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Ribosome quality control (RQC) acts as a critical cytoprotective mechanism in colorectal cancer (CRC) by resolving ribosome collisions and degrading aberrant nascent peptides induced by chemotherapeutic stress, particularly from 5-fluorouracil (5-FU). The activation of RQC pathways mitigates ribotoxic stress and prevents apoptosis, thereby modulating the efficacy and tolerance of standard-of-care treatments (Direct, High; PMID: 39351862).

Mechanisms of Ribosome Collision Induction by Chemotherapy

• 5-FU-Induced Ribosome Stalling: 5-FU is a pyrimidine analogue that misincorporates into RNA at significantly higher rates than into DNA (Direct, High; PMID: 39351862, PMID: 39378883). This misincorporation leads to abnormal RNA processing, aberrant post-transcriptional modifications, and stop-codon readthrough, all of which impede ribosome progression and cause stalling (Direct, High; PMID: 39351862).
• mTORC1-Driven Collisions: Treatment with 5-FU rapidly activates the mTORC1 signaling pathway, which elevates the rate of translation initiation. This increased initiation frequency increases the probability of trailing ribosomes colliding with stalled ribosomes, thereby generating disomes and trisomes (Direct, High; PMID: 39351862).
• Fluorinated Ribosomes: 5-FURD (5-fluorouridine) is incorporated into the rRNA of mature ribosomes. These "fluorinated ribosomes" are functional but exhibit altered translational specificity, favoring the translation of pro-survival genes such as IGF-1R through IRES-dependent mechanisms, which contributes to a drug-tolerant persistent phenotype in CRC cells (Direct, High; PMID: 35013311).

Key RQC Factors and Their Roles in CRC Therapy Response

• ZNF598 (Hel2): This E3 ubiquitin ligase acts as a sensor of collided ribosomes, promoting the mono-ubiquitination of the 40S subunit protein eS10 (uS10) (Direct, High; PMID: 39351862, PMID: 28685749). In CRC cells, 5-FU treatment triggers an mTOR-dependent post-translational upregulation of ZNF598 to bolster the cellular response to increased collisions (Direct, High; PMID: 39351862).
• GIGYF2 and 4EHP: ZNF598 recruits the GIGYF2-4EHP complex to collided ribosomes to repress further translation initiation on the aberrant mRNA, preventing the accumulation of additional trailing ribosomes (Direct, High; PMID: 39351862, PMID: 32657267). Like ZNF598, GIGYF2 is post-translationally stabilized by mTOR activation following 5-FU exposure (Direct, High; PMID: 39351862).
• hRQT Complex (ASCC3/2-TRIP4): In mammalian cells, the hRQT complex is required for the dissociation of ubiquitinated stalled ribosomes into subunits (Direct, High; PMID: 32099016). ASCC3 (the mammalian ortholog of yeast Slh1) is essential for RQC induction and resolution of 5-FU-induced collisions (Direct, High; PMID: 39351862).
• UFMylation Pathway: Ribosome stalls occurring during the cotranslational translocation of secretory proteins at the ER (common under 5-FU stress) trigger the UFMylation of RPL26. This modification is essential for RQC-mediated degradation of the arrested peptides via the LTN1 (Listerin) E3 ligase and p97/VCP ATPase (Direct, High; PMID: 37036982).

Impact of RQC Deficiency on Chemotherapy Efficacy

• Enhanced Sensitivity: Genetic knockout or shRNA-mediated depletion of ZNF598 or ASCC3 markedly increases the sensitivity of CRC cells (e.g., HCT116) to 5-FU (Direct, High; PMID: 39351862).
• Ribotoxic Stress Response: In RQC-deficient cells, unresolved collided ribosomes accumulate and activate the Ribotoxic Stress Response (RSR) via the p38 MAPK pathway, leading to enhanced apoptosis and impaired clonogenic survival (Direct, Medium; PMID: 39351862, PMID: 33713598).
• Restoration of Sensitivity: Inhibition of mTOR (using Torin-1) or repression of cap-dependent translation initiation (using 4EGI-1) reduces collision frequency and reverses the 5-FU sensitization seen in RQC-deficient cells, demonstrating that the toxicity is dependent on translational traffic (Direct, High; PMID: 39351862).

Clinical Relevance in Colorectal Cancer

• Protein Expression: Public datasets (CPTAC) show significantly increased protein expression levels of key RQC factors, including ZNF598, GIGYF2, and ASCC3, in colon adenocarcinoma samples compared to normal tissue (Direct, High; PMID: 39351862).
• Treatment Synergy: The dependency of CRC cells on RQC to survive 5-FU-induced RNA damage suggests that targeting the RQC machinery could synergistically enhance the anticancer efficacy of 5-FU-based chemotherapy regimens (Derived, Medium; PMID: 39351862, PMID: 39378883).

In summary, RQC is a vital homeostatic system that protects colorectal cancer cells from the RNA-dependent toxicity of 5-FU by resolving high-initiation-driven ribosome collisions and ensuring the degradation of arrested nascent chains. The mTORC1 pathway orchestrates a rapid post-translational stabilization of RQC factors to compensate for chemotherapy-induced stress, a mechanism that potentially contributes to the development of drug-tolerant phenotypes in colorectal tumors.

Unverified Citations

The following sources failed to support their assigned claims after 3 verification rounds designed to ensure only high-confidence, relevant references are retained:

• PMID:35013311 — The activation of RQC pathways mitigates ribotoxic stress and prevents apoptosis, thereby modulating the efficacy and to...
  Failed: mechanism,conclusion — This paper does not discuss the RQC (ribosome-associated quality control) pathway; it focuses on the creation of functional fluorinated ribosomes that promote survival by altering translation initiation specificity.
• PMID:35013311 — This misincorporation leads to abnormal RNA processing, aberrant post-transcriptional modifications, and stop-codon read...
  Failed: mechanism,conclusion — The paper discusses 5-FU incorporation into rRNA and its effects on ribosome maturation and selective translation initiation, but it does not support the claim that this leads to ribosome stalling.
• PMID:32099016 — ASCC3 (the mammalian ortholog of yeast Slh1) is essential for RQC induction and resolution of 5-FU-induced collisions **
  Failed: entities,conclusion — The paper characterizes ASCC3 as a component of the hRQT complex but does not study 5-FU or its induction of collisions.