Hussain Mehboob A, Porras Delia L, Rowe Matthew H, West Jason R, Song Woo-Jin, Schreiber Weston E, Wondisford Fredric E
Metabolism Division, Department of Pediatrics and Medicine, Johns Hopkins University, 600 N. Wolfe Street, CMSC 10-113, Baltimore, MD 21287, USA.
Mol Cell Biol. 2006 Oct;26(20):7747-59. doi: 10.1128/MCB.02353-05. Epub 2006 Aug 14.
The cyclic AMP (cAMP) signaling pathway is central in beta-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 beta 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 beta-cell masses were increased twofold, while pancreas weights were not different from the weights of wild-type littermates. beta-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, beta-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 beta-cell proliferation. In these beta cells, however, glucose-stimulated insulin secretion was diminished, resulting from concomitant CREB-CBP-mediated pgc1a gene activation.
环磷酸腺苷（cAMP）信号通路在β细胞基因表达和功能中起核心作用。在细胞核中，蛋白激酶A（PKA）使CREB磷酸化，导致转录共激活因子p300和CREB结合蛋白（CBP）的募集。响应胰岛素作用时，CBP而非p300在丝氨酸436处被磷酸化。CBP磷酸化会破坏CREB-CBP相互作用，从而降低细胞核cAMP的作用。为了具体在细胞核水平阐明cAMP-PKA-CREB-CBP通路在胰腺β细胞中的重要性，我们研究了缺乏CBP胰岛素依赖性磷酸化位点的突变小鼠。在这些小鼠中，无论有无cAMP刺激，CREB-CBP相互作用均增强。我们发现胰岛和β细胞质量增加了两倍，而胰腺重量与野生型同窝仔鼠的重量无差异。在分离的胰岛培养物中，β细胞增殖在体内和体外均增加。令人惊讶的是，灌注的分离突变胰岛的葡萄糖刺激胰岛素分泌减少。然而，用氯化钾使β细胞去极化可诱导突变型和野生型胰岛释放相似水平的胰岛素，表明胰岛素合成和储存正常。此外，破坏葡萄糖刺激胰岛素分泌的pgc1a转录本也显著升高。总之，CBP反应基因的持续激活导致β细胞增殖增加。然而，在这些β细胞中，葡萄糖刺激的胰岛素分泌减少，这是由CREB-CBP介导的pgc1a基因激活所致。
