The Role of Intracellular Signaling Pathways in the Pathogenesis of Multiple Myeloma and Novel Therapeutic Approaches

An mTORC1 to HRI signaling axis promotes cytotoxicity of proteasome inhibitors in multiple myeloma

Multiple myeloma (MM) causes approximately 20% of deaths from blood cancers. Notwithstanding significant therapeutic progress, such as with proteasome inhibitors (PIs), MM remains incurable due to the development of resistance. mTORC1 is a key metabolic regulator, which frequently becomes dysregulated in cancer. While mTORC1 inhibitors reduce MM viability and synergize with other therapies in vitro, clinically, mTORC1 inhibitors are not effective for MM. Here we show that the inactivation of mTORC1 is an intrinsic response of MM to PI treatment. Genetically enforced hyperactivation of mTORC1 in MM was sufficient to compromise tumorigenicity in mice. In vitro, mTORC1-hyperactivated MM cells gained sensitivity to PIs and hypoxia. This was accompanied by increased mitochondrial stress and activation of the eIF2α kinase HRI, which initiates the integrated stress response. Deletion of HRI elevated the toxicity of PIs in wt and mTORC1-activated MM. Finally, we identified the drug PMA as a robust inducer of mTORC1 activity, which synergized with PIs in inducing MM cell death. These results help explain the clinical inefficacy of mTORC1 inhibitors in MM. Our data implicate mTORC1 induction and/or HRI inhibition as pharmacological strategies to enhance MM therapy by PIs.

Targeting CAM-DR and Mitochondrial Transfer for the Treatment of Multiple Myeloma

The prognosis of patients with multiple myeloma (MM) has improved dramatically with the introduction of new therapeutic drugs, but the disease eventually becomes drug-resistant, following an intractable and incurable course. A myeloma niche (MM niche) develops in the bone marrow microenvironment and plays an important role in the drug resistance mechanism of MM. In particular, adhesion between MM cells and bone marrow stromal cells mediated by adhesion molecules induces cell adhesion-mediated drug resistance (CAM-DR). Analyses of the role of mitochondria in cancer cells, including MM cells, has revealed that the mechanism leading to drug resistance involves exchange of mitochondria between cells (mitochondrial transfer) via tunneling nanotubes (TNTs) within the MM niche. Here, we describe the discovery of these drug resistance mechanisms and the identification of promising therapeutic agents primarily targeting CAM-DR, mitochondrial transfer, and TNTs.

Contribution of the Tumor Microenvironment to Metabolic Changes Triggering Resistance of Multiple Myeloma to Proteasome Inhibitors

Virtually all patients with multiple myeloma become unresponsive to treatment with proteasome inhibitors over time. Relapsed/refractory multiple myeloma is accompanied by the clonal evolution of myeloma cells with heterogeneous genomic aberrations, diverse proteomic and metabolic alterations, and profound changes of the bone marrow microenvironment. However, the molecular mechanisms that drive resistance to proteasome inhibitors within the context of the bone marrow microenvironment remain elusive. In this review article, we summarize the latest knowledge about the complex interaction of malignant plasma cells with its surrounding microenvironment. We discuss the pivotal role of metabolic reprograming of malignant plasma cells within the tumor microenvironment with a subsequent focus on metabolic rewiring in plasma cells upon treatment with proteasome inhibitors, driving multiple ways of adaptation to the treatment. At the same time, mutual interaction of plasma cells with the surrounding tumor microenvironment drives multiple metabolic alterations in the bone marrow. This provides a tumor-promoting environment, but at the same time may offer novel therapeutic options for the treatment of relapsed/refractory myeloma patients.

Pathogenic Mechanisms of Myeloma Bone Disease and Possible Roles for NRF2

Osteolytic bone lesions are one of the central features of multiple myeloma (MM) and lead to bone pain, fractures, decreased quality of life, and decreased survival. Dysfunction of the osteoclast (OC)/osteoblast (OB) axis plays a key role in the development of myeloma-associated osteolytic lesions. Many signaling pathways and factors are associated with myeloma bone diseases (MBDs), including the RANKL/OPG and NF-κB pathways. NRF2, a master regulator of inflammatory signaling, might play a role in the regulation of bone metabolism via anti-inflammatory signaling and decreased reactive oxygen species (ROS) levels. The loss of NRF2 expression in OCs reduced bone mass via the RANK/RANKL pathway and other downstream signaling pathways that affect osteoclastogenesis. The NRF2 level in OBs could interfere with interleukin (IL)-6 expression, which is associated with bone metabolism and myeloma cells. In addition to direct impact on OCs and OBs, the activity of NRF2 on myeloma cells and mesenchymal stromal cells influences the inflammatory stress/ROS level in these cells, which has an impact on OCs, OBs, and osteocytes. The interaction between these cells and OCs affects the osteoclastogenesis of myeloma bone lesions associated with NRF2. Therefore, we have reviewed the effects of NRF2 on OCs and OBs in MBDs.

TAZ functions as a tumor suppressor in multiple myeloma by downregulating MYC

Multiple myeloma (MM) is an incurable blood cancer that is often characterized by amplification and overexpression of the MYC oncogene. Despite efforts, direct targeting of MYC is not yet possible; therefore, alternative strategies to inhibit MYC activity are necessary. TAZ is a transcriptional coactivator downstream of the Hippo-signaling pathway that functions as an oncogene in many solid tumors. However, its role in hematological malignancies is largely unexplored. Here, we show that, in contrast to solid tumors, expression of TAZ is lower in hematological malignancies, and that high expression of TAZ correlates with better patient outcomes. We further show that TAZ is hypermethylated in MM patient samples and in a panel of MM cell lines. Genetic overexpression of TAZ or pharmacological upregulation of TAZ by treatment with the demethylating agent decitabine induces apoptosis. Importantly, TAZ-induced apoptosis is independent of canonical Hippo components LATS1 or the TEA-domain family of transcription factors. Instead, RNA-sequencing analysis revealed that overexpression of TAZ represses a MYC transcriptional program and we show that increased TAZ expression correlates with decreased MYC expression in both cell-line models and patient samples. Furthermore, promoter derepression of TAZ expression sensitizes MM cell lines through a reciprocal reduction in MYC expression using additional therapeutics such as bortezomib, trichostatin A, and panobinostat. Our findings uncover an unexpected role for TAZ in MM tumorigenesis and provide a compelling rationale for exploring the therapeutic potential of upregulating TAZ expression to restore sensitivity to specific therapeutics in MM.

Overview of Current Targeted Anti-Cancer Drugs for Therapy in Onco-Hematology

The upgraded knowledge of tumor biology and microenviroment provides information on differences in neoplastic and normal cells. Thus, the need to target these differences led to the development of novel molecules (targeted therapy) active against the neoplastic cells' inner workings. There are several types of targeted agents, including Small Molecules Inhibitors (SMIs), monoclonal antibodies (mAbs), interfering RNA (iRNA) molecules and microRNA. In the clinical practice, these new medicines generate a multilayered step in pharmacokinetics (PK), which encompasses a broad individual PK variability, and unpredictable outcomes according to the pharmacogenetics (PG) profile of the patient (e.g., cytochrome P450 enzyme), and to patient characteristics such as adherence to treatment and environmental factors. This review focuses on the use of targeted agents in-human phase I/II/III clinical trials in cancer-hematology. Thus, it outlines the up-to-date anticancer drugs suitable for targeted therapies and the most recent finding in pharmacogenomics related to drug response. Besides, a summary assessment of the genotyping costs has been discussed. Targeted therapy seems to be an effective and less toxic therapeutic approach in onco-hematology. The identification of individual PG profile should be a new resource for oncologists to make treatment decisions for the patients to minimize the toxicity and or inefficacy of therapy. This could allow the clinicians to evaluate benefits and restrictions, regarding costs and applicability, of the most suitable pharmacological approach for performing a tailor-made therapy.

NRF2 Is One of the Players Involved in Bone Marrow Mediated Drug Resistance in Multiple Myeloma

Multiple myeloma with clonal plasma expansion in bone marrow is the second most common hematologic malignancy in the world. Though the improvement of outcomes from the achievement of novel agents in recent decades, the disease progresses and leads to death eventually due to the elusive nature of myeloma cells and resistance mechanisms to therapeutic agents. In addition to the molecular and genetic basis of resistance pathomechanisms, the bone marrow microenvironment also contributes to disease progression and confers drug resistance in myeloma cells. In this review, we focus on the current state of the literature in terms of critical bone marrow microenvironment components, including soluble factors, cell adhesion mechanisms, and other cellular components. Transcriptional factor nuclear factor erythroid-derived-2-like 2 (NRF2), a central regulator for anti-oxidative stresses and detoxification, is implicated in chemoresistance in several cancers. The functional roles of NRF2 in myeloid-derived suppressor cells and multiple myeloma cells, and the potential of targeting NRF2 for overcoming microenvironment-mediated drug resistance in multiple myeloma are also discussed.

Various Signaling Pathways in Multiple Myeloma Cells and Effects of Treatment on These Pathways

Multiple myeloma (MM) results from malignancy in plasma cells and occurs at ages > 50 years. MM is the second most common hematologic malignancy after non-Hodgkin lymphoma, which constitutes 1% of all malignancies. Despite the great advances in the discovery of useful drugs for this disease such as dexamethasone and bortezomib, it is still an incurable malignancy owing to the development of drug resistance. The tumor cells develop resistance to apoptosis, resulting in greater cell survival, and, ultimately, develop drug resistance by changing the various signaling pathways involved in cell proliferation, survival, differentiation, and apoptosis. We have reviewed the different signaling pathways in MM cells. We reached the conclusion that the most important factor in the drug resistance in MM patients is caused by the bone marrow microenvironment with production of adhesion molecules and cytokines. Binding of tumor cells to stromal cells prompts cytokine production of stromal cells and launches various signaling pathways such as Janus-activated kinase/signal transduction and activator of transcription, Ras/Raf/MEK/mitogen-activated protein kinase, phosphatidyl inositol 3-kinase/AKT, and NF-KB, which ultimately lead to the high survival rate and drug resistance in tumor cells. Thus, combining various drugs such as bortezomib, dexamethasone, lenalidomide, and melphalan with compounds that are not common, including CTY387, LLL-12, OPB31121, CNTO328, OSI-906, FTY720, triptolide, and AV-65, could be one of the most effective treatments for these patients.

YM155 exerts potent cytotoxic activity against quiescent (G0/G1) multiple myeloma and bortezomib resistant cells via inhibition of survivin and Mcl-1

YM155, a novel small molecule inhibitor of survivin, shows broad anticancer activity. Here, we have focused on the cytotoxic activity of YM155 against multiple myeloma (MM) including cytokinetically quiescent (G₀/G₁) cells and bortezomib resistant cells. YM155 strongly inhibited the growth of MM cell lines with the IC₅₀ value of below 10 nM. YM155 also showed potent anti-myeloma activity in mouse xenograft model. YM155 suppressed the expression of survivin and rapidly directed Mcl-1 protein for proteasome degradation. YM155 abrogated the interleukin-6-induced STAT3 phosphorylation, subsequently blocked Mcl-1 expression and induced apoptosis in MM cells. Triple-color flow cytometric analysis revealed that YM155 potently induced cell death of MM cells in G₀ phase. Quiescent primary MM cells were also sensitive to YM155. We established bortezomib-resistant MM cell line, U266/BTZR1, which possess a point mutation G322A. YM155 exhibited similar cytotoxic potency against U266/BTZR1 compared with parental cells. Interestingly, survivin expression was markedly elevated in U266/BTZR1 cells. Treatment with YM155 significantly down-regulated this increased survivin and Mcl-1 expression in U266/BTZR1 cells. In conclusion, our data indicate that YM155 exhibits potent cytotoxicity against quiescent (G₀/G₁) MM cells and bortezomib-resistant cells. These unique features of YM155 may be beneficial for the development of new therapeutic strategies to eliminate quiescent MM cells and overcome bortezomib resistance.