Antioxidant Defenses Confer Resistance to High Dose Melphalan in Multiple Myeloma Cells

Background: Multiple myeloma (MM) is the second most common hematological cancer after lymphoma. It is characterized by the accumulation of clonal malignant plasma cells within the bone marrow. The development of drug resistance remains a major problem for effective treatment of MM. Understand the mechanisms underlying drug resistance in MM is a focal point to improve MM treatment. Methods: In the current study, we analyzed further the role of redox imbalance induction in melphalan-induced toxicity both in human myeloma cell lines (HMCLs) and primary myeloma cells from patients. Results: We developed an in-vitro model of short-term resistance to high-dose melphalan and identified that pretreatment with physiological concentration of GSH protects HMCLs from melphalan-induced cell cycle arrest and cytotoxicity. We validated these results using primary MM cells from patients co-cultured with their bone marrow microenvironment. GSH did not affect the ability of melphalan to induce DNA damages in MM cells. Interestingly, melphalan induced reactive oxygen species, a significant decrease in GSH concentration, protein and lipd oxydation together with NRF2 (NF-E2-related factor 2) pathway activation. Conclusions: Our data demonstrate that antioxidant defenses confers resistance to high dose melphalan in MM cells, supporting that redox status in MM cells could be determinant for patients’ response to melphalan

Targeting the methyltransferase SETD8 impairs tumor cell survival and overcomes drug resistance independently of p53 status in multiple myeloma

International audienceAbstract Background Multiple myeloma (MM) is a malignancy of plasma cells that largely remains incurable. The search for new therapeutic targets is therefore essential. In addition to a wide panel of genetic mutations, epigenetic alterations also appear as important players in the development of this cancer, thereby offering the possibility to reveal novel approaches and targets for effective therapeutic intervention. Results Here, we show that a higher expression of the lysine methyltransferase SETD8, which is responsible for the mono-methylation of histone H4 at lysine 20, is an adverse prognosis factor associated with a poor outcome in two cohorts of newly diagnosed patients. Primary malignant plasma cells are particularly addicted to the activity of this epigenetic enzyme. Indeed, the inhibition of SETD8 by the chemical compound UNC-0379 and the subsequent decrease in histone H4 methylation at lysine 20 are highly toxic in MM cells compared to normal cells from the bone marrow microenvironment. At the molecular level, RNA sequencing and functional studies revealed that SETD8 inhibition induces a mature non-proliferating plasma cell signature and, as observed in other cancers, triggers an activation of the tumor suppressor p53, which together cause an impairment of myeloma cell proliferation and survival. However, a deadly level of replicative stress was also observed in p53-deficient myeloma cells treated with UNC-0379, indicating that the cytotoxicity associated with SETD8 inhibition is not necessarily dependent on p53 activation. Consistent with this, UNC-0379 triggers a p53-independent nucleolar stress characterized by nucleolin delocalization and reduction of nucleolar RNA synthesis. Finally, we showed that SETD8 inhibition is strongly synergistic with melphalan and may overcome resistance to this alkylating agent widely used in MM treatment. Conclusions Altogether, our data indicate that the up-regulation of the epigenetic enzyme SETD8 is associated with a poor outcome and the deregulation of major signaling pathways in MM. Moreover, we provide evidences that myeloma cells are dependent on SETD8 activity and its pharmacological inhibition synergizes with melphalan, which could be beneficial to improve MM treatment in high-risk patients whatever their status for p53