Objective To study the expression of skin aspartic protease (SASPase) in lesions of cutaneous lupus erythematosus (CLE) and its role in the pathogenesis of CLE.Methods Skin samples were resected from the lesions and normal skin of 9 patients with CLE,including 3 cases of subacute cutaneous lupus erythematosus (SCLE),4 cases of discoid lupus erythematosus (DLE) and 2 cases of acute cutaneous lupus erythematosus (ACLE).Keratinocytes were isolated from the tissue samples and cultured in serum-free medium.Total proteins were extracted from the keratinocytes and separated by two-dimensional gel electrophoresis.ImageMaster 2D analysis software was used to assess differentially expressed proteins in keratinocytes between the lesional and normal skin,which were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS).The expression levels of SASPase were further determined by Western blot.The data were analyzed statistically by Student's t test.Results Keratinocytes were isolated from the tissue samples and successfully cultured in vitro.Two-dimensional electrophoresis profiles of proteins from the keratinocytes were obtained with high resolution and reproducibility,and the average matching protein spots were about 1200 with the matching rate higher than 80％.As Western blot showed,the relative expression level of SASPase was 0.463 ± 0.018 in keratinocytes from the lesional skin,and 0.145 ± 0.011 in those from the normal skin (P ＜ 0.05).The Western blot results were consistent with those of two-dimensional electrophoresis.Conclusion The initiation and progression of CLE seem to be associated with the abnormal activation and overexpression of SASPase.