Selective Effects of Membrane Sterol Depletion on Surface Function Thymidine and 3-O-methyl-D-glucose Transport in a Sterol Auxotroph

Using a sterol-specific auxotroph of LM cells (mouse fibroblasts), we obtained cells with progressively lower sterol contents. 3-O-Methylglucose and thymidine were shown to enter the cells by saturable as well as by nonsaturable processes and it was possible to measure these processes separately at various stages of sterol depletion. Sterol depletion inhibited the saturable as well as the nonsaturable components for both 3-O-methylglucose and thymidine uptake. The extent of inhibition of the saturable process was much more than that of the nonsaturable component at any stage of the sterol depletion. Furthermore, in the course of progressive sterol depletion the saturable component of 3-O-methylglucose or thymidine uptake was inhibited well before the nonsaturable process. Sterol depletion produced reductions in the Vmax values but no change in the Km values for the saturable uptake of these two substrates. The depletion of cellular sterol to 75% resulted in 70% and 90% reduction of the saturable uptake of 3-O-methylglucose and thymidine, respectively, and 30% and 50% reduction of the nonsaturable uptake of 3-O-methylglucose and thymidine, respectively. DNA and protein synthesis, however, were not affected at all when the saturable component of thymidine uptake was inhibited by 50% demonstrating that the observed decrease of transport activity was selective and not a reflection of a gross alteration in cellular physiology.