INHIBITION OF THE CYCLIC-AMP SIGNALING CASCADE AND NUCLEAR FACTOR-BINDING TO CRE AND KAPPA-B ELEMENTS BY CANNABINOL, A MINIMALLY CNS-ACTIVECANNABINOID

Immune suppression by cannabinoids has been widely demonstrated in a v ariety of experimental models. The identification of two major types o f G-protein-coupled cannabinoid receptors expressed on leukocytes, CB1 and CB2, has provided a putative mechanism of action for immune modul ation by cannabinoid compounds. Ligand binding to both receptors negat ively regulates adenylate cyclase, thereby lowering intracellular cycl ic AMP (cAMP) levels. In the present studies, we demonstrated that can nabinol (CBN), a ligand that exhibits higher binding affinity for CB2, modulates immune responses and cAMP-mediated signal transduction in m ouse lymphoid cells. Direct addition of CBN to naive cultured splenocy tes produced a concentration-dependent inhibition of lymphoproliferati ve responses to anti-CD3, lipopolysaccharide, and phorbol-12-myristate -13-acetate/ionomycin stimulation. Similarly, a concentration-related inhibition of the in vitro anti-sheep red blood cell IgM antibody form ing cell response was also observed by CBN. Evaluation of cAMP signali ng in the presence of CBN showed a rapid and concentration-related inh ibition of adenylate cyclase activity in both splenocytes and thymocyt es. This decrease in intracellular cAMP levels produced by CBN resulte d in a reduction of protein kinase A activity, consequently leading to an inhibition of transcription factor binding to the cAMP response el ement and kappa B motifs in both cell preparations. Collectively, thes e results demonstrate that CBN, a cannabinoid with minimal CNS activit y, inhibited both cAMP signal transduction and immune function, furthe r supporting the involvement of CB2 receptors in immune modulation by cannabimimetic agents. (C) 1998 Elsevier Science Inc.