Subhepatotoxic exposure to arsenic enhances lipopolysaccharide-induced liver injury in mice

doi:10.1016/j.taap.2007.08.020

Toxicology and Applied Pharmacology

Volume 226, Issue 2, 15 January 2008, Pages 128-139

https://doi.org/10.1016/j.taap.2007.08.020 Get rights and content

Abstract

Exposure to arsenic via drinking water is a serious health concern in the US. Whereas studies have identified arsenic alone as an independent risk factor for liver disease, concentrations of arsenic required to damage this organ are generally higher than found in the US water supply. The purpose of the current study was to test the hypothesis that arsenic (at subhepatotoxic doses) may also sensitize the liver to a second hepatotoxin. To test this hypothesis, the effect of chronic exposure to arsenic on liver damage caused by acute lipopolysaccharide (LPS) was determined in mice. Male C57Bl/6J mice (4–6 weeks) were exposed to arsenic (49 ppm as sodium arsenite in drinking water). After 7 months of exposure, animals were injected with LPS (10 mg/kg i.p.) and sacrificed 24 h later. Arsenic alone caused no overt hepatotoxicity, as determined by plasma enzymes and histology. In contrast, arsenic exposure dramatically enhanced liver damage caused by LPS, increasing the number and size of necroinflammatory foci. This effect of arsenic was coupled with increases in indices of oxidative stress (4-HNE adducts, depletion of GSH and methionine pools). The number of apoptotic (TUNEL) hepatocytes was similar in the LPS and arsenic/LPS groups. In contrast, arsenic pre-exposure blunted the increase in proliferating (PCNA) hepatocytes caused by LPS; this change in the balance between cell death and proliferation was coupled with a robust loss of liver weight in the arsenic/LPS compared to the LPS alone group. The impairment of proliferation after LPS caused by arsenic was also coupled with alterations in the expression of key mediators of cell cycle progression (p27, p21, CDK6 and Cyclin D1). Taken together, these results suggest that arsenic, at doses that are not overtly hepatotoxic per se, significantly enhances LPS-induced liver injury. These results further suggest that arsenic levels in the drinking water may be a risk modifier for the development of chronic liver diseases.

Introduction

Inorganic arsenic is a ubiquitous element and a natural drinking water contaminant (National Research Council, 1999, National Research Council, 2001). Owing to its toxic potential to humans, it is a high priority hazardous substance in the United States. Chronic exposure to arsenic has been linked with a myriad of possible health effects, including skin lesions, hypertension, cardiovascular disease, respiratory disease, and malignancies of the skin and internal organs (Waalkes et al., 2004).

The liver has long been identified as a target organ of arsenic exposure. Non-malignant hepatic abnormalities include hepatomegaly, non-cirrhotic portal fibrosis, and portal hypertension (Santra et al., 1999, Santra et al., 2000, Mazumder, 2005). Furthermore, arsenic exposure has been linked to hepatic malignancies, namely hepatic angiosarcoma and hepatocellular carcinoma in both humans and in animal models (Smith et al., 1992, Waalkes et al., 2006). Straub et al. (2007) recently demonstrated that mouse liver is also sensitive to more subtle hepatic changes (e.g., SEC capillarization and vessel remodeling) at lower arsenic exposure levels (250 ppb) without any gross pathologic effects. However, with the exception of these endothelial changes (Straub et al., 2007) and hepatomegaly (Mazumder, 2005), the concentrations/doses of arsenic required to cause overt liver damage and/or cancer in experimental models or increase the risk of these diseases in humans have been generally higher than levels of arsenic present in the US water supply. It is therefore unclear at this time if liver disease is a primary risk of environmental arsenic exposure in the US.

It is now clear that the risk for developing a human disease derived from environmental exposure is not based solely on that environmental exposure but is rather modified by other mitigating conditions, such as other environmental or genetic factors. Indeed, it has been suggested that diet may contribute to the risk of developing arsenic toxicity (Smith et al., 1992). However, whether arsenic modifies the risk of developing liver damage owing to other insults has not been determined. Numerous studies have now established that physiological/biochemical changes to liver that are pathologically inert can enhance the hepatotoxic response caused by a second agent. This “2-hit” paradigm has been best exemplified in fatty liver diseases (Day and James, 1998). For example, Yang et al. (1997) demonstrated that livers from genetically obese (fa/fa) rats are exquisitely sensitive to hepatotoxicity caused by the injection of bacterial lipopolysaccharide (LPS) compared to their lean littermates; this exacerbation of liver damage was characterized by a more robust inflammatory response and enhanced cell death.

The purpose of the current study was to test the hypothesis that arsenic may also sensitize the liver to hepatotoxicity caused by a second hit. To test this hypothesis, the effect of chronic exposure to subhepatotoxic doses of arsenic on experimental liver damage caused by acute lipopolysaccharide (LPS) was determined in mice.

Section snippets

Animals and treatments

Four to six-week-old male C57BL/6J mice were purchased from Jackson Laboratory (Bar Harbor, ME). Mice were housed in a pathogen-free barrier facility accredited by the Association for Assessment and Accreditation of Laboratory Animal Care and procedures were approved by the local Institutional Animal Care and Use Committee. Food and tap water were allowed ad libitum. Some animals received arsenic (49 ppm as sodium arsenite) in the drinking water. Animals were weighed on a weekly basis during

Arsenic pre-exposure enhances liver damage after LPS

Table 1 summarizes the effect of arsenic and LPS on growth and liver weights in mice. Arsenic exposure (49 ppm) for 7 months caused a slight, but significant, decrease in growth rates in the mice, resulting in a ∼ 13% decrease in final body weight at the time of sacrifice (Table 1). Despite this decrease in growth rates, arsenic caused no detectable increase in other indices of systemic toxicity (e.g., behavioral changes and hair loss). LPS injection (± arsenic) had no significant effect on final

Subhepatotoxic doses of arsenic enhance liver damage caused by a 2nd hit of LPS

The role that arsenic in drinking water plays in disease is a major concern in the US because large areas of the country have elevated arsenic in the ground water. Consequently, nearly 4000 wells providing community water in the US have arsenic levels greater than the current WHO recommended MCL of 10 μg/L (Engel and Smith, 1994, Frost et al., 2003). Furthermore, even higher arsenic concentrations may be found in private artesian water supplies not regulated by the Safe Drinking Water Act. As

Summary and conclusions

There are many gaps in our understanding of the relative safety of arsenic to the human population. Importantly, most studies to date have focused on the effect of arsenic alone and not taken into consideration risk-modifying factors. Furthermore, even fewer studies have tested the possibility that arsenic may be a risk modifying factor for other diseases. It was shown here that arsenic enhances experimental LPS-induced liver injury in mice. Elevated LPS, with or without accompanying sepsis, is

Acknowledgments

This work was supported, in part, by a grant from the National Institute of Alcohol Abuse and Alcoholism (NIAAA) and the National Institute of Environmental Health Sciences (NIEHS).

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Subhepatotoxic exposure to arsenic enhances lipopolysaccharide-induced liver injury in mice

Abstract

Introduction

Section snippets

Animals and treatments

Arsenic pre-exposure enhances liver damage after LPS

Subhepatotoxic doses of arsenic enhance liver damage caused by a 2nd hit of LPS

Summary and conclusions

Acknowledgments

Best. Pract. Res. Clin. Gastroenterol.

Gastroenterology

Toxicol. Appl. Pharmacol.

Gastroenterology

Clin. Biochem.

Hepatology

Toxicology

Toxicol. Appl. Pharmacol.

J. Biol. Chem.

Arch. Biochem. Biophys.

J. Biol. Chem.

Anal. Biochem.

Hepatology

J. Hepatol.

Toxicol. Lett.

Am. J. Pathol.

Toxicol. Appl. Pharmacol.

Hepatology

Evidence that hypoxia markers detect oxygen gradients in liver: pimonidazole and retrograde perfusion of rat liver

Br. J. Cancer

Cell cycle control in breast cancer cells

J. Cell. Biochem.

Life span profiles of glutathione and acetaminophen detoxification

Drug Metab. Dispos.

Chronic inorganic arsenic exposure induces hepatic global and individual gene hypomethylation: implications for arsenic hepatocarcinogenesis

Carcinogenesis