Abstract

Inhibition of cell proliferation by fenoterol and fenoterol derivatives in 1321N1 astrocytoma cells is consistent with β₂-adrenergic receptor (β₂-AR) stimulation. However, the events that result in fenoterol-mediated control of cell proliferation in other cell types are not clear. Here, we compare the effect of the β₂-AR agonists (R,R′)-fenoterol (Fen) and (R,R′)-4-methoxy-1-naphthylfenoterol (MNF) on signaling and cell proliferation in HepG2 hepatocarcinoma cells by using Western blotting and [³H]thymidine incorporation assays. Despite the expression of β₂-AR, no cAMP accumulation was observed when cells were stimulated with isoproterenol or Fen, although the treatment elicited both mitogen-activated protein kinase and phosphatidylinositol 3-kinase/Akt activation. Unexpectedly, isoproterenol and Fen promoted HepG2 cell growth, but MNF reduced proliferation together with increased apoptosis. The mitogenic responses of Fen were attenuated by 3-(isopropylamino)-1-[(7-methyl-4-indanyl)oxy]butan-2-ol (ICI 118,551), a β₂-AR antagonist, whereas those of MNF were unaffected. Because of the coexpression of β₂-AR and cannabinoid receptors (CBRs) and their impact on HepG2 cell proliferation, these Gα_i/Gα_o-linked receptors may be implicated in MNF signaling. Cell treatment with (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-napthalenylmethanone (WIN 55,212-2), a synthetic agonist of CB₁R and CB₂R, led to growth inhibition, whereas inverse agonists of these receptors blocked MNF mitogenic responses without affecting Fen signaling. MNF responses were sensitive to pertussis toxin. The β₂-AR-deficient U87MG cells were refractory to Fen, but responsive to the antiproliferative actions of MNF and WIN 55,212-2. The data indicate that the presence of the naphthyl moiety in MNF results in functional coupling to the CBR pathway, providing one of the first examples of a dually acting β₂-AR-CBR ligand.