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Abstract:
This lecture explores the intricate relationship between viruses and cancer, differentiating between cell transformation (the acquisition of immortality and altered growth properties in culture) and oncogenesis (the multi-step process of cancer development). It delves into the historical discoveries and mechanisms involving both RNA and DNA tumor viruses. The lecture highlights how studies on RNA retroviruses, starting with Rous sarcoma virus (RSV), led to the discovery of oncogenes – viral genes derived from cellular proto-oncogenes (like src) that control cell growth – and elucidated mechanisms like oncogene transduction and insertional mutagenesis (e.g., Avian Leukosis Virus, ALV). It then explains how DNA tumor viruses (such as SV40, polyomavirus, papillomavirus, adenovirus) transform cells, typically in non-permissive or semi-permissive hosts, by expressing viral T antigens (e.g., SV40 Large T, HPV E6/E7, Adeno E1A/E1B). These T antigens are crucial for viral replication but also function by inactivating key cellular tumor suppressor proteins like RB and p53, thereby overriding cell cycle checkpoints and promoting uncontrolled cell division. The lecture emphasizes that viral transformation is often an accidental byproduct (epiphenomenon) of viral replication strategies and that studying these viruses provided foundational insights into fundamental mechanisms of cell cycle control and cancer biology.
Virology Lecture: Understanding Transformation and Oncogenesis
0:30 Defining Transformation: Transformation is experimentally defined by altered cell morphology and growth patterns in culture, such as loss of contact inhibition, where cells grow on top of each other, unlike normal cells.
1:56 Cell Lifespan & Telomeres: Mouse cells can spontaneously transform in culture due to longer telomeres allowing more divisions and mutation accumulation. Human cells typically senesce (die) due to telomere shortening unless immortalized, e.g., by reactivating telomerase (3:12, 4:41).
5:33 Cancer & Telomeres: Cancer cells often overcome senescence by maintaining telomere length, frequently through reactivated telomerase, enabling indefinite proliferation.
6:13 Properties of Transformed Cells: Key characteristics include immortality, loss of anchorage dependence (can grow without attachment), loss of contact inhibition (pile up), ability to form colonies in semi-solid media, and decreased requirement for external growth factors.
7:52 Transformation vs. Oncogenesis: Transformation is cellular immortalization in vitro. Oncogenesis is cancer development in vivo, a multi-step process requiring multiple (~12) genetic mutations affecting cell division, survival, and genome integrity (8:42). Viruses can contribute to these mutations.
10:52 Viruses & Cancer: Studying virus-induced transformation was key to understanding cancer mechanisms. Viruses are contributing factors in ~20% of human cancers (12:16). Transformation is usually an accident of viral infection, not required for replication (13:23), with a rare exception (walleye dermal sarcoma virus, 13:50).
14:05 Rous Sarcoma Virus (RSV): Peyton Rous (1909) showed a filterable agent (RSV, a retrovirus) from a chicken tumor could cause sarcomas in other chickens, a foundational discovery in viral oncology.
15:46 Cell Culture Models: Howard Temin adapted RSV studies to cell culture, observing "fosi" – distinct clumps of transformed cells – allowing quantitative study (17:44).
18:47 DNA Tumor Viruses: Polyomavirus and SV40 were also found to transform cells in culture in the 1960s, establishing that both RNA and DNA viruses could induce transformation.
19:24 Requirements for Viral Transformation: 1) Virus must not kill the cell (cytopathic effects reduced). 2) Virus particle production often reduced. 3) Cell must continue to divide (become immortal). This resembles some persistent infections (19:57).
22:04 Avian Leukosis Virus (ALV): A common retrovirus in chickens causing leukemia relatively slowly. It's the parent virus from which RSV and other acute transforming viruses are derived.
24:30 Oncogenes Discovery: Bishop and Varmas found RSV contains an extra gene, src, acquired from the chicken host genome. This cellular gene (proto-oncogene c-src) confers the rapid tumor-forming ability. Viral oncogenes are often mutated/deregulated versions (v-src) (25:03, 31:20).
26:16 Defective Transforming Retroviruses: Most retroviruses that pick up oncogenes are defective, meaning the host gene replaces essential viral genes (often env), requiring a helper virus (like ALV) for replication. RSV was non-defective by chance (26:29).
27:10 Diverse Oncogenes: Studying various animal retroviruses revealed many different cellular proto-oncogenes involved in growth control pathways (receptors, kinases, transcription factors) (28:29, 33:35).
42:43 Host Specificity: DNA viruses often cause tumors primarily in non-natural hosts or under conditions where infection is non-productive or semi-permissive (viral life cycle blocked before completion) (43:38).
44:50 Viral T Antigens: A key finding was that tumors/transformed cells always contain viral "Tumor antigens" (T antigens) – early viral proteins essential for viral replication (e.g., SV40 Large T, HPV E6/E7, Adenovirus E1A/E1B).
46:49 T Antigens Target Cellular Proteins: T antigens were found to bind crucial cellular regulators: SV40 TAg binds p53; Adenovirus E1A influences RB/E2F; HPV E7 binds RB, E6 targets p53 (47:03, 47:16).
48:06 Cell Cycle Checkpoints: Cell division is tightly regulated. Proto-oncogenes act like accelerators. Tumor suppressor genes like RB (Retinoblastoma) and p53 act as brakes or monitors (49:19).
50:59 RB Protein Function: RB normally binds the E2F transcription factor, preventing expression of genes needed for S-phase (DNA replication). Phosphorylation of RB releases E2F, allowing cell cycle progression (51:04).
51:43 DNA Viruses Overcome RB: T antigens bind to RB, functionally inactivating it and releasing E2F, thereby forcing the cell into S-phase, which the virus needs for its own DNA replication (51:56).
52:49 p53 Protein Function: p53 acts as a "guardian of the genome," sensing DNA damage or unscheduled DNA synthesis (as occurs during viral infection) and triggering cell cycle arrest or apoptosis (programmed cell death) (53:03).
54:02 DNA Viruses Overcome p53: T antigens also inactivate p53, either by binding/sequestering it (SV40 TAg, Ad E1B) or targeting it for degradation (HPV E6), preventing apoptosis and allowing viral replication to proceed.
56:01 Rarity of DNA Virus Transformation: Requires two low-probability events: 1) Failure to express lethal late viral genes (e.g., in semi-permissive cells). 2) Integration of the T antigen genes into the host genome, ensuring their continuous expression.
57:13 Transformation as an Epiphenomenon: For DNA viruses, transformation is a side effect. The virus needs T antigens to manipulate the cell cycle (inactivate RB/p53) for its own replication. If these T antigens become permanently expressed due to integration, the cell becomes transformed.
58:47 HeLa Cells Explained: Derived from Henrietta Lacks' cervical cancer, these cells are immortal because they contain integrated Human Papillomavirus (HPV) type 18 DNA, constitutively expressing the E6 and E7 T antigens, which inactivate cellular p53 and RB, respectively.