Association of increased radiocurability of murine carcinomas with low constitutive expression of p21(WAF1/CIP1) protein

Purpose: The study investigated whether basal, constitutive levels of p21(W AF1/CIP1) protein in murine carcinomas are related to in vivo tumor radiore sponse. The study is based on recent observations demonstrating that in vit ro cancer cell lines are resistant to cytotoxic drugs when they express hig h basal levels of p21(WAF1/CIP1) protein, and that the loss of the p21 gene in the HCT116 human colorectal cancer cell line results in increased radio response of xenografts derived from that cell line. Methods and Materials: Protein levels of p21(WAF1/CIP1), p53, bar, and bcl- 2 were determined in 8 carcinomas (3 mammary carcinomas designated MCa-4, M Ca-29, and MCa-35, 2 squamous cell carcinomas designated SCC-IV and SCC-VII , ovarian adenocarcinoma OCa-I, hepatocarcinoma HCa-I, and adenosquamous ca rcinoma ACa-SG) syngeneic to C3Hf/Kam mice using Western blot analysis. The tumors, growing in the right hind legs of mice, were 8 mm in diameter at t he time of analysis. These tumors greatly differ in their radioresponse, as sessed by TCD50 assay, and in their susceptibility to radiation-induced apo ptosis. Results: Protein levels of these oncogenes varied among tumors, with p21(WA F1/CIP1) showing the greatest variation: its mean densitometric value range d from 1 to 19, Bcl-2 levels also showed broad variation in densitometric v alues, from 1 to 10. In comparison, bar and p53 (7 of 8 tumors contained wi ld-type p53) varied much less among different tumor types; their variation was within a 5-fold range, and the level of p53 was similar in 6 of 8 tumor s. Tumor radioresponse correlated significantly (R = 0.77, p = 0.02) only w ith the magnitude of p21(WAF1/CIP1) expression: tumors with high levels of p21(WAF1/CIP1) were less radiocurable than those with lower levels. Tumor r adiocurability showed a significant positive correlation (p = 0.02) with th e extent of radiation-induced apoptosis, indicating that tumors that respon ded to radiation with higher percentages of apoptosis were more curable by radiation. Despite a strong trend to correlation, (p = 0.15), p21(WAF1/CIP1 ) expression did not correlate significantly with radiation-induced apoptos is, which suggested that p21(WAF1/CIP1) influenced tumor radioresponse by m echanisms beyond that of apoptosis induction. Conclusion: Our findings showed that murine tumors exhibit wide variation i n constitutive levels of p21(WAF1/CIP1) which had a significant relationshi p with tumor radioresponse: tumors with high levels of p21(WAF1/CIP1) were less radiocurable than those with lower levels. These findings support the concept that p21(WAF1/CIP1) is a major determinant of tumor radioresponse i n,vivo, and may have important clinical implications. The pretreatment asse ssment of p21(WAF1/CIP1) protein could serve as a useful predictor of radio therapy outcome and may assist in selecting an effective treatment modality . (C) 1999 Elsevier Science Inc.