Long Term Endocrine Regulation of Nucleoside Transporters in Rat Intestinal Epithelial Cells

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Abstract
We studied the regulation of nucleoside transporters in intestinal epithelial cells upon exposure to either differentiating or proliferative agents. Rat intestinal epithelial cells (line IEC-6) were incubated in the presence of differentiating (glucocorticoids) or proliferative (EGF and TGF-α) agents. Nucleoside uptake rates and nucleoside transporter protein and mRNA levels were assessed. The signal transduction pathways used by the proliferative stimuli were analyzed. We found that glucocorticoids induce an increase in sodium-dependent, concentrative nucleoside transport rates and in protein and mRNA levels of both rCNT2 and rCNT1, with negligible effects on the equilibrative transporters. EGF and TGF-α induce an increase in the equilibrative transport rate, mostly accounted for by an increase in rENT1 activity and mRNA levels, rENT2 mRNA levels remaining unaltered. This effect is mimicked by another proliferative stimulus that functions as an in vitro model of epithelial wounding. Here, rENT1 activity and mRNA levels are also increased, although the signal transduction pathways used by the two stimuli are different. We concluded that differentiation of rat intestinal epithelial cells is accompanied by increased mature enterocyte features, such as concentrative nucleoside transport (located at the brush border membrane of the enterocyte), thus preparing the cell for its ultimate absorptive function. A proliferative stimulus induces the equilibrative nucleoside activities (mostly through ENT1) known to be located at the basolateral membrane, allowing the uptake of nucleosides from the bloodstream for the increased demands of the proliferating cell.