Energy Substrate Modulates Mitochondrial Structure and Oxidative Capacity in Cancer Cells

TLDR

Cancer cells typically exhibit reduced mitochondrial content and oxidative phosphorylation capacity, yet the triggers and reversibility of these alterations remain poorly understood. The study investigates whether altering energy substrate availability can reverse mitochondrial deficits in cancer cells and compares the effects in HeLa cells versus nontransformed fibroblasts. Researchers examined substrate effects on HeLa cells by measuring oxidative phosphorylation with ratiometric GFP biosensors, assessing enzyme expression by Western blot, quantifying mitochondrial DNA via qPCR, and visualizing morphology through confocal and electron microscopy, then compared the results to fibroblasts. Substrate alteration markedly restored mitochondrial function in HeLa cells, enabling glutaminolysis‑driven oxidative phosphorylation, remodeling mitochondrial structure, and offering a possible explanation for enhanced oxidative capacity following tumor regression.

Abstract

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.