Long Term Phorbol Ester Treatment Down-regulates the β3-Adrenergic Receptor in 3T3-F442A Adipocytes

The role of protein kinase C (PKC) in the regulation of the beta 3-adrenergic receptor (beta 3-AR) gene was examined in murine 3T3-F442A adipocytes, which express this receptor subtype at a high level. We also investigated the involvement of this kinase in the modulation of beta 3-AR gene expression by insulin. Long term exposure of 3T3-F442A adipocytes to phorbol 12-myristate 13-acetate (PMA) decreased beta 3-AR mRNA content in a time- and concentration-dependent manner, with maximal changes observed at 6 h (6.5-fold decrease) and at 100 nM PMA. This inhibition was selective for beta 3-AR transcripts, since beta 1- and beta 2-AR mRNA content remained unchanged. Also, (-)-[125I]cyanopindolol saturation and competition binding experiments on adipocyte membranes indicated that PMA induced an approximately 2-fold decrease in beta 3-AR expression, while that of the two other subtypes was not affected. This correlated with a lower efficacy of beta 3-AR agonists to stimulate adenylyl cyclase. Conversely, long term exposure to PMA did not alter adenylyl cyclase activity in response to guanosine 5'-O-(3-thiotriphosphate) or forskolin. The inactive phorbol ester 4 alpha-phorbol 12,13-didecanoate did not repress beta 3-AR mRNA levels. Inhibition of beta 3-AR mRNA by PMA was suppressed by the PKC-selective inhibitor bisindolylmaleimide, and was not observed in PKC-depleted cells, indicating that PKC was involved in this response. mRNA turnover experiments showed that the half-life of beta 3-AR transcripts was not affected by long term PMA exposure. When 3T3-F442A adipocytes were pretreated with PMA for 24 h to down-regulate PKC, or with bisindolylmaleimide, the insulin-induced inhibition of beta 3-AR mRNA levels was reduced by 44-67%. These findings demonstrate that sustained PKC activation exerts a specific control of beta 3-AR gene expression and is involved, at least in part, in the modulation by insulin of this adrenergic receptor subtype.