Reversible independent alterations in glucose transport and metabolism in cultured human cells deprived of glucose

Abstract

We have measured uptake of 3H-hexoses into diploid human cells by exposing them to brief pulses of isotopic sugar during the log-growth, subconfluent-growth, and confluent-growth (contact inhibited) phases of the strain HSWP derived from human skin. 3H-deoxyglucose appears to be taken up three times faster than 3H-glucose. After exposure to 3H-glucose for longer than one minute, the cells excrete approximately 70% of the isotope into the medium as lactate. If lactate production (and hence excretion) is abolished by treating the cellls with iodoacetic acid or dinitrofluorobenzene, neither of which inhibits transport, the uptake of 3H-glucose is found to be in fact somewhat larger than that of 3H-deoxyglucose. If cells are deprived of glucose for 24 hours, apparent uptake of 3H-glucose is enhanced 10-fold or more. This latter increase is accounted for by 2- to 3-fold enhancement of true transport plus retention of greater than 90% of the radioactivity, since little lactate is formed or excreted in glucose-deprived cells. Deoxyglucose, galactose, or pyruvate when present during glucose deprivation each have quantitatively different effects on the cells' capacity to produce lactate from a short pulse of glucose, but none of them prevents the enhancement of hexose transport. After restoration of 5 mM glucose to starved cells, their metabolsim returns to normal (in the sense that approximately 70% of the glucose taken up in a pulse is again excreted as lactate), with a half-time of 0.5 hour; but the transport of hexoses returns to control levels much more slowly, with a half-time of approximately 6 hours. The two processes appear to be independently regulated.