The forkhead family of transcription factors of the Family A (formerly known as hepatocyte nuclear factors 3) in mammals includes three genes designated as Foxa1 (HNF-3a), Foxa2 (HNF-3β) and Foxa3 (HNF-3γ), which have overlapping patterns of tissue expression, including gut, central nervous system, neuroendocrine cells, and lung. They are all characterized by a 100 amino acid long, highly conserved winged helix motif that is responsible for monomeric recognition of specific DNA target sites, and is similar in structure to that of the linker histone H5. However, in contrast to linker histones that compact DNA in chromatin and repress gene expression, FoxA proteins are associated with transcriptionally active chromatin and may decompact DNA from the nucleosome.
FoxA factors have been shown to play crucial roles in organogenesis and cell differentiation during development. FoxA2, the earliest expressed forkhead, is essential for the development of the foregut endoderm and the notochord, a structure essential for sonic hedgehog signaling to pattern the neural tube. FoxA proteins are well positioned to regulate early gene expression because they can replace linker histones and affect chromatin structure directly and facilitate the binding of other nuclear receptors in a context dependent manner to their respective targets and hence initiate organ specific transcription factors. The unique ability of FoxA proteins to engage chromatin before other transcription factors has led to the concept that they act as “pioneer factors”.
Our group was the first to demonstrate that the activity of the FoxA family of transcription factors is regulated by extracellular signaling events. Specifically, we demonstrated that insulin signaling inactivates FoxA2 by specific phosphorylation of T156, which leads to the inactivation due to nuclear exclusion. We also demonstrated that Foxa2 functions as an “insulin sensor” in the liver and in the lateral hypothalamus. In the liver, Foxa2 is a key regulator of hepatic gene expression in response to fasting and feeding. Foxa2 is active in the fasted state promoting the expression of β-oxidation and ketogenic genes and inactive during feeding, when Foxa2 is inhibited in a phosphorylation-dependent manner via the insulin-PI3K-Akt signaling. In addition, FoxA2 activates the expression of microsomal transfer protein (Mttp), thereby increasing VLDL secretion and hepatic triglyceride export. Furthermore, we have shown that FoxA2 is a potent transcriptional activator of apolipoprotein M (apoM), a critical component of preb-HDL and mature HDL, which has been shown in mice to protect against atherosclerotic plaque formation due to augmented reverse cholesterol transport.
In the current grant proposal we will study the role of post translational modifications in response to hormonal and metabolic stress on the activity and function of Foxa-1 and -2. Specifically, we will biochemically characterize these modifications in Foxa1 and FoxA2 and study the physiological role in pancreatic beta-cells, liver, and hypothalamus in mice. We will also elucidate the effect of these modifications on longevity, since genetic studies in flies suggest that forkhead transcription factors regulate lifespan. Together, this research will shed light on the molecular mechanisms by which FoxA2 activity and downstream effector pathways are regulated and how they modify metabolic stress and diseases such as obesity and type 2 diabetes.