Hans-Joerg Burger

Prolonged blood glucose reduction in mrp-2 deficient rats (GY/TR(-)) by the glucose-6-phosphate translocase inhibitor S 3025.

Chlorogenic acid derivatives are potent inhibitors of hepatic glucose production by inhibition of the glucose-6-phosphate translocase component of the hepatic glucose-6-phosphatase system. The pharmacological proof of concept was clearly demonstrated during i.v. infusion of potent derivatives (S 4048, S 3483) in rats. However, the blood glucose lowering effect of S 4048 after bolus i.v. injection lasted only 60-90 min. Plasma clearance of S 4048 was very high, and the parent compound was rapidly and efficiently excreted into the bile of Wistar and GY/TR(-) rats, indicating that mrp-2 was not involved in this hepatobiliary elimination process. About 72% of the total administered radioactivity appeared in the bile within 20 min after i.v. bolus injection of the radiolabeled analogue [(3)H]S 1743 in a Wistar rat. However, in GY/TR(-) rats the dicarboxylic analogue of S 4048, S 3025, was cleared from the plasma less rapidly than its parent compound and its biliary elimination was comparatively low. In contrast, S 3025 exhibited comparable pharmacokinetics and biliary elimination profile as S 4048 in Wistar rats, suggesting that biliary elimination of S 3025 is facilitated by mrp-2, functionally absent in GY/TR(-) rats. Targeting to mrp-2 resulted in a significantly prolonged reduction of blood glucose levels in GY/TR(-) rats after i.v. bolus administration of S 3025.

Inhibition of autophagic proteolysis by inhibitors of phosphoinositide 3-kinase can interfere with the regulation of glycogen synthesis in isolated hepatocytes.

Amino acid-induced cell swelling stimulates conversion of glucose into glycogen in isolated hepatocytes. Activation of glycogen synthase (GS) phosphatase, caused by the fall in intracellular chloride accompanying regulatory volume decrease, and activation of phosphoinositide 3-kinase (PI 3-kinase), induced by cell swelling, have been proposed as underlying mechanisms. Because PI 3-kinase controls autophagic proteolysis, we examined the possibility that PI 3-kinase inhibitors interfere with glycogen production due to their anti-proteolytic action. The PI 3-kinase inhibitor wortmannin inhibited endogenous proteolysis, the production of glycogen from glucose and the activity of active (dephosphorylated) GS (GS a ) in the absence of added amino acids. The stimulation by amino acids of glycogen production and of GS a was only slightly affected by wortmannin. These effects of wortmannin could be mimicked by proteinase inhibitors. A combination of leucine, phenylalanine and tyrosine, which we showed previously to stimulate PI 3-kinase-dependent phosphorylation of ribosomal protein S6, did not stimulate glycogen production from glucose. In contrast with wortmannin, LY294002, another PI 3-kinase inhibitor, strongly inhibited both glycogen synthesis and GS a activity, irrespective of the presence of amino acids. Inhibition of glycogen synthesis by LY294002 could be ascribed in part to increased glycogenolysis and glycolysis. It is concluded that, in hepatocytes, activation of PI 3-kinase may not be responsible for the stimulation of glycogen synthesis by amino acids; LY294002 inhibits glycogen synthesis and stimulates glycogen breakdown by a mechanism that is unrelated to its action as an inhibitor of PI 3-kinase.