Smooth muscle actin is expressed by air space fibroblast-like cells in idiopathic pulmonary fibrosis and hypersensitivity pneumonitis

Animal model of sclerotic skin. V: Increased expression of alpha-smooth muscle actin in fibroblastic cells in bleomycin-induced scleroderma

Scleroderma is a connective tissue disorder with unknown etiology. Myofibroblasts appear during fibrotic processes such as scleroderma, hypertrophic scarring, and wound healing. We previously established a mouse model for scleroderma by local injections of bleomycin. To determine the phenotype of the fibroblasts in sclerotic skin after bleomycin treatment, we examined the expression of alpha-smooth muscle actin (alpha-SMA), a marker for myofibroblasts, in lesional skin as well as in fibrous lung in this model. Dermal sclerosis was induced by daily local injections of bleomycin (100 microg/ml) for 3 weeks in C3H mice. Immunohistochemical examination showed that alpha-SMA-reactive cells were detectable on fibroblastic cells in bleomycin-injected skin at 1 week. There was a significant increase in the immunoreactive fibroblastic cells for alpha-SMA in lesional skin in parallel with the induction of dermal sclerosis. After 3 weeks' treatment with bleomycin, the number of alpha-SMA-reactive fibroblasts showed an 11-fold increase compared with that in control PBS-treated mice. alpha-SMA-positive cells were also detected in lung parenchyma after bleomycin treatment. Following concomitant treatment with anti-transforming growth factor-beta (TGF-beta) antibody with bleomycin, the number of alpha-SMA-positive fibroblastic cells was significantly reduced up to 50%, along with the reduction of dermal sclerosis. To confirm the protein level of alpha-SMA, immunoblotting was carried out. Results showed an increase of alpha-SMA expression in lesional skin at 3 weeks of bleomycin treatment, which was reduced following anti-TGF-beta antibody treatment. These data suggest that fibroblastic cells are phenotypically altered into myofibroblasts during the fibrotic process in the experimental model of bleomycin-induced scleroderma, which was considered mediated, for the most part, by TGF-beta. Blockade of TGF-beta may be a therapeutic intervention for scleroderma.

Modulation of myofibroblast and smooth-muscle phenotypes in the lung

Considerable progress has been made in defining the phenotype of contractile cells of the lung during development, in the adult and during the remodeling process. The high degree of phenotypic heterogeneity of subgroups of these cells, such as SMCs, is appreciated. Recent studies also have explored the relationship between phenotype and cell function, though this remains an important area for research in the coming years. Similarly, though our understanding of the regulation of cell phenotype is expanding rapidly, much remains to be done, particularly at the level of gene regulation. New transgenic models, coupled with gene-promotor analyses in transgenic animals and in cultured cells should allow rapid progress. Studies of the regulation of specific contractile and cytoskeletal proteins at the gene level by specific cytokines and extracellular-matrix elements will be particularly important.

Smooth muscle actin and myosin expression in cultured airway smooth muscle cells

In this study, the expression of smooth muscle actin and myosin was examined in cultures of rat tracheal smooth muscle cells. Protein and mRNA analyses demonstrated that these cells express alpha- and gamma-smooth muscle actin and smooth muscle myosin and nonmuscle myosin-B heavy chains. The expression of the smooth muscle specific actin and myosin isoforms was regulated in the same direction when growth conditions were changed. Thus, at confluency in 1 or 10% serum-containing medium as well as for low-density cells (50-60% confluent) deprived of serum, the expression of the smooth muscle forms of actin and myosin was relatively high. Conversely, in rapidly proliferating cultures at low density in 10% serum, smooth muscle contractile protein expression was low. The expression of nonmuscle myosin-B mRNA and protein was more stable and was upregulated only to a small degree in growing cells. Our results provide new insight into the molecular basis of differentiation and contractile function in airway smooth muscle cells.

In vitro expression of the alpha-smooth muscle actin isoform by rat lung mesenchymal cells: regulation by culture condition and transforming growth factor-beta

alpha-Smooth muscle actin (alpha SM actin)-containing cells recently have been demonstrated in intraalveolar lesions in both rat and human tissues following lung injury. In order to develop model systems for the study of such cells, we examined cultured lung cell lines for this phenotype. The adult rat lung fibroblast-like "RL" cell lines were found to express alpha SM actin mRNA and protein and to organize this actin into stress fiber-like structures. Immunocytochemical staining of subclones of the RL87 line demonstrated the presence in the cultures of at least four cell phenotypes, one that fails to express alpha SM actin and three distinct morphologic types that do express alpha SM actin. The proportion of cellular actin that is the alpha-isoform was modulated by the culture conditions. RL cells growing at low density expressed minimal alpha SM actin. On reaching confluent densities, however, alpha SM actin increased to at least 20% of the total actin content. This effect, combined with the observation that the most immunoreactive cells were those that displayed overlapping cell processes in culture, suggests that cell-cell contact may be involved in actin isoform regulation in these cells. Similar to the response of some smooth muscle cell lines, alpha SM actin expression in RL cells also was promoted by conditions, e.g., maintenance in low serum medium, which minimize cell division. alpha SM actin expression was modulated in RL cells by the growth factor transforming growth factor-beta. Addition of this cytokine to growing cells substantially elevated the proportion of alpha SM actin protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Lung disease in systemic sclerosis (scleroderma)

Scleroderma (SSc) is a disease characterized by skin fibrosis but it is the end-organ effect of microvascular injury and fibrosis that is important prognostically. Pulmonary involvement in SSc patients, either of parenchymal fibrosis and/or pulmonary hypertension, is a major cause of morbidity and mortality. Interstitial lung disease occurs more commonly in patients with diffuse SSc and is associated with a loss of lung volume, as well as a defect of gas exchange. Parenchymal fibrosis may also cause pulmonary hypertension. Isolated pulmonary hypertension occurs exclusively in patients with limited SSc and is detectable by a reduced DCO. The early identification of either manifestation is difficult. Patients may have minimal symptoms, unremarkable physical findings, normal chest radiographs and/or minimally abnormal pulmonary function tests at a time when significant lung pathology is present. It is essential to attempt to identify pulmonary disease early, at a potentially reversible stage. Multiple therapeutic endeavours have yielded only short-term or minimal benefits in symptoms and pulmonary function, and thus a major alteration in SSc pulmonary prognosis has not been achieved. Further study of the pathogenesis of this disease manifestation will be helpful in its earlier identification and intervention.