THE ENDOGENOUS CANNABINOID ANANDAMIDE IS A LIPID MESSENGER ACTIVATINGCELL-GROWTH VIA A CANNABINOID RECEPTOR-INDEPENDENT PATHWAY IN HEMATOPOIETIC-CELL LINES

The effect of anandamide, an endogenous ligand for central (CBI) and p eripheral (CB2) cannabinoid receptors, was investigated on the growth of the murine IL-6-dependent lymphoid cell line B9 and the murine IL-3 -dependent myeloblastic cell line FDC-P1, In conditions of low serum l evel, anandamide potentiated the growth of both cytokine-dependent cel l lines. Comparison with other fatty acid cannabinoid ligands such as (R)-methanandamide, a ligand with improved selectivity for the CBI rec eptor, or palmitylethanolamide, an endogenous ligand for the CB2 recep tor, showed a very similar effect, suggesting that cell growth enhance ment by anandamide or its analogs could be mediated through either rec eptor subtype, However, several lines of evidence indicated that this growth-promoting effect mas cannabinoid receptor-independent. First, t he potent synthetic cannabinoid agonist CP 55940, which displays high affinity for both receptors, was inactive in this model. Second, SR 14 1716A and SR 144528, which are potent and specific antagonists of CB1 and CB2 receptors respectively, were unable, alone or in combination, to block the anandamide-induced effect. Third, inactivation of both re ceptors by pretreatment of cells with pertussis toxin did not affect t he potentiation of cell growth by anandamide, These data demonstrated that neither CB1 nor CB2 receptors were involved in the anandamide-ind uced effect. Moreover, using CB2-transfected Chinese hamster ovary cel ls, we demonstrated that after complete blockade of the receptors by t he specific antagonist SR 144528, anandamide was still able to strongl y stimulate a mitogen-activated protein (MAP) kinase activity, clearly indicating that the endogenous cannabinoid can transduce a mitogenic signal in the absence of available receptors, Finally, arachidonic aci d, a structurally related compound and an important lipid messenger wi thout known affinity for cannabinoid receptors, was shown to trigger M AP kinase activity and cell growth enhancement similar to those observ ed with anandamide. These findings provide clear evidence for a functi onal role of anandamide in activating a signal transduction pathway le ading to cell activation and proliferation via a non-cannabinoid recep tor-mediated process. (C) 1998 Federation of European Biochemical Soci eties.