Quantification of intracellular cathepsin activities in human lung tumor cell lines by flow cytometry

The cysteine proteases cathepsin B and cathepsin L are very likely involved in invasive processes of normal and malignant cells, they become relevant for a number of diseases and are possibly prognostic markers for the outcome of human lung cancer. Therefore, we have determined activities of these related enzymes in cells and in cell extracts of human lung carcinoma cell lines of different cathepsin composition by flow cytometry and by spectrophotometry, respectively. To this end we applied the synthetic dipeptidyl substrates benzoxycarbonyl-arginyl-arginine- and benzoxycarbonyl-phenyl-arginine- coupled to 4-methoxy-beta-naphthylamide, aminomethyl-coumarine or rhodamine R110. The apparent enzymatic activities were differentially defined by protease inhibitors, particularly E-64 and CA-074. Independent of the dipeptidyl-composition more than 99 per cent of the apparent activity was due to cathepsin B when 4-methoxy-beta-naphthylamide or aminomethylcoumarine were the leaving groups. The 4-methoxy-beta-naphthylamide precipitate used for detection of cell associated activities revealed a wide spectrum of excitation to fluorescence thwarting the application of other possible fluorescent tags. Therefore, its application is restricted to uniparametric fluorescence investigations. Both dipeptidylgroups coupled to rhodamine R110 were promiscuous: only 25 to 30% of the apparent activity were due to cathepsin B; the predominant activity came from cathepsin L, irrespective whether intracellular or activities of cellular extracts were analyzed. However, rhodamine R110-coupled substrates open the way for multiparametric fluorescent analysis of cathepsins B and L containing cells if appropriate inhibitors for specification of the enzymatic activities are additionally applied. In very contrast to 4-methoxy-beta-naphthylamide, which causes irreparable damage to the cells, the rhodamine substrates permit studies with living cells and live cell sorting.