p21CIP-1/WAF-1 induction is required to inhibit prostate cancer growth elicited by deficient expression of the Wnt inhibitor Dickkopf-1.

Osteoblastic bone metastases are the most common metastases produced by human prostate cancers (PCa). Deregulated activity of Wnt growth factors resulting from overexpression of the Wnt inhibitor Dickkopf-1 (DKK-1) is known to contribute to formation of the osteoblastic component of PCa skeletal bone metastases. In this study, we report that DKK-1 knockdown in osteolytic human PCa cells unexpectedly delays the development of both soft tissue and osseous lesions. PCa cells deficient in DKK-1 expression did not increase canonical Wnt signaling in target osteoblast cell lines; however, DKK-1 knockdown PCa cells exhibited increased expression of the CDK inhibitor p21(CIP1/WAF1) and a 32% increase in G(1) arrest compared with control cells. Ablating p21(CIP1/WAF1) in PCa cells deficient in DKK-1 was sufficient to rescue tumor growth. Collectively, our findings demonstrate that DKK-1 overexpression supports tumor growth in part by restricting expression of p21(CIP1/WAF1) through a mechanism independent of canonical Wnt signaling.