The transcription factor CREB (cAMP response element-binding protein) regulates metabolic gene expression in response to increases in cAMP signaling. CREB is thought to promote the expression of up to 5000 genes in the mammalian genome, but mechanisms by which CREB activates different sets of target genes in different contexts remain unclear. To address this problem, the Montminy lab will analyze CREB signaling output in the liver and pancreatic islets, as CREB activity in these two tissues is essential for glucose homeostasis. During periods of fasting, increases in circulating glucagon trigger the expression of gluconeogenic genes in the liver via induction of the CREB pathway. During feeding, intestinal enteroendocrine cells secrete glucagon like peptide 1 (GLP1), which stimulates insulin secretion and promotes beta cell viability, also via a CREB- dependent mechanism. Balancing these two opposing CREB-mediated physiological processes appears critical for glucose homeostasis. CREB activity is stimulated by the histone acetyl-transferase coactivators CBP and P300, and by a family of cAMP regulated transcriptional coactivators (CRTCs), which associate with CREB in response to cAMP. Here, the roles of different CRTCs and transcriptional co-factors in modulating CREB/CRTC activity to elicit tissue-specific responses (i.e., hepatic vs. pancreatic islet) will be examined. The overall hypothesis is that the CREB pathway functions as a central regulator of hepatic glucose production and pancreatic insulin secretion.
Aim 1 will focus on CREB-mediated glucose production by the liver in response to fasting. The Montminy lab will generate mice lacking CRTC2 and CRTC3 (alone and in combination) specifically in hepatocytes and assess effects on glucose output. The lab will then examine roles played by the transcription factor FOXO1 and the epigenetic regulator BRD4 in facilitating CRTC recruitment to a subset of fasting-induced genes in hepatocytes.
Aim 2 will explore the role of CRTC1 and CRTC2 in mediating the effects of glucose and GLP1 on insulin secretion and beta cell viability. The lab will generate mice lacking CRTC1 and CRTC2 (alone and in combination) specifically in pancreatic islet beta cells and assess the effects on insulin secretion and CREB target gene selection. The lab will characterize roles played by the beta cell- specific transcription factor NeuroD1 and the cAMP-regulated non-coding RNA, LINC00473, in potentiating CREB/CRTC signaling in the pancreas. The proposed studies will uncover how interactions between CREB, CRTCs, and their cofactors (e.g., lineage-specific transcription factors and epigenetic regulators) promote glucose homeostasis by differentially activating specific subsets of CREB target genes in liver and pancreatic islets. Targeting these cofactors via chemical inhibitors may provide therapeutic benefit to individuals with type II diabetes.
In healthy individuals, glucose levels in the blood stream are maintained within a narrow range by pancreatic beta cells, which release insulin when we eat, and by the liver, which produces glucose at night when we fast. Our work focuses on a specific signaling pathway, called the CREB/CRTC pathway, which coordinates both processes by turning on different sets of genes in the liver and pancreatic islets. We will test the roles played by different co-factors (transcription factors and epigenetic switches) in targeting CREB and CRTCs to different genes in these tissues, and we will examine the utility of chemical inhibitors against these co-factors in lowering blood glucose levels ? interventions that could provide therapeutic benefit to individuals with type II diabetes.
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