Viruses and cancer Flashcards
The Rb pathway (simplified)
Rb binds E2F, and also SW1 and HDAC.
Cyclin D and Cdk4 phosphorylate Rb, making the Rb E2F complex able to stimulate transcription of cyclin E.
Cyclin E hyperphosphorylates Rb, leading to its disassociation from E2F. E2F can then proceed to stimulate transcription at many promoters.
Extrinsic pathway of apoptosis
Death receptors activate death domains which cleave caspase 8/10. These cleave both Bid (which activates Bax, the mitochondrial pathway) and also caspase 3. Caspase 3 acts to cleave DNA and protein, causing apoptosis.
Intrinsic pathway of apoptosis
Start with Bax and Bak forming channels for cyt C release. Cytochrome C activates caspase 9.
Stimulated by extrinsic pathway and p53 activity.
Inhibited by growth factor signalling.
p53 stimulation of apoptosis
p53 is sequestered by mdm2. Proapoptotic signals lead to dissociation of this, and its stimulating action on BH3 only proteins.
These stimulate Bax and Bak channel formation.
growth factor inhibition of apoptosis.
Growth factors activate Bcl2 proteins via Ras, PI3K and Akt. Bcl2 proteins inhibit cyt C release.
Cell cycle (simple)
G0 = resting. –> G1 (checkpoint) –> G1 phase –> G1/S checkpoint –> S phase –> G2 phase –> G2/M checkpoint –> M phase –> G0.
Discuss viral modulations of the cell cycle
Draw the cell cycle
Different checkpoints
Consider specific viruses
Why don’t all oncogenic virus infections cause cancer?
Different checkpoints in the cell cycle
G1 control of growth. Does not provide substrates for replication.
G1-S transition - if DNA replication ability is needed. Consider Rb and p53.
Mitotic spindle assembly checkpoint.
G1 checkpoint
Mostly growth factors for control.
G1/S transition
Rb drives, p53 halts.
Rb: bind this, or regulate cyclins.
p53: target this, or downstream.
Mitosis checkpoint
MDC1 for DNA damage.
Various for spindle assembly.
Specific oncogenic viral proteins I should know about
Tax and HBZ
E6 and E7
HTLV causing neoplasia
Primary transforming gene is Tax
HBz also important
Importance of driving cellular proliferation.
HBV causing neoplasia
Liver damage: hypothesis is that immune response is initiated with antigen-specific responses, but as these are ineffective, non-specific responses begin to cause damage.
Viral integration and insertional mutagenesis.
Hbx role not unequivocally demonstrated.
HCV causing neoplasia
Mostly immune damage, but may have proteins hitting usual pathways.
EBV causing neoplasia
BL
NPC
EBV - BL
Underactive immune response, expression of latency pattern 1, inhibits apoptosis by LMP1/2, defined by myc relocation.
EBV NPC
Ethnicity contributes to low-grade pre-cancerous lesion
EBV infections with LMP1 and 2, telomerase activation and etc high grade pre-cancerous lesion.
Environmental factors other genetic changes, metastasis.
Evidence for role of viruses
- RSV
- HTLV – clonal expansion
- EBV
- Increased incidence in immunosuppression
EBV - evidence for role of viruses in cancers.
o Seroepidemiology
o Presence of viral DNA
o In vitro protein experiments
o Transgenic virus experiments.
Tax
Transcription
Activity
Downregulation
Tax activity
Degradation of IKK.
Activation of Akt.
Activation of Akt and NFkB promote cellular survival.
Reported to alter redox status of cells.
Tax - degradation of IKK
So nuclear localization of NFkB-RELB-p52 and RELA-p50 complexes.
Tax - PI3K
Activates PI3K makes PIP2, increases amount of PIP3, activates Akt.
HBz activity
Protein
RNA
HBz activity protein
Binds CREB2, which inhibits Tax-mediated transactivation.
Modulates activity of AP1 transcription.
HBz activity - RNA
Upregulates E2F.
HTLV - why drive cellular proliferation?
Cell associated virus: generating long-term pool of virus infected cells is crucial both to infect naive cells and clonally expand infected cells.
HBV in oncogenesis - viral integration.
- Integration first genetic hit.
* HBx and PreS could contribute to carcinogenesis in trans.
HBV in oncogenesis - Hbx.
Not unequivocally demonstrated. Impacts: • Apoptosis • Cell cycle regulation • Signalling pathways • DNA repair • Transactivation.
KSHV in oncogenesis
KS in immunosuppression. Presence of genome in >95% of cases.
Lytic infection important in maintaining KS: treatment with ganciclovir leads to decline.
Discuss mechanism whereby retroviruses cause neoplasia
Direct carcinogens
Insertional mutagenesis
Indirect carcinogens
Ways to alter G1 control
Growth factor pathway Viral homologues of growth factors. Viral receptors of growth factors. Viral downstream signalling proteins. Upregulating cellular versions. JSRV env proteins.
Ways to alter G1 control. Viral homologues of growth factors.
- Often in retroviruses: v-sis
* KSHV chemokine homologues – loads. Generally uses homologues to interfere with cell cycle.
Ways to alter G1 control. Viral receptors of growth factors.
- Retroviruses: v-kit, v-src.
* KSHV chemokine receptor homologues.
Ways to alter G1 control. Viral downstream signalling pathways
v-ras
Ways to alter G1 control. JSRV
env protein stimulates signaling pathways when it bind receptor for entry.
Ways to alter G1 control - upregulation of growth factor pathways.
Insertional retroviruses. ALV insertion eventually in position to upregulate Myc.
• Myc translocation in Burkitt’s lymphoma by EBV. EBV gene products inhibit apoptosis too. Also, EBNA2 transactivates expression of c-src, among other roles.
Activating cyclin D
Small T antigen: binds PP2A activating MAPK and resulting in growth stimulation due to transactivation of cyclin A and cyclin D1 promoters.
Activates Akt and telomerase.
KSHV make v-cyclin D.
Viruses and proteins interacting with Rb
HCMV - IE86, IE72.
SV40 large T binds Rb and p53.
Adenovirus E1A through CR1, CR2.
E7 in high risk papillomas.
G1/S transition - Rb, regulating cyclins.
Regulate cyclin D
Regulate cyclin E
Regulate CKIs (cyclin dependent kinase inhibitors p21 and p27).
Make cyclin homologues
A virus which makes cyclin homologues
KSHV makes v-cyclin D.
E7 in high risk papilloma types
Binds Rb proteins.
Regulates cyclin function A and E. - transformation.
Displaces HDAC which acts to hyperacetylate chromatin and repress transcription.
Causes centrosome amplification.
Suppresses STAT-1
Activates signalling pathways.
Displaces E2F4 and 5 from Rb, without phosphorylation.
Low risk E7
Does not drive proliferation.
Virus proteins targetting p53 and apoptosis pathways
E6, KSHV various, SV40 large T.
E6 in high risk papilloma types - p53 and apoptosis
Acts on p53, sequesters in cytoplasm, recruits E6AP to lead to its degradation. Hence inhibits growth arrest.
Targets apoptotic pathways by degrading proteins:
• Bak, TNFR1, FADD, p53.
E6 in low risk papilloma types
Low risk: in immunosuppression prevents apoptosis or DNA repair, so potentiates UV damage and hence cancer.
Some inhibition of p53 transactivation and acetylation.
KSHV - targetting p53 and apoptosis
LANA inhibits p53.
V-flip inhibits caspases
V-Bcl2 inhibits Bax
K1 binds Fas.
Targetting the mitosis checkpoint
Some viruses contribute to DNA damage - EBNA-1.
Centrosome amplification by E7.
Activation of telomerase by E6 in high risk papillomas.
KSHV LANA
Affects signallin, binds p53, binds hypophosphorylated Rb.
Expression of E6 and E7.
In HPV E6 and E7 are expressed in differentiated cells which have exited the cell cycle. They make viral particles and die as part of the epithelium. Carcinogenesis most often occurs when integration occurs in the basal layer, altering transcription of E2 and hence of E6 and E7 in basal cells.
Cause of delay in carcinogenesis by viruses.
Specific/rare insertions needed
Immune clearance of infected cells.
Usually proteins not sufficient.
Sometimes oncogenic via induction of DNA damage.
Cause of delay in carcinogenesis by viruses - immune clearance.
. virally-associated cancers that increase with HIV such as KSHV, EBV associated lymphomas and Merkel cell carcinoma. Also others. Thus Tax is highly immunogenic, so virus has to successfully switch from Tax expression to HBZ expression to not be cleared (as seen by the fact that most ATL cancer cells do not express Tax).
E6 activity, not p53 or apoptosis
Inhibits interferon response,
Activates signalling pathways
Telomerase activation
C-myc activation.
Signalling pathways activated by E6.
Akt
Wnt
Notch
mTORC1.
High risk E7 - binding Rb proteins.
Rb, p105 and p107.
Differential ability to bind.
Binds Rb through LXCXE leading to degradation: release of E2F to act as TF.
Differences between high and low risk E6 and E7
Activity
Expression
What changes expression of E6 and E7 between high and low risk types
Different promoter positions and regulation.
Different RNA splicing.
Increased expression of E6/E7 in high risk types
Low expression in CIN1 doesn’t facilitate cancer progression: needs increased expression for CIN3.
Cause of deregulation unclear. Possibly epigenetic methylation, possibly due to integration altering E2, a transcription factor for E6 and E7.
Papilloma life-cycles.
Microwound allows access
Basal cells form reservoir
Middle layers have increased copy number of episomes.
Upper epithelial cells.
Expression changes due to differentiation.
Basal cells and HPV
form reservoir with low copy number of episomes.
High risk E6 and E7 drive cell cycle stimulating division
Middle layers and HPV
Middle layers express E4 and have increased copy number of episomes. E1 and E2 necessary for initial amplification.
Upper epithelial layers and HPV
L2 and L1 expressed, leading to packaging of amplified viral genomes.
Acutely transforming retroviruses
Most are replication defective.
Need to reinitiate cell cycle to give cellular resources, but must avoid apoptosis.
Antivirus hypothesis.
Insertional mutagenesis
Promoter insertion or insertion of enhancer elements (U5, with read through, or U3).
Creation of fusion products. Altering mRNA transcript stability.
Indirect carcinogens
No cellular oncogenes, either viral forms or upregulated by insertion (random insertion).
Increased proliferation of cells.
Immunosuppressive retroviruses.
Immune mediated neoplasia
HepB, HepC.
