Targeting Constitutively Activated b1 Integrins Inhibits Prostate Cancer Metastasis

Disseminated prostate cancer cells must survive in circulation for metastasis to occur. Mechanisms by which these cells survive are not well understood. By immunohistochemistry of human tissues, we found that levels ofb1 integrins and integrin-induced autophosphorylation of FAK (pFAK-Y397) are increased in prostate cancer cells in primary prostate cancer and lymph node metastases, suggesting that b1 integrin activation occurs in metastatic progression of prostate cancer. A conformation-sensitive antibody, 9EG7,was used to examineb1 integrin activation.We found that b1 integrins are constitutively activated in highly metastatic PC3 and PC3-mm2 cells, with less activation in low metastatic LNCaP andC4-2B4 cells. Increasedb1 integrin activation as well as the anoikis resistance in prostate cancer cells correlated withmetastatic potential in vivo. Knockdown of b1 integrin abrogated anoikis resistance in PC3-mm2 cells. In agreement with b1 integrin activation, PC3-mm2 cells strongly adhered to type I collagen and fibronectin, a process inhibited by the b1 integrin-neutralizing antibody mAb 33B6. mAb 33B6 also inhibited the phosphorylation of b1 integrin downstream effectors, focal adhesion kinase (FAK) and AKT, leading to a 3-fold increase in PC3-mm2 apoptosis. Systemic delivery of mAb 33B6 suppressed spontaneous metastasis of PC3-mm2 from the prostate to distant lymph nodes following intraprostatic injection and suppressed metastasis of PC3-mm2 to multiple organs following intracardiac injection. Thus, constitutively activated b1 integrins play a role in survival of PC3-mm2 cells in circulation and represent a potential target for metastasis prevention. Visual Overview: http://mcr.aacrjournals.org/content/11/4/405/F1.large.jpg. Mol Cancer Res; 11(4); 405–17. 2013 AACR.