Overview
The transcript explains how the cell cycle is regulated by pro-growth signals, checkpoints, and tumor suppressor mechanisms. It focuses on signaling pathways, key genes, cyclins/CDKs, and checkpoint control, including the metaphase-to-anaphase transition.
Cell Cycle Phases and Checkpoints
- Phases: G1, S (DNA replication), G2, M (prophase, metaphase, anaphase, telophase), cytokinesis.
- Exit/entry: Some cells enter G0 (quiescent) and can re-enter with proper stimulus.
- Checkpoints: G1/S, G2/M, and M (metaphase-to-anaphase transition).
Growth Signals and Proto-oncogenes
- Proto-oncogenes: Genes that stimulate proliferation; produce products driving cycle progression.
- Mitogens/growth factors: Epidermal, vascular endothelial, platelet-derived growth factors; trigger signaling.
- Immediate early genes: myc, fos, jun respond to signaling; produce transcription factors.
Receptors and Intracellular Signaling Pathways
- Gq-coupled GPCR:
- Activates phospholipase C; PIP2 → DAG + IP3.
- DAG → PKC activation; IP3 → Ca2+ release; Ca2+-calmodulin kinases.
- Outcome: Phosphorylate transcription factors; activate genes.
- Gs-coupled GPCR:
- Activates adenylyl cyclase; ATP → cAMP.
- cAMP activates PKA; phosphorylates transcription factors.
- Receptor tyrosine kinase (RTK):
- Ligand-induced dimerization; tyrosine phosphorylation.
- Recruits GRB2 and Son of Sevenless (SOS) to activate Ras (GTP-bound).
- Ras → Raf → MAP kinase; activates transcription factors.
- JAK-STAT pathway:
- JAK activated by receptor; phosphorylates STAT.
- STAT drives transcription of target genes.
Cyclins and Cyclin-Dependent Kinases (CDKs)
- Cyclins: D, E, A, B; produced in response to upstream transcription factors.
- CDKs: Constantly present; require cyclin binding for activity.
- Cyclin-CDK complexes: Phosphorylate targets to drive progression.
Retinoblastoma (RB) and E2F Control
- RB: Tumor suppressor protein binding E2F; prevents E2F-driven transcription.
- Cyclin-CDK action: Hyperphosphorylates RB; releases E2F.
- E2F: Activates genes promoting cell cycle progression.
DNA Damage Sensing and Tumor Suppressors
- ATM gene: Ataxia-telangiectasia mutated; produces proteins that proofread DNA.
- ATM role: Detects mismatches; alerts p53 when damage is present.
- DNA repair genes: Produce enzymes for nucleotide excision and other repairs; p53 can induce them.
p53 Regulation and Functions
- p53: Central tumor suppressor; low at baseline due to inhibition.
- MDM2: E3 ubiquitin ligase; inhibits p53 by targeting its products.
- DNA damage: Inhibits MDM2; permits p53 activity.
- p53 outcomes:
- Induces DNA repair enzymes to correct damage.
- Activates pro-apoptotic genes (e.g., BAX) when damage is severe.
- Induces CDK inhibitors to pause the cycle and allow repair.
CDK Inhibitors (CKIs)
- CKIs: p15, p16, p18, p19, p21, p27, p57; inhibit cyclin-CDK complexes.
- Effect: Prevent RB phosphorylation; keep E2F sequestered; delay progression.
Cyclin/CDK Control Across the Cell Cycle
- Cyclin D-CDK4/6: Controls G1 progression before G1/S checkpoint.
- Cyclin E-CDK2: Regulates late G1 and G1/S checkpoint transition.
- Cyclin A-CDK2: Functions in S phase and into S→G2 transition.
- Cyclin B-CDK1: Controls G2/M and early mitosis through metaphase.
Metaphase Checkpoint and Anaphase Onset
- Structures: Kinetochores at centromeres; microtubule attachments required.
- Cohesin: Protein holding sister chromatids together at centromeres.
- Securin and separase: Securin binds separase; prevents cohesin cleavage.
- APC (anaphase-promoting complex): Targets securin, freeing separase.
- Outcome: Separase cleaves cohesin; chromatids separate; anaphase proceeds.
Cell Cycle Regulators: Summary Table
| Regulator | Type/Complex | Upstream Trigger | Primary Target/Action | Checkpoint/Phase Impacted |
|---|
| Cyclin D-CDK4/6 | Cyclin-CDK | Mitogen-induced transcription | RB phosphorylation (initial) | G1 progression |
| Cyclin E-CDK2 | Cyclin-CDK | E2F-driven transcription | RB hyperphosphorylation | G1/S transition |
| Cyclin A-CDK2 | Cyclin-CDK | S-phase entry programs | DNA replication control | S phase, S→G2 |
| Cyclin B-CDK1 | Cyclin-CDK | Pre-mitotic accumulation | Mitotic entry and progression | G2/M, early M |
| RB | Tumor suppressor | Basal expression | Sequesters E2F when unphosphorylated | Blocks G1/S |
| E2F | Transcription factor | RB phosphorylation/release | Activates S-phase genes | Promotes G1→S |
| ATM | DNA damage sensor | DNA mismatches/damage | Activates p53 pathway | Halts at G1/S or G2/M |
| p53 | Tumor suppressor | ATM signaling; MDM2 inhibition | Repairs, apoptosis, CKIs induction | Global checkpoint control |
| MDM2 | E3 ligase | Basal regulation | Inhibits p53 products | Limits p53 activity |
| CKIs (p15/p16/p18/p19/p21/p27/p57) | CDK inhibitors | p53 and other signals | Inhibit cyclin-CDKs | Pause G1/S or G2/M |
| APC | E3 ligase complex | Proper metaphase alignment | Degrades securin; activates separase | M checkpoint (meta→ana) |
| Separase | Protease | APC-mediated securin removal | Cleaves cohesin | Initiates anaphase |
Key Terms & Definitions
- Proto-oncogenes: Genes whose products promote cell proliferation under normal control.
- Mitogens: Growth factors that stimulate cell division and mitosis.
- GPCR: G protein-coupled receptor; activates second messenger cascades.
- PKC/PKA: Protein kinases C and A; phosphorylate transcription factors.
- RTK: Receptor tyrosine kinase; signals through Ras-Raf-MAPK.
- JAK-STAT: Pathway where JAK activates STAT to drive transcription.
- RB (Retinoblastoma protein): Gatekeeper binding E2F; blocks S-phase entry.
- E2F: Transcription factor promoting S-phase gene expression.
- ATM: DNA damage sensor kinase activating p53 pathway.
- p53: Master regulator of DNA repair, apoptosis, and cell cycle arrest.
- MDM2: E3 ubiquitin ligase inhibiting p53 products.
- CKIs: Cyclin-dependent kinase inhibitors; halt CDK activity.
- APC: Anaphase-promoting complex; triggers sister chromatid separation.
- Cohesin: Protein complex holding sister chromatids together.
- Securin: Inhibitor of separase; prevents premature chromatid separation.
- Separase: Protease cleaving cohesin to initiate anaphase.
Action Items / Next Steps
- Review the sequence D, E, A, B for cyclin timing and corresponding CDKs.
- Map each checkpoint to its primary regulators and outcomes.
- Practice tracing each signaling pathway from receptor to transcription factor.
- Connect p53 decisions to outcomes: repair, arrest, or apoptosis.