NICOTINIC ACETYLCHOLINE-RECEPTORS OF MUSCLE AND NEURONAL (ALPHA(7)) TYPES COEXPRESSED IN A SMALL-CELL LUNG-CARCINOMA

SCC-37 is a small cell lung carcinoma line that aberrantly expresses m uscle-type nicotinic acetylcholine receptors (nAChRs). It was establis hed from a patient with a paraneoplastic autoimmune neuromuscular diso rder, myasthenia gravis. When grown as a xenograft tumor, SCC-37 cells express plasma membrane receptors that bind I-125-labeled alpha-bunga rotoxin (I-125-alpha-BTx), cosediment with 9S nAChR pentamers, and bin d to a monoclonal antibody (MAb 35) specific for muscle-type (alpha(1) subunit) alpha-BTx receptors. The agonist carbamylcholine (carbachol) stimulates influx of Na-22(+) in SCC-37 cells; this is inhibited by a lpha-BTx and by d-tubocurarine. Long-term cultured SCC-37 cells have f unctional and ligand-binding evidence for surface coexpression of both al and neuronal-type (alpha(7) subunit) alpha-BTx receptors. A subclo ne of SCC-37, designated SCC-AS, expresses only the neuronal-type (alp ha(7) subunit) alpha-BTx receptors on its surface. Carbachol does not stimulate Na-22(+) influx in SCC-AS cells, but cytisine initiates a su stained influx of Ca2+. Activation of this response is inhibited by al pha-BTx and by the alpha(7)-selective antagonist methyllycaconitine. A ddition of Co2+ abrogates the sustained elevation of intracellular fre e Ca2+ concentration, implying that the cytisine-stimulated influx of Ca2+ is sustained by secondary opening of voltage-sensitive channels i n the plasma membrane. Surface receptors for I-125-alpha-BTx are block ed by methyllycaconitine and d-tubocurarine. Solubilized alpha-BTx rec eptors from plasma membranes of SCC-AS cells cosediment with 10S neuro nal nAChR pentamers and bind to an alpha(7)-specific monoclonal antibo dy (MAb P27) but not to the muscle nAChR-reactive MAb 35. However, MAb P27 and MAb 35 both bind to alpha-BTx receptors solubilized from the cytoplasmic compartments of SCC-A9 and the parental SCC-37 line. Rever se transcription-PCR analysis revealed RNA transcripts for alpha(7) an d alpha(1) subunits in both SCC-A9 and SCC-37 cells. The nAChRs that a re expressed in these novel human cell lines can regulate cation fluxe s directly as well as indirectly by synergizing with the activity of v oltage-sensitive Ca2+ channels. These activities may influence the sec retion of autocrine growth factors and the transcription of growth reg ulatory genes and thus be pertinent to the growth and metastasis of ma lignant neuroendocrine neoplasms.