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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Multiple pathways for cationic amino acid transport in rat thyroid epithelial cell line PC Cl3

¹Laboratory of General Physiology and ²Laboratory of General Pathology, Department of Biological and Environmental Sciences and Technologies, University of Lecce, Lecce; ³Department of Cellular and Molecular Biology and Pathology "L. Califano" and ⁵BioGeM Consortium, Federico II University of Naples, Naples; ⁴Institute of Endocrinology and Experimental Oncology "G. Salvatore," Italian National Research Council, Naples; and ⁶Laboratory of Molecular Genetics, Department of Biological and Environmental Sciences, University of Sannio, Benevento, Italy

Submitted 27 January 2004 ; accepted in final form 10 October 2004

Information regarding cationic amino acid transport systems in thyroid is limited to Northern blot detection of y⁺LAT1 mRNA in the mouse. This study investigated cationic amino acid transport in PC cell line clone 3 (PC Cl3 cells), a thyroid follicular cell line derived from a normal Fisher rat retaining many features of normal differentiated follicular thyroid cells. We provide evidence that in PC Cl3 cells plasmalemmal transport of cationic amino acids is Na⁺ independent and occurs, besides diffusion, with the contribution of high-affinity, carrier-mediated processes. Carrier-mediated transport is via y⁺, y⁺L, and b^0,+ systems, as assessed by L-arginine uptake and kinetics, inhibition of L-arginine transport by N-ethylmaleimide and neutral amino acids, and L-cystine transport studies. y⁺L and y⁺ systems account for the highest transport rate (with y⁺L > y⁺) and b^0,+ for a residual fraction of the transport. Uptake data correlate to expression of the genes encoding for CAT-1, CAT-2B, 4F2hc, y⁺LAT1, y⁺LAT2, rBAT, and b^0,+AT, an expression profile that is also shown by the rat thyroid gland. In PC Cl3 cells cationic amino acid uptake is under TSH and/or cAMP control (with transport increasing with increasing TSH concentration), and upregulation of CAT-1, CAT-2B, 4F2hc/y⁺LAT1, and rBAT/b^0,+AT occurs at the mRNA level under TSH stimulation. Our results provide the first description of an expression pattern of cationic amino acid transport systems in thyroid cells. Furthermore, we provide evidence that extracellular L-arginine is a crucial requirement for normal PC Cl3 cell growth and that long-term L-arginine deprivation negatively influences CAT-2B expression, as it correlates to reduction of CAT-2B mRNA levels.

cationic amino acid transporters; heteromeric amino acid transporters; system y⁺; system y⁺L; system b^0,+; thyrotropin; L-arginine

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