Hypoxia: Overview on Hypoxia-Mediated Mechanisms with a Focus on the Role of HIF Genes

Abstract

Hypoxia represents a frequent player in a number of malignancies, contributing to the development of the neoplastic disease. This review will discuss the means by which hypoxia powers the mechanisms behind cancer progression, with a majority of examples from lung cancer, the leading malignancy in terms of incidence and mortality rates (the frequent reference toward lung cancer is also for simplification purposes and follow up of the global mechanism in the context of a disease). The effects induced by low oxygen levels are orchestrated by hypoxia-inducible factors (HIFs) which regulate the expression of numerous genes involved in cancer progression. Hypoxia induces epithelial-to-mesenchymal transition (EMT) and metastasis through a complex machinery, by mediating various pathways such as TGF-β, PI3k/Akt, Wnt, and Jagged/Notch. Concomitantly, hypoxic environment has a vast implication in angiogenesis by stimulating vessel growth through the HIF-1α/VEGF axis. Low levels of oxygen can also promote the process through several other secondary factors, including ANGPT2, FGF, and HGF. Metabolic adaptations caused by hypoxia include the Warburg effect-a metabolic switch to glycolysis-and GLUT1 overexpression. The switch is achieved by directly increasing the expression of numerous glycolytic enzymes that are isoforms of those found in non-malignant cells.

Keywords: angiogenesis; cancer metabolism; drug resistance; hypoxia; metastasis.

Figure 1

Hypoxia-inducible factor (HIF) regulation. HIF-1α molecule presents 2 proline (Pro) residues in the 402 and 564 positions. In normoxia, hydroxylation of the proline residues allows the pVHL to ubiquitinate the substrate, leading to a 26S-dependent proteasome degradation of the complex. Contrarily, in hypoxic environment, the action of PHDs is blocked, the substrate is phosphorylated, which then binds the CBP/p300 complex. After HIF-1β binds HIF-1α, the dimer attacks the HRE of the target gene, exhibiting specific effects.

Figure 2

HIF-1α (A,B) and HIF-2α (C,D) expression in (A,C) lung adenocarcinoma and (B,D) lung squamous cell carcinoma tissue samples from TCGA database (*data was download from dataset: Gene expression RNAseq–HTSeq—Counts for both LUSC and LUAD and represented as scatter plot, mean with SD).

Figure 3

Pathways involved in hypoxia-dependent epithelial-to-mesenchymal transition (EMT). The main pathways that mediate EMT activation are TGF-β, PI3k/Akt, Wnt, and Jagged/Notch. The TGF-β pathway is SMAD-mediated; the SMAD complex binds specific DNA regions along with transcription factors such as SNAIL and zinc-finger E-box binding homeobox (ZEB) in order to modulate EMT-related gene expression. Another crucial pathway in hypoxia-mediated EMT is PI3k/Akt. HIF-2α is able to induce this pathway, with a concomitant activation of NF-κB/TWIST, and a downregulation of E-cadherin. Obviously, this downregulation leads to loss of cell-to-cell junctions and promotes EMT. The Wnt pathway, as well as the Jagged/Notch pathway, induces EMT in a SNAIL-dependent manner. Hypoxia can also stimulate EMT directly—HIF-1/2 can bind the HRE of the TWIST1 gene in order to promote its expression, thus leading to EMT.

Figure 4

The metabolic switch to glycolysis. Glucose enters the cell via the GLUT1 and GLUT3 transporters. In the intracellular environment, glucose is converted into glucose-6-phosphate and then into pyruvate following a succession of metabolic reactions catalyzed by HIF-induced enzymes HK II, PFK, PYK-M2. Consequently, pyruvate does not enter oxidative phosphorylation and is converted into lactate through the action of lactate dehydrogenase-A (LDHA). Lactate is transported towards the extracellular compartment through the MCT-4 transporter, whilst CO₂ resulted from the metabolic conversions diffuses extracellular. The membrane-bound carbonic anhydrase IX (CAIX) synthesizes carbonic acid which dissociates in bicarbonate and H+. The HCO3- is transported into the cell, increasing the intracellular pH, whilst the H+ decreases extracellular pH. The extracellular pH is further decreased by lactate-derived H+ which antiports Na+ through the NHE1 antiporter. The enzymes colored in red are HIF-1α-induced.