Pro-rat atrial natriuretic peptide-mimicking peptides as substrates for rat kallikreins rK2 (tonin) and rK9

"Corination" of the proANP converting enzyme

The membrane protease, corin, has been shown to be the proatrial natriuretic peptide (ANP)-converting enzyme. Like mice lacking ANP, corin-deficient animals are hypertensive and have cardiac hypertrophy.

Hypertension in mice lacking the proatrial natriuretic peptide convertase corin

Atrial natriuretic peptide (ANP) is a cardiac hormone that regulates blood pressure. In cardiomyocytes, the hormone is synthesized as a precursor, proatrial natriuretic peptide (pro-ANP), which is proteolytically converted to active ANP. Corin is a cardiac transmembrane serine protease that has been shown to process pro-ANP in vitro, but its physiological importance had not been established. Here, we show that corin-deficient (Cor-/-) mice develop normally during embryogenesis and survive to postnatal life. Cor-/- mice have elevated levels of pro-ANP but no detectable levels of ANP as compared with WT littermates. Infusion of an active recombinant soluble corin transiently restores pro-ANP conversion, resulting in the release of circulating biologically active ANP. Using radiotelemetry to assess blood pressure, we find that Cor-/- mice have spontaneous hypertension as compared with WT mice, and it is enhanced after dietary salt loading. Pregnant Cor-/- mice demonstrate late-gestation proteinuria and enhanced high blood pressure during pregnancy. In addition, Cor-/- mice exhibit cardiac hypertrophy resulting in a mild decline in cardiac function later in life. Thus, our data establish corin as the physiological pro-ANP convertase and indicate that corin deficiency may contribute to hypertensive heart disease.

Tonin in rat heart with experimental hypertrophy

The present study was undertaken to determine tonin expression and activity in rat heart presenting isoproterenol-induced hypertrophy. Renin, angiotensin-converting enzyme (ACE), and angiotensinogen (AG) expression were also determined. Wistar rats were treated with isoproterenol for 7 days (5 mg x kg(-1) x day(-1) sc). For untreated animals, the levels of tonin-specific activity in the atrium were 2.6- and 5.5-fold higher than those of the left and right ventricle, respectively. After treatment, the levels of tonin-specific activity increased twofold in the atrium but did not change in the ventricles. Renin expression was not detectable in these structures, and ACE expression levels did not change with treatment. AG expression was detected in the left ventricle at very low levels compared with the atrium and increased significantly only in the hypertrophied atrium (1.8-fold). Tonin mRNA was not detected in the ventricle but was found at low levels in the atrium, which increased after isoproterenol treatment. Our results permit us to conclude that tonin may play a role in the process of heart hypertrophy in the rat.

Processing of pro-atrial natriuretic peptide by corin in cardiac myocytes

Corin is a type II transmembrane serine protease abundantly expressed in the heart. In a previous study using transfected 293 cells, we showed that corin converted pro-atrial natriuretic peptide (pro-ANP) to atrial natriuretic peptide (ANP), suggesting that corin is likely the pro-ANP convertase. Because other serine proteases such as thrombin and kallikrein had previously also been shown to cleave pro-ANP in vitro, it remained to demonstrate that corin is indeed the endogenous pro-ANP convertase in cardiomyocytes. In this study, we examined pro-ANP processing in a murine cardiac muscle cell line, HL-5. Northern analysis showed that corin mRNA was present in HL-5 cells. In HL-5 cells transfected with a plasmid expressing pro-ANP, recombinant pro-ANP was converted to mature ANP as determined by Western analysis, indicating the presence of the endogenous pro-ANP convertase in these cells. The processed recombinant ANP was shown to be active in an enzyme-linked immunosorbent assay-based cGMP assay in baby hamster kidney cells. The processing of recombinant pro-ANP in HL-5 cells was highly sequence-specific, because mutation R98A, but not mutations R101A and R102A, in pro-ANP prevented the conversion of pro-ANP to ANP. Expression of recombinant wild-type corin enhanced the processing of pro-ANP in HL-5 cells. In contrast, overexpression of active site mutant corin S985A or transfection of oligonucleotide small interfering RNA duplexes directed against the mouse corin gene completely inhibited the processing of recombinant pro-ANP in HL-5 cells. These results indicate that corin is the physiological pro-ANP convertase in cardiac myocytes.

Purification and characterization of active recombinant rat kallikrein rK9

The rat tissue kallikrein rK9 is most abundant in the submandibular gland and the prostate. It has been successfully expressed in the Pichia pastoris yeast expression system. A full-length cDNA coding for the mature rK9 was fused in frame with yeast alpha-factor cDNA. The fusion protein was secreted into the medium with high yield without being processed by the yeast KEX2 signal peptidase. Mature rK9 was efficiently released from the fusion protein by trypsin and was purified to homogeneity by one-step affinity chromatography using soya bean trypsin inhibitor (SBTI) as affinity ligand. The identity of the recombinant enzyme was checked by N-terminal amino acid sequencing, Western blot analysis and kinetic studies. The dual trypsin- and chymotrypsin-like enzymatic specificity of rK9 was assessed by determining specificity constants (k(cat)/K(m)) for the hydrolysis of fluorogenic substrates, the peptide sequences of which were derived from proparathyroid hormone (pro-PTH) and from semenogelin-I. Our results confirmed the presence of an extended binding site in the rK9 active site. We also identified a far more sensitive substrate of this enzyme than those previously described, Abz-VKKRSARQ-EDDnp, which was hydrolysed with a catalytic efficiency k(cat)/K(m) of 420000 M(-1)s(-1). Finally, we showed that four of the five major proteins contained in secretions of rat seminal vesicles were rapidly degraded by recombinant rK9.

Human Kallikrein 2 (hK2) and prostate-specific antigen (PSA): two closely related, but distinct, kallikreins in the prostate

Recent studies on human kallikrein 2 (hK2) have revealed striking similarities and significant differences with the closely related kallikrein PSA. Both PSA and hK2 are primarily localized to the prostate and share close structural similarities. Although both kallikreins are produced by the same secretory epithelial cells in the prostate, hK2 is associated more with prostate tumors than PSA and is highly expressed in poorly differentiated cancer cells. The potent trypsin-like activity of hK2 contrasts with the weak chymotrypsin-like activity of PSA. The inactive precursor form of PSA, proPSA, is converted rapidly to active PSA by hK2, suggesting an important in vivo regulatory function by hK2 on PSA activity. The high homology between hK2 and PSA results in significant cross-reactivity to hK2 by polyclonal and some monoclonal antibodies to PSA. Future studies on both PSA and hK2 need to take into account this potential for cross-reactivity. Specific monoclonal antibodies to hK2 have now demonstrated that serum levels of hK2, like PSA, are correlated with prostate cancer. The production of hK2 protein in active protease form and specific monoclonal antibodies to the hK2 antigen will allow extensive future studies delineating the physiological and clinical utility of this new prostate antigen.