Traditional ‘rate‐limiting’ molecular indices of gluconeogenesis do not correlate with flux through the pathway in vivo

Abstract

Hyperglycemia in type 2 diabetes is due, in part, to elevated gluconeogenesis (GNG). This has been attributed, based largely on in vitro models, to increased mRNA transcription of supposed rate‐limiting enzymes PEPCK and G6Pase. However, in vivo metabolic substrate analysis in man and dog indicate that GNG flux to G6P varies minimally within the physiological range of insulin. We propose that GNG mRNA and protein expression would be decreased during hyperinsulinemic (2.0 mU/kg/min), euglycemic conditions in 24 h fasted conscious dogs subjected to a pancreatic clamp, but that the time course of expression would not correlate with functional GNG flux to G6P. Arterial and hepatic sinusoidal insulin levels increased 8‐fold and glucagon was clamped at basal levels. At 0.5 h, GNG flux to G6P was decreased 50% due to the suppression of lipolysis and a switch from net hepatic lactate uptake to output. Liver FOXO1 phosphorylation was increased 1.9‐fold and GNG enzyme mRNA expression was decreased ~40‐50% at 0.5 h but PEPCK protein levels did not change. By 4 h, GNG mRNA expression was decreased ~80%, and PEPCK protein was reduced by 45%, but GNG flux to G6P had stabilized at near‐basal levels. Thus, while insulin can inhibit GNG gene transcription and protein expression in vivo, these changes do not result in functional changes in substrate flux in the GNG pathway, indicating PEPCK was not a rate‐limiting marker for GNG in vivo.