Myeloid-derived suppressor cells (MDSCs) comprise a heterogeneous population of immature myeloid cells that suppress anti-tumor immune responses. The accumulation of CD33+HLA-DR- MDSCs correlates with tumor stage and metastatic burden in various cancers including hepatocellular carcinoma (HCC) (1). Cancer cells can secrete a variety of cytokines and chemokines to facilitate the peripheral expansion and tumor infiltration of MDSCs. However, the cancer cell-specific signaling cascades that promote MDSC expansion remain poorly understood. We previously demonstrated that cell cycle-related kinase (CCRK) acts as a new oncogenic signaling hub in hepatocellular proliferation and transformation (2-4). To investigate whether CCRK regulates tumor microenvironment in HCC, we determined the role of CCRK signaling in the crosstalk between cancer cells and MDSCs.

Our results showed that treatment of human peripheral blood mononuclear cells (PBMCs) with conditional medium of CCRK-over-expressing immortalized hepatocytes and HCC cells resulted in expansion of CD33+ HLA-DR-MDSCs with increased inhibitory effects on T cell responses. In contrast, knockdown of CCRK in hepatic cells downregulated MDSCs. Cytokine profiling analysis revealed that CCRK significantly induced hepatocellular interferon-6 (IL-6) expression and release, which mediated MDSC expansion as shown by IL-6 interfering experiments. Mechanistic studies demonstrated that CCRK triggered nuclear factor-kappa B (NF-κB) signaling in an enhancer of zeste homolog 2 (EZH2)-dependent manner. Simultaneously, the phosphorylation of p65 by CCRK facilitated the co-occupancy of IL-6 promoter by NF-κB-EZH2 complex for transcriptional activation. Using a Hepa1-6 orthotopic HCC model in immune-competent C57BL/6J mice, we showed that knockdown of Ccrk significantly decreased hepatic tumorigenicity and the levels of circulating and tumor-infiltrating MDSCs as well as related T cell responses. Notably, adoptive transfer of MDSCs rescued the effects of Ccrk knockdown. In a complementary experiment, we found that MDSC depletion by anti-Gr-1 antibody significantly reduced CCRK-induced tumorigenicity. Furthermore, transgenic over-expression of CCRK in mouse liver led to activation of EZH2/NF-κB signaling and elevated circulating levels of MDSC. As we also showed concordant over-expression of CCRK, IL-6 and MDSC markers (CD33, Arg-I) in human HCCs, our results uncover CCRK to be a critical immune regulator to promote MDSC, thereby providing a new mechanistic link between aberrant oncogenic signaling and tumor evasion for therapeutic exploitation.

Acknowledgement: This project was supported by the University Grants Committee through the Collaborative Research Fund C4017-14G.

References

1) Marigo I., et al., 2010. Immunity.

2) Feng H., et al., 2011. J Clin Invest.

3) Yu Z., et al., 2014. Gut.

4) Feng H., et al., 2015. J Hepatol.

Citation Format: Jingying Zhou, Man Liu, Hanyong Sun, Zhiwei Chen, Alfred Sze Lok Cheng. A novel kinase connection between oncogenic signaling and myeloid-derived suppressor cell functions in tumor evasion. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 529.