Diabetes causes pancreatic beta cell failure through hyperglycemia-induced oxidative stress, or "glucose toxicity." We show that the forkhead protein FoxO1 protects beta cells against oxidative stress by forming a complex with the promyelocytic leukemia protein Pml and the NAD-dependent deacetylase Sirt1 to activate expression of NeuroD and MafA, two Insulin2 (Ins2) gene transcription factors. Using acetylation-defective and acetylation-mimicking mutants, we demonstrate that acetylation targets FoxO1 to Pml and prevents ubiquitin-dependent degradation. We show that hyperglycemia suppresses MafA expression in vivo and that MafA inhibition can be prevented by transgenic expression of constitutively nuclear FoxO1 in beta cells. The findings provide a mechanism linking glucose- and growth factor receptor-activated pathways to protect beta cells against oxidative damage via FoxO proteins.

Summary Embryonic development relies on the capacity of progenitor cells to appropriately respond to inductive cues; a cellular property known as developmental competence. Here we report that epigenetic priming of enhancers signifies developmental competence during endodermal lineage diversification. Chromatin mapping during pancreatic and hepatic differentiation of human embryonic stem cells revealed the en masse acquisition of a poised chromatin state at enhancers specific to endoderm-derived cell lineages in gut tube intermediates. Experimentally, the acquisition of this poised enhancer state predicts the ability of endodermal intermediates to respond to inductive signals. Furthermore, these enhancers are first recognized by the pioneer transcription factors FOXA1 and FOXA2 when competence is acquired, while subsequent recruitment of lineage-inductive transcription factors, such as PDX1, leads to enhancer and target gene activation. Together, our results identify the acquisition of a poised chromatin state at enhancers as a mechanism by which progenitor cells acquire developmental competence.

Pancreatic endocrine cell differentiation depends on transcription factors that also contribute in adult insulin and glucagon gene expression. Islet cell development was examined in mice lacking MafB, a transcription factor expressed in immature ␣ (glucagon ؉ ) and ␤ (insulin ؉ ) cells and capable of activating insulin and glucagon expression in vitro. We observed that MafB ؊/؊ embryos had reduced numbers of insulin ؉ and glucagon ؉ cells throughout development, whereas the total number of endocrine cells was unchanged. Moreover, production of insulin ؉ cells was delayed until embryonic day (E) 13.5 in mutant mice and coincided with the onset of MafA expression, a MafB-related activator of insulin transcription. MafA expression was only detected in the insulin ؉ cell population in MafB mutants, whereas many important regulatory proteins continued to be expressed in insulin ؊ ␤ cells. However, Pdx1, Nkx6.1, and GLUT2 were selectively lost in these insulin-deficient cells between E15.5 and E18.5. MafB appears to directly regulate transcription of these genes, because binding was observed within endogenous control region sequences. These results demonstrate that MafB plays a previously uncharacterized role by regulating transcription of key factors during development that are required for the production of mature ␣ and ␤ cells.insulin ͉ MafA ͉ pancreas development

Cardiac progenitor cells are multipotent and give rise to cardiac endothelium, smooth muscle, and cardiomyocytes. Here, we define and characterize the cardiomyoblast intermediate that is committed to the cardiomyocyte fate, and we characterize the niche signals that regulate commitment. Cardiomyoblasts express Hopx, which functions to coordinate local Bmp signals to inhibit the Wnt pathway, thus promoting cardiomyogenesis. Hopx integrates Bmp and Wnt signaling by physically interacting with activated Smads and repressing Wnt genes. The identification of the committed cardiomyoblast that retains proliferative potential will inform cardiac regenerative therapeutics. In addition, Bmp signals characterize adult stem cell niches in other tissues where Hopx-mediated inhibition of Wnt is likely to contribute to stem cell quiescence and to explain the role of Hopx as a tumor suppressor.

The MafA transcription factor is both critical to islet ␤-cell function and has a unique pancreatic cell-typespecific expression pattern. To localize the potential transcriptional regulatory region(s) involved in directing expression to the ␤ cell, areas of identity within the 5 flanking region of the mouse, human, and rat mafA genes were found between nucleotides ؊9389 and ؊9194, ؊8426 and ؊8293, ؊8118 and ؊7750, ؊6622 and ؊6441, ؊6217 and ؊6031, and ؊250 and ؉56 relative to the transcription start site. The identity between species was greater than 75%, with the highest found between bp ؊8118 and ؊7750 (ϳ94%, termed region 3). Region 3 was the only upstream mammalian conserved region found in chicken mafA (88% identity). In addition, region 3 uniquely displayed ␤-cell-specific activity in cell-line-based reporter assays. Important regulators of ␤-cell formation and function, PDX-1, FoxA2, and Nkx2.2, were shown to specifically bind to region 3 in vivo using the chromatin immunoprecipitation assay. Mutational and functional analyses demonstrated that FoxA2 (bp ؊7943 to ؊7910), Nkx2.2 (bp ؊7771 to ؊7746), and PDX-1 (bp ؊8087 to ؊8063) mediated region 3 activation. Consistent with a role in transcription, small interfering RNA-mediated knockdown of PDX-1 led to decreased mafA mRNA production in INS-1-derived ␤-cell lines (832/13 and 832/3), while MafA expression was undetected in the pancreatic epithelium of Nkx2.2 null animals. These results suggest that ␤-cell-type-specific mafA transcription is principally controlled by region 3-acting transcription factors that are essential in the formation of functional ␤ cells.