Metformin confers sensitisation to syrosingopine in multiple myeloma cells by metabolic blockage and inhibition of protein synthesis.

Multiple myeloma (MM) remains an incurable haematological malignancy despite substantial advances in therapy. Hypoxic bone marrow induces metabolic rewiring in MM cells contributing to survival and drug resistance. Therefore, targeting metabolic pathways may offer an alternative treatment option. In this study, we repurpose two FDA-approved drugs, syrosingopine and metformin. Syrosingopine was used as a dual inhibitor of monocarboxylate transporter 1 and 4 (MCT1/4) and metformin as an inhibitor for oxidative phosphorylation (OXPHOS). Anti-tumour effects were evaluated for single agents and in combination therapy. Survival and expression data for MCT1/MCT4 were obtained from the Total Therapy 2, Mulligan, and Multiple Myeloma Research Foundation cohorts. Cell death, viability, and proliferation were measured using Annexin V/7-AAD, CellTiterGlo, and BrdU, respectively. Metabolic effects were assessed using Seahorse Glycolytic Rate assays and LactateGlo assays. Differential protein expression was determined using western blotting, and the SUnSET method was implemented to quantify protein synthesis. Finally, the syngeneic 5T33MMvv model was used for in vivo analysis. High-level expression of MCT1 and MCT4 both correlated with a significantly lower overall survival of patients. Lactate production as well as MCT1/MCT4 expression were significantly upregulated in hypoxia, confirming the Warburg effect in MM. Dual inhibition of MCT1/4 with syrosingopine resulted in intracellular lactate accumulation and reduced cell viability and proliferation. However, only at higher doses (>10 µm) was syrosingopine able to induce cell death. By contrast, combination treatment of syrosingopine with metformin was highly cytotoxic for MM cell lines and primary patient samples and resulted in a suppression of both glycolysis and OXPHOS. Moreover, pathway analysis revealed an upregulation of the energy sensor p-AMPKα and more downstream a reduction in protein synthesis. Finally, the combination treatment resulted in a significant reduction in tumour burden in vivo. This study proposes an alternative combination treatment for MM and provides insight into intracellular effects. © 2023 The Pathological Society of Great Britain and Ireland.

Metabolic cross-talk within the bone marrow milieu: focus on multiple myeloma.

Cancer cells are well-known for their capacity to adapt their metabolism to their increasing energy demands which is necessary for tumor progression. This is no different for Multiple Myeloma (MM), a hematological cancer which develops in the bone marrow (BM), whereby the malignant plasma cells accumulate and impair normal BM functions. It has become clear that the hypoxic BM environment contributes to metabolic rewiring of the MM cells, including changes in metabolite levels, increased/decreased activity of metabolic enzymes and metabolic shifts. These adaptations will lead to a pro-tumoral environment stimulating MM growth and drug resistance In this review, we discuss the identified metabolic changes in MM and the BM microenvironment and summarize how these identified changes have been targeted (by inhibitors, genetic approaches or deprivation studies) in order to block MM progression and survival.

System Xc- inhibition blocks bone marrow-multiple myeloma exosomal crosstalk, thereby countering bortezomib resistance.

Multiple myeloma (MM) cells derive proliferative signals from the bone marrow (BM) microenvironment via exosomal crosstalk. Therapeutic strategies targeting this crosstalk are still lacking. Bortezomib resistance in MM cells is linked to elevated expression of xCT (the subunit of system Xc-). Extracellular glutamate released by system Xc- can bind to glutamate metabotropic receptor (GRM) 3, thereby upregulating Rab27-dependent vesicular trafficking. Since Rab27 is also involved in exosome biogenesis, we aimed to investigate the role of system Xc- in exosomal communication between BM stromal cells (BMSCs) and MM cells. We observed that expression of xCT and GRMs was increased after bortezomib treatment in both BMSCs and MM cells. Secretion of glutamate and exosomes was simultaneously enhanced which could be countered by inhibition of system Xc- or GRMs. Moreover, glutamate supplementation increased exosome secretion by increasing expression of Alix, TSG101, Rab27a/b and VAMP7. Importantly, the system Xc- inhibitor sulfasalazine reduced BMSC-induced resistance to bortezomib in MM cells in vitro and enhanced its anti-MM effects in vivo. These findings suggest that system Xc- plays an important role within the BM and could be a potential target in MM.