Foxa2 Integrates The Transcriptional Response Of The Hepatocyte To Fasting

The activation of hepatic gluconeogenesis plays an important role in maintaining blood glucose level within a narrow range by hormone regulation in the long-term starvation. Inappropriate regulation of this process is a hallmark of diabetes and other metabolic diseases. The rate-limiting enzymes required for hepatic gluconeogenesis are regulated at the transcriptional level, with multiple transcription factors cooperating to ensure the appropriate adaptations to the metabolic and hormonal status of the organism. In response to fasting, blood insulin levels drop, while glucagons and glucocorticoid levels rise, promoting hepatic gluconeogenesis through activation of the cAMP response element binding protein (CREB) and the glucocorticoid receptor (GR), respectively. A long-standing paradox has been the fact that both CREB and GR are expressed in multiple organs, yet the activation of the gluconeogenic program occurs primarily in the liver, and to a lesser extent in the kidney. Analysis of cis-regulatory elements of genes encoding gluconeogenic enzymes has led to the finding that binding sites for CREB and GR are often located in close proximity to those of nuclear factors expressed predominantly in hepatocytes. However, neither a requirement for the Foxa factors in the hormone-dependent activation of the hepatic gluconeogenic program nor a mechanism for this process has been investigated until now. Here we using cell-type specific gene ablation as a vivo model investigate how Foxa2 regulates the hepatic response to fasting, we chose to evaluate the expression of three representative genes that are exquisitely sensitive to hormonal regulation: PEPCK, TAT and IGFBP-1. Concordant with previous findings, activation of all three genes is blunted in the absence of Foxa2 by RT-PCR. In order to define the specific contribution of Foxa2 in this complex in vivo process, we isolated primary hepatocytes and cultured them in a three-dimensional collagen matrix to assess responsiveness to individual hormones in defined conditions. In cultured hepatocytes the PEPCK gene is most responsive to cAMP, which mimics activation of the glucagons receptor signaling cascade, while both TAT and IGFBP-1 are induced more effectively by glucocorticoids (dexamethasone). In the absence of Foxa2, however, both the induction of PEPCK by cAMP, as well as the activation of TAT and IGFBP-1 by glucocorticoids is severely blunted. This is achieved by integrating the response of the hepatocyte to glucagons and glucocorticoids. Binding of Foxa2 to its target sites allows the tracscription factors CREB and GR and their potent co-activators access to cis-regulatory elements of genes involved in gluconeogenesis. In summary, we have shown that Foxa2 is required for the activation of the hepatic transcriptional program of gluconeogenesis, a major evolutionary adaptation to limiting and unpredictable food supply. The integration of the tissue-specific factor Foxa2 with the two ubiquitously expressed but hormone dependent activators CREB and GR assures that the gluconeogenic response occurs only in the liver, and only under the appropriate metabolic circumstances.