Insights into the New Cancer Therapy through Redox Homeostasis and Metabolic Shifts

Abstract

Modest levels of reactive oxygen species (ROS) are necessary for intracellular signaling, cell division, and enzyme activation. These ROS are later eliminated by the body's antioxidant defense system. High amounts of ROS cause carcinogenesis by altering the signaling pathways associated with metabolism, proliferation, metastasis, and cell survival. Cancer cells exhibit enhanced ATP production and high ROS levels, which allow them to maintain elevated proliferation through metabolic reprograming. In order to prevent further ROS generation, cancer cells rely on more glycolysis to produce ATP and on the pentose phosphate pathway to provide NADPH. Pro-oxidant therapy can induce more ROS generation beyond the physiologic thresholds in cancer cells. Alternatively, antioxidant therapy can protect normal cells by activating cell survival signaling cascades, such as the nuclear factor erythroid 2-related factor 2 (Nrf2)-Kelch-like ECH-associated protein 1 (Keap1) pathway, in response to radio- and chemotherapeutic drugs. Nrf2 is a key regulator that protects cells from oxidative stress. Under normal conditions, Nrf2 is tightly bound to Keap1 and is ubiquitinated and degraded by the proteasome. However, under oxidative stress, or when treated with Nrf2 activators, Nrf2 is liberated from the Nrf2-Keap1 complex, translocated into the nucleus, and bound to the antioxidant response element in association with other factors. This cascade results in the expression of detoxifying enzymes, including NADH-quinone oxidoreductase 1 (NQO1) and heme oxygenase 1. NQO1 and cytochrome b5 reductase can neutralize ROS in the plasma membrane and induce a high NAD⁺/NADH ratio, which then activates SIRT1 and mitochondrial bioenergetics. NQO1 can also stabilize the tumor suppressor p53. Given their roles in cancer pathogenesis, redox homeostasis and the metabolic shift from glycolysis to oxidative phosphorylation (through activation of Nrf2 and NQO1) seem to be good targets for cancer therapy. Therefore, Nrf2 modulation and NQO1 stimulation could be important therapeutic targets for cancer prevention and treatment.

Keywords: NQO1; Nrf2-Keap1; cancer; glycolysis; oxidative phosphorylation; oxidative stress.

Figure 1

Metabolic fates of glucose metabolism. Under aerobic conditions, glucose is converted to pyruvate and then moved to the mitochondria, where it undergoes oxidative phosphorylation for ATP production. The mitochondria are the main source of ROS, which cause oxidative damage to biomolecules. O₂^•− and its product H₂O₂ can be neutralized by antioxidant molecules (e.g., GSH and coenzyme Q) and antioxidant enzymes (e.g., SOD and catalase). Under anaerobic conditions (including in cancer cells), pyruvate is used for lactate fermentation to produce ATP. Glucose 6-phosphate is bypassed to the pentose phosphate pathway to generate NADPH. Oxidative stress and metabolic inhibitors are involved in metabolic reprogramming in cancer cells. Abbreviations: 2-DG, 2-deoxyglucose; 3-BP, 3-bromopyruvate; BZL101, Bezielle; Mito-Q, mitoquinone mesylate; PPP, pentose phosphate pathway; ROS, reactive oxygen species.

Figure 2

Proliferation, metastasis, and angiogenesis in cancer cells through the Akt, ERK, and STAT signaling cascades. PIP₂ is converted to PIP₃ by PI3K, which activates Akt signaling. Alternatively, PIP₂ can be degraded by PLC into DAG and IP₃. Remaining in the PM, DAG activates PKC. Next, IP₃ binds to the IP₃ receptor in the ER and triggers Ca²⁺ release from the ER, stimulating Ca²⁺-dependent PKC, which activates Raf, MEK, and ERK signaling cascades. After binding with the ligands, RTKs are autophosphorylated and recruit the GRB2 and SOS complex to change inactive Ras-GDP into active Ras-GTP. Activated Ras recruits p110 (a catalytic subunit of PI3K), and phosphorylates PIP₂, which initiates survival signaling via Akt. Autophosphorylated RTKs can bind to p85 (a regulatory subunit of PI3K), which then attaches to p110 and forms the active complex. mTOR-activated HIF-1a stimulates VEGF, which binds to VEGF ligand and triggers JAK-STAT5 signaling for angiogenesis. Abbreviations: Akt, protein kinase B; CDK, cyclin-dependent kinase; DAG, diacylglycerol; FOXO1, Forkhead family of transcription factor 1; GRB2, growth factor receptor-bound protein 2; HIF-1α, hypoxia-inducing factor 1α; JAK, Janus kinase; MEK1/2, mitogen-activated protein kinase kinase 1 and 2; mTOR, mammalian target of rapamycin; p100, phosphor- NF-kB2; p85, regulatory subunit of PI3K; PKC, protein kinase C; PIP₂, phosphatidylinositol 4,5-bisphosphate; PIP₃, phosphatidylinositol 3,4,5-trisphosphate; PLC, phospholipase C; PTEN, phosphatase and tensin homolog; Raf, rapidly accelerated fibrosarcoma; Ras; SOS, salt overly sensitive; STAT5, signal transducer and activator of transcription 5.

Figure 3

Oxygen-dependent regulation of HIF-1α in cancer cells. Under normoxia, HIF-1α is recognized and hydroxylated by PHD in the presence of Fe²⁺, ascorbate, and α-ketoglutarate. Hydroxylated proline in HIF-1α is recognized by VHL and subsequently ubiquitinated for proteasomal degradation. However, under hypoxic conditions, HIF-1α is stabilized and translocated into the nucleus, where it is dimerized with HIF-1β. Following the binding of the HIF-1α-HIF-1β complex with other cofactors (such as CBP/p300), the molecules can target the HRE, which is involved in the metabolic switch, angiogenesis, metastasis, and cell survival. Abbreviations: CBP, cAMP response element binding protein; HRE, hypoxia response elements; p300, E1A binding protein; PHD, proxyl hydroxylase protein; Ub, ubiquitin; VHL, von Hippel–Lindau protein.

Figure 4

Redox homeostasis and metabolic shift through activation of the Nrf2-Keap1 pathway and regulation of the NAD⁺/NADH ratio by PM redox enzymes. Disulfide bonds between Nrf2 and Keap1 are broken by oxidative stress and other Nrf2 activators. Activated Nrf2 is translocated into the nucleus, combined with CBP/p300 and FOXO3, and bound to ARE, which results in the expression of detoxifying enzymes. Under further oxidative/metabolic stress, expressed cytosolic NQO1 can be translocated into the inner surface of the PM. A high NAD+/NADH ratio is induced by stimulating b5R and PM NQO1 to activate SIRT1-PGC-1α signaling and enhance mitochondrial bioenergetics. Abbreviations: ARE, antioxidant response element; CBP, cAMP response element binding protein; Cyt. NQO1, cytosolic form of NADH-quinone oxidoreductase 1; FOXO3, Forkhead family of transcription factor 3; GST, glutathione S-transferase; p300, E1A binding protein; p53, tumor suppressor encoded by TP53; PKC, protein kinase C; PM NQO1, plasma membrane bound NQO1; PGC-1α, peroxisome proliferator-activated receptor-γ coactivator).