MIP binds to three different receptors CCR
MIP-1α binds to three different receptors: CCR1, CCR5 and CCR9. CCR1 and CCR5 are expressed on MM calcifediol and stromal cells. Inhibitors to CCR1 and CCR5 have been used to delineate the roles of each receptor in the myeloma bone marrow microenvironment. Menu and colleagues demonstrated that MM cell migration to MIP-1α in vitro and homing in vivo were mediated by binding to CCR5 and not CCR1. In vivo inhibition of CCR1 resulted in reduction of osteoclastic bone resorption, suggesting that CCR1 and CCR5 have differential effects on myeloma cell chemotaxis and stimulation of osteolysis. Small molecule antagonists to CCR1 have been studied in models of myeloma and have been shown to block both tumor growth and destruction [69,70], and to at least partially reverse MIP-1α osteoblast inhibitor effects in vitro and in vivo . These small molecule antagonists are currently in pharmaceutical development . IL-3/Activin A: IL-3 is produced by myeloma cells and T cells in the myeloma microenvironment and approximately 70% of MM patients have elevated IL-3 levels in marrow plasma. IL-3 is a bifunctional cytokine that can both stimulate osteoclastogenesis via an indirect effect  and inhibit osteoblast formation . Silbermann and coworkers  reported that IL-3 stimulates marrow macrophages in the myeloma microenvironment to produce activin A, a TGF-β family member, and that anti-activin A inhibits the effects of IL-3 on osteoclast formation. Activin A is known to directly induce osteoclast formation and enhance the effects of RANKL on osteoclast formation . Others have reported that levels of activin A are increased in marrow plasma and peripheral blood from patients with myeloma, and that marrow stromal cells and osteoclasts are the major source of activin A in myeloma patients . In addition, elevated circulating activin A levels have been described in newly diagnosed symptomatic myeloma patients as compared with controls and elevated activin A levels correlated with advanced disease stage and were associated with increased bone resorption and extensive bone disease . Interestingly, the mechanism of IL-3 suppression of osteoblast differentiation is also indirect, and requires the participation of CD45+ cells. Osteoclast precursors are also CD45+, suggesting that Activin A is involved as a cross-talk regulatory molecule between osteoclast and osteoblast precursors in both the directions . Activin A signals through the activin A type IIA receptor to increase bone resorption and suppress osteoblast differentiation by inhibiting production of the Dlx5. An activin A receptor antagonist (soluble activin receptor type IIA fusion protein, ActRIIA.muFc; RAP-011) has been shown to block bone destruction, stimulate bone formation, and decrease tumor growth in a murine model of myeloma . A humanized activin A soluble receptor antagonist (ACE-011) has been shown to inhibit bone resorption markers and stimulate bone formation in post menopausal women , and a trial of ACE-011 in myeloma patients is ongoing. Annexin II: Annexin II (AXII) is a recently identified factor produced by stromal cells and osteoclasts that is important in osteoclast formation, hematopoietic stem cell mobilization, and homing of prostate cancer cells to the bone . AXII is upregulated in MM, and myeloma-derived AXII increases proliferation of myeloma cell lines, possibly through an autocrine mechanism [80,81]. OCL and stromal cell derived AXII enhances the growth of MM cells in the bone marrow by binding to the AXII receptor on MM cells, primarily through a paracrine mechanism . In addition, AXII can induce stromal cell production of RANKL, further stimulating OCL formation. Ephrin B2/EphB4 bidirectional signaling: Bidirectional signaling between the ligand ephrin B2 from osteoclasts and its receptor EphB4 on bone marrow stromal cells and osteoblasts has been reported to negatively control OCL development from OCL precursors (reverse signaling) and to promote OBL differentiation (forward signaling) . EphrinB2 and EphB4 are decreased in stromal cells from myeloma patients . EphB4-Fc activates ephrinB2 in OCL, but not in stromal cells; and ephrinB2-Fc activates EphB4 in stromal cells. Administration of either peptide to a murine myeloma model stimulated osteoblastogenesis, bone formation, and angiogenesis, but only EphB4-Fc also inhibited osteoclastogenesis and myeloma growth. Thus, enhancing ephrinB2-EphB4 signaling is a possible therapeutic target for MMBD.