Exendin-4 induction of cyclin D1 expression in INS-1 beta-cells: involvement of cAMP-responsive element

초록

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 P-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 beta-cell line INS-1. We denionstrated 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, EX-induced 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 CB-E site, implicating a potential mechanism underlying beta-cell proliferation by EX.

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