Abstract
The effects of type I (insulin-dependent) diabetes mellitus on the hepatic cytochrome P450-dependent mixed-function oxidase system was investigated in both chemically-induced and spontaneously diabetic rats. Sex-specific influences on these changes were evaluated in male and female streptozotocin-induced diabetic rats. Further studies examined the ability of hepatic fractions from both chemically-induced and spontaneously diabetic rats to bioactivate known chemical carcinogens in the Ames mutagenicity assay. The role of ketone bodies and hyperglycaemia in the diabetes-induced alterations were investigated in rats rendered hyperketonaemic by dietary manipulation. Finally, the effects on hepatic drug metabolism of hypertension, using spontaneously hypertensive rats, type II (non insulin-dependent) diabetes using the spontaneously hyperglycaemic mouse and hyperinsulinaemia, by insulinoma transplantation in the rat were also determined.Streptozotocin-induced diabetes was associated with significant elevations of the enzyme activities catalysed by the cytochrome P450 IA, IIB, IIE, IIIA and IVA sub-families, a finding confirmed by immunological analysis. These alterations were a consequence of the diabetic state and not the diabetogen per se, as shown by the inclusion of a group of rats which received nicotinamide concurrently with streptozotocin to counter the effect of the diabetogen. Long-term streptozotocin-diabetic rats continued to display elevations of the cytochrome P450 IA and IIB families. However, after four weeks, the severity of ketosis declined and the elevation in activities of the cytochrome P450 IIE, IIIA and IVA families became less pronounced. Investigation of sex-related alterations in the diabetes-induced changes revealed that induction was always more pronounced in the male than the female. The spontaneously diabetic BB rat exhibited similar alterations of hepatic cytochrome P450 families as those observed in the streptozotocin-induced diabetic rats. However, in BB rats, all of the changes apparent at one month following onset of the disease were similarly observed in six month diabetic animals. Hepatic fractions from both chemically-induced and spontaneously-diabetic rats were markedly more efficient at activating selected chemical carcinogens in the Ames mutagenicity assay. However, these fractions were less efficient at activating the aromatic amine, 2-aminofluorene, suggesting that diabetes modulates other enzyme systems capable of drug metabolism. The mechanism responsible for the diabetes-induced changes in hepatic drug metabolism has been proposed to involve ketone bodies. In our experiments using rats rendered hyperketonaemic by dietary manipulation, we found that similar alterations of hepatic drug metabolism occurred as previously observed in diabetic animals. As the hyperketonaemic animals were normoglycaemic, it can be concluded that it is the ketone bodies that have a major role in the diabetes-induced changes in hepatic drug metabolism and not hyperglycaemia. However, hyperketonaemia failed to modulate the levels of cytochrome P450 IIIA as seen in the diabetic animals, indicating that other mechanisms are involved. Further work examining the effect of specific ketone bodies revealed that acetone was a potent inducer of cytochrome P450 IA, IIB and IIE proteins. This study also demonstrated that ketone bodies could not modulate the levels of cytochrome P450IVA. These studies provide compelling evidence for a direct link between ketone bodies and the induction of the cytochrome P450 IA, IIB and IIE subfamilies. Clearly, the mechanism of cytochrome P450 IIIA and IVA induction are dependent on other factors. However, in the case of cytochrome P450 IVA subfamily our studies suggest that the increased levels of circulating free fatty acids, accompanying type I diabetes, are responsible for the induction. Finally, our investigations into the effects of type II diabetes, hypertension and hyperinsulinaemia on hepatic drug metabolism revealed that type II diabetes and hypertension have little effect on the cytochrome P450 families investigated. However, hyperinsulinaemia was associated with a selective induction of the cytochrome P450 IA subfamily, indicating that perturbations of endogenous insulin levels may have marked influences on hepatic drug metabolism and consequently the toxicity / carcinogenicity of chemicals.