NFAT5 is involved in GRP‐enhanced secretion of GLP‐1 by high sodium

Abstract

Organ to organ communication is important in the maintenance of normal fluid and electrolyte balance and blood pressure (BP). Neural mechanisms and gastrointestinal (GIT) hormones mediate the natriuresis of an oral sodium load. The GIT is the first organ exposed to ingested nutrients and most likely to react initially to a sodium load. Sensing the amount of ingested sodium by the GIT may be an important mechanism by which sodium balance is regulated. We have reported that gastrin secreted by G‐cells in the stomach and duodenum is involved in the regulation of sodium balance and BP. Besides gastrin, glucagon‐like peptide‐1 (GLP‐1) is another gut hormone, which may also participate in regulation of sodium balance and BP. It is well known that glucose‐induced GLP‐1 secretion is sodium‐dependent. The increase in intracellular calcium needed for GLP‐1 secretion is also due to sodium‐dependent cell excitability. However, the specific effect of varying sodium concentration on GLP‐1 secretion and its potential mechanism remain to be elucidated. This study aimed to determine the effect of varying sodium concentrations on GLP‐1 secretion and its potential mechanism. Here, we show that sodium can stimulate GLP‐1 secretion and that gastrin‐releasing peptide (GRP) enhances the high sodium‐induced increase in GLP‐1 secretion in a concentration and time‐dependent manner in L‐cells. The exposure of L‐cells to high (170 mM) Na+ increases GLP‐1 protein (produced by degradation of glucagon) or proglucagon mRNA expression, while Low (90 mM) and high (170 mM) Na+ increase NFAT5 and GRP receptor (GRPR) mRNA and protein expression. Silencing NFAT5 abrogates the GRP‐mediated enhancement of high sodium‐induced increase in GLP‐1 secretion and blocks the sodium‐induced increase in GRPR mRNA and protein expressions in L‐cells. Furthermore, NFAT5 silencing reduces the GLP‐1 protein or proglucagon expression in L‐cells exposed to high sodium concentration. NFAT5 silencing also reduces GRPR expression, using the promoter reporter assay; n=3/group for all studies. These results suggest that the high sodium‐induced NFAT5 expression increases expression of GRPR; GRP acting on the increased GRPR expression increases GLP‐1 secretion from L‐cells. These data give a new perspective on the mechanisms of GLP‐1 secretion, and the direct relationship between sodium and GLP‐1. These may contribute to better utilization of existing GLP‐1 based drugs and further GLP‐1‐based drug development.Effect of NaCl or GRP on GLP‐1 secretion in CCL‐251 L cells (human colorectal adenocarcinoma cells=GLP‐1 secreting cells).The same amount of cells were exposed to 90 mM, 145 mM, or 170 mM NaCl medium, and A, treated with varying concentrations of GRP (10, 100, 500, or 1000 nmol) for 4 h, or B, treated with 1000 nmol GRP at different times (1 h, 2 h, 3 h, or 4 h). GLP‐1 in the cell culture supernatant was measured by ELISA. The results are expressed as ng/ml. *P<0.05 vs. 90 mM with or without GRP. #P<0.05 vs. others, one‐way ANOVA, Holm‐Sidak test, n=3/treatment.Figure 1