Mechanisms of metformin inhibiting lipolytic response to isoproterenol in primary rat adipocytes

Abstract

The mobilization of free fatty acids (FFA) from adipose tissue to the bloodstream primarily depends on triacylglycerol lipolysis in adipocytes. Catecholamines are major hormones that govern lipolysis through elevating cellular cAMP production and activating protein kinase, cAMP dependent, catalytic, alpha (PKA) and mitogen-activated protein kinase 1/2 (MAPK1/3). Obesity and type 2 diabetes are associated with elevated levels of systemic FFA, which restricts glucose utilization and induces insulin resistance. The biguanide metformin exerts its antihyperglycemic effect by enhancing insulin sensitivity, which is associated with decreased levels of circulating FFA. In this study, we examined the characteristics and basis of the inhibitory effect of metformin on adrenergic-stimulated lipolysis in primary rat adipocytes. We measured the release of FFA and glycerol as an index of lipolysis and examined the major signalings of the lipolytic cascade in primary rat adipocytes. Metformin at 250-500 mM efficiently attenuated FFA and glycerol release from the adipocytes stimulated with 1 mM isoproterenol. To elucidate the basis for this antilipolytic action, we showed that metformin decreased cellular cAMP production, reduced the activities of PKA and MAPK1/3, and attenuated the phosphorylation of perilipin during isoproterenol-stimulated lipolysis. Further, metformin suppressed isoproterenolpromoted lipase activity but did not affect the translocation of lipase, hormone-sensitive from the cytosol to lipid droplets in adipocytes. This study provides evidence that metformin acts on adipocytes to suppress the lipolysis response to catecholamine. This antilipolytic effect could be a cellular basis for metformin decreasing plasma FFA levels and improving insulin sensitivity.

Lipolysis Response to Endoplasmic Reticulum Stress in Adipose Cells

“…AMPK was proposed to activate lipolysis (43), but several studies debated that AMPK has anti-lipolytic activity (44,45). The latter view is supported by our previous work that antidiabetic biguanide metformin, which induces AMPK (44), can inhibit the lipolysis action of isoproterenol and TNF-␣ in primary rat adipocytes (46,47). Thus, by preferring an antilipolytic action of AMPK, we speculate that AMPK inactivation of H89, although significant, may not account for the lipolytic inhibition of H89 in ER-stressed adipocytes.…”

Biguanides and thiazolidinediones inhibit stimulated lipolysis in human adipocytes through activation of AMP-activated protein kinase

“…Human adipocytes were preincubated for 3 h in the absence or presence of triacsin C (10 μmol/l) before addition of 1 μmol/l isoprenaline or 100 nmol/l ANP for 1 h. Total lysates were analysed by western blotting for phosphorylation of AMPK Thr172 or ACC Ser80 and total ACC and AMPKα 1 content (b). These blots are representative of two independent experiments results showing an anti-lipolytic effect of both metformin and the related biguanide phenformin in rodent adipocytes [8,40], attributed for the latter drug to an activation of AMPK. Moreover, in humans, a perfusion of metformin in microdialysis experiments has an anti-lipolytic effect [22,41].…”

Suppression of adipose lipolysis by long-chain fatty acid analogs

“…As MEDICA analogs simulate the characteristics of LCFA in terms of AMPK activation ( 12 ) and in inducing UPR ( 27,28 ), we may assume that the mode of action proposed for MEDICA may account, at least partially, for inhibition of agonist-induced lipolysis by LCFA ( 4,5 ). In light of AMPK activation by metformin or octanoate ( 31,32 ), the proposed transduction pathway may also offer a mode of action for the recently reported activity of metformin ( 31 ) or octanoate ( 32 ) in suppressing isoproterenol-induced lipolysis in primary rat adipocytes or 3T3-L1 adipocytes, respectively.…”