Increased renal glucose‐6‐phosphatase gene expression and activity in mice lacking insulin receptors in the renal proximal tubule cells

Abstract

Enhanced gluconeogenesis hasbeen demonstrated in renal proximal tubules of insulin resistant Zucker rats. We have demonstrated reduced insulin receptor expression in renal‐tubular‐epithelial cells of insulin‐resistant rats. Since insulin has been shown to inhibit gluconeogenesis, we tested whether reduced insulin receptor in the proximal tubule affects renal gluconeogenesis. We analyzed gluconeogenesis components in the kidney tissues from mice with targeted deletion of insulin receptor from the proximal tubule (n=7/genotype). For glucose‐6‐ phosphatase (G6Pase) activity, male mice were fasted for 4 hours prior to study followed by intraperitoneal injection of insulin (0.5 U/kg·bw) in 300 μl saline plus 300 μl 25% dextrose in saline in order to amplify the difference between KO and WT (wild type) littermates. After 20 minutes, mice were euthanized and kidneys were removed for G6Pase activity and mRNA levels. Real time RT‐PCR revealed 2.5 fold higher expression of G6Pase mRNA in KO relative to WT littermates (p=0.02, n=7/genotype). In addition, renal G6Pase activity was trending higher (20%) in KO relative to WT; G6Pase activities (in μmolPi/min/mg of protein at 37°C) were 70.1 ± 7.4 in WT versus 84.5 ± 6.3 in KO). Furthermore, the KO mice were mildly hyperglycemic in the fasting state. These results support a physiologic role for the insulin receptor in renal proximal tubule to affect renal gluconeogenesis; and suggest that downregulation of the insulin receptor in renal proximal tubule in the insulin‐resistant state may further contribute to hyperglycemia through enhanced gluconeogenesis. Department of Biotechnology (Government of India) grant to S.T. (BT/HRD/35/02/17/2008).