Programmed cell death triggered by nucleotide pool damage and its prevention by MutT homolog-1 (MTH1) with oxidized purine nucleoside triphosphatase

The awakening of an advanced malignant cancer: an insult to the mitochondrial genome

BACKGROUND

In only months-to-years a primary cancer can progress to an advanced phenotype that is metastatic and resistant to clinical treatments. As early as the 1900s, it was discovered that the progression of a cancer to the advanced phenotype is often associated with a shift in the metabolic profile of the disease from a state of respiration to anaerobic fermentation - a phenomenon denoted as the Warburg Effect.

SCOPE OF REVIEW

Reports in the literature strongly suggest that the Warburg Effect is generated as a response to a loss in the integrity of the sequence and/or copy number of the mitochondrial genome content within a cancer.

MAJOR CONCLUSIONS

Multiple studies regarding the progression of cancer indicate that mutation, and/or, a flux in the copy number, of the mitochondrial genome content can support the early development of a cancer, until; the mutational load and/or the reduction-to-depletion of the copy number of the mitochondrial genome content induces the progression of the disease to an advanced phenotype.

GENERAL SIGNIFICANCE

Collectively, evidence has revealed that the human cell has incorporated the mitochondrial genome content into a cellular mechanism that, when pathologically actuated, can de(un)differentiate a cancer from the parental tissue of origin into an autonomous disease that disrupts the hierarchical structure-and-function of the human body. This article is part of a Special Issue entitled: Biochemistry of Mitochondria.

DNA glycosylase encoded by MUTYH functions as a molecular switch for programmed cell death under oxidative stress to suppress tumorigenesis

8-oxoguanine is a major base lesion in DNA or in nucleotides caused by oxidative stress, and is highly mutagenic because it can pair with adenine as well as cytosine. Adenine DNA glycosylase, encoded by the human mutY homolog gene, MUTYH, excises adenine in the nascent strand when inserted opposite 8-oxoguanine in template DNA, and thus suppresses mutagenesis caused by 8-oxoguanine that has accumulated in DNA due to oxidative stress. Several germ-line mutations in MUTYH are predisposed to MUTYH-associated polyposis, an autosomal recessive disorder characterized by multiple colorectal adenomas and carcinomas. Loss of function of MUTYH leads to an accumulation of somatic mutations in APC and KRAS genes, resulting in the development of adenomas/carcinomas. We recently demonstrated that accumulation of 8-oxoguanine in nuclear and mitochondrial DNA triggers two distinct cell death pathways that are independent of each other. Both pathways are initiated by the accumulation of MUTYH-generated single-strand breaks (SSBs) in nuclear or mitochondrial DNA. Our findings indicate that MUTYH-induced cell death due to oxidative stress results in an efficient elimination of mutagenic cells that have accumulated high levels of 8-oxoguanine in their DNAs. It is most likely that loss of function of MUTYH in stem or progenitor cells in the intestinal epithelium of MUTYH-associated polyposis patients results in escape from programmed cell death; however, accumulated 8-oxoguanine causes various mutations in APC or KRAS genes in these proliferative cells, thereby promoting tumorigenesis. We thus propose that MUTYH suppresses tumorigenesis under conditions of oxidative stress by inducing cell death and by suppressing mutagenesis.

Nucleotides function as endogenous chemical sensors for oxidative stress signaling

Oxidized and nitrated nucleotides including 8-oxogunanine and 8-nitroguanine derivatives such as 8-nitroguanosine 3',5'-cyclic monophosphate were generated by reactive nitrogen oxides and reactive oxygen species in cultured cells and in tissues. 8-oxoguanine and 8-nitroguanine in DNA and RNA are potentially mutagenic, and the former also induces cell death. Some derivative, 8-nitroguanosine 3',5'-cyclic monophosphate a major nitrated guanine nucleotide, was identified as a novel second messenger. Surprisingly, the amount of 8-nitroguanosine 3',5'-cyclic monophosphate generated was found to be higher than that of guanosine 3',5'-cyclic monophosphate in cells expressing inducible nitric oxide synthase. More important, 8-nitroguanosine 3',5'-cyclic monophosphate is electrophilic and reacted efficiently with sulfhydryls of proteins to produce a novel posttranslational modification (named S-guanylation) via guanosine 3',5'-cyclic monophosphate adduction. For example, 8-nitroguanosine 3',5'-cyclic monophosphate-induced S-guanylation of Kelch-like ECH-associated protein 1 led to NF-E2-related factor activation and induction of antioxidant enzymes. 8-nitroguanosine 3',5'-cyclic monophosphate may thus protect cells against oxidative stress-related cytotoxicity. Therefore, although chemically modified nucleotides produced via oxidative and nitrative stress are regarded simply as endogenous mutagens, the endogenous nucleotides stored in cells per se may serve functionally as a sensing mechanism for reactive nitrogen oxides and oxygen species to induce cellular adaptive responses to oxidative stress.

The biological and metabolic fates of endogenous DNA damage products

DNA and other biomolecules are subjected to damaging chemical reactions during normal physiological processes and in states of pathophysiology caused by endogenous and exogenous mechanisms. In DNA, this damage affects both the nucleobases and 2-deoxyribose, with a host of damage products that reflect the local chemical pathology such as oxidative stress and inflammation. These damaged molecules represent a potential source of biomarkers for defining mechanisms of pathology, quantifying the risk of human disease and studying interindividual variations in cellular repair pathways. Toward the goal of developing biomarkers, significant effort has been made to detect and quantify damage biomolecules in clinically accessible compartments such as blood and and urine. However, there has been little effort to define the biotransformational fate of damaged biomolecules as they move from the site of formation to excretion in clinically accessible compartments. This paper highlights examples of this important problem with DNA damage products.