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

Abstract

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 α-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 μM) or nitrendipine (1 μM), 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.