MECHANISM OF DIPEPTIDE STIMULATION OF ITS OWN TRANSPORT IN A HUMAN INTESTINAL-CELL LINE

The initial objective of this study was to investigate whether the pre sence of dipeptide in the culture medium stimulates the uptake of dipe ptide by a human intestinal cell line that expresses the oligopeptide transporter (Pept-1). The results showed that addition of glycylsarcos ine (Gly-Sar) for 24 hr to the culture medium significantly increased the rate of glycylglutamine (Gly-Gln) uptake by Caco-2 cells. Furtherm ore, this stimulation in transport was also observed when Cefadroxil ( beta-lactarn antibiotic) instead of Gly-Gln was used as a probe but di d not occur when Gly-Sar was added to the culture medium for only 2 hr or when Gly-Sar was substituted by a corresponding mixture of glycine plus sarcosine. The subsequent objective of the study was to investig ate the mechanism of stimulation in transport described earlier. The r esults showed that the addition of Gly-Sar for 24 hr to the culture me dium: (1) increased the V-max of Gly-Gln transport by two-fold without affecting its K-m, (2) increased the protein mass of Pept-1 by more t han two-fold, (3) increased the abundance of Pept-1 mRNA by three-fold , and (4) had no effect on Gly-Gln transport when an inhibitor of tran s-Golgi network (brefeldin) was added to the culture medium, but still increased the abundance of Pept-1 mRNA. In conclusion, the results. s how that dipeptides stimulate their own transport by increasing the me mbrane population of Pept-1. The molecular mechanism appears to be an increase in expression of the gene encoding Pept-1. A therapeutic appl ication of the present results is that if bioavailability of orally ad ministered peptidomimetic drugs is limited, patients may be tried on a high protein diet to enhance their absorption.