Muscarinic Receptors Mediated Stimulation and Intracellular Signaling Pathways Involved in Human Lung Fibroblast Proliferation

Abstract

Asthma and chronic obstructive pulmonary disease (COPD) are two diseases characterized by airflow limitation. The most important difference between asthma and COPD is the nature of inflammation, which is primarily eosinophilic and CD-4 driven in asthma, and neutrophilic and CD-8 driven in COPD. In spite of these differences fibrotic alterations are observed in both these diseases. Cholinergic pathway represents key mechanisms in control of airway smooth muscle tone. Anticholinergic bronchodilaters reduce vagal cholinergic tone, the main reversible component in COPD hence constitute an essential element in the therapy of obstructive airways diseases especially in COPD. Furthermore tiotropium bromide (Spiriva^®) a long acting muscarinic antagonist was found to delay the decline in airway function in COPD, suggesting that cholinergic mechanisms plays a pivotal role in the structural changes associated with airway remodeling. In studies carried out on human and bovine airway smooth muscle it was seen that muscarinic agonists enhanced the proliferative response to epidermal growth factor and platelet-dervied growth factor respectively. Moreover, tiotropium was found to attenuate the increase in airway smooth muscle mass and myosin expression induced by repeated allergen challenges. Almost every cell type in the airways expresses cholinoreceptor and hence could be a target for acetylcholine released from neuronal or nonneuronal sources. Expression of mRNA encoding different nicotinic receptor subunits and M₂ receptors in airway fibroblasts has been described, but a detailed study on expression of muscarinic receptor subtypes in lung and airway fibroblasts is still lacking. The purpose of this study was to determine the expression pattern of the muscarinic receptors in human lung fibroblasts and to investigate whether muscarinic receptors mediate effects on cell proliferation and if so, to elucidate the intracellular signaling pathways entailed. Our studies indicated that human lung fibroblasts indeed express muscarinic receptors with the most dominantly expressed subtype being the M₂ receptor, which appears to mediate proliferative stimulus via a Gi/o-Raf-1-MEK-MAPK/ERK pathway, an effect which might contribute to structural changes known to occur in chronic obstructive airway diseases. Prolonged blockade of these receptors may contribute to the long term beneficial effects of anticholinergics, as observed for example, for the long-acting muscarinic antagonist, tiotropium, in COPD.