Liver AMP-Activated Protein Kinase Is Unnecessary for Gluconeogenesis but Protects Energy State during Nutrient Deprivation

Abstract

AMPK is an energy sensor that protects cellular energy state by attenuating anabolic and promoting catabolic processes. AMPK signaling is purported to regulate hepatic gluconeogenesis and substrate oxidation; coordination of these processes is vital during nutrient deprivation or pathogenic during overnutrition. Here we directly test hepatic AMPK function in regulating metabolic fluxes that converge to produce glucose and energy in vivo. Flux analysis was applied in mice with a liver-specific deletion of AMPK (L-KO) or floxed control littermates to assess rates of hepatic glucose producing and citric acid cycle (CAC) fluxes. Fluxes were assessed in short and long term fasted mice; the latter condition is a nutrient stressor that increases liver AMP/ATP. The flux circuit connecting anaplerosis with gluconeogenesis from the CAC was unaffected by hepatic AMPK deletion in short and long term fasting. Nevertheless, depletion of hepatic ATP was exacerbated in L-KO mice, corresponding to a relative elevation in citrate synthase flux and accumulation of branched-chain amino acid-related metabolites. L-KO mice also had a physiological reduction in flux from glycogen to G6P. These results demonstrate AMPK is unnecessary for maintaining gluconeogenic flux from the CAC yet is critical for stabilizing liver energy state during nutrient deprivation.

Loss of hepatic AMP-activated protein kinase impedes the rate of glycogenolysis but not gluconeogenic fluxes in exercising mice

“…In contrast, recent work has added to the growing body of literature refuting such a regulatory role for liver AMPK in glucose homeostasis. We recently reported that 15-week-old mice lacking liver AMPK␣1␣2 subunits had no effect on endogenous glucose production or gluconeogenic flux following 7 h of fasting (29).…”

“…The 2 H/ 13 C MFA is sufficiently robust to detect an increase in CAC fluxes (V CS and V SDH ) (41). It is important to note that using this methodology, a higher V CS in liver-specific AMPK KO mice compared with WT mice following a 19-h fast was shown (29). Thus, the method is sensitive to experimental and physiological differences in CAC and gluconeogenic fluxes between WT and KO mice.…”

“…KO livers were shown to have reduced glycogen and increased triglycerides at 8.5 h but not 24 h of fasting. The short-term fasting phenotype resembles that of a longer fast (29), suggesting that the KO mice transition to the long-term fasted state quicker than the WT mice following food withdrawal. Extended fasting promotes the expression and secretion of FGF21 from the liver (56).…”

“…Previous studies in short-term fasted mice lacking liver AMPK␣ showed metabolic characteristics of long-term food withdrawal (29). FGF21 is secreted by the liver in response to starvation and exercise (30,31).…”

“…Energy charge is defined by the equation, energy charge ϭ ([ATP] ϩ 0.5[ADP])/([ATP] ϩ [ADP] ϩ [AMP]). Liver lipids, including triglycerides, diglycerides, phospholipids, and cholesterol esters, were measured as described previously (29). A cohort of 15-16-week-old, uncatheterized, male WT and KO mice were placed in an enclosed, stationary single-lane treadmill (Columbus Instruments) 4 h into a fast and were sacrificed for liver glycogen, lipids, adenine nucleotides, and immunoblotting in the sedentary state at 8.5 h of fasting (equivalent to the time ϭ 0 min of infusion studies).…”