Arsenic, a metalloid, presents a complex challenge in toxicology due to its varied oxidation states, solubility, and interactions with numerous biological and environmental factors [45]. The expression of arsenic’s toxic effects is intricately linked to factors such as exposure dose, frequency, duration, species, age, gender, individual genetic predispositions, and nutritional status [46]. Human arsenic toxicity is predominantly associated with inorganic forms, with trivalent arsenite (AsIII) exhibiting 2–10 times greater toxicity than pentavalent arsenate (AsV) [5]. As (III) disrupts cellular function by binding to thiol groups on proteins, inactivating over 200 enzymes – a mechanism underpinning its wide-ranging impact across organ systems. As (V), on the other hand, can substitute for phosphate in crucial biochemical pathways [5, 47]. Understanding these fundamental mechanisms is crucial as we progress in learning about the broader impact of metalloids and nonmetals on biological systems.
Core Mechanisms of Arsenic-Induced Toxicity
A primary toxic mechanism of arsenic involves the disruption of cellular respiration through the inhibition of mitochondrial enzymes and the uncoupling of oxidative phosphorylation. The interaction of arsenic with sulfhydryl groups of proteins and enzymes, alongside its ability to replace phosphorus in biochemical reactions, underpins much of its toxicity [48]. In vitro, arsenic reacts with protein sulfhydryl groups, inactivating key enzymes like dihydrolipoyl dehydrogenase and thiolase. This leads to inhibited pyruvate oxidation and beta-oxidation of fatty acids [49]. Methylation is the main metabolic pathway for inorganic arsenic in humans. Arsenic trioxide undergoes non-enzymatic methylation to monomethylarsonic acid (MMA), followed by enzymatic methylation to dimethyl arsenic acid (DMA) before urinary excretion [40, 47]. While initially considered a detoxification pathway, research now indicates that some methylated metabolites, particularly trivalent forms, may be even more toxic than arsenite [41]. This ongoing refinement of our understanding represents significant progress in learning about arsenic’s complex metabolic pathways and their toxicological implications.
Genotoxicity and Cellular Transformation
Genotoxicity studies reveal that arsenic compounds impede DNA repair and induce chromosomal aberrations, sister-chromatid exchanges, and micronuclei formation in both cultured human and rodent cells [50–52] and in cells from exposed humans [53]. Interestingly, arsenic compounds do not consistently induce mutations in Salmonella typhimurium reversion assays, suggesting they are weak mutagens in bacterial systems. However, they consistently display clastogenic properties in various cell types in vivo and in vitro [54]. In the absence of comprehensive animal models, in vitro cell transformation studies are vital for elucidating the carcinogenic mechanisms of arsenic toxicity. Arsenic and arsenical compounds are cytotoxic and induce morphological transformations in Syrian hamster embryo (SHE) cells, mouse C3H10T1/2 cells, and BALB/3T3 cells [55, 56]. These findings underscore the genotoxic potential of arsenic and its ability to induce cellular changes that can lead to malignancy, highlighting a key area of progress in learning about metalloid-induced carcinogenesis.
DNA Damage, Gene Expression, and Oxidative Stress
The comet assay has demonstrated that arsenic trioxide induces DNA damage in human lymphocytes [57] and mouse leukocytes [58]. Arsenic compounds are also implicated in gene amplification, mitotic arrest, DNA repair inhibition, and the induction of c-fos gene expression and heme oxygenase, an oxidative stress protein, in mammalian cells [58, 59]. They can act as promoters and comutagens for various toxic agents [60]. Recent research has shown arsenic trioxide to be cytotoxic and capable of transcriptionally inducing stress genes and associated proteins in human liver carcinoma cells [61]. These diverse effects on cellular processes, particularly the induction of oxidative stress and altered gene expression, represent crucial steps in our progress in learning about the multifaceted toxicity mechanisms shared by some nonmetals and metalloids.
Carcinogenesis and Cell Signaling Pathways
Epidemiological studies link long-term arsenic exposure to cancer promotion. Several hypotheses attempt to explain arsenic-induced carcinogenesis. Arsenic may act as a carcinogen by inducing DNA hypomethylation, which can lead to aberrant gene expression [62]. It is also a potent stimulator of extracellular signal-regulated protein kinase Erk1 and AP-1 transactivational activity, and an efficient inducer of c-fos and c-jun gene expression [63]. The induction of c-jun and c-fos by arsenic is associated with JNK activation [64], although the precise role of JNK activation in cell transformation and tumor promotion remains under investigation. Research into these signaling pathways represents significant progress in learning how environmental metalloids can contribute to cancer development.
Another study suggests that chronic exposure to high arsenic levels may sensitize cells to mitogenic stimulation, and alterations in mitogenic signaling proteins might contribute to arsenic’s carcinogenic action [65]. Accumulating evidence indicates that arsenic can disrupt cell signaling pathways, including the p53 pathway, frequently implicated in tumor promotion and progression in animal models and human cancers [66, 68]. However, the specific signal transduction alterations and target molecules driving arsenic-induced tumors in humans following chronic exposure are still being elucidated. Continued investigation into these areas reflects the ongoing progress in learning and understanding the complexities of arsenic-induced carcinogenesis.
Therapeutic Applications of Arsenic
Intriguingly, arsenic trioxide has demonstrated therapeutic efficacy in treating acute promyelocytic leukemia and is being explored for other cancers [69,70]. In acute promyelocytic leukemia, the specific molecular event driving malignancy is known. Studies have shown that increased BCR-ABL susceptibility in human lymphoblast cells dramatically enhances sensitivity to arsenic-induced apoptosis [71]. Arsenic trioxide appears to be tumor-specific, selectively inducing apoptosis in acute promyelocytic leukemia cells. Further studies indicate arsenic can induce apoptosis through alterations in other cell signaling pathways [72,73]. Beyond leukemia, arsenic shows therapeutic promise for myeloma [74]. In summary, cancer chemotherapy research in cell cultures and patients with acute promyelocytic leukemia demonstrates that arsenic trioxide can induce cell-cycle arrest and apoptosis in malignant cells. This therapeutic application represents a surprising and valuable area of progress in learning about and leveraging the biological effects of arsenic.
Arsenic’s Modulation of Gene Expression and Cellular Processes
Prior research has investigated p53 gene expression and mutation in tumors from individuals with a history of arsenic ingestion. p53 is crucial for cell-cycle control, DNA repair, differentiation, genomic stability, and programmed cell death. Multiple studies support the idea that arsenic can modulate gene expression [75,76]. Collectively, these studies provide further evidence that various arsenic forms can alter gene expression, and these changes may significantly contribute to the toxic and carcinogenic effects of arsenic in human populations [77]. This ongoing research into gene modulation is vital to our progress in learning the detailed mechanisms of arsenic toxicity.
In vitro studies have shown arsenic modulates DNA synthesis, gene and protein expression, genotoxicity, mitosis, and apoptosis in diverse cell lines, including keratinocytes, melanocytes, dendritic cells, dermal fibroblasts, microvascular endothelial cells, monocytes, and T-cells [78], colon cancer cells [79], lung cancer cells [80], human leukemia cells [81], Jurkat-T lymphocytes [82], and human liver carcinoma cells [83]. Oxidative stress is a key factor in arsenic-induced cytotoxicity, modulated by pro- and anti-oxidants like ascorbic acid and n-acetyl cysteine [84–86]. The toxicity of arsenic is also dependent on its chemical form, with inorganic forms being more toxic than organic ones [42]. These detailed in vitro investigations represent significant progress in learning about the specific cellular targets and pathways affected by arsenic.
Concluding Thoughts: Modes of Action and Future Directions
Various hypotheses attempt to explain inorganic arsenic’s carcinogenicity. However, the precise molecular mechanisms remain incompletely understood. Current evidence suggests inorganic arsenic may not act through classic genotoxic and mutagenic mechanisms, but rather as a tumor promoter modifying cell growth and proliferation signaling pathways [68]. Despite recent advances in understanding arsenic’s carcinogenic modes of action, a definitive scientific consensus remains elusive. A recent review outlines nine potential modes of action for arsenic carcinogenesis: chromosomal abnormalities, oxidative stress, altered DNA repair, altered DNA methylation patterns, altered growth factors, enhanced cell proliferation, promotion/progression, p53 suppression, and gene amplification [87]. Currently, chromosomal abnormality, oxidative stress, and altered growth factors show the strongest evidence across experimental systems and human tissues. Other proposed modes like progression of carcinogenesis, altered DNA repair, p53 suppression, altered DNA methylation patterns, and gene amplification require further investigation, particularly in in vivo animal studies, in vitro human cell studies, and human population studies. Current mode-of-action research suggests arsenic may act as a cocarcinogen, promoter, or progressor of carcinogenesis. Continued research into these complex mechanisms is essential for progress in learning how metalloids like arsenic, and potentially other nonmetals with similar properties, exert their toxic and carcinogenic effects. This ongoing progress in learning is critical for developing effective preventative and therapeutic strategies to mitigate arsenic’s harmful effects and understand the broader implications of metalloid and nonmetal toxicity.
