Is glucose transport enhanced in virus‐transformed mammalian cells? A dissenting view

Much of the literature on the uptake of glucose by untransformed and transformed animal cells is based on experiments carried out with 2‐deoxy‐D‐glucose (2‐DOG). Results obtained with this analog can be ambiguous, since 2‐DOG can be phosphorylated by hexokinases of animal cells. An intracellular trapping mechanism is thus provided. Therefore, the total flux of 2‐DOG into the cell is a resultant of both transport and hexokinase action, and the measurement of total 2‐DOG incorporation is a valid measurement of transport only if 2‐DOG is phosphorylated as rapidly as it enters the cell. Evidence is presented here that this is not necessarily the case; significant levels of free intracellular 2‐DOG approaching external concentrations were found in untransformed and transformed mouse 3T3 cells even at early times during uptake. Differences in total intracellular 2‐DOG between untransformed and transformed cells were accounted for entirely by 2‐deoxyglucose phosphate. Thus, it appears the apparent increase of 2‐DOG uptake accompanying transformation in these cell lines is not due to an effect on the transport process, but on enhanced phosphorylation, which is a reflection of an alteration in the regulation of glycolysis. The ambiguity introduced by phosphorylation can be obviated by the use of an analog that cannot be phosphorylated, such as 3‐O‐methyl‐D‐glucose. The rate of transport and efflux of this sugar was not found to be different in untransformed versus transformed 3T3 cells. Moreover, deficiencies of this analog as a substrate for the glucose transport system are pointed out.