5-fluorouracil: mechanisms of action (or resistance) in p53-proficient and -deficient colorectal cancer

Treatment of these models with clinically relevant doses of chemotherapy revealed that, in contrast to Oxaliplatin, 5- FU induced cell death is largely p53-independent. Interestingly, 5-FU treatment resulted in a dramatic increase in S-phase in p53-deficient cells, which coincides with increased DNA damage. This effect is not observed in p53-proficient cells due to the cell cycle inhibitory effects of p21.

Remarkably, we found that DNA damage and S-phase arrest are accompanied by a significant increase in the expression of the immune-regulator Programmed death-ligand 1 (PD-L1).<br><br>Integrative analysis of results of a genome-wide 5-FU GeCKO CRISPR screen in HCT116 p53 isogenic models, identified and validated a number of clinically tractable genes, which influence the sensitivity to 5-FU. These genes include the Caspase-8 inhibitor CFLARtFLIP (Fas-associated death domain (FADD)-like IL1 (3-converting enzyme inhibitory protein), which we identified as a negative regulator of both p53-dependent and -independent death.

Interestingly, FLIP expression can be attenuated by Entinostat (Class I HDAC inhibitor). Importantly, we observed that the combination of Entinostat with 5-FU significantly impacted long-term survival in a p53-independent manner, while its combination with Oxaliplatin enhanced p53-dependent effects on cell death and long-term survival.<br><br>In conclusion, our investigation supports two novel combination treatments for 5-FU-based therapies correlated to p53-status.

First, in p53-deficient patients, we propose the use of 5-FU-based treatments (5-FU, FOLFOX) in combination with anti-PD-L1 treatment. Second, in p53-proficient patients, we propose the combination of 5-FU or FOLFOX with class I Entinostat. Further investigations are necessary to support these proposals, particularly in light of the recently described transcriptomic sub-groups of CRC and emergent immunomodulatory effects of Entinostat.