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 significantly increase the total hormone-sensitive lipase (HSL) activity in these 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 homogenate fractions with an antiserum raised against synthetic peptide derived from rat HSL showed that HSL protein shifted from the supernatant to the fat layer in response to okadaic acid, which increased the HSL protein content on the fat layer and concomitantly reduced that of the supernatant, concentration- and time-dependently. Sonication of the fat cells abolished their responsiveness to okadaic acid. The lipolytic action of okadaic acid was examined and its site was identified using a cell-free system comprising lipid droplets isolated from rat fat cells and HSL. Okadaic acid induced lipolysis in this cell-free system and sonication of the lipid droplets caused disappearance of lipolytic action of okadaic acid. Okadaic acid failed to stimulate lipolysis in a cell-free system comprising HSL and artificial lipid droplets (trioleoylglycerol emulsified with gum arabic) instead of lipid droplets isolated from rat fat cells. These results suggest that okadaic acid does not increase the catalytic activity of HSL but induces translocation 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.