Myeloid-derived suppressor cells (MDSCs) and cancer stem cells (CSCs) are two important cellular components in the tumor microenvironment, which may modify the cancer phenotype and affect patient survival. However, the crosstalk between MDSCs and multiple myeloma stem cells (MMSCs) are relatively poorly understood. The frequencies of granulocytic-MDSCs (G-MDSCs) in MM patients were detected by flow cytometry and their association with the disease stage and patient survival were analyzed. RT-PCR, flow cytometry, western blot and sphere formation assays were performed to investigate the effects of G-MDSCs, piRNA-823 and DNA methylation on the maintenance of stemness in MM. Then a subcutaneous tumor mouse model was constructed to analyze tumor growth and angiogenesis after G-MDSCs induction and/or piRNA-823 knockdown in MM cells. Our clinical dataset validated the association between high G-MDSCs levels and poor overall survival in MM patients. In addition, for the first time we showed that G-MDSCs enhanced the side population, sphere formation and expression of CSCs core genes in MM cells. Moreover, the mechanism study showed that G-MDSCs triggered piRNA-823 expression, which then promoted DNA methylation and increased the tumorigenic potential of MM cells. Furthermore, silencing of piRNA-823 in MM cells reduced the stemness of MMSCs maintained by G-MDSCs, resulting in decreased tumor burden and angiogenesis in vivo. Altogether, these data established a cellular, molecular, and clinical network among G-MDSCs, piRNA-823, DNA methylation and CSCs core genes, suggesting a new anti-cancer strategy targeting both G-MDSCs and CSCs in MM microenvironment.

N⁶-methyladenosine (m⁶A), as the most pervasive internal modification of eukaryotic mRNA, plays a crucial role in various cancers, but its role in multiple myeloma (MM) pathogenesis has not yet been investigated. In this study, we revealed significantly decreased m⁶A methylation in plasma cells (PCs) from MM patients and showed that the abnormal m⁶A level resulted mainly from upregulation of the demethylase fat mass and obesity-associated protein (FTO). Gain- and loss-of-function studies demonstrated that FTO plays a tumor-promoting and pro-metastatic role in MM. Combined m⁶A and RNA sequencing (RNA-seq) and subsequent validation and functional studies identified heat shock factor 1 (HSF1) as a functional target of FTO-mediated m⁶A modification. FTO significantly promotes MM cell proliferation, migration, and invasion by targeting HSF1/HSPs in a YTHDF2-dependent manner. FTO inhibition, especially when combined with bortezomib (BTZ) treatment, synergistically inhibited myeloma bone tumor formation and extramedullary spread in NOD-Prkdc^em26Cd52il2rg^em26Cd22/Nju (NCG) mice. We demonstrated the functional importance of m⁶A demethylase FTO in MM progression, especially in promoting extramedullary myeloma (EMM) formation, and proposed the FTO-HSF1/HSP axis as a potential novel therapeutic target in MM.

Multiple myeloma (MM) is an incurable hematologic malignancy characterized by the clonal expansion of malignant plasma cells within the bone marrow. Activator Protein-1 (AP-1) transcription factors (TFs), comprised of the JUN, FOS, ATF and MAF multigene families, are implicated in a plethora of physiologic processes and tumorigenesis including plasma cell differentiation and MM pathogenesis. Depending on the genetic background, the tumor stage, and cues of the tumor microenvironment, specific dimeric AP-1 complexes are formed. For example, AP-1 complexes containing Fra-1, Fra-2 and B-ATF play central roles in the transcriptional control of B cell development and plasma cell differentiation, while dysregulation of AP-1 family members c-Maf, c-Jun, and JunB is associated with MM cell proliferation, survival, drug resistance, bone marrow angiogenesis, and bone disease. The present review article summarizes our up-to-date knowledge on the role of AP-1 family members in plasma cell differentiation and MM pathophysiology. Moreover, it discusses novel, rationally derived approaches to therapeutically target AP-1 TFs, including protein-protein and protein-DNA binding inhibitors, epigenetic modifiers and natural products.

Bone marrow (BM) angiogenesis significantly influences disease progression in multiple myeloma (MM) patients and correlates with adverse prognosis. The present study shows a statistically significant correlation of the AP-1 family member JunB with VEGF, VEGFB, and IGF1 expression levels in MM. In contrast to the angiogenic master regulator Hif-1α, JunB protein levels were independent of hypoxia. Results in tumor-cell models that allow the induction of JunB knockdown or JunB activation, respectively, corroborated the functional role of JunB in the production and secretion of these angiogenic factors (AFs). Consequently, conditioned media derived from MM cells after JunB knockdown or JunB activation either inhibited or stimulated in vitro angiogenesis. The impact of JunB on MM BM angiogenesis was finally confirmed in a dynamic 3D model of the BM microenvironment, a xenograft mouse model as well as in patient-derived BM sections. In summary, in continuation of our previous study (Fan et al., 2017), the present report reveals for the first time that JunB is not only a mediator of MM cell survival, proliferation, and drug resistance, but also a promoter of AF transcription and consequently of MM BM angiogenesis. Our results thereby underscore worldwide efforts to target AP-1 transcription factors such as JunB as a promising strategy in MM therapy.

Multiple myeloma (MM) is a clonal plasma cell malignancy that is characterized by genetic heterogeneity. The cytogenetic abnormality t(4;14) strongly predicts poor outcome in patients with MM, even in the era of novel drugs. Ferroptosis is a new approach to antitumor therapy, but the relationship between ferroptosis and MM cytogenetic abnormalities remains largely unclear. In this study, we show that t(4;14)-positive but not t(4;14)-negative MM cells are susceptible to class II ferroptosis inducers (FINs) in a preclinical setting, which is dependent on the significant upregulation of the MM SET domain-containing protein (MMSET). Mechanistically, MMSET upregulates acyl-coenzyme A synthetase long-chain family member 4 transcription by binding to its promoter region, leading to increased polyunsaturated fatty acid (PUFA) levels and enhanced sensitivity of t(4;14)-positive MM cells to ferroptosis. Supplementation with PUFAs efficiently restores the susceptibility of t(4;14)-negative MM cells to ferroptosis. In addition, combining class II FIN treatment with bortezomib in t(4;14)-positive MM cells attenuates cellular glutathione and induces both apoptosis and ferroptosis levels by inhibiting the increase in solute carrier family 7 member 11, demonstrating synergistic antitumor activity in vitro and in a xenograft model. Taken together, our findings suggest that targeting ferroptosis with class II FINs is a novel and promising therapeutic approach to improve the outcome of t(4;14)-positive patients with MM.

Multiple myeloma (MM) is the second most common hematologic malignancy, resulting from the clonal proliferation of malignant plasma cells within the bone marrow. Despite significant advances that have been made with novel drugs over the past two decades, MM patients often develop therapy resistance, especially to bortezomib, the first-in-class proteasome inhibitor that was approved for treatment of MM. As highly secretory monoclonal protein-producing cells, MM cells are characterized by uploaded endoplasmic reticulum stress (ERS), and rely heavily on the ERS response for survival. Great efforts have been made to illustrate how MM cells adapt to therapeutic stresses through modulating the ERS response. In this review, we summarize current knowledge on the mechanisms by which ERS response pathways influence MM cell fate and response to treatment. Moreover, based on promising results obtained in preclinical studies, we discuss the prospect of applying ERS modulators to overcome drug resistance in MM.