FBP1 is a nonenzymatic safety valve that curtails AKT activation to prevent insulin hyperresponsiveness

Insulin inhibits gluconeogenesis and stimulates glucose conversion to glycogen and lipids. How these activities are coordinated to prevent hypoglycemia and hepatosteatosis is not clear. Fructose-1,6 bisphosphatase (FBP1) is rate controlling for gluconeogenesis. However, inborn human FBP1 deficiency does not cause hypoglycemia unless accompanied by fasting or starvation, which also trigger paradoxical hepatomegaly, hepatosteatosis, and hyperlipidemia in affected individuals. Hepatocyte FBP1-ablated mice exhibit identical fasting-conditional pathologies along with AKT hyperactivation, whose inhibition reversed hepatomegaly, hepatosteatosis and hyperlipidemia but not hypoglycemia. Surprisingly, fasting-mediated AKT hyperactivation is insulin-dependent. FBP1 prevents insulin hyperresponsiveness, independently of its catalytic activity, by interacting with AKT, PP2A-C and Aldolase-B (ALDOB) to specifically accelerate AKT dephosphorylation. Enhanced by fasting and weakened by elevated insulin, FBP1:AKT:PP2A-C:ALDOB complex formation, which is disrupted by human FBP1 deficiency mutations or a C-terminal FBP1 truncation, prevents insulin-triggered liver pathologies and maintains lipid and glucose homeostasis. Conversely, a complex disrupting peptide reverses diet-induced insulin resistance. ### Competing Interest Statement M.K. and A.R.S. are founders and stockholders in Elgia Pharmaceuticals and M.K. had received research support from Merck and Janssen Pharmaceuticals. All other authors declare no competing interests.