Mitogenic signaling pathways in airway smooth muscle

Abstract

Increased airway smooth muscle mass has been demonstrated in patients with asthma, bronchopulmonary dysplasia and most recently, cystic fibrosis. These observations emphasize the need for further knowledge of the events involved in airway smooth muscle mitogenesis and hypertrophy. Workers in the field have developed cell culture systems involving tracheal and bronchial myocytes from different species. An emergent body of literature indicates that mutual signal transduction pathways control airway smooth muscle cell cycle entry across species lines. This article reviews what is known about mitogen-activated signal transduction in airway myocytes. The extracellular signal regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI 3-kinase) pathways appear to be key positive regulators of airway smooth muscle mitogenesis; recent studies have also demonstrated specific roles for reactive oxygen and the JAK/STAT pathway. It is also possible that growth factor stimulation of airway smooth muscle concurrently elicits signaling through negative regulatory intermediates such as p38 mitogen-activated protein (MAP) kinase and protein kinase C (PKC) delta, conceivably as a defense against extreme growth.

Introduction

Developing bronchi are enclosed by a well-formed layer of smooth muscle cells by the conclusion of the embryonic period of fetal lung development (Sparrow et al., 1999). Fetal airway smooth muscle is spontaneously contractile throughout gestation and this phasic activity is associated with the maintenance of a positive intraluminal pressure (Schittny et al., 2000). Positive intraluminal pressure, in turn, has been implicated in the process of lung growth and development (Nakamura and McCray, 2000). Conversely, mechanical stress promotes bronchial myogenesis via the alternative splicing of serum response factor, which is required for contractile protein expression (Yang et al., 2000). With airway circumferential and axial growth, this layer enlarges, a consequence of both cellular hypertrophy and hyperplasia. It has been shown that the quantity of smooth muscle is abnormally increased in the airways of premature infants with bronchopulmonary dysplasia (Hislop and Haworth, 1989, Sward-Comunelli et al., 1997), due in part to excess cell proliferation (Johnson et al., 1991). Information regarding airway smooth muscle growth during typical postnatal development is not available, however.

Increased airway smooth muscle mass has also been shown in non-fatal (Carroll et al., 1993) and fatal asthma (Dunhill et al., 1971, Takizawa and Thurlbeck, 1971, Heard and Hossain, 1973, Sobonya, 1984, James et al., 1989, Ebina et al., 1990, Saetta et al., 1991, Ebina et al., 1993). In the most compelling report to date, Ebina et al. (1993) studied the airway thickness and smooth muscle cell number of patients with fatal asthma with state-of-the-art stereological methods. Two asthmatic subtypes were found, one in which the smooth muscle thickness was increased only in the central bronchi (Type I) and another in which the quantity of smooth muscle was increased throughout the airway tree (Type II). In Type I, an increased number of smooth muscle cell nuclei was present in the central airways, whereas in Type II, cellular hypertrophy was present. However, it should be noted that earlier studies examining low-magnification circumferential profiles of airway tissue may not have taken into account the fact that airway smooth muscle bundles contain significant amounts of collagen which do not contribute to muscle shortening (Thomson et al., 1996).

Subepithelial myofibroblasts have been noted in the airways of patients with chronic severe asthma (Brewster et al., 1990), as well as those undergoing allergen challenge (Gizycki et al., 1997). Recent studies suggest that these cells may be the source of subepithelial fibrosis found in patients with chronic severe asthma (Morishima et al., 2001, Hastie et al., 2002). However, as far as we are aware there are no data suggesting that myofibroblasts contribute to increased airway smooth muscle mass in asthma.

Increased protein abundance of epidermal growth factor (EGF), a mitogen for human airway smooth muscle, has been found in asthmatic airways (Vignola et al., 1997, Amishima et al., 1998). EGF receptor expression is also increased (Amishima et al., 1998). Bronchoalveolar lavage fluid basic fibroblast growth factor concentrations are significantly higher in subjects with atopic asthma than in control subjects without asthma (Redington et al., 2001). Also, bronchoalveolar lavage fluid from asthmatic airways has been shown to increase the extracellular signal regulated kinase (ERK) activation, cyclin D₁ protein abundance, [³H]-thymidine incorporation and cell number of cultured human airway smooth muscle cells (Naureckas et al., 1999). Abnormally increased airway smooth muscle DNA synthesis has been shown in two animal models of airways disease, hyperoxic exposure and allergen sensitization (Hershenson et al., 1994, Wang et al., 1995, Panettieri et al., 1998). Finally, increased smooth muscle mass has recently been noted in the airways of patients with cystic fibrosis (Hays et al., 2001).

Together, these reports demonstrate that abnormal smooth muscle mitogenesis is present in the airways of patients with chronic airways disease, and emphasize the need for further study of airway smooth muscle mitogenesis. To that end, workers in the field have developed cell culture systems involving tracheal and bronchial myocytes from different species. A review of mitogen-activated signal transduction in airway smooth muscle follows below.