ERK pathway mediates P2X7 expression and cell death in renal interstitial fibroblasts exposed to necrotic renal epithelial cells

Ionotropic purinergic receptor 7 (P2X7) channel structure and pharmacology provides insight regarding non-nucleotide agonism

P2X7 is a member of the Ionotropic Purinergic Receptor (P2X) family. The P2X family of receptors is composed of seven (P2X1-7), ligand-gated, nonselective cation channels. Changes in P2X expression have been reported in multiple disease models. P2Xs have large complex extracellular domains that function as receptors for a variety of ligands, including endogenous and synthetic agonists and antagonists. ATP is the canonical agonist. ATP affinity ranges from nanomolar to micromolar for most P2XRs, but P2X7 has uniquely poor ATP affinity. In many physiological settings, it may be difficult to achieve the millimolar extracellular ATP concentrations needed for P2X7 channel activation; however, channel function is implicated in pain sensation, immune cell function, cardiovascular disease, cancer, and osteoporosis. Multiple high-resolution P2X7 structures have been solved in apo-, ATP-, and antagonist-bound states. P2X7 structural data reveal distinct allosteric and orthosteric antagonist-binding sites. Both allosteric and orthosteric P2X7 antagonists are well documented to inhibit ATP-evoked channel current. However, a growing body of evidence supports P2X7 activation by non-nucleotide agonists, including extracellular histone proteins and human cathelicidin-derived peptides (LL-37). Interestingly, P2X7 non-nucleotide agonism is not inhibited by allosteric antagonists, but is inhibited by orthosteric antagonists. Herein, we review P2X7 function with a focus on the efficacy of available pharmacology on P2X7 channel current activation by non-nucleotide agonists in effort to understand agonist/antagonist efficacy, and consider the impact of these data on the current understanding of P2X7 in physiology and disease given these limitations of P2X7-selective antagonists and incomplete knockout mouse models.

Guben Xiezhuo Decoction inhibits M1 polarization through the Raf1/p-Elk1 signaling axis to attenuate renal interstitial fibrosis

ETHNOPHARMACOLOGICAL RELEVANCE

Guben Xiezhuo Decoction (GBXZD) is an herbal compound used to treat chronic kidney disease (CKD) under the guidance of traditional Chinese medicine (TCM). Its main components are Astragalus membranaceus (Fisch.) Bunge, Codonopsis pilosula (Franch.) Nannf., Centella asiatica (L.) Urb., Salvia miltiorrhiza Bunge, Cuscuta chinensis Lam., and Rheum palmatum L.. Clinical studies have shown that it can relieve fatigue, nausea and other symptoms and improve kidney function in patients; however, its specific mechanism of action requires further study.

AIM OF THE STUDY

Renal interstitial fibrosis (RIF) is the ultimate characteristic manifestation of various CKD, that cannot be cured, and appropriate treatments to delay its progression require further exploration. GBXZD, widely used in clinical practice for RIF treatment, can effectively relieve the syndrome in patients with CKD. However, the specific mechanism of action of GBXZD in RIF is unknown and requires further study. This study aimed to explore the specific effects of GBXZD on RIF through the regulation of M1 macrophages.

MATERIALS AND METHODS

An in vivo RIF model was obtained through unilateral ureteral obstruction (UUO), and the Sprague-Dawley (SD) rats were randomly divided into sham operation, UUO, UUO + GBXZD-low dose (GBXZD-L) and UUO + GBXZD-high dose (GBXZD-H) groups. Pathological changes in rat kidney specimens were observed using hematoxylin and eosin (HE) and Masson staining. The expression of collagen I (COL I), fibronectin (FN), α-smooth muscle actin (α-SMA), interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumour necrosis factor-α (TNF-α) was detected using immunohistochemistry, and immunofluorescence was used to detect the expression of CD86 and inducible nitric-oxide synthase (iNOS) in kidney tissue. An in vitro experiment was performed using M1 polarization model in RAW264.7 macrophages induced by lipopolysaccharide (LPS). Cells were divided into control, LPS, LPS + GBXZD-low dose (GBXZD-L) and LPS + GBXZD-high dose (GBXZD-H) groups. The changes in expression of CD86, iNOS, IL-1β, IL-6, and TNF-α were measured using western blotting, flow cytometry, immunofluorescence and enzyme-linked immunosorbent assay (ELISA). We analyzed the action pathway of GBXZD in regulating M1 polarization of macrophages using antibody microarray and verified the results using western blotting.

RESULTS

Histopathological results showed that the UUO group exhibited significant fibrotic injury compared to the sham group. After GBXZD treatment, the degree of kidney injury, RIF, and inflammatory factor expression were lower than those in the UUO group. Compared with LPS-treated cells, the expression of the M1 markers CD86, iNOS, and pathway proteins Raf1 and p-Elk1 was down-regulated in RAW 264.7 cells treated with LPS and GBXZD. The secretion of the inflammatory factors IL-1β, IL-6, and TNF-α in the LPS group was more than that in the control group. However, the levels of these factors were significantly reduced in the GBXZD-H group compared to those in the LPS group.

CONCLUSIONS

This study indicates that GBXZD ameliorates RIF and inhibits the inflammatory response and macrophage M1 polarization by a potential mechanism related to the downregulation of Raf1 and p-Elk1. GBXZD therefore has therapeutic potential value for patients with CKD.

Functioning of Long Noncoding RNAs Expressed in Macrophage in the Development of Atherosclerosis

Chronic inflammation is part of the pathological process during atherosclerosis (AS). Due to the abundance of monocytes/macrophages within the arterial plaque, monocytes/macrophages have become a critical cellular target in AS studies. In recent decades, a number of long noncoding RNAs (lncRNAs) have been found to exert regulatory roles on the macrophage metabolism and macrophage plasticity, consequently promoting or suppressing atherosclerotic inflammation. In this review, we provide a comprehensive overview of lncRNAs in macrophage biology, highlighting the potential role of lncRNAs in AS based on recent findings, with the aim to identify disease biomarkers and future therapeutic interventions for AS.

Extracellular Nucleotides and P2 Receptors in Renal Function

The understanding of the nucleotide/P2 receptor system in the regulation of renal hemodynamics and transport function has grown exponentially over the last 20 yr. This review attempts to integrate the available data while also identifying areas of missing information. First, the determinants of nucleotide concentrations in the interstitial and tubular fluids of the kidney are described, including mechanisms of cellular release of nucleotides and their extracellular breakdown. Then the renal cell membrane expression of P2X and P2Y receptors is discussed in the context of their effects on renal vascular and tubular functions. Attention is paid to effects on the cortical vasculature and intraglomerular structures, autoregulation of renal blood flow, tubuloglomerular feedback, and the control of medullary blood flow. The role of the nucleotide/P2 receptor system in the autocrine/paracrine regulation of sodium and fluid transport in the tubular and collecting duct system is outlined together with its role in integrative sodium and fluid homeostasis and blood pressure control. The final section summarizes the rapidly growing evidence indicating a prominent role of the extracellular nucleotide/P2 receptor system in the pathophysiology of the kidney and aims to identify potential therapeutic opportunities, including hypertension, lithium-induced nephropathy, polycystic kidney disease, and kidney inflammation. We are only beginning to unravel the distinct physiological and pathophysiological influences of the extracellular nucleotide/P2 receptor system and the associated therapeutic perspectives.

LncRNA uc.48+ is involved in the diabetic immune and inflammatory responses mediated by P2X7 receptor in RAW264.7 macrophages

High glucose combined with high FFAs can contribute to the unfavorable development of type 2 diabetes mellitus (T2DM) and monocytes/macrophages are important in the occurrence and development of T2DM, which is regarded as a type of low‑grade inflammation. Although our previous study demonstrated that increased expression of P2X7 receptor (P2X7R) in peripheral blood monocytes may alter the innate immune system and that long non‑coding (lnc)RNA uc.48+ was involved in diabetic neuropathic pain, the involvement of uc.48+ mediated by the P2X7R in monocyte/macrophages during T2DM has not been reported. In the present study, the effectsof uc.48+ small interference RNA (siRNA) on factors, including the mRNA and protein expression of P2X7R, apoptosis and proliferation, levels of reactive oxygen species (ROS), cytokine levels, and expression of phosphorylated (p‑) extracellular signal‑regulated kinase (ERK)1/2, were examined in RAW264.7 macrophages following exposure to high glucose and high plasma free fatty acids (FFAs). After RAW264.7 cells were transfected with uc.48+ siRNA under high glucose conditions and FFAs treatment, the mRNA expression levels of uc.48+ and P2X7 receptor were detected by reverse transcription‑polymerase chain reaction. The protein mass of P2X7 receptor and ERK signaling pathway were assessed by western blotting. ROS and calcium concentrations, and culture supernatant cytokine content [tumor necrosis factor‑α, interleukin (IL)‑10, IL‑1β] were detected by fluorescent probes and ELISA respectively. Cell viability and apoptosis were determined by MTS test and flow cytometry, respectively. It was found that treatment of RAW264.7 cells with high glucose and FFAs, which exhibited increased expression of uc.48+, evoked P2X7R‑mediated immune and inflammatory responses through several means, including cytokine secretion, ROS formation, and activation of the ERK signaling pathway. The uc.48+ siRNA regulated these factors and thus influenced the course and outcome of the immune and inflammatory responses mediated by P2X7R.

Long non‑coding RNA BC168687 small interfering RNA reduces high glucose and high free fatty acid‑induced expression of P2X7 receptors in satellite glial cells

Purinergic signaling contributes to inflammatory and immune responses. The activation of the P2X purinoceptor 7 (P2X₇) in satellite glial cells (SGCs) may be an essential component in the promotion of inflammation and neuropathic pain. Long non-coding RNAs (lncRNAs) are involved in multiple physiological and pathological processes. The aim of the present study was to investigate the effects of a small interfering RNA for the lncRNA BC168687 on SGC P2X₇ expression in a high glucose and high free fatty acids (HGHF) environment. It was demonstrated that BC168687 small interfering (si)RNA downregulated the co-expression of the P2X₇ and glial fibrillary acidic protein and P2X₇ mRNA expression. Additionally, HGHF may activate the mitogen-activated protein kinase signaling pathway by increasing the release of nitric oxide and reactive oxygen species in SGCs. Taken together, these results indicate that silencing BC168687 expression may downregulate the increased expression of P2X₇ receptors in SGCs induced by a HGHF environment.

The role of microglial P2X7: modulation of cell death and cytokine release

Background

ATP-gated P2X7 is a non-selective cation channel, which participates in a wide range of cellular functions as well as pathophysiological processes including neuropathic pain, immune response, and neuroinflammation. Despite its abundant expression in microglia, the role of P2X7 in neuroinflammation still remains unclear.

Methods

Primary microglia were isolated from cortices of P0-2 C57BL/6 wild-type or P2X7 knockout (P2X7^−/−) mouse pups. Lipopolysaccharide, lipopolysaccharide plus IFNγ, or IL4 plus IL13 were used to polarize microglia to pro-inflammatory or anti-inflammatory states. P2rx7 expression level in resting or activated mouse and human microglia was measured by RNA-sequencing and quantitative real-time PCR. Microglial cell death was measured by cell counting kit-8 and immunocytochemistry, and microglial secretion in wild-type or P2X7^−/− microglia was examined by Luminex multiplex assay or ELISA using P2X7 agonist BzATP or P2X7 antagonist A-804598. P2X7 signaling was analyzed by Western blot.

Results

First, we confirmed that P2rx7 is constitutively expressed in mouse and human primary microglia. Moreover, P2rx7 mRNA level was downregulated in mouse microglia under both pro- and anti-inflammatory conditions. Second, P2X7 agonist BzATP caused cell death of mouse microglia, while this effect was suppressed either by P2X7 knockout or by A-804598 under both basal and pro-inflammatory conditions, which suggests the mediating role of P2X7 in BzATP-induced microglial cell death. Third, BzATP-induced release of IL1 family cytokines including IL1α, IL1β, and IL18 was blocked in P2X7^−/− microglia or by A-804598 in pro-inflammatory microglia, while the release of other cytokines/chemokines was independent of P2X7 activation. These findings support the specific role of P2X7 in IL1 family cytokine release. Finally, P2X7 activation was discovered to be linked to AKT and ERK pathways, which may be the underlying mechanism of P2X7 functions in microglia.

Conclusions

These results reveal that P2X7 mediates BzATP-induced microglial cell death and specific release of IL1 family cytokines, indicating the important role of P2X7 in neuroinflammation and implying the potential of targeting P2X7 for the treatment of neuroinflammatory disorders.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0904-8) contains supplementary material, which is available to authorized users.

C/EBPβ LIP augments cell death by inducing osteoglycin

Many types of tumor cell are devoid of the extracellular matrix proteoglycan osteoglycin (Ogn), but its role in tumor biology is poorly studied. Here we show that RNAi of Ogn attenuates stress-triggered cell death, whereas its overexpression increases cell death. We found that the transcription factor C/EBPβ regulates the expression of Ogn. C/EBPβ is expressed as a full-length, active form (LAP) and as a truncated, dominant-negative form (LIP), and the LIP/LAP ratio is positively correlated with the extent of cell death under stress. For example, we reported that drug-resistant tumor cells lack LIP altogether, and its supplementation abolished their resistance to chemotherapy and to endoplasmic reticulum (ER) stress. Here we further show that elevated LIP/LAP ratio robustly increased Ogn expression and cell death under stress by modulating the mitogen-activated protein kinase/activator protein 1 pathway (MAPK/AP-1). Our findings suggest that LIP deficiency renders tumor cell resistant to ER stress by preventing the induction of Ogn.

Rhein Inhibits Autophagy in Rat Renal Tubular Cells by Regulation of AMPK/mTOR Signaling

Rhubarb and its bioactive component rhein are frequently used for the treatment of chronic kidney diseases (CKD) in eastern Asia countries. However, the potential therapeutic mechanism remains unclear. Autophagy plays an important role in CKD. However, there were some important related issues that remained unresolved in the role of autophagy in CKD and treatment by rhubarb and rhein. We designed a number of experiments to examine whether rhubarb may reduce renal fibrosis and autophagy in rats with adenine (Ade)-induced renal tubular injury, and whether rhein could affect autophagic pathways in rat renal tubular cells. We found that, autophagic activation accompanied with renal fibrosis in rats with Ade-induced renal tubular injury, and both autophagy and renal fibrosis were attenuated by rhubarb. In addition, we observed that rhein could inhibit autophagy through regulating the key molecules in the AMPK-dependent mTOR signaling pathways, as well as the Erk and p38 MAPKs signaling pathways. These findings may partly explain the therapeutic mechanisms of rhubarb and rhein in treating CKD patients in clinic, and further suggest that targeting autophagy and related signaling pathways may provide new strategies for the treatment of renal fibrosis in CKD.

Long noncoding NONRATT021972 siRNA normalized abnormal sympathetic activity mediated by the upregulation of P2X7 receptor in superior cervical ganglia after myocardial ischemia

Previous studies showed that the upregulation of the P2X7 receptor in cervical sympathetic ganglia was involved in myocardial ischemic (MI) injury. The dysregulated expression of long noncoding RNAs (lncRNAs) participates in the onset and progression of many pathological conditions. The aim of this study was to investigate the effects of a small interfering RNA (siRNA) against the NONRATT021972 lncRNA on the abnormal changes of cardiac function mediated by the up-regulation of the P2X7 receptor in the superior cervical ganglia (SCG) after myocardial ischemia. When the MI rats were treated with NONRATT021972 siRNA, their increased systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), low-frequency (LF) power, and LF/HF ratio were reduced to normal levels. However, the decreased high-frequency (HF) power was increased. GAP43 and tyrosine hydroxylase (TH) are markers of nerve sprouting and sympathetic nerve fibers, respectively. We found that the TH/GAP43 value was significantly increased in the MI group. However, it was reduced after the MI rats were treated with NONRATT021972 siRNA. The serum norepinephrine (NE) and epinephrine (EPI) concentrations were decreased in the MI rats that were treated with NONRATT021972 siRNA. Meanwhile, the increased P2X7 mRNA and protein levels and the increased p-ERK1/2 expression in the SCG were also reduced. NONRATT021972 siRNA treatment inhibited the P2X7 agonist BzATP-activated currents in HEK293 cells transfected with pEGFP-P2X7. Our findings suggest that NONRATT021972 siRNA could decrease the upregulation of the P2X7 receptor and improve the abnormal changes in cardiac function after myocardial ischemia.

LncRNA uc.48+ siRNA improved diabetic sympathetic neuropathy in type 2 diabetic rats mediated by P2X7 receptor in SCG

Diabetic autonomic neuropathy includes the sympathetic ganglionic dysfunction. P2X7 receptor in superior cervical ganglia (SCG) participated in the pathological changes of cardiac dysfunction. Abnormal expression of long noncoding RNAs (lncRNAs) was reported to be involved in nervous system diseases. Our preliminary results obtained from rat lncRNA array profiling revealed that the expression of the uc.48+ was significantly increased in the rat SCG in response to diabetic sympathetic pathology. In this study, we found that lncRNAuc.48+ and P2X7 receptor in the SCG were increased in type 2 diabetic rats and were associated with the cardiac dysfunction. The uc.48+ small interference RNA (siRNA) improved the cardiac autonomic dysfunction and decreased the up-regulation P2X7 and the ratio of phosphorylated extracellular regulated protein kinases1/2 (p-ERK1/2) to ERK1/2 in SCG of type 2 diabetic rats. In conclusion, lncRNA uc.48+ siRNA improved diabetic sympathetic neuropathy in type 2 diabetic rats through regulating the expression of P2X7 and ERK signaling in SCG.

Vascular and inflammatory actions of P2X receptors in renal injury

P2 purinergic receptors are activated by extracellular ATP and subserve a plethora of roles in the body, including metabolism, inflammation and neuronal signalling. This review focuses on renal purinergic receptors and how different roles that they play may contribute to renal dysfunction and the progression of chronic kidney disease. Numerous studies have linked P2 receptors, particularly the P2X4R and P2X7R subtypes, to kidney injury and damage. However, the mechanisms underlying this association are not fully defined. Several studies show that activation of P2X4R and particularly P2X7R can have a pro-inflammatory effect, causing or exacerbating damage to renal tissue. However, clinical trials aiming to utilise P2X7R antagonists to treat inflammatory disease have been unsuccessful, and it is possible that other mechanisms besides inflammation tie P2X7R activation to disease progression. In this context, purinergic signalling is also involved in the control of vascular tone and our recent studies suggest that activation of P2X4R/P2X7R causes renal vascular dysfunction and contributes to chronic kidney disease. This brief review aims to summarise the complementary inflammatory and vascular roles of P2X receptors in the kidney, with emphasis on the subtypes P2X4R and P27XR, and how each contributes to and presents therapeutic targets in the progression of chronic kidney disease.

P2X7 receptor inhibition protects against ischemic acute kidney injury in mice

Activation of the purinergic P2X7 receptor (P2X7R) has been associated with the development of experimental nephritis and diabetic and hypertensive nephropathy. However, its role in acute kidney injury (AKI) remains unknown. In this study, we examined the effects of P2X7R inhibition in a murine model of ischemia-reperfusion (I/R)-induced AKI using A438079, a selective inhibitor of P2X7R. At 24 h after I/R, mice developed renal dysfunction and renal tubular damage, which was accompanied by elevated expression of P2X7R. Early administration of A438079 immediately or 6 h after the onset of reperfusion protected against renal dysfunction and attenuated kidney damage whereas delayed administration of A438079 at 24 h after restoration of perfusion had no protective effects. The protective actions of A438079 were associated with inhibition of renal tubule injury and cell death and suppression of renal expression of monocyte chemotactic protein-1 and regulated upon expression normal T cell expressed and secreted (RANTES). Moreover, I/R injury led to an increase in phosphorylation (activation) of extracellular signal-regulated kinases 1/2 in the kidney; treatment with A438079 diminished this response. Collectively, these results indicate that early P2X7R inhibition is effective against renal tubule injury and proinflammatory response after I/R injury and suggest that targeting P2X7R may be a promising therapeutic strategy for treatment of AKI.

Renal P2 receptors and hypertension

The regulation of extracellular fluid volume is a key component of blood pressure homeostasis. Long-term blood pressure is stabilized by the acute pressure natriuresis response by which changes in renal perfusion pressure evoke corresponding changes in renal sodium excretion. A wealth of experimental evidence suggests that a defect in the pressure natriuresis response contributes to the development and maintenance of hypertension. The mechanisms underlying the relationship between renal perfusion pressure and sodium excretion are incompletely understood. Increased blood flow through the vasa recta increases renal interstitial hydrostatic pressure, thereby reducing the driving force for transepithelial sodium reabsorption. Paracrine signalling also contributes to the overall natriuretic response by inhibiting tubular sodium reabsorption in several nephron segments. In this brief review, we discuss the role of purinergic signalling in the renal control of blood pressure. ATP is released from renal tubule and vascular cells in response to increased flow and can activate P2 receptor subtypes expressed in both epithelial and vascular endothelial/smooth muscle cells. In concert, these effects integrate the vascular and tubular responses to increased perfusion pressure and targeting P2 receptors, particularly P2X7, may prove beneficial for treatment of hypertension.

The dark side of extracellular ATP in kidney diseases

Intracellular ATP is the most vital source of cellular energy for biologic systems, whereas extracellular ATP is a multifaceted mediator of several cell functions via its interaction, in an autocrine or paracrine manner, with P2 purinergic receptors expressed on the cell surface. These ionotropic and metabotropic P2 purinergic receptors modulate a variety of physiologic events upon the maintenance of a highly sensitive "set point," the derangement of which may lead to the development of key pathogenic mechanisms during acute and chronic diseases. Growing evidence suggests that extracellular ATP signaling via P2 purinergic receptors may be involved in different renal pathologic conditions. For these reasons, investigators and pharmaceutical companies are actively exploring novel strategies to antagonize or block these receptors with the goal of reducing extracellular ATP production or accelerating extracellular ATP clearance. Targeting extracellular ATP signaling, particularly through the P2X7 receptor, has considerable translational potential, given that novel P2X7-receptor inhibitors are already available for clinical use (e.g., CE224,535, AZD9056, and GSK1482160). This review summarizes the current evidence regarding the involvement of extracellular ATP and its P2 purinergic receptor-mediated signaling in physiologic and pathologic processes in the kidney; potential therapeutic options targeting extracellular ATP purinergic receptors are analyzed as well.

Purinergic signalling in the kidney in health and disease

The involvement of purinergic signalling in kidney physiology and pathophysiology is rapidly gaining recognition and this is a comprehensive review of early and recent publications in the field. Purinergic signalling involvement is described in several important intrarenal regulatory mechanisms, including tuboglomerular feedback, the autoregulatory response of the glomerular and extraglomerular microcirculation and the control of renin release. Furthermore, purinergic signalling influences water and electrolyte transport in all segments of the renal tubule. Reports about purine- and pyrimidine-mediated actions in diseases of the kidney, including polycystic kidney disease, nephritis, diabetes, hypertension and nephrotoxicant injury are covered and possible purinergic therapeutic strategies discussed.

Sensory-sympathetic coupling in superior cervical ganglia after myocardial ischemic injury facilitates sympathoexcitatory action via P2X7 receptor

P2X receptors participate in cardiovascular regulation and disease. After myocardial ischemic injury, sensory-sympathetic coupling between rat cervical DRG nerves and superior cervical ganglia (SCG) facilitated sympathoexcitatory action via P2X7 receptor. The results showed that after myocardial ischemic injury, the systolic blood pressure, heart rate, serum cardiac enzymes, IL-6, and TNF-α were increased, while the levels of P2X7 mRNA and protein in SCG were also upregulated. However, these alterations diminished after treatment of myocardial ischemic (MI) rats with the P2X7 antagonist oxATP. After siRNA P2X7 in MI rats, the systolic blood pressure, heart rate, serum cardiac enzymes, the expression levels of the satellite glial cell (SGC) or P2X7 were significantly lower than those in MI group. The phosphorylation of ERK 1/2 in SCG participated in the molecular mechanism of the sympathoexcitatory action induced by the myocardial ischemic injury. Retrograde tracing test revealed the sprouting of CGRP or SP sensory nerves (the markers of sensory afferent fibers) from DRG to SCG neurons. The upregulated P2X7 receptor promoted the activation of SGCs in SCG, resulting in the formation of sensory-sympathetic coupling which facilitated the sympathoexcitatory action. P2X7 antagonist oxATP could inhibit the activation of SGCs and interrupt the formation of sensory-sympathetic coupling in SCG after the myocardial ischemic injury. Our findings may benefit the treatment of coronary heart disease and other cardiovascular diseases.

Necrotic renal epithelial cell inhibits renal interstitial fibroblast activation: role of protein tyrosine phosphatase 1B

Our recent studies showed that contents of necrotic renal proximal tubular cells (RPTC) from 2 × 10(6) cells/ml directly induced death of cultured renal interstitial fibroblasts. However, it remains unknown whether nonlethal number of necrotic RPTC would also alter the fate of renal interstitial fibroblasts. To address this issue, renal interstitial fibroblasts (NRK-49F) were exposed to necrotic RPTC supernatant (RPTC-Sup) obtained from 2 × 10(4) to 5 × 10(5) cells/ml. These concentrations of RPTC did not induce cell death, but led to inactivation of renal fibroblasts as indicated by reduced expression of α-smooth muscle actin and fibronectin, two hallmarks of activated fibroblasts. Concurrently, the same doses of necrotic RPTC-Sup suppressed phosphorylation of epidermal growth factor receptor (EGFR) and signal transducers and activators of transcription-3 (STAT3) in a time- and dose-dependent manner, but did not affect phosphorylation of platelet-derived growth factor receptor-β, AKT, and extracellular signal-regulated kinase 1/2. The presence of sodium orthovanadate, a general protein tyrosine phosphatase (PTP) inhibitor or TCS-401 (a selective PTP1B inhibitor), abrogated those effects of RPTC-Sup, whereas coincubation with the EGFR inhibitor (Gefitinib) or silencing of EGFR with siRNA preserved the ability of RPTC-Sup in suppressing renal fibroblast activation and STAT3 phosphorylation. Moreover, RPTC-Sup treatment induced PTP1B phosphorylation and its interaction with EGFR. Collectively, these results indicate that nonlethal necrotic RPTC-Sup can induce inactivation of renal interstitial fibroblasts, which occurs through a mechanism involved in PTP1B-mediated inhibition of EGFR signaling.

Production of reactive oxygen species by multipotent stromal cells/mesenchymal stem cells upon exposure to fas ligand

Multipotent stromal cells (MSCs) can be differentiated into osteoblasts and chondrocytes, making these cells candidates to regenerate cranio-facial injuries and lesions in long bones. A major problem with cell replacement therapy, however, is the loss of transplanted MSCs at the site of graft. Reactive oxygen species (ROS) and nonspecific inflammation generated at the ischemic site have been hypothesized to lead to MSCs loss; studies in vitro show MSCs dying both in the presence of ROS or cytokines like FasL. We questioned whether MSCs themselves may be the source of these death inducers, specifically whether MSCs produce ROS under cytokine challenge. On treating MSCs with FasL, we observed increased ROS production within 2 h, leading to apoptotic death after 6 h of exposure to the cytokine. N-acetyl cysteine, an antioxidant, is able to protect MSCs from FasL-induced ROS production and subsequent ROS-dependent apoptosis, though the MSCs eventually succumb to ROS-independent death signaling. Epidermal growth factor (EGF), a cell survival factor, is able to protect cells from FasL-induced ROS production initially; however, the protective effect wanes with continued FasL exposure. In parallel, FasL induces upregulation of the uncoupling protein UCP2, the main uncoupling protein in MSCs, which is not abrogated by EGF; however, the production of ROS is followed by a delayed apoptotic cell death despite moderation by UCP2. FasL-induced ROS activates the stress-induced MAPK pathways JNK and p38MAPK as well as ERK, along with the activation of Bad, a proapoptotic protein, and suppression of survivin, an antiapoptotic protein; the latter two key modulators of the mitochondrial death pathway. FasL by itself also activates its canonical extrinsic death pathway noted by a time-dependent degradation of c-FLIP and activation of caspase 8. These data suggest that MSCs participate in their own demise due to nonspecific inflammation, holding implications for replacement therapies.

Autophagy protects against necrotic renal epithelial cell-induced death of renal interstitial fibroblasts

We recently reported that necrotic renal proximal tubular cells (RPTC) can induce the death of renal interstitial fibroblasts. Since autophagy plays either cytoprotective or cytodestructive roles depending on the experimental condition, the present study was carried out to investigate whether necrotic RPTC would induce autophagy of renal interstitial fibroblasts and, if so, whether autophagy would contribute to cell death or exert a protective effect. Exposure of necrotic RPTC supernatant (RPTC-Sup) induced autophagy in renal interstitial fibroblast cells (NRK-49F) in a time- and dose-dependent manner, and its induction was earlier than caspase-3 activation. Inhibition of autophagy with 3-methyladenine (3-MA) or knockdown of Beclin-1, a molecule involved in the initiation of autophagosome formation, with small interference RNA (siRNA) significantly enhanced necrotic RPTC-Sup-induced cell death. Necrotic RPTC-Sup induced phosphorylation of extracellular signal-regulated kinases (ERK1/2), p38, c-Jun NH(2)-terminal kinases (JNKs), and AKT. Treatment with an ERK1/2 pathway inhibitor, but not with specific inhibitors for p38, JNKs, or AKT pathways, blocked NRK-49F autophagy and cell death upon exposure to necrotic RPTC-Sup. Furthermore, knockdown of MEK1 with siRNA also reduced autophagy along with cell death in NRK-49F exposed to necrotic RPTC-Sup. In contrast, overexpression of MEK1/2 increased RPTC-Sup-induced fibroblast cell death without enhancing autophagy. Collectively, this study demonstrates that necrotic RPTC induce both autophagy and cell death and that autophagy plays a cytoprotective or prosurvival role in renal fibroblasts. Furthermore, necrotic RPTC-induced autophagy and cell death in renal fibroblasts is mediated by the activation of the MEK1-ERK1/2 signaling pathway.