Deficiency of Either Carboxypeptidase B2 or Carboxypeptidase N Exacerbated Disease Activity in a Mouse Model of Hemolytic Uremic Syndrome

Two different basic carboxypeptidases circulate in blood - carboxypeptidase N (CPN) and proCPB2. CPN is constitutively active, while proCPB2 is a zymogen, (also termed thrombin activatable fibrinolysis inhibitor, TAFI), and is activated by the endothelial thrombin/thrombomodulin complex to CPB2. Their kinetics of substrate cleavage are distinct but both can efficiently inactivate the complement anaphylatoxins, C3a and C5a. Hemolytic uremic syndrome (HUS) is caused by Shiga toxin (stx) producing strains of E. coli and is characterized by the triad of hemolysis, thrombocytopenia and uremia. We hypothesized that in a mouse model of HUS, Cpb2-/- and Cpn-/- mice would have prolonged C5a anaphylatoxin activity thus causing disease exacerbation. HUS was induced by stx2 and LPS administration in WT, Cpn-/- and Cpb2-/- mice. In Cpb2-/- mice, median time to death was earlier (60 hours, n=15) than in WT mice (96 hours: n=42, p<0.0001), and had greater kidney and liver damage shown by increases in ALT, AST, BUN and creatinine levels at 48 hours (creatinine Cpb2-/-: 1.01 mg/dL, WT: 0.25 mg/dL; Cpb2-/- control: 0.20 mg/dL and WT control: 0.19 mg/dL; Cpb2-/- p<0.0001 vs. all other groups, n>9). An increase in hemolysis was demonstrated by reduced RBC count and hemoglobin level plus an increase in total bilirubin and LDH. Profound thrombocytopenia (Cpb2-/-: 121,000/μL vs. WT: 217,000/μL; p=ns) developed in both genotypes (control Cpb2-/-: 1,001,000/μL vs. control WT: 1,141,000/μL; p=ns but vs. either Cpb2-/- or WT with HUS, p<0.0001) and thus the HUS clinical triad was present. Histology showed tubular epithelial necrosis in the kidney ante-mortem. Administration of either toxin separately caused milder disease without the characteristics of HUS and with no observed mortality. Induction of the disease depended on co-administration of both toxins. Treatment with anti-murine C5 antibody (0.75 mg every 24 hours from 3 hours before disease initiation) improved survival of both WT and Cpb2-/- mice with a median survival time of 168 hours for both genotypes (n=11, p=0.003 and <0.0001 respectively) and normalized the outcomes between the genotypes. Cpn-/- mice also died sooner (median time to survival 81.5 hours, n=28) than WT mice (96 hours, n=42, p=0.0002). The median survival time between Cpb2-/- and Cpn-/- mice was also significantly different (60 vs. 81.5 hours, p=0.0083). This is a first direct comparison of the role of CPN vs. CPB2 in regulating C5a activity in a disease relevant mouse model. Our study suggests that both CPB2 and CPN protect against HUS by inactivation of C5a with CPB2 having a greater effect than CPN. When Cpb2+/-/Cpn+/- mice are crossed, all expected genotypes are recovered in the expected Mendelian ratios including double deficient Cpb2-/-/Cpn-/- mice. Thus absence of both plasma basic carboxypeptidases is not essential for murine life. We are currently evaluating the Cpb2-/-/Cpn-/- mice in our HUS model. Disclosures No relevant conflicts of interest to declare.

Basic carboxypeptidases of blood: significance for coagulology

This review considers the basic metallocarboxypeptidases of human blood and their role in coagulologic disorders. In includes information on the history of the discovery and biological characteristics of potential enzymes-regulators of the fibrinolytic process: carboxypeptidase U and carboxypeptidase N. Certain attention is paid to the biochemical mechanisms and the main modern concepts of the antifibrinolytic effects of these enzymes

Circulating Proteolytic Products of Carboxypeptidase N for Early Detection of Breast Cancer

BACKGROUND Carboxypeptidase N (CPN) is important in regulating vasoactive peptide hormones, growth factors, and cytokines by specifically cleaving their C-terminal basic residues. We investigated whether circulating peptides specifically cleaved by CPN in the tumor microenvironment can be stage-specific indicators of breast cancer. METHODS CPN activity was measured using an ex vivo peptide cleavage assay by incubating synthesized C3f peptide (His6-C3f_S1304-R1320-His6) in interstitial fluids of breast tumors and adjacent normal breast tissues in mice with orthotopic implantation of the human cell line MDA-MB-231. The nature and extent of peptide cleavage by CPN was investigated by fragment profiling using nanopore fractionation and mass spectrometry. The fragment profiles in interstitial fluid correlated with concentrations of CPN-catalyzed peptides in blood samples taken from the tumor-bearing mice, healthy women, and breast cancer patients. CPN expression in the same set of samples was further examined by immunohistochemistry and immunoblotting. RESULTS We showed that generation of C3f_R1310-L1319 specifically correlated with the CPN expression level. In both the mouse and clinical patient samples, CPN was clearly increased in tumor tissues compared with normal breast tissue, whereas corresponding CPN abundance in blood remained constant. Concentrations of 6 CPN-catalyzed peptides predominantly increased in sera taken from the mice (n = 8) at 2 weeks after orthotopic implantation. Six homologous peptides displayed significantly higher expression in the patients' plasma as early as the first pathologic stage of breast cancer. CONCLUSIONS Circulating CPN-catalyzed peptide concentrations reflect the CPN activity in tumors. These biomarkers show strong potential for the noninvasive and early diagnosis of breast cancer.