HIF-1-Independent Mechanisms Regulating Metabolic Adaptation in Hypoxic Cancer Cells

Abstract

In solid tumours, cancer cells exist within hypoxic microenvironments, and their metabolic adaptation to this hypoxia is driven by HIF-1 transcription factor, which is overexpressed in a broad range of human cancers. HIF inhibitors are under pre-clinical investigation and clinical trials, but there is evidence that hypoxic cancer cells can adapt metabolically to HIF-1 inhibition, which would provide a potential route for drug resistance. Here, we review accumulating evidence of such adaptions in carbohydrate and creatine metabolism and other HIF-1-independent mechanisms that might allow cancers to survive hypoxia despite anti-HIF-1 therapy. These include pathways in glucose, glutamine, and lipid metabolism; epigenetic mechanisms; post-translational protein modifications; spatial reorganization of enzymes; signalling pathways such as Myc, PI3K-Akt, 2-hyxdroxyglutarate and AMP-activated protein kinase (AMPK); and activation of the HIF-2 pathway. All of these should be investigated in future work on hypoxia bypass mechanisms in anti-HIF-1 cancer therapy. In principle, agents targeted toward HIF-1β rather than HIF-1α might be advantageous, as both HIF-1 and HIF-2 require HIF-1β for activation. However, HIF-1β is also the aryl hydrocarbon nuclear transporter (ARNT), which has functions in many tissues, so off-target effects should be expected. In general, cancer therapy by HIF inhibition will need careful attention to potential resistance mechanisms.

Keywords: 2-hydroxyglutarate; AMP-activated protein kinase (AMPK); Myc; cancer metabolism; creatine metabolism; glutamine metabolism; glycolysis; hypoxia; hypoxia-inducible factor-1 (HIF-1); lipid metabolism; phosphatidylinositol 3-kinase (PI3K).

Figure 1

HIF-1 in normoxia and hypoxia. (A) In normoxia, HIF-1α is hydroxylated by prolyl and asparaginyl hydroxylases in the cytoplasm in an oxygen-dependent manner. It then forms a complex with VHL and other proteins, allowing it to be ubiquitnylated and degraded by proteosomes. (B) In hypoxia, the lack of oxygen-dependent hydroxylation and proteasomal degradation of HIF-1α causes it to accumulate. It then translocates to the nucleus and complexes with HIF-1β to form a transcription factor complex that binds to hypoxia response elements (HRE) within the promoter regions of target genes involved in cell proliferation, metastasis, glycolysis, angiogenesis, growth, and survival.

Figure 2

Effects of the HIF-1 activation on the tricarboxylic acid cycle and associated pathways, including glutamine and citrate metabolism, in hypoxia. The mechanisms are highlighted in red, and the red arrows indicate the direction of flux via the metabolic pathways under the prevailing oxygen tension. (A) In normoxia, pyruvate produced via glycolysis enters the mitochondria, where it is converted to acetyl-CoA and enters the tri-carboxylic acid cycle (TCA). Citrate from the TCA can be exported from the mitochondria into the cytosol, where it is converted to acetyl-CoA for fatty acid synthesis. Cytosolic glutamine is converted to α-ketoglutarate and transported into the mitochondria where it enters the TCA. A key regulator of autophagy, Beclin 1 is inhibited by Bcl-2 and Bcl-XL, thereby decreasing autophagy. (B) In hypoxia, HIF-1 activation results in increased flux through the glycolytic pathway, as well as accumulation of pyruvate and conversion into lactate. BNIP3 and BNIP3L (not shown in this figure) are expressed under the control of HIF-1, which then interact with Bcl-2 and Bcl-XL to liberate Beclin 1. Beclin 1 induces mitochondrial autophagy, reducing the formation of mitochondrial reactive oxygen species under hypoxia. Fatty acid synthesis is maintained by reductive carboxylation of glutamine, formation of citrate, and conversion of citrate into acetyl-CoA in the cytoplasm.