Regulation of glutamine synthetase in cultured 3T3-L1 cells by insulin, hydrocortisone, and dibutyryl cyclic AMP

Abstract

The 3T3-L1 mouse fibroblast cell line develops morphological and biochemical characteristics of adipocytes when maintained at confluence. This conversion to adipocytes is accelerated by addition of insulin to the culture medium [Green, H. & Kehinde, O. (1975) Cell 5, 19-27]. During the course of the insulin-mediated adipocyte conversion, the specific activity (units/mg of protein) of glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] increases more than 100-fold. The specific activities of hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) and glucose-6-P dehydrogenase (D-glucose-6-phosphate:NADP(+) 1-oxidoreductase, EC 1.1.1.49) also increase but less dramatically (1.5- to 3-fold). In contrast, confluent cells maintained in the absence of insulin for the same time (12-20 days after confluence) display only minimal increases in the activity of these enzymes. Maintenance of confluent cells in culture medium lacking added L-glutamine has little, if any, effect on glutamine synthetase activity in either control or insulin-treated cultures. Treatment of confluent 3T3-L1 cultures with hydrocortisone (1 mug/ml) for 3 days prior to harvesting results in an increase in glutamine synthetase specific activity of 12-fold for control cultures maintained for 13 days in the absence of insulin and 1.4-fold for adipocyte cultures maintained for 13 days in the presence of insulin (10 mug/ml). Treatment of 3T3-L1 control cells and adipocytes with dibutyryl cyclic AMP (1 mM) plus theophylline (1 mM) decreases the glutamine synthetase specific activity and almost completely reverses the insulin- and hydrocortisone-mediated increases in enzyme activity. In contrast, treatment with dibutyryl cyclic AMP plus theophylline has relatively little effect on the specific activities of hexokinase or glucose-6-P dehydrogenase or on the protein content of the cultures. These data indicate that glutamine synthetase activity is hormonally regulated in 3T3-L1 cells.