## 4 Discussion  
Research into new malignancy markers is crucial for the development of clinical oncology. In this study, the focus is on the 20S proteasome, which is part of the ubiquitin-proteasome system, and the protein UCH-L1, which belongs to the family of deubiquitin enzymes (DUBs). The measurements presented in the paper are the first available data on concentrations of 20S proteasome and UCH-L1 protein, and their mutual correlation in the serum and urine of bladder cancer patients. The ubiquitin-proteasome system (UPS) regulates almost all cellular activities, including cell cycle progression, DNA replication and repair, transcription, protein quality control, immune response, and apoptosis [36]. UCH-L1 plays an important role in proteasome activity by cleaving the ubiquitin C-terminal from the ubiquitinated protein to facilitate protein degradation by the 20S proteasome. The serum and urine concentrations of both  
Figure 5: ROC curves for concentration ratio of UCH-L1 to 20S proteasome in serum as diagnostic tests for bladder cancer. substances were determined and correlated with clinicopathological parameters of patients with bladder tumour (transitional cell carcinoma, TCC). It was observed that proteasome and UCH-L1 concentrations were significantly elevated in both the serum and urine of TCC patients. Levels of 20S proteasome were six times higher in serum and seven times higher in urine, in TCC patients, as compared with healthy donors. UCH-L1 levels were eleven times higher in serum and three times higher in urine in TCC patients than in healthy donors (Table 2). In addition, there was a correlation between proteasome concentrations in serum and urine, as well as between serum proteasome concentration and UCH-L1 concentration (Figure 4). The results obtained are consistent with similar previous studies. It has been reported that serum 20S proteasome concentration is elevated in the case of, for example, renal cell carcinoma [37], multiple myeloma [38], breast cancer [39], ovarian cancer [11], and malignant melanoma [40]. Benign conditions, including autoimmune, vascular, and pulmonary conditions, can also alter circulating proteasome levels [41; 42]. [PERSON] et al. reported increased proteasome activity in TCC [43]. It is hypothesised that the 20S proteasome produces activity-related or tumour-associated effects [39]. There have been many reports confirming the relationship between UCH-L1 and cancer. It has been found to be overexpressed in prostate [44], renal [17], colorectal [19], and pancreatic [18] cancers, uterine serous cancer [45], and lymphoblastic leukaemia [46]. It has been confirmed that cancer cells are oxidative stressed. They produce more reactive oxygen species (ROS) and antioxidant systems are suppressed [47]. Oxidative stress can increase UCH-L1, proteasome subunit alpha, and proteasome activity [48; 49]. The UPS (ubiquitin-proteasome system) removes small damaged proteins and protein aggregates and protects tissues from oxidative damage [50]. The UPS is responsible for the selective degradation of proteins engaged in cellular metabolism, for example, in regulating the cell cycle, apoptosis, and gene expression [21; 51]. Higher expression of proteasome and its activity in malignant cells increase the degradation of cycle-controlling proteins and apoptosis, which leads to neoplasm growth. Increased proteasome levels in plasma result from reinforced secretion and disintegration of neoplasm cells. With respect to clinicopathological features, serum 20S proteasome concentration was significantly higher for high-grade than for low-grade TCC (\(p\) = 0.0036). It was also higher for primary than for recurrent TCC, and for muscle-invasive than for non-muscle-invasive tumours. Serum UCH-L1 concentration was significantly higher for high-grade than for low-grade TCC (\(p\) = 0.0001) and for muscle-invasive than for non-muscle-invasive TCC (\(p\) = 0.0001), and was higher for multiple than for single TCC.