Aims/hypothesis: Adipose tissue lipolysis plays a central part in total body fuel metabolism. Our study was to assess the long-term regulation of glycerol 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 release, 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 kinase cascades. This represented 50-60% of the response induced by 1 nmol/l TNF-alpha and approximately 40 % of the glycerol release maximally stimulated by isoproterenol (1 micromol/l, 30 min). The cellular mechanism seemed to be distinct from that of TNF-alpha: First, glycerol release in response to long-term insulin was progressive with time and did not display a lag-time characteristic of the effect of TNF-alpha. Second, pretreatment and co-treatment of the cells with troglitazone greatly inhibited TNF-alpha-induced glycerol release (128.5 +/- 10.2 to 35.4 +/- 2.1 nmol/mg protein per h) but not the effect of insulin, which was exaggerated. Third, hormone-sensitive lipase protein content was decreased (45 %) by TNF-alpha but not following long-term insulin. Finally, TNF-alpha was associated with NEFA release to the medium, whereas long-term insulin treatment was not. Moreover, glycerol release 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 short-term inhibitory effect, long-term insulin stimulates glycerol release with concomitant stimulation of NEFA re-esterification.