Received for publication July 2, 2007.
Revised September 26, 2007.
Accepted for publication October 2, 2007.

Salicylate Blocks Lipolytic Actions of Tumor Necrosis Factor-TNF- in Primary Rat Adipocytes

Abstract

Increased systemic free fatty acids (FFA) impair insulin sensitivity. In obese and diabetic subjects, production of a proinflammatory cytokine, tumor necrosis factor- (TNF-), is elevated. TNF- has a variety of effects by inducing inflammation, decreasing glucose utilization, and also stimulating adipocyte lipolysis to release FFA to plasma. High doses of nonsteroidal anti-inflammatory drug salicylates have been long recognized to lower blood FFA and glucose in humans, although the mechanisms are not fully understood. In this report, we show that sodium salicylate at therapeutic concentrations directly blocks TNF--stimulated lipolysis and therefore inhibits FFA release from primary rat adipocytes. To elucidate the cellular basis of this action, we show that salicylate suppresses TNF--induced extracellular signal-related kinase activation and intracellular cAMP elevation, two early events during the lipolysis response to TNF-. Further, salicylate prevents the downregulation of cyclic-nucleotide phosphodiesterase 3B, an enzyme responsible for cAMP hydrolysis. Perilipins coat intracellular lipid droplet surface by restricting lipase access to the triacylglycerol substrates. TNF- downregulates perilipin but promotes its phosphorylation during lipolysis stimulation; these actions are efficiently reversed by salicylate. Salicylate slightly reduces basal but completely inhibits TNF--liberated lipase activity. In contrast, neither salicylate nor TNF- alters the protein levels of hormone-sensitive lipase and adipose triglyceride lipase. In addition, sodium salicylate restricts basal lipolysis simulated by a high concentration of glucose and significantly diminishes the high glucose-enhanced lipolysis response to TNF-. These results provide novel evidence that salicylate directly blocks TNF--mediated FFA efflux from adipocytes, hence reducing plasma FFA levels and increasing insulin sensitivity.

Key words: Tumor necrosis factor, cAMP, Protein Kinase A, MAP Kinase

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