Synthesis of acetylcholine by lung cancer
Introduction
Lung cancer is the leading cause of cancer death for both men and women. Despite aggressive efforts, treatments are unsatisfactory and survival rates dismal. Clearly development of new approaches to studying and treating lung cancers are needed. The majority of lung cancers are associated with smoking, and beyond the well established causal connection with carcinogens in cigarette smoke, nicotine also appears to affect lung cancer growth [1], [2], [3], [4]. Nicotine is a ligand for nicotinic cholinergic receptors and lung cancers have been clearly shown to express nicotinic receptors [5], [6]. If lung cancers express acetylcholine (ACh), the endogenous ligand for cholinergic receptors, then ACh might be an autocrine growth factor for lung cancers - a finding which would explain why nicotine modulates lung cancer growth.
According to WHO classification, lung cancers are divided into small cell carcinoma (SCLC) and non-small cell lung carcinoma (NSCLC). NSCLC are further divided into squamous cell carcinoma, adenocarcinoma, adenosquamous carcinoma and large cell carcinoma. SCLC are strongly associated with smoking and represent 20–25% of lung cancers. SCLC secrete a variety of neuropeptides and growth factors [7], [8] and are believed to be related to pulmonary neuroendocrine cells (PNEC) [9]. Squamous cell carcinomas which arise from the epithelium of large airways are the most prevalent lung cancer (25–40%) and are characterized by stratified cells with pleomorphic nuclei and abundant eosinophilic cytoplasm with intracytoplasmic keratin. The relative frequency of lung adenocarcinomas has increased and now represents 25–40% of lung cancers. The reason for this increase is unclear but may be related to changes in smoking patterns in which smoke is drawn deeper into the lungs. Adenocarcinoma is the most common type of lung cancer in nonsmokers, but still greater than 75% occur in smokers [10]. Centrally-derived adenocarcinomas are further subdivided into acinar type, papillary type and solid carcinoma with mucus production. Another type of adenocarcinoma, known as bronchoalveolar carcinoma (BAC), arises primarily from peripheral lung and shares features of Clara cells and alveolar type II pneumocytes including expression of surfactant apoproteins. Large cell carcinomas represent 10–15% of lung cancers. Large cell carcinomas are more anaplastic but also frequently express neuroendocrine markers [11]. It is likely that the expression of cholinergic markers by lung cancers may relate to the expression of cholinergic markers by the cancer's cells of origin.
It is well established that lung cancers express nicotinic and muscarinic ACh receptors (nAChR, mAChR) [5], [6], [12] and that the activation of nAChR with nicotine [1], [2], [3] and ACh [2] stimulates the growth of lung cancer cell lines. For ACh to be an autocrine growth factor for lung cancers certain criteria must be met. First ACh must be synthesized and secreted by lung cancers. Second lung cancers must express cholinergic receptors and third, modification of cholinergic signaling must modify tumor growth. In this paper, we demonstrate that lung cancers synthesize ACh and that the synthesized, non-neuronal ACh meets the criteria of an autocrine growth factor for lung cancers.
Section snippets
Cell culture and SCLC tissue samples
Small cell carcinoma cell lines NCI H69, H82, H345, and H592 were generously provided by J. Minna and coworkers [8], [13]. H82 cells were grown in RPMI 1640 medium (Invitrogen, Carlsbad, CA) supplemented with 5 μg/ml insulin, 5μg/ml transferrin, 5 ng/ml sodium selenite, 100 units/ml penicillin, 100 μg/ml streptomycin. Routine processed, paraffin-embedded tumor samples were obtained from the Department of Pathology of the Oregon Health and Science University. Five micron sections were cut, and
Results
If ACh is expressed in lung cancers, then ACh should be expressed by the cells of origin of lung cancers. This was evaluated by immunohistochemistry for the enzymes and proteins necessary for ACh synthesis and secretion. The key proteins needed for ACh synthesis are ChAT (the enzyme that synthesizes ACh), VAChT (the transporter for packaging ACh into vesicles) and CHT1, (the high affinity transporter for choline uptake). As shown in Fig. 1A, bronchial epithelial cells of large airways, the
Discussion
Multiple reports have established that SCLC express cholinergic receptors and that nicotine modulates the growth of SCLC [1], [2], [3], [4], [5], [6]. The cholinergic regulation of lung cancer growth combined with the reports from Wessler and co-workers [16], [17] that non-neuronal cells in lung express ACh, raises the possibility that lung tumors may make ACh and that ACh may function as an autocrine growth factor for lung cancers. For ACh to be an autocrine growth factor for lung cancers,
Acknowledgements
This research supported by NIH grants RR00163, CA69533, HD/HL37131, AG09525, DA11203 and NS42793.
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