Glucagon signaling in metabolic homeostasis

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A hallmark of the bi-hormonal disease diabetes mellitus is a disturbed balance between the gluco- regulatory hormones insulin and glucagon. The liver is the principal organ where the metabolic actions of insulin and glucagon action are borne out. Insulin is the main hormone orchestrating and signaling during the anabolic (fed) state, while glucagon orchestrates a catabolic (fasting) metabolic state ensuring metabolic homeostasis and survival. The transition from the fasting state to the rapid supply of nutrients during a meal (feeding) poses a particular challenge to the liver - whereby insulin action regulates the rapid shift from a catabolic to an anabolic state including a switch from hepatic glucose production to glycogen storage, lipid and protein synthesis. While much is understood about the metabolic effects and signaling pathways of insulin acting via its receptor on hepatocytes, the molecular components of glucagon signaling in the liver are only beginning to be understood. The main signaling pathways activated upon glucagon binding to its receptor in the liver are the phospholipase C (PLC) -inositol-3-phosphate (IP3) pathway and the cyclic AMP (cAMP) pathway. The latter bifurcates into the cAMP – protein kinase A (PKA) pathway and the cAMP-EPAC (exchange protein activated by cAMP) pathway. EPAC2C (a guanine nucleotide exchange factor) is an isoform of EPAC2 that is uniquely expressed only in the liver. Importantly, homeostatic and metabolic role of EPAC2C signaling and its downstream effector Rap1 (Ras-proximate-1 or Ras-related protein 1) in the liver are poorly understood. Surprisingly, liver EPAC2C knockdown results in a disturbed transition from fasting to feeding accompanied by glucose intolerance, insulin resistance, hyperinsulinemia and mild hyperglucagonemia – changes reminiscent of type 2 diabetes mellitus. Based on these observations, we hypothesize that during fasting the hormone glucagon - via EPAC2C-Rap1 signaling - primes the liver for subsequent insulin action in response to feeding and that EPAC2C-Rap1 is an important signaling component to secure liver metabolic flexibility and metabolic homeostasis. Building on these observations, we also find virus transduced over-expression of a constitutively active EPAC2C isoform in the liver ameliorates glucose homeostasis in diet-induced obese mice. The present proposal aims to extend our exciting findings to further elucidate liver EPAC2C-Rap1 signaling in metabolic homeostasis. Our proposed studies will expand our understanding of glucagon action in the liver and also identify a novel therapeutic target for treating diabetes mellitus.