alpha(1)-Microglobulin: a yellow-brown lipocalin

iTRAQ-based mass spectrometry screen to identify serum biomarkers in systemic lupus erythematosus

OBJECTIVE

Systemic lupus erythematosus (SLE) is a complex systemic autoimmune disorder with no reliable serum biomarkers currently available other than autoantibodies.

METHODS

In the present study, isobaric tags for relative and absolute quantitation-based mass spectrometry was used to screen the sera of patients with SLE to uncover potential disease biomarkers.

RESULTS

85 common proteins were identified, with 16 being elevated (≥1.3) and 23 being decreased (≤0.7) in SLE. Of the 16 elevated proteins, serum alpha-1-microglobulin/bikunin precursor (AMBP), zinc alpha-2 glycoprotein (AZGP) and retinol-binding protein 4 (RBP4) were validated in independent cross-sectional cohorts (Cohort I, N=52; Cohort II, N=117) using an orthogonal platform, ELISA. Serum AMBP, AZGP and RBP4 were validated to be significantly elevated in both patients with inactive SLE and patients with active SLE compared with healthy controls (HCs) (p<0.05, fold change >2.5) in Cohort I. All three proteins exhibited good discriminatory power for distinguishing active SLE and inactive SLE (area under the curve=0.82-0.96), from HCs. Serum AMBP exhibited the largest fold change in active SLE (5.96) compared with HCs and correlated with renal disease activity. The elevation in serum AMBP was validated in a second cohort of patients with SLE of different ethnic origins, correlating with serum creatinine (r=0.60, p<0.001).

CONCLUSION

Since serum AMBP is validated to be elevated in SLE and correlated with renal disease, the clinical utility of this novel biomarker warrants further analysis in longitudinal cohorts of patients with lupus and lupus nephritis.

UCST-Type Soluble Immobilized Cellulase: A New Strategy for the Efficient Degradation and Improved Recycling Performance of Wastepaper Cellulose

This paper reports an innovative study that aims to address key issues in the efficient recycling of wastepaper cellulose. The research team utilized the temperature-responsive upper critical solution temperature (UCST) polymer P(NAGA-b-DMA) in combination with the LytA label's affinity for choline analogs. This innovative approach enabled them to successfully develop a novel soluble immobilized enzyme, P(NAGA-b-DMA)-cellulase. This new enzyme has proven highly effective, significantly enhancing the degradation of wastepaper cellulose while demonstrating exceptional stability. Compared with the traditional insoluble immobilized cellulase, the enzyme showed a significant improvement in the pH, temperature stability, recycling ability, and storage stability. A kinetic parameter calculation showed that the enzymatic effectiveness of the soluble immobilized enzyme was much better than that of the traditional insoluble immobilized cellulase. After the immobilization reaction, the Michaelis constant of the immobilized enzyme was only increased by 11.5%. In the actual wastepaper degradation experiment, the immobilized enzyme was effectively used, and it was found that the degradation efficiency of wastepaper cellulose reached 80% of that observed in laboratory conditions. This novel, thermosensitive soluble immobilized cellulase can efficiently catalyze the conversion of wastepaper cellulose into glucose under suitable conditions, so as to further ferment into environmentally friendly biofuel ethanol, which provides a solution to solve the shortage of raw materials and environmental protection problems in the paper products industry.

Toward Precision Medicine: Exploring the Landscape of Biomarkers in Acute Kidney Injury

Acute kidney injury (AKI) remains a complex challenge with diverse underlying pathological mechanisms and etiologies. Current detection methods predominantly rely on serum creatinine, which exhibits substantial limitations in specificity and poses the issue of late-stage detection of kidney injury. In this review, we propose an up-to-date and comprehensive summary of advancements that identified novel biomarker candidates in blood and urine and ideal criteria for AKI biomarkers such as renal injury specificity, mechanistic insight, prognostic capacity, and affordability. Recently identified biomarkers not only indicate injury location but also offer valuable insights into a range of pathological processes, encompassing reduced glomerular filtration rate, tubular function, inflammation, and adaptive response to injury. The clinical applications of AKI biomarkers are becoming extensive and serving as relevant tools in distinguishing acute tubular necrosis from other acute renal conditions. Also, these biomarkers can offer significant insights into the risk of progression to chronic kidney disease CKD and in the context of kidney transplantation. Integration of these biomarkers into clinical practice has the potential to improve early diagnosis of AKI and revolutionize the design of clinical trials, offering valuable endpoints for therapeutic interventions and enhancing patient care and outcomes.

Circulating extracellular vesicles are associated with pathophysiological condition including metabolic syndrome-related dysmetabolism in children and adolescents with obesity

Obesity of children and adolescents (OCA) is often accompanied by metabolic syndrome (MetS), which often leads to adult obesity and subsequent complications, yet the entire pathophysiological response is not fully understood. The number and composition of circulating extracellular vesicles (EV) reflect overall patient condition; therefore, we investigated the pathophysiological condition of OCA, including MetS-associated dysmetabolism, using circulating EVs. In total, 107 children and adolescents with or without obesity (boys, n = 69; girls, n = 38; median age, 10 years) were enrolled. Circulating EV number and EV protein composition were assessed via flow cytometry and liquid chromatography tandem-mass spectrometry, respectively. In a multivariate analysis, relative body weight (standardized partial regression coefficient (SPRC) 0.469, P = 0.012) and serum triglyceride level (SPRC 0.548, P < 0.001) were detected as independent parameters correlating with circulating EV number. Proteomic analysis identified 31 upregulated and 45 downregulated EV proteins in OCA. Gene ontology analysis revealed upregulated proteins to be involved in various biological processes, including intracellular protein transport, protein folding, stress response, leukocyte activation, innate immune response, and platelet degranulation, which can modulate lipid and glucose metabolism, skeletal and cardiac muscle development, inflammation, immune response, carcinogenesis, and cancer progression. Notably, several identified EV proteins are involved in neuro-development, neurotransmitter release, and neuro-protective agents in OCA. Circulating EVs were derived from adipocytes, hepatocytes, B cell lymphocytes, and neurons. Circulating EV number is significantly associated with MetS-related dysmetabolism and the EV protein cargo carries a special "signature" that reflects the alteration of various biological processes under the pathophysiological condition of OCA. KEY MESSAGES: Circulating EV number correlates with physical and laboratory parameters for obesity in children and adolescents. Relative body weight and triglyceride are independent factors for increased circulating EVs. EV composition is significantly changed in obesity of children and adolescents. Identified EV composition changes associated with obesity and involves in metabolism, immune response, and cancer progression. Circulating EVs are partially derived from adipocyte, hepatocytes, B cells, and neurons.

Biomarkers of aging

Aging biomarkers are a combination of biological parameters to (i) assess age-related changes, (ii) track the physiological aging process, and (iii) predict the transition into a pathological status. Although a broad spectrum of aging biomarkers has been developed, their potential uses and limitations remain poorly characterized. An immediate goal of biomarkers is to help us answer the following three fundamental questions in aging research: How old are we? Why do we get old? And how can we age slower? This review aims to address this need. Here, we summarize our current knowledge of biomarkers developed for cellular, organ, and organismal levels of aging, comprising six pillars: physiological characteristics, medical imaging, histological features, cellular alterations, molecular changes, and secretory factors. To fulfill all these requisites, we propose that aging biomarkers should qualify for being specific, systemic, and clinically relevant.

Structural and ligand binding analysis of the pet allergens Can f 1 and Fel d 7

Introduction

Pet lipocalins are respiratory allergens with a central hydrophobic ligand-binding cavity called a calyx. Molecules carried in the calyx by allergens are suggested to influence allergenicity, but little is known about the native ligands.

Methods

To provide more information on prospective ligands, we report crystal structures, NMR, molecular dynamics, and florescence studies of a dog lipocalin allergen Can f 1 and its closely related (and cross-reactive) cat allergen Fel d 7.

Results

Structural comparisons with reported lipocalins revealed that Can f 1 and Fel d 7 calyxes are open and positively charged while other dog lipocalin allergens are closed and negatively charged. We screened fatty acids as surrogate ligands, and found that Can f 1 and Fel d 7 bind multiple ligands with preferences for palmitic acid (16:0) among saturated fatty acids and oleic acid (18:1 cis-9) among unsaturated ones. NMR analysis of methyl probes reveals that conformational changes occur upon binding of pinolenic acid inside the calyx. Molecular dynamics simulation shows that the carboxylic group of fatty acids shuttles between two positively charged amino acids inside the Can f 1 and Fel d 7 calyx. Consistent with simulations, the stoichiometry of oleic acid-binding is 2:1 (fatty acid: protein) for Can f 1 and Fel d 7.

Discussion

The results provide valuable insights into the determinants of selectivity and candidate ligands for pet lipocalin allergens Can f 1 and Fel d 7.

Cardiac Hepatopathy: New Perspectives on Old Problems through a Prism of Endogenous Metabolic Regulations by Hepatokines

Cardiac hepatopathy refers to acute or chronic liver damage caused by cardiac dysfunction in the absence of any other possible causative reasons of liver injury. There is a large number of evidence of the fact that cardiac hepatopathy is associated with poor clinical outcomes in patients with acute or actually decompensated heart failure (HF). However, the currently dominated pathophysiological background does not explain a role of metabolic regulative proteins secreted by hepatocytes in progression of HF, including adverse cardiac remodeling, kidney injury, skeletal muscle dysfunction, osteopenia, sarcopenia and cardiac cachexia. The aim of this narrative review was to accumulate knowledge of hepatokines (adropin; fetuin-A, selenoprotein P, fibroblast growth factor-21, and alpha-1-microglobulin) as adaptive regulators of metabolic homeostasis in patients with HF. It is suggested that hepatokines play a crucial, causative role in inter-organ interactions and mediate tissue protective effects counteracting oxidative stress, inflammation, mitochondrial dysfunction, apoptosis and necrosis. The discriminative potencies of hepatokines for HF and damage of target organs in patients with known HF is under on-going scientific discussion and requires more investigations in the future.

Poly-IgA Complexes and Disease Severity in IgA Nephropathy

BACKGROUND AND OBJECTIVES

Poly-IgA immune complex formation and glomerular deposition play a key role in IgA nephropathy. Our study sought to develop a new methodology for one-step serologic detection of poly-IgA levels.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A novel ELISA method using recombinant CD89 as a "capturing" probe was established for detecting poly-IgA immune complex in plasma. We applied semiquantitative measurements of these poly-IgA indices in patients recruited at Peking University First Hospital who had IgA nephropathy or other kidney disease types, as compared with healthy controls. The longitudinal trend of the poly-IgA index and the association with pathologic parameters and treatment responses were evaluated. Finally, we analyzed the molecular composition of poly-IgA complexes in patients by mass spectrometry.

RESULTS

Recombinant CD89-mounted ELISA plates specifically captured plasma poly-IgA. The levels of poly-IgA immune complex (26.7 [interquartile range (IQR) 17.1-42.6] U/ml) in IgA nephropathy were significantly higher than those in healthy controls (15.5 [IQR 10.7-20.0] U/ml; P<0.001) or in controls with non-IgA nephropathy disease (14.8 [IQR 10.5-21.9] U/ml; P<0.001). Higher levels of poly-IgA immune complex were associated with lower eGFR and worse kidney outcome. Accuracy parameters and concordant statistics showed good discrimination between IgA nephropathy and healthy controls based on poly-IgA index levels (area under the curve [AUC], 0.78; 95% confidence interval [95% CI], 0.72 to 0.83; P<0.001), significantly outperforming galactose-deficient IgA1 levels (AUC, 0.70; P=0.05). Corticosteroid and immunosuppressant treatments lowered poly-IgA indices. After a recombinant CD89-directed workflow in conjunction with mass spectrometry, we also analyzed the molecular composition of IgA immune complex in patients with IgA nephropathy.

CONCLUSIONS

Higher level of recombinant CD89-bound poly-IgA immune complex was associated with the severity of the disease and with treatment response to steroids and immunosuppressants.

Mass spectrometry-based screening identifies circulating immunoglobulinA-α1-microglobulin complex as potential biomarker in immunoglobulin A nephropathy

BACKGROUND

Immunoglobulin A nephropathy (IgAN) is characterized by predominant IgA deposition in the glomerular mesangium. Previous studies have proved that renal-deposited IgA in IgAN came from circulating IgA1-containing complexes (CICs).

METHODS

To explore the composition of CICs in IgAN, we isolated CICs from IgAN patients and healthy controls and then quantitatively analyzed them by mass spectrometry. Meanwhile, the isolated CICs were used to treat human mesangial cells to monitor mesangial cell injury. Using the protein content and injury effects, the key constituent in CICs was identified. Then the circulating levels of identified key constituent-IgA complex were detected in an independent population by an in-house-developed enzyme-linked immunosorbent assay.

RESULTS

By comparing the proteins of CICs between IgAN patients and controls, we found that 14 proteins showed significantly different levels. Among them, α1-microglobulin content in CICs was associated with not only in vitro mesangial cell proliferation and monocyte chemoattractant protein 1 secretion, but also in vivo estimated glomerular filtration rate (eGFR) levels and tubulointerstitial lesions in IgAN patients. Moreover, we found α1-microglobulin was prone to bind aberrant glycosylated IgA1. Additionally, elevated circulating IgA-α1-microglobulin complex levels were detected in an independent IgAN population and IgA-α1-microglobulin complex levels were correlated with hypertension, eGFR levels and Oxford T- scores in these IgAN patients.

CONCLUSIONS

Our results suggest that the IgA-α1-microglobulin complex is an important constituent in CICs and that circulating IgA-α1-microglobulin complex detection might serve as a potential noninvasive biomarker detection method for IgAN.

Clinical and prognostic values of urinary alpha1-microglobulin as a tubular marker in acute heart failure

BACKGROUND

Although urinary alpha-1-microglobulin has been used as a marker of tubular dysfunction, its clinical and prognostic values in patients with acute heart failure have not been validated.

METHODS

We analyzed 623 patients (74 ± 13 years old, 60.0% male) with acute heart failure in whom urinary alpha-1-microglobulin (A1MG) levels were measured as tubular markers at the time of admission. The primary endpoint was all-cause mortality.

RESULTS

The median levels of urinary alpha-1-microglobulin with and without correction for urinary creatinine concentration were 8.80 (interquartile range: 4.20-17.7) mg/dL and 12.9 (5.92-30.7) mg/gCr, respectively. Urinary A1MG levels were significantly correlated with all of beta-2-microglobulin (r = 0.77), N-acetyl-β-D-glucosaminidase (r = 0.51), and estimated glomerular filtration rate (r = -0.42); however, alpha-1-microglobulin was most often predicted using beta-2-microglobulin or N-acetyl-β-D-glucosaminidase. During the 488-day (interquartile range: 185-938 days) follow-up, 141 deaths occurred. Higher A1MG levels were associated with higher mortality even after adjustment for other covariates. Only A1MG, but not beta-2-microglobulin or N-acetyl-β-D-glucosaminidase, yielded incremental prognostic information in addition to the preexisting prognostic factors (net-reclassification improvement: 0.254, P = 0.023; integrated discrimination improvement: 0.015, P = 0.028).

CONCLUSIONS

In patients hospitalized due to acute heart failure, urinary alpha-1-microglobulin was a marker of tubular dysfunction. High alpha-1-microglobulin was associated with all-cause mortality independent of glomerular function and was a better predictor of mortality than urinary beta-2-microglobulin.

Structure, Functions, and Physiological Roles of the Lipocalin α1-Microglobulin (A1M)

α₁-microglobulin (A1M) is found in all vertebrates including humans. A1M was, together with retinol-binding protein and β-lactoglobulin, one of the three original lipocalins when the family first was proposed in 1985. A1M is described as an antioxidant and tissue cleaning protein with reductase, heme- and radical-binding activities. These biochemical properties are driven by a strongly electronegative surface-exposed thiol group, C34, on loop 1 of the open end of the lipocalin barrel. A1M has been shown to have protective effects in vitro and in vivo in cell-, organ-, and animal models of oxidative stress-related medical conditions. The gene coding for A1M is unique among lipocalins since it is flanked downstream by four exons coding for another non-lipocalin protein, bikunin, and is consequently named α₁-microglobulin-bikunin precursor gene (AMBP). The precursor is cleaved in the Golgi, and A1M and bikunin are secreted from the cell separately. Recent publications have suggested novel physiological roles of A1M in regulation of endoplasmic reticulum activities and erythrocyte homeostasis. This review summarizes the present knowledge of the structure and functions of the lipocalin A1M and presents a current model of its biological role(s).

The application of neutrophil gelatin-related lipid delivery protein in evaluation of renal function, nutrition, anemia and inflammation in patients with CKD

OBJECTIVE

To investigate the role of neutrophil gelatinase-associated lipocalin in the evaluation of renal function, nutrition, anemia and inflammation in patients with chronic kidney diseases.

MATERIALS AND METHODS

A total of 302 patients with chronic kidney diseases were selected, and their clinical data, blood neutrophil gelatinase-associated lipocalin levels, renal function, nutrition, anemia, inflammation and calcium, and phosphorus metabolism were analyzed.

RESULT

Serum neutrophil gelatinase-associated lipocalin level increased with the progression of chronic kidney diseases. Higher neutrophil gelatinase-associated lipocalin levels were observed in patients with chronic kidney diseases stage 3b compared with healthy individuals (P<0.05), while the patients with chronic kidney diseases stage 5 showed higher levels compared with other chronic kidney diseases stages (P<0.01). Moreover, the ROC curve showed that neutrophil gelatinase-associated lipocalin had a better diagnostic performance from the chronic kidney diseases stage 3b to 5 (P<0.05). In addition, the serum neutrophil gelatinase-associated lipocalin levels in patient with chronic kidney diseases were negatively correlated with body mass index, number of red blood cells, hemoglobin, transferrin, the estimatedglomerular filtration rate (eGFR), serum calcium (P<0.01); and were positively correlated with mean arterial blood pressure, blood BUN, SCr and alpha 1 microglobulin, beta 2 microglobulin, urinary inhibition C, homocysteine, PTH levels, neutrophils ratio, free serum ferritin and c-reactive protein (P<0.01); while no significant correlation was found with gender, and age (P>0.05).

CONCLUSION

Serum neutrophil gelatinase-associated lipocalin levels are closely related to renal function injury, inflammatory response and anemia-related indicators in patients with chronic kidney diseases, and thus could be used as a diagnostic biomarker for evaluating the degree of renal injury and related complications in patients with chronic kidney diseases.

One ring to bring them all and in the darkness bind them: The trafficking of heme without deliverers

Heme, as a hydrophobic iron-containing organic ring, is lipid soluble and can interact with biological membranes. The very same properties of heme that nature exploits to support life also renders heme potentially cytotoxic. In order to utilize heme, while also mitigating its toxicity, cells are challenged to tightly control the concentration and bioavailability of heme. On the bright side, it is reasonable to envision that, analogous to other transition metals, a combination of membrane-bound transporters, soluble carriers, and chaperones coordinate heme trafficking to subcellular compartments. However, given the dual properties exhibited by heme as a transition metal and lipid, it is compelling to consider the dark side: the potential role of non-proteinaceous biomolecules including lipids and nucleic acids that bind, sequester, and control heme trafficking and bioavailability. The emergence of inter-organellar membrane contact sites, as well as intracellular vesicles derived from various organelles, have raised the prospect that heme can be trafficked through hydrophobic channels. In this review, we aim to focus on heme delivery without deliverers - an alternate paradigm for the regulation of heme homeostasis through chaperone-less pathways for heme trafficking.

Identification of a Prognostic Immune Signature for Esophageal Squamous Cell Carcinoma to Predict Survival and Inflammatory Landscapes

Immunotherapy has achieved success in the treatment of esophageal squamous cell carcinoma (ESCC). However, studies concerning immune phenotypes within the ESCC microenvironment and their relationship with prognostic outcomes are limited. We constructed and validated an individual immune-related risk signature for patients with ESCC. We collected 196 ESCC cases, including 119 samples from our previous public data (GSE53624) to use as a training set and an independent cohort with 77 quantitative real-time polymerase chain reaction (qRT-PCR) data, which we used for validation. Head and neck squamous cell carcinoma (HNSCC) and lung squamous cell carcinoma (LUSC) cohorts were also collected for validation. A least absolute shrinkage and selection operator (LASSO) model and a stepwise Cox proportional hazards regression model were used to construct the immune-specific signature. The potential mechanism and inflammatory landscapes of the signature were explored using bioinformatics and immunofluorescence assay methods. This signature predicted different prognoses in clinical subgroups and the independent cohort, as well as in patients with HNSCC and LUSC. Further exploration revealed that the signature was associated with specific inflammatory activities (activation of macrophages and T-cell signaling transduction). Additionally, high-risk patients exhibited distinctive immune checkpoints panel and higher regulatory T cell and fibroblast infiltration. This signature served as an independent prognostic factor in ESCC. This was the first applicable immune-related risk signature for ESCC. Our results furnished new hints of immune profiling of ESCC, which may provide some clues to further optimize associated cancer immunotherapies.

Hemopexin and α1-microglobulin heme scavengers with differential involvement in preeclampsia and fetal growth restriction

Hemopexin and α1-microglobulin act as scavengers to eliminate free heme-groups responsible for hemoglobin-induced oxidative stress. The present study evaluated maternal and fetal plasma concentrations of these scavengers in the different phenotypes of placenta-mediated disorders. Singleton pregnancies with normotensive fetal growth restriction [FGR] (n = 47), preeclampsia without FGR (n = 45) and preeclampsia with FGR (n = 51) were included prospectively as well as uncomplicated pregnancies (n = 49). Samples were collected at delivery and ELISA analysis was applied to measure the hemopexin and α1-microglobulin concentrations. In maternal blood in preeclampsia with and without FGR, hemopexin was significantly lower (p = 0.003 and p<0.001, respectively) and α1-microglobulin was significantly higher (p<0.001 in both) whereas no difference existed in normotensive FGR mothers compared to controls. In contrast, in fetal blood in growth restricted fetuses with and without preeclampsia, both hemopexin and α1-microglobulin were significantly lower (p<0.001 and p = 0.001 for hemopexin, p = 0.016 and p = 0.013 for α1-microglobulin, respectively) with no difference in fetuses from preeclampsia without FGR in comparison to controls. Thus, hemopexin and α1-microglobulin present significantly altered concentrations in maternal blood in the maternal disease -preeclampsia- and in cord blood in the fetal disease -FGR-, which supports their differential role in placenta-mediated disorders in accordance with the clinical presentation of these disorders.

Tubular Biomarkers and Chronic Kidney Disease Progression in SPRINT Participants

BACKGROUND

Kidney tubular atrophy on biopsy is a strong predictor of chronic kidney disease (CKD) progression, but tubular health is poorly quantified by traditional measures including estimated glomerular filtration rate (eGFR) and albuminuria. We hypothesized that urinary biomarkers of impaired tubule function would be associated with faster eGFR declines in persons with CKD.

METHODS

We measured baseline urine concentrations of uromodulin, β2-microglobulin (β2m), and α1-microglobulin (α1m) among 2,428 participants of the Systolic Blood Pressure Intervention Trial with an eGFR <60 mL/min/1.73 m2. We used linear mixed models to evaluate biomarker associations with annualized relative change in eGFR, stratified by randomization arm.

RESULTS

At baseline, the mean age was 73 ± 9 years and eGFR was 46 ± 11 mL/min/1.73 m2. In the standard blood pressure treatment arm, each 2-fold higher urinary uromodulin was associated with slower % annual eGFR decline (0.34 [95% CI: 0.08, 0.60]), whereas higher urinary β2m was associated with faster % annual eGFR decline (-0.10 [95% CI: -0.18, -0.02]) in multivariable-adjusted models including baseline eGFR and albuminuria. Associations were weaker and did not reach statistical significance in the intensive blood pressure treatment arm for either uromodulin (0.11 [-0.13, 0.35], p value for interaction by treatment arm = 0.045) or β2m (-0.01 [-0.08, 0.08], p value for interaction = 0.001). Urinary α1m was not independently associated with eGFR decline in the standard (0.01 [-0.22, 0.23]) or intensive (0.03 [-0.20, 0.25]) arm.

CONCLUSIONS

Among trial participants with hypertension and CKD, baseline measures of tubular function were associated with subsequent declines in kidney function, although these associations were diminished by intensive blood pressure control.

Expression, Purification and Initial Characterization of Functional α1-Microglobulin (A1M) in Nicotiana benthamiana

α₁-Microglobulin (A1M) is a small glycoprotein that belongs to the lipocalin protein family. A major biological role of A1M is to protect cells and tissues against oxidative damage by clearing free heme and reactive oxygen species. Because of this, the protein has attracted great interest as a potential pharmaceutical candidate for treatment of acute kidney injury and preeclampsia. The aim of this study was to explore the possibility of expressing human A1M in plants through transient gene expression, as an alternative or complement to other expression systems. E. coli, insect and mammalian cell culture have previously been used for recombinant A1M (rA1M) or A1M production, but these systems have various drawbacks, including additional complication and expense in refolding for E. coli, while insect produced rA1M is heavily modified with chromophores and mammalian cell culture has been used only in analytical scale. For that purpose, we have used a viral vector (pJL-TRBO) delivered by Agrobacterium for expression of three modified A1M gene variants in the leaves of N. benthamiana. The results showed that these modified rA1M protein variants, A1M-NB1, A1M-NB2 and A1M-NB3, targeted to the cytosol, ER and extracellular space, respectively, were successfully expressed in the leaves, which was confirmed by SDS-PAGE and Western blot analysis. The cytosol accumulated A1M-NB1 was selected for further analysis, as it appeared to have a higher yield than the other variants, and was purified with a yield of ca. 50 mg/kg leaf. The purified protein had the expected structural and functional properties, displaying heme-binding capacity and capacity of protecting red blood cells against stress-induced cell death. The protein also carried bound chromophores, a characteristic feature of A1M and an indicator of a capacity to bind small molecules. The study showed that expression of the functional protein in N. benthamiana may be an attractive alternative for production of rA1M for pharmaceutical purposes and a basis for future research on A1M structure and function.

Markers of kidney tubule function and risk of cardiovascular disease events and mortality in the SPRINT trial

AIMS

Biomarkers of kidney tubule injury, inflammation and fibrosis have been studied extensively and established as risk markers of adverse kidney and cardiovascular disease (CVD) outcomes. However, associations of markers of kidney tubular function with adverse clinical events have not been well studied, especially in persons with chronic kidney disease (CKD).

METHODS AND RESULTS

Using a sample of 2377 persons with CKD at the baseline Systolic Blood Pressure Intervention Trial (SPRINT) visit, we evaluated the association of three urine tubular function markers, alpha-1 microglobulin (α1m), beta-2 microglobulin (β2m), and uromodulin, with a composite CVD endpoint (myocardial infarction, acute coronary syndrome, stroke, acute decompensated heart failure, or death from cardiovascular causes) and mortality using Cox proportional hazards regression, adjusted for baseline estimated glomerular filtration rate (eGFR), albuminuria, and CVD risk factors. In unadjusted analysis, over a median follow-up of 3.8 years, α1m and β2m had positive associations with composite CVD events and mortality, whereas uromodulin had an inverse association with risk for both outcomes. In multivariable analysis including eGFR and albuminuria, a two-fold higher baseline concentration of α1m was associated with higher risk of CVD [hazard ratio (HR) 1.25; 95% confidence interval (CI): 1.10-1.45] and mortality (HR 1.25; 95% CI: 1.10-1.46), whereas β2m had no association with either outcome. A two-fold higher uromodulin concentration was associated with lower CVD risk (HR 0.79; 95% CI: 0.68-0.90) but not mortality (HR 0.86; 95% CI: 0.73-1.01) after adjusting for similar confounders.

CONCLUSION

Among non-diabetic persons with CKD, biomarkers of tubular function are associated with CVD events and mortality independent of glomerular function and albuminuria.

Distinct Dimensions of Kidney Health and Risk of Cardiovascular Disease, Heart Failure, and Mortality

Chronic kidney disease is a strong risk factor for cardiovascular disease (CVD), but clinical kidney measures (estimated glomerular filtration rate and albuminuria) do not fully reflect the multiple aspects of kidney tubules influencing cardiovascular health. Applied methods are needed to integrate numerous tubule biomarkers into useful prognostic scores. In SPRINT (Systolic Blood Pressure Intervention Trial) participants with chronic kidney disease at baseline (estimated glomerular filtration ratecr&cys <60 mL/minute per 1.73 m2), we measured 8 biomarkers from urine (α1M [α1M microglobulin], β2M [β2M microglobulin], umod [uromodulin], KIM-1 [kidney injury molecule-1], MCP-1 [monocyte chemoattractant protein-1], YKL-40 [chitinase-3-like protein-1], NGAL [neutrophil gelatinase-associated lipocalin], and IL-18 [interleukin 18]) and 2 biomarkers from serum (intact parathyroid hormone, iFGF-23 [intact fibroblast growth factor-23]). We used an unsupervised method, exploratory factor analysis, to create summary scores of tubule health dimensions. Adjusted Cox models evaluated each tubule score with CVD events, heart failure, and all-cause mortality. We examined CVD discrimination using Harrell C-statistic. Factor analysis of 10 biomarkers from 2376 SPRINT-chronic kidney disease participants identified 4 unique dimensions of tubular health: tubule injury/repair (NGAL, IL-18, YKL-40), tubule injury/fibrosis (KIM-1, MCP-1), tubule reabsorption (α1M, β2M), and tubular reserve/mineral metabolism (umod, intact parathyroid hormone, iFGF-23). After adjustment for CVD risk factors, estimated glomerular filtration rate, and albumin-to-creatinine ratio, 2 of the 4 tubule scores were associated with CVD (hazard ratio per SD; reabsorption, 1.21 [1.06-1.38]; reserve, 1.24 (1.08-1.38]), 1 with heart failure (reserve, 1.41 [1.13-1.74]), and none with mortality. Compared with a base model (C-statistic=0.674), adding estimated glomerular filtration rate and albumin-to-creatinine ratio improved the C-statistic (C=0.704; P=0.001); further adding tubule scores additionally improved the C-statistic (C=0.719; P=0.009). In the setting of chronic kidney disease, dimensions of tubule health quantified using factor analysis improved CVD discrimination beyond contemporary kidney measures. Clinical Trial Registration- URL: http://www.clinicaltrials.gov. Unique identifier: NCT01206062.

The Association of Serum hsCRP and Urinary Alpha1-Microglobulin in Patients with Type 2 Diabetes Mellitus

This study aimed to investigate the association of serum hsCRP and urinary A1MG in patients with T2DM. Numerous investigations have proven that serum hypersensitive C-reactive protein (hsCRP) concentration in patients with type 2 diabetes mellitus (T2DM) is increased. Also, increased urinary alpha-1 microglobulin (A1MG) can be an early sign of renal damage, primarily on the proximal tubules in T2DM. Little information is available with respect to the associations of serum hsCRP levels and urinary A1MG in T2DM. A total of 520 patients with T2DM were recruited to participate in this study. Serum hsCRP and UA1MG (urinary alpha1-microglobulin to creatinine ratio), UACR (urinary microalbumin to creatinine ratio), UIGG (urinary immunoglobulin G to creatinine ratio), and UTRF (urinary transferrin to creatinine ratio) were obtained. The association of serum hsCRP level and each urinary protein parameter was analyzed by using the regression analysis, respectively. LnhsCRP was positively associated with the lnUA1MG in all three linear regression models (adjusted β in model 3=0.122, SE=0.027, P<0.001). Furthermore, the high hsCRP group (hsCRP > 3mg/L) was associated with increasing risk of high UA1MG (adjusted OR in model 3=1.610, 95% CI 1.037-2.499, P=0.034) by logistic regression. This study suggests that serum hsCRP levels independently associate with UA1MG in patients with T2DM. Further research is warranted to elucidate these interactions.

Pre-exposure Prophylaxis With Tenofovir Disoproxil Fumarate/Emtricitabine and Kidney Tubular Dysfunction in HIV-Uninfected Individuals

BACKGROUND

Pre-exposure prophylaxis (PrEP) with tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) is becoming increasingly adopted for HIV prevention. Tenofovir can cause proximal tubular damage and chronic kidney disease in HIV-infected persons, but little is known regarding its nephrotoxic potential among HIV-uninfected persons. In this study, we evaluated the effects of PrEP on urine levels of the following: α1-microglobulin (α1m), a marker of impaired tubular reabsorption; albuminuria, a measure of glomerular injury; and total proteinuria.

SETTING

The Iniciativa Profilaxis Pre-Exposicion (iPrEx) study randomized HIV-seronegative men and transgender women who have sex with men to oral TDF/FTC or placebo. The iPrEx open-label extension (iPrEx-OLE) study enrolled former PrEP trial participants to receive open-label TDF/FTC.

METHODS

A cross-sectional analysis compared urine biomarker levels by study arm in iPrEx (N = 100 treatment arm, N = 100 placebo arm). Then, urine biomarker levels were compared before and after PrEP initiation in 109 participants of iPrEx-OLE.

RESULTS

In iPrEx, there were no significant differences in urine α1m, albuminuria, or proteinuria by treatment arm. In iPrEx-OLE, after 24 weeks on PrEP, urine α1m and proteinuria increased by 21% [95% confidence interval (CI): 10 to 33] and 18% (95% CI: 8 to 28), respectively. The prevalence of detectable α1m increased from 44% to 65% (P < 0.001) and estimated glomerular filtration rate declined by 4 mL/min/1.73 m (P < 0.001). There was no significant change in albuminuria (6%; 95% CI: -7% to 20%).

CONCLUSION

PrEP with TDF/FTC was associated with a statistically significant rise in urine α1m and proteinuria after 6 months, suggesting that PrEP may result in subclinical tubule dysfunction.

Biochemical evaluation of kidney disease

Different biochemical markers exist in both blood and urine for assessing renal function. Most of these biomarkers have advantages and limitations associated with their use, which is important to consider when ordering and utilising them in the clinical setting. The ideal marker should be able to detect acute kidney injury (AKI) at the onset and be used for the diagnosis and ongoing monitoring and management of kidney disease. The search for such a marker is ongoing, as all potential candidates thus far are associated with certain limitations. This article will attempt to compare and contrast established and emerging kidney disease markers.

Update on the human and mouse lipocalin (LCN) gene family, including evidence the mouse Mup cluster is result of an "evolutionary bloom"

Lipocalins (LCNs) are members of a family of evolutionarily conserved genes present in all kingdoms of life. There are 19 LCN-like genes in the human genome, and 45 Lcn-like genes in the mouse genome, which include 22 major urinary protein (Mup) genes. The Mup genes, plus 29 of 30 Mup-ps pseudogenes, are all located together on chromosome (Chr) 4; evidence points to an “evolutionary bloom” that resulted in this Mup cluster in mouse, syntenic to the human Chr 9q32 locus at which a single MUPP pseudogene is located. LCNs play important roles in physiological processes by binding and transporting small hydrophobic molecules —such as steroid hormones, odorants, retinoids, and lipids—in plasma and other body fluids. LCNs are extensively used in clinical practice as biochemical markers. LCN-like proteins (18–40 kDa) have the characteristic eight β-strands creating a barrel structure that houses the binding-site; LCNs are synthesized in the liver as well as various secretory tissues. In rodents, MUPs are involved in communication of information in urine-derived scent marks, serving as signatures of individual identity, or as kairomones (to elicit fear behavior). MUPs also participate in regulation of glucose and lipid metabolism via a mechanism not well understood. Although much has been learned about LCNs and MUPs in recent years, more research is necessary to allow better understanding of their physiological functions, as well as their involvement in clinical disorders.

Hepatokine α1-Microglobulin Signaling Exacerbates Inflammation and Disturbs Fibrotic Repair in Mouse Myocardial Infarction

Acute cardiac rupture and adverse left ventricular (LV) remodeling causing heart failure are serious complications of acute myocardial infarction (MI). While cardio-hepatic interactions have been recognized, their role in MI remains unknown. We treated cultured cardiomyocytes with conditioned media from various cell types and analyzed the media by mass spectrometry to identify α1-microglobulin (AM) as an Akt-activating hepatokine. In mouse MI model, AM protein transiently distributed in the infarct and border zones during the acute phase, reflecting infiltration of AM-bound macrophages. AM stimulation activated Akt, NFκB, and ERK signaling and enhanced inflammation as well as macrophage migration and polarization, while inhibited fibrogenesis-related mRNA expression in cultured macrophages and cardiac fibroblasts. Intramyocardial AM administration exacerbated macrophage infiltration, inflammation, and matrix metalloproteinase 9 mRNA expression in the infarct and border zones, whereas disturbed fibrotic repair, then provoked acute cardiac rupture in MI. Shotgun proteomics and lipid pull-down analysis found that AM partly binds to phosphatidic acid (PA) for its signaling and function. Furthermore, systemic delivery of a selective inhibitor of diacylglycerol kinase α-mediated PA synthesis notably reduced macrophage infiltration, inflammation, matrix metalloproteinase activity, and adverse LV remodeling in MI. Therefore, targeting AM signaling could be a novel pharmacological option to mitigate adverse LV remodeling in MI.

Cubilin, the Intrinsic Factor-Vitamin B12 Receptor in Development and Disease

Gp280/Intrinsic factor-vitamin B12 receptor/Cubilin (CUBN) is a large endocytic receptor serving multiple functions in vitamin B12 homeostasis, renal reabsorption of protein or toxic substances including albumin, vitamin D-binding protein or cadmium. Cubilin is a peripheral membrane protein consisting of 8 Epidermal Growth Factor (EGF)-like repeats and 27 CUB (defined as Complement C1r/C1s, Uegf, BMP1) domains. This structurally unique protein interacts with at least two molecular partners, Amnionless (AMN) and Lrp2/Megalin. AMN is involved in appropriate plasma membrane transport of Cubilin whereas Lrp2 is essential for efficient internalization of Cubilin and its ligands. Observations gleaned from animal models with Cubn deficiency or human diseases demonstrate the importance of this protein. In this review addressed to basic research and medical scientists, we summarize currently available data on Cubilin and its implication in renal and intestinal biology. We also discuss the role of Cubilin as a modulator of Fgf8 signaling during embryonic development and propose that the Cubilin-Fgf8 interaction may be relevant in human pathology, including in cancer progression, heart or neural tube defects. We finally provide experimental elements suggesting that some aspects of Cubilin physiology might be relevant in drug design.

Underexpression of α-1-microglobulin/bikunin precursor predicts a poor prognosis in oral squamous cell carcinoma

In the present study, in order to identify novel diagnostic biomarkers for the malignant behavior of oral squamous cell carcinoma (OSCC), we determined the proteomic profiles of several OSCC cell lines and keratinocytes by two-dimensional fluorescence difference gel electrophoresis and liquid chromatography tandem mass spectrometry. The protein expression level of α-1-microglobulin/bikunin precursor (AMBP) was found to be significantly lower in the OSCC cell lines than in the keratinocytes, and a significant decrease in AMBP mRNA expression was confirmed in the OSCC cell lines by RT-qPCR. To investigate the biological function of AMBP in OSCC, the cells were transiently transfected with an AMBP overexpression vector; the AMBP-overexpressing cells exhibited a significantly decreased invasion and migration in comparison to the mock-transfected control cells, although no significant changes in cell proliferation were observed. Immunohistochemistry revealed that the underexpression of AMBP was significantly associated with a high metastatic potential to cervical lymph nodes and a poor overall survival. Thus, the expression of AMBP is an independent predictive factor of cervical lymph node metastasis and a prognostic factor of overall survival, and it is involved in both cell invasion and metastasis in cervical lymph nodes in OSCC.

Urinary Biomarkers of Kidney Tubular Damage and Risk of Cardiovascular Disease and Mortality in Elders

RATIONALE & OBJECTIVE

Novel urinary biomarkers have enabled earlier detection of kidney tubular damage, but their prognostic value for adverse cardiovascular outcomes is uncertain. We hypothesized that tubular damage, measured by urine α₁-microglobulin (A1M), amino-terminal propeptide of type III procollagen (PIIINP), and neutrophil gelatinase-associated lipocalin (NGAL), would be associated with higher risks for cardiovascular events and mortality among elders.

STUDY DESIGN

Case-cohort study.

SETTING & PARTICIPANTS

This study included a randomly selected subcohort (n=502), cardiovascular disease (CVD) cases (n=245), and heart failure cases (n=220) from the Health, Aging, and Body Composition (Health ABC) Study.

PREDICTORS

Baseline urine A1M, PIIINP, and NGAL concentrations.

OUTCOMES

Incident CVD, heart failure, and all-cause mortality.

ANALYTICAL APPROACH

Cox proportional hazards models were used to evaluate biomarker associations with each outcome.

RESULTS

At baseline, mean age was 74 years and estimated glomerular filtration rate was 73mL/min/1.73m². After adjustment for demographics, estimated glomerular filtration rate, albumin-creatinine ratio, and other cardiovascular risk factors, each doubling in biomarker concentration was associated with the following adjusted HRs for CVD: A1M, 1.51 (95% CI, 1.16-1.96); PIIINP, 1.21 (95% CI, 1.00-1.46); and NGAL, 1.12 (95% CI, 1.05-1.20). There were 248 deaths in the subcohort during a median follow-up of 12.4 years. Adjusted associations of each biomarker (HR per doubling) with all-cause mortality were: A1M, 1.29 (95% CI, 1.10-1.51); PIIINP, 1.05 (95%, 0.94-1.18); and NGAL, 1.07 (95% CI, 1.02-1.12). Biomarker concentrations did not have statistically significant associations with heart failure after multivariable adjustment.

LIMITATIONS

Urine biomarkers were measured at a single time point; no validation cohort available.

CONCLUSIONS

Kidney tubular damage is an independent risk factor for CVD and death among elders. Future studies should investigate mechanisms by which kidney tubular damage may adversely affect cardiovascular risk.

Urinary Markers of Fibrosis and Risk of Cardiovascular Events and Death in Kidney Transplant Recipients: The FAVORIT Trial

Cardiovascular risk remains high in kidney transplant recipients (KTRs) despite improved kidney function after transplant. Urinary markers of kidney fibrosis and injury may help to reveal mechanisms of this risk. In a case-cohort study among stable KTRs who participated in the FAVORIT trial, we measured four urinary proteins known to correlate with kidney tubulointerstitial fibrosis on biopsy (urine alpha 1 microglobulin [α1m], monocyte chemoattractant protein-1 [MCP-1], procollagen type I [PINP] and type III [PIIINP] N-terminal amino peptide) and evaluated associations with cardiovascular disease (CVD) events (n = 300) and death (n = 371). In adjusted models, higher urine α1m (hazard ratio [HR] per doubling of biomarker 1.40 [95% confidence interval [CI] 1.21, 1.62]), MCP-1 (HR 1.18 [1.03, 1.36]), and PINP (HR 1.13 [95% CI 1.03, 1.23]) were associated with CVD events. These three markers were also associated with death (HR per doubling α1m 1.51 [95% CI 1.32, 1.72]; MCP-1 1.31 [95% CI 1.13, 1.51]; PINP 1.11 [95% CI 1.03, 1.20]). Higher concentrations of urine α1m, MCP-1, and PINP may identify KTRs at higher risk for CVD events and death. These markers may identify a systemic process of fibrosis involving both the kidney and cardiovascular system, and give new insights into mechanisms linking the kidney with CVD.

Alpha 1 Microglobulin: A Potentially Paradoxical Anti-Oxidant Agent

Alpha 1 microglobulin is a low molecular weight heme binding antioxidant protein with interesting, and potentially important, clinical applications. However, much remains to be learned about its in vivo effects. This invited review raises a number of physiologic issues regarding this compound as it pertains to clinical use.

Longitudinal characterization of renal proximal tubular markers in normotensive and preeclamptic pregnancies

Glomerular damage is common in preeclampsia (PE), but the extent and etiology of tubular injury are not well understood. The aim of this study was to evaluate tubular injury in patients with PE and to assess whether it predates clinical disease. We performed a prospective cohort study of 315 pregnant women who provided urine samples at the end of the second trimester and at delivery. This analysis included women who developed PE ( n = 15), gestational hypertension (GH; n = 14), and normotensive controls (NC; n = 44). Urinary markers of tubular injury, α1-microglobulin (A1M), retinol-binding protein (RBP), kidney-injury molecule-1 (KIM1), complement C5b-9, tissue inhibitor metalloproteinase-2 (TIMP-2), and insulin-like growth factor binding protein-7 (IGFBP-7) were measured by enzyme-linked immunosorbent assay (ELISA) and reported in relation to urine creatinine concentration. Second-trimester concentrations of all markers were similar among groups. At delivery, A1M concentrations were higher in the PE group than in the GH and NC groups as an A1M/creatinine ratio >13 (66.7, 8.3, and 35%, respectively, P = 0.01). Concentrations of C5b-9 were higher in the PE group than in the GH and NC groups (medians 9.85, 0.05, and 0.28 ng/mg, respectively, P = 0.003). KIM1, RBP, TIMP-2, and IGFBP-7 concentrations did not differ among groups at delivery. In conclusion, proximal tubular dysfunction, as assessed by A1M and C5b-9, developed during the interval between the end of the second trimester and delivery in patients with PE. However, this was not matched by abnormalities in markers previously associated with tubular cell injury (KIM-1, IGFBP-7, and TIMP-2).

Urine Fibrosis Markers and Risk of Allograft Failure in Kidney Transplant Recipients: A Case-Cohort Ancillary Study of the FAVORIT Trial

BACKGROUND

Kidney tubulointerstitial fibrosis marks risk for allograft failure in kidney transplant recipients, but is poorly captured by estimated glomerular filtration rate (eGFR) or urine albumin-creatinine ratio (ACR). Whether urinary markers of tubulointerstitial fibrosis can noninvasively identify risk for allograft failure above and beyond eGFR and ACR is unknown.

STUDY DESIGN

Case-cohort study.

SETTING & PARTICIPANTS

The FAVORIT (Folic Acid for Vascular Outcome Reduction in Transplantation) Trial was a randomized double-blind trial testing vitamin therapy to lower homocysteine levels in stable kidney transplant recipients. We selected a subset of participants at random (n=491) and all individuals with allograft failure during follow-up (cases; n=257).

PREDICTOR

Using spot urine specimens from the baseline visit, we measured 4 urinary proteins known to correlate with tubulointerstitial fibrosis on biopsy (urine α1-microglobulin [A1M], monocyte chemoattractant protein 1 [MCP-1], and procollagen type III and type I amino-terminal amino pro-peptide).

OUTCOME

Death-censored allograft failure.

RESULTS

In models adjusted for demographics, chronic kidney disease risk factors, eGFR, and ACR, higher concentrations of urine A1M (HR per doubling, 1.73; 95% CI, 1.43-2.08) and MCP-1 (HR per doubling, 1.60; 95% CI, 1.32-1.93) were strongly associated with allograft failure. When additionally adjusted for concentrations of other urine fibrosis and several urine injury markers, urine A1M (HR per doubling, 1.76; 95% CI, 1.27-2.44]) and MCP-1 levels (HR per doubling, 1.49; 95% CI, 1.17-1.89) remained associated with allograft failure. Urine procollagen type III and type I levels were not associated with allograft failure.

LIMITATIONS

We lack kidney biopsy data, BK titers, and HLA antibody status.

CONCLUSIONS

Urine measurement of tubulointerstitial fibrosis may provide a noninvasive method to identify kidney transplant recipients at higher risk for future allograft failure, above and beyond eGFR and urine ACR.

An evaluation of the antioxidant protein α1-microglobulin as a renal tubular cytoprotectant

α1-Microglobulin (A1M) is a low-molecular-weight heme-binding antioxidant protein that is readily filtered by the glomerulus and reabsorbed by proximal tubules. Given these properties, recombinant A1M (rA1M) has been proposed as a renal antioxidant and therapeutic agent. However, little direct evidence to support this hypothesis exists. Hence, we have sought “proof of concept” in this regard. Cultured proximal tubule (HK-2) cells or isolated mouse proximal tubule segments were challenged with a variety of prooxidant insults: 1) hemin, 2) myoglobin; 3) “catalytic” iron, 4) H2O2/Fenton reagents, 5) a Ca2+ ionophore, 6) antimycin A, or 7) hypoxia (with or without rA1M treatment). HK-2 injury was gauged by the percent lactate dehydrogenase release and 4,5-(dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide uptake. In vivo protection was sought in rA1M-treated mice subjected to 1) graded myohemoglobinura (2, 4, 8, or 9 ml/kg glycerol injection), 2) purified myoglobinemia/uria, or 3) endotoxemia. In vivo injury was assessed by blood urea nitrogen, creatinine, and the expression of redox-sensitive genes (heme oxygenase-1, neutrophil gelatinase-associated lipocalin, and monocyte chemoattractant protein-1 mRNAs). Although rA1M totally blocked in vitro hemin toxicity, equimolar albumin (another heme binder) or 10% serum induced equal protection. rA1M failed to mitigate any nonhemin forms of either in vitro or in vivo injury. A1M appeared to be rapidly degraded within proximal tubules (by Western blot analysis). Surprisingly, rA1M exerted select injury-promoting effects (increased in vitro catalytic iron/antimycin toxicities and increased in vivo monocyte chemoattractant protein-1/neutrophil gelatinase-associated lipocalin mRNA expression after glycerol or endotoxin injection). We conclude that rA1M has questionable utility as a renal antioxidant/cytoprotective agent, particularly in the presence of larger amounts of competitive free heme (e.g., albumin) binders.

A clinical, renal and immunological assessment of Surface Modifying Additive Treated (SMART™) cardiopulmonary bypass circuits

Biocompatible cardiopulmonary bypass (CPB) circuits aim to reduce contact activation and its physiological consequences. We investigated the hypothesis that use of Surface Modifying Additive (SMA)-treated circuits (Sorin Group Ltd) compared with non-SMA circuits would be associated with preservation of blood pressure during CPB and modulation of perioperative subclinical renal function (urinary α-1-microglobulin (α-1-m)) and plasma and urinary cytokine changes. In a study of low-risk CABG patients ( n=40), randomized to SMA ( n=20) versus non-SMA circuits ( n=20), we found better preserved blood pressure at CPB initiation in SMA patients (p <0.05), particularly in ACE-inhibited SMA patients ( n=11) versus ACE-inhibited non-SMA patients ( n=10) (p <0.05). Plasma anti-inflammatory IL-10, as well as urinary α-1-m, were elevated 48 hours postoperatively (p <0.05). SMA patients also had lower blood loss (p <0.05). SMA circuits have some clinical benefit, especially in ACE-inhibited patients.

HIV Infection, Tenofovir, and Urine α1-Microglobulin: A Cross-sectional Analysis in the Multicenter AIDS Cohort Study

BACKGROUND

Tenofovir disoproxil fumarate (TDF) can cause proximal tubular damage and chronic kidney disease in human immunodeficiency virus (HIV)-infected individuals. Urine α1-microglobulin (A1M), a low-molecular-weight protein indicative of proximal tubular dysfunction, may enable earlier detection of TDF-associated tubular toxicity.

STUDY DESIGN

Cross-sectional.

SETTING & PARTICIPANTS

883 HIV-infected and 350 -uninfected men enrolled in the Multicenter AIDS Cohort Study.

PREDICTORS

HIV infection and TDF exposure.

OUTCOME

Urine A1M level.

RESULTS

Urine A1M was detectable in 737 (83%) HIV-infected and 202 (58%) -uninfected men (P<0.001). Among HIV-infected participants, 573 (65%) were current TDF users and 112 (13%) were past TDF users. After multivariable adjustment including demographics, traditional kidney disease risk factors, and estimated glomerular filtration rate, HIV infection was associated with 136% (95% CI, 104%-173%) higher urine A1M levels and 1.5-fold (95% CI, 1.3- to 1.6-fold) prevalence of detectable A1M. When participants were stratified by TDF exposure, HIV infection was associated with higher adjusted A1M levels, by 164% (95% CI, 127%-208%) among current users, 124% (95% CI, 78%-183%) among past users, and 76% (95% CI, 45%-115%) among never users. Among HIV-infected participants, each year of cumulative TDF exposure was associated with 7.6% (95% CI, 5.4%-9.9%) higher A1M levels in fully adjusted models, a 4-fold effect size relative to advancing age (1.8% [95% CI, 0.9%-2.7%] per year). Each year since TDF treatment discontinuation was associated with 4.9% (95% CI, -9.4%--0.2%) lower A1M levels among past users.

LIMITATIONS

Results may not be generalizable to women.

CONCLUSIONS

HIV-infected men had higher urine A1M levels compared with HIV-uninfected men. Among HIV-infected men, cumulative TDF exposure was associated with incrementally higher A1M levels, whereas time since TDF treatment discontinuation was associated with progressively lower A1M levels. Urine A1M appears to be a promising biomarker for detecting and monitoring TDF-associated tubular toxicity.

Human plasma lipocalins and serum albumin: Plasma alternative carriers?

Lipocalins are an evolutionarily conserved family of proteins that bind and transport a variety of exogenous and endogenous ligands. Lipocalins share a conserved eight anti-parallel β-sheet structure. Among the different lipocalins identified in humans, α-1-acid glycoprotein (AGP), apolipoprotein D (apoD), apolipoprotein M (apoM), α1-microglobulin (α1-m) and retinol-binding protein (RBP) are plasma proteins. In particular, AGP is the most important transporter for basic and neutral drugs, apoD, apoM, and RBP mainly bind endogenous molecules such as progesterone, pregnenolone, bilirubin, sphingosine-1-phosphate, and retinol, while α1-m binds the heme. Human serum albumin (HSA) is a monomeric all-α protein that binds endogenous and exogenous molecules like fatty acids, heme, and acidic drugs. Changes in the plasmatic levels of lipocalins and HSA are responsible for the onset of pathological conditions associated with an altered drug transport and delivery. This, however, does not necessary result in potential adverse effects in patients because many drugs can bind both HSA and lipocalins, and therefore mutual compensatory binding mechanisms can be hypothesized. Here, molecular and clinical aspects of ligand transport by plasma lipocalins and HSA are reviewed, with special attention to their role as alterative carriers in health and disease.

Recent advances in pathophysiology and biomarkers of sepsis-induced acute kidney injury

Sepsis is a complex clinical syndrome characterized by a systemic inflammatory response to an infective insult. This process often leads to widespread tissue injury and multiple organ dysfunction. In particular, the development of acute kidney injury (AKI) is one of the most frequent complications, which increases the complexity and cost of care, and is an independent risk factor for mortality. Previous suggestions highlighting systemic hypotension, renal vasoconstriction and ischaemia-reperfusion injury as the primary pathophysiological mechanisms involved in sepsis-induced AKI have been challenged. Recently it has been shown that sepsis-induced AKI occurs in the setting of microvascular dysfunction with release of microparticles, inflammation and energetic adaptation of highly metabolic organs to cellular stress. The intolerable high mortality rate associated with sepsis-induced AKI is partially explained by an incomplete understanding of its pathophysiology and a delay in diagnosis. The aim of this review is to focus on advances in understanding the sepsis pathophysiology, with particular attention to the fundamental mechanisms of sepsis-induced AKI and the potential diagnostic and prognostic markers involved.

[Urinary protein detection by iTRAQ® associated with renal transplant complications and its modification with therapy]

BACKGROUND

After renal transplant, surgical, infection complications, as well as graft rejection may occur; early detection through non-invasive markers is the key to change therapy and avoid biopsy.

OBJECTIVE

The aime of the study is to determine urine protein profiles in patients undergoing renal transplant with complications and detect its variation when therapy is modified.

MATERIAL AND METHODS

Urine samples were collected from patients prior the transplant and various postoperative stages. Urinary protein profiles were obtained by peptide labeling using isobaric isotopes for relative quantification (iTRAQ(®)).

RESULTS

A total of 22 patients were included, of whom 12 developed post-transplant complication: 2 with graft rejection (one male and one female) and 10 (6 males and 4 females) in the group of post-transplant infections. Using iTRAQ(®) 15/345 and 28/113 proteins were identified and fulfilled the acceptance criteria, in graft rejection and post-transplant infections group, respectively.

CONCLUSIONS

Albumin was the only protein found in both groups, the remaining proteins were different. The 5 proteins with higher scores in graft rejection were: alpha-1-microglobulin, 5'-nucleotidase cytosolic III, retinol-binding protein 4, membrane protein palmitoylated 4, and serine carboxypeptidase, while post-transplant infections were: mitochondrial acetyl-coenzyme A synthetase, putative adenosyl homocysteinase 2, zinc finger protein GLIS1, putative protein FAM157B, and zinc finger protein 615. It remains to elucidate the involvement of each of these in patients with renal transplantation.

APOL1 Genotype and Glomerular and Tubular Kidney Injury in Women With HIV

BACKGROUND

APOL1 genotype is associated with advanced kidney disease in African Americans, but the pathogenic mechanisms are unclear. Here, associations of APOL1 genotype with urine biomarkers of glomerular and tubular injury and kidney function decline were evaluated.

STUDY DESIGN

Observational study.

SETTING & PARTICIPANTS

431 human immunodeficiency virus (HIV)-infected African American women enrolled in Women's Interagency HIV Study (WIHS).

PREDICTOR

APOL1 genotype.

OUTCOMES

Albumin-creatinine ratio (ACR), 4 tubular injury biomarkers (interleukin 18 [IL-18], kidney injury molecule 1 [KIM-1], neutrophil gelatinase-associated lipocalin [NGAL], and α1-microglobulin [A1M]), and kidney function estimated using the CKD-EPI cystatin C equation.

MEASUREMENTS

Participants were genotyped for APOL1 single-nucleotide polymorphisms rs73885319 (G1 allele) and rs71785313 (G2 allele). Urine biomarkers were measured using stored samples from 1999-2000. Cystatin C was measured using serum collected at baseline and 4- and 8-year follow-ups.

RESULTS

At baseline, ACRs were higher among 47 women with 2 APOL1 risk alleles versus 384 women with 0/1 risk allele (median, 24 vs 11mg/g; P<0.001). Compared with women with 0/1 risk allele, women with 2 risk alleles had 104% higher ACRs (95% CI, 29-223mg/g) and 2-fold greater risk of ACR>30 (95% CI, 1.17-3.44) mg/g after multivariable adjustment. APOL1 genotype showed little association with urine IL-18:Cr ratio, KIM-1:Cr ratio, and NGAL:Cr ratio (estimates of -5% [95% CI, -24% to 18%], -20% [95% CI, -36% to -1%], and 10% [95% CI, -26% to 64%], respectively) or detectable urine A1M (prevalence ratio, 1.13; 95% CI, 0.65-1.97) in adjusted analyses. Compared with women with 0/1 allele, women with 2 risk alleles had faster eGFR decline, by 1.2 (95% CI, 0.2 to 2.2) mL/min/1.73m(2) per year, and 1.7- and 3.4-fold greater rates of incident chronic kidney disease (95% CI, 1.1 to 2.5) and 10% annual eGFR decline (95% CI, 1.7 to 6.7), respectively, with minimal attenuation after adjustment for glomerular and tubular injury biomarker levels.

LIMITATIONS

Results may not be generalizable to men.

CONCLUSIONS

Among HIV-infected African American women, APOL1-associated kidney injury appears to localize to the glomerulus, rather than the tubules.

The menagerie of human lipocalins: a natural protein scaffold for molecular recognition of physiological compounds

While immunoglobulins are well-known for their characteristic ability to bind macromolecular antigens (i.e., as antibodies during an immune response), the lipocalins constitute a family of proteins whose role is the complexation of small molecules for various physiological processes. In fact, a number of low-molecular-weight substances in multicellular organisms show poor solubility, are prone to chemical decomposition, or play a pathophysiological role and thus require specific binding proteins for transport through body fluids, storage, or sequestration. In many cases, lipocalins are involved in such tasks. Lipocalins are small, usually monomeric proteins with 150-180 residues and diameters of approximately 40 Å, adopting a compact fold that is dominated by a central eight-stranded up-and-down β-barrel. At the amino-terminal end, this core is flanked by a coiled polypeptide segment, while its carboxy-terminal end is followed by an α-helix that leans against the β-barrel as well as an amino acid stretch in a more-or-less extended conformation, which finally is fixed by a disulfide bond. Within the β-barrel, the antiparallel strands (designated A to H) are arranged in a (+1)7 topology and wind around a central axis in a right-handed manner such that part of strand A is hydrogen-bonded to strand H again. Whereas the lower region of the β-barrel is closed by short loops and densely packed hydrophobic side chains, including many aromatic residues, the upper end is usually open to solvent. There, four long loops, each connecting one pair of β-strands, together form the entrance to a cup-shaped cavity. Depending on the individual structure of a lipocalin, and especially on the lengths and amino acid sequences of its four loops, this pocket can accommodate chemical ligands of various sizes and shapes, including lipids, steroids, and other chemical hormones as well as secondary metabolites such as vitamins, cofactors, or odorants. While lipocalins are ubiquitous in all higher organisms, physiologically important members of this family have long been known in the human body, for example with the plasma retinol-binding protein that serves for the transport of vitamin A. This prototypic human lipocalin was the first for which a crystal structure was solved. Notably, several other lipocalins were discovered and assigned to this protein class before the term itself became familiar, which explains their diverse names in the scientific literature. To date, up to 15 distinct members of the lipocalin family have been characterized in humans, and during the last two decades the three-dimensional structures of a dozen major subtypes have been elucidated. This Account presents a comprehensive overview of the human lipocalins, revealing common structural principles but also deviations that explain individual functional features. Taking advantage of modern methods for combinatorial protein design, lipocalins have also been employed as scaffolds for the construction of artifical binding proteins with novel ligand specificities, so-called Anticalins, hence opening perspectives as a new class of biopharmaceuticals for medical therapy.

Deeper insight into chronic kidney disease-related atherosclerosis: comparative proteomic studies of blood plasma using 2DE and mass spectrometry

Background

Atherosclerosis is a major cause of cardiac events and mortality in patients suffering from chronic kidney disease (CKD). Moreover, the risk of cardiovascular disease (CVD) development in patients with CKD increases as kidney function declines. Although the close connection between atherosclerosis and kidney dysfunction is undeniable, particular risk factors and specific mechanisms that promote CVD in patients with CKD remain unclear. To gain insight into better recognition of the mechanisms of accelerated atherosclerosis in patients with CKD, we performed a comparative proteomic analysis of blood plasma from patients in various stages of CKD and thus distinct progression of atherosclerosis (n = 90), patients with advanced CVD and normal renal function (n = 30) and healthy volunteers (n = 30).

Methods

Plasma samples were depleted using affinity chromatography and divided into three fractions: high-abundant, low-abundant and low-molecular weight proteins. The first two fractions were analyzed by two-dimensional gel electrophoresis and mass spectrometry, the last one has been subjected to direct MS/MS analysis. A proteomic profiles for high-abundant, low-abundant and low-molecular weight proteins fractions were obtained. Differential accumulated proteins were confirmed by selected reaction monitoring analysis (SRM). The Gene Ontology (GO) function and the interaction networks of differentially expressed proteins were then analyzed.

Results

Forty-nine proteins (13 high- and 36 low-molecular mass) showed differences in accumulation levels. For eleven of them differential expression were confirmed by selected reaction monitoring analysis. Bioinformatic analysis showed that identified differential proteins were related to three different processes: the blood coagulation cascade, the transport, binding and metabolism of lipoproteins and inflammatory processes.

Conclusions

Obtained data provide an additional line of evidence that different molecular mechanisms are involved in the development of CKD- and CVD-related atherosclerosis. The abundance of some anti-atherogenic factors revealed in patients with CKD suggests that these factors are not associated with the reduction of atherosclerosis progression in CKD that is typically observed in “classical” CVD. Moreover, obtained data also suggest that mechanism of CVD acceleration may be different in initial and advanced stages of CKD. Undoubtedly, in advanced stages of CKD inflammation is highly pronounced.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-014-0378-8) contains supplementary material, which is available to authorized users.

Association of urine α1-microglobulin with kidney function decline and mortality in HIV-infected women

BACKGROUND AND OBJECTIVES

Despite advances in therapy, HIV-infected individuals remain at higher risk for kidney dysfunction than uninfected individuals. It was hypothesized that urine levels of α1-microglobulin, a biomarker of proximal tubular dysfunction, would predict kidney function decline and mortality risk in HIV-infected and uninfected women.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In the Women's Interagency HIV Study, urine α1-microglobulin and creatinine concentrations were measured in 903 HIV-infected and 287 uninfected women using stored urine from 1999 to 2000, when prevalence of tenofovir use was <1%. Participants were categorized into three categories by level of α1-microglobulin-to-creatinine ratio, and associations with kidney decline and all-cause mortality over 8 years were evaluated.

RESULTS

Urine α1-microglobulin was detectable in 60% of HIV-infected and 40% of uninfected women (P<0.001). Among HIV-infected women, there were 177 (22%), 61 (7%), and 128 (14%) patients with incident CKD, with 10% annual eGFR decline, and who died, respectively. Compared with HIV-infected women in the lowest α1-microglobulin category, HIV-infected women in the highest α1-microglobulin category had a 2.1-fold risk of incident CKD (95% confidence interval, 1.3 to 3.4), 2.7-fold risk of 10% annual eGFR decline (95% confidence interval, 1.2 to 5.9), and 1.6-fold mortality risk (95% confidence interval, 1.0 to 2.6) in models adjusting for kidney risk factors, baseline eGFR, and albuminuria. Among uninfected women, the highest α1-microglobulin category was associated with 3% (relative risk, 2.2; 95% confidence interval, 1.4 to 3.5) and 5% (relative risk, 2.2; 95% confidence interval, 1.1 to 4.3) annual eGFR decline relative to the lowest α1-microglobulin category.

CONCLUSIONS

Proximal tubular dysfunction, indicated by urine α1-microglobulin, was independently associated with kidney function decline in HIV-infected and uninfected women and mortality risk among HIV-infected women.

High level of serum AMBP is associated with poor response to paclitaxel-capecitabine chemotherapy in advanced gastric cancer patients

Gastric cancer is one of the most common human cancers and ranks the second in the global cancer-related mortality. The clinical outcome of patients with advanced gastric cancer (AGC) is markedly dependent on their response to the chemotherapy. Paclitaxel plus capecitabine, as a first-line regimen, is widely administrated in AGC patients, but more than a half of the patients have a poor response, possibly due to their resistance to the treatment. Therefore, it is important to identify potential responders to improve the efficacy of the chemotherapy. In the present study, we used an isobaric tag approach for relative and absolute quantification combined with ESI-QUAD-TOF/MS to identify potential predictive biomarkers for the chemotherapy. We found 211 serum proteins, and confirmed 17 candidates that were differentially present in the progression of disease (PD) group and the partial response (PR) group to the treatment of paclitaxel plus capecitabine. In further validation of the 17 candidates in the set of 12 PD and 12 PR AGC patients, we identified a higher level of AMBP (Alpha-1-Microglobulin/Bikunin Precursor) in the sera of PD patients than of the PR patients assayed by ELISA (9.13 ± 0.45 vs. 8.11 ± 0.26 μg/mL, p = 0.06) and by the Western blotting (relative gray value 396.4 ± 39.1 vs. 275.0 ± 34.76, p = 0.03), respectively. The receiver operating characteristics curve showed 75% sensitivity and 75% specificity of AMBP in AGC patients treated with the chemotherapy. Our data indicated that the high level of serum AMBP could predict the poor response of the AGC patients treated with the paclitaxel-capecitabine chemotherapy, which could be used as a potential biomarker to identify patients who would benefit from this chemotherapeutic regimen.

Use of α1-microglobulin for diagnosing chronic interstitial nephropathy

α1-Microglobulin (α1M) is a low molecular weight protein and has been best characterized for detecting acute lesions of proximal tubules (Bonventre in Contrib Nephrol 156:213-219, 2007). This study has tried to evaluate the use of α1M for the differential diagnosis of chronic interstitial nephropathy. 145 patients were recruited [81 men and 64 women, mean age 61.8 ± 16.7 years, 64.8 % have an estimated glomerular filtration (GFR) <60 ml/min]. Urinary α1M was evaluated using an immunonephelometric assay. 82 patients were diagnosed as having chronic interstitial nephritis (CIN), and 46 patients have been previously diagnosed of glomerulonephritis (GN). A group of hypertensive patients without renal disease was used as control (n = 17). Patients in GN group had the highest α1M excretion (15.05 mg/24 h). When the α1M/albuminuria rates were calculated, the CIN group had the highest rate (1.03 mg/mg) and the GN group had the lowest rate (0.04 mg/mg) (p < 0.001). When the α1M/proteinuria rates were calculated, the results were rather similar. The AUC for CIN group was 0.785, and the one for GN group was 0.139. Patients with estimated GFR <60 ml/min showed a higher excretion of α1M (18.75, 8.75-40.00 mg/24 h). Nevertheless, α1M/albuminuria and α1M/proteinuria rates were still higher in CIN patients with GFR ≥60 ml/min. α1M urinary excretion is increased in chronic interstitial nephropathy and glomerulonephritis as well as in patients with GFR <60 ml/min. The α1M/albuminuria rate and the α1M/proteinuria quotient are increased in chronic interstitial nephropathies but decreased in glomerular diseases.

Renal uptake of 99mTc-dimercaptosuccinic acid is dependent on normal proximal tubule receptor-mediated endocytosis

(99m)Tc-labeled dimercaptosuccinic acid ((99m)Tc-DMSA) accumulates in the kidney cortex and is widely used for imaging of the renal parenchyma. Despite its extensive clinical use, the mechanism for renal targeting of the tracer is unresolved. Megalin and cubilin are cooperating receptors essential to the proximal tubule endocytic uptake of proteins from the glomerular ultrafiltrate. We have used megalin/cubilin-deficient mice produced by gene knockout to determine whether receptor-mediated endocytosis is responsible for the renal uptake of (99m)Tc-DMSA.

METHODS

Control or megalin/cubilin-deficient mice were injected intravenously with 0.5 MBq of (99m)Tc-DMSA or (99m)Tc-mercaptoacetyltriglycine (MAG3). Whole-body scintigrams and the activity in plasma, urine, and the kidneys were examined 6 h after injection. The size and identity of (99m)Tc-DMSA-bound proteins in urine were analyzed by fractionation by centrifugation and separation by sodium dodecyl sulfate polyacrylamide gel electrophoresis, followed by autoradiography and mass spectrometry.

RESULTS

No renal accumulation of (99m)Tc-DMSA was identified in scintigrams of megalin/cubilin-deficient mice. The renal accumulated activity of the tracer was reduced to 11.4% (± 2.5%, n = 7) of the normal uptake in control mice, correlating with a reduction in renal megalin/cubilin expression in knockout mice to about 10% of normal. The reduced renal uptake in megalin/cubilin-deficient mice was accompanied by an increase in the urinary excretion of (99m)Tc-DMSA. Size separation of the urine by ultracentrifugation and sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrated that in megalin/cubilin-deficient mice an increased amount of (99m)Tc-DMSA was excreted in an approximately 27-kDa form, which by mass spectrometry was identified as the plasma protein α1-microglobulin, an established megalin/cubilin ligand.

CONCLUSION

(99m)Tc-DMSA is filtered bound to α1-microglobulin and accumulates in the kidneys by megalin/cubilin-mediated endocytosis of the (99m)Tc-DMSA protein complex. Renal accumulation of (99m)Tc-DMSA is thus critically dependent on megalin/cubilin receptor function and therefore is a marker of proximal tubule endocytic activity.

Preparation of cysteine-34-nitroxide spin labeled human α₁-microglobulin

α(1)-Microglobulin (α(1)m) is a protein of yet unresolved function occurring in blood plasma and urine. It consists of a lipocaline type of fold with two cysteine residues forming a disulfide bridge and the third cysteine-34 remaining a free, somewhat reactive thiol. A number of investigations point to an interaction with heme and we have recently reported, that heme binding triggers the formation of a stable α(1)m trimer upon modification of cysteine-34 with 2-iodoacetamide, i.e., [α(1)m(heme)(2)](3) [J.F. Siebel, R.L. Kosinsky, B. Åkerström, M. Knipp, Insertion of heme b into the structure of the Cys34-carbamidomethylated human lipocalin α(1)-microglobulin-formation of a [(heme)(2)(α(1)-microglobulin)](3) complex, ChemBioChem 13 (2012) 879-887]. For further structural and functional investigations, an improved purification protocol for α(1)m was sought, in particular yielding an untagged amino acid sequence. The method reported herein improves the speed and the yield of the protein production even when an expression plasmid without tag was applied. Furthermore, for the purpose of future structural studies using electron paramagnetic resonance (EPR) techniques, in accordance to the modification with 2-iodoacetamide (α(1)m(AM)), the protein was modified with 3-(2-iodoacetamido)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (3-(2-iodoacetamido)-PROXYL) yielding the nitroxide spin labeled α(1)m(N-O). The extinction coefficient of the protein was calibrated using magnetic circular dichroism (MCD) spectroscopy of tryptophan (ε(280nm)=40,625M(-1)cm(-1)). The parallel quantification by absorbance spectroscopy (protein) and cw-EPR spectroscopy (radical spin) determined the degree of spin labeling to 90%. Characterization of the protein by circular dichroism (CD) spectroscopy and matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) upon tryptic digestion further demonstrated the similar fold of α(1)m(AM) and α(1)m(N-O), but also established the modification of cystein-34 as well as the formation of the cysteine-72-cysteine-169 disulfide bond.

Insertion of heme b into the structure of the Cys34-carbamidomethylated human lipocalin α(1)-microglobulin: formation of a [(heme)(2) (α(1)-Microglobulin)](3) complex

α(1)-Microglobulin (α(1)m) is a 26 kDa plasma and tissue protein belonging to the lipocalin protein family. Previous investigations indicate that the protein interacts with heme and suggest that it has a function in heme metabolism. However, detailed characterizations of the α(1)m-heme interactions are lacking. Here, we report for the first time the preparation and analysis of a stable α(1)m-heme complex upon carbamidomethylation of the reactive Cys34 by using recombinantly expressed human α(1)m. Analytical size-exclusion chromatography coupled with a diode-array absorbance spectrophotometry demonstrates that at first an α(1)m-heme monomer is formed. Subsequently, a second heme triggers oligomerization that leads to trimerization. The resulting (α(1)m[heme](2))(3) complex was characterized by resonance Raman and EPR spectroscopy, which support the presence of two ferrihemes, thus indicating an unusual spin-state admixed ground state with S=(3)/(2), (5)/(2).

The crystal structure of human α1-microglobulin reveals a potential haem-binding site

We describe the 2.3 Å (1 Å=0.1 nm) X-ray structure of α1m (α1-microglobulin), an abundant protein in human blood plasma, which reveals the β-barrel fold typical for lipocalins with a deep pocket lined by four loops at its open rim. Loop #1 harbours the residue Cys34 which is responsible for covalent cross-linking with plasma IgA. A single disulfide bond between Cys72 and Cys169 connects the C-terminal segment to the β-barrel, as in many other lipocalins. The exposed imidazole side chains of His122 and His123 in loop #4 give rise to a double Ni2+-binding site together with a crystallographic neighbour. The closest structural relatives of α1m are the complement protein component C8γ, the L-prostaglandin D synthase and lipocalin 15, three other structurally characterized members of the lipocalin family in humans that have only distant sequence similarity. In contrast with these, α1m is initially expressed as a bifunctional fusion protein with the protease inhibitor bikunin. Neither the electron density nor ESI–MS (electrospray ionization MS) provide evidence for a chromophore bound to the recombinant α1m, also known as ‘yellow/brown lipocalin’. However, the three side chains of Lys92, Lys118 and Lys130 that were reported to be involved in covalent chromophore binding appear to be freely accessible to ligands accommodated in the hydrophobic pocket. A structural feature similar to the well-known Cys–Pro haem-binding motif indicates the presence of a haem-binding site within the loop region of α1m, which explains previous biochemical findings and supports a physiological role in haem scavenging, as well as redox-mediated detoxification.