Ninjurin1 Is Up-regulated in Circulating Prostate Tumor Cells and Plays a Critical Role in Prostate Cancer Cell Motility

Abstract Ninjurin1 is a 17-kDa membrane protein that is highly expressed in circulating tumor cells (CTCs) obtained from locally-advanced prostate cancer patients. As CTCs are implicated in the initiation of distant metastasis, we examined the potential contribution of Ninjurin1 to the motility of prostate cancer cells. Ninjurin1 expression was evaluated in CTCs harvested from seven locally advanced patients with no metastatic hallmarks using real-time polymerase chain reaction (PCR). The role of Ninjurin1 in cell motility was investigated using small interfering RNA (siRNA), neutralizing antibodies against Ninjurin1 and Ninjurin1-overexpressing adenoviruses. Ninjurin1 was ranked as the most significantly up-regulated adhesion protein identified by RNA-Seq analysis. Both Ninjurin1 down-regulation by siRNA and neutralizing antibodies blocking Ninjurin1 homophilic interactions effectively inhibited cell motility. In contrast, cell motility was enhanced in prostate cancer cells infected with adenovirus enabling Ninjurin1 overexpression. Ninjurin1-neutralizing antibodies or Ninjurin1-targeting siRNA merit further development for patients with metastatic potential.