Constitutively Active Grp1 Promotes Basal Glucose Transport into White Adipose and Enhances Insulin Sensitivity in High-Fat Fed Mice

Abstract

Insulin-stimulated phosphorylation of Grp1 (general receptor for phosphoinositides; a.k.a Cyth3, cytohesin3) in cultured adipocytes is known to promote recycling of insulin-regulated glucose transporter (Glut4)-containing vesicles from the endosome to plasma membrane, and to enhance glucose uptake thereafter. We found that Cre-dependent expression of a constitutively active form of Grp1 (caGrp1) specifically increased basal glucose uptake into WAT of young chow-fed mice, without altering Akt phosphorylation or insulin-stimulated uptake. Although early growth and glucose homeostasis in chow-fed caGrp1 mice were normal, caGrp1 mice fed a high fat diet (HFD) became markedly less adipose, and showed improved glucose tolerance and insulin sensitivity vs. Cre-negative controls. Fasting insulinemia in HFD-fed caGrp1 mice was lower, whereas circulating adiponectin was higher, and FGF-21 and leptin were unchanged. Hepatic lipid disposition in HFD-fed caGrp1 mice was comparable to that in controls, though circulating and liver free fatty acids were slightly lower. However, epigonadal WAT from caGrp1 mice showed markedly reduced staining for inflammatory marker CD68, and adipocyte size was also significantly greater. Collectively, these features distinguish caGrp1 mice from those with adipose-specific over-expression of Glut4 described by others, but remain compatible with an adipose-specific basis of improved systemic insulin sensitivity. Given recently improved understanding of the structural/functional changes induced in Grp1 via Akt phosphorylation, drug-based activation of Grp1 in WAT might prove an attractive route by which to favorably alter systemic insulin sensitivity and glycemia. Disclosure K.D. Copps: None. J. Li: None.