Comparison of trophic effects of norepinephrine, insulin, and IGF-1 in mouse brown adipocytes

The objective of this work was to evaluate whether insulin, like norepinephrine (NE), exerts direct growth effects in brown adipocytes, as assessed by changes in rates of protein labeling with [35S]methionine. Mouse brown adipocytes isolated by tissue collagenase digestion were incubated for up to 24 h with or without NE in Dulbecco's modified Eagle's medium with albumin, calf serum, and antibiotics. There was a 40% cell loss and a 50% decrease in cell content of succinate dehydrogenase (SDH) activity over 24 h. Both cell recovery and SDH content significantly improved in the presence of NE. In addition, NE increased [35S]methionine incorporation into proteins in both cytosolic and mitochondrial compartments. These effects of NE were inhibited by propranolol. Both insulin and insulin-like growth factor-1 (IGF-1) receptors were detected in brown adipocytes, with insulin receptors in much greater concentration. Increased protein labeling was observed when brown adipocytes were incubated for 4 h with 0.2-5 nM insulin in the absence of serum. This effect was small (30% stimulation) compared with the 200-350% increase observed with NE, and 5 nM IGF-1 had no effect. These results indicate direct trophic actions of both NE and insulin in mouse brown adipocytes, with the effects of NE an order of magnitude greater than those of insulin.