A SELECTIVE INVERSE AGONIST FOR CENTRAL CANNABINOID RECEPTOR INHIBITSMITOGEN-ACTIVATED PROTEIN-KINASE ACTIVATION STIMULATED BY INSULIN OR INSULIN-LIKE-GROWTH-FACTOR-1 - EVIDENCE FOR A NEW MODEL OF RECEPTOR LIGAND INTERACTIONS/

In the present study, we showed that Chinese hamster ovary (CHO) cells transfected with human central cannabinoid receptor (CB1) exhibit hig h constitutive activity at both levels of mitogen-activated protein ki nase (MAPK) and adenylyl cyclase, These activities could be blocked by the CB1-selective ligand, SR 141716A, that functions as an inverse ag onist. Moreover, binding studies showed that guanine nucleotides decre ased the binding of the agonist CP-55,940, an effect usually observed with agonists, whereas it enhanced the binding of SR 141716A, a proper ty of inverse agonists, Unexpectedly, me found that CB1-mediated effec ts of SR 141716A included inhibition of MAPK activation by pertussis t oxin-sensitive receptor-tyrosine kinase such as insulin or insulin-lik e growth factor 1 receptors but not by pertussis toxin-insensitive rec eptor-tyrosine kinase such as the fibroblast growth factor receptor, W e also observed similar results when cells were stimulated with Mas-7, a mastoparan analog, that directly activates the G(i) protein. Furthe rmore, SR 141716A inhibited guanosine 5'-0-(thiotriphosphate) uptake i nduced by CP-55,940 or Mas-7 in CHO-CB1 cell membranes, This indicates that, in addition to the inhibition of autoactivated CB1, SR 141716A can deliver a biological signal that blocks the G(i) protein and conse quently abrogates most of the G(i)-mediated responses, By contrast, SR 141716A had no effect on MAPK activation by insulin or IGF1 in CHO ce lls lacking CB1 receptors, ruling out the possibility of a direct inte raction of SR 141716A with the G(i) protein, This supports the notion that the G(i) protein may act as a negative intracellular signaling cr oss-talk molecule, From these original results, which considerably enl arge the biological properties of the inverse agonist, we propose a no vel model for receptor/ligand interactions.