Figure 6

Model of redox homeostasis and metabolic changes during the adaptation to proteasome inhibitors. Excessive proteotoxic stress is the major mechanism of cytotoxicity of proteasome inhibitors in myeloma. The level of proteotoxic stress results from the degree of imbalance between proteasomal load and proteasomal proteolytic capacity. The proteasomal load of misfolded protein is tightly connected to the protein-folding capacity of the ER, which in turn relies on proper redox conditions. NAD(P)H is critical for providing oxidizing equivalents and thus for minimizing the proteasomal load in proteasome inhibitor-adapted myeloma cells that are viable with extremely low proteasome activity while producing high paraprotein amounts. NAD(P)H is mostly provided from intermediates of the TCA (Krebs Cycle), for example, malate–pyruvate and mitochondrial isocitrate dehydrogenase reactions, which we found consistently upregulated in proteasome inhibitor-adapted myeloma cells. In line with this, multiple other enzymes that are involved in NAD(P)H generation were upregulated, while enzymes mediating lipid biosynthesis from Acetyl-CoA, which removes tricarboxylic acid intermediates from the Krebs Cycle and thus decreases NAD(P)H generation, are downregulated. This way, we hypothesize that maximum NAD(P)H-producing intermediates may be provided, limiting proteotoxic stress in adapted/resistant cells. Cancer cells often use the faster, but less efficient, pathway for ATP production to generate ATP via pyruvate–lactate conversion (Warburg effect). When this is inhibited using the specific inhibitor of LDHA, FX-11, the sensitivity of myeloma cells to proteasome inhibitors decreases, presumably because glucose metabolism entirely switches to the TCA cycle, increasing NAD(P)H production and reducing proteotoxic stress. Upregulation of ABCB1 decreases proteotoxic stress via reduced intracellular concentrations of proteasome inhibitors, which can be reversed by specific ABCB1 inhibitors Verapamil or Reserpine. ABCB1, ATP-binding cassette, sub-family B (MDR/TAP) member 1; ACLY, ATP Citrate Lyase; ACSF2, Acyl-CoA synthetase family member 2; ACSF3, Acyl-CoA synthetase family member 3; ARE, antioxidant response elements; BVLRA, biliverdin reductase; CoQ, coenzyme Q; CoQH2, coenzyme Q-reduced; CytC, cytochrome C; GSSG, oxidized glutathione; GSH, reduced glutathione; GOT1, glutamic-oxaloacetic transaminase 1; GPX, glutathione peroxidase; LDHA, lactate dehydrogenase; NQO1, NAD[P]H dehydrogenase, quinone1; NRF2, nuclear factor (erythroid-derived 2)-like 2; PDI, protein disulfide isomerase; PEP, phosphoenolpyruvate; PI, proteasome inhibitors; PKM, pyruvate kinase, muscle; PRDX, peroxiredoxin; ROS, reactive oxygen species; SOD, superoxide dismutase; UQ, ubiquinone.