The role of calmodulin in hormone-stimulated lipolysis

To explore the role of calmodulin (CaM) in lipolysis, studies were carried out on effects of CaM inhibitors on hormone-stimulated lipolysis, the activity of cAMP-dependent protein kinase, and phosphorylation of endogenous substrate proteins. When adipocytes were incubated with trifluoperazine (TFP) and W-7 but not with W-5, stimulation of lipolysis by epinephrine was blunted. W-7 also inhibited lipolysis induced by ACTH, 1-methyl-3-isobutylxanthine (MIX) or (Bu)2 cAMP. The binding of 3H-cAMP to its receptor protein (the regulatory subunit of protein kinase) as well as the activity of cAMP-dependent protein kinase was suppressed by W-7, and the anti-CaM antibody, but not by W-5. The CaM-dependence of the protein kinase was also proved by the fact that the protein kinase activity that was markedly reduced in CaM-depleted cell extracts, was significantly restored by addition of exogenous CaM to them. Furthermore, W-7 decreased cAMP-stimulated phosphorylation of endogenous substrate proteins (mol wt 230k, 200k, 130k, 85k, 75k, and 50kdalton), among which the one of 85kdalton is most likely to be the hormone-sensitive lipase. These findings suggest that CaM is involved in the mechanism of hormone-induced lipolysis by exerting stimulatory effects on the activation of cAMP-dependent protein kinase in cell extracts capable of phosphorylating substrate proteins including hormone-sensitive lipase.