Abstract P227: Metformin Inhibits Glucose-Induced mTOR Activation and ER Stress in Rodent Heart

Abstract

Background: When subjected to increased workload, the heart remodels both metabolically and structurally by increasing glucose metabolism and activating the mTOR signaling pathway. Specifically, glucose activates mTOR in a TSC2-dependent manner. We now examined whether metformin regulation of AMPK, a major upstream regulator of TSC2, modulates mTOR activation and endoplasmic reticulum (ER) stress when hearts are subjected to increased workload. Methods: Working hearts from 12 week old male Sprague Dawley rats were perfused ex vivo (5 mM glucose) at physiologic pre- and afterload. At 30 minutes, the workload was doubled. Hearts were freeze-clamped at 60 minutes. Hearts received either the AMPK activator metformin (10 mM) or the ER stress relieving agent sodium 4-phenylbutyrate (10 mM) directly in the perfusate. Rates of glucose uptake and oxidation were measured with radiolabeled substrate (3H, 14C). Cardiac power, the mTOR signaling pathway, and markers of ER stress were assessed. Results: With increased workload rates of glucose uptake and oxidation doubled, with rates of glucose uptake exceeding those of glucose oxidation. Neither metformin nor sodium 4-phenylbutyrate significantly changed those rates. In control hearts, there was a downregulation of AMPK, an activation of mTOR, an induction of ER stress, and a decline in contractile function. Perfusion with media containing metformin activated AMPK, prevented glucose-mediated mTOR activation, alleviated ER stress and improved cardiac function (25% increase in cardiac power). In a separate group of animals nearly identical findings were obtained when metformin (300 mg/kg/day) was administered in vivo for one week. The functional improvement was also observed when ER stress was directly relieved with sodium 4-phenylbutyrate, as well. Conclusion: Our results implicate a critical role for glucose metabolism and AMPK in load-induced mTOR activation. Metformin improves contractile function in rat hearts subjected to increased workload by relieving ER stress. The relief of ER stress by metformin suggests previously unrecognized beneficial effects of AMPK activation in the hemodynamically stressed heart.