Arsenic, mode of action at biologically plausible low doses: What are the implications for low dose cancer risk?

Abstract

Arsenic is an established human carcinogen. However, there has been much controversy about the shape of the arsenic response curve, particularly at low doses. This controversy has been exacerbated by the fact that the mechanism(s) of arsenic carcinogenesis are still unclear and because there are few satisfactory animal models for arsenic-induced carcinogenesis. Recent epidemiological studies have shown that the relative risk for cancer among populations exposed to ≤60 ppb As in their drinking water is often lower than the risk for the unexposed control population. We have found that treatment of human keratinocyte and fibroblast cells with 0.1 to 1 μM arsenite (As^III) also produces a low dose protective effect against oxidative stress and DNA damage. This response includes increased transcription, protein levels and enzyme activity of several base excision repair genes, including DNA polymerase β and DNA ligase I. At higher concentrations (> 10 μM), As induces down-regulation of DNA repair, oxidative DNA damage and apoptosis. This low dose adaptive (protective) response by a toxic agent is known as hormesis and is characteristic of many agents that induce oxidative stress. A mechanistic model for arsenic carcinogenesis based on these data would predict that the low dose risk for carcinogenesis should be sub-linear. The threshold dose where toxicity outweighs protection is hard to predict based on in vitro dose response data, but might be estimated if one could determine the form (metabolite) and concentration of arsenic responsible for changes in gene regulation in the target tissues.

Introduction

Arsenic appears to function as a carcinogen or tumor promoter predominantly by disrupting or modulating cellular control mechanisms, such as control of redox status, transcriptional regulation, and DNA repair capacity. At very early times after exposure, trivalent arsenic (As^III) activates NADPH oxidase activity through a Ras-GTPase mechanism, which creates an intracellular burst of reactive oxygen (Smith et al., 2001, Hirano et al., 2003). At low levels, this produces increased oxidative stress and, because cells have evolved to respond to cellular and extracellular redox levels, this perturbation of the norm activates an adaptive response to re-establish the normal cellular redox status and to repair any excess damage that may be produced by the higher oxidation levels. Doses of As^III that induce a protective response in cultured human cells are generally in the range of 0.1 to 2 μM, which is similar to the levels of total arsenic found in the blood of heavily contaminated people (0.5–1.2 μM) (Smith et al., 2001, Hirano et al., 2003). This response includes changes in gene regulation at the level of transcription, which lead to parallel changes in protein levels and enzyme activity. At high (>10 μM) levels, As induces DNA and mitochondrial damage and apoptosis (Bode and Dong, 2002). It also causes a down-regulation of some of the same genes that were up-regulated by low doses of As, namely base excision repair genes (Snow et al., 1999, Snow et al., 2003) and telomerase (Zhang et al., 2003). As a consequence, low doses of arsenic can protect against oxidative damage caused by other endogenous or exogenous agents or processes. In at least one animal model, arsenic in drinking water at concentrations commonly found in Bangladesh was strongly protective against skin tumor development (Snow et al., 2003). This low dose adaptive response of a toxic agent is known as hormesis and is a characteristic response of many systems to agents that induce oxidative stress (Calabrese and Baldwin, 2003, Parsons, 2003). A mechanistic model for arsenic carcinogenesis based on our data would predict that the low dose risk for carcinogenesis should be sub-linear. Epidemiological studies that include low dose data also indicate that exposure to arsenic in drinking water at concentrations of less than approximately 60 ppb (0.8 μM) is associated with risks of bladder or lung cancer that are below control values (Lamm et al., 2004, Mahata et al., 2004). The threshold dose where toxicity outweighs protection is hard to predict based on an in vitro dose response, but might be estimated if one could determine the form (metabolite) and concentration of arsenic responsible for changes in gene regulation in the target tissues.