The Role of Cyclic-AMP in the Regulation of Hepatic Steroid Metabolism

Berry, Lesley A (1988) The Role of Cyclic-AMP in the Regulation of Hepatic Steroid Metabolism. PhD thesis, University of Glasgow.

Abstract

The hepatic monooxygenase system is responsible for the metabolism of xenobiotics and steroids (Kuntzman et al 1964). In vivo studies in rats have shown that the activities of the constituent enzymes of the hepatic monooxygenase system are not constant but are under the control of the endocrine system and the hormones which have been found to influence enzyme activity are (a) testosterone (Yates et. al. 1958) , (b) oestrogen (Einarsson et al 1973) , (c) growth hormone (Wilson and Frohman 1974 ; Wilson and Spelsberg, 1976 ; Vockentanz and Virgo 1985) , (d) thyroxine (Kato and Gillette 1965a ; Kato et al 1970) , (e) insulin (Kato et al 1971 ; Weiner et al 1972a ; Reinke et al 1978 ; Past and Cook 1983) , (f) glucocorticosteroids (Kato and Gillette 1965a ; Kato et al 1971) , (g) glucagon (Weiner et al 1972a) and (h) catecholamines (Fouts 1962 ; Dixon et al 1964 ; Kato and Gillette 1965a). The intracellular mechanisms by which these hormones produce their effects , however , have been little studied and extrapolation from in vivo data is complicated by the fact that hormones appear to have different effects in male and female rats. One of the mechanisms by which hormones are known to produce their intracellular effects is by altering turnover of cyclic AMP and it has been reported that hormones which , when administered in vivo , increase intracellular cyclic AMP levels by a variety of mechanisms e. g. thyroxine (Muller and Seitz 1987) , adrenaline (Exton et al,1971; Studer and Borle 1984 ; Kunos et al 1984 ; Kunos and Ishac 1987) tend to inhibit hepatic monooxygenase activity in the male rat but not in the female rat. Insulin , however , which decreases intracellular cyclic AMP levels by a variety of mechanisms (Heyworth et al 1985 ; O'Brien et al 1987) , stimulates hepatic monooxygenase activity when added to isolated rat hepatocytes (Hussin and Skett 1987). In addition , in vivo administration of the cyclic AMP analogue , dibutyryl cyclic AMP , to male rats has also been reported to inhibit hepatic monooxygenase activity (Weiner et al 1972a ; Ross et al 1973) which provides more direct evidence for a role of cyclic AMP in hormonally-mediated inhibition of hepatic monooxygenase activity in the male rat. This effect , however , does not occur in the female rat. It has recently been demonstrated that , in a reconstituted microsomal membrane system , the catalytic subunit of cyclic AMP-dependent protein kinase (PKa) can phosphorylate cytochrome P450 , the terminal oxidase in many hepatic monooxygenases (Pyerin et al 1983 , Pyerin et al 1986a ; Pyerin et al 1987) , resulting in its conversion to the inactive cytochrome P420 (Taniguchi et al 1985) with a concomitant inhibition of enzyme activity (Pyerin et al 1984). It is conceivable , therefore , that cyclic AMP , by activating PKa , phosphorylates cytochrome P450 and inhibits hepatic monooxygenase activity in the male rat but not in the female rat. In contradiction to the above findings it has been reported that the induction of hepatic monooxygenases by Arochlor-1254 (Costa et al 1976) , phenobarbitone and 3-methylcholanthrene (Byus et al 1976) are accompanied by large increase in intracellular cyclic AMP levels and activation of PKa. PKa may be producing its effects by turning on gene expression (Teng et al 1971 ; Dokas and Kleinsmith 1971). The purpose of this present study was to attempt to elucidate the previously contradictory implications for a role of cyclic AMP in the regulation of hepatic monooxygenase activity and to examine the mechanisms by which cyclic AMP produces its effects. The major problem of extrapolating from in vivo data is that the hormone may not necessarily be acting at the level of the liver but may be producing its effects by altering the synthesis or release of other hormones. Similarly the findings of in vitro studies may not be representative of the physiological situation. For the purpose of this study , therefore , we chose as our model the isolated rat hepatocyte which overcomes the major problems of in vivo and in vitro models. Cyclic AMP levels were elevated either indirectly using 8-bromocyclic AMP , adrenaline , isobutylmethylxanthine (IBMX) or forskolin in hepatocytes isolated from male or female (8-bromo-cyclic AMP only) rats , or directly by the addition of different concentrations of cyclic AMP to electropermeabilised hepatocytes isolated from male rats and subsequent effects upon the metabolism of the endogenous steroid , 4-androstene-3,17-dione , were determined after various periods of time. We were able to show that elevation of intracellular cyclic AMP levels by both direct and Indirect mechanisms had a time- and concentration-dependent biphasic effect upon steroid-metabolising enzymes. (Abstract shortened by ProQuest.).

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Pharmacology, Endocrinology
• Date of Award: 1988
• Depositing User: Enlighten Team
• Unique ID: glathesis:1988-77848
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2020 11:53
• Last Modified: 14 Jan 2020 11:53
• URI: https://theses.gla.ac.uk/id/eprint/77848