Glucagon-like peptide-1 (GLP-1) and its analog exendin-4 (EX) have been considered as a growth factor implicated in pancreatic islet mass increase and -cell proliferation. This study aimed to investigate the effect of EX on cyclin D1 expression, a key regulator of the cell cycle, in the pancreatic -cell line INS-1. We demonstrated that EX significantly increased cyclin D1 mRNA and subsequently its protein levels. Although EX induced phosphorylation of Raf-1 and extracellular-signal-regulated kinase (ERK), both PD98059 and exogenous ERK1 had no effect on the cyclin D1 induction by EX. Instead, the cAMP-elevating agent forskolin induced cyclin D1 expression remarkably and this response was inhibited by pretreatment with H-89, a protein kinase A (PKA) inhibitor. Promoter analyses revealed that the cAMP-responsive element (CRE) site (at position -48; 5 -TAACGTCA-3 ) of cyclin D1 gene was required for both basal and EX-induced activation of the cyclin D1 promoter, which was confirmed by site-directed mutagenesis study. For EX to activate the cyclin D1 promoter effectively, CRE-binding protein (CREB) should be phosphorylated and bound to the putative CRE site, according to the results of electrophoretic mobility shift and chromatin immunoprecipitation assays. Lastly, a transfection assay employing constitutively active or dominant-negative CREB expression plasmids clearly demonstrated that CREB was largely involved in both basal and EX-induced cyclin D1 promoter activities. Taken together, EXinduced cyclin D1 expression is largely dependent on the cAMP/PKA signaling pathway, and EX increases the level of phosphorylated CREB and more potently transactivates cyclin D1 gene through binding of the CREB to the putative CRE site, implicating a potential mechanism underlying -cell proliferation by EX.

BACKGROUND AND PURPOSERosiglitazone is a widely used oral hypoglycaemic agent, which improves insulin resistance in type 2 diabetes. Chronic rosiglitazone treatment is associated with a number of adverse cardiac events. The present study was designed to characterize the effects of rosiglitazone on cloned Kv4.3 potassium channels. EXPERIMENTAL APPROACHThe interaction of rosiglitazone with cloned Kv4.3 channels stably expressed in Chinese hamster ovary cells was investigated using whole-cell patch-clamp techniques. KEY RESULTSRosiglitazone decreased the currents carried by Kv4.3 channels and accelerated the current inactivation, concentrationdependently, with an IC50 of 24.5 mM. The association and dissociation rate constants for rosiglitazone were 1.22 mM -1 ·s -1 and 31.30 s -1 respectively. Block by rosiglitazone was voltage-dependent, increasing in the voltage range for channel activation; however, no voltage dependence was found in the voltage range required for full activation. Rosiglitazone had no effect on either the deactivation kinetics or the steady-state activation of Kv4.3 channels. Rosiglitazone shifted the steady-state inactivation curves in the hyperpolarizing direction, concentration-dependently. The Ki for the interaction between rosiglitazone and the inactivated state of Kv4.3 channels was 1.49 mM, from the concentration-dependent shift in the steady-state inactivation curves. Rosiglitazone also accelerated the kinetics of the closed-state inactivation of Kv4.3 channels. Rosiglitazone did not affect either use dependence or recovery from inactivation of Kv4.3 currents. CONCLUSION AND IMPLICATIONSOur results indicate that rosiglitazone potently inhibits currents carried by Kv4.3 channels by interacting with these channels in the open state and by accelerating the closed-state inactivation of Kv4.3 channels. LINKED ARTICLEThis article is commented on by Hancox, pp. 496-498 of this issue. To view this commentary visit http://dx