Stromal Cell-Derived Factor-1 Stimulates the Growth of Myeloma Cells in Concert with IL-6 in Both Autocrine and Paracrine Manners.

Stromal cell-derived factor-1 (SDF-1), which is constitutively expressed and produced in bone marrow (BM) stromal cells (BMSCs), functions via its receptor CXCR4 in the homing of myeloma cells to the bone marrow. In addition, SDF-1 inhibits dexamethasone-induced cell apoptosis. However, it has not been clarified whether SDF-1 stimulates myeloma cell growth. In this study, we explored the possibility that SDF-1 functions as an autocrine growth factor of myeloma cells. RPMI-8226 myeloma cells and primary CD138+ cells obtained from the BM of patients with multiple myeloma expressed SDF-1 mRNA, as revealed by RT-PCR analysis. SDF-1 was detected by ELISA in the supernatants of cultures of these cells grown for three days in serum-free medium, which indicates that myeloma cells are able to produce and secrete SDF-1. SDF-1 (added at a concentration of up to 100 ng/mL) alone had no discernible effects on the proliferation of RPMI-8226 cells and primary BM CD138+ cells in a serum-free condition. Interleukin-6 (IL-6) stimulated a moderate level of proliferation of these cells, while the combination of SDF-1 and IL-6 markedly enhanced the proliferation of these cells. IL-6 did not influence CXCR4 expression on myeloma cells. However, SDF-1 up-regulated significantly the expression of IL-6 receptor in these cells, as determined by Western blot analysis. SDF-1 and IL-6 enhanced in an additive fashion the phosphorylation of Akt and ERK1/2, but did not increase the phosphorylation of STAT3. Pertussis toxin (PTX, 200 ng/mL) and AMD3100 (100 nM), which is a specific antagonist of CXCR4, inhibited the proliferation of RPMI-8226 cells by 35% and 37%, respectively, in cultures grown for four days under serum-free conditions. To determine the effects of blocking the SDF-1/CXCR4 axis on the growth of myeloma cells in the presence of BMSCs, RPMI-8226 cells were co-cultured with MS-5 murine BMSCs. The addition of PTX and AMD3100 (100 nM) twenty-four hours after the beginning of the co-culture markedly decreased the number and size of cobblestone-like areas underneath the stromal cells. These results indicate that myeloma cells can produce SDF-1, and that SDF-1 in concert with IL-6 enhances the proliferation of myeloma cells in both a paracrine and autocrine manner.