The large Maf family of basic leucine-zipper-containing transcription factors are known regulators of key developmental and functional processes in various cell types, including pancreatic islets. Here, we demonstrate that within the adult pancreas, MafB is only expressed in islet ␣-cells and contributes to cell type-specific expression of the glucagon gene through activation of a conserved control element found between nucleotides ؊77 to ؊51. MafB was also shown to be expressed in developing ␣-and ␤-cells as well as in proliferating hormone-negative cells during pancreatogenesis. In addition, MafB expression is maintained in the insulin ؉ and glucagon ؉ cells remaining in mice lacking either the Pax4 or Pax6 developmental regulators, implicating a potentially early role for MafB in gene regulation during islet cell development. These results indicate that MafB is not only important to islet ␣-cell function but may also be involved in regulating genes required in both endocrine ␣-and ␤-cell differentiation. Diabetes 55:297-304, 2006 T he pancreatic islets of Langerhans are composed of ␣-, ␤-, ␦-, and pancreatic polypeptide cells, which independently produce the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide, respectively. Collectively, these hormones regulate both fuel and energy metabolism, with insulin and glucagon key to controlling glucose homeostasis (1). Thus, glucagon secreted from ␣-cells stimulates the mobilization of glucose through gluconeogenesis and glycogenolysis to prevent hypoglycemia, whereas ␤-cell-secreted insulin promotes glucose storage. Physiological glucose levels are maintained through the counter-regulatory actions of glucagon and insulin in peripheral tissues, with defects in ␣-and ␤-cell function playing a significant part in the ability of diabetic patients to maintain glycemic control.The identification and characterization of the transcription factors regulating insulin and glucagon expression have not only revealed their significance in islet function but also during pancreatogenesis. The pancreas develops from dorsal and ventral epithelial bud evaginations from the foregut, with glucagon-producing cells first appearing at mouse embryonic day (E) 9.5 in the dorsal bud (2-4). Insulin-producing cells emerge at E10.5 (3), whereas somatostatin and pancreatic polypeptide ϩ cells are not detected until E15.5 and E18.5, respectively (5). Insulin-and glucagon-producing cells appear in waves during development, with the functional ␣-and ␤-cells that will populate the islet produced starting at ϳE13.5 (5). This latter phase is termed the "secondary transition," and these cells continue to proliferate but are only organized into islet structures and become glucose-responsive shortly after birth (6).The transcription factors associated with controlling cell-specific expression of the insulin and glucagon genes are principal regulators of islet cell formation, including Pdx1 (7-10), Pax6 (11,12), Pax4 (13,14), and NeuroD1 (15,16). Pdx1 is necessary for the growth of the en...

Obesity is thought to promote insulin resistance in part via activation of the innate immune system. Increases in proinflammatory cytokine production by M1 macrophages inhibit insulin signaling in white adipose tissue. In contrast, M2 macrophages have been found to enhance insulin sensitivity in part by reducing adipose tissue inflammation. The paracrine hormone prostaglandin E2 (PGE2) enhances M2 polarization in part through activation of the cAMP pathway, although the underlying mechanism is unclear. Here we show that PGE2 stimulates M2 polarization via the cyclic AMPresponsive element binding (CREB)-mediated induction of Krupplelike factor 4 (KLF4). Targeted disruption of CREB or the cAMP-regulated transcriptional coactivators 2 and 3 (CRTC2/3) in macrophages downregulated M2 marker gene expression and promoted insulin resistance in the context of high-fat diet feeding. As re-expression of KLF4 rescued M2 marker gene expression in CREB-depleted cells, our results demonstrate the importance of the CREB/CRTC pathway in maintaining insulin sensitivity in white adipose tissue via its effects on the innate immune system.U nder obese conditions, macrophage infiltration and activation in adipose tissue leads to a chronic inflammatory state with increased secretion of proinflammatory cytokines (1). The activation of IkB and Jun N-terminal kinases impairs insulin signaling in metabolic tissues and thereby contributes to insulin resistance (2-4). Classically activated M1 macrophages secrete proinflammatory cytokines, such as TNF-α and IL-12, which promote insulin resistance. Alternatively activated M2 macrophages are thought to protect adipocytes from the development of insulin resistance in response to IL-4 signaling (5, 6). Increases in STAT6 activity stimulate the expression of Krupplelike factor 4 (KLF4), which in turn promotes expression of the M2 program. Obesity causes an M2-to-M1 shift in adipose tissue that leads to insulin resistance (7).The eicosanoid prostaglandin E2 (PGE2) has been found to promote M2 macrophage polarization in part via induction of the cAMP pathway. Indeed, circulating catecholamines also exert potent anti-inflammatory effects on macrophage function via cAMP signaling (8). In this regard, a number of bacteria appear to evade the innate immune system by producing toxins that enhance cAMP production. cAMP stimulates the expression of cellular genes in part via the phosphorylation of CREB at Ser133 and via the dephosphorylation of the cAMP regulated transcriptional coactivators (CRTC) family of coactivators (9). Following its activation, the cyclic AMP-responsive element binding (CREB) pathway appears to block M1 macrophage function in part via the induction of the anti-inflammatory cytokine IL-10 (10, 11). Superimposed on these effects, cAMP also inhibits the expression of proinflammatory cytokines via the induction of class IIa histone deacetylases (HDACs) and subsequent deacetylation of NF-κB.Here we explore the potential roles of the class IIa HDAC and CREB/CRTC pathways in M2 macrophage...

The adipocyte-derived hormone adiponectin signals from the fat storage depot to regulate metabolism in peripheral tissues. Inversely correlated with body fat levels, adiponectin reduction in obese individuals may play a causal role in the symptoms of metabolic syndrome. Adiponectin lowers serum glucose through suppression of hepatic glucose production, an effect attributed to activation of AMPK. Here, we investigated the signaling pathways that mediate the effects of adiponectin by studying mice with inducible hepatic deletion of LKB1, an upstream regulator of AMPK. We found that loss of LKB1 in the liver partially impaired the ability of adiponectin to lower serum glucose, though other actions of the hormone were preserved, including reduction of gluconeogenic gene expression and hepatic glucose production as assessed by euglycemic hyperinsulinemic clamp. Furthermore, in primary mouse hepatocytes, the absence of LKB1, AMPK, or the transcriptional coactivator CRTC2 did not prevent adiponectin from inhibiting glucose output or reducing gluconeogenic gene expression. These results reveal that whereas some of the hormone's actions in vivo may be LKB1 dependent, substantial LKB1-, AMPK-, and CRTC2-independent signaling pathways also mediate effects of adiponectin.

The recent discovery of betatrophin, a protein secreted by the liver and white adipose tissue in conditions of insulin resistance and shown to dramatically stimulate replication of mouse insulin-producing β-cells, has raised high hopes for the rapid development of a novel therapeutic approach for the treatment of diabetes. At present, however, the effects of betatrophin on human β-cells are not known. Here we use administration of the insulin receptor antagonist S961, shown to increase betatrophin gene expression and stimulate β-cell replication in mice, to test its effect on human β-cells. Although mouse β-cells, in their normal location in the pancreas or when transplanted under the kidney capsule, respond with a dramatic increase in β-cell DNA replication, human β-cells are completely unresponsive. These results put into question whether betatrophin can be developed as a therapeutic approach for treating human diabetes.

The global diabetes epidemic poses a major challenge. Epigenetic events contribute to the etiology of diabetes; however, the lack of epigenomic analysis has limited the elucidation of the mechanistic basis for this link. To determine the epigenetic architecture of human pancreatic islets we mapped the genome-wide locations of four histone marks: three associated with gene activation-H3K4me1, H3K4me2, and H3K4me3-and one associated with gene repression, H3K27me3. Interestingly, the promoters of the highly transcribed insulin and glucagon genes are occupied only sparsely by H3K4me2 and H3K4me3. Globally, we identified important relationships between promoter structure, histone modification, and gene expression. We demonstrated co-occurrences of histone modifications including bivalent marks in mature islets. Furthermore, we found a set of promoters that is differentially modified between islets and other cell types. We also use our histone marks to determine which of the known diabetes-associated single-nucleotide polymorphisms are likely to be part of regulatory elements. Our global map of histone marks will serve as an important resource for understanding the epigenetic basis of type 2 diabetes.