## 4 Discussion
The strong diagnostic potential of serum UCH-L1 concentration for distinguishing between muscle-invasive and non-muscle-invasive TCC is demonstrated by the corresponding ROC curve (Figure 3D and Table 5), with an AUC of 0.74. The cut-off value is 6.27 ng/mL. These results correspond with those reported by [PERSON] et al., in which variables representing tumour size, differentiation, and proliferation were associated with elevated 20S proteasome levels [40]. The results for urine 20S proteasome concentration (Table 4) were similar to those for serum: the concentration was significantly higher for high-grade than for low-grade TCC (\(p\) = 0.0033) and was higher for primary than for recurrent TCC and for muscle-invasive than for non-muscle-invasive tumours. Additionally, urine UCH-L1 concentration was significantly higher for tumour sizes above 30 mm than for smaller-sized tumours. In addition, 20S proteasome and UCH-L1 were rated for their predictive value. Proteasome appears to correlate with aggressiveness (sensitivity 82%, specificity 42%) and UCH-L1 with invasiveness (75.77%). Interestingly, the studies showed that the proteasome/UCH-L1 ratio in serum may be a useful additional marker. The cut-off value for ROC was calculated at 0.09. It may be speculated that UCH-L1 is a stronger bladder cancer predictor: a higher concentration of this marker relative to the 20S proteasome increases the probability of cancer. This is in line with previous considerations of the role of UCH-L1 in the oncogenic process. The results of this study may be useful in better understanding the biology of TCC tumours and in improving therapeutic methods. The 20S proteasome has been extensively explored as a drug target. Selective inhibition of the 20S proteasome has demonstrated a therapeutic benefit. The 20S proteasome inhibitor bortezomib has been used in lymphoma [52] and myeloma [53] treatment. This confirms the importance of investigations of the UPS in relation to cancer. The SPRi biosensors discussed here allow the detection of 20S proteasome and UCH-L1 in serum and urine. They are a simple and cost-effective tool for determining the concentration of these biomolecules. The use of SPRi biosensors makes it possible to analyse complex samples without special procedures for their preparation. The biomolecules can be quantified without the need for labelling. SPRi may therefore be an attractive alternative to conventional testing. The investigated UPS and DUB components do not appear to be highly specific markers of TCC but may be helpful in improving assessments of the progress of the disease and in making optimal therapeutic decisions. Conceptualisation, A.S., T.G. and E.G. ; methodology, A.S. and E.G. ; data collection, A.S. and T.G. ; statistical analysis, A.S. and T.G. ; data interpretation, A.S., E.G. and T.G. ; writing--original draft preparation, A.S. and T.G. ; writing--review and editing, E.G., A.S. and T.G. ; visualisation, A.S. and T.G. ; supervision, E.G. All authors have read and agreed to the published version of the manuscript. This article has received financial support from the Polish Ministry of Science and Higher Education under a subsidy for maintaining the research potential of the Faculty of Chemistry, University of Bialystok (Grant No. BST 162/2018). The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Bioethics Committee at the Medical University of Bialystok, nr R-I-002/409/2014. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data available on request due to restrictions privacy. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.