Neutrophils and their derived elastase are abundant in chronic inflammatory responses of asthma. This study aimed to investigate the mitogenic effect of elastase on airway smooth muscle (ASM) cells and the implicated signal transduction pathway. Near confluent cultured human ASM cells were treated with human neutrophil elastase (HNE, 0.01 to 0.5 microg/ml) or vehicle for 24 hours with or without extracellular signal-regulated kinase (ERK) inhibitor (PD98059, 30 microM), p38 kinase inhibitor (SB203580, 10 microM) or elastase inhibitor II (100 microg/ml). The ASM cell numbers were counted by a hemocytometer and DNA synthesis was assessed by flowcytometry. Western blots analysis for the expression of ERK, p38 and cyclin D1 was determined. HNE dose-dependently increased ASM cell numbers and the percentage of cells entering S-phase of cell cycle. This response was abolished by neutrophil elastase inhibitors and attenuated by PD98059, but not SB203580. HNE increased ERK phosphorylation and cyclin D1 expression. Pretreatment with PD98059 significantly inhibited elastase-induced cyclin D1 activity. The increased ASM cellular gap and cell shape change by proteolytic activity of HNE may be contributory to ERK activation and therefore cell proliferation. Our results demonstrate that HNE is mitogenic for ASM cells by increasing cyclin D1 activity through ERK signaling pathway.