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Arsenic is a naturally occurring and highly potent metalloid known to elicit serious public health concerns. Today, approximately 200 million people around the globe are exposed to arsenic-contaminated drinking water at levels greater than the World Health Organization's recommended limit of 10 parts per billion. As a class I human carcinogen, arsenic exposure is known to elicit various cancers, including lung, skin, liver, and kidney. Current evidence suggests that arsenic is capable of inducing both genotoxic and cytotoxic injury, as well as activating epigenetic pathways to induce carcinogenesis. Our study identifies a novel pathway that is implicated in arsenic-induced carcinogenesis. Arsenic down-regulated miRNA-31 and the release of this inhibition caused overexpression of special AT-rich sequence-binding protein 2 (SATB2). Arsenic is known to disrupt miRNA expression, and here we report for the first time that arsenic is capable of inhibiting miR-31 expression. As a direct downstream target of miR-31, SATB2 is a prominent transcription factor, and nuclear matrix binding protein implicated in many types of human diseases including lung cancer. Results from this study show that arsenic induces the overexpressing SATB2 by inhibiting miR-31 expression, which blocks the translation of SATB2 mRNA, since levels of SATB2 mRNA remain the same but protein levels decrease. Overexpression of SATB2 induces malignant transformation of human bronchial epithelial (BEAS-2B) cells indicating the importance of the expression of miR-31 in preventing carcinogenesis by suppressing SATB2 protein levels.

The ability for cells to harness alternative splicing enables them to diversify their proteome in order to carry out complex biological functions and adapt to external and internal stimuli. The spliceosome is the multiprotein-RNA complex charged with the intricate task of alternative splicing. Aberrant splicing can arise from abnormal spliceosomes or splicing factors and drive cancer development and progression. This review will provide an overview of the alternative splicing process and aberrant splicing in cancer, with a focus on serine/arginine-rich (SR) proteins and their recently reported roles in cancer development and progression and beyond. Recent mapping of the spliceosome, its associated splicing factors, and their relationship to cancer have opened the door to novel therapeutic approaches that capitalize on the widespread influence of alternative splicing. We conclude by discussing small molecule inhibitors of the spliceosome that have been identified in an evolving era of cancer treatment.

Arsenic occurs naturally in the environment, and exists predominantly as inorganic arsenite (As (III) and arsenate As (V)). Arsenic contamination of drinking water has long been recognized as a major global health concern. Arsenic exposure causes changes in skin color and lesions, and more severe health conditions such as black foot disease as well as various cancers originating in the lungs, skin, and bladder. In order to efficiently metabolize and excrete arsenic, it is methylated to monomethylarsonic and dimethylarsinic acid. One single enzyme, arsenic methyltransferase (AS3MT) is responsible for generating both metabolites. AS3MT has been purified from several mammalian and nonmammalian species, and its mRNA sequences were determined from amino acid sequences. With the advent of genome technology, mRNA sequences of AS3MT have been predicted from many species throughout the animal kingdom. Horizontal gene transfer had been postulated for this gene through phylogenetic studies, which suggests the importance of this gene in appropriately handling arsenic exposures in various organisms. An altered ability to methylate arsenic is dependent on specific single nucleotide polymorphisms (SNPs) in AS3MT. Reduced AS3MT activity resulting in poor metabolism of iAs has been shown to reduce expression of the tumor suppressor gene, p16, which is a potential pathway in arsenic carcinogenesis. Arsenic is also known to induce oxidative stress in cells. However, the presence of antioxidant response elements (AREs) in the promoter sequences of AS3MT in several species does not correlate with the ability to methylate arsenic. ARE elements are known to bind NRF2 and induce antioxidant enzymes to combat oxidative stress. NRF2 may be partly responsible for the biotransformation of iAs and the generation of methylated arsenic species via AS3MT. In this article, arsenic metabolism, excretion, and toxicity, a discussion of the AS3MT gene and its evolutionary history, and DNA methylation resulting from arsenic exposure have been reviewed.

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