REGULATION OF SYSTEM Y(+) ARGININE TRANSPORT CAPACITY IN DIFFERENTIATING HUMAN INTESTINAL CACO-2 CELLS

This study describes the ability of passaged human intestinal Caco-2 cells to regulate transport of L-arginine via system y(+). Subconfluent and confluent cells possessed system y(+) activity, as determined by the sodium independence of uptake and the pattern of inhibition by amino acid analogues or N-ethylmaleimide. Initial rates of arginine uptake via system y(+) decreased as the cells advanced from the undifferentiated to the differentiated state following culture passaging. Furthermore, kinetic analysis of the leucine-insensitive portion of uptake indicated that the Caco-2 system y(+) transport capacity decreased with cell age, dropping from a maximal velocity (V-max) = 1,094 pmol . mg(-1). min(-1) [Michaelis constant(K-m) = 41 mu M] in undifferentiated cells 2 days postseeding to V-max = 320 pmol . mg(-1). min(-1) (K, = 37 mu M) in confluent cells 9 days postseeding (from cells of the same passage). Northern analysis indicated that the levels of a single 7.9-kb mCAT-1 mRNA species were relatively constant over the course of Caco-2 differentiation and therefore were unsynchronized with the system y(+) relative transport activities. It is concluded that the Caco-2 capacity to transport arginine via system y(+) may be downregulated by posttransitional modifications in confluent cells compared with newly passaged undifferentiated cells. These data serve as a well-defined in vitro model for further studies regarding regulation of arginine transport in epithelial cells.