Bikunin in rat plasma, lymph and bile

Levels of Urinary Trypsin Inhibitor and Structure of Its Chondroitin Sulphate Moiety in Type 1 and Type 2 Diabetes

BACKGROUND

Diabetes mellitus is a global health problem representing the fifth leading cause of mortality and a major risk factor for cardiovascular diseases. In the last years, we reported an association among urinary trypsin inhibitor (UTI), a small proteoglycan that plays pleiotropic roles in many inflammatory processes, and both type 1 and 2 diabetes and developed a method for its direct quantitation and structural characterization.

METHODS

Urine from 39 patients affected by type 1 diabetes, 32 patients with type 2 diabetes, and 52 controls were analysed. UTI was separated from the main glycosaminoglycans physiologically present in urine by anion exchange chromatography, treated for chondroitin sulphate (CS) chain complete depolymerisation, and analysed for both UTI content and CS structure. UTI identification was performed by nano-LC-MS/MS analysis.

RESULTS

We evidenced increased UTI levels, as well as reduced sulphation of its CS moiety in association with diabetes, regardless of both age and medium-term glycaemic control. Furthermore, no association between UTI and albumin excretion rate was found.

CONCLUSIONS

Evidences suggest that UTI levels are not directly correlated with renal function or, otherwise, that they may increase before the onset of renal impairment in diabetes, representing a potential marker for the underlying inflammatory condition.

Heavy chains of inter alpha inhibitor (IαI) inhibit the human complement system at early stages of the cascade

Inter alpha inhibitor (IαI) is an abundant serum protein consisting of three polypeptides: two heavy chains (HC1 and HC2) and bikunin, a broad-specificity Kunitz-type proteinase inhibitor. The complex is covalently held together by chondroitin sulfate but during inflammation IαI may interact with TNF-stimulated gene 6 protein (TSG-6), which supports transesterification of heavy chains to hyaluronan. Recently, IαI was shown to inhibit mouse complement in vivo and to protect from complement-mediated lung injury but the mechanism of such activity was not elucidated. Using human serum depleted from IαI, we found that IαI is not an essential human complement inhibitor as was reported for mice and that such serum has unaltered hemolytic activity. However, purified human IαI inhibited classical, lectin and alternative complement pathways in vitro when added in excess to human serum. The inhibitory activity was dependent on heavy chains but not bikunin and detected at the level of initiating molecules (MBL, properdin) in the lectin/alternative pathways or C4b in the classical pathway. Furthermore, IαI affected formation and assembly of the C1 complex and prevented assembly of the classical pathway C3-convertase. Presence and putative interactions with TSG-6 did not affect the ability of IαI to inhibit complement thus implicating IαI as a potentially important complement inhibitor once enriched onto hyaluronan moieties in the course of local inflammatory processes. In support of this, we found a correlation between IαI/HC-containing proteins and hemolytic activity of synovial fluid from patients suffering from rheumatoid arthritis.

Identification of bikunin as an endogenous inhibitor of dynorphin convertase in human cerebrospinal fluid

Dynorphin-converting enzymes constitute a group of peptidases capable of converting dynorphins to enkephalins. Through the action of these enzymes, the dynorphin-related peptides bind to delta-opioid instead of kappa-opioid receptors, leading to a change in the biological function of the neuropeptides. In this article, we describe the identification of the protein bikunin as an endogenous, competitive inhibitor of a dynorphin-converting enzyme in human cerebrospinal fluid. This protein is present together with its target enzyme in the same body fluids. The K(M) value of the convertase was found to be 9 microm, and the K(i) value of the inhibitor was 1.7 nm. The finding indicates that bikunin may play a significant role as a regulatory mechanism of neuropeptides, where one bioactive peptide is converted to a shorter sequence, which in turn, can affect the action of its longer form.

Plasma bikunin: half-life and tissue uptake

Bikunin is a chondroitin sulfate-containing plasma protein synthesized in the liver. In vitro, it has been shown to inhibit proteases and to have additional activities, but its biological function is still unclear. Here we have studied the dynamics of plasma bikunin in rats and mice. A half-life of 7 +/- 2 min was obtained from the time course of the decrease of the plasma level of bikunin following hepatectomy. Clearance experiments with intravenously injected radiolabeled bikunin with or without the chondroitin sulfate chain showed that the polysaccharide had little influence on the elimination rate of the protein. The uptake of bikunin by different tissues was studied using bikunin labeled with the residualizing agent 125I-tyramine cellobiose; 60 min after intravenous injection, 49% of the radioactivity was recovered in the kidneys and 6-11% in the liver, bones, skin, intestine and skeletal muscle. The uptake in the liver was analyzed by intravenous injection of radiolabeled bikunin followed by collagenase perfusion and dispersion of the liver cells. These experiments indicated that bikunin is first trapped extracellularly within the liver before being internalized by the cells.

Plasma and urine levels of urinary trypsin inhibitor in patients with acute and fulminant hepatitis

BACKGROUND AND AIM

Urinary trypsin inhibitor (UTI) is synthesized by hepatocytes and excreted into urine. Plasma and urine UTI levels have been measured to evaluate whether these levels may be useful markers in various pathological conditions. However, there has been no study on plasma and urine UTI levels in patients with acute liver diseases. The aim of the present study was to evaluate plasma and urine UTI levels and their relationship with the severity of hepatic damage in patients with acute liver diseases.

METHODS

Plasma and urine UTI levels were measured by newly developed enzyme-linked immunosorbent assay in 15 patients with acute hepatitis (AH), 12 patients with acute severe hepatitis (ASH) and 10 patients with fulminant hepatitis (FH), as assessed on admission. The serial changes in plasma and urine UTI were also observed in some patients with AH and ASH.

RESULTS

Plasma UTI levels (U/mL, median [25-75th percentile]) were: 11.0, (9.5-16.1) in patients with AH; 7.8 (5.6-11.5) in those with ASH; 6.5 (4.0-9.5) in patients with FH; and 9.7 (7.3-11.0) in normal controls. Plasma UTI levels in patients with FH were significantly lower than in those with AH. Plasma UTI levels showed significant positive correlations with the levels of prothrombin time (PT), hepaplastin test, antithrombin III, alpha2-plasmin inhibitor, plasminogen (Plg) and fibrinogen. After the recovery of liver dysfunction, increased plasma UTI levels in patients with AH were decreased, whereas previously decreased plasma UTI levels in patients with ASH were increased. Urine UTI levels were significantly increased in patients with AH compared with those of normal controls. In patients with ASH and FH, urine UTI levels were increased but not significantly. Urine UTI levels significantly positively correlated with PT and Plg. After the recovery of liver dysfunction, previously increased urine UTI levels in patients with AH were decreased. The correlation between plasma UTI and urine UTI levels was not significant.

CONCLUSIONS

The findings of the present study suggested that the levels of plasma and urine UTI changed in patients with AH and were closely related to the abnormalities of coagulo-fibrinolysis, including PT. Further studies are needed to clarify whether these levels may be useful markers to predict the prognosis of acute hepatitis.

Expression of a Functional Proteinase Inhibitor Capable of Accepting Xylose: Bikunin

Bikunin is a Kunitz-type proteinase inhibitor, which is cross-linked to heavy chains via a chondroitin sulfate chain, forming inter-alpha-inhibitor and related molecules. Rat bikunin was produced by baculovirus-infected insect cells. The protein could be purified with a total yield of 20 mg/liter medium. Unlike naturally occuring bikunin the recombinant protein had no galactosaminoglycan chain. Endoglycosidase digestion also suggested that the recombinant form lacked N-linked oligosaccharides. Bikunin is translated as a part of a precursor, alpha1-microglobulin/bikunin, but the functional significance of the cotranslation is unknown. Our results indicate that the proteinase inhibitory function of bikunin is not regulated by the alpha1-microglobulin-part of the alpha1-microglobulin/bikunin precursor since recombinant bikunin had the same trypsin inhibitory activity as the recombinant precursor. Both free bikunin and the precursor were also functional as a substrate in an in vitro xylosylation system. This demonstrates that the alpha1-microglobulin-part is not necessary for the first step of galactosaminoglycan assembly.

Bikunin--not just a plasma proteinase inhibitor

Bikunin is a plasma proteinase inhibitor that has received little attention in the past, probably because its activity towards various proteinases was found to be relatively weak in early work. It was recently discovered, however, that bikunin effectively inhibits a proteinase that seems to be involved in the metastasis of tumour cells--cell surface plasmin--and that a fragment of bikunin inhibits two proteinases of the coagulation pathway--factor Xa and kallikrein. Furthermore, it has been found that bikunin has other properties, such as the ability to modulate cell growth and to block cellular calcium uptake. Most of the bikunin in the blood occurs as a covalently linked subunit of the proteins pre- and inter-alpha-inhibitor. In this form bikunin lacks some of its known activities, and there is evidence that its release by partial proteolytic degradation may function as a regulatory mechanism. Although the physiological function of bikunin still remains to be established, current data suggest that this protein plays a role in inflammation. Further studies could therefore lead to results of therapeutical value.

Binding of Zn(2+) to the plasma protein inter-alpha-inhibitor

Inter-alpha-inhibitor (IalphaI) is a serum protein consisting of a chondroitin-sulfate-containing protein of 25 kDa (bikunin) and two other polypeptides of 75-80 kDa (heavy chains 1 and 2). The physiological function of IalphaI is unclear but recent results suggest that it is required for the formation of the extracellular matrix of certain cell types and that it has anti-inflammatory activity. It was previously reported that IalphaI isolated from serum contains bound Zn(2+), but details of this binding are lacking. Using equilibrium dialysis, we have found that when the free Zn(2+) concentration is raised from 0.3 to 50 micromol/L, the number of bound ions increases from 0.1 to 7. The concentration of free Zn(2+) in plasma is in the nanomolar range; our results therefore suggest that inter-alpha-inhibitor does not contain stoichiometric amounts of zinc ions under normal in vivo conditions.

Generation of catalytically active granzyme K from Escherichia coli inclusion bodies and identification of efficient granzyme K inhibitors in human plasma

Granzymes are granule-stored lymphocyte serine proteases that are implicated in T- and natural killer cell-mediated cytotoxic defense reactions after target cell recognition. A fifth human granzyme (granzyme 3, lymphocyte tryptase-2), renamed as granzyme K (gene name GZMK), has recently been cloned from lymphocyte tissue. For its further characterization we successfully generated catalytically active enzyme in milligram quantities per liter of Escherichia coli culture. The natural proform of granzyme K with the amino-terminal propeptide Met-Glu was expressed as inclusion bodies and converted to its active enzyme by cathepsin C after refolding of precursor molecules. Recombinant granzyme K cleaves synthetic thiobenzyl ester substrates after Lys and Arg with k(cat)/K(m) values of 3.7 x 10(4) and 4.4 x 10(4) M(-1) s(-1), respectively. Granzyme K activity was shown to be inhibited by the synthetic compounds Phe-Pro-Arg-chloromethyl ketone, phenylmethylsulfonyl fluoride, PefablocSC, and benzamidine, by the Kunitz-type inhibitor aprotinin and by human blood plasma. The plasma-derived inter-alpha-trypsin inhibitor complex, its bikunin subunit, and the second carboxyl-terminal Kunitz-type domain of bikunin were identified as genuine physiologic inhibitors with K(i) values of 64, 50, and 22 nM, respectively. Inter-alpha-trypsin inhibitor and free bikunin have the potential to neutralize extracellular granzyme K activity after T cell degranulation and may thus control unspecific damage of bystander cells at sites of inflammatory reactions.

Urinary trypsin inhibitor suppresses vascular smooth muscle contraction by inhibition of Ca2+ influx

Urinary trypsin inhibitor (UTI) and its precursor form inter-alpha trypsin inhibitor (ITI) are present in plasma. To determine the action of UTI on blood vessels, we performed isometric vascular muscle contraction tests, microcirculation studies and measurement of cytosolic free Ca2+ in vascular smooth muscle cells. An isometric vascular muscle contraction test showed that the contractions stimulated by endothelin-1 or norepinephrine were suppressed in the presence of UTI, and that the contractions were not inhibited in the presence of ITI. The microcirculation study showed that the contraction of mesenteric arterioles of WKY rats induced by norepinephrine were inhibited by treatment of UTI, and that they did not alter by treatment of ITI. Pre-incubation of UTI, but not ITI, with vascular smooth muscle cells inhibited the increase of cytosolic free Ca2+ induced by endothelin-1 or norepinephrine. Cell-binding study by biotinylated UTI showed that vascular smooth muscle cells have specific binding site for UTI, but not for ITI. We propose that circulating UTI converted from ITI has a regulatory effect on local vascular tone by regulation of Ca2+ influx into smooth muscle cells.

Intracellular coupling of bikunin and the heavy chain of rat pre-alpha-inhibitor in COS-1 cells

Pre-alpha-inhibitor is a serum protein consisting of two polypeptides: bikunin of 16 kDa, which carries an 8 kDa chondroitin sulphate chain, and heavy chain 3 (H3) of 74 kDa. The two polypeptides are linked through an ester bond between an internal N-acetylgalactosamine residue of the chondroitin sulphate chain and the C-terminal aspartic acid residue of H3. Both bikunin and H3 are synthesized by hepatocytes and become linked as they pass through the Golgi complex. H3 is synthesized with both N- and C-terminal extensions which are released during intracellular transport. To be able to analyse the assembly of pre-alpha-inhibitor in detail, we have cloned and sequenced the cDNA of rat H3. Upon expression of the protein in COS-1 cells, both propeptides were found to be released. Furthermore, co-expression of H3 and bikunin resulted in the two polypeptides becoming coupled, indicating that cells other than hepatocytes may have the capacity to form chondroitin sulphate-containing links.

Increase of bikunin and alpha1-microglobulin concentrations in urine of rats during pregnancy is due to decreased tubular reabsorption

Bikunin and alpha1-microglobulin are two plasma proteins of about 25 kDa which are made in the liver from a common precursor. The concentration of bikunin in human urine has been shown to increase several fold during various conditions of stress. The mechanism behind this increase is unknown. We have studied pregnant rats and found that the bikunin and alpha1-microglobulin levels in their urine increased 3-fold towards the end of the pregnancy, whereas those of albumin and orosomucoid did not. There were no significant changes in either the bikunin/alpha1-microglobulin mRNA level or the concentrations of the two proteins in serum. These findings imply that the synthesis and the clearance rates of bikunin and alpha1-microglobulin are normal during pregnancy but that the tubular reabsorption of these proteins is decreased.

High glomerular permeability of bikunin despite similarity in charge and hydrodynamic size to serum albumin

Bikunin is a chondroitin-sulphate containing serum protein with a Stokes-Einstein radius and a negative net charge close to those of serum albumin. The plasma half life of bikunin is about 10 minutes, and approximately half of its clearance occurs in the kidneys. The quantitative role of glomerular filtration in the renal clearance of this protein has not been determined. To assess the glomerular permeability of bikunin we used isolated rat kidneys that were perfused with an albumin solution. The metabolic activities of the tubuli were inhibited by low temperature (8 degrees C). The clearances of radiolabeled bikunin and albumin were repeatedly determined under identical conditions. The fractional clearance of bikunin was found to be 80 times higher than that of albumin: 15% +/- 1% versus 0.18% +/- 0.02%. This value for bikunin can fully account for its renal clearance in vivo. It has previously been shown that uncharged flexible solutes, such as dextrans, have higher renal clearances than globular molecules with similar radii. The high glomerular permeability of bikunin is therefore probably due to its elongated and flexible configuration. Moreover, the observed clearance value of the anionic molecule bikunin is close to that of a neutral flexible dextran of similar size, indicating that the charge of bikunin is of little importance for its glomerular permeability.

Plasma clearance of rat bikunin: evidence for receptor-mediated uptake

Bikunin is a chondroitin sulphate-containing protease inhibitor with a molecular mass of 25 kDa. It is secreted into the blood by hepatocytes, and recent observations indicate that it may have an extravascular function. Here we have studied the plasma clearance of bikunin in rats and mice. On intravenous injection, radiolabelled bikunin was found to have a half-life of 10 min; in rats with ligated renal arteries, the clearance time was twice as long, implying that the kidneys account for half the uptake. As judged by gel filtration, the size of bikunin is similar to that of albumin. Autoradiographic analysis of kidneys removed 2 min after the injection of radiolabelled bikunin indicated that, despite its size, bikunin is cleared by glomerular filtration. On ligation of the renal arteries, the plasma concentration of bikunin increased linearly to at least four times normal. This finding shows that the non-renal uptake system is saturated and therefore presumably receptor-mediated. Most of the non-renal uptake of injected bikunin was found to occur in non-visceral tissues such as the skin. Analysis of skin samples by autoradiography after injection of radiolabelled bikunin suggested that bikunin had been transferred from the plasma to the interstitial space.

Inter-alpha-inhibitor is required for the formation of the hyaluronan-containing coat on fibroblasts and mesothelial cells

Cultured cells of various origins have been shown to be surrounded by a hyaluronan-containing coat, a structure that can be visualized by its ability to exclude large particles such as erythrocytes. When cultured in medium with no or low concentrations of serum, the cells lose their coats, although they still produce hyaluronan; upon the addition of serum, the coats are formed again. Here, we show that the serum protein inter-alpha-inhibitor can replace whole serum as an inducer of the formation of the coats on fibroblasts and mesothelial cells. The physiological role of inter-alpha-inhibitor has so far been unclear; our findings, together with those obtained with cumulus cell-oocyte complexes (Chen, L., Mao, S.J., and Larsen, W. J. (1992) J. Biol. Chem. 267, 12380-12386), suggest that inter-alpha-inhibitor and related proteins have a general function as stabilizers of hyaluronan-containing pericellular coats.