Antibody-based approaches to diagnosis and characterization of oxidative phosphorylation diseases

Metallothionein 2A affects the cell respiration by suppressing the expression of mitochondrial protein cytochrome c oxidase subunit II

Metallothioneins (MT) are involved in a broad range of cellular processes and play a major role in protection of cells towards various stressors. Two functions of MTs, namely the maintaining of the homeostasis of transition metal ions and the redox balance, are directly linked to the functioning of mitochondria. Dyshomeostasis of MTs is often related with malfunctioning of mitochondria; however, the mechanism by which MTs affect the mitochondrial respiratory chain is still unknown. We demonstrated that overexpression of MT-2A in HEK cell line decreased the oxidative phosphorylation capacity of the cells. HEK cells overexpressing MT-2A demonstrated reduced oxygen consumption and lower cellular ATP levels. MT-2A did not affect the number of mitochondria, but reduced specifically the level of cytochrome c oxidase subunit II protein, which resulted in lower activity of the complex IV.

Ultrastructure of the mitochondrion and its bearing on function and bioenergetics

The recently ascertained network and dynamic organization of the mitochondrion, as well as the demonstration of energy proteins and metabolites subcompartmentalization, have led to a reconsideration of the relationships between organellar form and function. In particular, the impact of mitochondrial morphological changes on bioenergetics is inseparable. Several observations indicate that mitochondrial energy production may be controlled by structural rearrangements of the organelle both interiorly and globally, including the remodeling of cristae morphology and elongation or fragmentation of the tubular network organization, respectively. These changes are mediated by fusion or fission reactions in response to physiological signals that remain unidentified. They lead to important changes in the internal diffusion of energy metabolites, the sequestration and conduction of the electric membrane potential (Delta Psi), and possibly the delivery of newly synthesized ATP to various cellular areas. Moreover, the physiological or even pathological context also determines the morphology of the mitochondrion, suggesting a tight and mutual control between mitochondrial form and bioenergetics. In this review, we delve into the link between mitochondrial structure and energy metabolism.

Crypt-restricted loss and decreased protein expression of cytochrome C oxidase subunit I as potential hypothesis-driven biomarkers of colon cancer risk

There is an increasing demand for the development of intermediate biomarkers to assess colon cancer risk. We previously determined that a live cell bioassay, which assesses apoptosis resistance in the nonneoplastic colonic mucosa, detects approximately 50% of patients with colon cancer. A hypothesis-driven biomarker that reflects apoptosis resistance in routine formalin-fixed, paraffin-embedded tissue would be easier to use. Cytochrome c oxidase is a critical enzyme that controls mitochondrial respiration and is central to apoptosis. We did an immunohistochemical study of cytochrome c oxidase subunit I expression in 46 colonic mucosal samples from 16 patients who had undergone a colonic resection. These included five patients without evidence of colonic neoplasia (three normal and two diverticulitis), three patients with tubulovillous adenomas, and eight patients with colonic adenocarcinomas. Analysis of aberrancies in expression of cytochrome c oxidase subunit I showed that, compared with nonneoplasia, the patients with neoplasia had a higher mean incidence of crypts having decreased expression (1.7 versus 22.8, P = 0.03) and a higher mean incidence having crypt-restricted loss (0.6 versus 3.2, P = 0.06). The percentage with segmented loss was low and was similar in the two groups. Combining these results, the mean % normal (i.e., with none of the three types of abnormality) was 96.7 in nonneoplasia versus only 73.2 in patients with neoplasia (P = 0.02). It should be noted that a defect in cytochrome c oxidase subunit I immunostaining was not detected in all biopsy samples from each patient for whom some abnormality was found, indicating a "patchiness" in the cytochrome c oxidase subunit I field defect. As a result of this "patchiness," the increased variability in the incidence of crypt-restricted loss of cytochrome c oxidase subunit I expression was a statistically significant feature of the neoplasia group. Crypt-restricted loss of cytochrome c oxidase subunit I has not been previously reported in colonic mucosa and is presumably the result of a crypt-restricted stem cell mutation. Decreased cytochrome c oxidase subunit I expression also significantly correlated with apoptosis resistance, a factor known to contribute to carcinogenesis. The results suggest, however, that aberrant cytochrome c oxidase subunit I expression may be a better biomarker than loss of apoptosis competence for increased colon cancer risk.

Energy substrate modulates mitochondrial structure and oxidative capacity in cancer cells

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.