Functional roles of superoxide and hydrogen peroxide generated by mitochondrial DNA mutation in regulating tumorigenicity of HepG2 cells

Multifaceted Roles of Mitochondrial Components and Metabolites in Metabolic Diseases and Cancer

Mitochondria are essential cellular components that ensure physiological metabolic functions. They provide energy in the form of adenosine triphosphate (ATP) through the electron transport chain (ETC). They also constitute a metabolic hub in which metabolites are used and processed, notably through the tricarboxylic acid (TCA) cycle. These newly generated metabolites have the capacity to feed other cellular metabolic pathways; modify cellular functions; and, ultimately, generate specific phenotypes. Mitochondria also provide intracellular signaling cues through reactive oxygen species (ROS) production. As expected with such a central cellular role, mitochondrial dysfunctions have been linked to many different diseases. The origins of some of these diseases could be pinpointed to specific mutations in both mitochondrial- and nuclear-encoded genes. In addition to their impressive intracellular tasks, mitochondria also provide intercellular signaling as they can be exchanged between cells, with resulting effects ranging from repair of damaged cells to strengthened progression and chemo-resistance of cancer cells. Several therapeutic options can now be envisioned to rescue mitochondria-defective cells. They include gene therapy for both mitochondrial and nuclear defective genes. Transferring exogenous mitochondria to target cells is also a whole new area of investigation. Finally, supplementing targeted metabolites, possibly through microbiota transplantation, appears as another therapeutic approach full of promises.

Mitochondrial ROS control of cancer

Mitochondria serves a primary role in energy maintenance but also function to govern levels of mitochondria-derived reactive oxygen species (mROS). ROS have long been established to play a critical role in tumorigenesis and are now considered to be integral to the regulation of diverse signaling networks that drive proliferation, tumor cell survival and malignant progression. mROS can damage DNA, activate oncogenes, block the function of tumor suppressors and drive migratory signaling. The mitochondrion's oxidant scavenging systems including SOD2, Grx2, GPrx, Trx and TrxR are key of the cellular redox tone. These mitochondrial antioxidant systems serve to tightly control the levels of the primary ROS signaling species, H2O2. The coordinated control of mROS levels is also coupled to the activity of the primary H2O2 consuming enzymes of the mitochondria which are reliant on the epitranscriptomic control of selenocysteine incorporation. This review highlights the interplay between these many oncogenic signaling networks, mROS and the H2O2 emitting and consuming capacity of the mitochondria.

Impairment of oxidative phosphorylation increases the toxicity of SYD-1 on hepatocarcinoma cells (HepG2)

Toxicity of the SYD-1 mesoionic compound (3-[4-chloro-3-nitrophenyl]-1,2,3-oxadiazolium-5-olate) was evaluated on human liver cancer cells (HepG2) grown in either high glucose (HG) or galactose (GAL) medium, and also on suspended cells kept in HG medium. SYD-1 was able to decrease the viability of cultured HepG2 cells in a dose-dependent manner, as assessed by MTT, LDH release and dye with crystal violet assays, but no effect was observed on suspended cells after 1-40 min of treatment. Respiration analysis was performed after 2 min (suspended cells) or 24 h (cultured cells) of treatment: no change was observed in suspended cells, whereas SYD-1 inhibited as well basal, leak and uncoupled states of the respiration in cultured cells with HG medium. These inhibitions were consistent with the decrease in pyruvate level and increase in lactate level. Even more extended results were obtained with HepG2 cells grown in GAL medium where, additionally, the ATP amount was reduced. Furthermore, SYD-1 appears not to be transported by the main ABC multidrug transporters. These results show that SYD-1 is able to change the metabolism of HepG2 cells, and suggest that its cytotoxicity is related to impairment of mitochondrial metabolism. Therefore, we may propose that SYD-1 is a potential candidate for hepatocarcinoma treatment.

A supersandwich fluorescence in situ hybridization strategy for highly sensitive and selective mRNA imaging in tumor cells

This strategy uses two fluorophore-labeled signal probes to generate a supersandwich product, which in turn generates numerous signal probes located at the target mRNA position, resulting in thein situfluorescence signal amplification.

Critical roles of the cAMP-responsive element-binding protein-mediated pathway in disorganized epithelial phenotypes caused by mitochondrial dysfunction

In most human cancers, somatic mutations have been identified in the mtDNA; however, their significance remains unclear. We recently discovered that NMuMG mouse mammary epithelial cells, when deprived of mitochondria or following inhibition of respiratory activity, undergo epithelial morphological disruption accompanied with irregular edging of E-cadherin, the appearance of actin stress fibers, and an altered gene expression profile. In this study, using the mtDNA-less pseudo ρ0 cells obtained from NMuMG mouse mammary epithelial cells, we examined the roles of two mitochondrial stress-associated transcription factors, cAMP-responsive element-binding protein (CREB) and C/EBP homologous protein-10 (CHOP), in the disorganization of epithelial phenotypes. We found that the expression of matrix metalloproteinase-13 and that of GADD45A, SNAIL and integrin α1 in the ρ0 cells were regulated by CHOP and CREB, respectively. Of note, knockdown and pharmacological inhibition of CREB ameliorated the disrupted epithelial morphology. It is interesting to note that the expression of high mobility group AT-hook 2 (HMGA2), a non-histone chromatin protein implicated in malignant neoplasms, was increased at the protein level through the CREB pathway. Here, we reveal how the activation of the CREB/HMGA2 pathway is implicated in the repression of integrin α1 expression in HepG2 human cancer cells, highlighting the importance of the CREB/HMGA2 pathway in malignant transformation associated with mitochondrial dysfunction, thereby raising the possibility that the pathway indirectly interferes with the cell-cell adhesion structure by influencing the cell-extracellular matrix adhesion status. Overall, the data suggest that mitochondrial dysfunction potentially contributes to neoplastic transformation of epithelial cells through the activation of these transcriptional pathways.