Studies examining many different cancers have demonstrated that inflammation plays a critical role in tumor progression, in part through the release of proteases from stromal cells that function to either remodel the tumor microenvironment or to directly stimulate cancer cells to grow. One specific protease, neutrophil elastase (NE), has been shown to be a critical regulator of cancer growth in several mouse models. Accordingly, our laboratory demonstrated that NE, most likely from granulocytic myeloid-derived suppressor cells, potentially promotes prostate cancer progression in several different in-vivo and in-vitro models. To date, however, little is known regarding the mechanisms utilized by NE to promote tumor growth. It has been suggested that NE might cleave epidermal growth factor (EGF) or transforming growth factor-α from the cell surface to induce activation of EGFR/ERK signal transduction in an autocrine fashion. Alternatively, NE has been shown to enter into early endosomes to degrade insulin receptor substrate-I, ultimately resulting in phosphoinositol 3-kinase hyperactivity and subsequent tumor cell proliferation. Here we demonstrate that NE triggered proliferative signals in six prostate cell lines representing the spectrum of prostate cell differentiation, including normal prostatic epithelium, benign prostatic hypertrophy, and metastatic prostate cancer. Focusing on ERK signaling, we found that the stimulatory effect of NE on ERK phosphorylation was dose dependent and was abrogated by small interfering RNA induced EGFR knockdown, as well as by pretreatment of cells with irreversible EGFR inhibitor AG1478. Unlike EGF, however, NE-initiated EGFR phosphorylation was minimal. Thus, while EGFR appears to be critical for NE-induced ERK activation, perhaps it is not extensively activated directly by NE. Notably, discoidin domain receptor-1 (DDR1) was strongly expressed in normal prostate epithelium cells, but gradually decreased and had little expression in benign and metastatic prostate cancer cells sequentially. Nevertheless, similar to EGFR knockdown, silencing of DDR1 in all cell types inhibited NE mediated pERK upregulation, suggesting that DDR1 may also be important for NE-induced action. Together, our data suggest that NE, in concert with low level signals from the EGFR and DDR1, play an important role in promoting prostate cell proliferation both in normal and cancerous prostate epithelial cells.