Portal hypertension and hepatocellular carcinoma: Des liaisons dangereuses…

Portal hypertension (PHT) and hepatocellular carcinoma (HCC) are major complication of cirrhosis which significantly contribute to morbidity and mortality. In this review, we aim to describe the consequences of both angiogenesis and inflammation in the pathogenesis of PHT and HCC, but also the difficulty to propose adapted treatment when PHT and HCC coexist in the same patients.


| INTRODUC TI ON
Portal hypertension (PHT) is a major complication of cirrhosis and is responsible for variceal bleeding, ascites and hepatorenal syndrome, which significantly contribute to morbidity and mortality. In the case of cirrhosis, PHT occurs secondary to an increase of intrahepatic vascular resistances due to sinusoidal alterations and the presence of fibrosis that cause distortion of the vascular architecture within the liver. The opening of portosystemic collateral vessels, and the formation of neovessels, related to vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF) production (so called angiogenic process), 1-3 as well as a subsequent increase of the splanchnic blood flow contribute and perpetuate PHT. Recently, bacterial translocation-mediated inflammation has also been identified as a major contributor to PHT occurrence. 4,5 In clinical practice, PHT is defined by an increase in portal pressure above 12 mm Hg or by a pressure difference between portal vein pressure and hepatic venous pressure higher than 5 mm Hg. Different methods to measure the pressure in the portal vein exist, but the most used technique in patients is the measurement of the hepatic venous pressure gradient (HVPG) through a transjugular approach. A HVPG higher than 10 mm Hg is associated with ascites and variceal occurrence, so-called clinically significant portal hypertension (CSPH) and a value higher than 12 mm Hg exposes the patient to a risk of variceal bleeding.
Hepatocellular carcinoma (HCC) is also a major complication of cirrhosis. Similar to PHT, an increased level of VEGF and chronic inflammation also contributes to HCC occurrence. 6 Moreover, patients with PHT present higher risk to develop HCC, and HCC through changes in the hepatic architecture and vascular invasion also contributes to PHT occurrence. 7,8 As PHT and HCC often coexist in the same patient, management of PHT and its related complications as well as HCC treatment appear more complex. Indeed, CSPH may hamper the access to curative treatment for HCC such as liver resection/ablation but also to locoregional therapies or systemic treatment in case of significant ascites. 6 Ascites and oesophageal varices have been identified as predictive factors of death in patients with HCC independently of the severity of the underlying liver disease and the HCC stage. 9,10 Conversely, the presence of HCC is also independently associated with poor prognosis in patients with acute variceal bleeding (AVB) related to PHT. 8,9,11,12 Thus, understanding the interaction between HCC and PTH is a major issue in order to improve the management of such complications and the outcome of these patients.
Recently, the positive results of the Imbrave 150 study, a randomized study comparing Atezolizumab (anti PD-L1) in combination with Bevacizumab (anti VEGF) versus Sorafenib (tyrosine kinase inhibitor), have prompted us to redefine our management strategy for advanced HCC by proposing the combination of Atezolizumab and Bevacizumab as a first-line treatment. 13 Patients were well selected, and few complications related to PHT were noticed in this phase 3 study. However, due to their respective mechanism of action, the combination of Atezolizumab and Bevacizumab may potentially impact the level of PHT and its related complications, and to date, no real-life data are available.
In this review, we will focus (i) on the consequences of both angiogenesis and inflammation in the pathogenesis of PHT and HCC and (ii) on the link between PHT and HCC, and (iii) we will discuss the potential impact of the combination Atezolizumab and Bevacizumab on PHT.
Studies for review in this article were retrieved from the PubMed database using the search terms 'hepatocellular carcinoma', 'liver cancer' and 'primary liver carcinoma', both individually and in combination with the terms 'portal hypertension', 'acute variceal bleeding', 'ascites' and 'TIPS'. The search included literature published in English until March 2021.

| ANG I OG ENE S IS AND CHRONI C INFL AMMATI ON CONTRIBUTE TO P ORTAL HYPERTENS ION PATHOG ENE S IS
In case of cirrhosis, PHT is initiated by an increased hepatic resistance caused by the distortion of liver vascular architecture due to fibrosis and regenerative nodules but also to an increase of vasoconstrictor stimuli. PHT is further perpetuated by opening of collateral vessels/ formation of neovessels and changes in the systemic circulation that culminate in an increased portal-tributary inflow. 1,14 The demonstration that angiogenesis is a main actor in the pathophysiology of PHT is relatively recent. 3 During decades, the formation of portosystemic collateral vessels in PHT has been related to the opening of pre-existing vascular channels secondary to the increased portal pressure. Recently, a neovascularization in the mesenteric vascular bed and in the portosystemic collateral vessels has been described and attributed to VEGF and PDGF production. 1 VEGF and VEGF receptor type 2 expression were observed in the mesentery from portal vein-stenosed rats, and the inhibition of VEGF receptor 2 signalling was associated with a 50% decrease in the formation of portal-systemic collateral blood vessels in rodent models with portal vein stenosis. 15,16 VEGF increases vascular permeability and induces the migration and survival of endothelial cells favouring angiogenesis process. 17,18 In addition to promoting neovessels formation in the mesentery and portosystemic system, VEGF also plays a role within the liver parenchyma. Increased hypoxia, inflammatory cytokines (IL-6, IL-1α), growth factors (epidermal growth factor, transforming growth factorα andβ, fibroblast growth factor, PDGF), shear and oxidative stress within the liver lead to increase VEGF production by hepatocytes but also Hepatic Stellate Cells (HSCs). 19 HSCs are activated through its VEGF receptor leading to fibrosis occurrence and also VEGF production that interacts with adjacent liver sinusoidal endothelial cells (LSECs). 3 HSC-signalling pathways involving PDGF regulate microvascular structure and function in liver and PHT. 20 Portal myofibroblasts also appear to be critical in pathological angiogenesis though the production of collagen that stabilizes the newly formed vessels. 21,22 Interestingly, sinusoidal angiogenesis depends on the liver stiffness, and mechanotransduction appears as an important pathway in liver fibrosis progression. [23][24][25] LSECs exhibit pro-angiogenic properties and promote fibrosis through HSCs activation on low stiffness substrates at early disease stage. On contrary, LSECs showed random migration and leaky sinusoids in high stiffness substrates. 24 The newly formed intrahepatic neovessels present varying diameter and flow pattern leading to the impairment of oxygen and nutrient supply to hepatocytes and to the recruitment of inflammatory cells. 3 Thus, angiogenesis mediated by VEGF promotes an extensive network of portosystemic collateral vessels, contribute to increased splanchnic and portal venous inflow that perpetuates and exacerbates PHT and favours liver fibrosis progression and inflammation ( Figure 1). The use of anti-VEGF such as Sorafenib was associated with PTH reduction and targeting angiogenesis at early stage of the liver disease may appear as a good strategy. [26][27][28][29] However, VEGF also plays a role in hepatic tissue repair and fibrosis resolution. 30 Chronic inflammation and PHT are also linked since bacterial infection increases portal pressure and PHT favours bacterial translocation. 31,32 PHT increases intestinal permeability, thus favouring bacterial translocation. Dysbiosis is associated with cirrhosis and increases with the severity of liver disease. 33,34 Bacterial translocation corresponds to the passage of microbial products through the mucosa into mesenteric lymph nodes and then to the liver. In the liver, the innate immune system recognizes damage-associated molecular pattern (DAMPs) and pathogen-associated molecular pattern (PAMPs) through pattern recognition receptors, such as Toll-like receptor (TLRs). 35 Kupffer cells are the first cells to respond to PAMPs through TLR4 and consequently adopt a pro-inflammatory phenotype leading to the production of proinflammatory cytokines such as TNF, IL-6 and IL-12 and to the generation of reactive oxygen species (ROS). 36 ROS production will conduct to a decrease in nitric oxide (NO) secretion, which usually regulates intrahepatic vascular tone by maintaining HSCs in a quiescent phenotype, and thus promote fibrosis process. 37 ROS will also contribute to local tissue damage and propagating innate immune signalling through DAMPs and promote VEGF production. 38 In addition, bacterial translocation will conduct F I G U R E 1 Impact of increased VEGF level on portal hypertension and hepatocellular occurrence. A, In physiological condition, VEGF plays an important role in cellular homeostasis, especially to maintain sinusoidal capillary network homeostasis; (B) inflammation, shear and oxidative stress within the liver induced by liver injury increase VEGF production by hepatocytes and HSCs. The augmentation of VEGF levels will favour fibrosis through activation of HSCs and angiogenesis through activation of LSECs that will increase intra hepatic resistance contributing to portal hypertension occurrence. Chronic inflammation favours by bacterial translocation also contribute to the phenomena. An increase of VEGF within the mesenteric vascular bed favours angiogenesis and lead to the creation of portal systemic collateral vessel, contributing to the increase splanchnic blood flow and portal hypertension; (C) With time, VEGF will promote hepatocytes proliferation and angiogenesis participating to the pathophysiology of hepatocellular carcinoma genesis. Chronic inflammation will also promote cell proliferation and creates a tumour microenvironment that supports the transformation of hepatocytes, and also induces their survival through activation of anti-apoptotic pathway and inhibition of immune surveillance. VEGF also mediates immunosuppression within the tumour microenvironment by promoting immunosuppressive cells such as Treg, MDSCs and TAMs and suppressing antigen suppressive cells and cytotoxic T lymphocyte. HCC, hepatocellular carcinoma; HSCs, hepatic stellate cells; LSECs, liver sinusoidal endothelial cells; MDSCs, myeloid derived suppressive cells; TAM, tumour associated macrophages; TReg, regulatory T cells; VEGF, vascular endothelial growth factor to stimulation of TLR4-mediated signalling in HSCs and LSECs directly promoting angiogenesis and fibrosis progression ( Figure 1). In animal models, TLR4 knockout mice are protected from fibrosis and PHT following bile duct ligation. 39,40 Similar results were observed in mice treated with rifaximin after bile duct ligation. 39,40 In humans, a trend towards HVPG reduction was observed in controlled study using rifaximin, but these finding remain controversial. 41,42 A similar reduction in portal pressure was observed in patients with acute on chronic liver failure treated with anti-TNF; however, this treatment was not adopted due to high rates of infection after anti-TNF treatment. 43 Human data are scarce; furthers studies are needed to confirm the dialogue between VEGF, DAMPS and PAMPS in PHT progression.

| A LINK B E T WEEN HEPATO CELLUL AR C ARCINOMA AND P ORTAL HYPERTENS ION
Clinically significant portal hypertension is predictive of HCC occurrence independently of the severity of the underlying cirrhosis. 7 in HCC patients, TIPS can also allow the access to HCC treatments (either alone or in a downstaging perspective before liver transplantation). 69,72,73 However, HCC is generally considered as a contra-indication to TIPS placement, due to the fear of HCC spread, and the poor outcome of these patients. [74][75][76][77] There is also a concern regarding the potential increased risk of liver failure after locoregional therapies and a decrease in efficacy of HCC treatments. [74][75][76][77] In fact, few data are available regarding these points, and whether or not TIPS is a real risk factor for higher toxicity after locoregional

| IMPAC T OF SYS TEMI C THER APY FOR HEPATOCELLUL AR C ARCINOMA ON P ORTAL HYPERTENS ION
During the last decades, tyrosine kinase inhibitors (TKIs) were the standard of treatment for advanced HCC. Sorafenib and Lenvatinib F I G U R E 2 TIPS placement in patients with HCC. TIPS placement in well-selected patients with HCC may be beneficial as TIPS increases survival (overall survival and transplant free survival in the setting of variceal bleeding and recurrent/refractory ascites). In addition, TIPS may allow to perform locoregional HCC treatment by modelling hypertension portal level and improving liver function in order to access to liver transplant. However, the decision of TIPS placement should consider a potential risk of liver dysfunction that may prevent patients to access to HCC treatments, but also a potential risk of HCC spread especially in case of central localization and vascular invasion and the risk of less effective transarterial HCC treatment. HCC, hepatocellular carcinoma were used in first line treatment while Regorafenib and Cabozantinib in second line. [78][79][80][81] All of these TKIs target the VEGFR and may impact PHT pathophysiology ( Table 1). Inhibition of VEGF-mediated angiogenesis by drugs such as sorafenib allowed to reduce portal pressure, portosystemic collateralization and fibrosis in animal models. 15,[26][27][28][29]82 Similar results were observed in rodents models treated with regorafenib and exposed to fibrotic models such as bile duct ligation, chronic carbon tetrachloride injections and partial portal vein ligation 83 and with Lenvatinib. 84 In humans, portal venous area but not portal venous flow velocity was also significantly decreased, as well as the congestion index (portal venous area divided by portal venous flow velocity) in 25 Child-Pugh A patients after 2 weeks administration of Sorafenib, suggesting a potential beneficial effect on PHT. 85 On the contrary, the congestion index was significantly worsened in 28 Child-Pugh A patients who received two weeks of Lenvatinib for advanced HCC. 86 Contrary to Sorafenib, Lenvatinib also targets fibroblast growth factor (FGF) receptors, and FGF19 and FGF 21 are known to promote liver regeneration and the maintenance of hepatic metabolism which could impact the underlying liver disease and favour PHT; however, further studies are needed to address this point. As discussed before, AVB is a major issue in patients with HCC, and similar PHT bleeding complications rates were observed in the phase 3 studies for advanced HCC (less than 3%) (Table 1). 13,55,62,[78][79][80][81] However, in these series, patients were well selected as recent history of bleeding was an exclusion criterion, and real-life data are needed (Table 2). Interestingly, neoplastic portal vein thrombosis was independently associated with AVB in patients treated with Sorafenib. 87 Such patients should benefit from regular screening and adapted bleeding prevention therapy. 88

| P ORTAL HYPERTEN S I ON AND THE NE W COMB INATI ON B E VACIZUMAB AND ATE ZOLIZUMAB
Bevacizumab is an antibody directed against VEGF who plays a major role in PHT pathophysiology as described before.
Atezolizumab is an immune checkpoint inhibitor (anti-PDL1) which aims to restore anti-tumour immunity and may contribute to PAMPs and DAMPs elimination. Therefore, these two  13 In this study, the patients were well selected, with a majority of patients with controlled viral hepatitis who benefitted from optimal PHT prophylaxis. In series where prophylactic treatment to prevent AVB was less standardized, 10% of patients included in Phase II trials of HCC using bevacizumab presented bleeding complications related to PHT. 54 Therefore, the impact of the combination of Bevacizumab and Atezolizumab on PHT remains to be determined. Indeed, even if VEGF contributes to PHT through angiogenesis process, this growth factor is also required for sinusoidal homeostasis. Inhibition of VEGF may impact preserved liver sinusoids in the non-tumoral liver leading to sinusoidal alterations, impairment of oxygen and nutrient supply to hepatocytes causing cell death and the recruitment of inflammatory cells, consequently worsening the underlying liver disease and PHT.
In addition, the use of Atezolizumab, which promotes cytotoxic lymphocytes, may also contribute to increase production of proinflammatory cytokines that might activate innate immune cells, as well as HSCs and LSECs, and consequently drive liver fibrosis, chronic inflammation and PHT, 89

ACK N OWLED G EM ENT
None.

CO N FLI C T O F I NTE R E S T
The authors do not have any disclosures to report.