Human leukocyte elastase induces keratinocyte proliferation in vitro and in vivo

Neutrophil infiltration and epidermal hyperproliferation are major histopathologic changes observed in psoriasis. Neutrophils contain human leukocyte elastase, which is thought to be released during neutrophil infiltration of the epidermis. As active human leukocyte elastase is known to be present in psoriatic lesions we were interested whether human leukocyte elastase induces hyperproliferation in keratinocytes in vitro and in vivo. In the cultured murine keratinocyte cell line PAM-212 concentrations of human leukocyte elastase from 1 to 30 nM induced significant proliferation as determined by 5-bromo-2'-deoxy-uridine-incorporation. Daily topical application of 0.043-434.8 pmol human leukocyte elastase per cm2 skin on hairless mice induced a concentration-dependent epidermal hyperproliferation and an increase in 5-bromo-2'-deoxy-uridine incorporation of up to 5-fold in basal keratinocytes within 3 d. Hyperproliferation resulted in a up to 2-fold increase of keratinocyte layers. Histologic analysis revealed marked vasodilatation but no inflammatory infiltrate. Application of porcine pancreatic elastase (3-300 pmol per cm2 skin) resulted in similar epidermal changes as observed for human leukocyte elastase. Hyperproliferative effects of human leukocyte elastase in vitro and in vivo were abolished by the addition of elastase inhibitors, such as elafin, anti-leukoprotease, and alpha1-protease inhibitor. In summary, human leukocyte elastase induces proliferation in murine keratinocytes in concentrations, which can be found on the skin surface of psoriatic lesions. These results may provide an explanation for the epidermal hyperproliferation observed in psoriasis.