Non-Canonical Roles of Cytosolic Phosphoenolpyruvate Carboxykinase in Small Intestine Metabolism

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2016-03-30

Authors

Potts, Austin Joseph

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Abstract

Phosphoenolpyruvate carboxykinase (PEPCK) is considered by many to be the quintessential regulatory enzyme of gluconeogenesis. This common perspective of PEPCK solely functioning in gluconeogenesis is too narrow and does not do justice to the complexity and number of pathways in which this enzyme participates. PEPCK performs important roles in a number of tissues: kidney, for acid-base balance with concomitant gluconeogenesis; adipose, with glyceroneogenesis for fatty acid esterification; enterocytes of the small intestine, for amino acid oxidation; and most famously, gluconeogenesis in the liver. Though amino acid, fatty acid, and glucose metabolism of the small intestine have been well studied, no investigation has emphasized the role of intestinal PEPCK in these pathways. The aim of this work is to examine the roles of PEPCK in small intestine function and metabolism. A mouse model with an intestine-specific deletion of PEPCK was generated by crossing existing models to produce Pck1ff and Pck1ff+Villin-cre+/-, named Small Intestine PEPCK Knockout (SIPKO). Intestinal metabolism was evaluated in vivo and in vitro with respect to glucose, triglycerides, amino acids, and TCA cycle metabolism using a novel everted intestine perfusion model, isolated enterocytes, 13C stable isotope tracers, GC-MS, LC-MSMS, 13C NMR spectroscopy, and mathematical modeling. The functions of PEPCK in small intestine enterocytes largely recapitulate known roles in other tissues. Although, intestinal gluconeogenesis was ablated in the SIPKO mouse, there was no change in whole body glucose homeostasis. However, postprandial triglyceride secretion was attenuated and intestinal amino acid profiles were dramatically different. Nearly all non-essential amino acid concentrations were increased, and nearly all essential amino acid concentrations were decreased. Intestinal TCA cycle and related pathway fluxes were decreased by approximately half, and enterocyte oxygen consumption was lower during a TCA cycle challenge. These studies indicate that the metabolic roles of PEPCK in the small intestine extend far beyond gluconeogenesis to include cataplerotic functions that not only include glucose production, but also triglyceride storage, amino acid catabolism and oxidative energy production. In summary, our studies reveal intestinal PEPCK is a metabolically influential enzyme that is at the hub of macronutrient metabolism and facilitates TCA cycle flexibility and adaptability.

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