New Mechanism Found That Could Reverse Drug Resistance in Colorectal Cancer
Scientists identify how the HMBOX1-HACE1-ATG5 axis suppresses autophagy to restore 5-FU chemosensitivity in colorectal cancer.
Summary
Colorectal cancer often becomes resistant to 5-fluorouracil (5-FU), the most commonly used chemotherapy. Researchers discovered that a transcription factor called HMBOX1 is frequently silenced in drug-resistant colorectal cancers, and this loss drives excessive autophagy that helps cancer cells survive chemotherapy. When HMBOX1 is active, it boosts production of an enzyme called HACE1, which tags another protein, ATG5, for destruction via the cell's protein-recycling machinery. Without ATG5, autophagy is suppressed, and cancer cells become vulnerable to 5-FU again. The findings were validated in cell lines, mouse models, and human tumor tissue, pointing to HMBOX1 as a potential target to overcome chemoresistance in colorectal cancer patients.
Detailed Summary
Colorectal cancer (CRC) is one of the leading causes of cancer-related death worldwide, and 5-fluorouracil (5-FU) remains the backbone of first-line chemotherapy for advanced or unresectable disease. A persistent clinical problem is that many tumors develop resistance to 5-FU, dramatically worsening patient outcomes. Dysregulated autophagy — a cellular self-digestion process — has been proposed as a key driver of this resistance, but the upstream regulators that control autophagy in drug-resistant CRC were poorly understood. This study set out to identify transcription factors that coordinate chemoresistance by modulating autophagy.
Using RNA sequencing of 5-FU-resistant versus parental CRC cell lines, the researchers found that HMBOX1 (homeobox containing 1) expression was significantly downregulated in resistant cells. Analysis of TCGA datasets and patient tumor cohorts confirmed that low HMBOX1 expression correlated with poor overall survival and worse progression-free survival. Single-cell RNA sequencing of clinical CRC tissues further showed that HMBOX1 levels were inversely associated with markers of active autophagy and 5-FU resistance, while positive correlation with HACE1 expression was also validated by multiplexed immunohistochemistry in patient biopsies.
Mechanistically, the team performed mass-spectrometry-based proteomics and RNA sequencing to map HMBOX1's downstream effectors. They found that HMBOX1 acts as a transcription factor directly promoting HACE1 (HECT domain and ankyrin repeat containing E3 ubiquitin protein ligase 1) gene expression through chromatin immunoprecipitation (ChIP)-confirmed binding to the HACE1 promoter. HACE1, in turn, catalyzes K63-linked ubiquitination of ATG5 — a critical autophagy-initiation protein — targeting it for proteasome-mediated degradation. Rescue experiments using ATG5 overexpression or HACE1 knockdown restored autophagy and 5-FU resistance even when HMBOX1 was overexpressed, confirming the HMBOX1→HACE1→ATG5 axis as a linear pathway.
Functionally, restoring HMBOX1 expression in resistant CRC cell lines (HCT116-R and SW480-R) markedly increased 5-FU sensitivity, elevated markers of apoptosis including cleaved caspase-3 and cleaved PARP, and reduced autophagic flux measured by LC3-II/LC3-I ratios and p62 accumulation. Treatment with the autophagy inhibitor 3-methyladenine (3-MA) phenocopied HMBOX1 overexpression, while autophagy induction with rapamycin reversed the sensitizing effects. In vivo xenograft mouse models confirmed that tumors expressing HMBOX1 showed significantly reduced growth under 5-FU treatment compared to controls, with lower ATG5 protein levels and reduced autophagy markers in tumor tissue.
The study also addressed a clinically actionable question: can the HMBOX1 pathway be leveraged therapeutically? The authors note that HMBOX1 downregulation is associated with copy number variation and potentially epigenetic silencing, suggesting that restoration strategies could be explored. While this is a preclinical study with no current clinical translation, the identification of the HMBOX1-HACE1-ATG5 axis provides a mechanistically coherent target for combination strategies pairing 5-FU with autophagy inhibitors or agents that restore HMBOX1 function. Limitations include reliance on cell line models of resistance and the absence of patient-derived organoid or clinical intervention data.
Key Findings
- HMBOX1 was significantly downregulated in 5-FU-resistant CRC cell lines and resistant patient tissues, with low HMBOX1 expression correlating with poor overall survival and progression-free survival in TCGA cohort analysis
- HMBOX1 overexpression reduced ATG5 protein levels and autophagic flux (LC3-II/LC3-I ratio) in resistant CRC cells, restoring sensitivity to 5-FU
- ChIP assays confirmed direct HMBOX1 binding to the HACE1 promoter, driving HACE1 transcription and subsequent K63-ubiquitination of ATG5 for proteasome-mediated degradation
- Knockdown of HACE1 or overexpression of ATG5 reversed HMBOX1-mediated resensitization to 5-FU, confirming the linear HMBOX1→HACE1→ATG5→autophagy axis
- In vivo xenograft experiments showed HMBOX1-expressing tumors exhibited significantly reduced growth under 5-FU treatment with lower ATG5 and LC3-II levels compared to controls
- Single-cell RNA sequencing and multiplexed IHC of clinical CRC tissues confirmed HMBOX1 positively correlated with HACE1 and inversely correlated with autophagy activity in patient tumors
- Autophagy inhibition with 3-MA phenocopied HMBOX1 overexpression effects, while rapamycin-induced autophagy negated HMBOX1-mediated chemosensitization
Methodology
The study used 5-FU-resistant CRC cell lines (HCT116-R, SW480-R) and their parental counterparts, validated in xenograft mouse models and clinical CRC tissue cohorts. Mechanistic dissection employed RNA sequencing, LC-MS/MS proteomics, co-immunoprecipitation, chromatin immunoprecipitation, cycloheximide chase assays, and ubiquitination assays. Clinical data were drawn from TCGA COAD datasets and single-cell RNA sequencing of patient tumor biopsies, with protein expression validated by multiplexed IHC. Statistical analyses included Kaplan-Meier survival curves with log-rank tests and standard in vitro cell viability (CCK8) and apoptosis assays.
Study Limitations
The study is entirely preclinical, relying on established resistant cell lines and xenograft models rather than patient-derived organoids or clinical trials, which limits direct translational applicability. The mechanism by which HMBOX1 itself is downregulated in resistant CRC (whether epigenetic, transcriptional, or copy-number driven) is not fully resolved. No conflicts of interest were declared by the authors.
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