Using specific ligands, we find that lung cancer cell lines of diverse histologic types express multiple, high-affinity (K(d) = 10-9-10-10 M) membrane receptors for μ, δ, and κ opioid agonists and for nicotine and α-bungarotoxin. These receptors are biologically active because cAMP levels decreased in lung cancer cells after opioid and nicotine application. Nicotine at concentrations (≃ 100 nM) found in the blood of smokers had no effect on in vitro lung cancer cell growth, whereas μ, δ, and κ opioid agonists at low concentrations (1-100 nM) inhibited lung cancer cell growth in vitro. We also found that lung cancer cells expressed various combinations of immunoreactive opioid peptides (β-endorphin, enkephalin, or dynorphin), suggesting the participation of opioids in a negative autocrine loop or tumor-expressing system. Due to the almost universal exposure of patients with lung cancer to nicotine, we tested whether nicotine affected the response of lung cancer cell growth to opioids and found that nicotine at concentrations of 100-200 nM partially or totally reversed opioid-induced growth inhibition in 9/14 lung cancer cell lines. These in vitro results for lung cancer cells suggests that opioids coulds function as part of a 'tumor suppresor' system and that nicotine can function to circumvent this system in the pathogenesis of lung cancer.
