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Antilipolytic actions of vanadate and insulin in rat adipocytes mediated by distinctly different mechanisms
1Department of Biochemistry, The Weizmann Institute of Science, Rehovot, Israel.
Abstract:
Vanadate, which mimics the biological effects of insulin, also inhibits lipolysis in rat adipocytes. Here we demonstrate that the antilipolytic effect of vanadate differs from that of insulin at least by the five following criteria: 1) vanadate inhibits lipolysis mediated by high (supraphysiological) concentrations of catecholamines; 2) vanadate antagonizes (Bu)2cAMP-mediated lipolysis; 3) vanadate antagonizes isobutylmethylxanthine-dependent lipolysis, 4) vanadate inhibits lipolysis mediated by okadaic acid; and 5) wortmannin, which blocks the antilipolytic effect of insulin, fails to block vanadate-mediated antilipolysis. Vanadate does activate phosphoinositol 3-kinase, and wortmannin blocks this activation. Our working hypothesis assumes that all of the insulin-like effects of vanadate, including antilipolysis, are initiated by the inhibition of protein phosphotyrosine phosphatases (PTPases). Among documented PTPase inhibitors we found that VOSO4 (oxidation state +4), several organic vanadyl compounds (+4), zinc (Zn2+), tungstate (W), and molybdate (Mo) also had antilipolytic activity. The order of potency was vanadyl acetylacetonate > or = VOSO4 > or = NaVO3 > or = vanadyl-dipicolinate > Zn2+ >> W > Mo, and it correlated better with the inhibition of adipose membranal-PTPases in cell-free experiments. We have concluded that the antilipolytic effect of vanadate is 1) mechanistically distinct from that of insulin, 2) independent of phosphoinositol 3-kinase activation, and 3) independent of the lipolytic cascade. We also strongly suggest that the antilipolytic effect of vanadate emanates from inhibiting adipose membranal, rather than cytosolic PTPases, and present preliminary data showing distinct differences in catalysis between these two PTPase categories. Overall, the study indicates that antilipolysis can be manifested via alternative, insulin-independent, signal-transducing pathways.
Insights
Vanadate inhibits fat breakdown (lipolysis) differently than insulin, acting independently of key signaling pathways. This suggests alternative, insulin-independent routes for controlling lipolysis.
Area of Science:
- Biochemistry
- Cellular Biology
- Metabolic Regulation
Background:
- Vanadate exhibits insulin-like effects, including the inhibition of lipolysis in rat adipocytes.
- The precise mechanisms by which vanadate inhibits lipolysis and its distinction from insulin's action require elucidation.
Purpose of the Study:
- To delineate the mechanistic differences between vanadate- and insulin-mediated antilipolysis.
- To investigate the role of phosphoinositol 3-kinase (PI3K) and the lipolytic cascade in vanadate's antilipolytic effect.
- To explore the involvement of protein phosphotyrosine phosphatases (PTPases) in vanadate's action.
Main Methods:
- Assessed vanadate's antilipolytic activity against various lipolytic stimuli, including catecholamines, (Bu)2cAMP, and isobutylmethylxanthine.
- Investigated the effect of wortmannin, a PI3K inhibitor, on vanadate-mediated antilipolysis.
- Examined the antilipolytic potential of other PTPase inhibitors, such as VOSO4, zinc, tungstate, and molybdate.
- Performed cell-free experiments to assess the inhibition of adipose membranal and cytosolic PTPases.
Main Results:
- Vanadate inhibited lipolysis induced by high catecholamine concentrations, (Bu)2cAMP, isobutylmethylxanthine, and okadaic acid, distinguishing it from insulin.
- Wortmannin blocked insulin's antilipolytic effect but not vanadate's, indicating PI3K-independent action of vanadate.
- Several PTPase inhibitors, notably vanadyl compounds and zinc, demonstrated antilipolytic activity, with potency correlating to PTPase inhibition.
Conclusions:
- Vanadate's antilipolytic effect is mechanistically distinct from insulin's, independent of PI3K activation and the lipolytic cascade.
- Vanadate likely exerts its antilipolytic effect by inhibiting adipose membranal PTPases.
- Antilipolysis can be achieved through alternative, insulin-independent signaling pathways.