The TGF- β Family Member Growth Differentiation Factor 15 (GDF15) Regulates the Self-Renewal of Multiple Myeloma Cancer Stem Cells

Multiple myeloma (MM) cancer stem cells (CSCs) possess both enhanced tumorigenic potential and relative drug resistance suggesting they play a major role in disease relapse and progression. Therefore, a better understanding of the processes regulating MM CSCs may lead to the development of novel therapies that prevent tumor regrowth and improve long-term outcomes. Normal stem cells are tightly regulated by factors within the local microenvironment that include both soluble factors and direct contact with accessory cells. However, external factors regulating MM CSCs have not been identified. Recent studies have demonstrated that stromal cells in the MM bone marrow microenvironment secrete growth differentiation factor 15 (GDF15), a member of the TGF-b family. We initially studied the role of this cytokine in the pathogenesis of MM by examining circulating GDF15 levels in MM patients. Compared to healthy volunteers, we found that median GDF15 levels were significantly increased in MM patients (821 vs. 390 pg/ml; n=16; p<0.05) and increased with disease stage (Stage II=585 pg/ml, Stage III=1, 004 pg/ml). To examine the functional effects of GDF15 on MM cells, we cultured human MM cell lines (NCI-H929, RPMI 8226) with recombinant GDF15 and found that it induced the expansion of isolated CD138^neg MM CSCs in a dose-dependent manner but had little impact on the growth of CD138⁺ plasma cells (Fig). Furthermore, GDF15 enhanced clonogenic myeloma growth as evidenced by increased colony formation that was maintained during serial replating, a surrogate for self-renewal. This effect appeared to be GDF15 specific since it could be blocked using anti-GDF15 antibody. Similarly, GDF15 treatment increased the in vitro clonogenic growth of MM CSCs from primary clinical bone marrow specimens. We also investigated the down-stream cellular pathways potentially mediating the effects of GDF15 and found that it activates the AKT signaling pathway known to improve the self-renewal of embryonic (ES) and normal hematopoietic stem cells. GDF15 also induced expression of the SOX2 transcription factor known to be upregulated in CD138^neg MM CSCs. Since SOX2 is required for the self-renewal of ES cells and the generation of induced pluripotent stem (iPS) cells, its induction by GDF15 may also increase the self-renewal of MM CSCs. GDF15 is the first soluble factor identified that regulates MM CSCs, and its effects are mediated by the activation of highly conserved self-renewal programs.