Cancer Therapy Vol 1, page 105 was usually associated with a loss of RB1CC1 expression in the present study, the absence of the latter seems to be a major cause of depleted expression of RB1 and subsequent breast cancer tumorigenesis. The loss of RB1CC1 expression was significantly correlated with a higher Ki-67 labeling index in our series (p < 0.0001). Since Ki-67 identifies proliferating cells by recognizing a nuclear antigen (Gerdes et al, 1987), our findings suggest that a loss of RB1CC1 expression promotes breast cancer progression through disruption ofits downstream pathways that normally suppress proliferative activity. Other studies have shown that Ki-67 staining levels are positively correlated with tumor size and nodal involvement in breast cancer (Wintzer, 1991; Molino et al, 1997). The level of Ki-67 may be an independent prognostic factor in breast cancer because Ki67 positivity is correlated with disease-free and overall survival rates (Rolio, 1993; Molino et al, 1997). RB1CC1negative cancers tended to show earlier metastasis than RB1CC1-positive ones. In our series, however, we could not conclude the sure relationship between RB1CC1 expression and the prognosis of breast cancer due to the short period of clinical observation and small numbers of cases. Therefore, longer-term clinical studies involving larger numbers of patients are required to confirm this issue. RB1CC1 expression and ER status were positively correlated. Estrogen regulates the proliferation and maturation of normal breast tissue through its receptors. In addition, both RB1CC1 and RB1 may contribute to the development and maturation of human embryonic cells (Chano, 2002d) and the RB1CC1-RB1 cascade may play a role in the maturation of breast tissues involving functional ER expression. About 70% of primary breast cancers are ER-positive (Andersen and Poulsen, 1989; Harvey et al, 1999) and such patients respond more favorably to endocrine therapy and survive longer than those with ER-negative cancer (Andersen and Poulsen, 1989; Harvey et al, 1999). Evaluating RB1CC1 expression in breast cancer may be helpful in predicting responses to adjuvant therapy. In conclusion, our findings suggest that RB1CC1 plays an important role in the RB1 pathway and that the absence of RB1CC1 expression accelerates cell proliferation in breast cancer. In addition, RB1CC1 status may be an important prognostic factor in breast cancer
Figure 3. RB1 and Ki-67 labeling indices in specimens with or without RB1CC1 expression. (a) RB1 labeling index is significantly higher with, than without RB1CC1 expression (RB1CC1-positive versus -negative specimens, 78.6 ± 13.9 versus 13.6 ± 12.1%, p < 0.0001; Student’s t-test). (b) Ki-67 labeling index is significantly higher in RB1CC1-negative than positive tumors (RB1CC1-positive versus -negative specimens, 20.3 ± 12.8 versus 65.0 ± 12.2%, p < 0.0001; Student’s t-test).
The 8 cases lacking RB1CC1 expression, showed no LOH at RB1 locus. These findings support those of previous reports (Chano et al, 2002a,b,c) suggesting that RB1CC1 functions as a key regulator of RB1 expression and that the dysfunction of RB1CC1 results in insufficient RB1 expression. The present study found a suspected RB1 abnormality in only one specimen with RB1CC1 expression, but we could not examine the LOH status at RB1 locus because of no matching tumor DNA. A LOH and other alterations at the RB1 locus were observed in 3% to 37% of breast cancers (Varley et al, 1989; Thorlacious et al, 1991; Lemoine, 1994; T’Ang, 1998; Kaelin et al, 1999; Bieche and Lidereau, 2000). However, irregular RB1 protein expression is not always linked to such RB1 gene derangement (Varley et al, 1989; Bieche and Lidereau, 2000). Since low or absent RB1 expression
Acknowledgements We would like to thank H. Chen, N. Takashima, H. Honjo and M. Sugimoto for excellent technical assistance. This study was partially supported by grant-in-aids for Scientific Research, the Ministry of Education, Science, Sports and Culture, Japan (08671356, 10671249, 13470520, 13671380 and 15591340).
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