Hypoxia-inducible factors: coupling glucose metabolism and redox regulation with induction of the breast cancer stem cell phenotype

Abstract

Reduced oxygen availability (hypoxia) leads to increased production of reactive oxygen species (ROS) by the electron transport chain. Here, I review recent work delineating mechanisms by which hypoxia-inducible factor 1 (HIF-1) mediates adaptive metabolic responses to hypoxia, including increased flux through the glycolytic pathway and decreased flux through the tricarboxylic acid cycle, in order to decrease mitochondrial ROS production. HIF-1 also mediates increased flux through the serine synthesis pathway and mitochondrial one-carbon (folate cycle) metabolism to increase mitochondrial antioxidant production (NADPH and glutathione). Dynamic maintenance of ROS homeostasis is required for induction of the breast cancer stem cell phenotype in response to hypoxia or cytotoxic chemotherapy. Consistently, inhibition of phosphoglycerate dehydrogenase, the first enzyme of the serine synthesis pathway, in breast cancer cells impairs tumor initiation, metastasis, and response to cytotoxic chemotherapy. I discuss how these findings have important implications for understanding the logic of the tumor microenvironment and for improving therapeutic responses in women with breast cancer.

Keywords: cancer; one‐carbon metabolism; pluripotency; progression; serine synthesis.

Figure 1. Glycolytic and oxidative metabolism of glucose

In well‐oxygenated cells, the Embden–Meyerhof pathway (EMP) reactions convert glucose (Glc) to pyruvate (Pyr), which is converted to acetyl‐CoA (AcCoA) by pyruvate dehydrogenase (PDH) for entry into the tricarboxylic acid (TCA) cycle, in which AcCoA is oxidized to CO₂ + H₂O and reducing equivalents are generated (NADH and FADH₂) for donation to the electron transport chain (complex I and complex II, respectively). Proton (H⁺) pumping during electron transport generates an electrochemical gradient that is used to synthesize ATP (complex V) with O₂ serving as the ultimate electron acceptor (complex IV). Premature transfer of electrons to O₂ (at complex I or complex III) results in the formation of superoxide anion. Under hypoxic conditions, superoxide production at complex III increases, leading to the hypoxia‐inducible factor 1 activation and increased expression of lactate dehydrogenase A (LDHA), which converts pyruvate to lactate (Lac), and PDH kinase 1 (PDK1), which inhibits the conversion of pyruvate to AcCoA, leading to decreased flux through the TCA cycle and decreased mitochondrial ROS production. CoQ, coenzyme Q (ubiquinone); Cyt c, cytochrome c; IMM, inner mitochondrial membrane; IMS, intermembrane space; OMM, outer mitochondrial membrane.

Figure 2. Glucose metabolic shunt pathways that generate NADPH

The first reaction of the EMP converts glucose (Glc) to glucose 6‐phosphate (G6P), which is either converted to fructose 6‐phosphate (F6P) or shunted to the pentose phosphate pathway (PPP) by glucose‐6‐phosphate dehydrogenase (G6PD), which converts G6P to 6‐phosphogluconate (6PG). Five steps later in the EMP, 3‐phosphoglycerate (3PG) is either converted to 2‐phosphoglycerate (2PG) or shunted to the serine synthesis pathway (SSP) by phosphoglycerate dehydrogenase (PHGDH), which converts 3PG to 3‐phosphohydroxypyruvate (3PHP). The PPP generates cytosolic NADPH, whereas the SSP can lead to the generation of either cytosolic or mitochondrial NADPH. Cellular O₂ availability determines flux through these pathways in breast cancer cells: Hypoxia represses G6PD expression and induces the expression of PHGDH, LDHA, and PDK1. PEP, phospho‐enol‐pyruvate; Pyr, pyruvate; Lac, lactate; GLY, glycolytic end‐product.

Figure 3. Mitochondrial one‐carbon (folate cycle) metabolism generates mitochondrial NADPH

The serine synthesis pathway converts 3PG to serine (Ser) through the activity of PHGDH, phosphoserine aminotransferase 1 (PSAT1), and phosphoserine phosphatase (PSPH). Ser is a substrate for mitochondrial one‐carbon metabolism, which generates NADPH while converting Ser + tetrahydrofolate (THF) into glycine (Gly) + THF + formate through the activity of serine hydroxymethyl transferase 2 (SHMT2), methylene‐THF dehydrogenase (MTHFD) 2, and MTHFD1L. Glu, glutamate; α‐KG, α‐ketoglutarate.

Figure 4. HIFs increase antioxidant production to maintain redox homeostasis, which is required for induction of the breast cancer stem cell phenotype under hypoxic conditions

Under conditions of acute hypoxia, the production of reactive oxygen species (ROS) by the electron transport chain (ETC) is increased. HIFs activate the transcription of the following: genes encoding enzymes of the serine synthesis pathway (SSP) and mitochondrial one‐carbon metabolism (m1CM) to increase the production of NADPH, which is used to convert oxidized glutathione (GSSG) to its reduced form (GSH); and genes encoding SLC7A11 and GCLM to increase glutathione production. Reduced glutathione is used to reverse the oxidation of cellular proteins (Protein_ox) to their reduced forms (Protein_red). HIFs also induce expression of pluripotency factors that specify the breast cancer stem cell (BCSC) phenotype. Hypoxia induces the BCSC phenotype, but only if mitochondrial redox homeostasis is maintained.