Intestinal function of gene-targeted mice lacking serum- and glucocorticoid-inducible kinase 1

Abstract

In vitro experiments have revealed the ability of serum- and glucocorticoid-inducible kinase 1 (SGK1) to stimulate intestinal Na⁺-coupled glucose cotransporter 1 (SGLT1) and intestinal Na⁺/H⁺ exchanger 3 (NHE3). The present study explored the contribution of SGK1 to the regulation of intestinal transport in vivo. SGK1 transcript levels were determined by real-time PCR and glucose-induced currents ( I_g) reflecting SGLT1 activity by Ussing chamber experiments. BCECF fluorescence was utilized for the determination of Na⁺-dependent pH recovery from an ammonium pulse (ΔpH_NHE) reflecting NHE activity. As a result, intestinal SGK1 transcript levels were significantly enhanced by a 4-day treatment with 10 μg·mg body wt⁻¹·day⁻¹ dexamethasone (Dex). I_g was, under control conditions, virtually identical in sgk1 knockout mice ( sgk1^−/−) and their wild type littermates ( sgk1^+/+). A 4-day treatment with Dex, however, increased I_g approximately threefold in sgk1^+/+ mice but not in sgk1^−/− mice. ΔpH_NHE was similar in sgk1^−/− and sgk1^+/+ mice before treatment. Dex increased ΔpH_NHE approximately threefold in sgk1^+/+ mice and approximately twofold in sgk1^−/−mice, an effect significantly blunted in the presence of the specific NHE3 blocker S-3226 (10 μM). According to Western blot analysis, Dex significantly enhanced SGLT1 and NHE3 protein abundance in brush-border membranes of sgk1^+/+ mice but not of sgk1^−/−mice. In conclusion, basic functions of SGLT1 and NHE3 in the intestine do not require stimulation by SGK1. However, the effects of glucocorticoids on SGLT1 are fully, and on NHE3 partially, dependent on SGK1.