Involvement of beta 2-microglobulin modified with advanced glycation end products in the pathogenesis of hemodialysis-associated amyloidosis. Induction of human monocyte chemotaxis and macrophage secretion of tumor necrosis factor-alpha and interleukin-1

Reduction of advanced glycation end product levels by on-line hemodiafiltration in long-term hemodialysis patients

BACKGROUND

Advanced glycation end products (AGEs) are thought to be involved in many complications of end-stage renal disease. This study analyzed serum AGE level reduction rates and corresponding long-term changes in serum levels among different dialysis modes.

METHODS

Eighty-one patients with chronic uremia were divided into 3 groups receiving conventional hemodialysis (HD), high-flux HD, or on-line hemodiafiltration (HDF). Serum AGE levels were measured by competitive enzyme-linked immunosorbent assay predialysis and postdialysis and after 6 months. Additionally, AGE clearance was measured in 11 uremic patients treated with alternative high-flux HD and on-line HDF.

RESULTS

Although predialysis serum AGE levels were similar, postdialysis levels were significantly lower in patients treated with on-line HDF (35.4 +/- 4.2 microg/mL) compared with those treated with conventional HD (82.2 +/- 11.4 microg/mL; P = 0.003), but not high-flux HD (56.7 +/- 5.9 microg/mL; P = 0.15). The serum AGE level reduction rate in on-line HDF (61.5% +/- 4.2%) was significantly greater than that in conventional HD (20.5% +/- 2.4%; P < 0.001) and high-flux HD patients (40.4% +/- 2.7%; P = 0.049). AGE clearance was increased 50% with on-line HDF compared with high-flux HD, reaching borderline significance (P = 0.07). In a 6-month study, predialysis serum AGE levels were significantly lower in patients treated with on-line HDF compared with those treated with conventional and high-flux HD.

CONCLUSION

On-line HDF may provide an improved form of treatment that achieves significantly better AGE level reduction than high-flux HD and conventional HD. Uremic patients treated with on-line HDF for longer than 6 months achieved a significant reduction in predialysis serum AGE levels.

Plasma pentosidine is associated with inflammation and malnutrition in end-stage renal disease patients starting on dialysis therapy

Pentosidine is an advanced glycation end-product (AGE), formed by glycosylation and oxidation, that accumulates markedly in end-stage renal disease (ESRD). It has been speculated that AGE and carbonyl stress contributes to long-term complications such as cardiovascular disease (CVD) in ESRD patients. This study determined plasma levels of pentosidine as well as the presence of inflammation (CRP > or = 10 mg/L), clinical CVD (CVD(clin)), and malnutrition (subjective global assessment [SGA] > 1) in a cohort of 191 ESRD patients, median age of 55 yr (range, 23 to 70 yr) and median GFR = 7 ml/min (range, 2 to 17 ml/min), close to start of renal replacement therapy. Fifty-one elderly subjects, median age of 82 yr (range, 71 to 110 yr), with mild renal impairment, median GFR = 67 ml/min (range, 38 to 113 ml/min), were also studied for comparative analysis of plasma pentosidine. The plasma pentosidine content was elevated in all patients compared with the levels in the elderly subjects and were negatively correlated with GFR both in the ESRD patients (Rho = -0.24; P < 0.01; n = 159) and in the elderly subjects (Rho = -0.31; P < 0.05). Moreover, the plasma pentosidine content was correlated with age in the ESRD patients (Rho = 0.26; P < 0.001) and in the elderly subjects (Rho = 0.44; P < 0.001). The 63 malnourished ESRD patients (35%) had a significantly higher (P < 0.05) median plasma pentosidine than the well-nourished patients (39 versus 27 pmol/mg albumin). Similarly, 73 inflamed patients (38%) had a significantly higher (P < 0.001) median pentosidine content compared with 118 non-inflamed patients (37 versus 24 pmol/mg albumin). Also, the plasma pentosidine content showed weak but significant positive correlations with CRP (Rho = 0.28; P < 0.0001), fibrinogen (Rho = 0.23; P < 0.01; n = 126), IL-6 (Rho = 0.22; P < 0.01; n = 169), and soluble vascular cellular adhesion molecule-1 (Rho = 0.38; P < 0.001; n = 74). On the other hand, no significant differences in plasma pentosidine content were noted between the patients with and those without CVD(clin) (32 versus 27 pmol/mg albumin, respectively). Analyses of all-cause mortality, by Kaplan-Meier, showed that mortality was not linked to the plasma pentosidine content. Moreover, survival analysis by the Cox regression model showed that age (P < 0.001), diabetes mellitus (P < 0.01), malnutrition (P < 0.01), and CVD(clin) (P < 0.01) independently predicted poor outcome, whereas an elevated plasma pentosidine content did not. The present study shows that an elevated plasma pentosidine content in ESRD patients is significantly associated with both inflammation and malnutrition and confirms that low residual renal function and high age further contribute to an increased plasma pentosidine content. However, in this small cohort, the plasma pentosidine content did not predict outcome. Thus, accumulation of plasma pentosidine is unlikely to be an appropriate clinically useful marker to predict mortality in ESRD patients.

Stimulation of osteoclastic bone resorption in a model of glycerol-induced acute renal failure: evidence for a parathyroid hormone-independent mechanism

This study was undertaken to evaluate the bone changes occurring in rats with acute renal failure (ARF). Acute renal failure was induced in rats 24 hours after dehydration by an intramuscular injection of glycerol. After induction of ARF, the rats were divided into two groups, one of which underwent parathyroidectomy (PTX). Rats with normal renal function, matched for age and weight, were used as controls and divided into two groups, one of them for PTX. At termination of the study blood and urine chemistry and bone histomorphometry were analyzed. Rats with glycerol-induced ARF developed bone changes compatible with mild hyperparathyroid bone disease, characterized mainly by increased osteoclastic bone resorption when compared with control rats having normal renal function. Rats with normal renal function following PTX developed bone disease showing complete suppression of forming and resorptive parameters. Rats with glycerol-induced ARF and PTX showed abolishment of all bone forming parameters, but a dramatic increase in osteoclastic resorption was apparent. Based on these observations we suggest that, in this model of glycerol-induced ARF, osteoclastic bone resorption may develop in the absence of parathyroid hormone, probably stimulated by other potent osteoclastogenic factors.

Low parathyroid hormone and pentosidine in hemodialysis patients

BACKGROUND

Accumulation of advanced glycation end products (AGEs) has been linked to the severity of osteoarticular and cardiovascular damage in patients with end-stage renal disease.

METHODS

We studied the relationship between plasma pentosidine and parathyroid hormone (PTH) levels and bone turnover in a group of hemodialysis patients (n = 85) with minimal aluminum exposure.

RESULTS

Plasma pentosidine levels were greater than the upper limit of normal range (cutoff value, 2.46 pmol/mg protein) in all dialysis patients. When patients were divided into three tertiles according to plasma pentosidine levels, serum PTH levels were approximately six times lower in patients in the third pentosidine tertile than in those in the first tertile (P = 0.008), and a similar association (P = 0.009) was found between pentosidine and bone alkaline phosphatase levels. Multivariate analysis confirmed that these relationships were independent of established risk factors for low bone turnover. Forty patients (47%) had serum PTH levels less than 125 pg/mL (13.2 pmol/L). Of note, in a multiple logistic regression model, the relative risk for low PTH level was 4.02 (95% confidence interval, 1.30 to 12.40; P = 0.02) times greater in patients in the third pentosidine tertile than in the first tertile.

CONCLUSION

Pentosidine, a reliable indicator of AGEs, is related inversely to circulating PTH and bone alkaline phosphatase levels. These associations are in agreement with recent experimental data indicating that AGE accumulation may be a factor involved in low bone turnover in dialysis patients.

Coexistence of beta2 microglobulin and lambda light chain in amyloid fibrils of dialysis-unrelated plasma cell dyscrasia-associated systemic amyloidosis

BACKGROUND

Systemic amyloidosis occurs as a result of amyloid deposition in various tissues. The amyloid fibrils in systemic amyloidosis have been reported to originate from immunoglobulin light chains.

OBJECTIVE

We studied the composition of amyloid fibrils from two patients with plasma cell-associated systemic amyloidosis (PASA).

METHODS

A double immunofluorescence study of the lesional skin of PASA was undertaken. Amyloid proteins were extracted with distilled water from one case of PASA.

RESULTS

The double immunofluorescence study showed that anti-lambda light chain and anti-beta2 microglobulin antibodies mostly reacted with the same area of amyloid deposit. Amyloid deposits from two patients with PASA who had never undergone haemodialysis showed a positive reaction with the antibodies for beta2 microglobulin as well as immunoglobulin lambda light chain. By the use of immunoblot assay of amyloid fibril proteins, polypeptides immunoreactive with antigamma light chain antibody (29 kDa) and with anti-beta2 microglobulin antibody (12 kDa) were detected.

CONCLUSIONS

These results indicate that beta2 microglobulin is a component of amyloid fibrils in PASA.

Association of the MCP-1 gene polymorphism A-2518G with carpal-tunnel syndrome in hemodialysis patients

Carpal-tunnel syndrome (CTS) in long-term hemodialysis patients is caused by the deposition of amyloid as well as by the local inflammatory process. The recruitment of monocytes/macrophages in the tenosynovium, promoted by chemokines such as monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-1alpha (MIP-1alpha), is thought to play an important role in CTS development. The genetic polymorphism of these chemokines has been identified and their clinical function has been partly revealed We attempted to analyze the relationship between these polymorphisms and their susceptibility to CTS. The subjects of this study were 366 patients who underwent hemodialysis. Ninety-five patients received surgery for CTS. No significant difference was observed in the genotype distributions of MCP-1 or MIP-1alpha between patients who received CTS surgery and those that did not. However, with the use of a logistic regression model, the MCP-1 GG genotype was identified as a risk factor for the development of CTS, in addition to the duration and the age of initiation of dialysis, as confirmed by a Cox proportional hazards model. In conclusion, homozygosity for G at -2518 in the MCP-1 gene might be a candidate for the genetic marker of CTS development in Japanese hemodialysis patients.

Characterization of antibody affinities using an AGE-modified dipeptide spot library

Recent immunological approaches have greatly helped understanding the significance of advanced glycation end products (AGEs) in age-related diseases. However, immunohistochemical localization of AGEs in tissues or their quantitative determination in biological fluids sometimes yields inconsistent results among different investigators. Since these differences might be caused by the heterogeneity of the AGE antibodies, a systematic mapping of their epitope recognition pattern would help in the evaluation of these different results. For this purpose, an N-terminally acetylated combinatorial dipeptide library with 400 positionally defined dipeptides was modified by glucose to yield AGEs, which are thought to be present on the protein side chains. Using this library, we have characterized six different AGE antibodies in respect to their immunoreactivity towards AGE-modified dipeptides. All antibodies predominantly recognized only one AGE-modified amino acid side chain (and not a particular dipeptide). In most cases, arginine- and lysine- derived AGEs, but also some epitopes on asparagine and on heterocyclic amino acids were recognized. Very interestingly, the immunization of different animals with the same antigen produced AGE antibodies with a totally different epitope recognition pattern. Defining the antigen recognition pattern with this method might help in the quality control of polyclonal AGE antibodies for immunodetection. In addition, defined AGE antibodies (as neutralizing antibodies) could also help to define "signal-active" AGEs in terms of specific AGE-mediated cellular responses.

Identification of the advanced glycation end products N(epsilon)-carboxymethyllysine in the synovial tissue of patients with rheumatoid arthritis

BACKGROUND

Generation of advanced glycation end products (AGEs) is an inevitable process in vivo and can be accelerated under pathological conditions such as oxidative stress. In serum and synovial fluid of patients with rheumatoid arthritis (RA) raised AGE levels have been found.

OBJECTIVE

To determine the presence of N(epsilon)-carboxymethyllysine (CML; marker of oxidative stress) in RA synovial tissue by immunohistology.

METHODS

Frozen synovial tissue samples from 10 patients with RA and eight controls (four patients without joint disease and four patients with osteoarthritis (OA)) were treated with rabbit-anti-CML-IgG and goat-antirabbit-IgG. Immunostaining was visualised by streptavidine-alkaline phosphatase (chromogen fuchsin). Cell differentiation was performed with antibodies against CD68, CD45RO, and CD20.

RESULTS

CML was detected in the synovial lining, sublining, and endothelium in 10/10 RA and 4/4 OA synovial specimens. In RA some macrophages (CD68+) and T cells (CD45RO+) showed positive immunostaining for CML, whereas B cells were negative. Staining in OA synovial sublining was weak compared with RA.

CONCLUSIONS

CML was detected for the first time in RA and OA synovial tissue. Different patterns of immunostaining in RA and OA and the presence of CML on macrophages and T cells, suggest a role for CML in the pathogenesis of RA. This might be due to presentation of new epitopes which can maintain or even trigger an autoimmune response.

Receptor for advanced glycation end products on human synovial fibroblasts: role in the pathogenesis of dialysis-related amyloidosis

An important component of amyloid fibrils in dialysis-related amyloidosis (DRA) is beta(2)-microglobulin (beta(2)m) modified with advanced glycation end products (AGE). The amyloid deposits are located principally in joint structures, with adjacent chronic inflammatory reaction characterized by monocyte infiltration. This study examined the interaction of AGE-beta(2)m with human synovial fibroblasts and investigated the proinflammatory effects of that interaction. It was demonstrated that human synovial fibroblasts constitutively expressed the receptor for AGE (RAGE). RAGE expression was detected mainly in synovial intima and was upregulated in DRA synovium. (125)I-AGE-beta(2)m bound to immobilized human synovial fibroblasts in a specific, dose-dependent manner (K(d) of approximately 138.0 nM), and binding was inhibited by anti-RAGE IgG. Incubation of human synovial fibroblasts with AGE-beta(2)m induced degradation of this AGE-modified protein, as well as increased monocyte chemoattractant protein-1 (MCP-1) mRNA and protein expression. The amount of MCP-1 produced by AGE-beta(2)m-stimulated human synovial fibroblasts was sufficient to induce the chemotaxis of monocytes. MCP-1 synthesis resulted from engagement of RAGE, because the increase in MCP-1 synthesis was attenuated by preincubation of human synovial fibroblasts with anti-RAGE IgG. These data provide evidence of RAGE-mediated perturbation of human synoviocytes, which may be involved in the pathogenesis of inflammatory processes associated with DRA.

Parathyroid hormone-independent osteoclastic resorptive bone disease: a new variant of adynamic bone disease in haemodialysis patients

BACKGROUND

Osteitis fibrosa cystica (OFC) caused by secondary hyperparathyroidism is the pre-eminent form of uraemic osteodystrophy. In recent years, however, new bone abnormalities have been described. Among them adynamic bone disease (ABD) has become a focus of growing interest. Marked suppression of dynamic bone measurements with normal or near-normal static bone-forming parameters are the hallmarks of this disorder. Depressed parathyroid hormone (PTH) levels, frequently evident in this entity, have been linked causally with low bone turnover.

METHODS

We reviewed bone biopsy specimens from 96 patients with end-stage renal disease undergoing chronic haemodialysis.

RESULTS

We found OFC in 50% of our patients, 20% had mixed bone disease, 24% showed bone morphology of ABD and a minority (6%) had osteomalacia, mostly due to aluminium accumulation. In the patients that were affected by ABD there was a distinct subgroup with bone morphology featuring a striking increase in osteoclast number and osteoclast surface, whereas the osteoid volume, osteoid thickness, osteoblast surface, tetracycline uptake and bone formation rates were diminished as in ordinary ABD. Similarly the PTH levels in this subgroup were low or undetectable.

CONCLUSION

We describe patients undergoing chronic haemodialysis with static and dynamic bone forming parameters, indistinguishable from that of ABD, but differing from the classic ABD by the presence of increased osteoclastic bone resorption. The suppressed PTH levels in this subgroup suggests that factors other than PTH activate osteoclasts in some patients on chronic haemodialysis. Uraemic cytokines and/or toxic metabolites, including beta-microglobulin, may be involved in this disorder. The precise nature of this bone abnormality remains to be defined by further studies.

Diabetes and advanced glycation endproducts

Bio-reactive advanced glycation endproducts (AGE) alter the structure and function of molecules in biological systems and increase oxidative stress. These adverse effects of both exogenous and endogenously derived AGE have been implicated in the pathogenesis of diabetic complications and changes associated with ageing including atherosclerosis, renal, eye and neurological disease. Specific AGE receptors and nonreceptor mechanisms contribute to these processes but also assist in the removal and degradation of AGE. The final disposal of AGE depends on renal clearance. Promising pharmacologic strategies to prevent AGE formation, reduce AGE toxicity, and/or inactivate AGE are under investigation.

Toward better dialysis compatibility: advances in the biochemistry and pathophysiology of the peritoneal membranes

Peritoneal dialysis (PD) has modified our concept of the peritoneal membrane, which is now a topic of active research. Peritoneal solute transport progressively increases with time on PD, enhances the dissipation of the osmotic gradient and, eventually, reduces ultrafiltration capacity. The causes of peritoneal membrane failure remain elusive. Recurrent episodes of peritonitis are not a prerequisite for the development of ultrafiltration failure. Functionally, the changes of the failing peritoneal membrane are best described as an increased functional area of exchange for small solutes between blood and dialysate. Histologically, these events are associated with vascular proliferation and structural changes of pre-existing vessels. Gathered evidence, including information on the composition of peritoneal cavity fluids and its dependence on the uremic environment, have cast a new light on the molecular mechanisms of decline in peritoneal membrane function. Chronic uremia per se modifies the peritoneal membrane and increases the functional area of exchange for small solutes. Biochemical alterations in the peritoneum inherent to uremia might be, at least in part, accounted for by severe reactive carbonyl compounds overload originating both from uremic circulation and PD fluid ("peritoneal carbonyl stress"). The molecular events associated with long-term PD are similar but more severe than those present in chronic uremia without PD, including modifications of nitric oxide synthase (NOS) and angiogenic growth factors expression, and advanced glycation and lipoxidation of the peritoneal proteins. This review focuses on reactive carbonyls and their association with a number of molecular changes observed in peritoneal tissues. This hypothetical approach will require further testing. Nevertheless, the insights gained on the peritoneal membrane offer a new paradigm to assess the effect of uremic toxins on serosal membranes. Furthermore, the progresses made in the dissection of the molecular events leading to peritoneal membrane failure open new avenues to develop safe, more biocompatible peritoneal dialysis technologies.

Hemodialysis Membranes: Interleukins, Biocompatibility, and Middle Molecules

ABSTRACT. Maintenance hemodialysis patients display evidence of elevated interleukin-1 (IL-1) and tumor necrosis factor alpha release after stimulation either by contaminated dialysate, bioincompatible membrane material, or both. This release is followed by the stimulated secretion of a large number of other interleukins, particularly IL-6, the cytokine principally responsible for acute-phase protein synthesis. It has been shown that high levels of the circulating proinflammatory cytokines IL-1, tumor necrosis factor alpha, IL-6, and IL-13 are associated with mortality in hemodialysis patients. Essential functions of polymorphonuclear leukocytes—that is, phagocytosis, oxygen species production, upregulation of specific cell surface receptor proteins, or apoptosis—are disturbed in patients with end-stage renal disease. These are further altered as a result of complement activation by the hemodialysis procedure, particularly if bioincompatible dialyzers are used. Polymorphonuclear leukocyte degranulation occurring during extracorporeal circulation does not depend on complement activation but rather on intracellular calcium and the presence or absence of the degranulation inhibitory proteins angiogenin and complement factor D. Clinical signs and symptoms of end-stage renal disease patients are at least in part related to the accumulation of middle molecules such as β2-microglobulin, parathyroid hormone, advanced glycation end products, advanced lipoxidation end products, advanced oxidation protein products (formed as a result of oxidative stress, carbonyl stress, or both), granulocyte inhibitory proteins, or leptin. Currently available membrane materials do not provide long-lasting, effective reduction of middle molecules in patients who require maintenance hemodialysis.

β2-Microglobulin Amyloidosis: Effects of Ultrapure Dialysate and Type of Dialyzer Membrane

ABSTRACT. The available data on the pathophysiology of β2-microglobulin amyloidosis (β2mA) suggest that this progressive disease associated with end-stage renal failure develops in several consecutive phases. First, declining kidney function leads to retention of β2 microglobulin (β2m) and its deposition preferentially in the synovial tissue of bigger joints such as wrists, shoulders, and hips. Second, at the site of deposition, formation of unique amyloid fibrils, whose major component is β2m, takes place. Deposition and fibril formation occur in the absence of modification of β2mA by advanced glycoxidation end products and also in the absence of a local inflammatory response. It is later, in the third phase, that advanced glycoxidation end product modification of β2m induces a local inflammatory response by attracting macrophages chemotactically and by stimulating these cells to produce and release proinflammatory cytokines. In addition, unmodified β2m itself induces inflammatory activities such as upregulation of cyclooxygenase-2 and metalloproteinase-1. The severity of the local inflammation seems to determine the degree of the destructive processes in tissue and bone accompanying β2mA.

Anti-inflammatory antioxidants attenuate the expression of inducible nitric oxide synthase mediated by advanced glycation endproducts in murine microglia

Advanced glycation endproducts (AGEs) accumulate on long-lived protein deposits including beta-amyloid plaques in Alzheimer's disease (AD). AGE-modified amyloid deposits contain oxidized and nitrated proteins as markers of a chronic neuroinflammatory condition and are surrounded by activated microglial and astroglial cells. We show in this study that AGEs increase nitric oxide production by induction of the inducible nitric oxide synthase (iNOS) on the mRNA and protein level in the murine microglial cell line N-11. Membrane permeable antioxidants including oestrogen derivatives (e.g. 17beta-oestradiol) thiol antioxidants (e.g. (R+)-alpha-lipoic acid) and Gingko biloba extract EGb 761, but not phosphodiesterase inhibitors such as propentophylline, prevent the up-regulation of AGE-induced iNOS expression and NO production. These results indicate that oxygen free radicals serve as second messengers in AGE-induced pro-inflammatory signal transduction pathways. As this pharmacological mechanism is not only relevant for Alzheimer's disease, but also for many chronic inflammatory conditions, such membrane-permeable antioxidants could be regarded not only as antioxidant, but also as potent therapeutic anti-inflammatory drugs.

Diabetes-associated sustained activation of the transcription factor nuclear factor-kappaB

Activation of the transcription factor nuclear factor-kappaB (NF-kappaB) has been suggested to participate in chronic disorders, such as diabetes and its complications. In contrast to the short and transient activation of NF-kappaB in vitro, we observed a long-lasting sustained activation of NF-kappaB in the absence of decreased IkappaBalpha in mononuclear cells from patients with type 1 diabetes. This was associated with increased transcription of NF-kappaBp65. A comparable increase in NF-kappaBp65 antigen and mRNA was also observed in vascular endothelial cells of diabetic rats. As a mechanism, we propose that binding of ligands such as advanced glycosylation end products (AGEs), members of the S100 family, or amyloid-beta peptide (Abeta) to the transmembrane receptor for AGE (RAGE) results in protein synthesis-dependent sustained activation of NF-kappaB both in vitro and in vivo. Infusion of AGE-albumin into mice bearing a beta-globin reporter transgene under control of NF-kappaB also resulted in prolonged expression of the reporter transgene. In vitro studies showed that RAGE-expressing cells induced sustained translocation of NF-kappaB (p50/p65) from the cytoplasm into the nucleus for >1 week. Sustained NF-kappaB activation by ligands of RAGE was mediated by initial degradation of IkappaB proteins followed by new synthesis of NF-kappaBp65 mRNA and protein in the presence of newly synthesized IkappaBalpha and IkappaBbeta. These data demonstrate that ligands of RAGE can induce sustained activation of NF-kappaB as a result of increased levels of de novo synthesized NF-kappaBp65 overriding endogenous negative feedback mechanisms and thus might contribute to the persistent NF-kappaB activation observed in hyperglycemia and possibly other chronic diseases.

Advanced glycation endproducts co-localize with inducible nitric oxide synthase in Alzheimer's disease

Advanced glycation endproducts (AGEs), protein-bound oxidation products of sugars, have been shown to be involved in the pathophysiological processes of Alzheimer's disease (AD). AGEs induce the expression of various pro-inflammatory cytokines and the inducible nitric oxide synthase (iNOS) leading to a state of oxidative stress. AGE modification and resulting crosslinking of protein deposits such as amyloid plaques may contribute to the oxidative stress occurring in AD. The aim of this study was to immunohistochemically compare the localization of AGEs and beta-amyloid (Abeta) with iNOS in the temporal cortex (Area 22) of normal and AD brains. In aged normal individuals as well as early stage AD brains (i.e. no pathological findings in isocortical areas), a few astrocytes showed co-localization of AGE and iNOS in the upper neuronal layers, compared with no astrocytes detected in young controls. In late AD brains, there was a much denser accumulation of astrocytes co-localized with AGE and iNOS in the deeper and particularly upper neuronal layers. Also, numerous neurons with diffuse AGE but not iNOS reactivity and some AGE and iNOS-positive microglia were demonstrated, compared with only a few AGE-reactive neurons and no microglia in controls. Finally, astrocytes co-localized with AGE and iNOS as well as AGE and were found surrounding mature but not diffuse amyloid plaques in the AD brain. Our results show that AGE-positive astrocytes and microglia in the AD brain express iNOS and support the evidence of an AGE-induced oxidative stress occurring in the vicinity of the characteristic lesions of AD. Hence activation of microglia and astrocytes by AGEs with subsequent oxidative stress and cytokine release may be an important progression factor in AD.

The AGE-receptor in the pathogenesis of diabetic complications

Native glucose-derived glycation derivatives (advanced glycation end products, AGE) in vascular, renal and neuronal tissues contribute to organ damage. Glycation derivatives include a number of chemically and cell-reactive substances, also termed glycoxidation products or glycotoxins (GT). Cell-associated AGE-specific receptors (AGE-Rs), AGE-R1-3, RAGE, as well as the scavenger receptors ScR-II and CD-36 that are present on vascular, renal, hemopoietic, and neuronal/glial cells, serve in the regulation of AGE uptake and removal. AGE-Rs also modulate cell activation, growth-related mediators, and cell proliferation, consequently influencing organ structure/function. This occurs via oxidant stress triggered via receptor-dependent or -independent pathways, and leads to signal activation pathways, resulting in pro-inflammatory responses. In susceptible individuals, the AGE-R expression/function may be subject to environmental or gene-related modulation, which in turn may influence tissue-specific gene functions. In this context, altered expression and activity of AGE-R components has recently been found in both mouse diabetes models and humans with diabetic complications. Although several gene polymorphisms are detected in most AGE-R components, no significant correlation to diabetic complications has as yet been found. Further investigation is underway to define whether primary or secondary genetic links of pathogenic significance exist in this system. Various AGE-binding peptides or soluble receptors have emerged as potential sequestering agents for toxic AGEs as potential therapies for diabetic complications.

Role of advanced glycation end products in adynamic bone disease in patients with diabetic nephropathy

Adynamic bone disease and elevated serum levels of advanced glycation end products (AGEs) often are found in patients with renal failure caused by diabetic nephropathy. To clarify the role of AGEs in adynamic bone disease, we investigated the effect of these substances on cultured human osteoblasts and parathyroid cells. After 72 hours of incubation with AGEs-bovine serum albumin (BSA) (1,000 microgram/mL), there was significant inhibition of the synthesis of type I collagen and osteocalcin in response to stimulation with 10(-10) to 10(-8) M of 1,25-dihydroxycholecalciferol. In a human osteoblastic cell line (MG 63), AGEs-BSA did not affect human osteocalcin promoter activity. In human parathyroid cells, a receptor for AGEs was detected by reverse-transcriptase polymerase chain reaction. Incubation with AGEs-BSA for 48 hours significantly inhibited parathyroid hormone secretion in response to a low calcium concentration of 0.81 mM (P < 0.01). In HEK-293 cells, expressing calcium-sensing receptors, the same AGE concentration caused a significant potentiation of the extracellular Ca(2+) induced-intracellular calcium concentration after 24 and 48 hours of incubation (P < 0.05 and P < 0.01). These data suggest that AGEs are involved in the pathogenesis of adynamic bone disease by inhibiting osteoblastic activity and by inhibiting parathyroid hormone secretion in response to hypocalcemia.

Filtration of native and glycated beta2-microglobulin by charged and neutral dialysis membranes

BACKGROUND

It has been postulated that protein glycation and formation of advanced glycation end products (AGE) are among toxic factors in chronic uremia, whether the renal disease is of diabetic or nondiabetic origin. In this setting, AGE-modified beta2-microglobulin (beta2m) may favor dialysis beta2m-related dialysis amyloidosis. Consequently, efficient removal of modified beta2m by highly permeable dialysis membranes is as important as removal of native beta2m to postpone the development of dialysis amyloidosis.

METHODS

To define the role of dialysis membrane surface electronegativity on plasma protein transfer, an in vitro model was used to test the interactions of native and glycated beta2m with various highly permeable dialysis membranes. An experimental circuit with minidialyzers was used. The neutral high-flux polysulfone membrane (PS), the electronegative polymethylmetacrylate membrane (PMMA), the electronegative AN69 membrane and a modified AN69 membrane, the surface of which was neutralized with polyethyleneimine (AN69-PEI), were tested using both native beta2m and the more acidic glycated beta2m. Protein mass transfer and binding to the membrane were measured.

RESULTS

Mass transfer of glycated beta2m was significantly decreased through all membranes tested when compared with native beta2m. This result was due to the increased molecular weight of beta2m, which became less permeable to porous membranes, whereas adsorption of both native and glycated beta2m to membranes, due to ionic interactions, decreased similarly with AN69 and AN69-PEI, but remained unchanged with PS and PMMA. Moreover, surface neutralization of AN69 membrane did not alter its core binding capacity, since beta2m absorption accounted for 98 and 97% and glycated beta2m for 83.7 and 81.4% of the protein removed with AN69 and AN69-PEI, respectively.

CONCLUSION

Clearance of glycated beta2m through highly permeable neutral and negatively charged membranes was lower than that of native beta2m, reflecting a decreased sieving coefficient for the neoformed higher molecular weight and conformationally altered molecule. The binding capacity of the neutral PS was roughly half that of the charged membranes. Neutralizing surface electronegativity of the AN69 membrane with PEI did not alter its binding capacity. These results suggest that it would be useful for dialysis protocols to include comparative studies of both serum native and modified beta2m in order to prevent beta2m-amyloidosis.

Accumulation of advanced glycation end products decreases collagen turnover by bovine chondrocytes

The integrity of the collagen network is essential for articular cartilage to fulfill its function in load support and distribution. Damage to the collagen network is one of the first characteristics of osteoarthritis. Since extensive collagen damage is considered irreversible, it is crucial that chondrocytes maintain a functional collagen network. We investigated the effects of advanced glycation end products (AGEs) on the turnover of collagen by articular cartilage chondrocytes. Increased AGE levels (by culturing in the presence of ribose) resulted in decreased collagen synthesis (P < 0.05) and decreased MMP-mediated collagen degradation (P < 0.02). The latter could be attributed to increased resistance of the collagen network to MMPs (P < 0.05) as well as the decreased production of MMPs by chondrocytes (P < 0.02). Turnover of a protein is determined by its synthesis and degradation rates and therefore these data indicate that collagen turnover is decreased at enhanced AGE levels. Since AGE levels in human cartilage increase approximately 50 fold between age 20 and 80, cartilage collagen turnover likely decreases with increasing age. Impaired collagen turnover adversely affects the capacity of chondrocytes to remodel and/or repair its extracellular matrix. Consequently, age-related accumulation of AGE (via decreased collagen turnover) may contribute to the development of cartilage damage in osteoarthritis.

Surgical therapy for dialysis-related spondyloarthropathy: review of 30 cases

Surgical therapy for dialysis-related spondyloarthropathy was investigated regarding its spinal manifestation. Between August 1985 and May 1998, 31 operations were performed on 16 male and 14 female patients; of these, 17 had cervical and 13 had lumbar spinal disorders. The average patient age was 59 years. The average period of hemodialysis was 14.8 years. Twenty-eight of 30 patients had cystic bone lesions and 24 had carpal tunnel syndrome. Four major postoperative complications occurred: death from paralysis and respiratory distress, severe kyphosis from the collapse of the grafted bone, deep infection from instrumentation, and wire breakage and bone fusion failure. Postoperative results with an average follow-up period of 2.7 years were good in 19 cases (63%), fair in 8 cases (27%), and poor in 3 cases (10%). As yet, surgical intervention for dialysis-related spondyloarthropathy is still regarded as a noncurative treatment; furthermore, the anterior approach to the cervical spine has a high risk for postoperative complications.

Pathogenesis of beta(2)-microglobulin amyloidosis: role of monocytes/macrophages

beta(2)-microglobulin (beta(2)M) amyloidosis (A beta(2)M) is a serious, often incapacitating complication for patients undergoing long-term hemodialysis. Amyloid deposits composed of beta(2)M fibrils as the major constituent protein are mainly localized in joints and periarticular bone and lead to chronic arthralgias, carpal tunnel syndrome, and eventually destructive arthropathy. Although recent histologic studies have shown the accumulation of monocytes/macrophages around amyloid deposits, the factor(s) causing their infiltration and pathologic involvement have yet to be fully elucidated. Immunohistochemical staining reveals that macrophages in tenosynovial tissues express CD13, CD14, CD33, HLA-DR, and CD68 antigens on their surfaces and express interleukin (IL)-1 beta, tumor necrosis factor (TNF)-alpha, and IL-6. Many of these cells also express LFA-1 (CD11a/CD18), Mac-1 (CD11b/CD18), and VLA-4 (CD49d/CD29) on their surfaces. AGE-modified beta(2)M enhances chemotaxis of monocytes and stimulates macrophages to release bone-resorbing cytokines, such as IL-1 beta, TNF-alpha and IL-6. Via a RAGE-mediated pathway, AGE-modified, but not unmodified beta(2)M, significantly delays constitutive apoptosis of human peripheral blood monocytes. Monocytes survival in an advanced glycation end product (AGE) beta(2)M-containing microenvironment is associated with their phenotypic alteration into macrophage-like cells that generate more reactive oxygen species and elaborate greater quantities of IL-1 beta and TNF-alpha. Thus through regulation of their survival and differentiation, AGE beta(2)M in amyloid deposits may be able to influence the presence and quantity of infiltrated monocytes, and hence their biologic effects.

beta(2)-Microglobulin modified with advanced glycation end products delays monocyte apoptosis

BACKGROUND

A local inflammatory reaction to beta(2)-microglobulin (beta(2)m) amyloid deposits by monocytes/macrophages is a characteristic histologic feature of dialysis-related amyloidosis (DRA). Since beta(2)m modified with advanced glycation end products (AGE-beta(2)m) is a major constituent of amyloid in DRA, we tested the hypothesis that AGE-beta(2)m affects apoptosis and phenotype of human monocytes.

METHODS

Human peripheral blood monocytes were incubated with or without in vitro-derived AGE-beta(2)m, and their viability, extent of apoptosis, morphology, and function examined over the subsequent four days.

RESULTS

AGE-modified but not unmodified beta(2)m significantly delayed spontaneous apoptosis of human peripheral blood monocytes in adherent and nonadherent cultures. The effect of AGE-beta(2)m on monocytes apoptosis was time- and dose-dependent and was attenuated by a blocking antibody directed against the human AGE receptor (RAGE). There was no difference in effect between AGE-beta(2)m and that of AGE-modified human serum albumin. Culture of monocytes with AGE-beta(2)m did not alter membrane expression of Fas or Fas ligand. Monocytes cultured with AGE-beta(2)m underwent substantial changes in morphology similar to those observed when monocytes differentiate into macrophages. The cultured cells increased in size and vacuolization, and their content of beta-glucuronidase and acid phosphatase increased by 5- to 10-fold at day 4. Expression of the monocyte--macrophage membrane antigens HLA-DR, CD11b, and CD11c also increased at day 4. Although exhibiting phenotypic characteristics of macrophages, monocytes cultured with AGE-beta(2)m functioned differently than macrophages cultured with serum. Superoxide production in response to phorbol myristic acetate was maintained in monocytes cultured with AGE-beta(2)m, but declined with time in cells cultured with serum. Constitutive synthesis of tumor necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1 beta) and prostaglandin E2 (PGE2) increased in monocytes cultured for four to six days with AGE-beta(2)m.

CONCLUSIONS

These findings support a novel role for AGE-modified proteins such as AGE-beta(2)m that may contribute to the development of a local inflammatory response, with predominant accumulation of monocytes/macrophages, in DRA.

Dialysis-related amyloidosis: pathogenesis focusing on AGE modification

Dialysis-related amyloidosis (DRA) is a serious complication in long-term dialysis patients, and presents with carpal tunnel syndrome, cystic bone lesions, destructive spondylarthropathy, diffuse arthritis and periarthritis, systemic organ involvement, and dialysis-related spinal canal stenosis (DSCS). Recently a new concept of DSCS has been proposed that includes both destructive spondylarthropathy and myeloradiculopathy induced by extradural thickness. beta(2)-microglobulin (beta(2)M) amyloid was demonstrated to be modified with advanced glycation end products (AGEs) such as imidazolone, N(epsilon)-(carboxymethyl)lysine (CML), and pentosidine. Imidazolone is a reaction product of arginine residue in proteins with 3-deoxyglucosone (3-DG), which is markedly accumulated in uremic serum. Imidazolone is generated under nonoxidative conditions, while CML and pentosidine are formed by oxidative processes. Immunoelectron microscopy demonstrated that AGEs were localized not only in dialysis amyloid but also in nonamyloid collagenous structures, supporting the hypothesis that AGE modification of collagen might have pathogenic relevance in the deposition of beta(2)M on collagen. Serum levels of AGEs are increased in uremic patients. The dimeric form of beta(2)M in the dialysate and urine of uremic patients is more susceptible to imidazolone modification as observed in dialysis amyloid. However, the major component of dialysis amyloid is a native form of beta(2)M, while AGE-modified beta(2)M and truncated beta(2)M are the minor components. Thus I propose that 3-DG and the other dicarbonyl compounds accumulating in uremic serum promote the modification of beta(2)M with AGEs mainly after deposition of beta(2)M as amyloid. For the prevention and treatment of DRA, beta(2)M should be efficiently eliminated from circulating blood by kidney transplantation, hemodialysis, or hemodiafiltration using high-flux membranes and an adsorbent (Lixelle) column.

The pathogenesis of beta(2)-microglobulin-induced bone lesions in dialysis-related amyloidosis

Dialysis-related amyloidosis (DRA), also referred to as beta(2)-microglobulin amyloidosis (A beta(2)M), is an important cause of morbidity in patients with chronic renal failure and in those who are on dialysis. Although DRA deposits from affected joints have been characterized as a unique amyloid fibril protein, beta(2)M, less is known about the pathologic role of beta(2)M as a mediator of bone and joint disease. Potential mechanisms for beta(2)M pathologic interaction in bone include bone growth factors, cytokines, and advanced glycation end products (AGEs). It appears that DRA is the result of a complex interaction between bone resorption and surrounding tissue destruction culminating in beta(2)M deposition and amyloid formation. More work is required to elucidate the relationship between beta(2)M accumulation and progressive tissue destruction.

Dynamic of beta(2)-microglobulin fibril formation and reabsorption: the role of proteolysis

Dialysis-related amyloidosis (DRA) is caused by the deposition, in target tissues, of beta(2)-microglobulin (beta(2)M) in fibrillar conformation. Several reports indicate that fibrillar beta(2)M is chemically heterogeneous and such heterogeneity is partially related to the presence of truncated species of the protein. In association with the full-length species, a beta(2)M isoform lacking six N-terminal residues is present in all the samples of our collection of ex vivo fibrils. The pattern of proteolytic cleavage in amyloidosis and in other diseases is completely different, as demonstrated by the absence in fibrillar beta(2)M of the cleavage at lysine 58, which is contrary to that described in rheumatoid arthritis and other diseases. The role of limited proteolysis of beta(2)M in the pathogenesis of the disease is uncertain. However, we have shown that the apparently minor modification of the intact protein, such as the removal of N-terminal hexapeptide, is capable of dramatically affecting its stability, protection from proteolytic digestion, and enhance its capacity to make in vitro amyloid fibrils. The structure, folding dynamic, and function of the truncated species of beta(2)M, peculiar of DRA, could shed new light on the mechanism of beta(2)M fibril formation and reabsorption.

Caloric restriction mimetics: metabolic interventions

Caloric restriction (CR) retards diseases and aging in laboratory rodents and is now being tested in nonhuman primates. One way to apply these findings to human health is to identify and test agents that may mimic critical actions of CR. Panel 2 focused on two outcomes of CR, reduction of oxidative stress and improved glucoregulation, for which candidate metabolic mimics exist. It was recommended that studies on oxidative stress should emphasize mitochondrial function and to test the efficacy of nitrone and other antioxidants in mimicking CR's effects. Studies should also focus on the long-term effects of compounds known to lower circulating glucose and insulin concentrations or to increase insulin sensitivity. Also, four other developing areas were identified: intermediary metabolism, response to infection, stress responses, and source of dietary fat. These areas are important because either they hold promise for the discovery of new mimetics or they need to be explored prior to initiation of CR trials in humans. Other recommendations were that transgenic approaches and adult-onset CR should be emphasized in future studies.

The role of galectin-3 in endocytosis of advanced glycation end products and modified low density lipoproteins

Galectin-3, a member of beta-galactoside-binding lectin family, is suggested to be an AGE-receptor. To examine this possibility, we prepared CHO cells overexpressing human galectin-3 (galectin-3-CHO cells). Galectin-3-CHO cells showed a specific and saturable binding to (125)I-AGE-BSA with Kd of 3.1 microg/ml. (125)I-AGE-BSA was endocytosed by galectin-3-CHO cells and underwent lysosomal degradation. The endocytosis of (125)I-AGE-BSA was inhibited not only by unlabeled AGE-BSA but also by acetylated LDL and oxidized LDL, ligands for the scavenger receptor family. Furthermore, (125)I-oxidized LDL and (125)I-acetylated LDL were actively endocytosed by galectin-3-CHO cells and the incubation with acetyl-LDL led to intracellular accumulation of cholesteryl esters, indicating the role of galectin-3 in endocytosis of AGE-proteins and modified LDLs. Since galectin-3 was localized and up-regulated in foam cells at human atherosclerotic lesions, the present results suggest that galectin-3 plays an important role in formation of atherosclerotic lesions in vivo, by modulating endocytic uptake of AGE-proteins and modified LDLs.

Cd36, a member of the class b scavenger receptor family, as a receptor for advanced glycation end products

Interaction of advanced glycation end products (AGE) with AGE receptors induces several cellular phenomena potentially relating to diabetic complications. Five AGE receptors identified so far are RAGE (receptor for AGE), galectin-3, 80K-H, OST-48, and SRA (macrophage scavenger receptor class A types I and II). Since SRA is known to belong to the class A scavenger receptor family, and the scavenger receptor collectively represents a family of multiligand lipoprotein receptors, it is possible that CD36, although belonging to the class B scavenger receptor family, can recognize AGE proteins as ligands. This was tested at the cellular level in this study using Chinese hamster ovary (CHO) cells overexpressing human CD36 (CD36-CHO cells). Cellular expression of CD36 was confirmed by immunoblotting and immunofluorescent microscopy using anti-CD36 antibody. Upon incubation at 37 degrees C, (125)I-AGE-bovine serum albumin (AGE-BSA) and (125)I-oxidized low density lipoprotein (LDL), an authentic ligand for CD36, were endocytosed in a dose-dependent fashion and underwent lysosomal degradation by CD36-CHO cells, but not wild-type CHO cells. In binding experiments at 4 degrees C, (125)I-AGE-BSA exhibited specific and saturable binding to CD36-CHO cells (K(d) = 5.6 microg/ml). The endocytic uptake of (125)I-AGE-BSA by these cells was inhibited by 50% by oxidized LDL and by 60% by FA6-152, an anti-CD36 antibody inhibiting cellular binding of oxidized LDL. Our results indicate that CD36 expressed by these cells mediates the endocytic uptake and subsequent intracellular degradation of AGE proteins. Since CD36 is one of the major oxidized LDL receptors and is up-regulated in macrophage- and smooth muscle cell-derived foam cells in human atherosclerotic lesions, these results suggest that, like oxidized LDL, AGE proteins generated in situ are recognized by CD36, which might contribute to the pathogenesis of diabetic macrovascular complications.

Effects of dialyzer membrane on interleukin-1beta (IL-1beta) and IL-1beta-converting enzyme in mononuclear cells

BACKGROUND

In vitro stimulation of mononuclear cells (peripheral blood mononuclear cells; PBMCs) with an endotoxin (lipopolysaccharide; LPS) revealed elevated cell-associated levels of interleukin-1beta (IL-1beta) in end-stage renal disease (ESRD) patients on Cuprophan hemodialysis (HD), suggesting a defect in the process of IL-1beta's release from activated cells. IL-1beta is initially synthesized as an inactive precursor called proIL-1beta. ProIL-1beta is processed into the biologically active mature form of IL-1beta (mIL-1beta) requiring the specific IL-1beta-converting enzyme (ICE).

METHODS

Using specific immunoassays (enzyme-linked immunosorbent assays), we measured cell-associated and extracellular proIL-1beta as well as mIL-1beta in LPS-stimulated PBMCs to test whether ICE-dependent processing of proIL-1beta and/or secretion of mIL-1beta was impaired in ESRD patients compared with healthy controls. PBMCs of healthy controls (N = 9), of ESRD patients on peritoneal dialysis (PD, N = 10), and of those patients on intermittent HD (N = 8) were studied. The same HD patients were studied three times with low-flux Cuprophan (GFS 12), low-flux polysulfon (F6 HPS), and high-flux polysulfon (F60S) in randomized order. PBMCs were separated from whole blood by Ficoll-Hypaque centrifugation and incubated in vitro for 18 hours in the presence LPS (10 ng/mL). Extracellular (PBMC culture supernatants) and cell-associated (cell lysates) levels of proIL-1beta and mIL-1beta were measured.

RESULTS

The total production (cell-associated plus extracellular) of LPS-induced IL-1beta (proIL-1beta plus mIL-1beta) was similar in healthy controls (25.96 +/- 0.84 ng/2.5 x 10(6) PBMC), PD patients (29.53 +/- 1.31 ng/2.5 x 106 PBMC), and in Cuprophan-treated HD patients (23.28 +/- 1.24 ng/2.5 x 10(6) PBMC). In normal controls, 43.6% of the total IL-1beta was processed into mIL-1beta, which was significantly more than that in PD patients (27.3%, P < 0.02) but was similar to that in Cuprophan-treated HD patients (37.1%). Comparing cell-associated and extracellular concentrations of mIL-1beta, PBMCs of normal controls secreted 82.2% of mIL-1beta; this was significantly more than that in PD patients (59.4%, P < 0.01) and that in Cuprophan HD patients (54.2%, P < 0.01). When HD patients were switched from Cuprophan to F6 HPS or F60S, neither total IL-1beta production nor processing of IL-1beta changed. However, secretion of mIL-1beta increased significantly with F6 HPS (80.6%, P < 0.01) as well as with F60S (76.6%, P < 0.02) compared with Cuprophan.

CONCLUSION

We conclude that the ability of PBMCs to produce IL-1beta in response to LPS is normal in PD patients as well as in HD patients. ICE-dependent processing of inactive proIL-1beta into biologically active mIL-1beta is reduced in PD patients, but not in HD patients. Secretion of mIL-1beta is impaired in PD and HD patients treated with Cuprophan. This impaired ability to secrete active mIL-1beta seems to be independent of ICE activity and is normalized when HD-patients are switched from Cuprophan to low- or high-flux polysulfon. Increased cell-associated levels of biologically active mIL-1beta in circulating PBMCs represent a state of inflammation that may contribute to chronic inflammatory diseases such as beta2-microglobulin amyloidosis. Replacement of Cuprophan by synthetic membranes normalizes PBMC function and reduces the state of inflammation in ESRD patients.

Inflammation and advanced glycation end products in uremia: simple coexistence, potentiation or causal relationship?

The causes for the high frequency of cardiovascular disease in dialysis patients are multifactorial in origin. Disturbances in the carbohydrate and lipid metabolism, the balance between oxidants and antioxidants and the immuno-inflammatory system are thought to play a role. Chronic uremia is characterized by the accumulation of advanced glycation end products (AGEs) and advanced oxidation products (AOPP) as well as activation of the acute phase response. High serum levels of these products and acute phase reactants such as C-reactive protein (CRP), fibrinogen and serum amyloid A can be found. CRP has been shown to predict cardiovascular and overall mortality in hemodialysis patients. Whether CRP is involved causally in atherosclerosis or merely represents a marker of disease is as yet unknown. Since CRP has been detected in colocalization with modified apolipoproteins or complement components in atherosclerotic lesions, a pathophysiological role seems very likely. AGEs as well have been detected in aortas of hemodialysis patients. Incubation of endothelial cells with AGEs induced expression of adhesion molecules with consecutive attraction of monocytes to the vessel wall. Thus far, clinical studies investigating the predictive effects of AGEs on cardiovascular mortality in hemodialysis patients are lacking. There is considerable debate about what factors turn on the acute phase response in this population. Proinflammatory effects of AGEs mediated through one receptor for AGEs, RAGE, have been described. We hypothesize that there may be a link between increased hepatic CRP production and the accumulation of AGEs in uremia. AGEs may stimulate CRP production in hepatocytes either directly or indirectly via interaction with monocytes.

Reactive carbonyl compounds related uremic toxicity ("carbonyl stress")

Many studies on uremic toxins have focused on enzymatic biochemistry. Recently, attention has turned to nonenzymatic biochemistry, especially progressive and irreversible modifications of proteins. Two different approaches opened the field of irreversible nonenzymatic modifications of proteins in uremia: the advanced glycation end products (AGEs) derived from the Maillard reaction and the advanced lipoxidation end products (ALEs) derived from lipid peroxidation. They have revealed the accumulation of reactive carbonyl compounds (RCOs) derived from carbohydrates and lipids and the subsequent carbonyl modifications of proteins ("carbonyl stress"). In this article, we describe the causal role of various RCOs and AGEs/ALEs accumulating in uremia, the clinical consequences of carbonyl stress in uremia, and finally, the therapeutic perspectives. We propose carbonyl stress as a new uremic toxicity.

Pathogenesis of beta2-microglobulin amyloidosis

Hemodialysis-related amyloidosis is a relatively new form of systemic amyloidosis, with beta2-microglobulin (B2M) being identified as the major constituent protein. Most of the clinical findings are related to amyloid deposition in osseo-articular tissues. B2M amyloid deposits first appear in the cervical intervertebral discs, which are well known to be susceptible to mechanical stress. A close relationship between changes of microenvironment caused by such stress and amyloid deposition is highly suggested. In advanced cases, an inflammatory reaction composed of macrophages, multinucleated giant cells, and granulation tissue, is observed around the amyloid deposits. Purified amyloid protein is native B2M, and mutations and proteolysis are not believed to be important for its deposition. Plasma levels of B2M are elevated as much as 5-10 times because of the inability of hemodialysis equipment removal of B2M from blood plasma, the duration being very important for B2M amyloid fibrillogenesis. Heparan sulfate proteoglycans, perlecan, is increased at the same sites of amyloid deposits from the early stages. In B2M amyloidosis, an increase of heparan sulfate proteoglycans is observed in the vascular wall and synovium, but in the discs, ligaments and cartilage, there is an increase of chondroitin sulfate proteoglycans predominantly. B2M has an affinity for heparan sulfate proteoglycans, although it is weaker than that for laminin and type IV collagen. This is related to the interactions between negative charges of sulfate groups of proteoglycans and positive charges of basic amino acids in N-terminal side of B2M. Increased cytokines production in the synovium, induced by advanced glycation end products as well as elevated plasma levels, is also linked to inflammatory reactions. Increased expression of matrix metalloproteinases (MMP), especially MMP-1 and -9, is related to the destructive changes of the bone and cartilage. The decrease of plasma levels by high flux membrane and control of inflammatory reactions are very important for prevention of B2M amyloidosis.

Effect of dwell time on carbonyl stress using icodextrin and amino acid peritoneal dialysis fluids

BACKGROUND

Deterioration of the peritoneal membrane limits the technical survival of peritoneal dialysis (PD). Advanced glycation of the membrane has been incriminated in this evolution. Advanced glycation end products (AGEs) develop under the influence of glucose and of its degradation products, mainly reactive carbonyl compounds (RCOs) such as glyoxal (GO), methylglyoxal (MGO), and 3-deoxyglucosone (3-DG). The present study was undertaken to evaluate the impact of recently developed glucose-free PD fluids on AGE generation.

METHODS

Recently developed glucose-free PD fluids containing either icodextrin or amino acids were investigated. GO, MGO, and 3-DG [high-performance liquid chromatography (HPLC)] and total RCOs (spectrophotometry) were measured in fresh solutions and in effluents after various dwell duration. The AGE formation potential of PD fluids and effluents was assessed by incubation at 37 degrees C, for one week, with bovine serum albumin and by the eventual measurement of pentosidine (HPLC) and Nepsilon-carboxymethyllysine (CML; gas chromatography/mass spectrometry).

RESULTS

GO, MGO, and 3-DG (P < 0. 001) as well as total RCOs levels (P < 0.01) were significantly lower in icodextrin and amino acid PD fluid than in commercial, heat-sterilized, 1.36% glucose PD fluid. Pentosidine and CML generation were also significantly lower (P < 0.001) in icodextrin and amino acid PD fluid than in conventional 1.36% glucose PD fluid. The levels of total RCOs, however, increased in icodextrin and amino acid PD fluid effluents with dwell time. AGE formation potential rose accordingly, as demonstrated by a parallel increase in the generation of pentosidine and CML during incubation of PD effluents.

CONCLUSION

The present data demonstrate lower RCO contents and AGE formation potential in fresh icodextrin and amino acid PD fluids than in fresh heat-sterilized glucose PD fluids. However, this difference decreases progressively during dwell time, mainly as a result of the influx of total RCOs.

Glycosylation of immunoglobulin light chains associated with amyloidosis

AL amyloidosis is a fatal disease caused by deposition of immunoglobulin light chains in a fibrillarforin (AL) in various organs. By searching the Kabat database of immunoglobulin sequences using the KabatMan software, we have shown that there is a preponderance of the consensus glycosylation sequon (AsnXxxSer/Thr) in the framework regions of amyloid light chains. We have characterised by computer graphics simulations, NMR spectroscopy and carbohydrate biochemistry the structure and conformation of the oligosaccharide from amyloid protein AL MS (lamba1) and from the amyloid associated Bence Jones protein of patient MH (kappa1). These proteins have glycosylation in the hypervariable complementarity-determining region versus framework region, respectively. Both contained a 2-6 sialylated core fucosylated biantennary chain mostly with bisecting GIcNAc. Together our results suggest that light chain glycosylation may be one of several modifications which may render the protein more prone to amyloid formation.

Mixed predilution and postdilution online hemodiafiltration compared with the traditional infusion modes

BACKGROUND

On postdilution hemodiafiltration (post-HDF), convective removal of medium-high molecular weight solutes is, at the highest ultrafiltration rates, limited by high blood viscosity and protein concentration. Prefilter reinfusion (pre-HDF) may overcome this problem, but plasma dilution may affect the overall efficiency of the technique. In this study, an experimental system of online HDF with combined prefilter and postfilter infusion (mixed HDF) was evaluated and compared with the traditional predilution and postdilution modes.

METHODS

Removal of urea (U), creatinine (Cr), phosphate (Phos), and beta(2)-microglobulin (beta(2)m), ultrafiltration coefficients of the dialyzer (K(UF)), and rheologic conditions of the blood circuit were evaluated during the three infusion modes (a total of 36 runs lasting 180 min), performed with a polysulfone hemofilter 1.8 m(2), blood flow (Q(b)) 400 mL/min, dialysate flow (Q(d)) 700 mL/min, and infusion rate 120 mL/min (pre-HDF and post-HDF), or 60 + 60 mL/min (mixed HDF).

RESULTS

The mean effective U and Cr clearances and urea index of dialysis dose (eKt/V) were significantly higher on post-HDF than on pre-HDF (K(WB) (U) 210 vs. 193 mL/min, K(DQ) (Cr) 152 vs. 142 mL/min, eKt/V 1.41 vs. 1.30), while mixed HDF did not show significant differences versus post-HDF (K(WB) (U) 201 mL/min, K(DQ) (Cr) 149 mL/min). K(DQ) for Phos and beta(2)m were higher on post-HDF in only absolute values. Similar differences were found for instantaneous dialyzer clearances (K(I)) at 60, 120, and 180 minutes of the sessions, with a common trend to decrease with time. K(UF) and the apparent beta(2)m sieving coefficient showed their lowest values toward the end of post-HDF sessions. Increasing filtration fractions (FFs) were associated with increasing transmembrane pressure (TMP) and solute clearances up to FF values of 0.45. These were values achieved in only post-HDF, at which point the curve of the relationship between TMP and FF assumed its steepest exponential trend.

CONCLUSIONS

Mixed HDF, by better preserving the characteristics of water and solute transport of the membrane, ensured safer operating conditions than post-HDF, while achieving similar removal of small- and large-size solutes. Optimizing the ratio of prefilter/postfilter infusion and the total infusion according to the relationships found in our study between solute clearances, FF, and TMP, convective flux and transport may avoid excessive hemoconcentration and dangerous pressure gradients.

Nitric oxide inhibits the formation of advanced glycation end products

BACKGROUND

Advanced glycation end products (AGEs) are elevated in renal failure and have been implicated in the pathogenesis of several uremic complications. Their formation is closely associated with oxidative stress. The recent observation that nitric oxide (NO) has an antioxidant effect led us to examine the possible role of NO in the generation of AGEs.

METHODS

We examined the effect of NO donors, 2, 2'-(hydroxynitrosohydrazono)bis-ethanamine (NOC18) and S-nitroso-N-acetyl-DL-penicillamine (SNAP), on the in vitro formation of pentosidine, which was used as a surrogate marker for AGEs. Bovine serum albumin was incubated under air at 37 degrees C in a medium containing either several AGE precursors or uremic plasma. To elucidate further the mechanism of the NO effect on AGE formation, we examined the generation of free radicals and carbonyls in pentose-driven pentosidine formation.

RESULTS

NO donors significantly inhibit the formation of pentosidine in a dose-dependent manner. The effect is abolished by the addition of a NO scavenging agent, 2-(4-carboxyphenyl)-4,4,5, 5-tetramethylimidazoline-1-oxyl 3-oxide (carboxy-PTIO). The inhibitory effect results from NO but not from the NO donor molecule. It is best explained by the ability of NO to scavenge carbon-centered radicals, hydroxyl radical, and carbonyl compounds.

CONCLUSIONS

NO inhibits pentosidine formation by scavenging free radicals and by inhibiting carbonyl compound formation. NO might be implicated in the atherogenic and inflammatory effects of AGEs: Reduced NO production and increased oxidative stress associated with atherosclerotic lesions may accelerate AGE formation and, thus, exacerbate endothelial dysfunction and accelerate the development of atherosclerosis in uremia.

Effects of probucol on cholesterol metabolism in mouse peritoneal macrophages: inhibition of HDL-mediated cholesterol efflux

Macrophage-derived foam cells are known to play an essential role in the development and progression of atherosclerotic lesions. Probucol prevents oxidative modification of low-density lipoprotein (LDL) and lowers plasma contents of LDL and high-density lipoprotein (HDL). A recent report using apoE -/- mice demonstrated that probucol treatment enhanced atherosclerosis in apoE -/- mice more rapidly than that in untreated apoE -/- mice, and a reduction in plasma cholesterol by probucol was not the cause of enhancement of atherosclerotic lesions in probucol-treated apoE -/- mice. Moreover, probucol was reported to inhibit apoA-I mediated cholesterol efflux from mouse macrophages. These reports suggested that probucol might directly affect cholesterol metabolism in mouse macrophages. Thus, we investigated the effects of probucol on cholesterol metabolism in mouse resident peritoneal macrophages. Probucol did not affect degradation of acetylated LDL (Ac-LDL), degradation of LDL and endogenous cholesterol synthesis in mouse macrophages. However, it significantly inhibited HDL-mediated cholesterol efflux. Moreover, probucol partially (30%) inhibited the binding of HDL to mouse macrophages, and significantly activated acyl-coenzyme A:cholesterol acyltransferase (ACAT). Our results suggested that probucol inhibited HDL-mediated cholesterol efflux by inhibiting the binding of HDL to mouse macrophages and reducing HDL-accessible free cholesterol content by ACAT activation, thereby worsening atherosclerotic lesions in apoE -/- mice. However, it remains unclear whether probucol inhibits HDL-mediated cholesterol efflux from human macrophages.

Dialysis-related spinal canal stenosis: a clinicopathological study on amyloid deposition and its AGE modification

Three cases operated for spinal canal stenosis induced by dialysis-related amyloidosis (DRA) were investigated clinicopathologically. Cases were all-male, and had undergone hemodialysis around 20 years. In two cases, cervical plain X-rays showed only minor spondylotic changes. However, magnetic resonance imaging (MRI), myelography, and computed tomography (CT) showed extradural thickness with compression on the cervical spinal cord and cauda equina. In one case cervical X-rays showed typical destructive spondyloarthropathy (DSA), and MRI showed compression myelopathy. Surgical treatment on both cervical and lumbar spine in two cases and on cervical spine only in one case successfully reduced the symptoms. Extradural thickened tissue and ligament flavum obtained during surgery were studied histopathologically and immunostained by using anti-CD68, anti-beta2-microglobulin (beta2m), and anti-advanced glycation end product (AGE) antibody. Congo red stain showed diffuse or nodular amyloid deposition, and immunostaining with anti-beta2m and anti-AGE antibodies also demonstrated the same distribution pattern. Thus, beta2m-positive amyloid tissue in the extradural thickness (extradural amyloid deposition) was immunohistochemically demonstrated to be modified with AGE. Inflammatory reaction with histiocytic and giant cell infiltration was also shown around the amyloid tissues. There were CD68-positive cells, and some cells were positive for AGE and beta2m. These findings suggest that beta2m accumulation and inflammatory reaction finally promote destruction of connective tissues. MRI, CT and/or myelography are necessary for diagnosing spinal canal stenosis accompanied by DRA. In conclusion, we propose a more comprehensive concept of dialysis-related spinal canal stenosis, which includes both DSA and myeloradiculopathy induced by extradural thickness.

The removal of uremic toxins

Three major groups of uremic solutes can be characterized: the small water-soluble compounds, the middle molecules, and the protein-bound compounds. Whereas small water-soluble compounds are quite easily removed by conventional hemodialysis, this is not the case for many other molecules with different physicochemical characteristics. Continuous ambulatory peritoneal dialysis (CAPD) is often characterized by better removal of those compounds. Urea and creatinine are small water-soluble compounds and the most current markers of retention and removal, but they do not exert much toxicity. This is also the case for many other small water-soluble compounds. Removal pattern by dialysis of urea and creatinine is markedly different from that of many other uremic solutes with proven toxicity. Whereas middle molecules are removed better by dialyzers containing membranes with a larger pore size, it is not clear whether this removal is sufficient to prevent the related complications. Larger pore size has virtually no effect on the removal of protein-bound toxins. Therefore, at present, the current dialytic methods do not offer many possibilities to remove protein-bound compounds. Nutritional and environmental factors as well as the residual renal function may influence the concentration of uremic toxins in the body fluids.

Relevance of oxidative and carbonyl stress to long-term uremic complications

Oxidative stress is a disturbance of balance between oxidants and antioxidant species. The existence of an increased oxidative stress in chronic renal failure is supported by evidence of increased lipid, carbohydrate, and protein oxidation products in plasma and cell membrane. Recent studies have implicated the oxidative stress in the nonenzymatic biochemistry leading to irreversible protein modifications. Reactive oxygen species may directly alter proteins with the eventual formation of oxidized amino acids. Alternatively, reactive carbonyl compounds formed by the oxidation of carbohydrates and lipids may indirectly lead to advanced glycation or lipoxidation of proteins. Chronic uremia is associated with increased modification of protein caused by reactive carbonyl compounds derived from both carbohydrates and lipids. Increased carbonyl modification of proteins subsequently results in the rise of plasma and tissue contents of advanced glycation end products and advanced lipoxidation end products, in which the deleterious biological effects have been revealed. This article focuses on the irreversible nonenzymatic modification of proteins, which might, at least in part, contribute to the development of complications associated with chronic renal failure and long-term dialysis, such as atherosclerosis and dialysis-related amyloidosis.