Genome‐wide protein QTL mapping identifies human plasma kallikrein as a post‐translational regulator of serum uPAR levels

Abstract

The soluble cleaved urokinase plasminogen activator receptor (scuPAR) is a circulating protein detected in multiple diseases, including various cancers, cardiovascular disease, and kidney disease, where elevated levels of scuPAR have been associated with worsening prognosis and increased disease aggressiveness. We aimed to identify novel genetic and biomolecular mechanisms regulating scuPAR levels. Elevated serum scuPAR levels were identified in asthma (n=514) and chronic obstructive pulmonary disease (COPD; n=219) cohorts when compared to controls (n=96). In these cohorts, a genome-wide association study of serum scuPAR levels identified a human plasma kallikrein gene (KLKB1) promoter polymorphism (rs4253238) associated with serum scuPAR levels in a control/asthma population (P=1.17×10−7), which was also observed in a COPD population (combined P=5.04×10−12). Using a fluorescent assay, we demonstrated that serum KLKB1 enzymatic activity was driven by rs4253238 and is inverse to scuPAR levels. Biochemical analysis identified that KLKB1 cleaves scuPAR and negates scuPAR's effects on primary human bronchial epithelial cells (HBECs) in vitro. Chymotrypsin was used as a proproteolytic control, while basal HBECs were used as a control to define scuPAR-driven effects. In summary, we reveal a novel post-translational regulatory mechanism for scuPAR using a hypothesis-free approach with implications for multiple human diseases.—Portelli, M. A., Siedlinski, M., Stewart, C. E., Postma, D. S., Nieuwenhuis, M. A., Vonk, J. M., Nurnberg, P., Altmuller, J., Moffatt, M. F., Wardlaw, A. J., Parker, S. G., Connolly, M. J., Koppelman, G. H., Sayers, I. Genome-wide protein QTL mapping identifies human plasma kallikrein as a post-translational regulator of serum uPAR levels.

Translating Lung Function Genome-Wide Association Study (GWAS) Findings

“…bronchial epithelial cells and observed increased proliferation as a result of this manipulation which was suggested to contribute to airway remodelling. (Portelli et al, 2014). This fits well with the observation of elevated levels of PLAUR in the airway epithelium in asthma patients (Stewart et al, 2012) and association of PLAUR levels with worsening prognosis and increased disease aggressiveness in other diseases such as cancer and COPD (Ivancso et al, 2013;Smith & Marshall, 2010).…”

“…This approach identified a key SNP present in the promoter of human plasma kallikrein gene (KLKB1) as a determinant of suPAR protein levels, and subsequent biochemical analyses demonstrated that this SNP was an eQTL for KLKB1. We were able to show that KLKB1 subsequently cleaves uPAR from the surface of the cell providing a regulatory genetic posttranslational mechanism (Portelli et al, 2014). This study, using a protein eQTL approach, therefore identified a novel way whereby an asthma susceptibility gene can be modified by a genetically driven post-translational mechanism.…”

“…A good example is the recent protein eQTL analyses of serum urokinase plasminogen activator receptor (suPAR) levels (Portelli et al, 2014). uPAR has been identified as an asthma susceptibility gene (Barton et al, 2009) and has been shown to be differentially expressed in the airways (Stewart, Nijmeh, Brightling, & Sayers, 2012) and blood (Portelli et al, 2014) in asthma patients. The receptor exists as a glycosyl-phosphatidylinositol (GPI)-linked receptor and is involved in a wide range of functions including migration, proliferation and adhesion and is also involved in fibrinolysis (Portelli et al, 2014).…”

“…uPAR has been identified as an asthma susceptibility gene (Barton et al, 2009) and has been shown to be differentially expressed in the airways (Stewart, Nijmeh, Brightling, & Sayers, 2012) and blood (Portelli et al, 2014) in asthma patients. The receptor exists as a glycosyl-phosphatidylinositol (GPI)-linked receptor and is involved in a wide range of functions including migration, proliferation and adhesion and is also involved in fibrinolysis (Portelli et al, 2014). Importantly, the receptor exists as both a soluble form generated by splicing and a soluble form generated by cleavage and these soluble forms are thought to be involved in regulating overall receptor function ).…”

Soluble urokinase-type plasminogen activator receptor is a novel biomarker predicting acute exacerbation in COPD

“…5,6 Migration of inflammatory cells from the bloodstream into tissues is an essential component of inflammation and the immune response against infection in which the uPAR/uPA system is directly involved. 7 During inflammatory stimulation, uPAR is cleaved from the cell surface by proteases to create the soluble form of the receptor, suPAR, which can be detected in blood, urine, and cerebrospinal fluid. 8 High suPAR levels were shown to be associated with an undesirable outcome in HIV infection, 9 active pulmonary tuberculosis, 10 bacterial meningitis, 8 and pneumococcal bacteremia, 11 suggesting that it may be a good biomarker of inflammation.…”