l-Proline transport into renal OK epithelial cells: a second renal proline transport system is induced by amino acid deprivation |
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Authors: | A. Nickel U. Klein D. Weitz H. Daniel |
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Affiliation: | (1) Molecular Nutrition Unit, Technical University of Munich, Am Forum 5, 85350 Freising-Weihenstephan, Germany; |
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Abstract: | Influx of [3H]-l-proline into renal OK cells revealed that basal transport was mediated by the transporter SIT1. When cells were submitted for 8 h to amino acid deprivation, uptake of l-proline was now dominated by a low-affinity system with an apparent K m of 4.4 ± 0.6 mM and a V max of 10.2 ± 0.6 nmol/mg of protein/min operating in addition to the high-affinity SIT1 system with a K m of 0.12 ± 0.01 mM and a V max of 0.28 ± 0.04 nmol/mg of protein/min. The low- and high-affinity proline transporting systems were sensitive to inhibitors of JNK and PI-3 kinases, whereas a GSK-3 inhibitor affected only the upregulated transport system. Ion-replacement studies and experiments assessing substrate specificities for both systems provided strong evidence that SNAT2, that showed two- to threefold increased mRNA levels, is the responsible transporter mediating the increased proline influx under conditions of amino acid deprivation. |
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