Increased Pancreatic β-Cell Proliferation Mediated by CREB Binding Protein Gene Activation

Abstract

The cyclic AMP (cAMP) signaling pathway is central in ␤-cell gene expression and function. In the nucleus, protein kinase A (PKA) phosphorylates CREB, resulting in recruitment of the transcriptional coactivators p300 and CREB binding protein (CBP). CBP, but not p300, is phosphorylated at serine 436 in response to insulin action. CBP phosphorylation disrupts CREB-CBP interaction and thus reduces nuclear cAMP action. To elucidate the importance of the cAMP-PKA-CREB-CBP pathway in pancreatic ␤ cells specifically at the nuclear level, we have examined mutant mice lacking the insulin-dependent phosphorylation site of CBP. In these mice, the CREB-CBP interaction is enhanced in both the absence and presence of cAMP stimulation. We found that islet and ␤-cell masses were increased twofold, while pancreas weights were not different from the weights of wild-type littermates. ␤-Cell proliferation was increased both in vivo and in vitro in isolated islet cultures. Surprisingly, glucose-stimulated insulin secretion from perfused, isolated mutant islets was reduced. However, ␤-cell depolarization with KCl induced similar levels of insulin release from mutant and wild-type islets, indicating normal insulin synthesis and storage. In addition, transcripts of pgc1a, which disrupts glucose-stimulated insulin secretion, were also markedly elevated. In conclusion, sustained activation of CBP-responsive genes results in increased ␤-cell proliferation. In these ␤ cells, however, glucose-stimulated insulin secretion was diminished, resulting from concomitant CREB-CBP-mediated pgc1a gene activation.Under circumstances of increased metabolic demand, such as pregnancy (2, 4), or in insulin-resistant states (3, 30), islet and ␤-cell masses increase to meet metabolic requirements. The failure of ␤ cells to adapt to metabolic requirements results in relative insulin deficiency and diabetes mellitus. Within the pancreas, three conceptual mechanisms may lead to an increase in ␤-cell mass: (i) neogenesis from nonendocrine pancreatic tissue, such as pancreatic-duct epithelium (2, 3, 27, 28), pancreatic acinar cells (␤ cells derived from non-␤ cells) (3, 38, 39), or undifferentiated cells within the islet (1, 9, 15, 44); (ii) proliferation of ␤ cells (␤ cells derived from existing ␤ cells) (6,11,38); and (iii) reduced ␤-cell apoptosis in the context of normal ␤-cell turnover (5, 26).Cyclic AMP (cAMP) signaling is critical in the physiologic function of ␤ cells. This is exemplified by the effects of the incretin hormone glucagon-like peptide-1 (GLP-1), which improves glucose-stimulated insulin secretion from pancreatic ␤ cells, in part by raising intracellular cAMP levels (13,18,28). In pharmacologic studies, GLP-1 also stimulated PDX-1 gene expression in pancreatic-duct epithelial cells and stimulates proliferation of ␤ cells (5,7,22,26,38). Binding of GLP-1 to its ␤-cell receptor elevates intracellular cAMP levels; cAMP in turn binds to the regulatory subunit of protein kinase A (PKA) and releases the PKA catalytic subunit. Elevation of cAMP als...

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