Inhibitory effect of bikunin on calcium oxalate crystallization in vitro and urinary bikunin decrease in renal stone formers

Kidney stone matrix proteins: Role in stone formation

Stone formation is induced by an increased level of urine crystallization promoters and reduced levels of its inhibitors. Crystallization inhibitors include citrate, magnesium, zinc, and organic compounds such as glycosaminoglycans. In the urine, there are various proteins, such as uromodulin (Tamm-Horsfall protein), calgranulin, osteopontin, bikunin, and nephrocalcin, that are present in the stone matrix. The presence of several carboxyl groups in these macromolecules reduces calcium oxalate monohydrate crystal adhesion to the urinary epithelium and could potentially protect against lithiasis. Proteins are the most abundant component of kidney stone matrix, and their presence may reflect the process of stone formation. Many recent studies have explored the proteomics of urinary stones. Among the stone matrix proteins, the most frequently identified were uromodulin, S100 proteins (calgranulins A and B), osteopontin, and several other proteins typically engaged in inflammation and immune response. The normal level and structure of these macromolecules may constitute protection against calcium salt formation. Paradoxically, most of them may act as both promoters and inhibitors depending on circumstances. Many of these proteins have other functions in modulating oxidative stress, immune function, and inflammation that could also influence stone formation. Yet, the role of these kidney stone matrix proteins needs to be established through more studies comparing urinary stone proteomics between stone formers and non-stone formers.

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.

Nephrolithiasis: molecular mechanism of renal stone formation and the critical role played by modulators

Urinary stone disease is an ailment that has afflicted human kind for many centuries. Nephrolithiasis is a significant clinical problem in everyday practice with a subsequent burden for the health system. Nephrolithiasis remains a chronic disease and our fundamental understanding of the pathogenesis of stones as well as their prevention and cure still remains rudimentary. Regardless of the fact that supersaturation of stone-forming salts in urine is essential, abundance of these salts by itself will not always result in stone formation. The pathogenesis of calcium oxalate stone formation is a multistep process and essentially includes nucleation, crystal growth, crystal aggregation, and crystal retention. Various substances in the body have an effect on one or more of the above stone-forming processes, thereby influencing a person's ability to promote or prevent stone formation. Promoters facilitate the stone formation while inhibitors prevent it. Besides low urine volume and low urine pH, high calcium, sodium, oxalate and urate are also known to promote calcium oxalate stone formation. Many inorganic (citrate, magnesium) and organic substances (nephrocalcin, urinary prothrombin fragment-1, osteopontin) are known to inhibit stone formation. This review presents a comprehensive account of the mechanism of renal stone formation and the role of inhibitors/promoters in calcium oxalate crystallisation.

Bikunin and α1-microglobulin/bikunin precursor (AMBP) gene mutational screening in patients with kidney stones: a case-control study

OBJECTIVE

Bikunin is an inhibitor of kidney stone formation synthesized in the liver together with α(1)-microglobulin from the α(1)-microglobulin/bikunin precursor (AMBP) gene. The aim of this study was to investigate the possible association between bikunin/AMBP gene polymorphisms and urinary stone formation.

MATERIAL AND METHODS

To analyse the DNA, blood samples were taken from 75 kidney stone formers who had a familial stone history, 35 sporadic stone formers and 101 healthy individuals. Four exons of bikunin gene and five parts of the promoter region of the AMBP gene were screened using single-strand conformation polymorphism and nucleotide sequence analysis.

RESULTS

The Init-2 region of the promoter of AMBP gene had polymorphisms at positions -218 and -189 nt giving three different genotypes having 1,3, 2,4 and 1,2,3,4 alleles with frequencies of 17.06%, 60.19% and 22.75%, respectively, in all groups. Therefore, the Init-2 region appears to be polymorphic. As a result, the 1,3 allele has -218G and -189T complying with the reference database sequence, the 2,4 allele has -218G and T-189C substitution and the allele 1,2,3,4 genotype has substitutions at positions G-218C and T-189C.

CONCLUSIONS

There were no significant differences in allele distribution between patients and controls. These common alleles exist in the Turkish population independent of stone formation. These results are the first to demonstrate the existence of bikunin and AMBP promoter polymorphism. Although the Init-2 region of the AMBP gene is the binding site for various transcription factors, the results showed no association between these observed genotypes and stone-forming phenotypes.

Crystallization of calcium oxalates is controlled by molecular hydrophilicity and specific polyanion-crystal interactions

To gain more insight into protein structure-function relationships that govern ectopic biomineralization processes in kidney stone formation, we have studied the ability of urinary proteins (Tamm-Horsfall protein, osteopontin (OPN), prothrombin fragment 1 (PTF1), bikunin, lysozyme, albumin, fetuin-A), and model compounds (a bikunin fragment, recombinant-, milk-, bone osteopontin, poly-L-aspartic acid (poly asp), poly-L-glutamic acid (poly glu)) in modulating precipitation reactions of kidney stone-related calcium oxalate mono- and dihydrates (COM, COD). Combining scanning confocal microscopy and fluorescence imaging, we determined the crystal faces of COM with which these polypeptides interact; using scanning electron microscopy, we characterized their effects on crystal habits and precipitated volumes. Our findings demonstrate that polypeptide adsorption to COM crystals is dictated first by the polypeptide's affinity for the crystal followed by its preference for a crystal face: basic and relatively hydrophobic macromolecules show no adsorption, while acidic and more hydrophilic polypeptides adsorb either nonspecifically to all faces of COM or preferentially to {100}/{121} edges and {100} faces. However, investigating calcium oxalates grown in the presence of these polypeptides showed that some acidic proteins that adsorb to crystals do not affect crystallization, even if present in excess of physiological concentrations. These proteins (albumin, bikunin, PTF1, recombinant OPN) have estimated total hydrophilicities from 200 to 850 kJ/mol and net negative charges from -9 to -35, perhaps representing a "window" in which proteins adsorb and coat urinary crystals (support of excretion) without affecting crystallization. Strongest effects on crystallization were observed for polypeptides that are either highly hydrophilic (>950 kJ/mol) and highly carboxylated (poly asp, poly glu), or else highly hydrophilic and highly phosphorylated (native OPN isoforms), suggesting that highly hydrophilic proteins strongly affect precipitation processes in the urinary tract. Therefore, the level of hydrophilicity and net charge is a critical factor in the ability of polypeptides to affect crystallization and to regulate biomineralization processes.

Genetic basis of renal cellular dysfunction and the formation of kidney stones

Nephrolithiasis is a result of formation and retention of crystals within the kidneys. The driving force behind crystal formation is urinary supersaturation with respect to the stone-forming salts, which means that crystals form when the concentrations of participating ions are higher than the thermodynamic solubility for that salt. Levels of supersaturation are kept low and under control by proper functioning of a variety of cells including those that line the renal tubules. It is our hypothesis that crystal deposition, i.e., formation and retention in the kidneys, is a result of impaired cellular function, which may be intrinsic and inherent or triggered by external stimuli and challenges. Cellular impairment or dysfunction affects the supersaturation, by influencing the excretion of participating ions such as calcium, oxalate and citrate and causing hypercalciuria, hyperoxaluria or hypocitraturia. The production and excretion of macromolecular promoters and inhibitors of crystallization is also dependent upon proper functioning of the renal epithelial cells. Insufficient or ineffective crystallization modulators such as osteopontin, Tamm-Horsfall protein, bikunin, etc. are most likely produced by the impaired cells.

Structural analysis of bikunin glycosaminoglycan

The structure of an intact glycosaminoglycan (GAG) chain of the bikunin proteoglycan (PG) was analyzed using a combined top-down and bottom-up sequencing strategy. PGs are proteins with one or more linear, high-molecular weight, sulfated GAG polysaccharides O-linked to serine or threonine residues. GAGs are often responsible for the biological functions of PGs, and subtle variations in the GAG structure have pronounced physiological effects. Bikunin is a serine protease inhibitor found in human amniotic fluid, plasma, and urine. Bikunin is posttranslationally modified with a chondroitin sulfate (CS) chain, O-linked to a serine residue of the core protein. Recent studies have shown that the CS chain of bikunin plays an important role in the physiological and pathological functions of this PG. While no PG or GAG has yet been sequenced, bikunin, the least complex PG, offers a compelling target. Electrospray ionization Fourier transform-ion cyclotron resonance mass spectrometry (ESI FTICR-MS) permitted the identification of several major components in the GAG mixture having molecular masses in a range of 5505-7102 Da. This is the first report of a mass spectrum of an intact GAG component of a PG. FTICR-MS analysis of a size-uniform fraction of bikunin GAG mixture obtained by preparative polyacrylamide gel electrophoresis, allowed the determination of chain length and number of sulfo groups in the intact GAGs.

Frequent expression loss of Inter-alpha-trypsin inhibitor heavy chain (ITIH) genes in multiple human solid tumors: a systematic expression analysis

Background

The inter-alpha-trypsin inhibitors (ITI) are a family of plasma protease inhibitors, assembled from a light chain – bikunin, encoded by AMBP – and five homologous heavy chains (encoded by ITIH1, ITIH2, ITIH3, ITIH4, and ITIH5), contributing to extracellular matrix stability by covalent linkage to hyaluronan. So far, ITIH molecules have been shown to play a particularly important role in inflammation and carcinogenesis.

Methods

We systematically investigated differential gene expression of the ITIH gene family, as well as AMBP and the interacting partner TNFAIP6 in 13 different human tumor entities (of breast, endometrium, ovary, cervix, stomach, small intestine, colon, rectum, lung, thyroid, prostate, kidney, and pancreas) using cDNA dot blot analysis (Cancer Profiling Array, CPA), semiquantitative RT-PCR and immunohistochemistry.

Results

We found that ITIH genes are clearly downregulated in multiple human solid tumors, including breast, colon and lung cancer. Thus, ITIH genes may represent a family of putative tumor suppressor genes that should be analyzed in greater detail in the future. For an initial detailed analysis we chose ITIH2 expression in human breast cancer. Loss of ITIH2 expression in 70% of cases (n = 50, CPA) could be confirmed by real-time PCR in an additional set of breast cancers (n = 36). Next we studied ITIH2 expression on the protein level by analyzing a comprehensive tissue micro array including 185 invasive breast cancer specimens. We found a strong correlation (p < 0.001) between ITIH2 expression and estrogen receptor (ER) expression indicating that ER may be involved in the regulation of this ECM molecule.

Conclusion

Altogether, this is the first systematic analysis on the differential expression of ITIH genes in human cancer, showing frequent downregulation that may be associated with initiation and/or progression of these malignancies.

Inhibitors of calcification in blood and urine

In bone and teeth formation, coordinated calcification is a highly desirable biological process. However, heterotopic calcification at unwanted tissue sites leads to dysfunction, disease and, potentially, to death and therefore requires prevention and treatment. With the recent discovery of calcification inhibitors we now know that biological calcification is not passive but a complex, active and highly regulated process. Calcification at vascular sites is the most threatening localization and manifests as part of atherosclerosis or arteriosclerosis. Atherosclerosis is often accompanied by intimal plaque calcification, whereas arteriosclerosis is characterized by calcification of the media. The severity of calcification of cerebral or coronary atherosclerotic plaques is associated with an increased incidence of events such as stroke or myocardial infarction. Medial calcification is the major cause of arterial stiffness, which contributes to left ventricular dysfunction and heart failure. Patients with chronic kidney disease are at especially increased risk for both intimal and medial calcification. In this context, it is currently thought that calcium-regulatory factors including fetuin-A, matrix Gla protein, osteoprotegerin, and pyrophosphates act in a local or systemic manner to prevent calcifications of the vasculature, and that dys-regulations of such calcification inhibitors may contribute to progressive calcifications. Nephrolithiasis represents another process of unwanted calcification responsible for significant morbidity. More than 80% of renal stones contain calcium. Urinary factors inhibiting calcification are citrate, glycosaminoglycans, Tamm-Horsfall protein, and osteopontin. This review summarizes current experimental and clinical data underlining the biological importance of these calcification inhibitors.

Urine protein markers distinguish stone-forming from non-stone-forming relatives of calcium stone formers

We have investigated urine protein inhibitors of calcium oxalate crystallization to determine whether variations in these proteins are associated with kidney stone disease and whether protein measurements improve the identification of stone formers compared with conventional risk factors (RF). Using Western blotting, we studied variations in the electrophoretic mobility patterns and relative abundances of crystallization-inhibitory proteins in urine from 50 stone-forming (SF) and 50 non-stone-forming (NS) first-degree relatives of calcium SF patients, matched by gender and age. Standard urine chemistry stone risk measurements were also made. Multivariate discriminant analysis was used to test the association of these proteins with nephrolithiasis. Differences in form and abundance of several urine proteins including inter-alpha-trypsin inhibitor (ITI), prothrombin fragment 1 (PF1), CD59, and calgranulin B (calB) were found to be associated with stone formation. By multivariate discriminant analysis, measurements of forms of PF1, ITI, and calB in men and ITI and CD59 in women, classified 84% of men and 76% of women correctly by stone status. In contrast, standard urine chemistry RF identified only 70% of men correctly and failed to distinguish female SF from NS. Thus a small subset of protein measurements distinguished SF from NS far better than conventional RF in a population of relatives of calcium SF, illustrating the significant association of these proteins with stone disease. Variations in these proteins may serve as markers of stone disease activity or vulnerability to recurrence and may provide new insights into mechanisms of stone formation.

A simple procedure for isolating microgram quantities of biologically active bikunin from human urine

OBJECTIVE

To report a simple, relatively rapid protocol to isolate biologically active bikunin from human urine using ion-exchange-trypsin affinity chromatography. Bikunin is a protease inhibitor which has been shown to play a role in various processes, including inhibition of calcium oxalate crystallization, the regulation of proliferation and modulation of carcinogenesis. The unavailability of the purified protein has hampered studies on bikunin's expanding role in these processes.

MATERIALS AND METHODS

Female human urine was dialysed (15 kDa threshold) and crudely fractionated with a double-saturated ammonium sulphate precipitation. The first precipitation was with 35% saturated ammonium sulphate, and the supernatant was harvested, and the second with 90% saturated ammonium sulphate, and the precipitate collected. The protein mixture was then passed over Sepharose SP-fast-flow cation exchange and Sepharose Q-fast-flow anion exchange columns connected in series. The final purification was with a trypsin-affinity column which selectively bound bikunin.

RESULTS

This procedure could recover 1 microg of bikunin per 2 mL of urine, and the final product was essentially free of contaminating inter-alpha-trypsin inhibitor heavy chains or bikunin-heavy chain conjugates. Product purity was confirmed by two-dimensional polyacrylamide gel electrophoresis combined with silver staining or Western blot. All isolations contained the 17 kDa minimally glycoslyated/sulphated form of bikunin and the 28 kDa form of bikunin. Some preparations also contained 33-48 kDa forms of bikunin. The protein cores of all three proteins were confirmed to be bikunin by mass spectrometry and Western blot. Harvested bikunin retained its trypsin inhibitory activity (L-benzoylarginine-p-nitroanilide assay). Preparations containing the 33-45 kDa form had two to three times more trypsin inhibitory activity than preparations without this band.

CONCLUSIONS

This novel ion exchange-trypsin affinity chromatography protocol uses only two chromatographic steps. The product consists of three isomers of biologically active bikunin, free of contaminating heavy chains or bikunin-heavy chain conjugates. The ready availability of purified bikunin should facilitate future studies of bikunin's emerging role in urolithiasis, proliferation and carcinogenesis.

The role of glycoproteins in calcium oxalate crystal development

OBJECTIVE

To assess the effects of a glycoprotein (mucine) on calcium oxalate crystal development in different conditions and situations, to clarify some of its possible effects.

MATERIALS AND METHODS

Crystallization was assessed using a batch system in presence of mucine suspensions, by kinetic-turbidimetric measurements, and using a flow system in the presence of retained agglomerates of mucine, evaluating the precipitated calcium oxalate.

RESULTS

In batch conditions low mucine concentrations (<15 mg/L) inhibited calcium oxalate nucleation and higher concentrations (<250 mg/L) inhibited calcium phosphate nucleation, whereas at high concentrations there was also promotion. The presence of an aggregate of mucine in the flow system provoked calcium oxalate monohydrate crystallization at 0.691 microg/h per mg of mucine. In flow conditions pyrophosphate at 11.5 micromol/L caused a decrease of 84% in the calcium oxalate crystallized on mucine, 1.32 mmol/L of citrate a decrease of a 83%, 20 mg/L of pentosan polysulphate a decrease by 80%, and 7.58 micromol/L phytate totally prevented the crystallization of calcium oxalate on mucine.

CONCLUSION

All substances inhibiting calcium oxalate crystallization with the capacity to interact with calcium ions also have crystallization promoting properties when they are at sufficiently high concentrations, because of their capacity to form agglomerates or the insolubility of their calcium salts.

Identification of bikunin isolated from human urine inhibits calcium oxalate crystal growth and its localization in the kidneys

BACKGROUND

We previously reported a high molecular weight substance purified from human urine that strongly inhibited calcium oxalate (CaOx) crystal growth in vitro. In the study present herein, we identified and investigated a protein purified from human urine that strongly inhibits CaOx crystal growth using a column chromatography series.

METHODS

The protein was identified by amino acid sequencing and was investigated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), western blotting and immunohistochemical staining.

RESULTS

The molecular weight of this protein was approximately 35 KDa, and it also had another band around 20 KDa. We determined that the amino acid sequence of the protein was homologous with that of bikunin, the light chain of the inter-alpha-trypsin inhibitor, which is known as a strong CaOx crystallization inhibitor in vitro. On western blotting analysis, the molecular weight was also found to be around 35K Da, the same as that of bikunin. Immunohistochemical staining revealed that it was mainly located in the epithelial cells of the proximal tubules and the thin descending segment near the loop of Henle, but not in the glomeruli, distal tubules or the collecting ducts.

CONCLUSION

In the present study, the protein extracted from human urine was identical to bikunin, which may be expressed mainly in the proximal tubules and the thin descending segment near the loop of Henle, and which prevents CaOx crystallization in vitro.

Urinary bikunin determination provides insight into proteinase/proteinase inhibitor imbalance in patients with inflammatory diseases

Bikunin (BK) is a Kunitz-type proteinase inhibitor responsible for most of the antitryptic activity of urine and so is known as the urinary trypsin inhibitor. As its excretion increases in inflammatory conditions, it is often considered to be a positive acute phase protein (APP). However, the gene for BK is downregulated in inflammation. In human plasma the major part of BK is covalently linked through a glycosaminoglycan chain to one or two homologous peptide heavy chains, thus forming high molecular weight proteinase inhibitors called pre-alpha-inhibitor (PalphaI) and inter-alpha-inhibitor (IalphaI), respectively. The C-terminal parts of these heavy chains are very sensitive to proteolysis. Neutrophil proteinases in particular are able to release from IalphaI and PalphaI BK (M, about 25,000) which retains its antitryptic activity and is quickly excreted in urine. It was therefore an early supposition that the higher urinary excretion of BK occurring during inflammatory diseases should be, at least in some respect, related to a partial proteolysis of IalphaI and PalphaI. In this study we observed that BK, determined as antitryptic activity, was clearly increased in urine from 35 patients with inflammatory diseases varying in origin and severity (76.5 +/- 75.5 IU/g vs. reference value <10 IU/g creatinine). This increase seems mainly to be associated with polymorphonuclear leukocyte activation, monitored by human leukocyte elastase (HLE) determination rather than with the acute phase response assessed by C-reactive protein (CRP) measurement. For all the patients we found that the urinary levels of BK and serum concentration of intact IalphaI correlated inversely (r=-0.36; p=0.03), in agreement with the presumed precursor-product relationship linking IalphaI and BK. We also proved that urinary BK was significantly higher, and serum IalphaI was significantly lower, in samples with plasma HLE values above the reference: 90 microg/l. Taken together, our results demonstrate that BK, the urinary excretion of which is increased in inflammatory conditions, originates, at least partly, from IalphaI and PalphaI by proteolytic cleavage. Consequently, urinary BK determination provides information on the severity of systemic proteolysis occurring in inflammation. We also demonstrated that during inflammatory diseases IalphaI and PalphaI concentrations in serum are dependent on their increased utilization as well as on the regulation of their biosynthesis.

Changing concepts in the aetiology of renal stones

In the past two decades an increasing number of nephrolithiasis-related urinary proteins have been identified. This paper focuses on two of them, namely prothrombin fragment 1 and bikunin, members of the prothrombin and inter-alpha-trypsin inhibitor families of proteins, respectively. Besides their role as inhibitors of crystallization, these proteins are also involved in inflammation-mediated tissue repair. This is the basis for the concept that the response of renal tissue to injury might play an important role in the aetiology of kidney stones.

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.