1002. L1 Cell Adhesion Molecule, a Novel Target of Prostate Cancer Gene Therapy

Abstract

Bone metastasis is one of the major causes of morbidity and eventual mortality in prostate cancer patients. Unfortunately none of the currently available therapies is effective for curing bone metastasis in these patients. Tumor cell migration and metastasis is a highly coordinated process involving alternation of cell-cell junction and cell adhesion to extracellular matrix (ECM) proteins. Adhesionmolecules that involve in the metastatic cascade of prostate cancer presents an attractive target for therapeutic intervention. The L1 cell adhesion molecule has been implicated in a variety of motile processes in the developing nervous system and malignant diseases. In this communication we demonstrated that L1 was expressed on cell surface and in conditioned medium by androgen-independent and highly metastatic human prostate cancer cell lines DU145, PC3 but not androgen-dependent LNCaP and its derivative C4-2. Using genetic approach to manipulate the expression of L1 in PC3 and C4-2 cells, we found that unlike member-bound L1, which inhibited prostate cancer cell adherence to ECM and migration, due to the L1-mediated homotypic cell-cell interaction, the ectodomain of L1 secreted by cells showed chemotactic activity for L1-defected cancer cells in transwell migration assay. The possible role of soluble L1 on prostate cancer progression and dissemination was also investigated in vivo. The localization of L1 protein in clinical prostate cancer specimens was found at tumor-adjacent stroma and the invasive front of tumors but not the central, differentiated area of the tumor by immunohistochemistry analysis. The mean serum L1 levels in the bone metastatic prostate cancer patients (45.0|[plusmn]|27.2 ng/ml, N=19) were significantly higher than patients with organ-confined tumors (28.4|[plusmn]|22.2 ng/ml, N=30, p<0.05) and normal controls (12.1 |[plusmn]| 8.6 ng/ml, N=10, p<0.001), as detected by ELISA method. These results suggest that L1 released by tumor and/or tumor-associated stroma acted as a chemoattractant and adhesive matrix for metastatic cell dissemination. Also L1 in serum could be an important biomaker for prostate cancer dissemination, in particular bone metastasis. The biological function of L1 in prostate cancer cell progression and metastasis was further demonstrated in an experimental animal model, in which intracardiac injection of luciferase-expressing PC3 (PC3-Luc) cells that were pre-transfected with L1-targeting siRNA resulted in reduction of the overall tumor uptake as well as the number of metastases in athymic nude mice, as determined by bioluminescence imaging. The antitumor activities of liposome-encapsulated L1 targeting siRNAs on the growth of established PC3-Luc tumors was also shown in subcutaneous and bone xenograft model when administrated intratumorally. Taken together, these results strongly support our concept that L1 CAM is a potential therapeutic target of prostate cancer. Liposome-encapsulated L1-targeting siRNAs is a promising gene therapy approach and may be applied for the treatment of localized and bone metastatic prostate cancers.