Dimethylnitrosamine-induced DNA damage and toxic cell death in cultured mouse hepatocytes

Matrix metalloproteinase-9, -10, and -12, MDM2 and p53 expression in mouse liver during dimethylnitrosamine-induced oxidative stress and genomic injury

Treatment during early tumor development has greater success because tissue growth remains largely confined to its original locus. At later stages, malignant cells migrate from their original location, invade surrounding normal areas, and can disseminate widely throughout the body. Remodeling of the extracellular matrix (ECM) serves as a key facilitator of this dissemination. Proteolytic enzymes including plasmin and matrix metalloproteinases (MMPs) play an integral role in degrading the surrounding ECM proteins and clearing a path for tumor cell migration. Specific MMPs are highly expressed late during malignant tumor invasion. It is not understood whether early changes in MMPs influence apoptotic and necrotic cell death, processes known to govern the early stages of carcinogenesis. Similarly, the interaction between MDM2 and p53 is tightly controlled by a complex array of post-translational modifications, which in turn dictates the stability and activity of both p53 and MDM2. The present studies examine the hypothesis that model hepatotoxin dimethylnitrosamine (DMN), which is also a model carcinogen, will induce the MMP family of proteins after administration in hepatotoxic doses. Doses of 25, 50, and 100 mg/kg DMN were administered i.p. to male C3H mice. Changes in parameters associated with apoptotic and necrotic cell death, DNA damage, cell proliferation, and extracellular proteinases were examined in liver at 24 h. Serum ALT activity, oxidative stress [malondialdehyde], and caspase-activated DNAse mediated DNA laddering increased in a dose-dependent manner, as did the level of MDM2 protein. MMP-9, -10 and -12 (gelatinase-B, stromelysin-2, macrophage elastase), and p53 protein levels increased following 25 mg/kg DMN, but were successively decreased after higher DMN doses. The results of this study demonstrate changes in MDM2 and MMPs during DMN-induced acute liver injury and provide a plausible linkage between DMN-induced oxidative stress-mediated genomic injury and its likely involvement in setting the stage for initiating subsequent metastatic disease at later circumstances.

Toxicity of hepatotoxins: new insights into mechanisms and therapy

Liver injury caused by hepatotoxins, such as carbon tetrachloride (CCl4), ethanol, and acetaminophen (APAP), is characterised by varying degrees of hepatocyte degeneration and cell death via either apoptosis or necrosis. The generation of reactive intermediate metabolites from the metabolism of hepatotoxins, and the occurrence of reactive oxygen species (ROS) during the inflammatory reaction account for a variety of pathophysiologic pathways leading to cell death, such as covalent binding, disordered cytosolic calcium homeostasis, glutathione (GSH) depletion, onset of mitochondrial permeability transition (MPT) and associated lipid peroxidation. The metabolism of hepatotoxins by cytochrome P-450 enzyme subtypes is a key step of the intoxication; therefore, enzyme inhibitors are shown to minimise the hepatotoxin-associated liver damage. Understanding the function of transcription factors, such as nuclear factor kappaB (NF-kappaB) in acute liver injury, may provide some answers as to the molecular mechanisms of toxic insults. Moreover, substantial evidence exists that MPT is involved in ROS-associated hepatocellular injury and new findings offer a novel therapeutic approach to attenuate cell damage by blocking the onset of MPT. Thus, oxidant stress and lipid peroxidation are crucial elements leading to hepatotoxin-associated liver injury. In addition to specific treatment for a given hepatotoxin, the general strategy for prevention and treatment of the damage includes reducing the production of reactive metabolites of the hepatotoxins, using anti-oxidative agents, and selectively targeting therapeutics to Kupffer cells or hepatocytes for on-going processes, which play a role in mediating a second phase of the injury.

N-Nitrosodimethylamine-Mediated Formation of Oxidized and Methylated DNA Bases in a Cytochrome P450 2E1 Expressing Cell Line

The metabolic activation of N-nitrosodimethylamine (NDMA) to reactive metabolites is a critical step for the expression of its toxic and carcinogenic potential. We have previously reported that a P450 2E1 expressing cell line, GM2E1, can metabolize NDMA to toxic reactive metabolites and cause apoptotic cell death. To investigate whether DNA is a critical target for the reactive metabolites of NDMA, we measured the levels of DNA adducts in untreated and NDMA-treated GM2E1 cells. 8-Hydroxydeoxyguanosine (8-OHdG), a biomarker for oxidative DNA damage, was analyzed following enzymatic hydrolysis of DNA. 7-Methylguanine (7-mGua), the most suitable marker for the DNA adducts formed by methylating agents, was released by thermal depurination of DNA. The modified guanine adducts were separated by HPLC and quantified using electrochemical detection. The levels of 8-OHdG and 7-mGua in GM2E1 cells treated with NDMA increased up to approximately 4- and 100-fold over those in the untreated cells, respectively. The addition of ascorbic acid, an antioxidant, to the NDMA-treated cells resulted in a significant decrease in the cytotoxicity with a concomitant decrease in the levels of 8-OHdG, but not the levels of 7-mGua. Our results demonstrate that the metabolism of NDMA in GM2E1 cells causes both DNA methylation and oxidation and support the hypothesis that NDMA-mediated DNA damage may play an important role in its toxic effects.

An evaluation of the roles of metabolic denitrosation and alpha-hydroxylation in the hepatotoxicity of N-Nitrosodimethylamine

N-Nitrosodimethylamine (NDMA) is a potent hepatotoxic agent in rats. NDMA causes cell death that does not correlate with known mechanisms of toxicity such as the production of oxidative stress or covalent binding to proteins. The following studies were designed to determine whether NDMA cytotoxicity is the result of metabolic denitrosation or alpha-hydroxylation of the nitrosamine. We determined the toxicity of various metabolites of NDMA in monolayer cultures of primary rat hepatocytes. NDMA was toxic at 0.1 mM in our cultures, but the metabolites formaldehyde, methanol, and methylamine were not toxic at this concentration. We used diazeniumdiolates that spontaneously release nitric oxide (NO) in aqueous media to deliver NO to hepatocytes in culture. The results show that, while NO released from diazeniumdiolates causes death in hepatocytes, the levels of NO produced during NDMA metabolism are insufficient to account for the toxicity of the nitrosamine. NDMA-d6, the fully deuteriated form of NDMA that undergoes approximately twice as much denitrosation in vivo as NDMA, was significantly less cytotoxic than NDMA. In contrast, N-nitroso(acetoxymethyl)methylamine (AcO-NDMA), a stable precursor of the methanediazonium ion, was found to cause toxicity equivalent of NDMA on a molar basis. Altogether, our results with methylamine, formaldehyde, methanol, the diazeniumdiolates, and NDMA-d6 indicate that NDMA toxicity is not the result of metabolic denitrosation, while the toxicity of AcO-NDMA provides strong strong evidence that the formation of the methanediazonium ion via alpha-hydroxylation leads to cell death.