Modulating the Crosstalk between the Tumor and Its Microenvironment Using RNA Interference: A Treatment Strategy for Hepatocellular Carcinoma

Abstract

Hepatocellular carcinoma (HCC) is the most common primary liver malignancy with one of the highest mortality rates among solid cancers. It develops almost exclusively in the background of chronic liver inflammation, which can be caused by viral hepatitis, chronic alcohol consumption or an unhealthy diet. Chronic inflammation deregulates the innate and adaptive immune responses that contribute to the proliferation, survival and migration of tumor cells. The continuous communication between the tumor and its microenvironment components serves as the overriding force of the tumor against the body's defenses. The importance of this crosstalk between the tumor microenvironment and immune cells in the process of hepatocarcinogenesis has been shown, and therapeutic strategies modulating this communication have improved the outcomes of patients with liver cancer. To target this communication, an RNA interference (RNAi)-based approach can be used, an innovative and promising strategy that can disrupt the crosstalk at the transcriptomic level. Moreover, RNAi offers the advantage of specificity in comparison to the treatments currently used for HCC in clinics. In this review, we will provide the recent data pertaining to the modulation of a tumor and its microenvironment by using RNAi and its potential for therapeutic intervention in HCC.

Keywords: HCC; RNAi; miRNA; shRNA; siRNA; tumor microenvironment.

Figure 1

Scheme of three different RNA interference (RNAi) pathways: micro(mi)RNA, short hairpin (sh)RNA and small interfering (si)RNA. shRNA and miRNA mediated silencing are based on a modification of the genomic content within the nucleus whereas siRNAs act directly in the cytoplasm of cells and do not require nuclear import. shRNAs and miRNAs are introduced into the cell in the form of DNA, and it is essential that they are transported into the nucleus to be transcribed into dsRNAs with a hairpin-like structure and are thus termed pri-shRNAs and pri-miRNAs, respectively, which are further cleaved in the nucleus by the DROSHA enzyme. The resulting transcripts are termed pre-shRNAs and pre-miRNAs, respectively, and are exported to the cytoplasm via Exportin 5. siRNAs, on the other hand, are introduced in the form of dsRNA sequences and are not delimited by a nuclear transport step; they are cleaved by DICER enzyme to attain their final form. In the cytoplasm, the three pathways converge as they are all loaded to the RNA induced silencing (RISC) complex. After that, they diverge again in their mechanism of action to silence a target gene: a miRNA-RISC complex inhibits translation, whereas sh/siRNA-RISC complexes bind to mRNA sequencesresulting in their degradation.

Figure 2

Tumor microenvironment in hepatocellular carcinoma (HCC). CAFs: Cancer associated fibroblasts; TAM: tumor associated macrophages; HSC: hepatic stellate cell; T-regs: T-regulatory cells; CTLs: cytotoxic T-Lymphocytes; TANs: tumor associated neutrophils; DC: dendritic cell; ECM: extracellular matrix; siRNA: small interfering ribonucleic acid. The cellular and non-cellular components of the tumor microenvironment of HCC are modulated by the tumor–tumor microenvironment communication and help the cancer to thrive by reciprocal communication. siRNAs can serve as a communication barrier by targeting the effector molecules exchanged. CAFs arise from resident cells in the tumor microenvironment and support tumor growth, survival and metastasis. TAMs are a macrophage subset that exhibits a tumor-supporting role. HSCs are modulated by the tumor to sustain its invasiveness and growth. T-regs depotentiate the immune response against the tumor. CTLs aim to combat the tumor but are often depotentiated. TANs recruit TAMs and T-regs to the tumor’s vicinity. DCs impair the T-cell response against the tumor. The ECM acts as a mediator of communication between the various cells. Exosomes shuttle effector molecules (proteins and/or nucleic acids) between the tumor and the cell of origin and vice versa.