Human Bronchial Epithelial and Endothelial Cells Express α7 Nicotinic Acetylcholine Receptors

Abstract

The epithelial or endothelial cells that line the human bronchi and the aorta express nicotinic acetylcholine receptors (nAChRs) of alpha3 subtypes. We report here that human bronchial epithelial cells (BEC) and aortic endothelial cells (AEC) express also the nAChR alpha7 subunit, which forms functional nAChRs. Polymerase chain reaction and in situ hybridization experiments detected alpha7 subunit mRNA in cultured human BEC and AEC and in sections of rat trachea. The binding of radiolabeled alpha-bungarotoxin revealed a few thousand binding sites per cell in cultured human BEC and human and bovine AEC. Western blot and immunohistochemistry experiments demonstrated that cultured BEC and AEC express a protein(s) recognized by anti-alpha7 antibodies. Whole-cell patch-clamp studies of cultured human BEC demonstrated the presence of fast-desensitizing currents activated by choline and nicotine that were blocked reversibly by methyllycaconitine (1 nM) and irreversibly by alpha-bungarotoxin (100 nM), consistent with the expression of functional alpha7 nAChRs. In some cells, choline activated also slowly decaying currents, confirming previous reports that BEC express functional alpha3beta4 nAChRs. Exposure of cultured BEC to nicotine (1 microM) for 3 days up-regulated functional alpha7 and alpha3 nAChRs, as indicated by the increased number of cells responding to acetylcholine and choline, with both fast-desensitizing currents, which were blocked irreversibly by alpha-bungarotoxin, and with slowly desensitizing currents, which are alpha-bungarotoxin-insensitive currents. The presence of alpha7 nAChRs in BEC and AEC suggests that some toxic effects of tobacco smoke could be mediated through these nicotine-sensitive receptors.

Nicotinic receptor signaling in nonexcitable cells

“…In the lung, bronchial epithelial cells also express a wide array of nAChR subunits and show significant [ 3 H]-epibatidine and [ 125 I]-␣-Bungarotoxin (␣Bgt) binding (Table 1). Application of nicotine and ACh elicits measurable inward currents in these cells further strengthening the evidence for functional nAChRs (Maus et al, 1998;Wang et al, 2001).…”

“…In human airways, both basal and ciliated epithelial cells produce ACh as detected by HPLC measurements from acutely isolated cells . This represents a possible source of ligand for nAChRs present on bronchial epithelial cells (Wang et al, 2001). Vascular endothelial cells themselves synthesize acetylcholine suggesting a source of the ligand for autocrine activation of nAChRs (Kawashima et al, 1990).…”

Autoimmune diseases involving nicotinic receptors

“…Thus far, no diseases have been described that are associated with autoimmune responses to brain AChRs. Neuronal AChRs have been detected in several non-neuronal tissues (Grando, 2001;Corriveau et al, 1995;Wang et al, 2001;Sekhon et al, 1999;Kawashima et al, 2000). Recently an autoimmune response to ␣9 AChRs has been found in the skin disease pemphigus (Grando, 2000).…”

“…Neuronal AChRs are expressed at low levels throughout the CNS, PNS, and enteric nervous systems as well as in skin, lymphocytes, bronchial and vascular epithelia and other lung tissues (Corriveau et al, 1995;Fischer et al, 1999;Macklin et al, 1998;Maus et al, 1998;Wang et al, 2001;Sekhon et al, 1999;Grando, 2001;Battaglioli et al, 1998;Kawashima et al, 2000;Lustig et al, 2001).…”

“…The limited accessibility and high complexity of neuronal tissues are further barriers to studies. Neuronal AChRs are being discovered in many nonneuronal tissues, such as muscle (Corriveau et al, 1995;Fischer et al, 1999), bronchial and vascular endothelia Maus et al, 1998;Wang et al, 2001), other lung tissues (Sekhon et al, 1999), keratinocytes (Grando, 2001), and lymphocytes (Battaglioli et al, 1998;Kawashima and Fujii, 2000;Lustig et al, 2001). In skin, many components for cholinergic signaling have been found, including choline acetyltransferase, AChRs, and acetylcholinesterase; but synapses have not yet been identified (Grando, 2001).…”