Mechanism of the stimulatory action of okadaic acid on lipolysis in rat fat cells

Okadaic acid was found to induce concentration and time-dependent lipolysis in rat fat cells in the absence of Lipolytic hormones, but it did not sign ificantly increase the total hormone-sensitive lipase (HSL) activity in the se fat cells, the activity of HSL extracted from fat layer and that of HSL in the supernatant of homogenized fat cells. Western blotting of fat cell h omogenate fractions with an antiserum raised against synthetic peptide deri ved from rat HSL showed that HSL protein shifted from the supernatant to th e fat layer in response to okadaic acid, which increased the HSL protein co ntent on the fat layer and concomitantly reduced that of the supernatant, c oncentration- and time-dependently. Sonication of the fat cells abolished t heir responsiveness to okadaic acid. The Lipolytic action of okadaic acid w as examined and its site was identified using a cell-free system comprising lipid droplets isolated from rat fat cells and HSL. Okadaic acid induced l ipolysis in this cell-free system and sonication of the lipid droplets caus ed disappearance of lipolytic action of okadaic acid, Okadaic acid failed t o stimulate Lipolysis in a cell-free system comprising HSL and artificial l ipid droplets (trioleoylglycerol emulsified with gum arabic) instead of lip id droplets isolated from rat fat cells. These results suggest that okadaic acid does not increase the catalytic activity of HSL but induces transloca tion of HSL to the lipid droplets isolated from rat fat cells. The site of the lipolytic action of okadaic acid in relation to the interaction between HSL and lipid droplet is discussed.