Background: HCC is the most common primary malignant type of liver cancer. Median survival for
patients with advanced tumors is less than one year. Novel treatment options are urgently
needed. The cytosolic helicase retinoic acid-inducible gene I (RIG-I) is an immune
receptor for viral 5'-triphosphat-RNA (ppp-RNA) and its activation leads to innate
and adaptive immunity via type I IFN and proinflammatory cytokines. In addition, it
promotes the intrinsic pathway of apoptosis in tumor cells. This project focuses on
the development of 5'ppp-RNAs targeting RIG-I for HCC therapy.
Methods: A synthetic 5'ppp-RNA was generated via in vitro transcription and used to analyze functional RIG-I expression in human (Hep3B) and murine (Hepa1 – 6) HCC cells. Therapeutic efficacy
was evaluated in vivo in the orthotopic Hepa1 – 6 mouse model of HCC. Seven days after tumor induction
mice were treated intravenously with 5'ppp-RNA complexed to polyethylenimine. 24h
later mice were sacrificed, tumors removed and size measured. Tumor cell death was
assessed by TUNNEL staining and cytokine expression by qRT-PCR. In another experiment
survival of RNA-treated mice was monitored.
Results: Treatment of human and murine HCC cells with ppp-RNA induced phosphorylation of IRF3,
production of IFN-β and IFN-induced genes, such as CXCL10, indicative of intact RIG-I
signaling. In addition, RIG-I activation led to profound tumor cell death. A single
treatment of mice with orthotopic HCC with ppp-RNA significantly reduced tumor size
in comparison to control RNA, induced type I IFN production in the tumor tissue and
led to massive tumor cell death. Treatment of mice with repeated ppp-RNA injections
significantly prolonged survival.
Conclusion: We could demonstrate that the RIG-I pathway is functional in HCC cells. RIG-I activation
leads to type I IFN production and massive tumor cell death both in vitro and in vivo. RIG-I-based immunotherapy holds promise for HCC therapy deserving further evaluation.
