In vitro silencing of the insulin receptor attenuates cellular accumulation of fibronectin in renal mesangial cells
© Yano et al.; licensee BioMed Central Ltd. 2012
Received: 16 July 2012
Accepted: 5 October 2012
Published: 12 October 2012
Insulin receptor (InsR) and insulin signaling proteins are widely distributed throughout the kidney cortex. Insulin signaling can act in the kidney in multiple ways, some of which may be totally independent of its primary role of the maintenance of whole-body glucose homeostasis. However, descriptions of the insulin signaling in renal glomerular mesangial cells (MCs) are quite limited and the roles of insulin signaling in MC functions have not been sufficiently elucidated.
InsR silencing induced a unique phenotype of reduced fibronectin (FN) accumulation in renal glomerular MCs. Transcription level of FN was not significantly changed in the InsR silenced cells, suggesting the phenotype switching was caused by post-transcriptional modification. The decreased expression of InsR was associated with enhanced activity of insulin-like growth factor-1 receptor (IGF-1R)/PI3K/Akt signaling pathway which contributed in part to the attenuation of cellular FN accumulation. Formation of IGF-1R homodimer was increased in the InsR silenced cells. The InsR silenced cells also showed increased sensitivity to exogenous IGF-1, and increased PI3K activity was reversed significantly by incubating cells with IGF-1R specific antagonist, AG538. PI3K/Akt dependent activation of cAMP responsive element-binding protein (CREB)-1 induced expression of matrix metalloproteinase (MMP)-9 and suppressing MMP activity by doxycycline partially reversed FN accumulation in the InsR silenced cells.
The effects of InsR silencing on cellular FN accumulation in vitro are, at least partially, mediated by increased degradation of FN by MMPs which is induced by enhanced signaling sequence of IGF-1R/PI3K/Akt/CREB-1.
One of the common local pathologic changes of glomerulonephropathy is accumulation of extracellular matrix (ECM) components, including fibronectin (FN), which results in glomerulosclerosis. Although the mechanisms responsible for the ECM protein deposition are still inconclusive, the role of renal glomerular mesangial cells (MCs) in this sclerotic change has been gathering increasing attentions. Glomerular mesangium is an area which shows the most prominent ECM accumulations in diseased kidney. And the resulting glomerular fibrosis has been recognized as the major degenerative event in glomerulonephropathies regardless of their etiologies [1–3]. MCs are specialized pericytes located among the mesangium area within the renal corpuscle of the kidney . ECM protein deposition could be caused by matrix deposition exceeding matrix degradation. Many studies have focused on a possible imbalance of in situ synthesis and degradation of ECM proteins in glomeruli [5, 6]. Indeed, some animal experiments have shown that the accumulation of ECM proteins in focal segmental glomerulosclerosis is associated with increased ECM protein synthesis coupled with suppression of expression or activity of proteolytic enzymes, including cathepsins and matrix metalloproteinases (MMPs) [6–8]. Mesangial cells secrete MMPs that degrade intact glomerular basement membrane, gelatin, soluble type IV collagen and FN at neutral pH [5, 9]. MMPs and their specific inhibitors, tissue inhibitor of metalloproteinases (TIMPs), play an important role in regulating glomerular matrix remodeling [5, 9, 10]. Based on these findings, besides their three primary functions, that is, filtration, structural support, and phagocytosis, MCs have been postulated to be a key player for FN regulation in the kidney and is speculated as one of the major contributors to the sclerotic lesion in glomeruli .
The insulin receptor (InsR) and insulin signaling proteins are widely distributed throughout kidney cortex [12, 13]. Insulin signaling can act in the kidney in multiple ways, some of which may be totally independent of its primary role of the maintenance of whole-body glucose homeostasis. As for renal tissue, the roles for insulin signaling in tubular epithelial cells have been described extensively in previous studies. The signaling is clearly anti-natriuretic, affecting sodium reabsorption in the proximal tubule , thick ascending limb [15, 16], and collecting duct . In contrast, descriptions of insulin signaling in MCs are limited and the roles of insulin signaling in MC functions have not been sufficiently elucidated.
The InsR and IGF-1R are structurally related transmembrane glycoproteins with approximately 50% amino acid sequence identity . Post-translational processing results in dimerization and disulphide linkage of proreceptors. This is followed by proteolytic cleavage, which generates α and β subunits . Mature and functional receptors thus have the subunit composition of (αβ)2. The extracellular α subunit contains a ligand binding site and the transmembrane β subunit possesses tyrosine kinase activity. The distinct physiological functions of insulin and IGFs depend on differences in the distribution and/or signaling potential of their respective receptors. Despite of this, it has been shown that a proportion of InsR and IGF-1R assemble as hybrid structures containing an (αβ) half of the InsR disulphide-linked to an (α’β’) half of the IGF-1R [20–22]. These hybrid receptors are functional, in that they bind IGF-1 with high affinity and insulin with somewhat lower affinity [23, 24], and display IGF-1-induced autophosphorylation both in vitro and in situ[24, 25]. MCs express both InsR and IGF-1R [11, 26] and therefore have the potential to form functional hybrid receptors. However, the significance of hybrid receptors in MCs remains unclear.
Here, we explore the roles of InsR/IGF-1R signaling in cellular FN accumulation, using a SV40 immortalized mouse mesangial cell line, MES-13. With silencing InsR expression by shRNA, the cells showed significant reduction of cellular FN accumulation. The aim of the present work was to identify responsible alterations of the signaling pathway for the phenotype switching. We provide evidences that rearrangement in the balance between insulin and IGF-1 receptor homodimers and hybrid receptors contributed to the phenotype switching. We also demonstrate cAMP responsive element-binding protein (CREB-1) and MMP-9 as downstream factors for the attenuation of cellular FN accumulation.
InsR silencing in MES-13 cells
Cellular FN accumulation is attenuated in the InsR silenced cells
Activation of PI3K/Akt pathway and suppression of Ras/Erk1/2 in the InsR silenced cells
Attenuation of FN accumulation depends on PI3K/Akt signaling in the InsR silenced cells
PI3K/Akt activation depends on IGF-1R signaling in the InsR silenced cells
Next, we investigated sensitivity of the cells to exogenous IGF-1. The cells were stimulated with vehicle (H2O) or a recombinant IGF-1 (rIGF-1, 10ng/mL) for 5mintes, and the cell lysates were subject to in vitro immunoprecipitation lipid kinase assay to measure PI3K activity. The ratio of PI3K activity in rIGF-1 treated cells and vehicle treated cells were used as an indicator for cell sensitivity to IGF-1. In SC transfected cell, PI3K activity was significantly increased by rIGF-1 pretreatment (lanes 1, 3, Figure 5B). In InsR silenced cells, however, the increase in PI3K activity in response to rIGF-1 was even more significant (lanes 2, 4, Figure 5B). These results suggest an increase in sensitivity to IGF-1 in InsR silenced cells.
InsR silencing induces alteration in InsR/IGF-1R hybrid formation
CREB-1-mediated MMP-9 expression leads to increased degradation of cellular FN in the InsR silenced cells
Next, we incubated cells with doxycycline (10μg/mL), a broad-spectrum MMP inhibitor, for 72 hours, and then evaluated intracellular FN by Western blotting. As expected cellular accumulation of FN in InsR silenced cells was about 20% of that of SC transfected cells. Inhibition of MMP with doxycycline in InsR silenced cells effectively reversed the reduction in intracellular FN levels to about 70% of that of SC cells (Figure 6D). Taken together, these findings suggest that the attenuated cellular accumulation of FN in the InsR silenced cells was partially dependent on increased degradation by MMP which was induced by IGF-1R/PI3K/Akt/CREB-1 signaling pathways.
In cells expressing both InsR and IGF-1R, InsR hemireceptors may heterodimerize with IGF-1R hemireceptors, leading to the formation of hybrid receptors (HRs). Heterodimerization of the two receptors is due to the high degree of homology between InsR and IGF-1R, which ranges from 27 to 84% depending on the region that is compared [27, 29]. Heterodimerization is believed to occur with a similar efficiency as homodimerization. So the cellular proportions of homodimers and hybrids are supposed to be dependent on the expression ratios of each hemidimers [21–24, 28, 30]. Although, the physiological role of HRs is still unclear, early studies carried out with affinity chromatography-purified HRs indicated that these receptors mostly bind IGF-1 and that they bind insulin with much lower affinity [24, 31].
Findings in the present study suggested that silencing InsR caused increased formation of IGF-1R homodimers in MCs. It’s possible that the change in the balance of the homodimers and hybrid receptors induced remarkable phenotypic changes of the attenuated cellular FN accumulation. Regardless of etiology, most end-stage glomerular diseases are characterized by accumulation of ECM proteins, including FN, in mesangium and other areas in glomeruli [1–3]. Over accumulation of the ECMs leads to sclerotic non-functional status of the kidney. So controlling the local ECM accumulation is believed to be one of the key therapeutic targets to prevent the development and progression of glomerular diseases. In spite of the extensive studies in the last decades, the mechanisms underlying this abnormal accumulation of ECM have not been fully elucidated. Our in vitro data suggested that alteration in the balance between InsR and IGF-1R and the resultant changes in PI3K/Akt signaling pathway contributed to the attenuation of cellular FN accumulation in MCs.
Expression of InsR is influenced by multiple factors. Estrogen showed inhibitory effect on Ir promoter activity . In accordance with the negative effect of the estrogen, monocyte InsR content is higher during the luteal phases in adult females. This elevation is abolished by use of oral contraceptives or pregnancy . Moreover, glucocorticoids and thyrotropin have been reported to enhance InsR expression while insulin down-regulates its cognate ligand [34–37]. Finally, nutrition and exercise have been reported to influence IR expression . Various chronic glomerulopathy prone conditions such as poorly controlled diabetes, malignant tumors, systemic autoimmune diseases, pregnancy are inclinable to be accompanied endocrinological/metabolic disturbance which may affect expression of InsR or IGF-1R [39–42]. In these patients, appropriate control of the metabolic disturbance may potentially contribute to prevent the renal complications by correcting the InsR/IGF-1R balances.
IGF-1 has been shown to regulate protein synthesis in renal proximal tubular epithelial cells by activating both PI3K and Erk pathways . Whereas, in the present study, InsR silencing caused increased PI3K activity and decreased Erk1/2 pathway activity. The reason for these discrepant findings has not been fully investigated but can be explained for the following: 1) we used renal glomerular MCs, which are specialized pericytes, whereas others used renal proximal tubular epithelial cells; 2) In our model, we showed silencing of InsR is associated with reduced Erk1/2 and increased PI3K activity. The latter can be blocked with IGF-1R inhibition. Hence, our experimental model is different from others that reported acute treatment with IGF-1 activates both PI3K and Erk1/2; 3) Altered InsR/IGF-1R balance may also be a factor for this characteristic phenotype of the InsR silenced MCs. Elucidating further details in the mechanisms will benefit understanding significance of InsR/IGF-1R balance in the fibrotie phenotype switching in MCs.
The ECM accumulation can result from either increased synthesis or decreased degradation of ECM components or both. MMPs are one of the main contributors to degrade ECMs. Quite a few numbers of mechanisms for regulation of MMP activities and expressions in various tissues have been reported and our study is the first to describe the involvement of InsR/IGF-1R signaling in MMP expression. We also showed CREB-1 as a specific transcription factor to regulate MMP expression in our model. Based on these findings, we proposed a novel signaling model for regulation of cellular FN accumulation. In this model, silencing InsR promotes increased formation of IGF-1R homodimer which alters the activation status of downstream kinases. Enhanced activity of PI3K/Akt induces activation of CREB-1 and results in increased MMP expression to promote FN degradation. Since the MES-13 cells synthesize IGF-1 ( Additional file 1) and since sensitivity to IGF-1 is increase in the InsR silenced MCs, in our in vitro model these endogenously produced IGF-1 possibly contributes to the observed changes in signaling pathways in an autocrine manner.
In summary, we have identified the phenotype of reduced cellular accumulation of FN in InsR silenced MCs. We explored the mechanisms underlying this unique phenotype. Our data suggest that the altered balance in the formation of InsR/IGF-1R homodimer and hybrid receptor is a crucial factor in the phenotype switching in MCs. We also demonstrated that resultant change in PI3K/Akt signaling pathway was involved in the induction of this phenotype. We further established that PI3K/Akt induced CREB-1 activation lead to enhanced MMP-9 expression which was likely involved in reduced FN accumulation by enhancing degradation of FN. These findings provide important information for largely unknown mechanisms in InsR/IGF-1R mediated myofibroblast transdifferentiation (MFT) of MCs in glomerulopathies. Elucidating further details in this signaling pathway will benefit future choices of treatment for glomerulosclerosis.
Materials and methods
MES-13 mouse mesangial cells (ATCC #CRL-1927) were grown in a 3:1 mixture of Dulbecco's modified Eagle's and Ham's F-12 media (Invitrogen) supplemented with 5% (v/v) fetal bovine serum containing 50 units/ml penicillin G and 50 μg/ml streptomycin in a humidified atmosphere containing 5% CO2 at 37°C. Cells were first grown up to 90% confluence and synchronized overnight in serum-free medium prior to treatment. In some experiments cells were treated with recombinant IGF-1 (Invitrogen) or AG538 (EMD Chemical), an IGF-1R selective inhibitor, for the indicated durations.
PI3K activity in cell lysate was determined with in vitro mmunoprecipitation lipid kinase assay as described previously . Briefly, cell lysates (0.5 mg) were immunoprecipitated with anti-phosphotyrosine (anti-pY) antibody (Millipore), and L-α-phosphoinositide (Avanti Polar Lipids) was used as the lipid substrate (2 μg/reaction). After incubation, the final extracted reaction mixtures were spotted onto silica gel-coated TLC plates (Whatman) and run in TLC buffer (65% n-propanol, 0.54 M acetic acid).
Antibody against fibronectin was purchased from Millipore. Antibodies against InsRα, InsRβ, IGF-1Rα and phospholylated or total CREB-1 were purchased from Santa Cruz Biotechnology. Antibodies against phosphorylated or total Akt, phosphorylated or total Erk1/2 and phosphorylated or total p70S6, IGF-1Rβ and MMP-9 were purchased from Cell Signaling Technology.
For immunoprecipitation, protein lysates (0.5mg) were incubated with an antibody specific for InsRα or IGF-1Rα at 4°C for 4 hours with continuous rotation. After the incubation, a 20-μl packed volume of protein G-Sepharose (GE-Health Care) was added to the lysates and incubated for another 4 hours at 4°C. After washing, 45 μl of 1× Laemmli sample buffer was added to the beads. The sample was heated in boiling water for 5 minutes and quenched on ice for 2 minutes. After vortex and centrifuge, 20 μl of the supernatant was resolved on a 7.5% SDS-PAGE gel and immunoblotted with appropriate antibody.
Pull down assay for Ras activity
Cells were lysed in Mg2+ IP buffer (25mM HEPES, pH7.7, 150mM NaCl, 1% Igepal CA-630, 10mM MgCl2, 1mM EDTA and 2% glycerol). Cleared cell lysates were normalized with the BCA assay (Santa Cruz Biotechnology). Normalized cell lysates were incubated with Raf-1 RBD preconjugated agarose (Millipore) for 2 hours at 4°C. Pelleted agarose were washed three times with IP buffer. Protein bound to the beads was eluted with 1xLaemmli’s buffer and subject to 12% SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Ras was detected with a specific monoclonal antibody (clone Ras10, Millipore).
In vitro IP Akt activity assay
Cells were serum-starved for 16 hours, and then washed twice with phosphate-buffered saline and lysed in ice-cold lysis buffer (20mM Tris, pH 7.4, 150mM NaCl, 1% Triton X-100, 1mM EDTA, 1mM EGTA, 2.5mM sodium pyrophosphate, 1mM β-glycerolphosphate, 1mM sodium orthovanadate, 1μg/ml leupeptin, and 1mM phenylmethylsulfonyl fluoride). The extracts were centrifuged to remove cellular debris, and normalized by BCA assay (Santa Cruz). 250μg of protein from the lysate samples was incubated with gentle rocking at 4°C overnight with immobilized anti-phospho-Akt (Ser473) antibody cross-linked to sepharose beads (Cell Signaling Technology). After phosphorylated Akt was selectively immunoprecipitated from the cell lysates, the immunoprecipitated products were washed twice in lysis buffer and twice in kinase assay buffer (25mM Tris, pH 7.5, 10mM MgCl2, 50mM β-glycerolphosphate, 0.1mM sodium orthovanadate, and 2mM dithiothreitol), and the samples were resuspended in 40μl of kinase assay buffer containing 200μM ATP and 1μg of GSK-3α fusion protein. The kinase reaction was allowed to proceed at 30°C for 30 minutes and stopped by the addition of Laemmli SDS sample buffer. Reaction products were resolved by 15% SDS-PAGE followed by Western blotting with an anti-phospho-GSK-3α/β antibody (Cell Signaling Technology).
A vector expressing shRNA against InsR was purchased from Santa Cruz Biotechnology. A vector expressing shRNA against CREB1 was purchased from SABioscience. A vector expressing dominant negative Akt1 was engineered by inserting a K179M mutant dominant negative Akt1 cDNA (Millipore) into a multiple cloning site of a eukaryotic expression vector, pUSEamp(+) (Millipore). A 529F mutant constitutively activated H-Ras cDNA (Millipore) was processed similarly to engineer a vector expressing constitutively activated H-Ras. Stable transfection of the constructs in MES-13 cells was performed using Lipofectamine 2000 (Invitrogen) according to the manufacturer's instructions. Individual single cells were isolated and screened for neomycin or puromycin resistance. Phenotypes of the transfected cells were examined by Western blotting, IP Akt assay or Ras pull-down assay (Figure 1, Additional files 2, 3, 4).
Total RNA was isolated from cells using TRIzol reagent (Invitrogen). Reverse transcription was carried out on total RNA using SuperScript III (Invitrogen) followed by PCR using gene specific primers ( Additional file 5).
Statistical significance of the difference among groups was analyzed by the paired Student’s t test or parametric ANOVA and Ryan’s multiple comparison test using Microsoft Exel (Microsoft) and ANOVA4 on the Web (http://www.hju.ac.jp/~kiriki/anova4/). All data were represented as the mean ± SEM of four different experiments unless otherwise noted in the figure legends. A probability of p<0.05 was considered to represent a significant difference.
Funding: (1) National Center for Research Resources (5P20RR018728-10) and the National Institute of General Medical Sciences (8 P20 GM103537-10) from the National Institutes of Health (Y-TT and JFP). (2) Natural Science Foundation of China (81200087 to WZ).
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