Amino acid sensing by enteroendocrine STC-1 cells: role of the Na+-coupled neutral amino acid transporter 2

Young SH, Rey O, Sternini C, Rozengurt E. Amino acid sensing by enteroendocrine STC-1 cells: role of the Na+-coupled neutral amino acid transporter 2. Am J Physiol Cell Physiol 298: C1401-C1413, 2010. First published March 10, 2010; doi:10.1152/ajpcell.00518.2009.-The results presented here show that STC-1 cells, a model of intestinal endocrine cells, respond to a broad range of amino acids, including L-proline, L-serine, L-alanine, L-methionine, L-glycine, L-histidine, and alpha-methyl-amino-isobutyric acid (MeAIB) with a rapid increase in the intracellular Ca2+ concentration ([Ca2+](i)). We sought to identify the mechanism by which amino acids induce Ca2+ signaling in these cells. Several lines of evidence suggest that amino acid transport through the Na+-coupled neutral amino acid transporter 2 (SNAT2) is a major mechanism by which amino acids induced Ca2+ signaling in STC-1 cells: 1) the amino acid efficacy profile for inducing Ca2+ signaling in STC-1 cells closely matches the amino acid specificity of SNAT2; 2) amino acid-induced Ca2+ signaling in STC-1 cells was suppressed by removing Na+ from the medium; 3) the nonmetabolized synthetic substrate of amino acid transport MeAIB produced a marked increase in [Ca2+](i); 4) transfection of small interfering RNA targeting SNAT2 produced a marked decrease in Ca2+ signaling in response to L-proline in STC-1 cells; 5) amino acid-induced increase in [Ca2+](i) was associated with membrane depolarization and mediated by Ca2+ influx, since it depended on extracellular Ca2+; 6) the increase in [Ca2+](i) in response to L-proline, L-alanine, or MeAIB was abrogated by either nifedipine (1-10 mu M) or nitrendipine (1 mu M), which block L-type voltage-sensitive Ca2+ channels. We hypothesize that the inward current of Na+ associated with the function of SNAT2 leads to membrane depolarization and activation of voltage-sensitive Ca2+ channels that mediate Ca2+ influx, thereby leading to an increase in the [Ca2+](i) in enteroendocrine STC-1 cells.