Rapid and reversible uncoupling of the hepatic alpha 1-adrenergic receptor and guanine-nucleotide-binding protein by phorbol 12,13-dibutyrate

Alpha 1-adrenoceptors: function and phosphorylation

This review focuses on alpha(1)-adrenoceptor phosphorylation and function. Most of what is currently known is based on studies on the hamster alpha(1B)-adrenoceptor. It is known that agonist stimulation leads to homologous desensitization of these receptors and current evidence indicates that such decrease in receptor activity is associated with receptor phosphorylation. Such receptor phosphorylation seems to involve G protein-receptor kinases and the receptor phosphorylation sites have been located in the carboxyl tail (Ser(404), Ser(408), and Ser(410)). There is also evidence showing that in addition to desensitization, receptor phosphorylation is associated with internalization and roles of beta-arrestins have been observed. Direct activation of protein kinase C leads to receptor desensitization/internalization associated with phosphorylation; the protein-kinase-C-catalyzed receptor phosphorylation sites have been also located in the carboxyl tail (Ser(394) and Ser(400)). Activation of G(q)-coupled receptors, such as the endothelin ET(A) receptor induces alpha(1B)-adrenoceptor phosphorylation and desensitization. Such effect involves protein kinase C and a yet unidentified tyrosine kinase. Activation of G(i)-coupled receptors, such as the lysophosphatidic acid receptor, also induces alpha(1B)-adrenoceptor phosphorylation and desensitization. These effects involve protein kinase C and phosphatidyl inositol 3-kinase. Interestingly, activation of epidermal growth factor receptors also induces alpha(1B)-adrenoceptor phosphorylation and desensitization involving protein kinase C and phosphatidyl inositol 3-kinase. A pivotal role of these kinases in heterologous desensitization is evidenced.

Effects of PKC activation and receptor desensitization on neurosteroid modulation of GABA(A) receptors

The effect of calcium-phospholipid-dependent protein kinase (PKC) activation on neurosteroid modulation of the GABA(A) receptor was examined in Xenopus oocytes expressing human recombinant alpha1beta2gamma2L GABA(A) receptors. GABA-gated chloride currents were measured using the two-electrode voltage-clamp technique. The peak amplitude of GABA-gated chloride currents was reduced by the PKC activator phorbol 12-myristate 13-acetate (PMA), but not by the inactive analog phorbol 12-mono-myristate (PMM). This effect of PMA was inhibited by the protein kinase inhibitor staurosporine. To investigate whether the activation of PKC could alter neurosteroid modulation of the GABA(A) receptor, the effect of PMA was studied on the positive allosteric modulatory steroid 3alpha,21-dihydroxy-5alpha-pregnan-20-one (THDOC) and the negative modulatory neurosteroid pregnenolone sulfate (PS). THDOC potentiation of GABA-gated chloride currents was found to be increased by approximately 120% following PMA treatment, while PS inhibition was not affected. The increase in THDOC potentiation by PMA was blocked by staurosporine. No change in THDOC potentiation was observed following PMM treatment. The enhancement of THDOC potentiation following PMA treatment was not due to a shift in the GABA EC50. In addition to inhibiting the peak amplitude of the GABA response, PMA treatment resulted in non-desensitizing GABA responses. Similarly, GABA responses of receptors which had been desensitized with prolonged GABA application also showed a reduction in peak amplitude and reduced desensitization. THDOC potentiation of desensitized receptors was enhanced approximately 70% with respect to non-desensitized receptors. The present results demonstrate that protein phosphorylation and receptor desensitization alter modulation of the GABA(A) receptor complex by some neurosteroids.

Regulation of hepatocyte plasma membrane alpha 1-adrenergic receptors by 4 beta-phorbol 12-myristate 13-acetate

The effect of phorbol 12-myristate 13-acetate (PMA) on hepatocyte alpha 1-adrenergic receptors was determined by [3H]prazosin binding to plasma membranes from control and PMA-treated hepatocytes. Membranes from hepatocytes incubated with PMA (1 microgram/ml) for 1 h exhibited a 40% decrease in alpha 1-adrenergic receptors (481 +/- 10 fmol/mg of protein; mean +/- S.E.M. for three separate experiments) relative to vehicle-treated (dimethylformamide) hepatocytes (802 +/- 91 fmol/mg of protein; n = 3), with no significant effect on the KD. The PMA-induced decrease in alpha 1-adrenergic receptors was maximal by 30 min and half-maximal inhibition of [3H]prazosin binding occurred with a PMA concentration of approx. 15 ng/ml. Pretreatment of hepatocytes with staurosporine (5 microM) blocked the effect of PMA, and 4 beta-phorbol 13-monoacetate was ineffective, suggesting the involvement of protein kinase C (PKC). Treatment of hepatocytes with primaquine (300 microM) for 15 min decreased hepatocyte plasma membrane alpha 1-adrenergic receptors by 34.0 +/- 2.4% (mean +/- S.E.M. of three experiments). Removal of primaquine allowed essentially complete recovery (98 +/- 4%; mean +/- S.E.M. for five separate experiments) of plasma membrane [3H]prazosin binding within 20 min, suggesting that the alpha 1-adrenergic receptor undergoes endocytotic recycling. Addition of PMA (1 microgram/ml) to hepatocytes immediately after removal of primaquine, completely inhibited the increase in plasma membrane alpha 1-adrenergic receptors relative to control cells, but had no effect on hepatocytes whose cell surface alpha 1-receptors remaining after primaquine treatment had been inactivated by alkylation. These observations suggested that activation of PKC may facilitate the internalization of the alpha 1-adrenergic receptor in hepatocytes.