p53 (TP53) is the "guardian of the genome" — a transcription factor that senses cellular stress (DNA damage, oncogene activation, hypoxia, nutrient deprivation, ribosomal stress) and triggers cell-cycle arrest, senescence, or apoptosis. Mutated in ~50% of human cancers (most frequently mutated tumor suppressor). Steady-state regulation: MDM2 (HDM2 in humans) is an E3 ubiquitin ligase that polyubiquitinates p53 → proteasomal degradation. p53 transcribes MDM2 → negative-feedback loop; basal p53 levels stay low. Stress-induced stabilization: (1) DNA damage — ATM/ATR sense DSBs + replication stress → phosphorylate p53 (Ser15, Ser20) + MDM2 → disrupts MDM2-p53 binding. (2) Oncogene activation — ARF (p14ARF) binds + sequesters MDM2 → p53 stabilizes. (3) Ribosomal stress — RPL5/RPL11 bind MDM2. Stabilized p53 → tetramerizes + binds p53RE → transcribes p21 (cell-cycle arrest), PUMA + BAX + NOXA (apoptosis), GADD45, MDM2 (feedback), p53R2 (DNA repair). Decision logic: low / transient stress → p21 → reversible arrest. Moderate persistent stress → senescence (cross-link senescence sasp senolytics). High / unrepairable → BAX/PUMA → apoptosis (cross-link apoptosis bcl2 axis). Hotspot mutations (R175H, R248Q/W, R273H/C, R282W) — loss of DNA binding + gain-of-function. Li-Fraumeni: germline TP53 → early-onset cancers. Therapeutic landscape: MDM2-p53 interaction inhibitors (idasanutlin, milademetan, navtemadlin — reactivate WT p53; toxicity is challenging); reactivators of mutant p53 (APR-246 in MDS); many natural products (curcumin, resveratrol, quercetin, withaferin-A) restore p53 function in preclinical cancer models. Cross-links: apoptosis bcl2 axis, senescence sasp senolytics, cell cycle cdk, ubiquitin proteasome (MDM2-mediated p53 turnover).
Organ Systems
immune-hematologic
integumentary
nervous
reproductive
Pathway Steps
cellular stress (DNA damage, oncogene, hypoxia, ribosomal) → ATM/ATR or ARF or RPL5/11 activation — via distinct sensors → converge on p53 stabilization. Diverse stresses feed p53 through distinct sensors that converge on its stabilization: DNA double-strand breaks activate ATM/ATR kinases; oncogene-driven hyperproliferation induces ARF (which sequesters MDM2); and impaired ribosome biogenesis frees RPL5/RPL11 to bind MDM2. This sensor multiplicity lets p53 act as a central integrator of cellular damage.
ATM/ATR phosphorylation of p53 Ser15/Ser20 + MDM2 → disrupted p53-MDM2 binding → p53 stabilization — via p53 half-life rises from minutes to hours. Normally the E3 ligase MDM2 keeps p53 low by constant ubiquitination/degradation (half-life minutes). ATM/ATR phosphorylation of p53 (Ser15/20) and of MDM2 disrupts their binding, so p53 escapes degradation and accumulates within hours. This brake is exploited by MDM2 antagonists (nutlins) that reactivate wild-type p53.
stabilized p53 → tetramerization + DNA binding at p53-RE — via transcription factor active state. Stabilized p53 assembles into tetramers and binds p53 response elements as a sequence-specific transcription factor. Most cancer-associated TP53 mutations are missense changes in the DNA-binding domain that cripple this step — making p53 the most frequently mutated gene in human cancer.
p53 transcriptional output → p21 + MDM2 + PUMA/BAX + GADD45 + p53R2 — via p21 = arrest; PUMA/BAX = apoptosis; MDM2 = feedback. Active p53 transactivates a program spanning opposing fates: p21 (CDKN1A) for cell-cycle arrest, PUMA/BAX for apoptosis, GADD45 and p53R2 for repair, and MDM2 for negative feedback. The balance of these targets — tuned by p53 levels, modifications, and cofactors — decides whether a cell pauses, dies, or senesces.
low / transient damage → p21 → reversible G1/G2 arrest → repair — via pro-survival outcome; cells re-enter cycle after repair. With low or transient damage, p21 inhibits CDKs to impose a reversible G1/G2 arrest, giving time for repair before cells re-enter the cycle — the pro-survival outcome. p21 is the principal effector linking p53 to the cell-cycle machinery.
persistent moderate stress → senescence (p21 + p16-cyclin) — via permanent arrest + SASP (cross-link senescence_sasp_senolytics). Persistent moderate stress tips the outcome toward senescence — a permanent arrest enforced by p21 plus the p16-cyclin/Rb axis and accompanied by the senescence-associated secretory phenotype (SASP). This is a potent tumor-suppressive barrier that also contributes to aging.
high / unrepairable damage → BAX / PUMA → mitochondrial apoptosis — via cross-link to apoptosis_bcl2_axis; pro-death p53 outcome. High or unrepairable damage drives p53 toward pro-apoptotic targets (BAX, PUMA), which engage the mitochondrial (intrinsic) apoptosis pathway — committing the cell to death rather than risking propagation of a damaged genome.
Known Modulators
rapamycin (activator) — p53-mTOR cross-talk; rapalogs can stabilize WT p53 in some contexts