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. 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- 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 .mu.g/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 .mu.g/ml). Treatment of 3T3-L1 control cells and adipocytes with dibutyryl cyclic[dbc]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 dbcAMP plus the theophylline has relatively little effect on the specific activities of hexokinase or glucose-6-P dehydrogenase or on the protein content of the cultures. Glutamine synthetase activity is hormonally regulated in 3T3-L1 cells.