Soluble Urokinase Receptor and Liver Disease

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Abbreviations

ALF

acute liver failure

ALT

alanine aminotransferase

AST

aspartate aminotransferase

FSGS

focal segmental glomerulosclerosis

HCC

hepatocellular carcinoma

LPS

lipopolysaccharide

suPAR

soluble form of urokinase plasminogen activator receptor

Tg

transgenic

uPAR

urokinase plasminogen activator receptor

Soluble urokinase receptor is the soluble form of urokinase plasminogen activator receptor (suPAR), which is a membrane bound protein, expressed on various immunologically active cells such as macrophages, neutrophils, and activated T lymphocytes, as well as on endothelial cells and podocytes.1, 2 It is largely generated from urokinase plasminogen activator receptor (uPAR) by enzymatic cleavage of glycosylphosphatidylinositol anchor, but also exists as a secreted isoform due to alternative transcription.3 Circulating suPAR levels are elevated in various medical conditions associated with inflammation such as kidney disease, increasing age, diabetes, atherosclerosis, heart failure, sepsis, human immunodeficiency virus, autoimmune diseases, and smoking.4 High suPAR levels cause kidney disease in mice; in people, they are strongly predictive of incident kidney disease and accelerated progression of renal decline.5, 6 Given the plethora of conditions in which high suPAR levels can be observed (Fig. 1),7 we further discuss the association of suPAR in liver disease.

Figure 1.

Association of elevated circulating suPAR levels with multiple organ systems. Although the increase of suPAR is implicated in many organ systems or disease states, its pathogenesis is not yet clear in most cases except kidney disease.

Elevation of suPAR Levels in Liver Disease

Increase of circulating suPAR levels has been documented in different chronic and acute liver diseases. Zhou et al.8 reported the elevation of suPAR in patients with acute or chronic hepatitis B in 2009. suPAR was found to be associated with progressive liver fibrosis in a cohort of 146 patients with hepatitis C infection.9 In a prospective cross‐sectional study of 159 cases with chronic liver disease, including hepatitis B, hepatitis C, and alcoholism, Zimmermann et al.10 not only observed a stepwise elevation of serum suPAR levels from healthy control patients without cirrhosis to patients with compensated cirrhosis to patients with decompensated cirrhosis, but also identified suPAR as a strong predictor for mortality or need for transplantation. Later, the same group found that suPAR levels in ascitic fluid correlate poorly with circulating suPAR, but rather indicate spontaneous bacterial peritonitis and a worse outcome. Thus, they postulated that suPAR is compartmentally regulated in decompensated cirrhosis.11 The association of increased circulating suPAR with the progression of fibrosis was reported by other groups as well in chronic hepatitis B and alcoholic liver disease.12, 13 Similarly, elevation of circulating suPAR was found in nonalcoholic fatty liver disease, and its levels were also associated with fibrosis severity.14 In addition, suPAR has been evaluated in acute liver failure (ALF). Koch et al.15 observed that serum suPAR concentration was significantly increased in patients with ALF, regardless of underlying etiology.

Lastly, increase of circulating suPAR levels was found in liver malignancy, both primary and metastatic. Chounta et al.16 had observed higher suPAR levels in patients with hepatocellular carcinoma (HCC), compared with that in patients with minimal liver inflammation. The authors then prospectively monitored 267 patients without any imaging signs of HCC for up to 7 years. They found that patients with suPAR levels >9.5 ng/mL at the beginning of the study progressed earlier to HCC; thus, they suggested that serum suPAR serves as a potential screening tool for the early diagnosis of HCC in patients with chronic liver disorders.16 In terms of liver metastases, Loosen et al.17 measured suPAR levels in 104 patients undergoing hepatic resection for colorectal liver metastases. They found that the preoperative, but not postoperative, suPAR level was a prognostic factor for mortality. Moreover, preoperative suPAR levels may help to identify patients at risk for postoperative kidney disease.

Cellular Source of suPAR in Liver Disease

Even though the increase of circulating suPAR has been reported in many disease conditions, its origin or cellular source is still unclear in most cases. For proteinuric kidney diseases, Hahm et al.18 showed that an increase in plasma suPAR level largely originated from bone marrow–derived immature myeloid cells. What is the source of suPAR implicated in liver disease? Flow cytometry of peripheral blood and ascitic fluid demonstrated monocytes, neutrophils, and CD14⁺ peripheral peritoneal macrophages as a potential source of suPAR. Cultured monocytes released much more suPAR than neutrophils in response to lipopolysaccharide (LPS).11 In ALF, accumulation of distinct immune cell subsets was noted in liver. Collectively, uPAR expression on hepatic leucocytes was largely restricted to proinflammatory monocytes and unconventional lymphocyte subsets, indicating a distinct role of uPAR for innate immune responses in ALF.15 In vitro, LPS promoted suPAR release from monocytes, but not from other hepatic cell compartments including hepatocytes (hematoma cell lines).15 In liver malignancies, induction of uPAR was observed in colorectal liver metastases, whereas normal liver tissues showed very weak uPAR staining. However, serum suPAR levels increased after surgical resection of the tumor, suggesting that liver metastases are not a major source of circulating suPAR.17

Role of suPAR in the Management of Liver Disease

What is the role of suPAR in liver disease? As reviewed earlier, suPAR has been associated with many types of liver diseases that show with an elevated level of circulating suPAR. As a result, suPAR was proposed as a biomarker indicative of disease severity or prognosis, however absent multicenter, large, prospective, cohort studies with comorbidities and/or confounders sufficiently adjusted. Circulating suPAR was closely and inversely related to glomerular filtration ratio (calculated from cystatin C value) in chronic liver disease.10 In particular, the incidence rate of renal failure in ALF varies from 40% to 85% depending on etiology.19 Considering ALF displayed significantly higher serum suPAR levels independent of underlying causes,15 impaired kidney function at least in part accounts for the increase because of a decreased renal clearance.

Does elevated circulating suPAR cause any liver pathology? As described earlier, circulating suPAR was increased in patients with chronic hepatitis C, yet viral control by direct‐acting antiviral therapy failed to reduce suPAR levels even though liver function was improved and the levels of other inflammatory mediator decreased,20 suggesting that elevated circulating suPAR may not be the culprit for liver damage in this scenario. In regard to kidney disease, our group has identified suPAR as a causative circulating factor that contributes to the development of glomerular diseases such as focal segmental glomerulosclerosis (FSGS) via activating podocyte αvβ3 integrin and c‐src.5, 21 Moreover, as a biomarker, we found that baseline suPAR levels predict the incidence and progression of chronic kidney disease.6 To uncover the mechanisms underlying the multifaceted functions of suPAR in kidney disease, we developed two suPAR transgenic (Tg) mouse models, msuPAR1‐Tg for uPAR isoform 1 (canonical form) and msuPAR2‐Tg for isoform 2 (secreted form), in which suPAR is driven by adipocyte protein 2 (AP2) promoter and released into blood circulation thereafter.18, 22 With circulating suPAR evidently increased in both msuPAR‐Tg models, we examined liver tissues for any functional and histological changes. Surprisingly, both aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels were lower in either msuPAR1‐Tg or msuPAR2‐Tg mice, as compared with the same‐age, same‐sex wild‐type controls. Concomitantly, there was no fibrosis or cirrhosis appreciated from the liver sections of msuPAR1‐Tg or msuPAR2‐Tg mice as indicated by Masson trichrome staining (Fig. 2). Note that these msuPAR2‐Tg mice acquired a renal pathology characteristic of FSGS.22 Taken together, these findings indicate that although an elevated circulating suPAR level causes kidney glomerular sclerosis, it does not lead to any obvious liver fibrosis in mice. However, it is still unclear whether circulating suPAR is implicated in any liver injury models.

Figure 2.

Liver function and histochemistry in msuPAR‐Tg mice. Twelve‐month‐old msuPAR1‐Tg and msuPAR2‐Tg mice maintained on regular chow were sacrificed for liver function test and liver tissue histochemistry analyses. (A) The serum AST and ALT levels decreased in msuPAR‐Tg mice, as compared with wild‐type controls. **P < 0.01. (B) Masson trichrome staining of liver sections did not reveal any fibrosis in msuPAR‐Tg mice.

In summary, our current findings do not support a causative role of circulating suPAR in liver pathology. However, because elevation of circulating suPAR levels has been reported in many liver diseases, suggesting its application as a valuable biomarker for risk stratification, multicenter, prospective, cohort studies are warranted to sufficiently adjust confounders and develop clinically relevant suPAR cutoff values to better risk‐stratify patients with hepatobiliary diseases with respect to cardiorenal and mortality outcomes.