Inhibition of prostaglandin E 2 catabolism and potentiation of hepatic prostaglandin E 2 action in rat hepatocytes by inhibitors of oxidative metabolism

Abstract

Prostaglandin E 2 (PGE 2) can modulate the actions of a number of hormones in liver. PGE 2 is rapidly metabolized in liver tissue, and thus alterations in the rate of PGE 2 catabolism might exert a short-term influence on the concentration of PGE 2 in liver. The present study examined the effects of inhibitors of oxidative metabolism on PGE 2 catabolism and action in isolated rat hepatocytes. [ 3H]-PGE 2 was metabolized to three major products by the hepatocyte system as assessed by reverse-phase high performance liquid chromatography. Metyrapone (5 mM), aminopyrine (5 mM), SKF-525A (20 μM) and α-napthoflavone (20 μM) each inhibited the breakdown of [ 3H]-PGE 2. The inhibition of oxidative metabolism by these compounds was not limited to action at cytochrome P-450, and metyrapone, aminopyrine and SKF-525A each was shown to inhibit [1- 14C]-palmitate β-oxidation in the hepatocyte system. To determine the contribution of β-oxidation to the rapid catabolism of [ 3H]-PGE 2, studies were performed using [1− 14C]-PGE 2 as substrate. Two major product peaks seen with [ 3H]-PGE 2 as substrate lacked radioactivity when [1− 14C]-PGE 2 was the substrate, and thus these two products did not contain the 1-position carbon, consistent with their identity as β-oxidation products. Furthermore, [1− 14C]-PGE 2 also yielded 14CO 2 and a [ 14C]-PGE 2 metabolite not seen with [ 3H]-PGE 2. It was calculated that 60% of the rapid PGE 2 inactivation in the hepatocyte system occurred via β-oxidation. An additional, non-β-oxidation, metyrapone-sensitive, pathway accounted for 26% of PGE 2 disappearance. The effect of PGE 2 to inhibit glucagon-stimulated glycogenolysis was potentiated when metyrapone was included in the incubation, consistent with increased survival of intact PGE 2. In summary, PGE 2 was rapidly inactivated by intact hepatocytes via oxidative metabolism, primarily β-oxidation. Inhibition of prostaglandin catabolism can have short-term effects on PGE 2 concentrations and result in potentiation of PGE 2 effects on hepatic glucose metabolism.