Endogenous beta(3)- but not beta(1)-adrenergic receptors are resistant to agonist mediated regulation in human SK-N-MC neurotumor cells

Although there is considerable interest in the regulation of the different beta-adrenergic receptor (AR) subtypes, most previous studies have utilized stably transfected cells expressing recombinant receptors under the control of viral promoters. Human SK-N-MC neurotumor sells appear to be novel, since they express both endogenous beta(1)AR and beta(3)AR based on radioligand binding and on functional response. Saturation binding of either the hydrophilic ligand (-)-[H-3]CGP-12177 or the more hydrophobic (-)-[I-125] iodocyanopindolol indicated the presence of two populations of binding sites with high and low affinities. With either ligand, the beta(1)AR antagonist CGP-20712A preferentially inhibited binding to the high-affinity sites. This is consistent with the latter representing beta(1)AR whereas the low-affinity sites represent beta(3)AR. Both subtypes appeared to be functional on the basis of isoproterenol stimulation of cyclic adenosine monophosphate (cAMP) in intact cells and adenylyl cyclase activity in cell membranes in the absence and presence of CGP-20712A. SK-N-MC-IXC cells, derived by twice subcloning the parental cells, also expressed both beta AR subtypes, indicating that they so-exist in the same cell. SK-N-MC cells exposed to isoproterenol exhibited a rapid sequestration and a slower downregulation of beta(1)AR. The latter subtype also underwent desensitization, as indicated by a rightward shift to less sensitivity in the EC(50) for isoproterenol stimulation of adenylyl cyclase activity. In contrast, the beta(3)AR subtype was resistant to agonist-mediated sequestration, downregulation, and desensitization. Thus, when endogenously expressed in the same cell line, human beta(1)AR and beta(3)AR display differences in their ability to be regulated by agonist.