Enhanced myofibroblastic differentiation and survival in Thy-1(-) lung fibroblasts

Thy-1 is a glycosylphosphatidyl-inositol-linked cell surface glycoprotein whose exact biological role remains unclear. Differential expression of Thy-1 affects fibroblast proliferation and fibrogenic signaling. In idiopathic pulmonary fibrosis, the proliferating myofibroblasts within the fibroblastic foci are Thy-1(-), whereas normal lung fibroblasts are predominantly Thy-1(+). In this study, we used rat lung fibroblasts sorted for Thy-1 expression to examine myofibroblastic differentiation in response to fibrogenic stimuli. We examined the effects of transforming growth factor-beta, endothelin-1, and connective tissue growth factor on the expression of myofibroblast proteins and myogenic regulatory factors by real-time RT-PCR and immunoblotting. Thy-1(-) cells have significantly higher myofibroblast and myogenic regulatory factor gene and protein expression compared with Thy-1(+) cells, confirmed by immunofluorescence. We also used floating collagen matrix contraction assays to assess the functional differentiation of the fibroblasts. At baseline and after stimulation with transforming growth factor-beta and endothelin-1, Thy-1(-) cells caused significantly greater collagen contraction than did Thy-1(+) cells, supporting the hypothesis that Thy-1(-) cells are more fully differentiated myofibroblasts. Because apoptosis has been implicated in the regression of myofibroblasts, we examined the percentage of apoptotic cells in the contracted collagen matrices at baseline and after stimulation with fibrogenic agents. A significantly greater proportion of Thy-1(+) cells underwent apoptosis in all conditions compared with Thy-1(-) fibroblasts. Transfection of Thy-1 into Thy-1(-) cells inhibits collagen matrix contraction and reduces cell survival. Our data indicate that Thy-1 regulates myogenic gene expression, myofibroblastic differentiation, and survival in lung fibroblasts.

Loss of fibroblast Thy-1 expression correlates with lung fibrogenesis

Fibroblasts consist of heterogeneous subpopulations that have distinct roles in fibrotic responses. Previously we reported enhanced proliferation in response to fibrogenic growth factors and selective activation of latent transforming growth factor (TGF)-beta in fibroblasts lacking cell surface expression of Thy-1 glycoprotein, suggesting that Thy-1 modulates the fibrogenic potential of fibroblasts. Here we report that compared to controls Thy-1-/- C57BL/6 mice displayed more severe histopathological lung fibrosis, greater accumulation of lung collagen, and increased TGF-beta activation in the lungs 14 days after intratracheal bleomycin. The majority of cells demonstrating TGF-beta activation and myofibroblast differentiation in bleomycin-induced lesions were Thy-1-negative. Histological sections from patients with idiopathic pulmonary fibrosis demonstrated absent Thy-1 staining within fibroblastic foci. Normal lung fibroblasts, in both mice and humans, were predominantly Thy-1-positive. The fibrogenic cytokines interleukin-1 and tumor necrosis factor-alpha induced loss of fibroblast Thy-1 surface expression in vitro, which was associated with Thy-1 shedding, Smad phosphorylation, and myofibroblast differentiation. These results suggest that fibrogenic injury promotes loss of lung fibroblast Thy-1 expression, resulting in enhanced fibrogenesis.