THE ROLE OF SDF-1&alpha AS A VASCULOGENIC CHEMOKINE AND ENDOTHELIUM-ASSOCIATED CELL ADHESION MOLECULE FOR THE RECRUITMENT OF BONE MARROW-DERIVED PROGENITOR CELLS TO DEVELOPING TUMORS

Tumor vascularization is an important control point in cancer progression and its inhibition is a promising approach to cancer therapy. Stromal-derived factor-1 alpha (SDF-1 alpha or CXCL12) is a chemoattractant for lymphocytes, hematopoietic progenitor cells, and vascular endothelial cells. SDF-1 alpha binds to the CXCR4 receptor on cell surfaces and to heparan sulfate in cell membranes and the extracellular matrix. We hypothesized that SDF-1 alpha is an important mediator of tumor vasculogenesis, recruiting bone marrow-derived endothelial progenitor cells to tumors. We evaluated the role of SDF-1 alpha in the adhesion of bone marrow-derived lineage-negative cells to the murine Lewis lung adenocarcinoma (LLCaB subclone), both in vitro and in vivo. LLCaB lung and liver metastases expressed significantly elevated levels of SDF-1 alpha compared to normal tissue from naive controls and primary tumors. SDF-1 alpha expression inversely correlated with tumor size, implicating involvement in early tumor development. SDF-1 alpha was detected on ~30% of blood vessels in primary and metastatic tumors by double immunocytochemistry staining and on endothelial cells within primary cultures of dissociated LLCaB tumors (DTC). Lineage-negative BMC enriched for Sca1+cKit+ (LSK) cells, including endothelial progenitors and CXCR4+ cells, were used for subsequent adhesion studies. LSK cells preferentially adhered in vitro to DTC, compared to other target cell types. Pre-incubation with anti-CXCR4 antibody or the CXCR4 antagonist AMD3100 decreased adhesion of LSK cells by 50%. Biolocalization of adoptively-transferred EGFP-Lineage-negative BMC in tumor-bearing wild-type mice was evaluated by qPCR for the egfp-transgene. A significantly greater number of Lineage-negative BMC localized to metastases 2-3 days after intravenous injection than to adjacent healthy tissue, normal organs, or primary tumors. Control CXCR4-negative EGFP-dermal fibroblasts showed significantly less tumor localization. One-week post-injection, Lineage-negative BMC were still detected in every metastasis, suggesting that BMC were retained and/or had replicated. These data support the hypothesis that the biolocalization of circulating Lineage-negative BMC to tumors is SDF-1 alpha-mediated. We have also put forth the novel proposal that SDF-1 alpha promotes early tumor vasculogenesis as a tumor endothelium-associated adhesion molecule for circulating CXCR4+ Lineage-negative BMC. These findings may aid our understanding of the mechanism of tumor vascularization and the design of tumor-targeted therapy.