Med14 phosphorylation shapes genomic response to GLP-1 agonist [ChIP-seq]

Under feeding conditions, increases in circulating glucose concentrations trigger the release of glucagon-like peptide (GLP-1) from intestinal L cells. GLP-1 promotes insulin secretion and pancreatic beta cell viability in part via triggering of the beta cell GLP-1 receptor and subsequent induction of the cAMP signaling pathway, leading to the protein kinase A (PKA) mediated phosphorylation of CREB and induction of CREB target genes. By contrast with the acute effects of this pathway on immediate early CREB target genes, which attenuate the cAMP-CREB response, sustained exposure of beta cells to GLP-1 agonist (exenatide-4; Ex-4) or adenyl cyclase activator (Forskolin; FSK) stimulates the expression of beta cell specific CREB target genes with delayed kinetics. In a proteomic screen for transcriptional co-regulators that mediate the long-term effects of GLP-1, we identified Med14, a backbone subunit of the Mediator complex. Exposure to either Ex-4 or FSK stimulates Med14 phosphorylation at Ser983, corresponding to a conserved PKA recognition site (RRXS) that is located within an intrinsically disordered region of Med14. Phosphorylation of Med14 is essential for maintenance of enhancers that drive induction of beta cell-specific and diabetes-linked genes. Mutation of Med14 at Ser983 to alanine decreased beta cell numbers and repressed growth factor signaling in primary mouse islets. Our work reveals how phosphorylation of a general transcription factor in response to GLP-1 analogs triggers a broad genomic response with salutary effects on beta cell function.在喂养状态下，循环血糖浓度升高会触发肠道L细胞释放胰高血糖素样肽（glucagon-like peptide, GLP-1）。GLP-1可通过激活β细胞表面的GLP-1受体，继而诱导环腺苷酸（cyclic adenosine monophosphate, cAMP）信号通路，进而介导蛋白激酶A（protein kinase A, PKA）对环腺苷酸应答元件结合蛋白（cAMP response element-binding protein, CREB）的磷酸化，并诱导CREB靶基因的表达，以此促进胰岛素分泌并维持胰腺β细胞的存活能力。尽管该通路对即刻早期CREB靶基因的急性效应会削弱cAMP-CREB应答反应，但与之不同的是，β细胞持续暴露于GLP-1激动剂艾塞那肽-4（exenatide-4, Ex-4）或腺苷酸环化酶激活剂福司柯林（Forskolin, FSK）时，会以延迟动力学特征诱导β细胞特异性CREB靶基因的表达。在针对介导GLP-1长期效应的转录共调控因子开展的蛋白质组学筛选中，我们鉴定出了中介体复合物（Mediator complex）的骨架亚基Med14。经Ex-4或FSK处理后，Med14会在Ser983位点发生磷酸化；该位点对应一个保守的PKA识别基序（RRXS），且处于Med14的内在无序区域内。Med14的磷酸化对于维持可驱动β细胞特异性及糖尿病相关基因诱导的增强子功能至关重要。将Med14的Ser983位点突变为丙氨酸后，原代小鼠胰岛中的β细胞数量减少，且生长因子信号通路受到抑制。本研究阐明了通用转录因子的磷酸化如何响应GLP-1类似物，进而触发广泛的基因组应答，并对β细胞功能产生有益影响。