Inhibition of the MUC1-C oncoprotein induces multiple myeloma cell death by down-regulating TIGAR expression and depleting NADPH

Abstract

The MUC1-C oncoprotein is aberrantly expressed in most multiple myeloma cells. However, the functional significance of MUC1-C expression in multiple myeloma is not known. The present studies demonstrate that treatment of multiple myeloma cells with a MUC1-C inhibitor is associated with increases in reactive oxygen species (ROS), oxidation of mitochondrial cardiolipin, and loss of the mitochondrial transmembrane potential. The MUC1-C inhibitor-induced increases in ROS were also associated with downregulation of the p53-inducible regulator of glycolysis and apoptosis (TIGAR). In concert with the decrease in TIGAR expression, which regulates the pentose phosphate pathway, treatment with the MUC1-C inhibitor reduced production of NADPH, and in turn glutathione (GSH) levels. TIGAR protects against oxidative stress-induced apoptosis. The suppression of TIGAR and NADPH levels thus contributed to ROS-mediated late apoptosis/necrosis of multiple myeloma cells. These IntroductionMultiple myeloma is an incurable hematologic disorder that is characterized by the clonal proliferation of malignant plasma cells. Targeted agents, such as the proteosome inhibitor, bortezomib, and the immunomodulatory agent lenalidamide, have extended overall survival for patients with multiple myeloma. 1 However, invariably, patients eventually relapse and succumb to this disease, emphasizing the need for additional therapeutic targets.Mucin 1 (MUC1) is a heterodimeric protein that is aberrantly expressed in most multiple myeloma cell lines and primary patient samples. [2][3][4][5][6][7][8] The extracellular MUC1 N-terminal subunit (MUC1-N) contains glycosylated tandem repeats that are a characteristic of mucin family members. 9 MUC1-N forms a complex with the transmembrane MUC1 C-terminal subunit (MUC1-C) at the cell surface. 9 MUC1-C consists of a 58 amino acid extracellular domain that associates with galectin-3, 10 and a 72 amino acid cytoplasmic domain that interacts with diverse effectors that have been linked to transformation. 9 In this regard, MUC1-C expression is sufficient to induce anchorage-independent growth and tumorigenicity. 11,12 In addition to localization in the cell membrane, MUC1-C is detectable in the cytoplasm of multiple myeloma cells and is targeted to the nucleus. 13 Of functional relevance, MUC1-C expression in multiple myeloma cells is associated with activation of the Wnt/␤-catenin and NF-B RelA pathways. 8 Moreover, silencing MUC1-C in multiple myeloma cells results in slowing of proliferation and enhanced sensitivity to apoptosis, and loss of self-renewal in the response to melphalan and dexamethasone. 8 These findings thus provided support for the involvement of MUC1-C in multiple myeloma cell growth and survival.The MUC1-C cytoplasmic domain contains a CQC motif that is necessary for its oligomerization and thereby its nuclear localization. 14 Based on these findings, cell-penetrating peptide drugs were developed to inhibit MUC1-C oligomerization and its oncogenic function. 15 The peptide inhibitors contain t...

MUC1-C drives MYC in multiple myeloma

“…This observation is in concert with the demonstration that targeting MUC1-C in MM is associated with marked suppression of GSH levels and thereby death as a result of increases in reactive oxygen species. 27,28 These findings thus collectively demonstrate that targeting MUC1-C is associated with suppression of multiple MYC-driven genes of importance to MM cell growth and survival.…”

“…27,28 In addition, targeting MUC1-C is synergistic with bortezomib in inducing reactive oxygen speciesmediated MM cell death. 27 These findings have supported the importance of MUC1-C for MM cell survival.…”

“…19,26 Based on these observations, peptide drugs containing the MUC1-C CQCRRKN sequence linked to Arg residues for cell penetration have been developed to inhibit MUC1-C homodimerization and its function. 27 Notably, treatment of MM cell lines and primary MM cells, but not normal B cells, with the MUC1-C inhibitor is associated with arrest of growth in predominantly G1 phase and induction of late apoptosis/necrosis that is mediated in part by disruption of redox balance. 27,28 In addition, targeting MUC1-C is synergistic with bortezomib in inducing reactive oxygen speciesmediated MM cell death.…”

“…27 Notably, treatment of MM cell lines and primary MM cells, but not normal B cells, with the MUC1-C inhibitor is associated with arrest of growth in predominantly G1 phase and induction of late apoptosis/necrosis that is mediated in part by disruption of redox balance. 27,28 In addition, targeting MUC1-C is synergistic with bortezomib in inducing reactive oxygen speciesmediated MM cell death. 27 These findings have supported the importance of MUC1-C for MM cell survival.…”

Loss of TIGAR Induces Oxidative Stress and Meiotic Defects in Oocytes from Obese Mice

“…The major sources of reactive oxygen species (ROS) production are mitochondrial oxidative respiration (12,13), where they are generated as a result of release of electrons from the electron transport chain (12,14). However, excessive amounts of ROS can be damaging to the cell, inducing DNA damage, lipid oxidation, and, ultimately, cell death (15).…”