Hypercortisolemia and glucocorticoid treatment cause elevated level of circulating free fatty acids (FFAs). The basis of this phenomenon has long been linked to the effect of glucocorticoids permitting and enhancing the adipose lipolysis response to various hormones. In this study, we demonstrate that glucocorticoids directly stimulate lipolysis in rat primary adipocytes in a dose- and time-responsive manner; this lipolytic action was attenuated by treatment with the glucocorticoid antagonist RU486. Dexamethasone down-regulates mRNA and protein levels of cyclic-nucleotide phosphodiesterase 3B, thereby elevating cellular cAMP production and activating protein kinase A (PKA). On inhibition of PKA but not other kinases, the lipolysis response ceases. Furthermore, dexamethasone induces phosphorylation and down-regulation of perilipin, a lipid droplet-associating protein that modulates lipolysis; this effect is restored by RU486 or PKA inhibitor H89. Dexamethasone up-regulates mRNA and protein levels of hormone-sensitive lipase (HSL) and adipose triglyceride lipase; these effects, parallel to increased lipolysis, are attenuated by RU486 or actinomycin D. Phosphorylation at Ser-563 and Ser-660 residues of HSL and activity of cellular lipases are elevated on dexamethasone stimulation but abrogated by the coaddition of H89. However, dexamethasone does not induce HSL translocation to the lipid droplet surface in differentiated adipocytes. We show that elevated FFA concentration in plasma is associated with increased lipase activity and lipolysis in vivo in adipose tissues of dexamethasone-treated rats. Therefore, the lipolytic action of glucocorticoids liberates FFA efflux from adipocytes to the bloodstream, which could be a cellular basis of systemic FFA elevation in response to glucocorticoid challenge.
Bacterial endotoxin/lipopolysaccharide elicits inflammatory responses and also elevates circulating levels of free fatty acids (FFAs) and impairs insulin sensitivity. Serum FFA elevation in acute endotoxemia has long been thought to be due to endotoxin dysregulating lipid disposal and counterregulatory hormones and cytokines. Here, we investigated the direct lipolysis effect of endotoxin in rodents and in isolated primary adipocytes. Endotoxin increases lipolysis in vivo in adipose tissues, elevates circulating FFA level, induces insulin resistance in rats, and directly stimulates chronic lipolysis in vitro in adipocytes. The lipolytic action of endotoxin is mediated via its lipid A moiety and is blocked by anti-endotoxin peptides. Neither adipocytokine secretion nor nuclear factor-B activation is involved in endotoxin-induced lipolysis. Different from catecholamine, endotoxin stimulates lipolysis without elevating cAMP production and activating protein kinase A and protein kinase C. Instead, endotoxin induces phosphorylation of Raf-1, MEK1/2, and ERK1/2. Upon inhibition of ERK1/2 but not JNK and p38 MAPK, endotoxin-stimulated lipolysis ceases. Endotoxin causes perilipin down-regulation and phosphorylation and increases the activity and protein levels of hormone-sensitive lipase and adipose triglyceride lipase but does not induce hormonesensitive lipase translocation to intracellular lipid droplets. In TLR4 (Toll-like receptor 4)-deficient mice and adipocytes, endotoxin fails to increase in vivo and in vitro lipolysis. These findings suggest that endotoxin stimulates lipolysis via TLR4 and ERK1/2 signaling in adipocytes. The lipolytic action of endotoxin liberates FFA efflux from adipocytes to the bloodstream, which is a possible basis for systemic FFA elevation and insulin resistance in endotoxemia or Gram-negative bacterial infection.
In patients with type 2 non-insulin-dependent diabetes mellitus (NIDDM), the biguanide, metformin, exerts its antihyperglycemic effect by improving insulin sensitivity, which is associated with decreased level of circulating free fatty acids (FFA). The flux of FFA and glycerol from adipose tissue to the blood stream primarily depends on the lipolysis of triacylglycerols in the adipocytes. Adipocyte lipolysis is physiologically stimulated by catecholamine hormones. Tumor necrosis factor-a (TNF-a), a cytokine largely expressed in adipose tissue, stimulates chronic lipolysis, which may be associated with increased systemic FFA and insulin resistance in obesity and NIDDM. In this study, we examined the role of metformin in inhibiting lipolytic action upon various lipolytic stimulations in primary rat adipocytes. Treatment with metformin attenuated TNF-a-mediated lipolysis by suppressing phosphorylation of extracellular signal-related kinase 1/2 and reversing the downregulation of perilipin protein in TNF-a-stimulated adipocytes. The acute lipolytic response to adrenergic stimulation of isoproterenol was also restricted by metformin. A high concentration of glucose in the adipocyte culture promoted the basal rate of glycerol release and significantly enhanced the lipolytic action stimulated by either TNF-a or isoproterenol. Metformin not only inhibits the basal lipolysis simulated by high glucose, but also suppresses the high glucose-enhanced lipolysis response to TNF-a or isoproterenol. The antilipolytic action in adipocytes could be the mechanism by which cellular action by metformin reduces systemic FFA concentration and thus improves insulin sensitivity in obese patients and the hyperglycemic conditions of NIDDM.
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