Tumor‐associated macrophages (TAMs) are recognized as antitumor suppressors, but how TAMs behave in the hypoxic environment of hepatocellular carcinoma (HCC) remains unclear. Here, we demonstrated that hypoxia inducible factor 1α induced increased expression of triggering receptor expressed on myeloid cells‐1 (TREM‐1) in TAMs, resulting in immunosuppression. Specifically, TREM‐1‐positive (TREM‐1+) TAMs abundant at advanced stages of HCC progression indirectly impaired the cytotoxic functions of CD8+ T cells and induced CD8+ T‐cells apoptosis. Biological and functional assays showed that TREM‐1+ TAMs had higher expression of programmed cell death ligand 1 (PD‐L1) under hypoxic environment. However, TREM‐1+ TAMs could abrogate spontaneous and PD‐L1‐blockade‐mediated antitumor effects in vivo, suggesting that TREM‐1+ TAM‐induced immunosuppression was dependent on a pathway separate from PD‐L1/programmed cell death 1 axis. Moreover, TREM‐1+ TAM‐associated regulatory T cells (Tregs) were crucial for HCC resistance to anti‐PD‐L1 therapy. Mechanistically, TREM‐1+ TAMs elevated chemokine (C‐C motif) ligand 20 expression through the extracellular signal‐regulated kinase/NF‐κβ pathway in response to hypoxia and tumor metabolites leading to CCR6+Foxp3+ Treg accumulation. Blocking the TREM‐1 pathway could significantly inhibit tumor progression, reduce CCR6+Foxp3+ Treg recruitment, and improve the therapeutic efficacy of PD‐L1 blockade. Thus, these data demonstrated that CCR6+Foxp3+ Treg recruitment was crucial for TREM‐1+ TAM‐mediated anti‐PD‐L1 resistance and immunosuppression in hypoxic tumor environment. Conclusion: This study highlighted that the hypoxic environment initiated the onset of tumor immunosuppression through TREM‐1+ TAMs attracting CCR6+Foxp3+ Tregs, and TREM‐1+ TAMs endowed HCC with anti‐PD‐L1 therapy resistance.

Background Multi-drug resistance is the major cause of chemotherapy failure in hepatocellular carcinoma (HCC). YAP, a critical effector of the Hippo pathway, has been shown to contribute to the progression, metastasis and invasion of cancers. However, the potential role of YAP in mediating drug resistance remains obscure. Methods RT-qPCR and western blot were used to assess YAP expression in HCC cell lines. CCK-8 assays, flow cytometry, a xenograft tumour model, immunochemistry and GFP-mRFP-LC3 fusion proteins were utilized to evaluate the effect of YAP on multi-drug resistance, intracellular ROS production and the autophagy of HCC cells in vitro and in vivo. Autophagy inhibitor and rescue experiments were carried out to elucidate the mechanism by which YAP promotes chemoresistance in HCC cells. Results We found that BEL/FU, a typical HCC cell line with chemoresistance, exhibited overexpression of YAP. Moreover, the inhibition of YAP by shRNA or verteporfin conferred the sensitivity of BEL/FU cells to chemotherapeutic agents through autophagy-related cell death in vitro and in vivo. Mechanistically, YAP silencing significantly enhanced autophagic flux by increasing RAC1-driven ROS, which contributed to the inactivation of mTOR in HCC cells. In addition, the antagonist of autophagy reversed the enhanced effect of YAP silencing on cell death under treatment with chemotherapeutic agents. Conclusion Our findings suggested that YAP upregulation endowed HCC cells with multi-drug resistance via the RAC1-ROS-mTOR pathway, resulting in the repression of autophagy-related cell death. The blockade of YAP may serve as a promising novel therapeutic strategy for overcoming chemoresistance in HCC. Electronic supplementary material The online version of this article (10.1186/s12935-019-0898-7) contains supplementary material, which is available to authorized users.

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