2 DISTINCT PATHWAYS FOR HISTAMINE H-2-RECEPTOR DOWN-REGULATION

Pretreatment of Chinese hamster ovary cells expressing the histamine H -2 receptor (CHOrH(2) cells) with histamine resulted in a time-depende nt (t(1/2) approximate to 7 h) and dose-dependent (EC(50) = 18 nM) H-2 receptor down-regulation measured as [I-125]iodoaminopotentidine bind ing (44 +/- 10% down-regulation). Pretreatment of CHOrH(2) cells with cholera toxin or forskolin also led to H-2 receptor down-regulation. F orskolin time-dependently (t(1/2) approximate to 7 h) and dose-depende ntly (EC(50) = 0.3 mu M) induced H-2 receptor down-regulation. Both hi stamine and forskolin induced rapid down-regulation of H-2 receptor mR NA levels, probably caused by mRNA destabilization. Recently, Moro et al. (Moro, O., Lameh, J., Hogger, P., and Sadee, W. (1993) J. Biol. Ch em. 268, 22273-22276) showed that hydrophobic amino acids in a conserv ed G-protein-coupled receptor motif in the second intracellular loop a re implicated in G-protein coupling, To uncouple the H-2 receptor from the G(s)-protein, we introduced the Leu(124) --> Ala mutation in the second intracellular loop of the H-2 receptor. The H-2 Leu(124) --> Al a mutant showed altered agonist-binding parameters, attenuated histami ne-induced cAMP production, and was down-regulated by concentrations o f histamine that did not give rise to cAMP production, Taken together, in CHOrH(2) cells, H-2 receptor down-regulation appears to be induced by two distinct pathways, a cAMP-dependent and cAMP-independent pathw ay.