Laboratory of Molecular Medical Bioscience, Graduate School of Medical Life Science, Yokohama City University, Yokohama, Japan.
J Mol Endocrinol. 2024 Oct 4;73(3). doi: 10.1530/JME-24-0023. Print 2024 Oct 1.
MafA is a key transcriptional regulator of pancreatic islet β-cell function. Its target genes include those encoding preproinsulin and the glucose transporter Glut2 (Slc2a2); thus, MafA function is essential for glucose-stimulated insulin secretion. Expression levels of MafA are reduced in β-cells of diabetic mouse models and human subjects, suggesting that β-cell dysfunction associated with type 2 diabetes is attributable to the loss of MafA. On the other hand, MafA is transcriptionally upregulated by incretin hormones through activation of CREB and its co-activator CRTC2. β-cell-specific expression of MafA relies on a distal enhancer element. However, the precise mechanism by which CREB-CRTC2 regulates the enhancer and proximal promoter regions of MafA remains unclear. In this report, we analyzed previously published ChIP-seq data and found that CREB and NeuroD1, a β-cell-enriched transactivator, bound to both the promoter and enhancer regions of human MAFA. A series of reporter assays revealed that CREB activated the enhancer through a conserved cAMP-responsive element (CRE) but stimulated MAFA promoter activity even when the putative CRE was deleted. Two E-box elements and a CCAAT motif, which bind NeuroD1 and ubiquitous NF-Y transcription factors, respectively, were necessary for transcriptional activation of the MAFA promoter by CREB. Genome-wide analysis of CREB-bound loci in β-cells revealed that they were enriched with CCAAT motifs. Furthermore, promoter analysis of the Isl1 gene encoding a β-cell-enriched transcription factor revealed that a CRE-like element and two CCAAT motifs, but not the E-box, were necessary for activation by CREB. These results provide clues to elucidate the detailed mechanism by which CREB regulates MafA as well as β-cell-specific genes.
MafA 是胰腺胰岛 β 细胞功能的关键转录调节因子。其靶基因包括编码前胰岛素和葡萄糖转运体 Glut2（Slc2a2）的基因；因此，MafA 功能对于葡萄糖刺激的胰岛素分泌是必不可少的。在糖尿病小鼠模型和人类受试者的 β 细胞中，MafA 的表达水平降低，这表明与 2 型糖尿病相关的 β 细胞功能障碍归因于 MafA 的丧失。另一方面，肠促胰岛素激素通过激活 CREB 及其共激活因子 CRTC2 而上调 MafA 的转录。MafA 在 β 细胞中的特异性表达依赖于远端增强子元件。然而，CREB-CRTC2 调节 MafA 的增强子和近端启动子区域的确切机制仍不清楚。在本报告中，我们分析了先前发表的 ChIP-seq 数据，发现 CREB 和 NeuroD1（一种富含 β 细胞的转录激活因子）结合到人类 MAFA 的启动子和增强子区域。一系列报告基因实验表明，CREB 通过保守的 cAMP 反应元件（CRE）激活增强子，但即使删除了假定的 CRE，也能刺激 MAFA 启动子活性。两个 E 盒元件和一个 CCAAT 基序分别与 NeuroD1 和普遍存在的 NF-Y 转录因子结合，是 CREB 激活 MAFA 启动子所必需的。β 细胞中 CREB 结合位点的全基因组分析表明，它们富含 CCAAT 基序。此外，编码富含 β 细胞转录因子的 Isl1 基因的启动子分析表明，CRE 样元件和两个 CCAAT 基序（而非 E 盒）对于 CREB 的激活是必需的。这些结果为阐明 CREB 调节 MafA 以及 β 细胞特异性基因的详细机制提供了线索。
