Using specific ligands, we find that lung cancer cell lines of diverse histologic types express multiple, highaffinity (Kd = 10-9-10-1o M) membrane receptors for Iz, 8, and K opioid agonists and for nicotine and a-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 IA, 8, and Kopioid agonists at low concentrations (1-100 nM) inhibited lung cancer growth in vitro. We also found that lung cancer cells expressed various combinations of immunoreactive opioid pep-
Although nicotine has been implicated as a potential factor in the pathogenesis of human lung cancer, its mechanism of action in the development of this cancer remains largely unknown. The present study provides evidence that nicotine (a) activates the mitogen-activated protein (MAP) kinase signalling pathway in lung cancer cells, specifically extracellular signal-regulated kinase (ERK2), resulting in increased expression of the bcl-2 protein and inhibition of apoptosis in these cells; and (b) blocks the inhibition of protein kinase C (PKC) and ERK2 activity in lung cancer cells by anticancer agents, such as therapeutic opioid drugs, and thus can adversely affect cancer therapy. Nicotine appears to have no effect on the activities of c-jun NH 2 -terminal protein kinase (JNK) and p38 MAP kinases, which have also been shown to be involved in apoptosis. While exposure to nicotine can result in the activation of the two major signalling pathways (MAP kinase and PKC) that are known to inhibit apoptosis, nicotine regulation of MAP (ERK2) kinase activity is not dependent on PKC. These effects of nicotine occur at concentrations of 1 µM or less, that are generally found in the blood of smokers, and could lead to disruption of the critical balance between cell death and proliferation, resulting in the unregulated growth of cells. The findings suggest caution in the use of smokeless tobacco products to treat smoking addiction, as they could have a potentially deleterious effect in patients with undetectable early tumour development.
The effect of insulin-like growth factor I (IGF-I) on growth of small cell lung cancer (SCLC) cell lines was studied. Western blot analysis of whole cell lysates of cell lines NCI-H345 and NCI-N417 demonstrated the presence of a 16-kD band consistent with an IGF-I precursor molecule. Scatchard plot analysis of cell line NCI-H345 using '25I-labeled IGF
Methadone was found to significantly inhibit the in vitro and in vivo growth of human lung cancer cells. The in vitro growth inhibition (occurring at 1-100 nM methadone) was associated with changes in cell morphology and viability detectable within 1 hr and was irreversible after a 24-hr exposure to the drug. These effects of methadone could be reversed in the first 6 hr by naltrexone, actinomycin D, and cycloheximide, suggesting involvement of opioid-like receptors and the requirement for de novo mRNA and protein synthesis. The inhibitory effects of methadone on the growth of lung cancer cells also could be achieved by the less addictive (+) isomer of methadone. Characterization of the methadone binding to lung cancer cell membranes revealed high-affinity (nM), saturable binding sites for (±)-[3H]methadone, which crossreacted with ligands for ec, phencyclidine, or, but not jp, and 8 opioid receptors, and the binding characteristics appeared to be different from methadone sites present in rat brain. Methadone decreases cAMP levels in lung cancer cells, but the receptors are not coupled to a pertussis toxin-sensitive guanine nucleotide-binding regulatory protein. We conclude that the lung cancer growth inhibitory effects of methadone are significant, occur at low concentrations, and are mediated by a nonconventional type of opioid binding site distinct from methadone receptors found in the brain.
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