THYROXINE 5'-DEIODINATION MEDIATES NOREPINEPHRINE-INDUCED LIPOGENESISIN DISPERSED BROWN ADIPOCYTES

In euthyroid rats, maximal sympathetic nervous system stimulation (e.g . during cold exposure) results in a 3- to 4-fold increase in brown ad ipose tissue lipogenesis, a response that is blunted in hypothyroid ra ts. To further investigate this phenomenon, the role of local type II 5'-deiodinase (5'-DII) was studied in freshly isolated brown adipocyte s. In a typical experiment, 1.5 x 10(6) cells were incubated for up to 48 h in a water-saturated 5% CO2-95% O-2 atmosphere. After incubation with medium alone or with different concentrations of T-4, T-3, and/o r norepinephrine (NE), lipogenesis was studied by measuring 1) the rat e of fatty acid synthesis as reflected by (H2O)-H-3 incorporation into lipids and 2) the activity of key rate-limiting enzymes, i.e. acetyl coenzyme A carboxylase and malic enzyme, and the results are reported in terms of DNA content per tube. Lipogenesis decreased progressively over time (similar to 40%) when no additions were made to the incubati on medium. T-4 or T-3 partially prevented that inhibition at physiolog ical concentrations (65 x 10(-9) and 0.77 x 10(-9) ill, respectively), whereas a receptor-saturating concentration of T-3 (154 x 10(-9) M) d oubled the lipogenesis rate. The addition of 10(-6) M NE inhibited lip ogenesis acutely (similar to 50% by 12 h) and was followed by a progre ssive stimulation that reached similar to 2-fold by 48 h, but only in the presence of T-4. Furthermore, NE did not attenuate T-3 (154 x 10(- 9) nz)-induced lipogenesis. Both the inhibition and the stimulation of lipogenesis caused by NE showed a strong dose-response relationship w ithin the range of 10(-11)-10(-5) hi. The role of local 5'-DII was fur ther tested by incubating brown adipocytes with 10(-6) hi NE and T-4 ( 65 x 10(-9) M) in the presence of 100 mu M iopanoic acid, a potent inh ibitor of 5'-DII. Although iopanoic acid did not affect the T-3 stimul ation oflipogenesis, it did block the similar to 2-fold stimulation of lipogenesis triggered by NE in the presence of T-4, confirming the me diation of 5'-DII in this process. In conclusion, lipogenesis in brown adipose tissue is under complex hormonal control, with key roles play ed by NE, thyroid hormones, and local 5'-DII. As in other tissues, NE- generated signals acutely (12 h) inhibited lipogenesis. However, the p resence of the 5'-DII generated enough T-3 to stimulate lipogenesis an d gradually reverse the short-lived NE-induced inhibition, leading to the 2- to 3-fold response observed at later time points.