Cathepsin B fraction active at physiological pH of 7.5 is of prognostic significance in squamous cell carcinoma of human lung

In this study we examined both the pH dependence of cathepsin B (cath B) activity and its stability at physiological pH of 7.5 in lung tumours and normal lung tissue by means of fluorogenic assays with Z-Arg-Arg-AMC as specific substrate. Specificity was verified with the cath B blocking inhibitors E-64 and CA-074. With respect to pH dependence of activity, we found a deviation from a normal-shaped pH- activity curve. Besides the typical activity peak at pH 6.0, there were shoulders at pH 4.5-5.5 and at pH 7.0-7.5. This heterogeneity was found in both tumour and normal tissue. To test the stability of cath B at physiological pH of 7.5, homogenates were kept at pH 7.5 for 60 min. Altogether, 82-100% of residual cath B activity was found at pH 5.0-5.5, whereas activity in the range between 5.5 and 7.4 dropped drastically to 26-42%. At pH 7.5, there was still 20-34% residual cath B activity detectable. To test the hypothesis whether the cath B fraction active at pH 7.5 is more abundant in tumour tissues compared with the normal counterparts, we determined this fraction in 91 pairs of lung tumour and normal lung tissue. We found a 2.3-fold increase of median cath B fraction active at pH 7.5 in tumour tissue, although this fraction represented only a small part (about 16%) of the native, acidic (pH 6.0) cath B activity. However, in contrast to native cath B at 6.0, the cath B fraction active at pH 7.5 was related to post-operative probability of survival in curatively operated patients, since activity values higher than 292 (muEU mg(-1) protein) were significantly associated with poor prognosis in patients with squamous cell carcinomas (n = 33, P= 0.04). It is concluded that in lung tumour and in normal lung tissue, cath B activity can be divided into at least three fractions with stability optima at different pH values, indicating various forms of cath B. The cath B fraction active at pH 7.5 provides prognostic information in patients with squamous cell carcinoma.