Polybrominated diphenyl ethers in dust, hair and urine: Exposure, excretion

Polybrominated diphenyl ethers (PBDEs) have been detected in various environmental media and human tissues. PBDEs concentrations in dust from college buildings and homes and in paired hair and urine samples from students were determined. This is of great significance to explore the accumulation and excretion patterns of PBDEs in the human body. The median PBDEs concentrations in the dust (College: 84.59 ng/g; Home: 170.32 ng/g) and hair (undergraduate: 6.16 ng/g; Home: 3.25 ng/g) samples were generally lower than were found in the majority of previous studies. The PBDEs concentrations in the hair and urine samples were subjected to principal component analysis, and the results combined with the PBDEs detection rates confirmed that hair is a useful non-invasive sampling medium for assessing PBDEs exposure and the risks posed. Body mass indices (BMIs) were used to divide students who had not been exposed to large amounts of PBDEs into groups. Body fat percentage is an important factor affecting the accumulation of PBDE in the human body. Environmental factors were found to affect the PBDEs concentrations in the hair and urine samples less for normal-weight students (BMI≤24) than overweight students (BMI>24). Short-term environmental changes to more readily affect the PBDEs concentrations in the tissues of the normal-weight than overweight students. PBDEs with seven or more bromine substituents were found not to be readily excreted in urine. Performing molecular docking simulations of the binding of isomers BDE-99 and BDE-100 to megalin. The binding energy was higher for BDE-100 and megalin than for BDE-99 and megalin, meaning BDE-99 would be more readily excreted than BDE-100.

Megalin: A bridge connecting kidney, the renin-angiotensin system, and atherosclerosis

Megalin is a member of the low-density lipoprotein receptor superfamily. It has been recognized as an endocytic receptor for a large spectrum of ligands. As a consequence, megalin regulates homeostasis of many molecules and affects multiple physiological and pathophysiological functions. The renin-angiotensin system is a hormonal system. A number of studies have reported contributions of the renin-angiotensin system to atherosclerosis. There is evolving evidence that megalin is a regulator of the renin-angiotensin system, and contributes to atherosclerosis. This brief review provides contemporary insights into effects of megalin on renal functions, the renin-angiotensin system, and atherosclerosis.

Urinary megalin in association with progression factors of diabetic nephropathy

AIM

The aim of this study is to evaluate the association between urinary megalin, renal function, blood pressure, lipid profile, vitamin D and glycemic control in patients with type 2 diabetes mellitus (T2DM).

METHODS

. This was a cross-sectional study which recruited 209 patients with T2DM. Urinary megalin was positively associated with systolic blood pressure (SBP) (r=0.218, p=0.04) but negatively with glomerular filtration rate (GFR) (r=-0.16, p=0.023). The levels of urinary albumin, triglycerides (TGs) and glycosylated hemoglobin (HbA1c) were higher in the "high-megalin" group, compared to those in "low-megalin" group. Moreover, there was a significant inverse association between vitamin D3 levels and megalin levels in urine (OR=0.281, p=0.047).

CONCLUSION

Our study showed for the first time that megalin is associated with progression factors of diabetic nephropathy as well as vitamin D deficiency (Tab. 3, Fig. 1, Ref. 15).

Skeletal muscle vitamin D in patients with end stage osteoarthritis of the knee

Muscle function is often impaired in patients with knee osteoarthritis (OA), with reduced strength and increased pain. The role of vitamin D and the vitamin D-endocrine pathway in muscle health has recently been placed in the spotlight, with various groups reporting positive effects on muscle development, function and health. Recently, it has been shown that uptake into muscle of the specialized vitamin D binding protein (DBP) is dependent on the endocytic receptor, megalin. Here we analyse circulating vitamin D, and muscle DBP, megalin and the cognate vitamin D receptor (VDR) in patients with knee OA and compare them to asymptomatic controls. Muscle and blood samples were collected from 19 patients with end-stage OA of the knee and 10 age-matched controls. Muscle biopsies from the OA group were performed during knee replacement surgery and a needle biopsy was used on control volunteers. Immunoblots performed with specific antibodies were used to detect the presence of DBP, megalin, VDR (using the specific D-6 antibody) and albumin in the muscle biopsies. Results were correlated with FoxO1, a key regulator of the ubiquitin-proteasome degradation pathway in muscle. There were no differences in circulating levels of 25 (OH) vitamin D3 between the groups, and no subjects were vitamin D deficient. We found increased VDR, DBP and albumin protein in the muscle from patients with OA compared to controls, with no change in muscle megalin expression. Furthermore, DBP levels in the muscle correlated with FoxO1, suggesting an association between muscle protein breakdown and the activation of the vitamin D-endocrine pathway in muscle surrounding an OA affected joint. We show, for the first time, that the factors involved in the vitamin D-endocrine-pathway are present at higher levels in muscles from OA patients compared to asymptomatic controls. This is despite no differences in circulating 25 (OH) vitamin D levels between the groups. These findings indicate the activation of vitamin D pathway in these muscles that may provide a beneficial compensatory stimulation of the repair process in muscles that are subject to inflammatory and proteolytic processes.

Evidence for a Clathrin-Mediated Recycling of Albumin in Human Term Placenta1

During human pregnancy, the trophoblast layer is in direct contact with maternal albumin. In contrast to immunoglobulins, albumin does not cross the placental barrier. However, albumin affects the trophoblast placental lactogen and chorionic gonadotroph secretion. The present study investigated the interaction between albumin and syncytiotrophoblast using human term placental explants. Bovine serum albumin, labeled with either 125I or fluorescein isothio-cyanate, was taken up rapidly by placental explants. This process was temperature-sensitive. The internalized labeled BSA quickly outflowed from the tissue at the maternal side, largely without any major modification in molecular weight. Colchicine (1 mM), which disrupts the microtubule network, or cytochalasin B (40 microM), which disassembles filamentous actin, did not interfere with the placental transmembrane movements of labeled BSA. Megalin, clathrin, and caveolin 1 are three membrane proteins associated with albumin endocytosis in other tissues, but only megalin and clathrin were detected in the syncytiotrophoblast layer by immunohistochemistry. The uptake of labeled BSA into placental explants was not modified by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (1 mM) or 5-nitro-2-(3-phenylpropylamino)benzoic acid (100 microM), two pharmacological tools known to disturb megalin-mediated albumin endocytosis. By contrast, methyl-beta-cyclodextrin (10 mM) and chlorpromazine (1.4 mM), both of which disrupt the clathrin-mediated endocytotic system, significantly reduced the uptake of labeled BSA. These data suggest, to our knowledge for the first time, that maternal albumin is actively internalized into the human trophoblast according to an apical recycling pathway. This temperature-sensitive process does not depend on an intact cytoskeleton, but it is associated with a clathrin-mediated endocytotic system.

The renal Na+/phosphate cotransporter NaPi-IIa is internalized via the receptor-mediated endocytic route in response to parathyroid hormone

The major renal Na(+)/phosphate cotransporter, NaPi-IIa, is regulated by a number of factors including parathyroid hormone (PTH), dopamine, and dietary phosphate intake. PTH induces the acute internalization of NaPi-IIa from the brush border membrane (BBM) and its routing to and subsequent degradation in lysosomes. Previous work indicated that megalin, part of the apical receptor-mediated endocytic apparatus, may play a role in the PTH-induced removal of NaPi-IIa. Here we examined in rats the time-dependent internalization route of NaPi-IIa after acute PTH application using immunohistochemistry and markers of several endocytic compartments. NaPi-IIa removal from the BBM was detectable as early as 5 min after PTH injection. After 10-15 min, NaPi-IIa was localized in subapical compartments positive for clathrin. Shortly thereafter, NaPi-IIa appeared in endosomes stained for EEA1 (early endosomal antigen 1). After 45-60 min, NaPi-IIa was found in late endosomes/lysosomes marked with lgp120. In contrast, no change in the subcellular localization of megalin and the Na(+)/H(+) exchanger NHE3 was detected up to 60 min after PTH injection. To further characterize the internalization route, insulin, as a marker for receptor-mediated endocytosis, and horseradish peroxidase (HRP) and fluorescein isothiocyanate (FITC)-dextran (10 kDa), as markers for fluid-phase mediated endocytosis, were used. NaPi-IIa colocalized with insulin 5-30 min after PTH injection but did not overlap with HRP or FITC-dextran. These results demonstrate a distinct internalization route of NaPi-IIa in response to acute PTH application that may involve the receptor-mediated endocytic pathway including clathrin-coated vesicles and EEA1-positive early endosomes, and routes NaPi-IIa to lysosomes for degradation.

Overlapping expression patterns of the multiligand endocytic receptors cubilin and megalin in the CNS, sensory organs and developing epithelia of the rodent embryo

Cubilin and megalin are multiligand epithelial endocytic receptors well characterized in the adult kidney and ileum where they form a complex essential for protein, lipid and vitamin uptake. Although inactivation of the megalin gene leads to holoprosencephaly and administration of anti-cubilin antibodies induces fetal resorptions or cranio-facial malformations their function in the developing embryo remains unclear. We recently showed that both proteins are strongly expressed by the maternal-fetal interfaces and the neuroepithelium of the early rodent embryo where they co-localize and form a complex important for nutrient uptake. The aim of the present study was the further investigation of cubilin expression at later developmental stages of the rodent embryo and its correlation to that of megalin. Immunohistochemical and in situ hybridization analysis showed striking similarities in the spatial and temporal expression patterns of cubilin and megalin. The electrophoretic mobility of both proteins was identical to that of the adult as revealed by Western blot analysis. Cubilin and megalin were strongly expressed in the sensory organs, the central nervous system, the respiratory and urogenital tracts as well as in the thymus, parathyroids and thyroid. In each site, the expression mainly concerned epithelial structures and correlated with the onset of epithelial induction. Depending on the site, a decreased or restricted expression was observed by the end of the gestation for both proteins.

Immunocytochemical characterization of the incubated rat renal cortical slices

The use of renal cortical slices in vitro and the data obtained in these studies have been subjects of controversy, largely due to uncertain viability, e.g., structural and functional integrity of the proximal and other tubules. However, detailed studies of tubule integrity have not been reported. To correlate functional and structural viability of the hand-cut rat renal cortical slices, incubated in optimally conditioned media for up to 25 h, we studied the time course of p-aminohippurate (PAH) uptake, the immunocytochemical distribution of several proteins that reside in the proximal tubule basolateral [Na/K-ATPase, organic anion transporters (OAT)1 and OAT3], or brush border [megalin, sodium-proton exchanger (NHE)3] membrane, as well as the general integrity of the tubule epithelium and its cytoskeleton (actin filaments, microtubules). PAH uptake in slices was proportional to time within 1 h of incubation and gradually declined thereafter. The immunostaining experiments indicated a fast, time-dependent loss of basolateral transporters, at a rate of OAT1 > Na/K-ATPase > OAT3. In the brush border membrane, the loss of megalin was faster than that of NHE3, and a partial redistribution of NHE3 into the basolateral domain indicated the loss of cell polarity. The loss of intracellular actin and tubulin cytoskeleton in the proximal tubule was already visible after 15 min of incubation and gradually increased with time, whereas a partial redistribution of actin to the basolateral domain indicated a compromised polarity of the cells. The data also revealed very early (after 15 min) necrotic events in the proximal tubule epithelium, with sloughing of brush border and cell debris into the tubule lumen, detachment of cells from the basal membrane, and opening and widening of the tubule lumen. We conclude that the loss of cellular structure, cytoskeleton, and cell membrane transporters in the nephron epithelium is a very early event in the incubated rat renal cortical slices.

Differential distribution of cubilin and megalin expression in the mouse embryo

Cubilin and megalin are cell surface proteins that work cooperatively in many absorptive epithelia to mediate endocytosis of lipoproteins, vitamin carriers, and other proteins. Here we have investigated the coordinate expression of these receptors during mouse development. Our findings indicate that while there are sites where the receptors are co-expressed, there are other tissues where expression is not overlapping. Apical cubilin expression is pronounced in the extraembryonic visceral endoderm (VE) of 6-9.5 days postcoitum (dpc) embryos. By contrast, little megalin expression is evident in the VE at 6 dpc. However, megalin expression in the VE increases as development progresses (7.5-9.5 dpc), although it is not as uniformly distributed as cubilin. Punctate expression of megalin is also apparent in the region of the ectoplacental cone associated with decidual cells, whereas cubilin expression is not seen in association with the ectoplacenta. Strong expression of megalin is observed in the neural ectoderm, neural plate and neural tube (6-8.5 dpc), but cubilin expression is not apparent in any of these tissues. At 8.5 dpc, megalin is expressed in the developing endothelial cells of blood islands, whereas cubilin is absent from these cells. Finally, cubilin, but not megalin, is expressed by a subpopulation of cells dispersed within the 7.5 dpc embryonic endoderm and having a migratory morphology. In summary, the co-expression of cubilin and megalin in the VE is consistent with the two proteins functioning jointly in this tissue. However, the differential distribution pattern indicates that the proteins also function independent of one another. Furthermore, the finding of megalin expression in blood island endothelial cells and cubilin expression in embryonic endoderm highlight potential new developmental roles for these proteins.

Different types of glomerulopathic light chains interact with mesangial cells using a common receptor but exhibit different intracellular trafficking patterns

Patients with plasma cell dyscrasias may have circulating light chains (LCs), some of which are nephrotoxic. Nephrotoxic LCs can affect the various renal compartments. Some of these LCs may produce predominantly proximal tubular damage, while others are associated with distal nephron obstruction (the so-called "myeloma kidney"). Both these are considered tubulopathic (T) LCs. A receptor has been found in proximal tubular cells (cubilin/megalin complex), which mediates the absorption of LCs and is involved in the pathogenesis of tubulopathies that occurs in these patients. Another group of nephrotoxic LCs is associated with glomerular damage and these are considered as glomerulopathic (G). These patients with G-LCs may develop AL-amyloidosis (AL-Am) or LC deposition disease (LCDD). Recent evidence indicates that the physicochemical characteristics (amino-acid composition and conformation of the variable region) of a given nephrotoxic LC may be the most significant factor in determining the type and location of renal damage within the nephron. Other factors may also be involved, including yet uncharacterized host factors that may include genetic polymorphism, among others. Interestingly, the amount of LC production by the clone of plasma cells does not correlate directly with the severity of the renal alterations. Understanding the nature of the interactions between G-LCs and mesangial cells (MCs) is crucial to define key steps that may be targeted for therapeutic purposes. Experimental studies have delineated important aspects pertaining to interactions between G-LCs and MCs, indicating that these interactions are receptor mediated. The data presented in the current study support a single receptor present on MCs for both LCDD and AL-LCs, as clearly demonstrated with competition and colocalization immunofluorescence (IF) studies. This receptor resides in caveolae present on the plasma membrane of HMCs and is overexpressed when HMCs are incubated with G-LCs but not TLCs. Caveolae play a fundamental role in receptor-mediated endocytosis, a crucial process in the internalization of AL-LCs and amyloidogenesis. LC internalization is clathrin mediated. The data also indicate that intracellular trafficking in MCs is different for AL-LCs and LCDD-LCs. AL-LCs are delivered to the mature lysosomal compartment where amyloid formation occurs. LCDD-LCs alter mesangial function and phenotype by interacting with the MC surface membranes through similar receptors as the AL-LCs. The data also demonstrated that cubilin and megalin were absent on MCs, so the receptor involved is different from the one already characterized in the proximal tubules.

Rat enterocytes secrete SLPs containing alkaline phosphatase and cubilin in response to corn oil feeding

Surfactant-like particles (SLP) are unilamellar secreted membranes associated with the process of lipid absorption and isolated previously only from the apical surface of enterocytes. In this paper, the intracellular membrane has been isolated from corn oil-fed animals, identified by its content of the marker protein intestinal alkaline phosphatase (IAP). Another brush-border protein, cubilin, and its anchoring protein megalin have been identified as components of extracellular SLP, but only cubilin is present to any extent in intracellular SLP. During fat absorption, IAP is modestly enriched in intracellular SLP, but full-length cubilin (migrating at 210 kDa in fat-fed mucosal fractions) falls by one-half, although fragments of cubilin are abundant in the intracellular SLP. Both IAP and cubilin colocalize to the same cells during corn oil absorption and colocalize around lipid droplets. This localization is more intense during feeding of corn oil with Pluronic L-81, a detergent that allows uptake of fatty acids and monoglycerides from the lumen, but blocks chylomicron secretion. Confocal microscopy confirms the colocalization of IAP and the ligand for cubilin, intrinsic factor. Possible roles for cubilin in intracellular SLP include facilitating movement of the lipid droplet through the cell and binding to the basolateral membrane before reverse endocytosis.

NHE3 serves as a molecular tool for cAMP-mediated regulation of receptor-mediated endocytosis

Receptor-mediated, clathrin-dependent endocytosis (RME) is important for macromolecular transport and regulation of cell-surface protein expression. Pharmacological studies have shown that the plasma membrane transport protein Na(+)/H(+) exchanger 3 (NHE3), which shuttles between the plasma membrane and the early endosomal compartment by means of clathrin-mediated endocytosis, contributes to endosomal pH homeostasis and endocytic fusion events. Furthermore, it is known that NHE3 is phosphorylated and inhibited by cAMP-dependent kinase (protein kinase A). Here, we show, in a cellular knockout/retransfection approach, that NHE3 supports RME and confers cAMP sensitivity to RME, using megalin/cubilin-mediated albumin uptake in opossum kidney cells. RME, but not fluid-phase endocytosis, was dependent on NHE3 activity and expression. Furthermore, NHE3 deficiency or inhibition reduced the relative surface expression of megalin without altering total expression. In wild-type cells, cAMP inhibits NHE3 activity, leads to endosomal alkalinization, and reduces RME. In NHE3-deficient cells, endosomal pH is not sensitive to NHE3 inhibition, and cAMP does not affect endosomal pH or RME. NHE3 transfection into deficient cells restores RME and the effects of cAMP. Thus our data show that NHE3 is important for cAMP sensitivity of clathrin-dependent RME.

Megalin is essential for renal proximal tubule reabsorption and accumulation of transcobalamin-B(12)

Megalin has previously been shown to bind and mediate endocytosis of transcobalamin (TC)-B(12). However, the physiological significance of this has not been established, and other TC-B(12) binding proteins have been suggested to mediate renal uptake of this vitamin complex. The present study demonstrates by the use of megalin-deficient mice that megalin is, in fact, essential for the normal renal reabsorption of TC-vitamin B(12) and for renal accumulation of this highly conserved vitamin. Megalin-deficient mice excrete increased amounts of TC and B(12) in the urine, revealing a defective renal tubular uptake of TC-B(12). The urinary B(12) excretion is increased approximately 4-fold, resulting in an approximately 28-fold higher renal B(12) clearance. This is associated with an approximately 4-fold decrease in B(12) content in megalin-deficient kidney cortex. Thus megalin is important to prevent urinary loss of vitamin B(12). In addition, light- and electron-microscopic immunocytochemistry demonstrate lysosomal accumulation of B(12) in rat and mouse proximal tubules. In rats this accumulation is correlated with vitamin intake. Thus renal lysosomal B(12) accumulation is dependent on vitamin status, indicating a possible reserve function of this organelle in the rat kidney.

Megalin- and cubilin-mediated endocytosis of protein-bound vitamins, lipids, and hormones in polarized epithelia

Polarized epithelia have several functional and morphological similarities, including a high capacity for uptake of various substances present in the fluids facing the apical epithelial surfaces. Studies during the past decade have shown that receptor-mediated endocytosis, rather than nonspecific pinocytosis, accounts for the apical epithelial uptake of many carrier-bound nutrients and hormones. The two interacting receptors of distinct evolutionary origin, megalin and cubilin, are main receptors in this process. Both receptors are apically expressed in polarized epithelia, in which they function as biological affinity matrices for overlapping repertoires of ligands. The ability to bind multiple ligands is accounted for by a high number of replicated low-density lipoprotein receptor type-A repeats in megalin and CUB (complement C1r/C1s, Uegf, and bone morphogenic protein-1) domains in cubilin. Here we summarize and discuss the structural, genetic, and functional aspects of megalin and cubilin, with emphasis on their function as receptors for uptake of protein-associated vitamins, lipids, and hormones.

Cubilin and megalin expression and their interaction in the rat intestine: effect of thyroidectomy

Cubilin is a 460-kDa multipurpose, multidomain receptor that contains an NH(2)-terminal 110-residue segment followed by 8 epidermal growth factor (EGF)-like repeats and a contiguous stretch (representing nearly 88% of its mass) of 27 CUB (initially found in complement components C1r/C1s, Uegf, and bone morphogenic protein-1) domains. Cubilin binds to intrinsic factor (IF)-cobalamin (cbl, vitamin B(12)) complex and promotes the ileal transport of cbl. The 460-kDa form of cubilin is the predominant form present in the apical brush-border membranes of rat intestine, kidney, and yolk sac, but a 230-kDa form of cubilin is also noted in the intestinal membranes. In thyroidectomized (TDX) rats, levels of intestinal brush-border IF-[(57)Co]-labeled cbl binding, 460-kDa cubilin protein levels and tissue (kidney) accumulation of cbl were reduced by approximately 70%. Immunoblot analysis using cubilin antiserum of intestinal total membranes from TDX rats revealed cubilin fragments with molecular masses of 200 and 300 kDa. Both of these bands, along with the 230-kDa band detected in the total membranes of control rats and unlike the 460-kDa form, failed to react with antiserum to EGF. Mucosal membrane cubilin associated with megalin was reduced from approximately 12% in control to approximately 4% in TDX rats, and this decreased association was not due to altered megalin levels. Thyroxine treatment of TDX rats resulted in reversal of all of these effects, including an increase to nearly 24% of cubilin associated with megalin. In vitro, megalin binding to cubilin occurred with the NH(2)-terminal region that contained the EGF-like repeats and CUB domains 1 and 2 but not with a downstream region that contained CUB domains 2-10. These studies indicate that thyroxine deficiency in rats results in decreased uptake and tissue accumulation of cbl caused mainly by destabilization and deficit of cubilin in the intestinal brush border.

Reduced albumin reabsorption in the proximal tubule of early-stage diabetic rats

The aim of this study is to investigate the role of the proximal tubule in microalbuminuria in the early stage of diabetic nephropathy. Diabetes was induced in male Sprague-Dawley rats by an injection of streptozotocin (50 mg/kg, i.v.). After 2 weeks, albumin delivery in the proximal tubule was measured using micropuncture and the endocytosis process of FITC-labeled albumin was evaluated with immunoelectron microscopy. Albumin was significantly reabsorbed in the proximal convoluted tubule (PCT) of controls (0.39+/-0.05 ng/min at early PCT to 0.17+/-0.08 at late PCT, P<0.05), whereas albumin reabsorption was inhibited in diabetic rats (0.27+/-0.05 to 0.21+/-0.08). Immunogold study revealed that FITC-albumin was significantly less reabsorbed in endosomes and lysosomes of S1 segments in diabetic rats than in controls (endosome: 1.20+/-0.10 vs 2.16+/-0.15 microm-1, P<0.0001; lysosome: 0.26+/-0.03 vs 0.83+/-0.07, P<0.0001). The expression of megalin, an endocytosis receptor, was decreased at the apical membrane of PCT in diabetic rats. The lipid peroxidation production in the proximal tubule was significantly increased in diabetic rats. In conclusion, albuminuria in early-stage diabetic rats can be partly explained by a decreased albumin endocytosis with reduced megalin expression and with increased lipid peroxidation in the proximal tubule.