bile acids, and we find that fusidic acid stimulates PKC activity in the presence of TPA with a Ka of 350 μM. There is substantial evidence that bile acids are endogenous tumor promoters, and that colon carcinogenesis is influenced by the composition of bile acids in vivo. Thus, fusidic acid may be a prototype of bile acids which could mediate tumor promotion, at least in part, by replacing the requirement for PS in the activation of PKC.