HEPATIC GLUTAMINASE GENE-EXPRESSION IN THE TUMOR-BEARING RAT

Previous studies have documented an increase in hepatic plasma membran e glutamine transport in the tumor-bearing rat, but the effects of tum or burden on hepatic glutaminase expression have not been carefully st udied. The purpose of this study was to examine the effects of tumor b urden and food intake on hepatic glutaminase expression. Rats were imp lanted with syngeneic methylcholanthrene-induced fibrosarcoma tumor ti ssue; control rats were sham operated and pair-fed every 24 hr. Northe rn blotting was used to assay the effect of tumor burden and fasting o n hepatic glutaminase mRNA levels, using beta-actin mRNA as a control. Hepatic glutaminase mRNA levels in livers of pair-fed controls were f ound to be 4-fold greater than levels in livers of tumor-bearing anima ls. Examination of food intake patterns in these animals indicated tha t pair-fed controls ate their allotted chow quickly while tumor-bearin g rats ate small amounts throughout each 24 hr period. This observatio n suggested that the differences in glutaminase mRNA levels may be due to a period of fasting by pair-fed animals which was not experienced by the tumor-bearing group. Hepatic glutaminase mRNA levels rapidly in creased in normal rats during acute fasting to levels 5.5-fold greater than fed animals. Glucose feeding and insulin injection rapidly rever sed the effect of fasting on hepatic glutaminase mRNA levels in normal rats. Tumor-bearing rats also exhibited upregulation of hepatic gluta minase mRNA levels in response to fasting. Conclusions: (1) Tumor burd en itself does not alter hepatic glutaminase expression, at least at t he pre-translational level. Instead, differences in hepatic glutaminas e mRNA content are due to differences in food intake patterns. (2) Hep atic glutaminase mRNA levels are rapidly upregulated in response to fa sting, an effect which appears to be linked to a decrease in plasma in sulin concentrations. Because tumor-bearing rats eat regularly over a 24 hr period (albeit in small increments), thereby maintaining the pla sma insulin concentration, hepatic glutaminase mRNA may not rise as it does in pair-fed controls whose daily chow intake is complete within hours of food allocation. (3) This study indicates that differences in the timing of food intake between tumor-bearing rats and pair-fed con trols can alter the expression of genes that are influenced by nutrien t availability. These differences should be taken into account when de signing studies which involve pair-feeding to control nutrient intake. (C) 1997 Academic Press.