GENE EXPRESSION AND ARSENIC

1. GENE EXPRESSION AND
ARSENIC
Present by:
Sajjad Moradi-MS Student
Department of Community Nutrition, School of
Nutritional Sciences and Dietetics, Tehran Unive
rsity of Medical Sciences (TUMS), Tehran, Iran

2. 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

3. Introduction
• Chronic arsenic exposure is a
worldwide health problem
• The International Agency for
Research on Cancer (IARC)
classified arsenic, a toxic
metalloid

4. Introduction
• It is widely accepted that
exposure to arsenic is
associated with lung, bladder,
kidney, liver, and non
melanoma skin cancers

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

6. 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.

8. Usage
• Pharmaceuticals : Neosalvarsan
• Feed additives : Roxarsone
• pesticides
• Copper arsenates: wood preservative
• As a preservative in animal hides
• Arsenide semiconductors

9. Resource
• Humans may be exposed to
arsenic mainly through food
and water, particularly in cer
tain areas where the ground
water is in contact with arse
nic-containing minerals.

10. 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 th
e cereals eaten by infants and young children.

11. In water
• Drinking water is an important and potentially controllable source of
arsenic exposure. In fact, drinking water is a major source of arsenic e
xposure 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 source
s, such as lakes or reservoirs.

13. 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.

14. Metabolism of arsenic
• In the human arsenic metabolic
pathway, inorganic pentavalent
arsenic (AsV) is converted to tri vale
ntarsenic (AsIII), with subsequent m
ethylation to mono methylated and di
methylated arsenicals (MMA, DMA, r
espectively)

15. 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)

17. 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 pentavale
nt 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

18. 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

19. 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, l
eading to the inactivation of many enzymes

20. 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 e
nzymes 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

22. Epigenetic mechanisms
• Chromatin remodeling by epigenetic reprogramming controls the
regulation of gene expression and has important implications in the d
evelopment 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 arsenic
als on histone modifications, chromatin structure, and microRNA

24. 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

25. 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

26. 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

28. 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.

29. Epigenetic mechanisms
• Hcy can be condensed with serine to form
cystathionine in a reaction catalyzed by cystathionin
e β-synthase (cBS)
• Cystathionine can be then utilized to form the
antioxidant compound glutathione

32. 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 t
wo 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 numbe
r and type of covalent modifications in these residues.

33. Arsenic Exposure and Histone
Modification
• These covalent modifications, found in the tails of the histone chains,
influence many fundamental biological processes including acetylation, m
ethylation, phosphorylation, citrullination, ubiquitination, ADP ribosylati
on, 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

35. 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 m
ore exist. Each miRNA is thought to target several hundred genes, and
as many as 30% of mammalian genes are regulated by miRNA

36. Arsenic Exposure and miRNA Expression
• 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 synt
hesis, the degradation of target mRNAs, and the trans location of
target mRNAs into cytoplasmic processing bodies

37. Arsenic Exposure and miRNA Expression
• Because of the suppressive effect of miRNA on gene expression, a
reduction or elimination of miRNAs that target oncogenes could result i
n 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 reduc
e the expression of such genes. A prime example of this is the observati
on of the miR-34 family on the p53 tumor suppressor pathway

38. Arsenic Exposure and miRNA Expression
• 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.

39. 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

40. 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 ind
uce histone modifications such as methylation, acetylation, a
nd phosphorylation of histone tails, changing the expression
of several genes

41. Concluding remarks
• Furthermore, several investigators observe that the exposure to
arsenic induces gene-specific alteration of miRNA expression lik
ely resulting in an impaired expression of all the genes whose exp
ression is regulated by those miRNAs