Transcript Medical Biochemistry Cancer and Oncogenes Lecture 74 Properties of cancer cells • Diminished or unrestrained control of growth – benign tumors also show diminished.

Medical Biochemistry
Cancer and Oncogenes
Lecture 74
Properties of cancer cells
• Diminished or unrestrained control of growth
– benign tumors also show diminished growth control
• invasion of local tissues
• spread, or metastasis to other parts of the body
Agents causing cancer
• Three broad groups of agents
– radiant energy
– chemical compounds
– viruses
• Act by causing mutations or introducing genes into cells
• Familial conditions also cause cancer
– mutations in specific genes (e.g., tumor suppressor genes)
• Spontaneous mutations (may predispose to cancer)
– frequency ~10-7 to 10-6 per cell per generation
– will increase in tissues subject to high rate of proliferation,
increasing generation of potential cancer cells from affected stem
cells
– oxidative damage to DNA (e.g., release of OH• from
mitochondria) may be factor  mutation rate
Radiant Energy
• Ultraviolet rays, x-rays, and g-rays are mutagenic
and carcinogenic
• Result in DNA damage
– formation of pyrimidine dimers (elimination of
corresponding bases may form apurinic or apyrimidinic
sites
– single- and double-strand breaks or cross-linking
strands may occur
– x-rays and g-rays also cause free radicals to form in
tissues
• resultant OH•, superoxide, and other radicals can interact with
DNA leading to molecular damage and contribute to
carcinogenic effects
Chemical Carcinogens
• Estimated that ~80% of human cancers caused by
environmental factors, principally chemicals
• Exposure by:
– occupation (e.g., benzene, asbestos)
– diet (e.g., aflatoxin B1, mold sometimes contaminant of
peanuts
– life-style (e.g., cigarette smoking)
– other ways (e.g., some therapeutic drugs can be
carcinogenic)
Chemical Carcinogens
• Both organic and inorganic molecules may be
carcinogenic - no common structural feature
• Some compounds interact directly with target
molecules (direct carcinogens)
Chemical Carcinogens
• Others may require metabolic activation
procarinogen  proximate carcinogen  ultimate carcinogen
• procarcinogen not chemically reactive
• ultimate carcinogen often highly reactive
– usually electrophiles (i.e., molecules deficient in electrons)
– readily attack nucleophilic (electron-rich) groups in DNA,
RNA, and proteins
– form adducts (most common site of attack is guanine)
Chemical Carcinogens
• principally species of cytochrome P450 (in
ER) responsible for metabolic activation of
procarcinogens
– normal function to detoxify noxious chemicals
(xenobiotics)
• therapeutic drugs, insecticides, polycyclic
hydrocarbons
– many of these compounds so fat-souble they would
accumulate continually in fat cells and lipid
membranes, not be excreted from body
– detoxification by converting to water-soluble
derivatives that can be excreted
Mutagens
• Most chemical carcinogens are mutagens
• Ames assay - used to detect mutagenicity of
chemical carcinogens
– Salmonella typhimurium auxotroph
• strain with mutation (His-) in gene
involved in synthesis of histidine
– Grow on medium lacking His in
presence of test compound
• screen for mutations that cause
reversion to His+
– Compound may be activated by incubation with
postmitochondrial supernatant (contains microsomes)
Initiation and Promotion
• Chemical Carcinogen Experiment on Skin:
– Paint skin of mice with benzo[a]pyrene  no tumors
– After benzo[a]pyrene, apply croton oil several
times  many tumors develop
– Apply croton oil alone  no skin tumors
• Conclusions:
– stage of carcinogenesis caused by
benzo[a]pyrene called initiation
• rapid and irreversible (modification of DNA?)
– second, slower stage (months or years) is promotion
• promoters incapable of causing initiation
• Most carcinogens capable of acting as both
initiators and promoters
Initiation and Promotion
• Large number of compounds can act as promoters
in various organs
– phenobarbital, saccharin
• Active agent of croton oil is a mixture of phorbol
esters
– most active phorbol esters is
12-O-tetradecanoylphorbol-13-acetate (TPA)
– protein kinase C (PKC) can act as a receptor for TPA
– activation of PKC with TPA may lead to
phosphorylation of signaling molecules, loss of growth
control
– may increase proliferation of stem cells
DNA and RNA Viruses are Carcinogenic
• Transforming activity of tumor virus
resides in particular gene(s) carried in
viral genome
• Response of cell to infection by DNA tumor virus
depends on whether cells are permissive or
nonpermissive
– permissive cells productively infected  virus lytic
cycle (e.g., adenovirus in humans)
– in nonpermissive cells, viral replication abortive, viral
DNA integrates, may transforms cell (EBV associated
with Burkitt’s lymphoma)
DNA and RNA Viruses are Carcinogenic
• RNA tumor viruses (retroviruses)
– during life cycle, RNA genome
converted to DNA by viral reverse
transcriptase, can be integrated into
host genome
• Transforming retrovirus carries copy
of cellular sequence in place of some
of its own gene(s)
– usually replication defective
– expression of transduced cellular gene(s) may alter
phenotype of infected cell (may stimulate growth of
infected cells, enhance proliferation of virus)
Oncogenes of retroviruses
• c-onc (cellular oncogene) - oncogene present in tumor
cells
• proto-oncogene - gene present in normal cells
• v-onc (viral oncogene) - oncogene present in virus
• viral oncogenes represent wide variety of signaling
molecules (mostly protein tyrosine kinases)
Oncogenes of retroviruses
• Two theories may explain difference between
v-onc and c-onc genes
– quantitative model - viral genes functionally the
same as cellular genes, but oncogenic because
expressed in much greater amounts, in
inappropriate cell types, or at inappropriate times
– qualitative model - c-onc genes intrinsically lack
oncogenic properties, but can be converted by
mutation into oncogenes (acquire, or lose,
important property)
Activation of Proto-oncogenes
• Five mechanisms alter the expression or structure
of proto-oncogenes and participate in their
conversion to oncogenes
–
–
–
–
–
Promoter insertion
Enhancer insertion
Chromosomal Translocations
Gene Amplification
Point Mutation
Quantitative model
Qualitative model
Promoter Insertion
• Certain retroviruses lack oncogenes (e.g., avian
leukemia viruses) but may cause cancer over longer
period of time
• Infect cell, reverse transcriptase synthesizes cDNA of
RNA genome, integrates into host genome (provirus)
• cDNA of provirus flanked by long-terminal repeats
(LTRs) that can promote transcription
• provirus integration near c-myc gene
leads to increased constitutive
expression and formation of B cell
tumor
Enhancer Insertion
• Provirus inserted downstream of myc gene, or
upstream in opposite orientation
• myc gene activated because enhancer sequences in
LTR increase rate of transcription
Chromosomal Translocations
• Many tumor cells exhibit chromosomal abnormalities
• One type seen in cancer cells is translocation
– piece of one chromosomes is split off and joined to
another chromosome
– if second chromosome donates material to the first,
translocation is “reciprocal”
• Burkitt’s lymphoma is fast-growing cancer of
human B lymphocytes
– In certain cases, chromosomes 8 and 14 involved
in reciprocal translocation
– Segment of chromosome 8 that moves to 14
contains c-myc
– Transposition places inactive c-myc under
influence of enhancers from immunoglobulin
heavy chain genes
Gene Amplification
• Amplification of certain genes is
found in a number of tumors
• Methotrexate, an inhibitor of dihydrofolate reductase
(DHFR), is administered as an anticancer drug
– Tumor cells that become resistant to the action of this
drug amplify the gene for DHFR resulting in increased
enzyme activity
– Appear either as homogeneously staining
regions (HSR) or as self-replicating doubleminute chromosomes (lacking centromeres)
Point Mutation
• DNA sequence of c-ras proto-oncogene
from normal human cells and c-ras
oncogene from human bladder cancer
showed difference in only one base
(resulting in amino acid substitution at
Gly12)
• Mutation results in loss of GTPase
activity, may lead to persistent activation
of MAPK pathway (mitogenic pathway)