Glycerol Uptake in HCT-15 Human Colon Cancer Cell Line by Na⁺-Dependent Carrier-Mediated Transport

Abstract

It has recently been suggested that an Na⁺-dependent carrier-mediated transport system is involved in intestinal glycerol absorption. Such a transport system is of general interest as a possible pathway of drug delivery and a target of drug development. However, the Na⁺-dependent mechanism of cellular glycerol uptake has not been fully clarified in the small intestine or in any other organ. The purpose of the present study was to examine glycerol uptake in the HCT-15 human colon cancer cell line, which was found to be able to perform Na⁺-dependent glycerol uptake, to determine the transport characteristics and help identify such glycerol transport systems. The uptake of glycerol in HCT-15 cells was highly saturable with a Michaelis constant of 15.0 μM and a maximum uptake rate of 11.9 pmol/min/mg protein, accompanied by minimal unsaturable transport; it was reduced markedly under Na⁺-free conditions, indicating Na⁺ requirement. Glycerol uptake was also reduced by 2,4-dinitrophenol, a metabolic inhibitor. These results suggest that a carrier-mediated glycerol transport system, which is Na⁺-dependent and secondarily active, is present in HCT-15 cells. The transport system could be specific for glycerol and some analogous compounds with hydroxyl groups, since glycerol uptake was inhibited by some alcohols and compounds related to glycerol, such as 1,2-propanediol and glycerol 3-phosphpate. However, it may represent a high affinity transport system, which is different from the one in the small intestine, because the Michaelis constant of 15.0 μM is about 50-fold lower than that observed in the rat small intestine. In conclusion, this is the first study to demonstrate an Na⁺-dependent carrier-mediated glycerol transport in an established cell line. This will help in identifying a group of Na⁺-dependent glycerol transport systems and elucidating their transport mechanisms, although the one found in HCT-15 cells in this study seems to be different from one previously found in the rat small intestine.