Interruption of Hepatic Glucagon Signaling Reveals a Hepatic-a-Islet Cell Axis Where L-Glutamine Stimulates a-Cell Proliferation

Decreasing glucagon action lowers blood glucose and may be a useful therapeutic approach for diabetes. However, interrupted glucagon signaling in mice leads to hyperglucagonemia and a-cell hyperplasia. We show using islet transplantation, mouse and zebrafish models, an in vitro islet culture assay that a hepatic-derived, circulating factor in mice with interrupted glucagon signaling stimulates a-cell proliferation, which was dependent on mTOR signaling and the FoxP transcription factors. a-cells of transplanted human islets also proliferated in response to this signal in mice. A combination of liver transcriptomics and serum fractionation with proteomics/metabolomics found changes in hepatic gene expression relating to amino acid catabolism predicting the observed increase in serum amino acid levels. Amino acid concentrations that mimicked the levels in mice with interrupted glucagon signaling, specifically L-glutamine, stimulated a-cell proliferation. These results indicate a hepatic-a-islet cell axis where glucagon regulates serum amino acid availability and L-glutamine regulates a-cell proliferation via mTOR-dependent nutrient sensing. Overall design: Ten days of GCGR mAb treatment increased the percentage of proliferating a-cell approximately 14-fold while Gcgr-/- mice also have increased a-cell proliferation (Dean et al., unpublished; Longuet et al., 2013). Therefore, we used these two models for liver transcriptomics. Mice were fasted for 6 hours prior to tissue collection. For RNA isolation, approximately 100mg punches of liver tissue were stored in RNAlater (Ambion) according to the manufacturer’s instructions until isolation using RNeasy Mini kit with DNase I (Qiagen Hilden, DE) digestion. RNA purity and quantity were determined by Bioanalyzer. Transcriptome expression was determined as described previously (Reinert et al, 2014; Brissova et al., 2014).