Modification of serum, pancreatic, and microbial lipase activities by phorbol diesters

The influence of phorbol diesters on the in vitro hydrolysis of diacylglycerols was examined using enzymes from rat serum, porcine pancreas, and Rhizopus delemar. Two main phenomena were observed: 12-O-tetradecanoylphorbol-13-acetate (TPA), when added to the enzyme assay system, stimulated 2- to 3-fold the hydrolysis of [9,10-3H]dioleoylglycerol by serum lipase. The hydrolysis of dioleoylglycerol by either pancreatic or R. delemar lipase was, on the other hand, inhibited by TPA. A 50% inhibition of the pancreatic and R. delemar enzymes was attained with 10 and 2.0 microM TPA, respectively. The pattern of enzyme stimulation (rat serum), with regard to increasing TPA concentrations, was hyperbolic. Stimulation was not influenced by Triton X-100, but it was highly dependent on the structure of the phorbol ester: TPA greater than phorbol didecanoate greater than tetradecanoylphorbol. Phorbol dibutyrate, phorbol acetate, myristic acid, and mezerein were without influence. Lipase activity was inhibited most strongly by TPA and the nonpromoter 4-O-methyl-TPA; the weaker promoter, phorbol dibutyrate, was relatively inactive. The inhibition of R. delemar lipase by TPA was reversible. Collectively, these data show that phorbol diesters can interact with enzymes other than protein kinase C. It is believed, by virtue of their structural similarity to diacylglycerols, that phorbol diesters may serve directly as intracellular regulators of lipid metabolism. In such a manner phorbol esters could sustain or attenuate the second messenger signal by modifying diacylglycerol metabolism, a manifestation of the pleiotropic action.