Human leukocyte elastase induces keratinocyte proliferation by epidermal growth factor receptor activation

Epidermal hyperproliferation and neutrophil infiltration are major histopathological changes observed in psoriasis. Neutrophils contain human leukocyte elastase (HLE), which is released at sites of inflammation. HLE is present in psoriatic lesions and induces keratinocyte hyperproliferation in vitro and in vivo. To determine the molecular mechanisms linking a proteolytic effect of HLE and epidermal hyperproliferation, we examined the effects of HLE-induced signaling in human keratinocytes. Application of 100 nM HLE resulted in a transient calcium influx in FURA2-loaded human HaCaT keratinocytes observed by single-cell fluorescence imaging. The calcium signal was concentration dependent and was inhibited by addition of the HLE inhibitors elafin and secretory leukocyte protease inhibitor. The calcium signal was neither inhibited by pertussis toxin, cholera, or by pre-stimulation with trypsin. Incubation with the tyrosine kinase inhibitor genistein, a protein kinase C inhibitor, as well as incubation with neutralizing EGFR antibodies abolished the HLE-induced calcium influx. The supernatants of HLE-treated keratinocytes induced a calcium signal in separately cultured keratinocytes. This could be inhibited by the addition of anti-TGF-alpha antibodies. Application of HLE-induced keratinocyte proliferation, which could be inhibited by neutralizing of anti-EGFR and anti-TGF-alpha antibodies. Herein we demonstrate that HLE induces keratinocyte proliferation by proteolytic activation of an EGFR signaling cascade involving TGF-alpha.