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Reduced expression of the CDK inhibitor p27(KIP1) in rat two-stage bladder carcinogenesis and its association with expression profiles of p21(WAF1/Cip1) and p53.

The cyclin-dependent kinase (CDK) inhibitor p27(KIP1) exerts its growth suppressive effects by targeting the cyclin-CDK complexes. Reduced protein levels of p27(KIP1) have been reported in numerous human cancers and this has been attributed to increased degradation. However, few reports have addressed the significance of p27(KIP1) expression in chemical carcinogenesis of rodents. In a rat two-stage urinary bladder carcinogenesis model, with N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) initiation followed by promotion with sodium L-ascorbate (Na-AsA), we evaluated the expression of p27(KIP1) protein using immunohistochemistry during various stages of urinary bladder carcinogenesis. In addition, we evaluated the mRNA expression profiles for p27(KIP1), p21(WAF1/Cip1) and p53 in tumors. Fisher 344 rats were initiated with 0.05% BBN in the drinking water for 4 weeks and then administered 5% Na-AsA in the diet. Immunohistochemical examination revealed p27(KIP1) protein to be constitutively expressed in normal urothelium, simple hyperplasia and in most papillary and nodular (PN) hyperplasias and small papillomas, but diminished or absent in large papillomas and in transitional cell carcinomas. An inverse correlation between expression of p27(KIP1) and cell proliferation was generally observed. Quantitation of mRNA by multiplex reverse transcription-PCR showed a significant downregulaton of p27(KIP1), p21(WAF1/Cip1) and p53 mRNA in tumors. More than 50% reduction in p27(KIP1) mRNA expression was observed in 42 and 47% of tumors at weeks 18 and 24, respectively; similar reduction in p21(WAF1/Cip1) mRNA expression was observed in 58 and 73% of tumors at weeks 18 and 24, and in p53 mRNA expression in 50 and 73% of tumors at weeks 18 and 24, respectively. None of the 25 tumors we examined by PCR-single-strand conformational polymorphism analysis had p53 mutations. These data imply that abnormal down-regulation of p27(KIP1), p21(WAF1/Cip1) and/or p53 in tumor cells may contribute to the malignant progression of tumors during rat two-stage bladder carcinogenesis.