Expression of Bmi-1 is a prognostic marker in bladder cancer

Abstract

Background: The molecular mechanisms of the development and progression of bladder cancer are poorly understood. The objective of this study was to analyze the expression of Bmi-1 protein and its clinical significance in human bladder cancer.

Methods: We examined the expression of Bmi-1 mRNA and Bmi-1 protein by RT-PCR and Western blot, respectively in 14 paired bladder cancers and the adjacent normal tissues. The expression of Bmi-1 protein in 137 specimens of bladder cancer and 30 specimens of adjacent normal bladder tissue was determined by immunohistochemistry. Statistical analyses were applied to test the relationship between expression of Bmi-1, and clinicopathologic features and prognosis.

Results: Expression of Bmi-1 mRNA and protein was higher in bladder cancers than in the adjacent normal tissues in 14 paired samples (P < 0.01). By immunohistochemical examination, five of 30 adjacent normal bladder specimens (16.7%) versus 75 of 137 bladder cancers (54.3%) showed Bmi-1 protein expression (P < 0.05). Bmi-1 protein expression was intense in 20.6%, 54.3%, and 78.8% of tumors of histopathological stages G1, G2, and G3, respectively (P < 0.05). Expression of Bmi-1 protein was greater in invasive bladder cancers than in superficial bladder cancers (81.5% versus 32.5%, P < 0.05). In invasive bladder cancers, the expression of Bmi-1 protein in progression-free cancers was similar to that of cancers that have progressed (80.0% versus 82.4%, P > 0.5). In superficial bladder cancers, the expression of Bmi-1 protein in recurrent cases was higher than in recurrence-free cases (62.5% versus 13.7%, P < 0.05). Bmi-1 expression was positively correlated with tumor classification and TNM stage (P < 0.05), but not with tumor number (P > 0.05). Five-year survival in the group with higher Bmi-1 expression was 50.8%, while it was 78.5% in the group with lower Bmi-1 expression (P < 0.05). Patients with higher Bmi-1 expression had shorter survival time, whereas patients with lower Bmi-1 expression had longer survival time (P < 0.05).

Conclusion: Expression of Bmi-1 was greater in bladder cancers than in the adjacent normal tissues. The examination of Bmi-1 protein expression is potentially valuable in prognostic evaluation of bladder cancer.

Figure 1

Expression of Bmi-1 mRNA in paired bladder cancers and the adjacent normal tissues by RT-PCR analysis. A: RT-PCR analysis in 14 paired bladder cancers and the adjacent normal tissues (upper panel); β-actin as the internal control (showed in lower panel). B: The relative expression level of Bmi-1 in comparison to the expression level of β-actin.

Figure 2

Expression of Bmi-1 protein in paired bladder cancers and the adjacent normal tissues by Western Blot analysis. A: Western Blot analysis in 14 paired bladder cancers and the adjacent normal tissues (upper panel); α-tubulin as the loading control (showed in lower panel). B: The relative expression level of Bmi-1 in comparison to the expression level of α-tubulin.

Figure 3

Expression analysis of Bmi-1 protein by immunohistochemistry. Bmi-1 staining was mainly localized within the nuclei, and it expressed was observed in cancer cell. A: Negative Bmi-1 staining in non-cancerous tissue (100×). B: Weak Bmi-1 staining in non-cancerous cells(400×). C: strong Bmi-1 staining in tumor nests(400×). D: Strong nuclear stainingin most of the tumor cells (400×).

Figure 4

Survival curves of patients with bladder cancer, subdivided according to Bmi-1 protein expression. The 5-year survival rate was 79.0% in the Bmi-1(low) group (n = 62) and it was 50.8% in the Bmi-1(high) group (n = 75). The longest follow-up time is 86 months.