Distinct long-term regulation of glycerol and non-esterified fatty acid release by insulin and TNF-alpha in 3T3-L1 adipocytes

Aims/hypothesis. Adipose tissue lipolysis plays a central part in total bod y fuel metabolism. Our study was to assess the long-term regulation of glyc erol and non-esterified fatty acid (NEFA) release by insulin or TNF-alpha. Methods. Fully differentiated 3T3-L1 adipocytes were exposed for up to 22 h to insulin or TNF-alpha. Results. Long-term insulin treatment resulted in increased basal glycerol r elease, reaching sixfold at 22 h with 1 nmol/l insulin. Partial inhibition was observed by pharmacologically inhibiting phosphatidylinositol 3-kinase or the mitogen-activated kinase kinase - extracellular signal-regulated kin ase cascades, This represented 50-60 % of the response induced by 1 nmol/l TNF-alpha and approximately 40 % of the glycerol release maximally stimulat ed by isoproterenol (1 mu mol/l, 30 min). The cellular mechanism seemed to be distinct from that of TNF-alpha First, glycerol release in response to l ong-term insulin was progressive with time and did not display a lag-time c haracteristic of the effect of TNF-alpha. Second, pretreatment and co-treat ment of the cells with troglitazone greatly inhibited TNF-alpha -induced gl ycerol release (128.5 +/- 10.2 to 35.4 +/- 2.1 nmol/mg protein per h) but n ot the effect of insulin, which was exaggerated. Third, hormone-sensitive l ipase protein content was decreased (45 %) by TNF-alpha but not following l ongterm insulin. Finally, TNF-alpha was associated with NEFA release to the medium, whereas long-term insulin treatment was not. Moreover, glycerol re lease during isoproterenol-stimulated lipolysis was additive to the effect of long-term insulin, whereas NEFA release was inhibited by nearly 90 %. Conclusions interpretation. Contradictory to its shortterm inhibitory effec t, long-term insulin stimulates glycerol release with concomitant stimulati on of NEFA re-esterification.