Protein kinase C attenuates β-adrenergic receptor-mediated lipolysis, probably through inhibition of the β1-adrenergic receptor system

Lipolysis in rat white adipocytes is stimulated by β-adrenergic agonists. Phorbol 12-myristate 13-acetate (PMA) attenuated the receptor-mediated lipolysis by causing a shift of the dose–response curve to the higher concentrations of norepinephrine and isoproterenol. Although the adipocytes possess β1-, β2-, and β3-adrenergic receptor subtypes, the effect of PMA was observed only when a β1-agonist (dobutamine) was used. No lipolysis-attenuating effect of PMA was found when cells were exposed to a β2-agonist (procaterol) and β3-agonists (BRL 37344 and CL 316243), or to forskolin and 8-bromo cAMP. CGP 20712A (β1-antagonist) efficiently inhibited lipolysis by norepinephrine, isoproterenol, and dobutamine, but did not affect lipolysis by the β2- and β3-agonists. ICI 118551 (β2-antagonist) had no significant effect on lipolysis by the β-agonists examined. CGP 20712A abolished the lipolysis-attenuating effect of PMA, but ICI 118551 did not. The protein kinase C (PKC) inhibitors, GF 109203X or Gö 6976, suppressed the effect of PMA. Pretreatment of adipocytes with PMA for 6 h caused downregulation of conventional and novel PKCs in association with a decrease in the lipolysis-attenuating effect of PMA. These results indicate that conventional and novel PKCs attenuate lipolysis mediated by β-adrenergic receptors, probably through inhibition of the β1-adrenergic receptor system.