#p53

Cell Cycle Regulation

The G1/S transition is tightly regulated by:

Commitment to S phase involves transcriptional and posttranslational steps driven by Cyclin E-CDK2 activity pulse. During early G1, Cyclin D-CDK4/6 initiates partial Rb phosphorylation after mitogen induction, allowing modest E2F-dependent synthesis of Cyclin E. Rising Cyclin E binds CDK2, fully activating kinase normally restrained by inhibitors p21 and p27. Cyclin E-CDK2 hyperphosphorylates Rb at additional sites, liberating large pool of E2F1-3 which amplify expression of Cyclin A, Cdc6, MCM helicase components, dihydrofolate reductase and Pol alpha. Concurrently Cyclin E-CDK2 phosphorylates p27 at Thr187 creating phosphodegron recognized by SCF-Skp2-Cks1, leading to degradation and reinforcement of CDK2 activity. It also phosphorylates NPAT regulator of histone genes, Cdc45 recruitment factors Treslin and RecQL4, directly promoting origin firing and replication complex assembly. Cyclin E levels then fall through autophosphorylation induced recognition by SCF-Fbw7 ligase, producing transient spike. Amplification of Cyclin E in cancers causes premature S entry, centrosome overduplication, replication stress and chromosome instability. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.

Ref: Hinds & Weinberg, Cell Cycle Control by Cyclin E-CDK2, Curr Opin Cell Biol; Alberts et al., Chapter 17, G1/S Transition.

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Cell Cycle Regulation

What happens if p53 is mutated?

Functional p53 integrates stress signals, including DNA damage, oncogene activation, and hypoxia, to decide between cell cycle arrest, senescence, or apoptosis. Under normal conditions, wild-type p53 accumulates after ATM phosphorylates serine 15 and CHK2 phosphorylates serine 20, displacing E3 ubiquitin ligase MDM2 that normally targets p53 for proteasomal degradation. Transcriptional program includes CDKN1A encoding p21 inhibitor of CDK2-cyclin E and CDK4-cyclin D, causing Rb to remain hypophosphorylated and E2F-dependent S-phase entry genes repressed, imposing G1 arrest to allow repair. If TP53 gene is mutated, common missense mutations in DNA-binding domain producing dominant-negative tetramers, cells lose ability to transactivate p21 and other checkpoint targets, so G1 arrest fails despite DNA damage. Damaged templates proceed into replication, accumulating mutations and chromosomal aberrations driving tumor progression. Mutant p53 does not accelerate apoptosis or permanently block CDK1; instead it disables G1 surveillance and also compromises apoptosis via BAX and PUMA reduction. Consequently, p53 mutation abolishes G1 checkpoint stringency and genomic stability, explaining high prevalence in cancers.

Ref: Vogelstein et al., Nature 2000, Surfing p53 Network; Levine & Oren, Nature Rev Cancer 2009, p53 Mutations.

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Cell Cycle Regulation

What is the role of p53 in the G1 checkpoint?

G1 progression toward S phase is driven by cyclin-dependent kinases CDK4/6-cyclin D and CDK2-cyclin E that phosphorylate retinoblastoma protein Rb, releasing transcription factor E2F to induce genes for DNA replication enzymes, nucleotide biosynthesis, and replication licensing factors such as Cdc6. DNA damage in G1 would risk copying lesions, necessitating arrest. Tumor suppressor p53 serves as guardian by acting as transcription factor stabilized upon ATM-CHK2 signaling after double-strand breaks. One of its principal targets is CDKN1A encoding p21CIP1/WAF1, a potent inhibitor of CDK2-cyclin E and CDK2-cyclin A complexes. p21 binds and inhibits kinase active site, preventing phosphorylation of Rb and downstream substrates like Cdc6 and keeping E2F repressed, thereby imposing G1 arrest and allowing time for repair. p53 also induces GADD45 and 14-3-3 sigma contributing to pause. p53 does not degrade Rb, directly phosphorylate Cdc25, or ubiquitinate securin; those actions belong to other regulators. Through p21 induction, p53 enforces G1 checkpoint, and loss of this axis contributes to unchecked proliferation and genomic instability in majority of human cancers.

Ref: El-Deiry et al., Cell 1993, p21 as p53 Target; Bertoli et al., Nature Rev Mol Cell Biol 2013, G1-S Regulation.

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Cancer

Loss of p53 cooperates with Ras by

The tumor-suppressor protein p53 is kept at low levels in unstressed cells by continuous MDM2-mediated ubiquitination and degradation. DNA damage or oncogenic stress leads to phosphorylation of p53 or induction of ARF, both of which block the p53–MDM2 interaction. Stabilized p53 then transcriptionally activates genes that induce cell-cycle arrest, DNA repair or apoptosis, thereby preventing propagation of damaged genomes.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cancer

HPV E6 oncoprotein promotes cancer by

High-risk human papillomavirus oncoproteins E6 and E7 cooperate to drive cervical carcinogenesis. E7 binds and inactivates Rb, releasing E2F and promoting cell-cycle progression. E6 recruits the ubiquitin ligase E6AP to target p53 for degradation, thereby disabling the apoptotic and checkpoint responses that would otherwise eliminate the proliferating infected cell. The combined action removes both major tumor-suppressor barriers.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cancer

p53 induces apoptosis mainly via activation of

The tumor-suppressor protein p53 is kept at low levels in unstressed cells by continuous MDM2-mediated ubiquitination and degradation. DNA damage or oncogenic stress leads to phosphorylation of p53 or induction of ARF, both of which block the p53–MDM2 interaction. Stabilized p53 then transcriptionally activates genes that induce cell-cycle arrest, DNA repair or apoptosis, thereby preventing propagation of damaged genomes.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cancer

Active p53 can induce cell cycle arrest by upregulating

The tumor-suppressor protein p53 is kept at low levels in unstressed cells by continuous MDM2-mediated ubiquitination and degradation. DNA damage or oncogenic stress leads to phosphorylation of p53 or induction of ARF, both of which block the p53–MDM2 interaction. Stabilized p53 then transcriptionally activates genes that induce cell-cycle arrest, DNA repair or apoptosis, thereby preventing propagation of damaged genomes.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cancer

DNA damage stabilizes p53 mainly by

The tumor-suppressor protein p53 is kept at low levels in unstressed cells by continuous MDM2-mediated ubiquitination and degradation. DNA damage or oncogenic stress leads to phosphorylation of p53 or induction of ARF, both of which block the p53–MDM2 interaction. Stabilized p53 then transcriptionally activates genes that induce cell-cycle arrest, DNA repair or apoptosis, thereby preventing propagation of damaged genomes.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cancer

p53 is normally kept at low levels due to

The tumor-suppressor protein p53 is kept at low levels in unstressed cells by continuous MDM2-mediated ubiquitination and degradation. DNA damage or oncogenic stress leads to phosphorylation of p53 or induction of ARF, both of which block the p53–MDM2 interaction. Stabilized p53 then transcriptionally activates genes that induce cell-cycle arrest, DNA repair or apoptosis, thereby preventing propagation of damaged genomes.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cancer

Loss of p53 function leads to

The tumor-suppressor protein p53 is kept at low levels in unstressed cells by continuous MDM2-mediated ubiquitination and degradation. DNA damage or oncogenic stress leads to phosphorylation of p53 or induction of ARF, both of which block the p53–MDM2 interaction. Stabilized p53 then transcriptionally activates genes that induce cell-cycle arrest, DNA repair or apoptosis, thereby preventing propagation of damaged genomes.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cancer

p53 is often referred to as

The tumor-suppressor protein p53 is kept at low levels in unstressed cells by continuous MDM2-mediated ubiquitination and degradation. DNA damage or oncogenic stress leads to phosphorylation of p53 or induction of ARF, both of which block the p53–MDM2 interaction. Stabilized p53 then transcriptionally activates genes that induce cell-cycle arrest, DNA repair or apoptosis, thereby preventing propagation of damaged genomes.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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