Cancer

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Cancer

• M.Prasad Naidu
• MSc Medical Biochemistry,
• Ph.D.Research Scholar

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Introduction

• Cancer cells are characterized by three
  properties
• Diminished or unrestrained control of growth ,
• Invasion of local tissues ,
• Spread or metastasis to other parts of the body
  .
• Nearly all cancers originate from a single cell
  this clonal origin is critical discriminating
  feature between neoplasia hyperplasia .

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contd

• Physical , chemical , biological agents can
  cause cancer .
• Agents causing cancer fall into 3 broad groups
• Radiant energy ,
• Chemical compound ,
• Viruses .
• Spontaneous mutations
• Oxidative damage to DNA .

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Carcinogenic chemicals
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Both organic inorganic molecules are
carcinogenic . Carcinogens do not share
structural similarity .
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Ultimate carcinogens are electrophiles (
molecules deficient in electrons ) they readily
attack nucleophilic groups ( electron rich ) in
DNA , RNA , proteins .Metabolism of
procarcinogens involves cytochrome P
450 system .

• Procarcinogen
• Proximate carcinogen
• Ultimate carcinogen

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contd

• Carcinogens interact covalently with cellular
  macro molecules .
• Carcinogens found to interact with purine ,
  pyrimidine , or phosphodiester groups of DNA .
• The most common site of attack is guanine
  addition of various carcinogens to N2 , N3 , N7 ,
  O6 , O8 atoms of this base .

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contd

• Persistant unrepaired lesions are important in
  generating mutations critical for
  carcinogenesis.
• Ames assay is used to detect the mutagenecity of
  the chemical carcinogen .
• Ames assay uses specially constructed strain of
  salmonella typhimurium , that has a mutation in
  gene that codes for one of the enzyme involved in
  synthesis of histidine .

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• S .typhimurium of the above strain can not
  synthesize histidine , needs histdine added to
  the medium for its growth .
• Chemical carcinogen by its mutagenic nature
  restore synthesis of histidine in the
  S . Typhimurium .
• S .typhimurium along with mitochondrial
  supernatant provides activation by monooxygenases
  .

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Initiation promotion

• In organs such as skin liver carcinogenesis
  has 2 stages
• Intiation ( rapid irreversible stage ),
• Promotion .
• Most carcinogens are capable of acting as both
  intiating promoting agents .

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DNA the critical macromolecule in carcinogenesis

• Cancer cells beget cancer cells that is
  essential changes responsible for cancer are
  transmitted from mother to daughter cells .
• Both irradiation chemical carcinogens damage
  DNA are capable of causing mutations in DNA .
• Many tumor cells exhibit abnormal chromosomes .

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• Transfection experiments indicate that purified
  DNA ( oncogens ) from cancer cells can transform
  normal cells into ( potential )cancer cells .
• Genes that increase susceptibility to cancer
  have been isolated .
• Epigenetics is defined as changes that alter the
  pattern of gene expression that persists across
  at least one cell division eg methylation of CpG
  dinucleotides , histone acetylation .

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Oncogenes play a crucial role in carcinogenesis

• An oncogene is a gene that, when mutated or
  expressed at high levels, turn a normal cell into
  a tumor cell.
• Oncogenes were first recognized as unique genes
  of tumor causing viruses that are responsible for
  the process of transformation.

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Oncogenes of Rous Sarcoma virus

• Rous sarcoma virus is retrovirus containing 4
  genes named gag , pol , env , src .

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contd

• gag gene encodes for group specific antigens
  of the virus .
• pol for reverse transcriptase that
  characterizes retro viruses .
• env for certain glycoproteins of viruses .
• src ( sarcoma causing gene ) produces a
  protein tyrosine kinase .

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contd

• The abnormal phosphorylation of vinculin a
  protein in focal adhesion plaques could help
  explain the rounding - up of cells their
  diminished adhesion to substratum to one
  another during transfromation .
• Certain glycolytic enzymes appear to be target
  proteins for src tyrosine kinases .

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contd

• The products of src interact with the kinase
  catalyzing phosphorylation of phosphotidyl
  inositol to phosphotidyl inositol 4 , 5 bis
  phosphate .
• Inositol triphosphate releases calcium from
  intracellular storage .

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Contd

• Diacyl glycerol activates protien kinase C which
  inturn phosphorylates number of proteins some
  of which are ion pumps .
• Mild alkalinization of the cell brought about by
  activation Na / H antiport system could play a
  role in stimulating mitosis .

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Protein tyrosine kinase in normal transformed
cells

• Insulin , PDGF , EGF found in both normal
  transformed cells have tyrosine kinase activity .
• The amount of phosphotyrosine in most normal
  cells is low but is usually elevated in cells
  transformed by oncogenic virus containing protein
  tyrosine kinase .

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contd

• The product of ras oncogene of murine sarcoma
  virus binds GTP , has GTPase activity regulate
  activity of adenylyl cyclase .
• myc oncogen encodes a DNA binding protein .
• abl , src , sis , erb B , oncogene products
  have tyrosine kinase activity .

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Proto - Oncogene

• A proto-oncogene is a normal gene that can become
  an oncogene due to mutations or increased
  expression .
• Products of proto oncogenes believed to play
  important roles in normal differentiation other
  cellular process.

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Oncogenes from tumor cells

• DNA isolated from tumor cell
• added
• Recipient cells often a line of mouse fibroblasts
  NIH/3T3
• microscopic observation for1-2
  wk
• Foci of transformed cell

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contd

• The procedure is repeated several times using DNA
  extracted from transformed cells ,thus reducing
  the amount of DNA not involved in transformation
  , that was transfected facilitating
  identification by southern blot technique using
  suitable probe .

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Activation of Proto - oncogenes

• 5 mechanisms of activation
• Promoter insertion ,
• Enhancer insertion ,
• Chromosomal translocation ,
• Gene amplification ,
• Point mutations .

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Promoter Insertion

• Certain retro viruses lack oncogenes ( eg
  avian leukemia viruses ) but may cause cancer
  over a long period of time .
• Retroviruses infect cells a DNA copy ( cDNA )
  of their RNA genome is synthesized by reverse
  transcriptase the cDNA is integrated into the
  host genome .

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Enhancer Insertion
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Chromosomal translocation
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Mechanism of action of oncogens

• Oncogens act on key intracellualr pathways
  involved on growth control , uncoupling them from
  the need fro an exogenous stimuli .
• Products of src acting as tyrosine kinase ,
  products of ras acting to stimulate adenylyl
  cyclase act by phosphorylating key regulatory
  proteins of cell cycle .

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• Elevations of various cyclins , decrease of
  kinase inhibitory proteins ( KIP p21 ,p27, p57
  ) decrease of inhibitors of CDK4 ( INK4
  p16 )has been documented in cancers .

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Growth factors

• Polypeptide growth factors are mitogenic .
• Growth factors act in an endocrine , paracrine ,
  or autocrine manner .
• Growth factors act on the cell cycle mitosis
  via transmembrane signal transduction.

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Growth factors oncogenes interact in several
ways

• The products of several oncogenes are either
  growth factors or parts of the receptors for
  growth factors .
• Product of sis oncogene truncated B chain of PDGF
  .
• B chain is biologically active as homodimer
  without involvement of A chain .

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contd

• Autocrine stimulation by PDGF gives a chronic
  mitogenic stimulus could be an imporatnt factor
  in transformation of cell .
• The product of erb B is truncated EGF with much
  of deletion of external domain of EGF but
  retaining the tyrosine kinase part .
• This abnormal form is continuously active
  tyrosine kinase when present in cells causing
  chronic mitogenic stimulus .

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Transforming growth factor ß

• TGFß known to inhibit the growth of most cell
  types except fibrobalsts .
• In fibroblast TGFß promotes growth by activating
  sis gene .
• TGFß inhibitory effect is on cell cycle
  progression is by phosphorylating pRB , reduction
  of the levels of mRNA s of cyclin E A ,
  inhibition of cyclin E cyclin A dependent
  kinases .

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The p53 tumor suppressor gene acts as a guardian
of the genome

• Mutations in p53 occurs frequently in many human
  tumors .

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• Selenium has cancer preventive action .
• Selenomethionine the main form of selenium in
  our diets , participates in a redox reaction
  resulting in the reduction of 2 cysteine residues
  within p53 leading to an induction of p53 DNA
  binding activity .
• HPV proteins E6 E7 bind inactivate cellular
  tumor suppressors p53 pRB respectively .

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Genetic model of colorectal cancer suggest poly
gene etiology for its development

• Multiple cumulative mutational events are
  invariably required for the progression from
  normal to fully malignant phenotype .

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Telomerase

• DNA polymerase is unable to replicate the ends of
  chromosomes , resulting in loss of DNA at
  specialized ends of chromosomes called telomere.
• Telomeres composed of tandem repeates of six
  nucleotide sequences ( TTAGGG ) .

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contd

• Telomere binds with specialized telomere binding
  proteins to form a T loop structure that prevents
  the ends of chromosomes from being recognized as
  broken or damaged DNA.
• Loss of telomere repeats with each cell division
  cycle causes gradual telomere shortening leading
  to growth arrest.

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contd

• Critically short telomere triggers a p53
  regulated DNA damage check point , this is called
  replicative senescence .
• Cells can bypass this growth arrest if pRB or
  p53 are nonfunctional .

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contd

• The ability to bypass telomere based growth
  limitations is thought to be a critical step in
  the evolution of most malignancies .
• This occurs by the expansion of telomerase in
  cancer cells .
• Telomerase is an enzyme that adds TTAGGG repeats
  onto the 3 end of chromosome .

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Contd

• More than 90 of human cancers express high
  levels of telomearse .
• Most normal do not express sufficient telomerase
  to prevent telomere attrition with each cell
  division .
• Stems cell have high telomerase activity .

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Malignancy of tumor cells tends to progress

• Once a cell becomes tumor cell , the composition
  behavior of its progeny do not remain static ,
  there is tendency for malignancy to increase .
• The important phenomenon of progression appears
  to reflect a fundamental instability of the
  genome of tumor cells .

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Metastasis is the most dangerous property of
tumor cells

• Metastasis is spread of cancer cell from primary
  site of origin to other tissue where they grow as
  secondary tumors .
• There is failure of cell - cell interaction much
  attention is on comparison of the biochemistry of
  normal malignant cells .

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