Regulation of Hepatic Steroid Metabolism by Protein Kinase C

The oxidative metabolism of both drugs and steroids occurs mainly as a consequence of utilising the hepatic monooxygenase system (Kuntzman et al, 1964). It has been shown that the activities of various hepatic enzymes are not constant, but are subject to regulation by the endocrine system, and also by hormones. The main classes of hormones which have been implicated in the regulation of hepatic metabolism are; (a) androgens, as extensively reviewed by Skett & Gustafsson (1979) and Gustafsson and coworkers (1980); (b) the pituitary hormones, including prolactin (Colby et al, 1974) and growth hormone (Colby et al, 1974; Mode et al, 1981); and (c) the pancreatic hormones, particularly insulin (Kato & Gillette, 1965; Kato et al, 1970). As the different hormonal pathways within the body are subject to complex interactions, and since some hormones have different secretion profiles in male and female animals, the precise mechanism by which hormones act to regulate the activity of hepatic enzymes has not yet been fully elucidated. One mechanism by which hormones can produce their intracellular effects is by stimulating the hydrolysis of membrane-bound phospholipids. The consequence of this hydrolysis is that two intracellular second messengers are produced, inositol-1,4,5-trisphosphate, which causes an elevation in intracellular free calcium levels, and 1,2-diacylglycerol, which interacts with phosphatidylserine and calcium to activate the phosphorylating enzyme, protein kinase C. Activation of protein kinase C by receptor-mediated inositol phospholipid hydrolysis, relays information across the cell membrane to regulate a variety of intracellular processes, and hormonal activation of protein kinase C in the liver has been reported. Vasopressin is known to affect hepatic enzymes, such as glycogen synthetase (Blackmore et al, 1986a; Ahmad et al, 1984) and glycogen phosphorylase (Barritt et al, 1988), and vasopressin also has general effects upon hepatic events, especially a1-mediated adrenergic effects (Garcia-Sainz et al, 1986). Angiotensin II has also been reported to have hepatic effects mediated through activation of protein kinase C (Garcia-Sainz et al, 1986; Garcia-Sainz & Hernandez-Sotomayor, 1987). Recently, some actions of insulin have also been attributed to protein kinase C activation (Acevedo-Duncan et al, 1989; Cooper et al, 1987; Gomez et al, 1988), although evidence implicating hepatic effects of insulin through protein kinsae C are still contradictory. Recently, it has been demonstrated that, in a reconstituted microsomal membrane system, protein kinase C, and protein kinase A, are capable of phosphorylating cytochrome P450, the terminal oxidase in hepatic monooxygenases (Pyerin et al, 1983; Pyerin et al, 1987). It seemed possible, therefore, that stimulation of inositol phospholipid hydrolysis, and therefore of protein kinase C and subsequent phosphorylation of cytochrome P450, or other proteinaceous components, may be one mechanism by which hepatic monooxygenase activity could be regulated by hormones. The proposed aim of the present study was to try to determine whether or not activation of protein kinase C in the rat liver could influence the activity of hepatic monooxygenases, and to try to elucidate the underlying mechanism of any effect which was observed. This study was conducted using isolated rat hepatocytes in an attempt to avoid complex hormonal interactions, and to try to represent a more physiological situation than is often encountered during in vitro studies. To activate protein kinase C directly, tumor-promoting phorbol esters and synthethic diacylglycerols were used. Phorbol esters are reported to act solely through protein kinase C (Niedel et al, 1983; Castagna et al, 1982) and they are convenient probes of protein kinase C activity in isolated cell systems. Hormonal activation of protein kinase C, via stimulation of inositol phospholipid hydrolysis, was investigated with vasopressin and angiotensin II, and the role of elevating intracellular calcium was investigated by the use of the ionophore A23187. The effect of activating protein kinase C upon the metabolism of the endogenous steroid 4-androstene-3,17-dione was ascertained after various times of incubation with the different compounds. It was found that activation of protein kinase C with all of the compounds, except angiotensin II, produced a time-dependent inhibition in the activity of all of the enzymes metabolising 4-androstene-3,17-dione. This inhibition of enzymes activity was not sex-dependent, males and female were both affected to the same extent. With vasopressin, a biphasic pattern of enzymes inhibition was observed, an effect which implies a dependency on diacylglycerol initially, with the later effect being more dependent upon calcium. (Abstract shortened by ProQuest.)