Energy Substrate Modulates Mitochondrial Structure and Oxidative Capacity in Cancer Cells

Abstract

Comparative analysis of cytoplasmic organelles in a variety of tumors relative to normal tissues generally reveals a strong diminution in mitochondrial content and in oxidative phosphorylation capacity. However, little is known about what triggers these modifications and whether or not they are physiologically reversible. We hypothesized that energy substrate availability could play an important role in this phenomenon. The physiological effects of a change in substrate availability were examined on a human cancer cell line (HeLa), focusing specifically on its ability to use glycolysis versus oxidative phosphorylation, and the effect that energy substrate type has on mitochondrial composition, structure, and function. Changes in oxidative phosphorylation were measured in vivo by a variety of techniques, including the use of two novel ratiometric green fluorescent protein biosensors, the expression level of oxidative phosphorylation and some glycolytic enzymes were determined by Western blot, mitochondrial DNA content was measured by real-time PCR, and mitochondrial morphology was monitored by both confocal and electron microscopy. Our data show that the defective mitochondrial system described in cancer cells can be dramatically improved by solely changing substrate availability and that HeLa cells can adapt their mitochondrial network structurally and functionally to derive energy by glutaminolysis only. This could also provide an explanation for the enhancement of oxidative phosphorylation capacity observed after tumor regression or removal. Our work demonstrates that the pleomorphic, highly dynamic structure of the mitochondrion can be remodeled to accommodate a change in oxidative phosphorylation activity. We compared our finding on HeLa cells with those for nontransformed fibroblasts to help distinguish the regulatory pathways.

Quantitative analysis of mitochondrial DNA 4977-bp deletion in sporadic breast cancer and benign breast diseases

“…We believe the higher detection rate for the mtDNA 4977 mutation in our study is a result of the increased sensitivity and specificity of the quantitative assay we employed, in contrast to the methods used in previous studies (long PCR or semi-quantitative PCR methods). Our finding is also biologically plausible, due to the fact that cancer cells are metabolically adapted for rapid growth and proliferation under conditions of low pH and oxygen tension, solid cancer cells generate energy by glycolysis in strong preference to oxidative phosphorylation (33)(34)(35). Furthermore, previous in vitro studies have suggested that mitochondrial protein synthesis is defective and the activity of cytochrome oxidase decreases when the proportion of the mtDNA 4977 mutation was greater than 60% in the cancer cell (23).…”

Cardiolipin or MTCH2 can serve as tBID receptors during apoptosis

“…To test this, cells were incubated in a medium containing galactose in which cell growth depends on functional oxidative phosphorylation. 40 As a control, cells were grown in normal glucose-containing medium in which energy production occurs mainly via glycolysis. Strikingly, in the absence of CL, cells were unable to grow in galactose medium (Figure 2c), which points to a defective mitochondrial respiratory chain.…”

Mitochondria and cancer: is there a morphological connection?

“…This corresponds to low ADP and minimal O 2 consumption. Further evidence that mitochondria remodel according to changes in OXPHOS activity came from the work of Rossignol et al (2004) who showed that the mitochondrial network extended within cells and became more ramified and interconnected in the presence of galactose.…”