gene expression and arsenic

GENE EXPRESSION AND ARSENIC

Present by:Sajjad Moradi-MS Student

Department of Community Nutrition, School of Nutritional Sciences and Dietetics, Tehran

University of Medical Sciences (TUMS), Tehran, Iran

Introduction• Arsenic is a poisonous substance, which is released both from

certain human activities and naturally from the Earth's crust.• Arsenic is found in the natural environment in some

abundance in the Earth’s crust and in small quantities in rock, soil, water and air. It is present in many different minerals.• Industrial processes such as mining, smelting and coal-fired

power plants all contribute to the presence of arsenic in air, water and soil

Introduction• Chronic arsenic

exposure is a worldwide health problem

• The International Agency for Research on Cancer (IARC) classified arsenic, a toxic metalloid

Introduction• It is widely accepted

that exposure to arsenic is associated with lung, bladder, kidney, liver, and non melanoma skin cancers

Introduction Arsenic does not directly damage DNA, but may act as a carcinogenthrough inhibition of DNA repair mechanisms, leading indirectly to increased mutations from other DNA damaging agents.

Arsenic exposure• Exposure to inorganic arsenic can cause various health effects:• Irritation of the stomach and intestines• decreased production of red and white blood cells• skin changes and lung irritation• The chances of development of skin cancer, lung cancer, liver cancer and

lymphatic cancer.

Usage• Pharmaceuticals : Neosalvarsan• Feed additives : Roxarsone• pesticides• Copper arsenates: wood

preservative• As a preservative in animal hides• Arsenide semiconductors

Resource• Humans may be

exposed to arsenic mainly through food and water, particularly in certain areas where the groundwater is in contact with arsenic-containing minerals.

In food• The highest levels of arsenic (in all forms) in foods can

be found in seafood, rice, rice cereal (and other rice products), mushrooms, and poultry, although many other foods can contain low levels of arsenic• Rice is of particular concern because it is a major part of

the diet in many parts of the world. It is also a major component of many of the cereals eaten by infants and young children.

In water• Drinking water is an important and potentially controllable

source of arsenic exposure. In fact, drinking water is a major source of arsenic exposure in some parts of the world.

• In parts of Taiwan, Japan, Bangladesh, and western South America, high levels of arsenic occur naturally in drinking water

• Arsenic levels tend to be higher in drinking water that comes from ground sources, such as wells, as opposed to water from surface sources, such as lakes or reservoirs.

Arsenicals and Their Metabolism

• sulfur as red arsenic (As2S2)• yellow arsenic (As2S3)• White arsenic or arsenic trioxide (As2O3)• pentavalent oxidation states →chemically unstable sulfide or

oxide, or as a salt of sodium, potassium, or calcium• trivalent oxidation states →including sodium arsenite and the

more soluble arsenic trioxide inhibit many enzymes by reacting with biological ligands that possess available sulfur groups.

Metabolism of arsenic

• In the human arsenic metabolic pathway, inorganic pentavalent arsenic (AsV) is converted to tri valentarsenic (AsIII), with subsequent methylation to mono methylated and dimethylated arsenicals (MMA, DMA, respectively)

Metabolism of arsenic

• Arsenic uptake strongly depends on cell type and on its oxidation state; in particular, trivalent forms are more membrane-permeable than pentavalent ones

• Arsenite is methylated by arsenic-3-methyl transferas enzyme (As3MT) with S-adenosyl-l-methionine (SAM as the methyl-donating cofactor)

• The reduction of MMA5 to MMA3 is catalyzed by glutathione-S-transferase omega (GSTO)

Metabolism of arsenic• The inorganic arsenicals are known to be taken up by the

liver, transformed to MMA and DMA, and then excreted into urine as pentavalent methylated arsenic form.

• Half-life of approximately 4 days in humans• Even after cessation of arsenic exposure, it was found

that about 40–60 % of arsenic may be retained in skin, hair, nails, and muscle and small amounts may be retained in teeth and bones

Metabolism of arsenic• The key point is that, whatever the pathway of arsenic metabolism

may be, the balance between the intake and excretion of arsenic• This balance in turn, coupled with the genetic makeup of the

subject, determines whether a particular subject will be susceptible to arsenic toxicity or not.• polymorphisms in genes implicated in arsenic metabolism can

make a person more susceptible to the damaging effects of arsenic

Genotoxic effects• Several mechanisms have been proposed to

explain the genotoxicity of arsenic, including the induction of oxidative stress and altered patterns of DNA repair• Arsenic can also induce oxidative damage in

proteins and enzymes due to its high affinity for their sulfhydryl groups, leading to the inactivation of many enzymes

Genotoxic effects• Arsenic-mediated oxidative damage in enzymes is also

reported to interfere with the DNA repair mechanisms by either inhibiting ligation or down-regulating the gene expression of DNA repair enzymes such as DNA polymerase β • The mechanisms by which arsenic exerts its effects are

complex because its metabolism involves more than five metabolites, all of which can produce toxic effects

Epigenetic mechanisms

• Chromatin remodeling by epigenetic reprogramming controls the regulation of gene expression and has important implications in the development of human cancers.

• The effects of arsenic and arsenic metabolites on global and gene specific DNA methylation, as well as the effects of exposure to arsenicals on histone modifications, chromatin structure, and microRNA

Arsenic Exposure and DNA Methylation

• DNA methylation is tightly regulated in mammalian development and is essential for maintaining the normal functioning of the adult organism

• Global genomic DNA hypo methylation is a hallmark of many types of cancers , resulting in illegitimate recombination events

• In mammalian systems, DNA methylation occurs predominantly in cytosine-rich gene regions, known as CpG islands, and serves to regulate gene expression and maintain genome stability

Epigenetic mechanisms• The main epigenetic mechanism studied in relation to

arsenic exposure is DNA methylation• Methylation refers to the addition of a methyl group to

the fifth carbon position of a cytosine residue that is followed on the same strand by guanine, also known as a cpG dinucleotide.

• cpG dinucleotides occur in concentrations known as cpG islands, and cpG islands can be found in the promoter region of approximately half of all human genes

Epigenetic mechanisms• In pathologically normal cells, promoter cpG islands

regions are typically unmethylated. Several mechanisms are theorized to underline epigenetic changes in DNA methylation by arsenic

• Arsenic, by its metabolism, affects the activity of DNA methyltransferase (DNMT) enzymes

• The bulk of circulating arsenic undergoes biotransformation in hepatocytes where arsenite is subjected to a series of sequential oxidative methylation and reduction steps yielding several methylation products

Epigenetic mechanisms• These methylation steps facilitate arsenic excretion but at

the same time consume S-adenosyl methyionine (SAM)• SAM is generated in one-carbon metabolism that requires

homocysteine (Hcy), folate, and other vitamins and cofactors, such as cobalamin (vitamin B12), from the diet to function

• Folate is reduced in the 5-methyltetrahydrofolate (5-methyl THF) form that can be used by one-carbon metabolism to form SAM as well as to synthesize DNA and RNA precursors.

Epigenetic mechanisms

• Hcy can be condensed with serine to form cystathionine in a reaction catalyzed by cystathionine β-synthase (cBS)• Cystathionine can be then utilized to form

the antioxidant compound glutathione

Arsenic Exposure and Histone Modification

• Chromatin is structured within the cell nucleus in units called nucleosomes, in which DNA is packaged within the cell

• The nucleosome core particle consists of stretches of DNA wrapped in left-handed super helical turns around a histone octamer consisting of two copies each of the core histones H2A, H2B, H3, and H4

• From a structural and functional perspective, histones have different characteristics depending on the number of amino acids and the number and type of covalent modifications in these residues.

Arsenic Exposure and Histone Modification

• These covalent modifications, found in the tails of the histone chains, influence many fundamental biological processes including acetylation, methylation, phosphorylation, citrullination, ubiquitination, ADP ribosylation, deimination, and proline isomerization

• To date, published studies on histone modifications and arsenic toxicity have focused on acetylation, methylation, and phosphorylation

• Histone modifications affected by AsIII and MMAIII exposure

Arsenic Exposure and miRNA Expression

• In the past few years, several laboratories have discovered a small class of non-protein-coding RNAs, called microRNAs (miRNAs), that participate in diverse biological regulatory events and are transcribed mainly from non-protein-coding regions of the genome

• More than 700 human miRNAs have been identified to date, as documented in the miRBase database, and it is predicted that many more exist. Each miRNA is thought to target several hundred genes, and as many as 30% of mammalian genes are regulated by miRNA


• miRNAs deactivate gene expression by binding to the 3´-untranslated region of mRNA with incomplete base pairing • The exact mechanisms by which expression is repressed

are still under investigation but may include the inhibition of protein synthesis, the degradation of target mRNAs, and the trans location of target mRNAs into cytoplasmic processing bodies


• Because of the suppressive effect of miRNA on gene expression, a reduction or elimination of miRNAs that target oncogenes could result in the inappropriate expression of those oncoproteins

• Conversely, the amplification or over expression of miRNAs that have a role in regulating the expression of tumor suppressor genes could reduce the expression of such genes. A prime example of this is the observation of the miR-34 family on the p53 tumor suppressor pathway


• exposure of cells to iron sulfate or aluminum sulfate, which generate reactive oxygen species (ROS), led to the up-regulation of a specific set of miRNAs, including miR-9, miR125b, and miR-128

• ROS generation resulting from arsenic exposure is thought to play a large role in arsenic induced carcinogenesis and toxicity and could potentially alter these miRNAs in a similar manner.

Concluding remarks• Arsenic is a human carcinogen that induces tumors through

mechanisms not yet completely understood. elevated concentrations of inorganic arsenic in drinking water pose public health threat to millions of people worldwide.

• Reviewed literature indicates that the main mechanisms by which inorganic arsenic causes negative health effects are induction of genotoxicity,oxidative stress, and inhibition of DNA repair

• A growing body of evidence indicates that epigenetic modifications have a role in arsenic-inducing adverse effects on human health

Concluding remarks• epidemiological studies show that arsenic induces

genotoxic effects acting as a clastogen • Arsenic induces epimutations both at a genome-

wide level and at specific gene promoter regions and is also able to induce histone modifications such as methylation, acetylation, and phosphorylation of histone tails, changing the expression of several genes

Concluding remarks

• Furthermore, several investigators observe that the exposure to arsenic induces gene-specific alteration of miRNA expression likely resulting in an impaired expression of all the genes whose expression is regulated by those miRNAs

gene expression and arsenic

Health & Medicine