Blocking PCSK9 suppresses hepatocellular carcinoma immune escape by decreasing FLI1-mediated SPP1 and PD-L1 expression

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Background Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a newly identified immunosuppressive regulator, but its mechanism of suppressing antitumor immunity remains ambiguous. This study aims to uncover the underlying mechanism by which PCSK9 promotes hepatocellular carcinoma (HCC) immune escape and to explore potential intervention strategies. Methods Co-culture assay assessed the cytotoxicity of CD8 + T cells against PCSK9-knockout HCC cells. Hepa1-6, H22, and HepG2 cells were used to establish HCC mouse models. Tumor microenvironment changes were evaluated using flow cytometry and single-cell RNA sequencing. Additionally, we developed a CRISPR adenine base editing (ABE) base editor and screened small molecule inhibitors for PCSK9 inhibition in HCC treatment. Results We found that PCSK9 was highly expressed and correlated with poor survival in patients with HCC. While PCSK9 deficiency did not affect HCC growth in vitro, it significantly enhanced CD8 + T cell-mediated selective killing in vitro and in vivo. This selective killing of PCSK9-deficient HCC cells could not be explained by existing theories related to major histocompatibility complex-I and T-cell receptor (TCR) degradation. Instead, our study revealed that PCSK9 knockout inhibited the expression of secreted phosphoprotein 1 (SPP1) and programmed death-ligand 1 (PD-L1) in HCC cells, and identified friend leukemia virus integration 1 (FLI1) as their co-transcription factor. Overexpression of FLI1 reversed the PCSK9 knockout-induced downregulation of SPP1 and PD-L1, thereby promoting HCC immune escape. Furthermore, PCSK9 upregulated FLI1 expression through the neurogenic locus notch homolog protein 3 (NOTCH3) pathway. Additionally, we designed an all-in-one ABE base editor with thyroxine-binding globulin promoter (ABE-TBG-PCSK9) to knock down PCSK9 and identified parecoxib as a small molecule inhibitor. We confirmed both approaches enhanced CD8 + T-cell antitumor activity, significantly inhibiting HCC tumor growth and prolonging mouse survival. Conclusions PCSK9 promoted HCC immune escape by upregulating SPP1 and PD-L1 via NOTCH3/FLI1 signaling. CRISPR ABE-mediated PCSK9 deficiency and PCSK9 inhibitor parecoxib may serve as effective strategies to inhibit HCC.

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