Role of chemokine pathways in hepatobiliary cancer

Highlights

• Chemokines and their corresponding receptors regulate critical aspects in the pathogenesis of hepatobiliary cancer.

• Chemokines modulate the hepatic microenvironment toward an inflammatory, fibrotic, angiogenic and thus pre-neoplastic milieu.

• Liver neoplasms provoke pro- and anti-tumor immune responses regulated through differential activation of chemokine pathways.

• Malignant hepato-/cholangiocytes respond to distinct chemokine signals, which influence their proliferation and invasiveness.

• Pharmaceutical targeting of chemokine pathways is a promising approach to treat or even prevent hepatobiliary cancer.

Abstract

Persistent hepatic inflammation resulting from hepatitis B or C virus infections (HBV or HCV, respectively), obesity-associated non-alcoholic steatohepatitis (NASH) or alcohol abuse is a hallmark feature of chronic liver diseases and appears to be an essential prerequisite of hepatocarcinogenesis. The inflammatory processes in the liver are regulated by various chemokines, which orchestrate the interaction between parenchymal liver cells, Kupffer cells (resident macrophages), hepatic stellate cells (HSC), endothelial cells, and infiltrating immune cells. In consequence, these cellular interactions result in the re-modeling of the hepatic microenvironment toward a pro-inflammatory, pro-fibrotic, pro-angiogenic and thus pre-neoplastic milieu. Once developed, liver neoplasms provoke pro- and anti-tumor immune responses that are also critically regulated through differential activation of chemokine pathways. With respect to hepatobiliary cancers, including hepatocellular carcinoma (HCC), gallbladder cancer and cholangiocellular carcinoma (cholangiocarcinoma), together belonging to the highest causes of cancer-related deaths worldwide, this review article will give an overview of chemokine pathways involved in both the establishment of a pro-tumorigenic microenvironment as well as the development and progression of hepatobiliary cancer. Pharmaceutical targeting of chemokine pathways is a promising approach to treat or even prevent hepatobiliary cancer.

Introduction

Hepatobiliary cancers, including hepatocellular carcinoma (HCC), gallbladder cancer as well as intra- and extrahepatic cholangiocarcinomas, are among the highest cause of cancer-related deaths worldwide [1]. In 2014, approximately 33,190 persons in the United States were estimated to be diagnosed with liver or intrahepatic bile duct cancer and 10,650 with gallbladder cancer or other biliary tract cancer. Tragically, approximately 23,000 deaths from the former cancers and 3630 deaths from the latter cancers were estimated to occur in 2014 [2]. Treatment options include surgical tumor resection, organ transplantation, and in particular for intrahepatic hepatobiliary cancers, transarterial chemoembolization (TACE), radiofrequency thermal ablation (RFTA) and percutaneous ethanol injection (PEI) [3]. However, despite these different therapeutic options, the overall 5-year survival rate does not exceed 14% [4]. Regarding pharmacological treatment options, the multi-targeted tyrosine kinase inhibitor sorafenib has been shown to prolong the survival of patients with advanced HCC, albeit this drug is associated with various side effects [5] and restricted to patients with fairly preserved liver function [6]. In consequence, novel systemic pharmaceutical treatments are required for the treatment of hepatobiliary cancer. Due to a close association between hepatic inflammation and the pathogenesis of primary liver malignancies, it seems to be obligatory to take chemokine pathways that orchestrate not only the immunological responses upon liver injury, but that also promote the initiation and progression of malignancies, into account when developing novel therapeutic approaches in order to treat hepatobiliary cancer.

Chemokines are chemotactic cytokines specialized in regulating the migration of immune cells into damaged or diseased organs in response to pro-inflammatory stimuli. Together with their corresponding receptors, chemokines promote the extravasation of immune cells from the circulation into injured tissue and regulate the migration of immune cells through the tissue. Chemokines are small molecular weight proteins (approximately 8–13 kDa) and are classified into four groups according to the presence of cysteine motifs near their amino-termini: CC chemokines containing two adjacent cysteine (C) residues, CXC chemokines containing two cysteines separated by one amino acid (X), CX3C chemokines containing three amino acids separating their cysteine residues, and C chemokines containing only one cysteine residue near the NH₂ terminus [7]. To date, about 50 different chemokines and 20 different chemokine receptors have been identified [8].

Chemokine receptors located on the surface of leukocytes are seven-transmembrane spanning G protein-coupled proteins, whose Gα1 and Gß-γ subunits dissociate upon receptor binding leading to the activation of down-stream pathways [8], [9]. The latter include the activation of the phosphatidylinositol 3-kinase (PI3K), cellular calcium influx and conformational changes of leukocytes' integrins facilitating the adhesion of leukocytes to endothelial cells as well as their extravasation and migration into the interstitial space along a chemotactic gradient toward the site of injury/inflammation [10], [11]. Chemokines, which have been identified to play a key role in the development and progression of hepatobiliary cancer, and their corresponding receptors are listed in Table 1.

Whereas the production and release of inflammatory chemokines is specifically induced upon pro-inflammatory stimuli in order to attract leukocytes to the site of injury or infection, a broad variety of chemokines is constitutively expressed under homeostatic conditions and participates in the formation and accurate organization of lymphoid tissue. Inflammatory chemokines released upon tissue injury form a concentration gradient and attract leukocytes, which express the corresponding chemokine receptor(s), enabling them to migrate toward the highest concentration of the chemokines. Inflammatory chemokines can be released by several cell types including endothelial cells, fibroblasts and parenchymal cells upon injury, immune cells themselves (to orchestrate the interactions of subsets involved in the inflammatory response) and tumor cells, which can utilize chemokines as positive feedback mechanisms in an autocrine manner. These complex cellular mechanisms orchestrated by chemokines lead to the establishment of an inflammatory microenvironment. In the context of chronic liver diseases, the chemokine-mediated inflammation also induces the establishment of a pro-fibrotic and pro-angiogenic milieu, which significantly contributes to the initiation and progression of liver cancer [56]. It is striking that several parenchymal and non-parenchymal cell populations in the liver, such as hepatocytes, liver sinusoidal endothelial cells (LSEC), hepatic stellate cells (HSC) or Kupffer cells, readily release an array of chemokines in conditions of altered tissue homeostasis [8].

Persistently inflamed hepatic tissue is a hallmark condition promoting the development of hepatobiliary cancer, in particular for HCC. Common causes of chronic liver inflammation are hepatitis B or C virus (HBV or HCV, respectively) infections, obesity-associated non-alcoholic steatohepatitis (NASH) and alcohol abuse, and all of them dramatically increase the risk for developing HCCs. Conversely, the inhibition of hepatic inflammation and the restoration of physiological microenvironmental conditions reduce the risk for HCC development, as i.e. exemplified by reduced HCC incidences in HBV or HCV-infected patients who are successfully treated with antiviral therapy [57], [58], corroborating the assumption that chronic hepatic injury resulting in persistent hepatic inflammation rather than a specific carcinogen promotes a pro-tumorigenic microenvironment in livers [59]. Chronic hepatic injury and the continuous death of hepatocytes lead to a complex inflammatory response orchestrated by chemokines. In case of persistency, the continuous cell death of hepatocytes and infiltrating immune cells, the inflammatory signaling as well as the compensatory proliferation of hepatocytes facilitates genetic alteration of transformed and proliferating hepatocytes as well as the establishment of a pro-tumorigenic milieu, enabling the expansion of dysplastic and/or neoplastic parenchymal liver cells. In this context, oncogenes such as members of the RAS and MYC family have been identified as key mediators for remodeling the hepatic microenvironment toward a pro-inflammatory milieu resulting in the recruitment of leukocytes and lymphocytes, mediating pro- and anti-tumor immune responses [56].

This article focuses on chemokine pathways involved in both the initiation and the progression of hepatobiliary cancer and gives an overview of current insights into the link between chemokine-mediated hepatic inflammation, the liver disease-specific establishment of a pro-tumorigenic microenvironment and invasive cancer with respect to hepatocellular carcinoma (HCC), (intra- or extrahepatic) cholangiocarcinoma and gallbladder cancer (Fig. 1).