The clinical value of angiopoietin‐2 in liver diseases

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Abbreviations

AKI

acute kidney injury

ALD

alcoholic liver disease

Ang2

angiopoietin‐2

CLD

chronic liver disease

HBV

hepatitis B virus

HCC

hepatocellular carcinoma

HCV

hepatitis C virus

MELD

Model for End‐Stage Liver Disease

NAFLD

nonalcoholic fatty liver disease

NASH

nonalcoholic steatohepatitis

NF‐B

nuclear factor‐κ light‐chain enhancer of activated B cells

PI3K/AKT

phosphatidylinositol 3‐kinase/protein kinase B

VEGF

vascular endothelial growth factor

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INTRODUCTION

Angiopoietin‐2 (Ang2) is a type of endothelial cell–specific growth factor that plays a significant role in the maturation, stabilization, and remodeling of blood vessels. ¹ Ang2 belongs to the angiopoietin/tyrosine kinase (Tie) signaling pathway, which induces angiogenesis through antagonism of the Tie2 receptor. ²

Recent data suggest that angiogenesis plays a critical role in the evolution of chronic liver diseases (CLDs). ³ Serum and hepatocyte Ang2 levels have been shown to be significantly higher in patients with CLD, and inhibitors of angiogenesis have demonstrated antifibrotic effects. ³ , ⁴

PATHOPHYSIOLOGY

The signal transduction pathway for angiogenesis includes the Tie receptors (Tie1 and Tie2) and the vascular endothelial growth factors (VEGF 1‐3), which are two distinct endothelial cell receptors sharing a common downstream receptor tyrosine kinase pathway. ⁵ The angiopoietin growth factors (Ang1 and Ang2) have a uniquely distinct mechanism from the VEGF‐induced receptor activation pathway. Ang2 is released in response to inflammatory stimuli, such as hypoxia, malignancy, and thrombin. ⁶ The expression of Ang2 is typically low in inactive, mature vessels but is increased in the setting of inflammatory and angiogenic factors ⁷ (Figure 1).

FIGURE 1.

Schematic showing the signal transduction with Tie2 and Ang1/2. Proposed model showing stimulatory effect (green arrows) and inhibitory effect (red arrows). Tie2 is expressed on the blood vessel and, when Ang1 binding causes a stimulatory effect, thus downstream signal transduction. Ang2 has an inhibitory effect on Ang1. Ang1, Angiopoietin 1; Ang2, Angiopoietin 2; VEGF, vascular endothelial Growth factor; NF‐κβ, nuclear factor kappa‐light‐chain‐enhancer of activated B cells; PI3K/AKT, Phosphatidylinositol‐3‐Kinase/Protein kinase B ²⁵

ANG2 MEASUREMENT

Ang2 is an approximately 70‐kDa secreted glycoprotein. Typically, human Ang2 immunoassay, for instance, Human Angiopoietin‐2 Quantikine ELISA, is a 4.5‐h solid‐phase enzyme‐linked immunosorbent assay designed to measure human Ang2 in cell culture supernatants, serum, plasma, and saliva. ⁸ A monoclonal antibody specific for human Ang2 cab is used. There are no Ang2 assays available for clinical use to our knowledge.

ACUTE LIVER FAILURE

Haden et al. ⁹  demonstrated a correlation between increased circulating Ang2 levels in 37 patients with multiple organ dysfunction and acute liver failure. They showed that the median Ang2 serum concentrations were consistently augmented in the subsequent groups as follows: healthy control (1.4 [0.9‐1.7] ng/ml), recovered patients without liver transplantation (10.0 [4.7‐12.1] ng/ml), and patients requiring emergency liver transplantation or resulting in death (16.8 [11.3‐39.5] ng/ml). Ang2 was thus recognized as a predictor of the composite endpoint of liver transplantation or death. ⁹

NONALCOHOLIC FATTY LIVER DISEASE AND NONALCOHOLIC STEATOHEPATITIS

Lefere et al. ⁴  measured serum Ang2 levels in 102 obese patients status post–bariatric surgery with concomitant liver biopsy. When compared with obese patients without steatosis and control subjects, they demonstrated significantly higher levels of Ang2 in patients with nonalcoholic steatohepatitis (NASH). Furthermore, serum Ang2 levels also correlated with the severity of steatosis (p < 0.05) seen on liver biopsy. ⁴

VIRAL HEPATITIS

Sanz‐Cameno et al. ¹⁰ measured serum Ang2 levels in 15 patients with chronic hepatitis B virus (HBV) infection, demonstrating increased Ang2 expression in areas of inflammation compared with control normal liver tissue. Osawa et al. ¹¹ demonstrated that plasma levels of Ang2 were greater in 20 patients with chronic hepatitis C virus (HCV) compared with healthy volunteers. In addition, Ang2 levels were also shown to decline in response to appropriate treatment and elimination of HCV. ¹¹

Hernandez‐Bartolome et al. ¹² examined 179 patients with chronic HCV infection and showed serum Ang2 levels to be considerably higher with progression to cirrhosis (p < 0.01). The Ang2/Ang1 ratio was remarkably augmented and meaningfully associated with fibrosis (p < 0.0001). ¹²

ALCOHOLIC LIVER DISEASE

Pauta et al. ¹³  also found higher circulating levels of Ang2 in patients with cirrhosis related to alcohol (alcoholic liver disease [ALD]), and the Ang1/Ang2 ratio was inversely correlated with predictive models of the disease. In addition, data showed significantly higher levels of Ang2 and VEGF‐A in 147 patients with ALD compared with healthy control subjects. ¹⁴

HEPATOCELLULAR CARCINOMA

Torimura et al. ¹⁵ showed increased expression of Ang2 mRNA in patients with hepatocellular carcinoma (HCC), which was also associated with tumor dedifferentiation. Moreover, in nude mice, Tanaka et al. ¹⁶ observed that injecting human HCC tumor cells with Ang2 led to extensive intraperitoneal bleeding and the development of large tumors.

Pestana et al. ¹⁷ reported that serum Ang2 levels were significantly higher in patients with HCC (mean = 15.3 ng/ml; 95% confidence interval, 14.1‐16.4 ng/ml) compared with healthy control groups (p < 0.001). They concluded that levels of Ang2 could be a potential biomarker for the diagnosis and prognosis of HCC. ¹⁷

DECOMPENSATED CIRRHOSIS

In a prospective study, serum Ang2 was measured in 191 patients with decompensated cirrhosis with acute kidney injury (AKI). They demonstrated that elevated Ang2 levels were linked with higher mortality rate (alive versus dead: 15.2 [9.8, 23.0] versus 21.9 [13.9, 30.3] ng/ml; p < 0.001) in these patients. They concluded that serum Ang2 levels were positively correlated with poorer survival and AKI severity. ¹⁸

ANG2 AS A THERAPEUTIC TARGET

The blockage of Ang2 with a purified single‐chain variable fragment against human Ang2 caused a reduction in endothelial migration and angiogenesis and growth inhibition and, more importantly, decreased development of intrahepatic metastasis. ¹⁹ Data showed that treatment with the Tie2/Ang2 interaction‐inhibiting peptibody L1‐10 (Amgen) in a mouse model with nonalcoholic fatty liver disease (NAFLD) can reduce HCC development ⁴ (Table 1).

TABLE 1.

Ang2 in HCC treatment

HCC treatment	Ang2 marker
Yttrium‐90 radioembolization	Baseline circulating Ang2 levels inversely correlate with posttreatment survival 21
Surgical resection	Ang2 expression in HCC tissue correlates with postsurgery recurrence 22
Liver transplantation	Ang2 expression in HCC tissue correlates with postsurgery graft rejection 23 , 24
Sorafenib	Baseline circulating Ang2 levels inversely correlate with response to sorafenib 25

Li et al. ²⁰ documented that Ang2/Tie2 signaling reduces doxorubicin‐induced apoptosis in human HCC cell lines by increasing the expression of Survivin and Ref‐1. Accordingly, they proposed that Ang2 targeting not only signifies a promising antiangiogenic therapeutic strategy but may also increase chemosensitivity in patients with HCC. ²⁰

CONCLUSION

Numerous studies have reported angiogenesis and fibrosis as essential processes in the pathogenesis of CLD and HCC. Elevated levels of Ang2, which likely originates from the injured liver, may potentially be used as a biomarker in various liver diseases. However, most of the data require further experimental study to validate the Ang2 mechanism in patients with liver disease. They suggested a large number of patients, a more extended follow‐up period, and a standardized assay system to gain greater insight into the potential usefulness of Ang2 in patients. Further studies will need to be conducted to establish a standardized assay system to measure Ang2 level in the multicenter setting and largest population (Table 2).

TABLE 2.

Summary of Ang2 level in liver diseases

Author	Population	Study	Results
Haden et al. 9	37 with acute liver failure	Retrospective clinical and immunohistological study	Circulating Ang2 correlates with several features of multiple‐organ dysfunction syndromes and independently predicts outcome
Lefere et al. 4	104 with obesity	Prospective cohort	Serum Ang2 levels increased significantly with increasing histological grades of steatosis, lobular inflammation, and ballooning, but not with the stage of fibrosis
			Importantly, patients with histological NASH had significantly higher serum Ang2 levels than those without NAFLD
Sanz‐Cameno et al. 10	15 patients with chronic HBV	—	Ang2 expression was up‐regulated at both mRNA and protein levels in the liver of patients with Chronic Hepatitis B
Hernandez‐Bartolome et al. 12	179 patients with and without cirrhosis with chronic HCV	—	Ang2 was significantly increased as HCV progressed to the end stage of liver disease
			Ang2/Ang1 ratio might constitute a useful minimally invasive indicator of cirrhosis in patients with chronic HCV
Kasztelan‐Szczerbinska et al. 14	147 patients with ALD	Cohort	High Ang2 concentrations had an independent impact on the severity of liver failure (MELD ≥ 20) and the development of two major ALD complications: Hepatic Encephalopathy and renal impairment
Pestana et al. 17	767 patients with HCC	Case‐control	Lower plasma Ang2 levels correlated with prolonged overall survival
			Higher plasma Ang2 was significantly associated with advanced clinicopathological features of advanced HCC
Allegretti et al. 18	191 patients with decompensated cirrhosis	Prospective cohort	Serum Ang2 levels were strongly associated with mortality in patients with cirrhosis. All adjusted models demonstrated a stable association between Ang2 level and increased mortality, including adjustment for MELD score, cause of AKI, presence of infection, and age

CONFLICT OF INTEREST

Nothing to report.

Farshidpour M, Pace S, Volk ML. The clinical value of angiopoietin‐2 in liver diseases. Clinical Liver Disease. 2022;19:244–247. 10.1002/cld.1218