Mitogen-activated signaling in airway smooth muscle. A central role for Ras.

The first detailed report of airway structural changes in asthma was published over 75 years ago in the Archives of Internal Medicine by Huber and Koessler (1). In this report, the authors demonstrated that patients with fatal asthma had substantial thickening of the airway smoothmuscle layer. Half a century later, this early observation was confirmed by a number of investigators (2–9). Carroll and colleagues (10) found that in both fatal and nonfatal cases of asthma, the airway smooth-muscle area of the larger membranous bronchioles was significantly greater than in control cases, suggesting that increases in airway smooth-muscle mass are not exclusive to fatal asthma. Few studies have examined the local mechanism of muscular thickening (i.e., hyperplasia versus hypertrophy). Heard and Hossain (4) found a threefold increase in cell number in the bronchi of asthmatic patients, suggesting that smooth-muscle hyperplasia is present in the airways of patients with fatal asthma. Although the realization that objects must be counted directly in three-dimensional space to obtain unbiased estimates cast doubt on the validity of this early work (11), Ebina and colleagues (9) examined the airways of patients with fatal asthma using a combination of the dissector method with a serial sectioning technique. Two subgroups of asthmatic airways were found: in Type I, smooth-muscle hyperplasia was responsible for central airway smooth-muscle thickening, whereas in Type II, cellular hypertrophy was evident over the entire length of the airway. Finally, excess airway smooth-muscle DNA synthesis has been demonstrated in two animal models of airways disease, hyperoxic exposure, and antigen challenge (12–14). The above data, which strongly suggest that excessive smooth-muscle proliferation is present in the airways of patients with asthma, highlight the need for a precise understanding of the events involved in airway smooth-muscle mitogenesis. To that end, numerous investigators have developed cell culture systems adopting tracheal and bronchial myocytes from different species. A large number of smooth-muscle mitogens have been identified, some of which are species specific in their effect. For example, histamine is mitogenic for human airway smooth muscle (15, 16), but does not induce proliferation in bovine cells (17). Nevertheless, a growing body of literature suggests that common signal transduction pathways regulate airway smooth-muscle cell cycle entry across species lines. Indeed, the signaling pathways regulating airway smoothmuscle proliferation may not be substantially different from those regulating the growth of other mesenchymal cells such as fibroblasts. Perhaps this is to be expected, as many aspects of mitogen-activated protein kinase (MAPK) cascades, guanine triphosphatase (GTPase) signaling pathways, and cell-cycle regulation are highly conserved in eukaryotic species, including mammals, Drosophila , nematodes, and yeast (18–24). A major signal transduction pathway activated by growth factors is the extracellular signal-regulated kinase (ERK) pathway. ERKs (p44 ERK1 and p42 ERK2 ) are cytosolic serine/threonine kinases of the MAPK superfamily. ERKs participate in the transduction of growth and differentiation-promoting signals to the nucleus. Studies in airway smooth-muscle cells using selective overexpression of either dominant-negative or constitutively active forms of Ras, Raf-1, and MAPK/ERK kinase-1 (MEK1) suggest that, as in other mesenchymal cells, these signaling intermediates constitute the major route toward ERK activation (25–27). These data, combined with additional studies using selective chemical inhibitors of MEK1 (27–30), suggest that signaling through the ERK pathway is required for the airway smooth-muscle cell cycle progression. However, activation of ERK and expression of cyclin D 1 , a downstream affector of ERK signaling (31), may not be sufficient for cell-cycle entry. In NIH3T3 cells, constitutive activation of MEK1, although sufficient to induce cyclin D 1 protein accumulation, is insufficient for maximal phosphorylation of retinoblastoma protein, degradation of the cyclin-dependent kinase inhibitor p27, and cyclin A expression—additional key events required for the G 1 -toS-phase transition (32). Moreover, in IIC9 fibroblasts, Ras, but not ERK, is required for growth-factor–induced degradation of p27 (33). Together, these data suggest that Ras coordinates cell-cycle progression by regulating signaling through both ERK-dependent and -independent signaling pathways. In this month’s issue of the Journal , Ammit and colleagues examined the requirement of Ras isoforms for human airway smooth-muscle DNA synthesis (34). Microin( Received in original form August 12, 1999 )