title: Beta-adrenergic receptor levels and function after growth of S49 lymphoma cells in low concentrations of epinephrine.
abstract: Growth of S49 wild-type (WT) lymphoma cells for 24 hr with 3 nM epinephrine produces a very pronounced attenuation of cAMP accumulation in response to subsequent challenges with much higher concentrations of the catecholamine [Mol. Pharmacol. 36:459-464 (1989)]. We report here the effects of this treatment, in S49 WT, cyc-, and kin- cells, on the responsiveness of adenylate cyclase in partially purified membranes. The desensitization of adenylate cyclase in the S49 WT cells after 24-hr treatment was homologous, in that only responses to epinephrine were attenuated. The EC50 for epinephrine stimulation of adenylate cyclase was 54 +/- 8% (mean +/- standard error) higher in treated cells than in controls, and there was a 32 +/- 3% reduction in Vmax at supramaximal epinephrine concentrations. The treatment also caused a 34 +/- 9% reduction in the levels of the beta-adrenergic receptor (beta AR), which was of a sufficient magnitude to account for the homologous desensitization seen. The 24-hr treatment had similar effects in S49 kin- cells, where we observed a 28 +/- 4% decrease in Vmax, a 35 +/- 6% increase in EC50 for epinephrine stimulation of adenylate cyclase, and a 25 +/- 3% decrease in beta AR. In contrast, the 24-hr treatment had no measurable effect on adenylate cyclase activity in S49 cyc- cells. That is, the responsivity of adenylate cyclase reconstituted with Gs from S49 WT cells was not attenuated, although beta AR levels were significantly decreased. The desensitization of S49 cells with the 24-hr treatment was additive with that mediated by the cAMP-dependent protein kinase (cAPK). Further, unlike the cAPK-mediated attenuation, it was relatively insensitive to the levels of free Mg2+ in the adenylate cyclase reaction mixture. The characteristics of the desensitization produced by 24-hr treatment with 3 nM epinephrine, together with the observation that it is similar in S49 WT and kin- cells, demonstrates that the process in WT cells is, at least in part, independent of the rapid cAPK-mediated desensitization. It is also most likely that it is unrelated to the rapid cAMP-independent processes involving sequestration/internalization or the beta AR kinase, because those mechanisms require much higher receptor occupancies than the 0.2% occurring with 3 nM epinephrine. Thus, 24-hr treatment appears to produce attenuation of adenylate cyclase by causing down-regulation of beta AR, without involving any other known form of desensitization.
authors.affiliation: Graduate School of Biomedical Sciences, University of Texas Health Science Center, Houston 77225.
authors.forename: M A
authors.initials: MA
authors.lastname: Proll
authors.forename: R B
authors.initials: RB
authors.lastname: Clark
authors.forename: T J
authors.initials: TJ
authors.lastname: Goka
authors.forename: R
authors.initials: R
authors.lastname: Barber
authors.forename: R W
authors.initials: RW
authors.lastname: Butcher
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D015536:Down-Regulation; D004789:Enzyme Activation; D004837:Epinephrine / Q000494:pharmacology*; D008223:Lymphoma / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism; D011943:Receptors, Adrenergic, beta / Q000187:drug effects / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: D011943:Receptors, Adrenergic, beta; D000242:Cyclic AMP; D011494:Protein Kinases; D000262:Adenylyl Cyclases; D004837:Epinephrine