Receptor-mediated endocytosis of immunoglobulin light chains by renal proximal tubule cells

Cellular antioxidant mechanisms control immunoglobulin light chain-mediated proximal tubule injury

A major cause of morbidity and mortality in multiple myeloma is kidney injury from overproduction of monoclonal immunoglobulin light chains (FLC). FLC can induce damage through the production of hydrogen peroxide, which activates pro-inflammatory and pro-apoptotic pathways. The present study focused on catalase, a highly conserved antioxidant enzyme that degrades hydrogen peroxide. Initial findings were that FLC increased hydrogen peroxide levels but also decreased catalase levels and activity in proximal tubule epithelium. In order to clarify, we showed that the phosphatidylinositol 3-kinase inhibitor, LY294002, inhibited FLC-induced Akt-mediated deactivation of Forkhead box O class 3a (FoxO3a) and increased catalase activity in proximal tubule cells. Augmented catalase activity decreased FLC-mediated production of hydrogen peroxide as well as the associated increase in High Mobility Group Box 1 (HMGB1) protein release and caspase-3 activity. Coincubation of cells with FLC and an allosteric activator of Sirtuin 1 (SIRT1) was also sufficient to increase catalase activity and promote similar cytoprotective effects. Our studies confirmed that the mechanism of downregulation of catalase by FLC involved deactivation of FoxO3a and inhibition of SIRT1. Mechanistic understanding of catalase regulation allows for future treatments that target pathways that increase catalase in the setting of proximal tubule injury from FLC.

Free light chains injure proximal tubule cells through the STAT1/HMGB1/TLR axis

Free light chains (FLCs) induce inflammatory pathways in proximal tubule cells (PTCs). The role of TLRs in these responses is unknown. Here we present findings on the role of TLRs in FLC-induced PTC injury. We exposed human kidney PTC cultures to κ and λ FLCs and used cell supernatants and pellets for ELISA and gene expression studies. We also analyzed tissues from Stat1-/- and littermate control mice treated with daily i.p. injections of a κ FLC for 10 days. FLCs increased the expression of TLR2, TLR4, and TLR6 via HMGB1, a damage-associated molecular pattern. Countering TLR2, TLR4, and TLR6 through GIT-27 or specific TLR siRNAs reduced downstream cytokine responses. Blocking HMGB1 through siRNA or pharmacologic inhibition, or via STAT1 inhibition, reduced FLC-induced TLR2, TLR4, and TLR6 expression. Blocking endocytosis of FLCs through silencing of megalin/cubilin, with bafilomycin A1 or hypertonic sucrose, attenuated FLC-induced cytokine responses in PTCs. IHC showed decreased TLR4 and TLR6 expression in kidney sections from Stat1-/- mice compared with their littermate controls. PTCs exposed to FLCs released HMGB1, which induced expression of TLR2, TLR4, and TLR6 and downstream inflammation. Blocking FLCs' endocytosis, Stat1 knockdown, HMGB1 inhibition, and TLR knockdown each rescued PTCs from FLC-induced injury.

Chromogranin A Tubulopathy: Differing Histopathologic Patterns of Acute Tubular Injury in the Setting of Neuroendocrine Neoplasms

Introduction

Neoplasms of neuroendocrine derivation or differentiation may express specific peptides, some of which are capable of producing clinical symptomatology and others used as biomarkers: one such peptide being chromogranin A (CGA). Herein, we describe histopathologic changes present in kidney specimens from patients with such neoplasms, and illustrate 2 patterns of acute tubular injury (ATI) attributable to CGA.

Methods

Eleven patients with a history of a neoplasm of neuroendocrine derivation or differentiation and having histopathologic sampling of the kidney were retrospectively identified, 3 of whom had ATI with either engorgement of the proximal tubular epithelium by resorbed material or tubular cast formation.

Results

Two patterns of ATI were observed. One characterized by acutely injured proximal tubular cells engorged with resorption granules that expressed CGA via immunoperoxidase staining. Another pattern was characterized by intraluminal tubular cast material associated with ATI that did not exhibit restriction of immunoglobulin light chains (LCs), but immunoperoxidase staining for CGA revealed that the cast material was composed of the neuroendocrine-associated peptide. The level of serum CGA does not appear to necessarily equate to developing either of these 2 patterns of ATI.

Conclusions

Patients with a neoplasm of neuroendocrine derivation or differentiation may develop ATI, and in certain cases may be secondary to CGA renal tubular deposition.

Bortezomib overcomes the negative prognostic impact of renal impairment in a newly diagnosed elderly patient with multiple myeloma: A case report

Multiple myeloma (MM) is a common B-cell hematological malignancy in the clinic. Bortezomib is the first-in-class proteasome inhibitor that has been approved for the treatment of patients with MM in the bone marrow. The present study report the case of an 83-year-old man who showed marked weakness, fatigue and a poor appetite. The patient was admitted to the Department of Nephrology due to severe renal impairment (RI). Immunofixation electrophoresis indicated a λ light chain-positive status. There were 19.2% plasmablasts and proplasmacytes in the bone marrow. Positivity for the cell surface markers cluster of differentiation (CD)13, CD33, CD38 and human leukocyte antigen-antigen D-related was detected by flow cytometry. The patient was diagnosed with MM, λ light chain type, stage IIIB, and received bortezomib and dexamethasone regimen chemotherapy. RI was improved following the chemotherapy, and plasmablasts and proplasmacytes were almost eliminated. The Hb level was maintained at ~90 g/l. Overall, the present case report suggests that bortezomib may be safe and effective for elderly patients, even those >80 years of age, with severe RI.

The Scope of Kidney Affection in Monoclonal Gammopathies at All Levels of Clinical Significance

Multiple myeloma (MM) is one of the most important clonal malignant plasma cell disorders and renal involvement is associated with poor prognosis. Although there are several reasons for renal impairment in MM, the main cause is the toxic effects of monoclonal proteins. Although cast nephropathy is the best known and unchallenged diagnosis for hematologists and pathologists, the renal effects of monoclonal gammopathy can be various. Monoclonal gammopathy of renal significance was proposed by the International Kidney and Monoclonal Gammopathy Research Group for renal lesions in monoclonal gammopathy in recent years. Renal lesions in monoclonal gammopathy can be grouped as follows: light chain (cast) nephropathy, acute tubular injury/necrosis, tubulointerstitial nephritis, amyloidosis, monoclonal Ig deposition diseases, immunotactoid glomerulopathy, type I cryoglobulinemia, proliferative glomerulonephritis with monoclonal IgG deposits, C3 glomerulopathy with monoclonal gammopathy, and crystal-storing histiocytosis, considering the previous and new terminology. In this study, renal involvement of monoclonal gammopathies, in terms of previous and new terminology, was reviewed.

Natural history and outcome of light chain deposition disease

Light chain deposition disease (LCDD) is characterized by the deposition of monotypic immunoglobulin light chains in the kidney, resulting in renal dysfunction. Fifty-three patients with biopsy-proven LCDD were prospectively followed at the UK National Amyloidosis Center. Median age at diagnosis was 56 years, and patients were followed for a median of 6.2 years (range, 1.1-14.0 years). Median renal survival from diagnosis by Kaplan-Meier analysis was 5.4 years, and median estimated patient survival was 14.0 years; 64% of patients were alive at censor. Sixty-two percent of patients required dialysis, and median survival from commencement of dialysis was 5.2 years. There was a strong association between hematologic response to chemotherapy and renal outcome, with a mean improvement in glomerular filtration rate (GFR) of 6.1 mL/min/year among those achieving a complete or very good partial hematologic response (VGPR) with chemotherapy, most of whom remained dialysis independent, compared with a mean GFR loss of 6.5 mL/min/year among those achieving only a partial or no hematologic response (P < .009), most of whom developed end-stage renal disease (ESRD; P = .005). Seven patients received a renal transplant, and among those whose underlying clonal disorder was in sustained remission, there was no recurrence of LCDD up to 9.7 years later. This study highlights the need to diagnose and treat LCDD early and to target at least a hematologic VGPR with chemotherapy, even among patients with advanced renal dysfunction, to delay progression to ESRD and prevent recurrence of LCDD in the renal allografts of those who subsequently receive a kidney transplant.

The endocytic receptor megalin and its associated proteins in proximal tubule epithelial cells

Receptor-mediated endocytosis in renal proximal tubule epithelial cells (PTECs) is important for the reabsorption and metabolization of proteins and other substances, including carrier-bound vitamins and trace elements, in glomerular filtrates. Impairment of this endocytic process results in the loss of such substances and development of proteinuria, which is an important clinical indicator of kidney diseases and is also a risk marker for cardiovascular disease. Megalin, a member of the low-density lipoprotein receptor gene family, is a multiligand receptor expressed in the apical membrane of PTECs and plays a central role in the endocytic process. Megalin interacts with various intracellular adaptor proteins for intracellular trafficking and cooperatively functions with other membrane molecules, including the cubilin-amnionless complex. Evidence suggests that megalin and the cubilin-amnionless complex are involved in the uptake of toxic substances into PTECs, which leads to the development of kidney disease. Studies of megalin and its associated molecules will be useful for future development of novel strategies for the diagnosis and treatment of kidney diseases.

Receptor-associated protein blocks internalization and cytotoxicity of myeloma light chain in cultured human proximal tubular cells

Background

Free light chains (LCs) are among the many ligands that bind to cubilin/megalin for endocytosis via the clathrin-dependent endosomal/lysosomal pathway. Receptor associated protein (RAP), is a 39 kDA high-affinity, chaperone-like ligand for megalin that assists in the proper folding and functioning of megalin/cubilin. Although RAP is known to inhibit ligand binding to megalin/cubilin, its effect on LC endocytosis has not been shown directly.

Methods and Principal Findings

We investigated whether RAP can block the endocytosis of LC in cultured human proximal tubule cells and whether this can prevent LC cytotoxicity. Immunofluorescence microscopy and flow cytometry showed that fluorescently labeled LC endocytosis was markedly inhibited in HK-2 cells pretreated with human RAP. The effect of RAP was dose-dependent, and was predominantly on endocytosis as it had no effect on the small acid-washable fraction of LC bound to cell membrane. RAP significantly inhibited LC induced cytokine production and phosphorylation of ERK1/2 and p38 MAPK. Prolonged exposure to LC for 48 h resulted in epithelial-to-mesenchymal transformation in HK-2 cells as evidenced by marked reduction in the expression of the epithelial cell marker E-cadherin, and increased the expression of the mesenchymal marker α-SMA, which was also prevented by RAP in the endocytosis medium.

Conclusions

RAP inhibited LC endocytosis by ∼88% and ameliorated LC-induced cytokine responses and EMT in human PTCs. The results not only provide additional evidence that LCs endocytosis occurs via the megalin/cubilin endocytic receptor system, but also show that blocking LC endocytosis by RAP can protect proximal tubule cells from LC cytotoxicity.

Prostaglandin D2 synthase: Apoptotic factor in alzheimer plasma, inducer of reactive oxygen species, inflammatory cytokines and dialysis dementia

BACKGROUND

Apoptosis, reactive oxygen species (ROS) and inflammatory cytokines have all been implicated in the development of Alzheimer's disease (AD).

OBJECTIVES

The present study identifies the apoptotic factor that was responsible for the fourfold increase in apoptotic rates that we previously noted when pig proximal tubule, LLC-PK1, cells were exposed to AD plasma as compared to plasma from normal controls and multi-infarct dementia.

PATIENTS AND METHODS

The apoptotic factor was isolated from AD urine and identified as lipocalin-type prostaglandin D2 synthase (L-PGDS). L-PGDS was found to be the major apoptotic factor in AD plasma as determined by inhibition of apoptosis approximating control levels by the cyclo-oxygenase (COX) 2 inhibitor, NS398, and the antibody to L-PGDS. Blood levels of L-PGDS, however, were not elevated in AD. We now demonstrate a receptor-mediated uptake of L-PGDS in PC12 neuronal cells that was time, dose and temperature-dependent and was saturable by competition with cold L-PGDS and albumin. Further proof of this endocytosis was provided by an electron microscopic study of gold labeled L-PGDS and immunofluorescence with Alexa-labeled L-PGDS.

RESULTS

The recombinant L-PGDS and wild type (WT) L-PGDS increased ROS but only the WTL-PGDS increased IL6 and TNFα, suggesting that differences in glycosylation of L-PGDS in AD was responsible for this discrepancy.

CONCLUSIONS

These data collectively suggest that L-PGDS might play an important role in the development of dementia in patients on dialysis and of AD.

Pivotal role of apoptosis signal-regulating kinase 1 in monoclonal free light chain-mediated apoptosis

Renal failure, a major complication associated with multiple myeloma, is usually related to deposition of monoclonal immunoglobulin free light chains (FLCs) and directly contributes to morbidity and mortality in this disease. The present study focused on the cytotoxic effects of monoclonal FLCs. Human proximal tubular epithelial cells (HK-2) were examined after incubation with two human monoclonal FLCs (termed κ2 and λ3). Incubation of HK-2 cells for 24 and 48 hours with either FLCs at 1 mg/mL promoted activation of caspase-9 and caspase-3 and increased the rate of apoptosis. Because prior studies demonstrated that FLCs generated intracellular oxidative stress, our studies focused on the redox-sensitive mitogen-activated protein kinase kinase kinase known as apoptosis signal-regulating kinase 1 (ASK1). A time-dependent increase in phosphorylation of ASK1 at T845, indicating activation of this enzyme, was observed. Small interfering RNA designed to reduce ASK1 expression in HK-2 cells successfully decreased ASK1, which was confirmed by Western blot analysis. Incubation of ASK1-depleted HK-2 cells with the two FLCs prevented the increase in apoptosis while pretreating HK-2 cell with nontargeting small interfering RNA did not prevent FLCs-mediated apoptosis. The combined data demonstrate that monoclonal FLCs activated the intrinsic apoptotic pathway in renal epithelial cells by activation of ASK1.

The pathogenesis and diagnosis of acute kidney injury in multiple myeloma

Renal failure remains a principal cause of morbidity for patients with multiple myeloma. Once reversible factors such as hypercalcemia have been corrected, the most common cause of severe renal failure in these patients is a tubulointerstitial pathology that results from the very high circulating concentrations of monoclonal immunoglobulin free light chains. These endogenous proteins can result in isolated proximal tubule cell cytotoxicity, tubulointerstitial nephritis and cast nephropathy (myeloma kidney). Less frequently, high levels of free light chains can lead to immunoglobulin light chain amyloidosis and light chain deposition disease, although these conditions are usually associated with insidious progression of renal failure rather than acute kidney injury. Unless there is rapid intervention, progressive and irreversible damage occurs, particularly interstitial fibrosis and tubular atrophy. Despite advances in our understanding of the pathogenesis of these processes there has been a gap in translating these achievements into improved patient outcomes. The International Kidney and Monoclonal Gammopathy Research Group was formed to address this need. In this Review, we discuss the mechanisms of disease and diagnostic approaches to patients with acute kidney injury complicating multiple myeloma.

Immunoglobulin light chains activate nuclear factor-κB in renal epithelial cells through a Src-dependent mechanism

One of the major attendant complications of multiple myeloma is renal injury, which contributes significantly to morbidity and mortality in this disease. Monoclonal immunoglobulin free light chains (FLCs) are usually directly involved, and tubulointerstitial renal injury and fibrosis are prominent histologic features observed in myeloma. The present study examined the role of monoclonal FLCs in altering the nuclear factor κ light chain enhancer of activated B cells (NF-κB) activity of renal epithelial cells. Human proximal tubule epithelial cells exposed to 3 different human monoclonal FLCs demonstrated Src kinase-dependent activation of the NF-κB pathway, which increased production of monocyte chemoattractant protein-1 (MCP-1). Tyrosine phosphorylation of inhibitor of κB kinases (IKKs) IKKα and IKKβ and a concomitant increase in inhibitor of κB (IκB) kinase activity in cell lysates were observed. Time-dependent, Src kinase-dependent increases in serine and tyrosine phosphorylation of IκBα and NF-κB activity were also demonstrated. Proteasome inhibition partially blocked FLC-induced MCP-1 production. These findings fit into a paradigm characterized by FLC-induced redox-signaling events that activated the canonical and atypical (IKK-independent) NF-κB pathways to promote a proinflammatory, profibrotic renal environment.

Immunoglobulin light chains activate tubular epithelial cells through redox signaling

The renal proximal tubule metabolizes circulating low-molecular-weight proteins such as Ig free light chains. In the setting of plasma cell dyscrasias, the burden of filtered protein can be very high. Endocytosis of certain nephrotoxic light chains induces H(2)O(2) production and monocyte chemoattractant protein-1 (MCP-1) release, leading to recruitment of inflammatory cells and interstitial fibrosis, but how these processes are linked mechanistically is not well understood. This study investigated the relationship between H(2)O(2) generated after light chain endocytosis by human proximal tubular (HK-2) cells and activation of c-Src, a redox-sensitive tyrosine kinase. HK-2 cells exposed to two different light chains upregulated c-Src activity, which increased the production of MCP-1. In parallel, we observed a time-dependent oxidation of c-Src. Inhibition of c-Src activity and silencing c-Src expression abrogated the light chain-induced MCP-1 response, but had no effect on H(2)O(2), indicating that production of H(2)O(2) is upstream of c-Src in the signaling cascade. Silencing megalin and cubilin expression inhibited the MCP-1 response, whereas extracellular catalase did not, indicating that endocytosis is required and that intracellular generation of reactive oxygen species activates c-Src. These data show that intracellular H(2)O(2) induced by endocytosis of monoclonal free light chains oxidizes and activates c-Src, which promotes release of MCP-1.

Regulation of the V-ATPase in kidney epithelial cells: dual role in acid-base homeostasis and vesicle trafficking

The proton-pumping V-ATPase is a complex, multi-subunit enzyme that is highly expressed in the plasma membranes of some epithelial cells in the kidney, including collecting duct intercalated cells. It is also located on the limiting membranes of intracellular organelles in the degradative and secretory pathways of all cells. Different isoforms of some V-ATPase subunits are involved in the targeting of the proton pump to its various intracellular locations, where it functions in transporting protons out of the cell across the plasma membrane or acidifying intracellular compartments. The former process plays a critical role in proton secretion by the kidney and regulates systemic acid-base status whereas the latter process is central to intracellular vesicle trafficking, membrane recycling and the degradative pathway in cells. We will focus our discussion on two cell types in the kidney: (1) intercalated cells, in which proton secretion is controlled by shuttling V-ATPase complexes back and forth between the plasma membrane and highly-specialized intracellular vesicles, and (2) proximal tubule cells, in which the endocytotic pathway that retrieves proteins from the glomerular ultrafiltrate requires V-ATPase-dependent acidification of post-endocytotic vesicles. The regulation of both of these activities depends upon the ability of cells to monitor the pH and/or bicarbonate content of their extracellular environment and intracellular compartments. Recent information about these pH-sensing mechanisms, which include the role of the V-ATPase itself as a pH sensor and the soluble adenylyl cyclase as a bicarbonate sensor, will be addressed in this review.

Silencing megalin and cubilin genes inhibits myeloma light chain endocytosis and ameliorates toxicity in human renal proximal tubule epithelial cells

Using target-specific short interfering (si) RNAs, we silenced the tandem endocytic receptors megalin and cubilin genes in cultured human renal proximal tubule epithelial cells. Transfection by siRNA resulted in up to 90% suppression of both megalin and cubilin protein and mRNA expression. In HK-2 cells exposed to kappa-light chain for up to 24 h, light chain endocytosis was reduced in either megalin- or cubilin-silenced cells markedly but incompletely. Simultaneous silencing of both the cubilin and megalin genes, however, resulted in near-complete inhibition of light chain endocytosis, as determined by measuring kappa-light chain protein concentration in cell cytoplasm and by flow cytometry using FITC-labeled kappa-light chain. In these cells, light chain-induced cytokine responses (interleukin-6 and monocyte chemoattractant protein-1) and epithelial-to-mesenchymal transition as well as the associated cellular and morphological alterations were also markedly suppressed. The results demonstrate that light chain endocytosis is predominantly mediated by the megalin-cubilin tandem endocytic receptor and identify endocytosis as a key step in light chain cytotoxicity. Blocking light chain endocytosis prevents its nephrotoxic effects on human kidney proximal tubule cells.

Application of periodic acid-Schiff fluorescence emission for immunohistochemistry of living mouse renal glomeruli by an "in vivo cryotechnique"

To identify the distribution of endogenous serum proteins in living mouse renal glomeruli under various hemodynamic conditions, we used the periodic acid-Schiff (PAS) and its fluorescence emission as a marker for the glomerular basement membrane (GBM). The immunostaining for collagen type IV was hardly observed without microwave treatment in specimens prepared by an "in vivo cryotechnique". However, PAS staining and its fluorescence emission could be clearly visualized at the GBM with the "in vivo cryotechnique". Under normotensive conditions, immunoreaction products of albumin and immunoglobulin G heavy and light chains (IgG(H+L)) were localized within glomerular capillary loops (GCL) but not colocalized with the PAS fluorescence emission of the GBM. Under heart-arrest conditions and with quick-freezing of resected tissues, albumin, IgG (H+L), immunoglobulin kappa light chain, and IgG1 heavy chain (IgG1) were immunolocalized within the GCL and mesangial areas, but only albumin and the kappa light chain were additionally immunolocalized in Bowman's space, indicating their passage through the GBM. Under acute hypertensive conditions, both albumin and the kappa light chain, but not IgG1, were clearly immunolocalized along the GBM and in the Bowman's space, indicating their increased passage through the GBM. The overlapping areas of PAS fluorescence emission and the albumin or kappa light chain appeared to be larger with quick-freezing and under the heart arrest or acute hypertensive conditions than under normal circulation, whereas those of PAS emission and IgG1 did not differ among these conditions. The serum proteins passing through the GBM were clearly visualized with the "in vivo cryotechnique", immunofluorescence staining, and PAS fluorescence emission.

Immunolocalization of serum proteins in living mouse glomeruli under various hemodynamic conditions by "in vivo cryotechnique"

Distribution of serum proteins in renal glomeruli is important for histopathology in medical and biological fields, but mechanisms of their passage through glomerular capillary loops (GCL) are still difficult to clarify. We have tried to visualize topographical changes of the serum proteins passing through GCL by "in vivo cryotechnique" in combination with immunohistochemistry. Albumin and immunoglobulin G (IgG), Ig kappa light chain and IgG1 heavy chain were mainly immunolocalized in GCL, but not colocalized with zonula occludens-1 (ZO-1) under normotensive condition. Under heart-arrest condition and in quick-frozen fresh tissues, albumin and kappa light chain were immunolocalized in Bowman's space, indicating their passage caused by the stoppage of blood supply. However, under acute hypertensive condition, they were more clearly immunolocalized along basement membranes and in the Bowman's space, indicating their increased passage through GCL. IgG was also more clearly localized in mesangial areas under acute hypertension, compared with that under the normotensive or heart-arrest condition. This study is the first direct visualization for glomerular passage of serum proteins under abnormal hemodynamic conditions by the "in vivo cryotechnique", and the experimental protocol will be useful for morphofunctional examination of living mouse GCL and immunohistochemical analyses of dynamically changing proteins.

Potential protective action of pituitary adenylate cyclase-activating polypeptide (PACAP38) on in vitro and in vivo models of myeloma kidney injury

The most common type of renal injury in multiple myeloma is chronic tubulointerstitial nephropathy associated with casts in tubule lumens, an entity referred to as “myeloma kidney” that often progresses to end-stage kidney diseases. Myeloma kidney is associated with a significant increase in all-cause mortality, yet no effective intervention, except a limited use of steroid, is available. Here, we report that pituitary adenylate cyclase-activating polypeptide with 38 residues (PACAP38) dramatically prevents injury of cultured renal proximal tubule cells caused by myeloma light chains through suppression of proinflammatory cytokines production, by inhibiting p38 MAPK and translocation of NFκB via both PAC1 and VPAC1 receptors. The suppressive effects of PACAP was as effective as dexamethasone in all of their cytokine assays and demonstrated both in vitro and in vivo. Furthermore, PACAP38 inhibits myeloma cell growth directly and may also indirectly by suppressing production of the growth factor, IL-6, from bone marrow stromal cells, that is stimulated by adhesion of myeloma cells. These findings render PACAP38 worth evaluation as a promising candidate for an effective and safe renoprotectant in myeloma kidney, and possibly other nephropathy, and also as a new antitumor agent in multiple myeloma.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Role of MAPK pathways in light chain-induced cytokine production in human proximal tubule cells

We previously demonstrated that light chain (LC) endocytosis by human proximal tubule cells (PTCs) leads to production of cytokines through activation of NF-kappaB. Here, we examined the role of MAPK pathways in these responses using four species of myeloma LCs (kappa(1), kappa(2), kappa(3), and lambda(1)) previously shown to induce cytokine production by PTCs. Among these, kappa(1)-LC, which yielded the strongest cytokine responses, was selected for detailed studies. Activation of MAPKs was probed by Western blot analysis for the active kinases, ERK 1/2, JNK 1/2, and p38 in kappa(1)-LC-exposed human PTCs. To evaluate the functional role of MAPKs in LC-induced cytokine responses, we tested the effects of U-0126, an ERK inhibitor; SP-600125, an inhibitor of JNK; SB-203580, a p38 inhibitor; and curcumin, a JNK-AP-1 inhibitor, all added to media before 4-h exposure to 1.5 mg/ml kappa(1)-LC. IL-6 and monocyte chemotactic protein-1 (MCP-1) were determined by ELISA. Both LC and human serum albumin (HSA) activated ERK, although the HSA effect was weaker. kappa(1)-LC stimulated all three MAPKs, although phosphorylation of ERK was more pronounced and sustained than others. Inhibitors of ERK, JNK, and p38 reduced LC-induced IL-6 and MCP-1 production. These findings suggest that activation of MAPKs plays a role in LC-induced cytokine responses in PTCs. Activation of MAPKs may be involved in cytokine responses induced by other proteins as well as LCs and may be pivotal in the pathophysiology of tubulointerstitial injury in proteinuric diseases.

FcRn-mediated transcytosis of immunoglobulin G in human renal proximal tubular epithelial cells

In the kidney, proteins filtered through glomeruli are reabsorbed by endocytosis along the proximal tubules to avoid renal loss of large amounts of proteins. Recently, neonatal Fc receptor (FcRn), which is involved in the transport of IgG across several epithelial and endothelial cells, was reported to be expressed in renal proximal tubular epithelial cells (RPTECs). However, there has been no direct evidence for receptor-mediated endocytosis of IgG in human RPTECs. To explore physiological roles of FcRn in the proximal tubules, we used the human RPTECs to examine IgG transport. FcRn was expressed in RPTECs and physically associated with beta(2)-microglobulin, preserving the capacity of specific pH-dependent IgG binding. Human IgG was bound to the cell surface of RPTECs in a pH-dependent manner. The human IgG transport assay revealed that receptor-mediated transepithelial transport of intact IgG in RPTECs is bidirectional and that it requires the formation of acidified intracellular compartments. With the use of double immunofluorescence, the internalized human IgG was marked in cytoplasm of RPTECs and colocalized with FcRn. These data define the mechanisms of FcRn-associated IgG transport in RPTEC monolayers. It was suggested that the intact pathway for human IgG transepithelial transport may avoid lysosomal degradation of IgG.