[Urotensin II inhibits glucokinase expression and glucose-induced insulin secretion]

The purpose of the present study is to investigate the effects of urotensin II (UII) on insulin secretion in islet beta cells and the underlying mechanism. Glucose tolerance test was performed in Wistar rats to evaluate the effect of UII on the levels of plasma glucose and insulin. Static incubation experiment was employed to investigate the effect of UII on glucose-induced insulin secretion (GIIS) in betaTC-6 cells. After the incubation, insulin content and mRNA level in betaTC-6 cells were analyzed. Finally, Western blot was used to find out if UII could change the expression levels of pancreatic duodenal homeobox-1 (PDX-1) and glucokinase (GCK). It was observed that intravenous administration of UII (30, 300 nmol/kg) resulted in a significant decrease in insulin level 15 min after glucose load, and induced an obvious increase in plasma glucose 90 min after the load. In vitro, two hours of UII incubation inhibited GIIS in betaTC-6 cells without affecting insulin content and mRNA levels. The inhibitory effect of UII was blocked by UII receptor antagonist (urantide), and partially blunted by protein kinase C (PKC) inhibitor (chelerythrine) and somatostatin receptor antagonist (cyclosomatostatin). Moreover, we found that GCK protein level was significantly reduced by UII, while PDX-1, a key regulator of insulin gene transcription in beta cells, was not affected. These results suggest that UII-induced inhibition of GIIS in betaTC-6 cells are mediated by UII receptor and PKC pathway, as well as somatostatin receptor which could be activated by high dose of UII. The inhibitory effect of UII on insulin secretion is rather associated with a suppression of GCK expression than a regulation on PDX-1 expression.