Pregnancy-associated plasma protein-A as potential novel target and serum marker for hepatic fibrosis

Pregnancy-associated plasma protein-A (PAPP-A), a member of the metzincin metalloproteinase superfamily, can enhance local insulin-like growth factor (IGF) bioavailability through proteolytic cleavage of IGF binding proteins. Recently, we have discovered PAPP-A as a tumor promotor in hepatocellular carcinoma applying causal modeling (PLoS Comput Biol. 2015;11(5):e1004293). The IGF-system is known to play a role in hepatic fibrosis, however, the expression and function of PAPP-A in chronic liver disease is unknown.

The aim of this study was to analyze the expression and function of PAPPA in liver fibrosis.

Methods and Results:

RT-PCR revealed that PAPP-A is solely expressed by hepatic stellate cells (HSC) but not by hepatocytes, Kupffer cells or endothelial cells. PAPP-A expression was significantly increased on the mRNA and protein level in different murine models of hepatic fibrosis (TAA-induced liver injury, bile duct ligation, dietary NASH-models). Furthermore, PAPPA expression was increased in cirrhotic compared with non-cirrhotic human liver tissues. PAPPA expression levels correlated with markers of HSC-activation (collagen I and alpha-smooth-muscle actin expression) and fitting to this, PAPPA expression significantly increased in hepatic stellate cells (HSC) during in vitro activation. PAPPA suppression in activated HSC with siRNA resulted in significantly reduced proliferation. Similarly, HSC primary HSC isolated from PAPPA-deficient mice showed significantly reduced proliferation compared with control HSC. Moreover, already the activation process of PAPPA-deficient HSC was significantly retarded compared with HSC isolated from wild-type control mice. Since PAPP-A is a secreted protein, we analyzed its expression levels also in the supernatant of PAPP-A-depleted HSC and confirmed significantly reduced levels compared with control HSC. Interestingly, we also found significant differences in PAPP-A between human HSC isolated from 14 different donors. Furthermore, PAPP-A serum levels were significantly increased in patients with cirrhosis as compared to individuals without liver disease. In addition to PAPP-A, we newly discovered strong expression of IGFBP-4 (a protein cleaved by PAPP-A) in activated HSC, and found a significant correlation between PAPP-A and IGFBP-4 expression.

Summary and Conclusions:

PAPP-A expression and secretion increase during HSC activation, and our data suggest that this leads to an autocrine induction of HSC proliferation, potentially via cleavage of IGFBP-4 and consecutive release of (bioactive) IGF. Thus, PAPP-A appears as novel therapeutic target and circulating PAPP-A levels could have potential as novel biomarker for hepatic fibrosis.