MSc Medical Biochemistry,
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 .
Physical , chemical , & biological agents can cause
Agents causing cancer fall into 3 broad groups :
1. Radiant energy ,
2. Chemical compound ,
3. Viruses .
Oxidative damage to DNA .
Both organic & inorganic molecules are carcinogenic .
Carcinogens do not share structural similarity .
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
Carcinogens interact covalently with cellular macro
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 .
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
S .typhimurium of the above strain can not synthesize
histidine , needs histdine added to the medium for it’s
Chemical carcinogen by its mutagenic nature restore
synthesis of histidine in the S .
S .typhimurium along with mitochondrial supernatant
provides activation by monooxygenases .
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 .
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 .
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
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 .
Oncogenes play a crucial role in
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
Oncogenes of Rous Sarcoma
virus Rous sarcoma virus is retrovirus containing 4 genes
named gag , pol , env , src .
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
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 .
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
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 .
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 .
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 .
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.
Oncogenes from tumor cellsDNA isolated from tumor cell
Recipient cells often a line of mouse fibroblasts NIH/3T3
microscopic observation for1-2 wk
Foci of transformed cell
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 .
Activation of Proto - oncogenes
5 mechanisms of activation :
1. Promoter insertion ,
2. Enhancer insertion ,
3. Chromosomal translocation ,
4. Gene amplification ,
5. Point mutations .
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 .
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 .
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 .
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.
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 .
Autocrine stimulation by PDGF gives a chronic mitogenic
stimulus could be an imporatnt factor in transformation of
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 .
Transforming growth factor β
TGFβ known to inhibit the growth of most cell types except
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
The p53 tumor suppressor gene
acts as a guardian of the
Mutations in p53 occurs frequently in many human
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 .
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 .
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 ) .
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
Critically short telomere triggers a p53 regulated DNA
damage check point , this is called replicative
Cells can bypass this growth arrest if pRB or p53 are
The ability to bypass telomere based growth limitations
is thought to be a critical step in the evolution of most
This occurs by the expansion of telomerase in cancer
Telomerase is an enzyme that adds TTAGGG repeats
onto the 3’ end of chromosome .
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 .
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 .
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