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 CXCL12γ 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, CXCL12γ, unlike the CXCL12α isoform, was retained on the surface of these BMSCs. This membrane retention of CXCL12γ is HSPG-mediated, since it was completely annulated by CRISPR-Cas9 mediated deletion of the heparan-sulfate (HS) co-polymerase EXT1. Recombinant CXCL12γ was found to induce strong adhesion of MM cells to vascular cell-adhesion molecule 1 (VCAM-1) coated plates. Furthermore, CXCL12γ expressed by BMSCs and membrane-retained by HSPGs, supported robust adhesion of MM cells to the BMSCs. Specific genetic deletion of either CXCL12γ 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 CXCL12γ 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 CXCL12γ and associated HSPGs as potential therapeutic targets in MM.