NITRIC-OXIDE MEDIATES DOWN-REGULATION OF LIPOPROTEIN-LIPASE ACTIVITY-INDUCED BY TUMOR-NECROSIS-FACTOR-ALPHA IN BROWN ADIPOCYTES

We previously reported that tumor necrosis factor-alpha (TNF-alpha)/ca chectin suppresses lipoprotein lipase activity and its gene expression in brown adipocytes differentiated in culture. Recent evidence sugges ts that the effect of TNF-alpha over various cells is related to the e nhanced production of nitric oxide (NO). The present study examined wh ether the suppressive effect of TNF-alpha on lipoprotein lipase activi ty is mediated by production of NO in the brown adipocytes. A reverse transcription-polymerase chain reaction (RT-PCR) assay revealed that T NF-alpha caused a concentration-and time-dependent expression of induc ible NO synthase in brown adipocytes. Increasing concentrations of TNF -alpha (0.5-50 ng/ml) for 24 h resulted in a concentration-dependent d ecrease in lipoprotein lipase activity with reciprocal increase in nit rite production in the medium. The suppressive effect of TNF-alpha on lipoprotein lipase activity was significantly prevented by NO synthase inhibitors, N-G-nitro-L-arginine methyl ester (L-NAME) and aminoguani dine, but not by D-NAME, an inactive isomer. Furthermore, 8-bromoguano sine 3',5'-cyclic monophosphate, cell permeant cGMP, suppressed lipopr otein lipase activity and 1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one , a selective inhibitor for soluble guanylate cyclase, restored the TN F-alpha-suppressed lipoprotein lipase activity. These results suggest that TNF-alpha stimulates brown adipocytes to express inducible NO syn thase, followed by production of NO, which in turn mediates the suppre ssive effect of TNF-alpha on lipoprotein lipase activity. The effect o f NO is mediated, at least partly, through production of cGMP. (C) 199 7 Elsevier Science B.V.