Inhibition of hepatic alpha 1-adrenergic effects and binding by phorbol myristate acetate.

C J Lynch,R Charest,S B Bocckino,J H Exton,P F Blackmore

doi:10.1016/s0021-9258(18)89441-8

Abstract

Treatment of isolated hepatocytes with the tumor-promoting agent, 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) produced a time- and dose-dependent, non-competitive inhibition of alpha 1-adrenergic responses, including the activation of phosphorylase, increase in Ca2+ efflux, increase in free cytosolic Ca2+, and release of myo-inositol-1,4,5-P3. The actions of [8-arginine] vasopressin (AVP) on liver cells were also inhibited by PMA, but the inhibition could be overcome by high AVP concentrations. No significant inhibition of beta-adrenergic and glucagon-mediated activation of phosphorylase was induced by PMA and no inhibitory or synergistic effects of PMA were observed on the dose-dependent activation of phosphorylase by the Ca2+ ionophore A23187. In radioligand binding studies, PMA did not directly interfere with [3H]prazosin specific binding, the displacement of [3H]prazosin by (-)-norepinephrine nor with [3H]AVP specific binding to purified liver plasma membranes. Plasma membranes prepared from livers perfused with PMA exhibited a 30-44% reduction in [3H]prazosin binding capacity. Under identical conditions [3H]AVP binding was unchanged. The alpha 1-receptors remaining in membranes from PMA-treated livers had equivalent affinities for [3H]prazosin and (-)-norepinephrine, and were unaffected in terms of coupling to guanine nucleotide-regulating proteins as indicated by the ability of guanosine 5'-(beta, gamma-imido)triphosphate to promote the conversion of the remaining alpha 1-receptors into a low affinity state. These data indicate that tumor promoters are potent antagonists of alpha 1-adrenergic and vasopressin (low dose) responses in liver. It is proposed that PMA acting via protein kinase C (which presumably mediates the action of PMA) exerts its inhibitory action on alpha 1-adrenergic responses at the alpha 1-adrenergic receptor itself and also at a site close to or before myo-inositol-1,4,5-P3 release.

Highlights

From the Laboratoriesfor the Studies of Metabolic Disorders, Howurd Hughes Medica1 Institute and the Department of Physiology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232
The actions of [%arginine] vasopressin (AVP) on liver cells were inhibited by PMA, but the inhibition could be overcome by high AVP concentrations
Enouslyto hepatocytes.This effect is oppositoe thaotbserved in turkey erythrocytes with the @-adrenergicreceptor, where desensitization is seen following phosphorylation by the CAMP-dependent protein kinase [2]

Summary

Introduction

The effect of PMA to stimulate a partially purified preparation of rat liver protein kinase C in vitro' is shown (Fig. 2A). The inhibition of vasopressin effects can be observed if the time course of phosphorylase activation is examined.

Results

Conclusion