Targeting the metabolic changes in multiple myeloma and its tumor microenvironment to conquer drug resistance

Multiple myeloma (MM) is a hematological cancer characterized by the uncontrolled growth of plasma cells in the bone marrow. Although many advances in therapy have been made, MM remains incurable due to drug resistance. Many cancer cell types undergo metabolic alterations to match their increasing energy demands. In this dissertation, we investigated how metabolic changes, induced by the hypoxic environment, contribute to MM progression and how targeting these changes can sensitize the MM cells to current therapy. An initial tracer metabolomics study revealed two differentially regulated metabolic pathways in MM upon hypoxic culture: an increased glutamine-to-proline conversion and an increased glucose-to-lactate conversion, which we both further investigated.
First, we attributed the increased glutamine-to-proline conversion to an increased activity of the PYCR1 enzyme. We investigated the role of PYCR1 in the MM cells themselves (manuscript 1), but also in the surrounding stromal cells (manuscript 2). In our first manuscript, inhibition of PYCR1 in the MM cells reduced their viability and proliferation. These effects were due to an inhibition of several survival pathways; c-MYC, p-AKT and p-p42/44 MAPK and the protein synthesis-mediated PRAS40 pathway. When combined with bortezomib, apoptotic cell death was increased in MM cell lines and in primary patient material due to an upregulation of the unfolded protein response pathway. In our second manuscript, we focused on how PYCR1 in stromal cells affected MM metabolism and viability. We inhibited PYCR1 in stromal cells and then isolated conditioned medium to treat MM cells. Stromal PYCR1 inhibition reduced oxidative phosphorylation in the MM cells, while also limiting the stromal release of activin A, leading to a decrease in phosphorylation of SMAD3 in MM cells. These two features combined sensitized the MM cells to bortezomib. Finally, the combination therapy consisting of bortezomib and the PYCR1 inhibitor pargyline successfully reduced tumor load in 5TGM1-bearing mice compared to both single agents, with observed reductions in p-mTOR, its downstream targets and c-MYC, as well as a reduction in serum activin A.
In our third manuscript, we investigated the role of lactate transport in MM. Lactate is used as an energy source and is transported in and out of the cells by monocarboxylate transporters 1 and 4 (MCT1 and MCT4). We evaluated the therapeutic effects of MCT inhibition by using the dual MCT1/4 inhibitor syrosingopine, which led to a decrease in viability and increase in apoptotic cell death in MM cell lines. In combination with metformin, an inhibitor of the electron transport chain, effects on viability were even more pronounced in MM cell lines and primary patient samples. Moreover, metformin also successfully sensitized MM to syrosingopine in 5T33MM-bearing mice. Mechanistically, syrosingopine inhibited glycolysis, while metformin inhibited oxidative phosphorylation. Moreover, the combination therapy increased the phosphorylation of the energy sensor AMPK, leading to a decrease p-mTOR, p-4EBP1, p-p70S6K and associated protein synthesis.
In conclusion, this dissertation identified new metabolic alterations in MM, influenced by the hypoxic bone marrow niche. Targeting these alterations led to a reduction in MM viability and drug resistance in vitro and in vivo by altering metabolic pathways and reducing protein synthesis.