Cathepsin activity and cystatin C amount in primary and metastatic human melanoma cells. (A) Quantitative cytofluorimetric analysis of plasma membrane cathepsin B, D and L in four different primary (white columns) and metastatic (black columns) melanoma cell lines. Numbers represent the mean values among four different primary or metastatic melanoma cell lines analyzed. (B) Fluorimetric assays for cathepsin B (black columns), cathepsin D (grey columns) and cathepsin L (white columns) activity in the cell medium of two representative primary and metastatic melanoma cell lines. Results are reported as fluorescence units. (C) Concentration of cystatin C in the growth medium of two representative primary and metastatic melanoma cell lines obtained by ELISA test. Results are reported as ng/ml. (*) p < 0.01 for PM cell lines vs. MM cell lines by Student's t-test. (D) Quantitative cytofluorimetric analysis of cystatin C intracellular content in two representative primary (light grey histograms) and metastatic (deep grey histograms) melanoma cell lines. Numbers represent the median fluorescence values ± SD among four independent measurements. Histograms from a representative experiment are shown.