Abstract 16930: Inhibition of Glutamine Synthase Causes Glucose Intolerance in Mice

Abstract

Background: We recently performed metabolite profiling in 2,346 non-diabetic participants in the Framingham Offspring cohort. The glutamine/glutamate ratio was among the strongest predictors of the development of type II diabetes mellitus (DMII) during a 12-year follow-up period. Glutamate and glutamine are in a precursor-product relationship via the enzyme glutamine synthase (GS). Hypothesis: We hypothesized that inhibition of systemic GS activity would decrease the circulating glutamine/glutamate ratio and worsen glucose homeostasis in vivo. Methods: We administered 20 mg/kg of methionine sulfoximine (MSO), an irreversible inhibitor of GS, to 12 - 16 week old C57Bl6 mice intraperitoneally (i.p.) over 14 days (every other day for 7 injections). We then performed an intraperitoneal glucose tolerance test (IPGTT) with glucose measurements at 0, 30, 60, 90, 120 min from the tail vein. In a separate cohort of mice, we administered a single dose of 300 mg/kg of glutamine to both MSO-treated and control mice to assess reversibility of MSO effects. Results: Administration of MSO decreased circulating glutamine levels (n = 18, -64 ± 19% from baseline, p < 0.01), whereas glutamate levels increased significantly (n = 18, +25 ± 8, p = 0.03). The glutamine/glutamate ratio decreased from 2.16 ± 0.16 to 0.44 ± 0.03 (n = 12, p < 0.001). In the MSO-treated mice, the area under the curve of glucose excursion during the IPGTT increased markedly (n = 24, 12,098 ± 452 vs. 8,158 ± 368 AU, p < 0.001). In livers isolated from MSO-treated animals, we documented reduced liver GS activity by 94 ± 3% (n = 24, p < 0.001). Further, glutamine treatment of the MSO-treated animals at a dose of 300 mg/kg i.p. reversed the diabetogenic effects of MSO treatment (n = 12, 12,687 ± 568 vs 9,235 ± 399 AU, p < 0.01). Conclusion: Inhibition of systemic GS causes a reversal of the plasma glutamine/glutamate ratio and worsening of glucose tolerance. Glutamine administration rescues MSO-induced glucose intolerance. These studies implicate glutamine/glutamate metabolism in the control of glucose homeostasis. Further studies will focus on the mechanism of MSO-induced glucose intolerance and relate the murine findings to human pathology.