Loss of tumor intrinsic PD-L1 confers resistance to drug-induced apoptosis in human colon cancer

Abstract

Colorectal cancer (CRC) with BRAF (V600E) is associated with microsatellite instability (MSI) that predicts response to immune checkpoint inhibitors. We demonstrated the interrogation of TCGA RNA-seq human datasets revealed that BRAFV600E tumors had significantly higher Programmed Death Ligand 1 (PD-L1) mRNA compared to non-mutated BRAF CRCs. Also, MSI-H tumors were evaluated as higher PD-L1 than MSS CRCs. Inhibition of MEK/ERK by cobimetinib or CDK inhibitor dinaciclib was shown to attenuate mutant BRAF-induced PD-L1 coincident with reduced c-JUN and YAP expression whose combined knockdown reduced PD-L1. Using TCGA datasets, PD-L1 mRNA expression in human colon cancers was significantly associated with YAP expression. The deletion of PD-L1 can reduce tumor cell growth shown by clonogenic assay. Analysis of the role of PD-L1 as a mediator of chemosensitivity was then performed. Knockout of PD-L1 was shown to attenuate the induction of DNA double-strand breaks (pH2AX) and caspase-3 cleavage by 5fluorouracil (5-FU) and paclitaxel compared to parental CRC cells. Results were confirmed in PD-L1 knockout MC38 murine CRC cells where re-expression of wild-type PD-L1 promoted DNA damage and apoptosis. We also performed the clonogenic assay and flow cytometry to prove that loss of PD-L1 attenuated DNA damage and apoptosis induced by diverse anti-cancer drugs that could be reversed by restoration of wild-type PD-L1. Mechanistically, knockout of PD-L1 reduced chemosensitivity in association with reductions in p-AKT and in BH3-only proteins BIM and BIK, rather than STAT3 in CRC cells. However, STAT3 had a significant role in melanoma, which shows the heterogeneity of cancers. In summary, BRAFV600E can upregulate PD-L1 expression that was induced by c-jun and YAP to enhance chemotherapy-induced apoptosis. Together, we demonstrate a potential role for PD-L1 as a regulator of chemotherapy-induced apoptosis whose deletion or suppression confers chemoresistance. These findings expand the understanding of PD-L1 functions to include nonimmune mechanisms and suggest the potential use of PD-L1 as a biomarker of response to cytotoxic chemotherapy.

Colorectal cancer biomarkers and their impact on the clinical practice

“…A recent study suggested that PD-L1 expression has a broader role as a potential biomarker for predicting response to immune treatment. [97] PD-L1 expression was significantly correlated with lymphatic metastasis, tumor diameter and differentiation, vascular invasion, and could act as an independent poor prognostic factor in CRC. [98] Nonetheless, the correlation between PD-L1 expression and clinicopathological features and prognosis of CRC is still controversial.…”

Immune checkpoint blockade therapy for BRAF mutant metastatic colorectal cancer: the efficacy, new strategies, and potential biomarkers

“…Additionally, a recent study has examined the relationship between PD-L1 expression and BRAF mutation [ 100 ]. Surprisingly, a high level of PD-L1 expression in MSS colorectal cancer cell lines (DiFi) was induced by BRAF V600E [ 101 ], suggesting that PD-L1 expression may not only exist in MSI-H BRAF -mutated CRC.…”

“…Feng et al manifested that oncogenic BRAF V600E in colon cancers can transcriptionally upregulate intrinsic PD-L1 expression, which enhances chemotherapy-induced apoptosis by inducing BIM and BIK proteins [ 102 ]. Subsequently, Feng et al proved the roles of c-JUN and YAP in inducing PD-L1 expression in BRAF V600E colon cancers [ 101 ]. Although PD-L1 expression fails to be a predictive biomarker for immunotherapy in colorectal cancer, the immune-independent function broadens our horizons regarding tumor therapy.…”