Two Distinct Types of Enhancement of Galactose Uptake into Hamster Cells: Tumor-Virus Transformation and Hexose Starvation

Abstract

Enhancement of hexose uptake seems well correlated with transformation of cell cultures by tumor viruses and the absence of contact inhibition. Enhancement of sugar uptake has also been observed as a result of hexose starvation. Both types of enhancement can clearly be demonstrated in cultures of hamster cells when uptake of (14)C-labeled galactose is monitored after 10 or 20 min. The profiles of accumulation products are strikingly different. In cultures of hamster NIL cells transformed with polyoma virus much of the (14)C is accumulated as UDPhexose. Untransformed cells accumulate galactose-l-phosphate as well as UDPhexose. Hexose-starved cells show enhanced uptake of galactose; however, this marked enhancement was only observed in NIL cultures close to contact inhibition. The novel and common feature seen in hexose-starved cells when incubated briefly with (14)C-labeled galactose is the occurrence of a marked accumulation of [(14)C]UDPglucuronic acid at the expense of UDPhexose. The ratio [(14)C]UDPglucuronic acid/UDPhexose in cultures fed glucose or galactose was invariably low (0.15-0.2) regardless of the presence or absence of contact inhibition. 20 hr of hexose starvation invariably changed this ratio by a factor of 10 or more, due to accumulation of UDPglucuronic acid. This result was also observed in cultures transformed with polyoma virus. The presence of 3-O-methylglucose in the growth medium did not alter the typical "sugar starvation pattern" (i.e., the UDPglucuronic acid/UDPhexose ratio averaged 1.7). Enhancement of galactose uptake by hexose starvation was very pronounced in NIL cultures that were close to contact inhibition, but was not a prominent feature in the polyoma-transformed cultures. The transformed cells grown on glucose or galactose growth medium showed the usual enhanced rate of uptake of galactose as compared with nontransformed near-confluent cultures that had been fed hexose. The polyoma-induced enhancement showed none of the features characteristic of hexosestarved cells.