ACTA1 and pulmonary fibrosis: To examine the involvement of neutrophil elastase in PM-induced increased severity of bleomycin-induced pulmonary fibrosis, we first showed that neutrophil elastase induced differentiation of primary mouse lung fibroblasts into myofibroblasts as demonstrated by the increased expression of α-smooth muscle actin (α-SMA), a marker of myofibroblast [16], after treatment with neutrophil elastase (Figure 2D).