Hepatic TGFbeta1 in Control of Type 2 Diabetes and NASH via FoxO1 Signaling

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Project Summary Overnutrition induces insulin resistance, which is a high-risk factor for T2D and NASH. The mechanism underlying insulin resistance-induced hepatic lipogenesis, fibrosis, apoptosis, and inflammation for NASH is unclear. High levels of transforming growth factor-β1 (TGFβ1) in blood and tissues were observed in both human and mice with T2D and NASH. TGF-β1 plays a pivotal role in a diverse range of cellular responses, including extracellular matrix (ECM) synthesis and apoptosis which are essential for tissue homeostasis, but the molecular mechanism by which TGFβ1 regulates glucose metabolism and liver function is incompletely understood. The forkhead transcription factor Foxo1 is a key downstream target of the insulin→ PI3K→protein kinase B (Akt) signaling pathway and the glucagon→cAMP→protein kinase A (PKA) signaling pathway. Akt phosphorylates Foxo1-Ser253, triggering Foxo1 nuclear export and cytoplasmic sequestration for ubiquitination. By contrast, PKA phosphorylates Foxo1-Ser273 to promote Foxo1 nuclear translocation and protein stability in hepatocytes. Upon metabolic stress such as overnutrition, Foxo1 hyperactivation promotes hepatic glucose production (HGP) and hyperglycemia. In this proposal, the PI hypothesizes that hepatic TGFβ1 plays key role in control of Foxo1 via phosphorylation at S273 (Foxo1-pS273), enhancing Foxo1 nuclear activity, and inducing hyperglycemia, liver fibrosis and inflammation, and promoting T2D and NASH. In Aim 1, PI and his team will use genetic approaches to 1) delete the TGFβ1 gene in the liver of mice (L-TGFβ1KO) with or without active Foxo1-S273D/D mutation, or 2) generate liver-specific overexpression TGFβ1 mice (L-TGFβ1OE) with or without inactive Foxo1-S273A/A mutation, investigating whether hepatic TGFβ1 is a key controller for Foxo1-pS273 in promoting HGP, apoptosis, liver fibrosis and inflammation following the NASH diet feeding. Aim2 is to use mouse primary hepatocytes, bone marrow- derived macrophages, and hepatic stellate cells (HSC) to determine the mechanisms by which TGFβ1 promotes macrophage depolarization and HSC activation via Foxo1-pS273 in hepatocytes. The specific- domain interactions of Smad3 and Foxo1 and related target genes expression in hepatocytes will be further investigated. Aim 3 is to investigate whether suppression of systematic or hepatic TGFβ1 signaling is sufficient for the prevention of T2D and NASH in mice, in a Foxo1 dependent manner. Overall, using the genetic, genomic, bioinformatic, and pharmacological approaches, the PI and his team are fully equipped to investigate the new pathophysiological role of hepatic TGF-β1→Foxo1-pS273→TGFβ1 looping system in control of T2D and NASH, which will provide novel insights on mechanism of diabetes and NASH.