Epithelial vs myofibroblast differentiation in immortal rat lung cell lines--modulating effects of bleomycin

Two alveolar epithelial cell lines R3/1 and L2 were screened by immunocytochemical and RT-PCR analysis of epithelial and mesenchymal/contractile marker proteins. R3/1 and L2 cells were tested for their sensitivity to bleomycin (BLM), an anticancer drug, which is proposed to induce changes in lung cell differentiation. Both epithelial cell lines exhibited a mixed phenotype consisting of epithelial (E-cadherin, aquaporin-5 and cytokeratin 8) and myofibroblast-like (vimentin, alpha-SMA and caveolin-3) properties suggesting that the cell lines are arrested in vitro at a certain developmental stage during epithelial-mesenchymal transition (EMT). BLM treatment of R3/1 cells resulted in a partial reversal of this process modifying the cells in an epithelial direction, e.g., upregulation of E-cadherin, aquaporin-5 and other lung epithelial antigens at the mRNA and protein level. L2 cells showed similar alterations following BLM exposure.Immunohistochemical investigation of lung tissue from two different animal models of BLM-induced fibrosis (mouse and rat), revealed no signs of EMT, e.g., myofibroblastic differentiation of alveolar epithelial cells in situ. Immunohistological analysis of tissue samples of the rat model showed a heterogeneous population of myofibroblasts (alpha-SMA+/caveolin-3+, alpha-SMA-/caveolin-3+, and alpha-SMA+/caveolin-3-). These results suggest that BLM, on one hand, induces fibrosis and on the other hand possibly suppresses EMT during fibrogenesis.