Loss of tumor intrinsic PD-L1 confers resistance to drug-induced apoptosis in human colon cancer
Abstract
Colorectal cancer (CRC) with the BRAF V600E mutation is associated with microsatellite instability (MSI), which predicts a favorable response to immune checkpoint inhibitors. Our analysis of TCGA RNA-seq datasets revealed that BRAFV600E-mutant tumors exhibited significantly higher levels of Programmed Death Ligand 1 (PD-L1) mRNA compared to non-mutated BRAF CRCs. Additionally, MSI-high (MSI-H) tumors showed elevated PD-L1 expression compared to microsatellite stable (MSS) CRCs. Inhibition of the MEK/ERK pathway using cobimetinib or the CDK inhibitor dinaciclib reduced PD-L1 expression in BRAF-mutant tumors, alongside decreased c-JUN and YAP expression, which contributed to further suppression of PD-L1. PD-L1 mRNA expression in human colon cancers was found to correlate significantly with YAP levels. Furthermore, the deletion of PD-L1 impaired tumor cell growth, as shown by clonogenic assays.
We also investigated PD-L1’s role in mediating chemosensitivity. In CRC cells, knockout of PD-L1 attenuated the induction of DNA double-strand breaks (pH2AX) and caspase-3 cleavage in response to 5-fluorouracil (5-FU) and paclitaxel, compared to parental CRC cells. These findings were confirmed in PD-L1 knockout MC38 murine CRC cells, where re-expression of wild-type PD-L1 restored DNA damage and apoptosis. Clonogenic assays and flow cytometry further showed that loss of PD-L1 reduced DNA damage and apoptosis induced by various anticancer agents, with these effects reversed upon restoration of wild-type PD-L1. Mechanistically, PD-L1 knockout led to reduced chemosensitivity, associated with decreased p-AKT levels and lower BH3-only proteins BIM and BIK, but not STAT3. However, STAT3 was found to play a significant role in melanoma, highlighting the cancer-specific variability.
In summary, BRAFV600E mutation enhances PD-L1 expression, which is driven by c-JUN and YAP and plays a role in facilitating chemotherapy-induced apoptosis. Our results suggest that PD-L1 acts as a regulator of chemotherapy-induced apoptosis, and its deletion or suppression confers chemoresistance. These findings broaden our GDC-0973 understanding of PD-L1’s functions, extending its role beyond immune mechanisms and suggesting its potential as a biomarker for chemotherapy response.