Consistent with recent independent studies we

Consistent with recent independent studies, we confirm that the GCG-induced increase in oxygen consumption is only partially blunted in Fgf21−/− mice [32], implicating additional GCGR-dependent mechanisms contributing to the acute GCG-dependent enhancement of energy expenditure. Notably, glucagon administration may increase oxygen consumption in the liver, adipose tissue, and possibly other tissues [31]. Whether these actions, partially independent of FGF21, reflect additional recruitment of downstream mediators, or direct signaling from the GCGR to mitochondrial pathways, requires further exploration. Our analysis of relative Gcgr gene expression across a range of tissues revealed that levels of adipose tissue Gcgr mRNA transcripts were not abundant, being several orders of magnitude lower in WAT compared to liver, and even lower in BAT. Nevertheless, acute administration of GCG to mice increased oxygen consumption not only in the liver, but also in BAT. Furthermore, our studies with BAT Ivermectin in vitro demonstrated that enhanced or reduced Gcgr expression was associated with the regulation of lipolysis and the expression of genes linked to the control of energy expenditure. Hence, to identify a role for the BAT Gcgr in the pharmacological response to acute Gcgr administration in vivo, we generated and characterized GcgrBAT−/− mice. Unexpectedly, GCG administration robustly induced oxygen consumption to a similar extent in GcgrBAT+/+ vs. GcgrBAT−/− mice. Taken together, these results suggest that the murine BAT GCGR is not an important molecular target for the acute pharmacological actions of GCG administration, findings consistent with the lack of increased neck temperature or BAT glucose uptake in healthy male humans following transient GCG infusion [15]. The generation of mice with a marked reduction of BAT Gcgr expression provided an opportunity to determine the physiological importance of the GCGR within the cellular domains targeted by Myf5-Cre. Notably, the BAT Gcgr is not required for control of body weight, adiposity, core temperature, or energy expenditure, either when housed at RT or 4 °C. Moreover, we detected no differences in these parameters in mice maintained on a RCD or HFD. Furthermore, glucose homeostasis and lipid tolerance was not different in GcgrBAT+/+ vs. GcgrBAT−/− mice, whether maintained in the cold, or at RT and studied after exposure to RCD or HFD conditions. Hence, the totality of the data is consistent with the very low level of BAT Gcgr expression and reveals that the BAT Gcgr is dispensable for metabolic adaptation to energy excess or a cold environment.
Limitations Our studies have several limitations. First, we focused on acute pharmacological GCG administration in young male C57Bl/6 mice and cannot rule out a role for the BAT GCGR in transduction of a component of weight loss observed with more sustained GCGR agonism in a wider range of mice and conditions [32]. Second, we did not study mice with experimental diabetes; hence, our conclusions are limited to mice analyzed under RCD and HFD feeding. Third, we did not examine the actions of GCG-containing multi-agonists, and it is possible that BAT GCGR activation may be more important when GCG is combined with one or more peptides with complementary metabolic activity. Nevertheless, within the context of these limitations, the results of our studies are quite conclusive and suggest that the BAT GCGR is dispensable for the acute metabolic response to GCG administration and the physiological control of energy homeostasis.
Acknowledgements J.L.B. and E.M.V. have received fellowship funding from Diabetes Canada. E.E.M. has received fellowship funding from the Canadian Diabetes Association and the Canadian Institutes of Health Research. J.E.C. has received fellowships from the Banting and Best Diabetes Centre, University of Toronto, and the Canadian Institutes of Health Research. J.H.S. was supported by the US National Institutes of Health (NIH) grant R00AG055649. P.E.S. was supported by US National Institutes of Health (NIH) grants R01-DK55758, P01-DK088761, R01-DK099110, P01-AG051459 and by an unrestricted grant from the Novo Nordisk Research Foundation.