Abstract

Inhibition of cell proliferation by fenoterol and fenoterol derivatives in 1321N1 astrocytoma cells is consistent with beta2-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 using Western blotting and [3H]-thymidine incorporation assays. Despite expression of β2-AR, no cAMP accumulation was observed when cells were stimulated with isoproterenol or Fen, although the treatment elicited both MAPK and PI3K/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 ICI118,551, a β2-AR antagonist, while those of MNF were unaffected. Because of the co-expression of β2-AR and cannabinoid receptors (CBRs) and their impact on HepG2 cell proliferation, these Gαi/o-linked receptors may be implicated in MNF signaling. Cell treatment with WIN 55,212-2, a synthetic agonist of the 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 anti-proliferative actions of MNF and WIN 55,212-2. The data indicates 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.