The CXCL12gamma chemokine immobilized by heparan sulfate on stromal niche cells controls adhesion and mediates drug resistance in multiple myeloma

The homing/retention, survival and proliferation of multiple myeloma (MM) cells critically depends on interaction with CXCL12 expressing stromal cells in the bone marrow (BM) niche. Here, we report a unique role in this interaction for the recently characterized CXCL12gamma isoform, which contains an extended C-terminal domain that binds heparan-sulfate proteoglycans (HSPGs) with an extraordinary high affinity. We observed that CXCL12{gamma} is expressed in situ by reticular stromal cells in both normal and MM BM, as well as by primary BM stromal-cell (BMSC) isolates and BMSC lines. Importantly, upon secretion, CXCL12{gamma}, unlike the CXCL12 isoform, was retained on the surface of these BMSCs. This membrane retention of CXCL12{gamma} is HSPG-mediated, since it was completely annulated by CRISPR-Cas9 mediated deletion of the heparan-sulfate (HS) co-polymerase EXT1. Recombinant CXCL12{gamma} was found to induce strong adhesion of MM cells to vascular cell-adhesion molecule 1 (VCAM-1) coated plates. Furthermore, CXCL12{gamma} expressed by BMSCs and membrane-retained by HSPGs, supported robust adhesion of MM cells to the BMSCs. Specific genetic deletion of either CXCL12{gamma} or of EXT1 significantly attenuated the ability of BMSCs to support MM cell adhesion and, in addition, impaired their capacity to protect MM cells from bortezomib-induced cell death. Our data indicate that CXCL12{gamma} functions as a membrane-bound niche chemokine, which plays a unique role in the interaction of MM cells with the stromal niche by controlling adhesion/retention as well as cell adhesion-mediated drug resistance (CAM-DR). These findings designate CXCL12{gamma} and associated HSPGs as potential therapeutic targets in MM.