Purpose: The prevention and treatment of prostate cancer metastasis continue to provide a significant clinical challenge. Identification of the rate limiting steps of metastasis and their underlying molecular mechanisms may lead to new therapeutic targets and also allow more accurate risk stratification for clinical metastases. We review the literature supporting growth of disseminated tumor cells at the secondary site as a key rate limiting step in metastasis. We also reviewed the definition, identification and characterization of metastasis suppressor genes, and discuss their evolving role in regulating this step.
Materials and methods: We performed MEDLINE searches and manual bibliographic reviews on the specific steps of metastasis, including growth at the secondary site. In addition, we performed a comprehensive literature review to identify genes fitting the classic definition of a metastasis suppressor gene. The literature was also searched to assess the status of each gene in clinical cancer and evaluate functional support for the potential involvement of each gene in regulating growth at the secondary site.
Results: Clinical studies in prostate cancer and other cancer types suggest that dissemination to the secondary site is often an early clinical event. However, not all patients with tumor cells at the secondary site have overt metastatic lesions even in the absence of therapy, suggesting that growth at the secondary site may be highly inefficient. Complimentary approaches have allowed researchers to document and quantify the inefficiency of cancer cell growth at the secondary site. Regarding the mechanism of growth control, many studies support a role for the interaction of a cancer cell and the microenvironment at the secondary site influencing whether growth into metastasis may occur. The 7 genes that suppress metastasis without affecting primary tumor growth that have been identified are KAI1, CD44, mitogen activated protein kinase (MAPK) kinase 4, nm23-H1, nm23-H2, KiSS1 and BrMS1. Three of these genes (KAI1, CD44 and MAPK kinase 4) act as metastasis suppressor genes of prostate cancer, while the remainder have yet to be tested in this cancer type. Loss of expression has been demonstrated for most of these genes during the clinical progression of prostate cancer to metastasis. MAPK kinase 4 and KiSS1 appear to suppress metastasis by inhibiting cancer cell growth at the secondary site. Interestingly many metastasis suppressor genes have common roles in growth control, adhesion and cytoskeletal reorganization, suggesting a common mechanism of metastasis suppression. Proposed candidate pathways include signaling through Src kinase and Rac GTPase.
Conclusions: The findings discussed support growth at the secondary site as a clinical target for metastasis treatment and prevention. Metastasis suppressor genes may offer valuable mechanistic insight for guiding specific therapeutic strategies, which may include drug induced reactivation of metastasis suppressor genes and their signaling pathways. Clinical assessment of metastasis suppressor gene product status in disseminated cancer cells may improve the accuracy of predicting the prognosis in patients with clinically localized disease.