Respiratory complex I is essential to induce a Warburg profile in mitochondria-defective tumor cells

Abstract

BackgroundAerobic glycolysis, namely the Warburg effect, is the main hallmark of cancer cells. Mitochondrial respiratory dysfunction has been proposed to be one of the major causes for such glycolytic shift. This hypothesis has been revisited as tumors appear to undergo waves of gene regulation during progression, some of which rely on functional mitochondria. In this framework, the role of mitochondrial complex I is still debated, in particular with respect to the effect of mitochondrial DNA mutations in cancer metabolism. The aim of this work is to provide the proof of concept that functional complex I is necessary to sustain tumor progression.MethodsComplex I-null osteosarcoma cells were complemented with allotopically expressed complex I subunit 1 (MT-ND1). Complex I re-assembly and function recovery, also in terms of NADH consumption, were assessed. Clones were tested for their ability to grow in soft agar and to generate tumor masses in nude mice. Hypoxia levels were evaluated via pimonidazole staining and hypoxia-inducible factor-1α (HIF-1α) immunoblotting and histochemical staining. 454-pyrosequencing was implemented to obtain global transcriptomic profiling of allotopic and non-allotopic xenografts.ResultsComplementation of a truncative mutation in the gene encoding MT-ND1, showed that a functional enzyme was required to perform the glycolytic shift during the hypoxia response and to induce a Warburg profile in vitro and in vivo, fostering cancer progression. Such trigger was mediated by HIF-1α, whose stabilization was regulated after recovery of the balance between α-ketoglutarate and succinate due to a recuperation of NADH consumption that followed complex I rescue.ConclusionRespiratory complex I is essential for the induction of Warburg effect and adaptation to hypoxia of cancer cells, allowing them to sustain tumor growth. Differently from other mitochondrial tumor suppressor genes, therefore, a complex I severe mutation such as the one here reported may confer anti-tumorigenic properties, highlighting the prognostic values of such genetic markers in cancer.

Stable nuclear expression ofATP8andATP6genes rescues a mtDNA Complex Vnullmutant

“…To our knowledge this is the first successful demonstration of allotopic expression of a mitochondrial gene in cells that are both null for a protein and homoplasmic for that mutation. Previously, the MT - ND1 gene was allotopically expressed in ND1 null cells, but these cells were not completely homoplasmic and the purpose of that study was to identify the role of ND1 in tumorigenisis rather than to establish the biochemical functionality of allotopically expressed ND1 (20). Our approach completely removes the ambiguity that might be caused by competing with either WT or mutant proteins.…”

“…ATP6 expression was also able to partially rescue mutant CHO cells (16) while exogenous ND4 expression has been claimed to rescue rodent models of LHON (17–19). Mutant MT-ND1 cells (OST-93 ND1 cells) were complemented by allotopic expression of ND1 with dramatic changes in the bioenergetics state and tumorgenic potential of the mutant cells (20). These cells are not perfectly homoplasmic (reported to carry a 93% mutation load) though the authors showed that this mutation load was sufficient to induce a null phenotype for the ND1 protein (21).…”

How do changes in the mtDNA and mitochondrial dysfunction influence cancer and cancer therapy? Challenges, opportunities and models

“…Data analysis by Larman et al showed that, across 5 different cancer types, displayed somatic mtDNA mutations ranging, from 13% in glioblastoma to 63% frequency in rectal adenocarcinomas [133]. Some data suggests that the effect of the somatic mtDNA mutations and the degree or nature of the tumorigenesis effect depend on the functional and threshold effect of the mutation [134][135][136]. For instance, the m.3460G>A/MT-ND1 mutation (decrease in complex I activity) result in different tumorigenic potential as determined by colony forming efficiency and tumor growth of osteosarcoma cybrids (cytoplasmic hybrids, Fig.…”

Gene Therapy for Leber Hereditary Optic Neuropathy

“…11–14 Many experimental approaches have been proposed. 15–17 However, only the allotopic expression approach 18–27 has reached human testing in China (NCT01267422) and France (NCT02064569). We describe the preliminary findings from the first 5 participants enrolled in this trial based in the United States (NCT02161380).…”