Regulation of the New CXCR7 Receptor in Plasma Cell Dyscrasias.

Background: Multiple Myeloma (MM) and Waldenstrom Macroglobulinemia (WM) are characterized by widespread involvement of the bone marrow (BM) as the result of successful homing, engraftment and growth of tumor cells at that site. The receptor for SDF-1, CXCR4 has already been implicated by us and others in the migration and homing of tumor cells to the bone marrow. Recently, a new receptor for the chemokine SDF-1 has been described, namely CXCR7. To date, it has been implicated in the migration of embryonic cells during neurogenesis in zebrafish. It has been reported to be expressed in cancer cells but its function remains unknown. We have previously shown that disruption of the CXCR4/SDF-1 axis interferes with homing of MM cells to the BM. To better understand the homing and migration of MM and WM tumor cells, we sought to study the expression and function of CXCR7.

Methods: MM (U266, MM1.S, RPMI, OPM2, OPM1, KMS12BM) and WM (BCWM.1 and WSU-WM) cell lines were used. CD19+ and CD138+ cells were extracted from patient bone marrow samples by microbeads selection after informed consent. To determine the expression of CXCR7 on MM and WM cell lines as well as primary samples, we used flow cytometry and RT-PCR analysis. The migration of tumor cells towards SDF-1 was studied using the transwell boyden chamber migration assay. The adhesion of tumor cells to fibronectin using a fluorometric assay. The CXCR7 inhibitor CCX-754 (ChemoCentryx Inc., Mountain View, CA) was used.

Results: CXCR7 was expressed in all cell lines tested except WSU-WM by flow cytometry and RT-PCR. Interestingly, U266 cell line did not express surface CXCR4 and expressed CXCR7 and therefore was used in further experiments to determine the differential role of CXCR7 in SDF-1 related function. Primary WM (n=14) and MM (n=7) cells expressed low intensity CXCR7 by flow cytometry. Use of the above cells in experiments utilizing small molecule antagonists of both CXCR4 and CXCR7 suggested that inhibiting the binding of SDF-1 to one receptor could impact the signaling functions of the other. Experiments are ongoing to clarify the nature of the interaction between these two SDF receptors. Furthermore, adhesion of U266 cells to fibronectin was increased in presence of SDF-1, and inhibited in the presence of CCX-7754. Further analysis to examine CXCR7 and CXCR4 antagonism in adhesion are ongoing.

Conclusion: we showed for the first time CXCR7 expression on MM and WM tumor cells. Our results lead us to conclude that through binding of a shared ligand, CXCR7 and CXCR4 may modulate the biological activity of the other.