Neo3 Flashcards
Mechanisms of carcinogenesis
Cancer as a multistep process:
Initiation: DNA alteration or cell change
Tumor-promotion: from single mutated cell to formation of tumor
Tumor-progression: development of malignancy
Results from mutation of DNA to activate a proto-oncogene or inactivate a tumor suppressor gene
Initiation
- Initiation alone does not result in tumors
complete carcinogens”).
Some initiators can subsequently act as promoters (are “complete carcinogens”).
are usually substances that produce cell activation and proliferation
Promotors #Effects of promotors are reversible.
Promotors cannot induce neoplasia if
i) alone
ii) if applied before initiator
iii) if applied in too small amount
iv) if too much time elapses between applications
does not cause mutation, but it leads to clonal expansion of the initiated (mutated) cells
Promotion
Ex : hormones (estrogen) , growth factors
: Progression
Karyotypic instability: Increased growth rates Increased invasiveness Increased hormonal response Anaplasia
multiple genetic alterations involving activation of several oncogenes and loss of two or more cancer suppressor genes.
Progression
colon cancer:
Progression
Colon epithelial hyperplasia
Formation of adenomas
Malignant transformation
Karyotypic Changes in Tumors
Point mutation: e.g. ras oncogene Balanced translocations Gene amplification Deletions Whole chromosomes may be gained or lost.
Molecular Basis of Multistep Mechanism
Genes that regulate entry of cells into the multi-step carcinogenesis are called…..
E.g. mutations of ….
gatekeepers
Rb, NF-1, VHL, or APC gives rise to retinoblastoma, schwannomas, renal cell cancer, and colon cancer
In contrast to gatekeeper genes, those that affect genomic stability are called…… genes
E.g…
caretaker
the DNA repair genes
A) Chemical Carcinogens
Direct agents:
They require no metabolic conversion to become carcinogenic
Indirect agents:
They become active after metabolic conversion
These agents react with RNA, cellular proteins, and DNA
Some chemical carcinogens are augmented by certain agents that are called promoters
Direct-Acting Carcinogens
1. Alkylating agents Anticancer drugs (cyclophosphamide , chlorambucil , nitrosoureas, and others) 2. Acylating agents 1-Acetyl-imidazole Dimethylcarbamyl chloride
Indirect Acting Agents
Polycyclic and heterocyclic aromatic hydrocarbons
Aromatic amines, amides, azo dyes
2-Naphthylamine (β-naphthylamine)
bladder cancer in workers exposed to aniline dye & rubber industry
Natural plant and microbial products
Aflatoxin B1 hepatocellular carcinoma
Others
Asbestos, nickel, chromium lung cancer
HPV (types 1, 2, 4, 7): cause
benign squamous papillomas (warts in human)
HPV (types 16, 18, 31)
involved in the genesis of oropharyngeal, cervical, anal, perianal, and penile cancers
Epstein-Barr Virus:
• Involved in the pathogenesis of several human tumors
– Burkitt lymphoma
– AIDS-related lymphomas
– Nasopharyngeal carcinoma
Helicobacter Pylori
• gram negative spiral bacteria
• associated with 90% of duodenal ulcers,and 70-90% of gastric ulcers
• the likely cause for gastric carcinoma & lymphoma (MALToma: marginal zone associated lymphoma)
.
Four classes of genes are targets of genetic damage
- The growth promoting proto-oncogenes
- The growth-inhibiting tumor-suppressor genes
(anti-oncogenes) - Genes that regulate apoptosis
- The DNA repair genes
Six fundamental changes in cell physiology that together dictate malignant phenotype
- Self-sufficiency in growth signals (oncogenes)
- Insensitivity to growth-inhibitory signals (Cancer
Suppressor Genes) - Evasion of apoptosis
- Limitless replicative potential
- Sustained angiogenesis
- Ability to invade and metastasize
Mutations and pathologic overexpression of normal forms of growth factor receptors have been detected in several tumors. e.g
the epidermal growth factor (EGF) receptor is overexpressed in 80% of squamous cell carcinomas of the lung.
Knudson (two hit theory):
• In hereditary retinoblastoma, children are born with one normal and one defective copy of the Rb gene (first hit).
• They lose the normal copy by some somatic mutation
(second hit).
,
Tumor Suppressor Genes: p53
• located on chromosome 17
• It’s the most common Tumor Suppressor Gene target for genetic alterations in human tumors
• More than 50% of human tumors contain mutations in this gene
• Familial loss causes Li-Fraumeni syndrome (multiple tumors) includes sarcomas, breast cancer, leukemia,
brain tumors, and carcinomas of the adrenal cortex
.
Tumor Angiogenesis
• Angiogenesis is required for: 1. continued tumor growth
2. metastasis
• Tumors can not enlarge beyond 1-2 mm in diameter or thickness unless they are vascularized
• The 1-2 mm zone represents the maximal distance across which oxygen and nutrients can diffuse from blood vessels. Beyond this distance the tumor fails to enlarge without vascularization. Hypoxia will induce apoptosis by activation of p53
.
How Do Growing Tumor Develop a Blood Supply
• Tumor growth is controlled by the balance between angiogenic factors, & antiangiogenesis molecules.
• Examples of angiogenic factors:
– Vascular endothelial growth factor (VEGF) – Basic fibroblast growth factor (bFGF)
• Examples of antiangiogenesis are:
– thrombospondin-1, angiostatin, endostatin, & vasculostatin.
.
Hypoxia within the growing tumor favors angiogenesis by release of hypoxia-inducibale factor–1 (HIF-1).
HIF-1 controls transcription of VEGF
Transcription of VEGF is under the control of Ras oncogene
Ras oncogene activation upregulates the production of VEGF
Proteases are involved in regulating the balance between angiogenic and antiangiogenic factors
. Hypoxia within the growing tumor favors angiogenesis by release of hypoxia-inducibale factor–1 (HIF-1).
HIF-1 controls transcription of VEGF
Transcription of VEGF is under the control of Ras oncogene
Ras oncogene activation upregulates the production of VEGF
Proteases are involved in regulating the balance between angiogenic and antiangiogenic factors
P53 inhibits angiogenesis by inducing the synthesis of the antiangiogenic molecule thrombospondin-1
With mutational inactivation of both p53 alleles, the level of thrombospondin-1 drop markedly tilting the balance in favor of angiogenic factors
Some success in the treatment of cancer has been achieved through the use of angiogenesis inhibitors such as endostatin
.
Metastatic process can be divided into two phases:
Invasion of the extracellular matrix
Vascular dissemination and homing of tumor cells
E-cadherins acts as intercellular glue that bind cells together.
E-cadherin function is lost in almost all epithelial cancer
Invasion of the cellular matrix in 4 steps:
Detachment of tumor cells from each other
Attachment of tumor cells to matrix components
Degradation of ECM
Migration of tumor cells
Attachment of tumor cells to ECM proteins such as laminin and fibronectin
Local degradation of the basement membrane and interstitial connective tissue
Tumor cells either secrete proteolytic enzymes themselves or induce the host cells fibroblasts to elaborate proteases
Several of metalloproteinases including gelatinases, collagenases, and stromelysins, cathepsin-D are involved
Benign tumors show little type IV collagenase activity, whereas their malignant counterpart overexpresses this enzyme
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