Regulation of endothelin-A receptor sensitivity by cyclic adenosine monophosphate in rat hepatic stellate cells

Abstract

Sensitization of the endothelin-A receptor (ET_A) occurs during HSC transdifferentiation, but the underlying mechanisms remained unclear. Sensitization of ET_A was studied in quiescent and activated hepatic stellate cells (HSC) at the levels of receptor phosphorylation, localization, endothelin (ET)-1-induced Ca²⁺ signals, and cell contraction. The endothelin-1 (ET-1) concentrations required to obtain an ET_A-mediated Ca²⁺ signal in 50% of HSC cultured for 1 to 2 or 10 days were approximately 1.2 and 0.012 nmol/L, respectively. This transdifferentiation-dependent sensitization of ET_A was accompanied by receptor translocation to the plasma membrane. Cyclic AMP rapidly desensitized ET_A in activated HSC and shifted their ET-1 responsiveness from picomolar to nanomolar concentrations with respect to Ca²⁺ signals and HSC contraction. ET_A desensitization also occurred in response to prostaglandin E₂, adenosine, or ET_B stimulation. Desensitization by cAMP in activated HSC was accompanied by an increased Ser/Thr phosphorylation of ET_A and their rapid internalization. Quiescent HSC exhibited Ser/Thr phosphorylation of the ET_A protein, which was not affected by cAMP. In conclusion, the ET_A response in HSC is regulated by protein kinase A (PKA)-dependent receptor phosphorylation and internalization. This may explain the transdifferentiation-dependent sensitization of HSC towards ET-1 and its reversal by cAMP and ET_B activation.