cancer and health

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

2. Introduction Cancer cells are characterized by three properties :
1. Diminished or unrestrained control of growth ,
2. Invasion of local tissues ,
3. 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 .

3. contd
 Physical , chemical , & biological agents can cause
cancer .
 Agents causing cancer fall into 3 broad groups :
1. Radiant energy ,
2. Chemical compound ,
3. Viruses .
 Spontaneous mutations
 Oxidative damage to DNA .

4. Carcinogenic chemicals

5. Both organic & inorganic molecules are carcinogenic .
Carcinogens do not share structural similarity .

6. 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

7. 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 .

8. 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 .

9.  S .typhimurium of the above strain can not synthesize
histidine , needs histdine added to the medium for it’s
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 .

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

12. 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 .

13.  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 .

16. 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.

17. Oncogenes of Rous Sarcoma
virus Rous sarcoma virus is retrovirus containing 4 genes
named gag , pol , env , src .

18. 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 .

19. 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 .

20. 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 .

21. 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 .

22. 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 .

24. 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 .

25. 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.

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

27. 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 .

28. Activation of Proto - oncogenes
 5 mechanisms of activation :
1. Promoter insertion ,
2. Enhancer insertion ,
3. Chromosomal translocation ,
4. Gene amplification ,
5. Point mutations .

29. 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 .

31. Enhancer Insertion

32. Chromosomal translocation

35. 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 .

39.  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 .

40. 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.

41. 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 .

42. 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 .

45. Transforming growth factor β
 TGFβ known to inhibit the growth of most cell types except
fibrobalsts .
 In fibroblast TGFβ promotes growth by activating sis gene .
 TGFβ 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 .

48. The p53 tumor suppressor gene
acts as a guardian of the
genome
 Mutations in p53 occurs frequently in many human
tumors .

50.  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 .

52. 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 .

58. 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 ) .

59. 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.

60. 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 .

61. 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 .

62. 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 .

63. Malignancy of tumor cells tends to
progress Once a cell becomes tumor cell , the composition & behavior
of it’s 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 .

68. 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
.