Involvement of inflammation and its related microRNAs in hepatocellular carcinoma

Abstract

Hepatocellular carcinoma (HCC) is the fifth most commonly diagnosed type of cancer. The tumor inflammatory microenvironment regulates almost every step towards liver tumorigenesis and subsequent progression, and regulation of the inflammation-related signaling pathways, cytokines, chemokines and non-coding RNAs influences the proliferation, migration and metastasis of liver tumor cells. Inflammation fine-tunes the cancer microenvironment to favor epithelial-mesenchymal transition, in which cancer stem cells maintain tumorigenic potential. Emerging evidence points to inflammation-related microRNAs as crucial molecules to integrate the complex cellular and molecular crosstalk during HCC progression. Thus understanding the mechanisms by which inflammation regulates microRNAs might provide novel and admissible strategies for preventing, diagnosing and treating HCC. In this review, we will update three hypotheses of hepatocarcinogenesis and elaborate the most predominant inflammation signaling pathways, i.e. IL-6/STAT3 and NF-κB. We also try to summarize the crucial tumor-promoting and tumor-suppressing microRNAs and detail how they regulate HCC initiation and progression and collaborate with other critical modulators in this review.

Keywords: cancer stem cells; cell signaling; epithelial-mesenchymal transition; inflammation; microRNAs.

Figure 1. The dominating interconnected signaling pathways and transcriptional network that promote EMT during tumorigenesis

TGF-β signaling pathway is initiated by binding of TGF-β ligands to TβRII and TβRI. The Smad pathway is mediated by phosphorylation of TβRI by TβRII and subsequent activation of Smad2/3. Activated Smad2/3 form complexes with Smad4 and translocate into the nucleus. The Non-Smad pathway takes place through multiple intracellular signaling cascades such as Par6-Smurf1-RhoA, RAS-RAF-MEK-ERK and PI3K/Akt pathway. Other signaling pathways, such as Wnt, Notch and HIF-1α, are also involved in EMT. Wnt signaling promotes EMT by inhibiting GSK3β to stabilize β-catenin, which translocates to the nucleus with LEF/TCF. The interaction between Delta/Jagged and its receptor Notch induces the release of Notch ICD. Hypoxia in the tumor microenvironment promotes EMT through HIF-1α and crosstalks with Wnt and Notch pathways. Activation of above pathways induces the expression of master regulators of EMT including Snail1/2, Twist and ZEB1/2 families, which can initiate a coordinated transcriptional network leading to suppression of epithelial marker and up-regulation of mesenchymal marker expressions.

Figure 2. The roles of TAMs in the pro-inflammatory microenvironment

Macrophages can be classified into two main classes according to their phenotypic polarization: M1 macrophages respond to IL-6, TNF-α, M-CSF, INFγ and LPS whereas they differentiate into M2 in response to TGF-β, VEGF, CCL2, IL-4, IL-10 and IL-13. M1 and M2 macrophages exert different functions. M1 macrophages with powerful antigen presentation potential can secrete IL-1, IL-6, IL-12 and TNF-α, and are able to exert cytotoxic activity on microbes and tumor cells. M2 macrophages can secrete VEGF, MMPs, IL-10 and TGF-β and promote angiogenesis, tissue remodeling, tumor progression, invasion and metastasis as well as suppression of anti-tumor immune response. TAMs can be recruited to tumor lesions and interact with both stromal and tumor cells within the tumor microenvironment, which will amplify the inflammation and accelerate tumor progression.

Figure 3. The role of IL-6/STAT3 signaling pathway and interactions with other pathways in hepatocarcinogenesis

IL-6 secreted by Kupffer cells or hepatocytes binds to IL-6Rα and induces the homodimerization of IL-6Rα with gp130, activating downstream signaling pathways such as JAK/STAT3, PI3K/Akt and MAPK pathways, which promote proliferation and survival of cells, inflammatory amplification and tumor invasion and metastasis.

Figure 4. The activation of canonical and non-canonical NF-κB signaling pathways in the liver tumorigenesis

In the canonical NF-κB pathway, IL-1, LPS or TNF-α activate IL-1R, TLRs and TNFR respectively, leading to the activation of the IKK complex which can phosphorylate IκB-α. This phosphorylation results in the polyubiquitination and subsequent proteasomal degradation of IκB-α. The released NF-κB p50-p65 dimers then translocate into nucleus and activate target gene transcription. In the non-canonical pathway, activation of CD40, LTβR, etc. leads to activation of IKK-α by NIK. IKK-α homodimers can then phosphorylate p100 subunit, which is a prerequisite for the polyubiquitination of p100 and its proteasomal processing to p52. Then RelB-p52 heterodimers translocate into nucleus and activate transcription of target genes.

Figure 5. The critical crosstalks between important transcriptional factors, oncogenic and tumor suppressive proteins, and inflammation-related miRNAs that regulate key processes during HCC initiation, progression and metastasis

The core associated proteins and miRNAs can constitute positive or negative feedback circuits to sustain the malignant state when there is an exogenous stimulus triggering the malignant transformation, and even when the stimulus is removed.

Figure 6. A hypothetical illustration delineating the connection between activation of inflammatory pathways, miRNAs and liver tumorigenesis

Once extrinsic stimuli such as HBV/HCV, alcohol and DEN damage the liver, Kupffer cells can be activated and produce several inflammatory cytokines such as IL-6 and TGF-β1. On one hand, IL-6 can stimulate LPCs residing in the canal of hering to proliferate to restore the injured liver; however, if gene mutations happen to proliferating LPCs, they will have the potential to develop to CSCs. On the other hand, TGF-β1 can act on HSCs and activated HSCs proliferate and generate ECM to reconstitute the liver and promote hepatic fibrosis if the dynamic balance of ECM synthesis and decomposition is disrupted. Meanwhile, TGF-β1 can also stimulate hepatocytes to respond to either cell death or proliferation signals under different conditions. Several miRNAs such as miR-122, miR-155 and miR-21 could join to regulate correlated pathologic processes. All the cytokines, miRNAs and other inflammatory mediators together generate an inflammatory microenvironment which will amplify the oncogenic mutations and self-reinforce the pro-inflammatory signals, finally leading to the irreversible liver tumorigenesis.