Abstract
mtDNA variations often result in bioenergetic dysfunction inducing a metabolic switch toward glycolysis resulting in an unbalanced pH homeostasis. In hypoxic cells, expression of the tumor-associated carbonic anhydrase IX (CAIX) is enhanced to maintain cellular pH homeostasis. We hypothesized that cells with a dysfunctional oxidative phosphorylation machinery display elevated CAIX expression levels. Increased glycolysis was observed for cytoplasmic 143B mutant hybrid (m.3243A>G, >94.5%) cells (p < 0.05) and 143B mitochondrial DNA (mtDNA) depleted cells (p < 0.05). Upon hypoxia (0.2%, 16 h), genetic or pharmacological oxidative phosphorylation (OXPHOS) inhibition resulted in decreased CAIX (p < 0.05), vascular endothelial growth factor (VEGF) and hypoxia-inducible factor 1-alpha (HIF-1α) expression levels. Reactive oxygen species (ROS) and prolyl-hydroxylase 2 (PHD2) levels could not explain these observations. In vivo, tumor take (>500 mm3) took longer for mutant hybrid xenografts, but growth rates were comparable with control tumors upon establishment. Previously, it has been shown that HIF-1α is responsible for tumor establishment. In agreement, we found that HIF-1α expression levels and the pimonidazole-positive hypoxic fraction were reduced for the mutant hybrid xenografts. Our results demonstrate that OXPHOS dysfunction leads to a decreased HIF-1α stabilization and subsequently to a reduced expression of its downstream targets and hypoxic fraction in vivo. In contrast, hypoxia-inducible factor 2-alpha (HIF-2α) expression levels in these xenografts were enhanced. Inhibition of mitochondrial function is therefore an interesting approach to increase therapeutic efficacy in hypoxic tumors.
Highlights
As the tumor microenvironment is a heterogeneous and dynamic entity, cells within a tumor have different gene expression profiles, metabolism and oxygen supply
In order to investigate if extreme changes in oxidative phosphorylation (OXPHOS) capacity would lead to a change in carbonic anhydrase IX (CAIX) expression, we used 143B and A549 mitochondrial DNA depleted cancer cell lines (ρ0 cells) (Figure 1; Figure S1)
Where MELAS 1 cells were completely dependent on glycolysis, since no glycolytic reserve capacity could be observed (Figure 2B), MELAS 2 did display a glycolytic reserve capacity with comparable levels to the control cell lines, suggesting that the OXPHOS was still functional in these cells
Summary
As the tumor microenvironment is a heterogeneous and dynamic entity, cells within a tumor have different gene expression profiles, metabolism and oxygen supply. Frontiers in Oncology | www.frontiersin.org van Gisbergen et al. Inhibiting HIF-1α by Mitochondrial Dysfunction a tumor and its metabolic consequences, is critical for tumor progression. Tumors cells in a hypoxic tumor microenvironment have often the capability to alter their metabolism and favor metabolic pathways, which are less dependent on oxygen, to meet their energy demand. Hypoxic tumor cells often produce large amount of acids due to their increase in glycolysis and have a decreased extracellular pH [2,3,4]. One important family of proteins involved in tumor pH maintenance are carbonic anhydrases (CA) [5]. The membrane bound zincmetallo-enzyme CAIX is capable of re-hydrating CO2 into bicarbonate
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