Effects of Combined Glutamine and Serum Deprivation on Glucose Control of Hexose Transport in Mammalian Fibroblast Cultures

Regulation of hexose transport in NIL hamster fibroblasts has been studied in confluent cultures preconditioned for 24 hr in media deprived of glutamine or of serum or of both. Cultures maintained in media containing dialyzed fetal calf serum and 4 mM glutamine accumulated up to 72 nmol of glutamine per mg of cell protein; in contrast, cells deprived of glutamine contained less than 1 nmol/mg of cell protein. Glutamine elicited a general enhancement of hexose transport compared with transport in glutamine-deprived cultures. This enhancement was particularly pronounced in glucose-fed cultures which in the absence of glutamine showed conspicuously low transport activity. When maintained in glucose media, cultures deprived of serum also showed a marked loss of hexose transport which, in this case, was not compensated for by addition of glutamine. However, regardless of the presence or absence of glutamine, these cultures were able to develop the usual transport enhancement response to glucose starvation. Moreover, 2,4-dinitrophenol was also able to elicit a pronounced enhancement of hexose transport in the glucose-fed cultures; this effect surpassed even the transport derepression observed in the glucose-starved cultures. In polyoma-transformed cultures maintained in serum-free media, hexose transport remained relatively high, even in the presence of glucose. However, addition of glutamine brought about an enhancement in both the presence and absence of serum. The various phenomena are discussed in regard to protein turnover in general and more specifically the turnover of hexose transport carriers.