Regulation of protein synthesis by IGF-I in proximal tubular epithelial cells

Female Protection Against Diabetic Kidney Disease Is Regulated by Kidney-Specific AMPK Activity

Reduced kidney AMPK activity is associated with nutrient stress-induced chronic kidney disease (CKD) in male mice. In contrast, female mice resist nutrient stress-induced CKD. The role of kidney AMPK in sex-related organ protection against nutrient stress and metabolite changes was evaluated in diabetic kidney tubule-specific AMPKγ2KO (KTAMPKγ2ΚΟ) male and female mice. In wild-type (WT) males, diabetes increased albuminuria, urinary kidney injury molecule-1, hypertension, kidney p70S6K phosphorylation, and kidney matrix accumulation; these features were not exacerbated with KTAMPKγ2ΚΟ. Whereas WT females had protection against diabetes-induced kidney injury, KTAMPKγ2ΚΟ led to loss of female protection against kidney disease. The hormone 17β-estradiol ameliorated high glucose-induced AMPK inactivation, p70S6K phosphorylation, and matrix protein accumulation in kidney tubule cells. The mechanism for female protection against diabetes-induced kidney injury is likely via an estrogen-AMPK pathway, as inhibition of AMPK led to loss of estrogen protection to glucose-induced mTORC1 activation and matrix production. RNA sequencing and metabolomic analysis identified a decrease in the degradation pathway of phenylalanine and tyrosine resulting in increased urinary phenylalanine and tyrosine levels in females. The metabolite levels correlated with loss of female protection. The findings provide new insights to explain evolutionary advantages to females during states of nutrient challenges.

ARTICLE HIGHLIGHTS

Marmoset as a Model to Study Kidney Changes Associated With Aging

We evaluated whether the marmoset, a nonhuman primate, can serve as a good model to study aging-related changes in the kidney by employing healthy young and aged marmosets of both sexes. Aging was associated with glomerulosclerosis, interstitial fibrosis, and arteriolosclerosis in both sexes; correspondingly, the content of matrix proteins was increased. Functionally, aging resulted in an increase in urinary albumin and protein excretion. There was a robust correlation between markers of fibrosis and functional changes. We explored signaling pathways as potential mechanistic events. Aging in males, but not in females, was associated with reduced renal cortical activity of AMP-activated protein kinase (AMPK) and a trend toward activation of mechanistic target of rapamycin complex 1 (mTORC1); upstream of AMPK and mTORC1, Akt and IGF-1 receptor were activated. In both sexes, aging promoted kidney activation of transforming growth factor β-1 signaling pathway. While the expression of cystathionine β-synthase (CBS), an enzyme involved hydrogen sulfide (H2S) synthesis, was reduced in both aged males and females, decreased H2S generation was seen in only males. Our studies show that the marmoset is a valid model to study kidney aging; some of the signaling pathways involved in renal senescence differ between male and female marmosets.

The silk gland damage and the transcriptional response to detoxifying enzymes-related genes of Bombyx mori under phoxim exposure

Silk gland is a major organ of Bombyx mori for the synthesis and secretion of silk protein. Phoxim exposure can be toxic to B. mori and causes a decrease of fibroin synthesis, finally affecting the silk production in industry. To study the mechanism of metabolism and detoxification of silk gland under phoxim exposure, we measured the residual quantity of phoxim in silk gland and hemolymph after phoxim exposure, and the detoxifying enzymes-related genes and enzyme activity were also investigated. Results indicated that the residual amount of phoxim existed up to 24 h in silk gland compared with that in hemolymph, suggesting that phoxim can accumulate in the silk glands within a certain time course. The transcriptional levels of PI3K/Akt genes, including Akt, Tor1, p70s6k and 4e-bp, were up-regulated by 6.919, 1.358, 10.766 and 7.708-fold, respectively. The expression of two downstream genes (CncC and Keap1) was up-regulated by 1.939 and 3.373-fold, respectively. In addition, the transcriptional levels of detoxification-related genes including CYP6AB, CYP306A, CarE2, GST1 and GSTd1 were up-regulated by 1.731, 1.221, 1.366, 1.376 and 6.591-fold, respectively. The enzymatic activity of CYP450, CarE and GST were increased over time. These results provided possible insights into the injury of silk gland and the transcriptional response to detoxifying enzymes-related genes in silkworm after phoxim exposure.

Protein Translation in the Nucleus Accumbens Is Dysregulated during Cocaine Withdrawal and Required for Expression of Incubation of Cocaine Craving

Exposure to drug-associated cues can induce drug craving and relapse in abstinent addicts. Cue-induced craving that progressively intensifies ("incubates") during withdrawal from cocaine has been observed in both rats and humans. Building on recent evidence that aberrant protein translation underlies incubation-related adaptations in the NAc, we used male rats to test the hypothesis that translation is dysregulated during cocaine withdrawal and/or when rats express incubated cocaine craving. We found that intra-NAc infusion of anisomycin, a general protein translation inhibitor, or rapamycin, an inhibitor of mammalian target of rapamycin, reduced the expression of incubated cocaine craving, consistent with previous results showing that inhibition of translation in slices normalized the adaptations that maintain incubation. We then examined signaling pathways involved in protein translation using NAc synaptoneurosomes prepared after >47 d of withdrawal from cocaine or saline self-administration, or after withdrawal plus a cue-induced seeking test. The most robust changes were observed following seeking tests. Most notably, we found that eukaryotic elongation factor 2 (eEF2) and eukaryotic initiation factor 2α (eIF2α) are dephosphorylated when cocaine rats undergo a cue-induced seeking test; both effects are consistent with increased translation during the test. Blocking eIF2α dephosphorylation and thereby restoring its inhibitory influence on translation, via intra-NAc injection of Sal003 just before the test, substantially reduced cocaine seeking. These results are consistent with dysregulation of protein translation in the NAc during cocaine withdrawal, enabling cocaine cues to elicit an aberrant increase in translation that is required for the expression of incubated cocaine craving.SIGNIFICANCE STATEMENT Cue-induced cocaine craving progressively intensifies (incubates) during withdrawal in both humans and rats. This may contribute to persistent vulnerability to relapse. We previously demonstrated a role for protein translation in synaptic adaptations in the NAc closely linked to incubation. Here, we tested the hypothesis that translation is dysregulated during cocaine withdrawal, and this contributes to incubated craving. Analysis of signaling pathways regulating translation suggested that translation is enhanced when "incubated" rats undergo a cue-induced seeking test. Furthermore, intra-NAc infusions of drugs that inhibit protein translation through different mechanisms reduced expression of incubated cue-induced cocaine seeking. These results demonstrate that the expression of incubation depends on an acute increase in translation that may result from dysregulation of several pathways.

Role of circadian rhythm and REM sleep for memory consolidation

Although sleep is strongly implicated in memory consolidation, the molecular basis for the role of sleep in memory is not known. It has been established that the consolidation of hippocampus-dependent memory depends on the activation of the Erk1,2 MAP kinase (MAPK) pathway which activates de novo CRE-mediated transcription and translation, two processes required for memory consolidation pathway. The activation of MAPK during memory formation and its nuclear translocation both depend upon cAMP signals generated by the calmodulin-stimulated adenylyl cyclases, type 1 and type 8 (AC1 and AC8). This signaling pathway undergoes a circadian oscillation in the hippocampus with maximal activation during REM sleep. This data supports the hypothesis that the persistence of long-term memory traces may depend upon the reactivation and circadian oscillation of the cAMP/MAP kinase/CRE transcriptional pathway in tagged neurons which reaches a maximum during REM sleep.

Phosphorylation of eukaryotic translation initiation factor 4E and eukaryotic translation initiation factor 4E-binding protein (4EBP) and their upstream signaling components undergo diurnal oscillation in the mouse hippocampus: implications for memory persistence

Translation of mRNA plays a critical role in consolidation of long-term memory. Here, we report that markers of initiation of mRNA translation are activated during training for contextual memory and that they undergo diurnal oscillation in the mouse hippocampus with maximal activity observed during the daytime (zeitgeber time 4-8 h). Phosphorylation and activation of eukaryotic translation initiation factor 4E (eIF4E), eIF4E-binding protein 1 (4EBP1), ribosomal protein S6, and eIF4F cap-complex formation, all of which are markers for translation initiation, were higher in the hippocampus during the daytime compared with night. The circadian oscillation in markers of mRNA translation was lost in memory-deficient transgenic mice lacking calmodulin-stimulated adenylyl cyclases. Moreover, disruption of the circadian rhythm blocked diurnal oscillations in eIF4E, 4EBP1, rpS6, Akt, and ERK1/2 phosphorylation and impaired memory consolidation. Furthermore, repeated inhibition of translation in the hippocampus 48 h after contextual training with the protein synthesis inhibitor anisomycin impaired memory persistence. We conclude that repeated activation of markers of translation initiation in hippocampus during the circadian cycle might be critical for memory persistence.

20-HETE and EETs in diabetic nephropathy: a novel mechanistic pathway

Diabetic nephropathy (DN), a major complication of diabetes, is characterized by hypertrophy, extracellular matrix accumulation, fibrosis and proteinuria leading to loss of renal function. Hypertrophy is a major factor inducing proximal tubular epithelial cells injury. However, the mechanisms leading to tubular injury is not well defined. In our study, we show that exposure of rats proximal tubular epithelial cells to high glucose (HG) resulted in increased extracellular matrix accumulation and hypertrophy. HG treatment increased ROS production and was associated with alteration in CYPs 4A and 2C11 expression concomitant with alteration in 20-HETE and EETs formation. HG-induced tubular injury were blocked by HET0016, an inhibitor of CYPs 4A. In contrast, inhibition of EETs promoted the effects of HG on cultured proximal tubular cells. Our results also show that alteration in CYPs 4A and 2C expression and 20HETE and EETs formation regulates the activation of the mTOR/p70S6Kinase pathway, known to play a major role in the development of DN. In conclusion, we show that hyperglycemia in diabetes has a significant effect on the expression of Arachidonic Acid (AA)-metabolizing CYPs, manifested by increased AA metabolism, and might thus alter kidney function through alteration of type and amount of AA metabolites.

Insulin receptor substrate-2 is expressed in kidney epithelium and up-regulated in diabetic nephropathy

Diabetic nephropathy (DN) is a progressive fibrotic condition that may lead to end-stage renal disease and kidney failure. Transforming growth factor-β1 and bone morphogenetic protein-7 (BMP7) have been shown to induce DN-like changes in the kidney and protect the kidney from such changes, respectively. Recent data identified insulin action at the level of the nephron as a crucial factor in the development and progression of DN. Insulin requires a family of insulin receptor substrate (IRS) proteins for its physiological effects, and many reports have highlighted the role of insulin and IRS proteins in kidney physiology and disease. Here, we observed IRS2 expression predominantly in the developing and adult kidney epithelium in mouse and human. BMP7 treatment of human kidney proximal tubule epithelial cells (HK-2 cells) increases IRS2 transcription. In addition, BMP7 treatment of HK-2 cells induces an electrophoretic shift in IRS2 migration on SDS/PAGE, and increased association with phosphatidylinositol-3-kinase, probably due to increased tyrosine/serine phosphorylation. In a cohort of DN patients with a range of chronic kidney disease severity, IRS2 mRNA levels were elevated approximately ninefold, with the majority of IRS2 staining evident in the kidney tubules in DN patients. These data show that IRS2 is expressed in the kidney epithelium and may play a role in the downstream protective events triggered by BMP7 in the kidney. The specific up-regulation of IRS2 in the kidney tubules of DN patients also indicates a novel role for IRS2 as a marker and/or mediator of human DN progression.

Psychological stress and aging: role of glucocorticoids (GCs)

Psychological stress has extreme adverse consequences on health. However, the molecular mechanisms that mediate and accelerate the process of aging due to stress hormone are not well defined. This review has focused on diverse molecular paths that come out in response to chronic psychological stress via releasing of excessive glucocorticoids (GCs), involved in the aging process. GCs suppress transcription of nuclear cell adhesion molecules which impair synaptic plasticity, memory formation, and cognitive ability. Again, GCs promote muscle atrophy by means of motivating ubiquitin proteasome system and can repress muscle protein synthesis by inhibition of PI3-kinase/Akt pathway. GCs also inhibit interleukin-2 synthesis through suppressing T cell receptor signal that leads to loss of T cell activation, proliferation, and B-cell activation. Moreover, GCs increase the expression of collagenase-3, RANK ligand, and colony stimulating factor-1 that induce bone resorption. In general, stress-induced GCs can play causal role for aging and age-related disorders.

In vitro silencing of the insulin receptor attenuates cellular accumulation of fibronectin in renal mesangial cells

Background

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.

Results

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.

Conclusions

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.

IGF-I increases the expression of fibronectin by Nox4-dependent Akt phosphorylation in renal tubular epithelial cells

Extracellular matrix accumulation contributes to the progression of chronic kidney disease. Many growth factors including insulin-like growth factor-I (IGF-I) enhance matrix protein accumulation. Proximal tubular epithelial cells (PTCs) synthesize matrix proteins. NADPH oxidases are major sources of reactive oxygen species (ROS), important signaling molecules that mediate biological responses in a variety of cells and tissue. We investigated the mechanism by which IGF-I regulates fibronectin accumulation in PTCs and the role of a potential redox-dependent signaling pathway. IGF-I induces an increase in NADPH-dependent superoxide generation, enhances the release of hydrogen peroxide, and increases the expression of NADPH oxidase 4 (Nox4) in PTCs. IGF-I also stimulates phosphorylation of Akt, and inhibition of Akt or its upstream activator phosphatidylinositol 3-kinase attenuates IGF-I-induced fibronectin accumulation. Expression of dominant negative Akt also inhibits IGF-I-induced expression of fibronectin, indicating a role for this kinase in fibronectin accumulation. Expression of dominant negative adenovirus Nox4 inhibits IGF-I-induced NADPH oxidase activity, Akt phosphorylation, and fibronectin protein expression. Moreover, transfection of small interfering RNA targeting Nox4 decreases Nox4 protein expression and blocks IGF-I-induced Akt phosphorylation and the increase in fibronectin, placing Nox4 and ROS upstream of Akt signaling pathway. To confirm the role of Nox4, PTCs were infected with adenovirus construct expressing wild-type Nox4. Ad-Nox4, but not control Ad-green fluorescent protein, upregulated Nox4 expression and increased NADPH oxidase activity as well as fibronectin expression. Taken together, these results provide the first evidence for a role of Nox4 in IGF-I-induced Akt phosphorylation and fibronectin expression in tubular epithelial cells.

PRAS40 acts as a nodal regulator of high glucose-induced TORC1 activation in glomerular mesangial cell hypertrophy

Diabetic nephropathy manifests aberrant activation of TORC1, which senses key signals to modulate protein synthesis and renal hypertrophy. PRAS40 has recently been identified as a raptor-interacting protein and is a component and a constitutive inhibitor of TORC1. The mechanism by which high glucose stimulates TORC1 activity is not known. PRAS40 was identified in the mesangial cells in renal glomeruli and in tubulointerstitium of rat kidney. Streptozotocin-induced diabetic renal hypertrophy was associated with phosphorylation of PRAS40 in the cortex and glomeruli. In vitro, high glucose concentration increased PRAS40 phosphorylation in a PI 3 kinase- and Akt-dependent manner, resulting in dissociation of raptor-PRAS40 complex in mesangial cells. High glucose augmented the inactivating and activating phosphorylation of 4EBP-1 and S6 kinase, respectively, with concomitant induction of protein synthesis and hypertrophy. Expression of TORC1-nonphosphorylatable mutant of 4EBP-1 and dominant-negative S6 kinase significantly inhibited high glucose-induced protein synthesis and hypertrophy. PRAS40 knockdown mimicked the effect of high glucose on phosphorylation of 4EBP-1 and S6 kinase, protein synthesis, and hypertrophy. To elucidate the role of PRAS40 phosphorylation, we used phosphorylation-deficient mutant of PRAS40, which in contrast to PRAS40 knockdown inhibited phosphorylation of 4EBP-1 and S6 kinase, leading to reduced mesangial cell hypertrophy. Thus, our data identify high glucose-induced phosphorylation and inactivation of PRAS40 as a central node for mesangial cell hypertrophy in diabetic nephropathy.

Regulation of mRNA translation in renal physiology and disease

Translation, a process of generating a peptide from the codons present in messenger RNA, can be a site of independent regulation of protein synthesis; it has not been well studied in the kidney. Translation occurs in three stages (initiation, elongation, and termination), each with its own set of regulatory factors. Mechanisms controlling translation include small inhibitory RNAs such as microRNAs, binding proteins, and signaling reactions. Role of translation in renal injury in diabetes, endoplasmic reticulum stress, acute kidney injury, and, in physiological adaptation to loss of nephrons is reviewed here. Contribution of mRNA translation to physiology and disease is not well understood. Because it is involved in such diverse areas as development and cancer, it should prove a fertile field for investigation in renal science.

IGF-1-stimulated protein synthesis in oligodendrocyte progenitors requires PI3K/mTOR/Akt and MEK/ERK pathways

Insulin-like growth factor-1 (IGF-1) interacts with the Type I receptor to activate two main signaling pathways, the mitogen-activated protein kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) and the phosphatidylinositol 3-kinase (PI3K)-Akt cascades, which mediate proliferation or survival of oligodendrocyte (OL) progenitors (OLPs). In other cellular systems, mammalian target of rapamycin (mTOR) and the p70 S6 kinase are downstream effectors that phosphorylate translation initiation factors (e.g. eIF-4E), their regulators (e.g. 4E-binding protein 1, 4E-BP1) and ribosomal protein S6 (S6). The aim of this study was to determine whether these pathways are involved in IGF-1-stimulated protein synthesis, important for growth and differentiation of OLs. Rat cultured OLPs were treated with IGF-1 with or without inhibitors of PI3K (LY294002 or Wortmannin), mTOR (rapamycin), MEK (PD98059), and Akt (III or IV), as well as an adenovirus encoding a dominant negative form of Akt. Protein synthesis, as assessed by [(35)S]-methionine incorporation, was stimulated by IGF-1 and required the upstream activation of PI3K, Akt, mTOR and MEK/ERK. Concordant with the experiments using protein kinase inhibitors, western blotting revealed that IGF-1 stimulates phosphorylation of Akt, mTOR, ERK, S6 and 4E-BP1. Activation of S6 and inactivation of 4E-BP1, necessary for protein synthesis to take place, were dependent on the upstream activation of PI3K and mTOR. Finally, IGF-1 consistently stimulated protein synthesis through mTOR in differentiating OLPs but mRNA transcription was not required at day 4, indicating a differential role of IGF-1 throughout OL development.

Glycogen synthase kinase 3beta is a novel regulator of high glucose- and high insulin-induced extracellular matrix protein synthesis in renal proximal tubular epithelial cells

High glucose (30 mM) and high insulin (1 nM), pathogenic factors of type 2 diabetes, increased mRNA expression and synthesis of lamininbeta1 and fibronectin after 24 h of incubation in kidney proximal tubular epithelial (MCT) cells. We tested the hypothesis that inactivation of glycogen synthase kinase 3beta (GSK3beta) by high glucose and high insulin induces increase in synthesis of laminin beta1 via activation of eIF2Bepsilon. Both high glucose and high insulin induced Ser-9 phosphorylation and inactivation of GSK3beta at 2 h that lasted for up to 48 h. This was associated with dephosphorylation of eIF2Bepsilon and eEF2, and increase in phosphorylation of 4E-BP1 and eIF4E. Expression of the kinase-dead mutant of GSK3beta or constitutively active kinase led to increased and diminished laminin beta1 synthesis, respectively. Incubation with selective kinase inhibitors showed that high glucose- and high insulin-induced laminin beta1 synthesis and phosphorylation of GSK3beta were dependent on PI 3-kinase, Erk, and mTOR. High glucose and high insulin augmented activation of Akt, Erk, and p70S6 kinase. Dominant negative Akt, but not dominant negative p70S6 kinase, inhibited GSK3beta phosphorylation induced by high glucose and high insulin, suggesting Akt but not p70S6 kinase was upstream of GSK3beta. Status of GSK3beta was examined in vivo in renal cortex of db/db mice with type 2 diabetes at 2 weeks and 2 months of diabetes. Diabetic mice showed increased phosphorylation of renal cortical GSK3beta and decreased phosphorylation of eIF2Bepsilon, which correlated with renal hypertrophy at 2 weeks, and increased laminin beta1 and fibronectin protein content at 2 months. GSK3beta and eIF2Bepsilon play a role in augmented protein synthesis associated with high glucose- and high insulin-stimulated hypertrophy and matrix accumulation in renal disease in type 2 diabetes.

Renal activity of Akt kinase in obese Zucker rats

Insulin resistance (IR) and consequent hyperinsulinemia are hallmarks of Type 2 diabetes (DM2). Akt kinase (Akt) is an important molecule in insulin signaling, implicated in regulation of glucose uptake, cell growth, cell survival, protein synthesis, and endothelial nitric oxide (NO) production. Impaired Akt activation in insulin-sensitive tissues contributes to IR. However, Akt activity in other tissues, particularly those affected by complications of DM2, has been less studied. We hypothesized that hyperinsulinemia could have an impact on activity of Akt and its effectors involved in regulation of renal morphology and function in DM2. To address this issue, renal cortical Akt was determined in obese Zucker rats (ZO), a model of DM2, and lean controls (ZL). We also studied expression and phosphorylation of the mammalian target of rapamycin (mTOR) and endothelial NO synthase (eNOS), molecules downstream of Akt in the insulin signaling cascade, and documented modulators of renal injury. Akt activity was measured by a kinase assay with GSK-3 as a substrate. Expression of phosphorylated (active) and total proteins was measured by immunoblotting and immunohistochemistry. Renal Akt activity was increased in ZO as compared to ZL rats, in parallel with progressive hyperinsulinemia. No differences in Akt were observed in the skeletal muscle. Corresponding to increases in Akt activity, ZO rats demonstrated enhanced phosphorylation of renal mTOR. Acute PI3K inhibition with wortmannin (100 mug/kg) attenuated renal Akt and mTOR activities in ZO, but not in ZL rats. In contrast to mTOR, eNOS phosphorylation was similar in ZO and ZL rats, despite higher total eNOS expression. In conclusion, ZO rats demonstrated increases in renal Akt and mTOR activity and expression. However, eNOS phosphorylation did not follow this pattern. These data suggest that DM2 is associated with selective IR in the kidney, allowing pro-growth signaling via mTOR, whereas potentially protective effects mediated by eNOS are blunted.

Raptor-rictor axis in TGFbeta-induced protein synthesis

Transforming growth factor-beta (TGFbeta) stimulates pathological renal cell hypertrophy for which increased protein synthesis is critical. The mechanism of TGFbeta-induced protein synthesis is not known, but PI 3 kinase-dependent Akt kinase activity is necessary. We investigated the contribution of downstream effectors of Akt in TGFbeta-stimulated protein synthesis. TGFbeta increased inactivating phosphorylation of Akt substrate tuberin in a PI 3 kinase/Akt dependent manner, resulting in activation of mTOR kinase. mTOR activity increased phosphorylation of S6 kinase and the translation repressor 4EBP-1, which were sensitive to inhibition of both PI 3 kinase and Akt. mTOR inhibitor rapamycin and a dominant negative mutant of mTOR suppressed TGFbeta-induced phosphorylation of S6 kinase and 4EBP-1. PI 3 kinase/Akt and mTOR regulated dissociation of 4EBP-1 from eIF4E to make the latter available for binding to eIF4G. mTOR and 4EBP-1 modulated TGFbeta-induced protein synthesis. mTOR is present in two multi protein complexes, mTORC1 and mTORC2. Raptor and rictor are part of mTORC1 and mTORC2, respectively. shRNA-mediated downregulation of raptor inhibited TGFbeta-stimulated mTOR kinase activity, resulting in inhibition of phosphorylation of S6 kinase and 4EBP-1. Raptor shRNA also prevented protein synthesis in response to TGFbeta. Downregulation of rictor inhibited serine 473 phosphorylation of Akt without any effect on phosphorylation of its substrate, tuberin. Furthermore, rictor shRNA increased phosphorylation of S6 kinase and 4EBP-1 in TGFbeta-independent manner, resulting in increased protein synthesis. Thus mTORC1 function is essential for TGFbeta-induced protein synthesis. Our data also provide novel evidence that rictor negatively regulates TORC1 activity to control basal protein synthesis, thus conferring tight control on cellular hypertrophy.

Eukaryotic translation initiation factor 4E (eIF4E) expression in the brain tissue is induced by infusion of nerve growth factor into the mouse cisterna magnum: an in vivo study

In many cell types translation can be regulated by an expression of the translation initiation factor. Eukaryotic translation initiation factor eIF4E, which binds to the 5' cap structure of mRNA, plays an important role in translation regulation and it has been suggested that it is implicated in increased protein synthesis promoted by growth factors. In this study the effects of nerve growth factor (NGF) infusion into the cerebrospinal fluid (CSF) on eIF4E expression and phosphorylation in mouse brain tissue have been investigated. We investigated NGF as it is one of the most important growth factors and it is an important factor in cerebral cortical development, stimulating neuronal precursor proliferation. eIF4E level is also increased in response to infusion of NGF into the CSF. The present study shows that eIF4E is phosphorylated in the brain tissues treated with NGF. It is concluded that NGF regulates protein synthesis in the nervous tissue by enhancing expression and phosphorylation of eIF4E.

Novel roles of the IGF-IGFBP axis in etiopathophysiology of diabetic nephropathy

Mechanisms contributing to development of diabetic nephropathy (DN) remain unclear. High ambient glucose level transforms intracellular pathways, promoting stable phenotypic changes in the glomerulus such as mesangial cell hypertrophy, podocyte apoptosis, and matrix expansion. Insulin-like growth factors (IGFs) and the high affinity IGF binding proteins (IGFBPs) exert major effects on cell growth and metabolism. Compared with diabetic patients without microalbuminuria (MA), MA diabetic patients display perturbed GH-IGF-IGFBP homeostasis, including increased circulating IGF-I and IGFBP-3 protease activity, increased excretion of bioactive GH, IGF-I, and IGFBP-3, but decreased circulating IGFBP-3 levels. In diabetic animal models, expression of IGF-I and IGFBP-1 to -4 increases in key renal tissues and glomerular ulrafiltrate. Epithelial, mesangial, and endothelial cells derived from the kidney respond to IGF-I binding with increased protein synthesis, migration, and proliferation. This article reviews classic and emerging concepts for the roles of the GH-IGF-IGFBP axis in the etiopathophysiology, treatment, and prevention of diabetic renal disease. We report IGF-independent actions of IGFBP-3 in the podocyte for the first time.

High glucose, high insulin, and their combination rapidly induce laminin-beta1 synthesis by regulation of mRNA translation in renal epithelial cells

Laminin is a glycoprotein that contributes to renal extracellular matrix expansion in diabetes. We investigated regulation of laminin-beta1 synthesis in murine renal proximal tubular epithelial cells by 30 mmol/l glucose (high glucose), 1 nmol/l insulin (high insulin), and their combination (high glucose+high insulin), simulating conditions observed during progression of type 2 diabetes. Compared with 5 mmol/l glucose and no insulin (control), high glucose alone, high insulin alone, or high glucose+high insulin together increased laminin-beta1 chain protein synthesis within 5 min, lasting for up to 60 min with no change in laminin-beta1 mRNA levels. Cycloheximide, but not actinomycin-D, abrogated increased laminin-beta1 synthesis. High glucose, high insulin, and high glucose+high insulin stimulated phosphorylation of 4E-BP1, a repressor binding protein for eukaryotic initiation factor 4E (eIF4E), that was dependent on activation of phosphatidylinositol 3-kinase, Akt, and mammalian target of rapamycin. High glucose, high insulin, and high glucose+high insulin also promoted release of eIF4E from 4E-BP1, phosphorylation of eIF4E, and increase in eIF4E association with eIF4G, critical events in the initiation phase of mRNA translation. High glucose, high insulin, and high glucose+high insulin increased Erk phosphorylation, which is an upstream regulator of eIF4E phosphorylation, and PD098059, which is a MEK inhibitor that blocks Erk activation, abolished laminin-beta1 synthesis. This is the first demonstration of rapid increment in laminin-beta1 synthesis by regulation of its mRNA translation by cells exposed to high glucose, high insulin, or high glucose+high insulin.

Gene expression profiles of human proximal tubular epithelial cells in proteinuric nephropathies

In kidney disease renal proximal tubular epithelial cells (RPTEC) actively contribute to the progression of tubulointerstitial fibrosis by mediating both an inflammatory response and via epithelial-to-mesenchymal transition. Using laser capture microdissection we specifically isolated RPTEC from cryosections of the healthy parts of kidneys removed owing to renal cell carcinoma and from kidney biopsies from patients with proteinuric nephropathies. RNA was extracted and hybridized to complementary DNA microarrays after linear RNA amplification. Statistical analysis identified 168 unique genes with known gene ontology association, which separated patients from controls. Besides distinct alterations in signal-transduction pathways (e.g. Wnt signalling), functional annotation revealed a significant upregulation of genes involved in cell proliferation and cell cycle control (like insulin-like growth factor 1 or cell division cycle 34), cell differentiation (e.g. bone morphogenetic protein 7), immune response, intracellular transport and metabolism in RPTEC from patients. On the contrary we found differential expression of a number of genes responsible for cell adhesion (like BH-protocadherin) with a marked downregulation of most of these transcripts. In summary, our results obtained from RPTEC revealed a differential regulation of genes, which are likely to be involved in either pro-fibrotic or tubulo-protective mechanisms in proteinuric patients at an early stage of kidney disease.

A role for AMP-activated protein kinase in diabetes-induced renal hypertrophy

We tested the hypothesis that AMP-activated protein kinase (AMPK), an energy sensor, regulates diabetes-induced renal hypertrophy. In kidney glomerular epithelial cells, high glucose (30 mM), but not equimolar mannitol, stimulated de novo protein synthesis and induced hypertrophy in association with increased phosphorylation of eukaryotic initiation factor 4E binding protein 1 and decreased phosphorylation of eukaryotic elongation factor 2, regulatory events in mRNA translation. These high-glucose-induced changes in protein synthesis were phosphatidylinositol 3-kinase, Akt, and mammalian target of rapamycin (mTOR) dependent and transforming growth factor-beta independent. High glucose reduced AMPK alpha-subunit theronine (Thr) 172 phosphorylation, which required Akt activation. Changes in AMP and ATP content could not fully account for high-glucose-induced reductions in AMPK phosphorylation. Metformin and 5-aminoimidazole-4-carboxamide-1beta-riboside (AICAR) increased AMPK phosphorylation, inhibited high-glucose stimulation of protein synthesis, and prevented high-glucose-induced changes in phosphorylation of 4E binding protein 1 and eukaryotic elongation factor 2. Expression of kinase-inactive AMPK further increased high-glucose-induced protein synthesis. Renal hypertrophy in rats with Type 1 diabetes was associated with reduction in AMPK phosphorylation and increased mTOR activity. In diabetic rats, metformin and AICAR increased renal AMPK phosphorylation, reversed mTOR activation, and inhibited renal hypertrophy, without affecting hyperglycemia. AMPK is a newly identified regulator of renal hypertrophy in diabetes.

mRNA translation: unexplored territory in renal science

Ambient protein levels are under coordinated control of transcription, mRNA translation, and degradation. Whereas transcription and degradation mechanisms have been studied in depth in renal science, the role of mRNA translation, the process by which peptide synthesis occurs according to the genetic code that is present in the mRNA, has not received much attention. mRNA translation occurs in three phases: Initiation, elongation, and termination. Each phase is controlled by unique eukaryotic factors. In the initiation phase, mRNA and ribosomal subunits are brought together. During the elongation phase, amino acids are added to the nascent peptide chain in accordance with codon sequences in the mRNA. During the termination phase, the fully synthesized peptide is released from the ribosome for posttranslational processing. Signaling pathways figure prominently in regulation of mRNA translation, particularly the phosphatidylinositol 3 kinase-Akt-mammalian target of rapamycin pathway, the AMP-activated protein kinase-tuberous sclerosis complex protein 1/tuberous sclerosis complex protein 2-Rheb pathway, and the extracellular signal-regulated kinase 1/2 type mitogen-activated protein kinase signaling pathway; there is significant cross-talk among these pathways. Regulation by mRNA translation is suggested when changes in mRNA and protein levels do not correlate and in the setting of rapid protein synthesis. Ongoing work suggests an important role for mRNA translation in compensatory renal growth, hypertrophy and extracellular matrix synthesis in diabetic nephropathy, growth factor synthesis by kidney cells, and glomerulonephritis. Considering that mRNA translation plays an important role in cell growth, development, malignancy, apoptosis, and response to stress, its study should provide novel insights in renal physiology and pathology.

Mesangial cell hypertrophy by high glucose is mediated by downregulation of the tumor suppressor PTEN

Diabetic nephropathy is characterized early in its course by glomerular hypertrophy and, importantly, mesangial hypertrophy, which correlate with eventual glomerulosclerosis. The mechanism of hypertrophy, however, is not known. Gene disruption of the tumor suppressor PTEN, a negative regulator of the phosphatidylinositol 3-kinase/Akt pathway, in fruit flies and mice demonstrated its role in size control in a cell-specific manner. Here, we investigated the mechanism of mesangial hypertrophy in response to high extracellular glucose. We link early renal hypertrophy with significant reduction in PTEN expression in the streptozotocin-induced diabetic kidney cortex and glomeruli, concomitant with activation of Akt. Similarly, exposure of mesangial cells to high concentrations of glucose also decreased PTEN expression and its phosphatase activity, resulting in increased Akt activity. Expression of PTEN inhibited high-glucose-induced mesangial cell hypertrophy, and expression of dominant-negative PTEN was sufficient to induce hypertrophy. In diabetic nephropathy, the hypertrophic effect of hyperglycemia is thought to be mediated by transforming growth factor-beta (TGF-beta). TGF-beta significantly reduced PTEN expression in mesangial cells, with a reduction in its phosphatase activity and an increase in Akt activation. PTEN and dominant-negative Akt attenuated TGF-beta-induced hypertrophy of mesangial cells. Finally, we show that inhibition of TGF-beta signal transduction blocks the effect of high glucose on PTEN downregulation. These data identify a novel mechanism placing PTEN as a key regulator of diabetic mesangial hypertrophy involving TGF-beta signaling.

Angiotensin II stimulation of VEGF mRNA translation requires production of reactive oxygen species

ANG II, a mediator of renal injury in diabetic renal disease, promotes vascular endothelial growth factor (VEGF) mRNA translation in proximal tubular epithelial (MCT) cells (Feliers D, Duraisamy S, Barnes JL, Ghosh-Choudhury G, and Kasimath BS. Am J Physiol Renal Physiol 288: F521–F529, 2005). The mechanism by which ANG II elicits this effect is not known. ANG II is known to induce oxidative stress and the rapidity of the effect suggested a role for reactive oxygen species (ROS). The aim of this study is to test the hypothesis that ANG II regulates VEGF mRNA translation in MCT cells through ROS production. In MCT cells exposed to 1 nM ANG II, ROS production was increased in a time-dependent manner. Inhibition of ROS production by N-acetylcysteine (NAC), a precursor of glutathione, and diphenyleneiodonium (DPI), an inhibitor of flavoproteins that include NAD(P)H oxidase, prevented ANG II-stimulated VEGF protein expression. NAC and DPI also inhibited phosphorylation of 4E-BP1 on Thr46 and association of eIF4E with eIF4G, steps that are important in the initiation phase of mRNA translation. NAC and DPI also blocked Akt activation which is required for 4E-BP1 phosphorylation. LY-294002, a selective phosphatidylinositol (PI 3-kinase) inhibitor, did not prevent ROS accumulation in response to ANG II, whereas DPI blocked ANG II activation of PI 3-kinase, demonstrating that ROS production is upstream of the PI 3-kinase signaling pathway. Preincubation with catalase abolished ANG II stimulation of VEGF expression and mRNA translation, suggesting involvement of hydrogen peroxide (H2O2). H2O2reproduced the effects of ANG II on VEGF expression and aforementioned parameters of mRNA translation. Finally, neither preincubation of MCT cells with specific inhibitors of the mitochondrial respiratory chain nor inactivation of the mitochondrial respiratory chain in MCT cells prevented ANG II stimulation of VEGF expression. Inhibition of nitric oxide synthase by l-NAME had no effect on ANG II stimulation of VEGF expression. These data show that ROS, generated probably through activation of an NAD(P)H oxidase, mediate ANG II stimulation of VEGF mRNA translation.

Translational event mediates differential production of tumor necrosis factor-α in hyaluronan-stimulated microglia and macrophages

Recent evidence has demonstrated that hyaluronan synthase 2 mRNA is up-regulated after brain ischemia. After a cerebral ischemic event, microglia and macrophages are the major inflammatory cells and are activated by hyaluronan (HA). However, it is unclear how these cells compare with regard to HA responsiveness. We show here that peritoneal macrophages and RAW 264.7 macrophages produced more than five- and 10-fold more tumor necrosis factor-alpha (TNF-alpha) than primary microglia and BV-2 microglia, respectively. Antibody blockade study showed that CD44, Toll-like receptor-4 receptor and the receptor for HA-mediated motility were responsible for HA-induced TNF-alpha release. Furthermore, HA induced higher levels of phosphorylated MAPK in RAW 264.7 cells when compared with BV-2 cells. HA-mediated TNF-alpha production required p38 MAPK, extracellular-regulated kinase and c-Jun N-terminal kinase phosphorylation in both cell types. The levels of HA-induced TNF-alpha mRNA expression in BV-2 cells were only twofold lower compared with RAW 264.7 cells, suggesting that a translational event is involved in the differential production of TNF-alpha. Western blot analysis revealed that HA treatment resulted in more rapid phosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) and more effective dissociation of 4E-BP1 from eukaryotic initiation factor 4E in RAW 264.7 cells than in BV-2 cells. Additionally, HA-induced phosphorylation of 4E-BP1 was dependent on MAPK signaling, indicating that RAW 264.7 cells exhibited higher levels of hyperphosphorylated 4E-BP1 possibly due to the overactivation of MAPK. The results suggest that resident microglia and blood-derived monocytes/macrophages exhibit differential sensitivities in response to extracellular mediators after brain ischemia.

VEGF regulation of endothelial nitric oxide synthase in glomerular endothelial cells

BACKGROUND

Vascular endothelial growth factor (VEGF) regulation of endothelial nitric oxide synthase (eNOS) and signaling pathways involved have not been well studied in glomerular endothelial cells (GENCs).

METHODS

GENCs grown from tsA58 Immortomice were used. Immunoblotting and in-cell Western blot analysis were employed to assess changes in VEGF receptor signaling pathway and eNOS phosphorylation of ser1177. Immunokinase assay and immunoblotting with phosphospecific antibodies were performed to assess activity of kinases.

RESULTS

VEGF rapidly induced tyrosine phosphorylation of type 1 and type 2 VEGF receptors. Physical association between VEGF-receptor 2 (VEGF-R2) and insulin receptor substrate (IRS-1) and phosphatidylinositol 3'-kinase (PI3K) was induced by VEGF, which augmented PI3K activity in VEGF-R2 immunoprecipitates. VEGF stimulated Akt phosphorylation in a PI3K-dependent manner. VEGF increased eNOS phosphorylation on Ser1177. Activation of eNOS was associated with nitric oxide generation as measured by medium nitrite content. Signaling mechanisms involved in VEGF stimulation of eNOS were explored. VEGF-induced eNOS phosphorylation was abolished by SU1498, a VEGF-R2 inhibitor, LY294002, a PI3K inhibitor, and infection of cells with an adenovirus carrying a dominant negative-mutant of Akt, demonstrating the requirement of the VEGF-R2/IRS-1/PI3K/Akt axis for activation of eNOS. VEGF also activated extracellular signal-regulated protein kinase (ERK) in a time-dependent manner; and VEGF-stimulated eNOS phosphorylation on Ser1177 was prevented by PD098059, an upstream inhibitor of ERK, demonstrating that ERK was involved in VEGF regulation of eNOS. ERK phosphorylation was abolished by LY294002, suggesting ERK was downstream of PI3K in VEGF-treated GENC.

CONCLUSIONS

Our data demonstrate that in GENC, VEGF stimulates VEGF-R2/IRS-1/PI3K/Akt axis to regulate eNOS phosphorylation on Ser1177 in conjunction with the ERK signaling pathway.

Involvement of tyrosine kinase and PI3K in the regulation of OAT3-mediated estrone sulfate transport in isolated rabbit renal proximal tubules

It was shown previously that OAT3 activity was differentially regulated by protein kinases including MAPK, PKA, and PKC. The present study investigated the short-term effect of tyrosine kinase and phosphatidylinositol 3-kinase (PI3K) on OAT3-mediated organic anion transport in S2 segments of renal proximal tubules. Genistein, a tyrosine kinase inhibitor, and wortmannin, a PI3K inhibitor, inhibited transport of estrone sulfate, a prototypic substrate for OAT3, in a dose-dependent manner. Previously, we showed that epidermal growth factor (EGF) stimulated OAT3 activity via the MAPK pathway. In the present study, we investigated whether EGF-stimulated OAT3 activity was dependent on tyrosine kinase and PI3K. We showed that EGF stimulation of OAT3 was reduced by inhibition of tyrosine kinase or PI3K, suggesting that they play a role in the stimulatory process. Inhibitory effects also indicated that tyrosine kinase and PI3K are involved in the MAPK pathway for EGF stimulation of OAT3 in intact renal proximal tubules, with PI3K acting upstream and tyrosine kinase acting downstream of mitogen-activated/extracellular signal-regulated kinase kinase activation.

Alcohol impairs leucine-mediated phosphorylation of 4E-BP1, S6K1, eIF4G, and mTOR in skeletal muscle

Acute alcohol (EtOH) intoxication impairs skeletal muscle protein synthesis. Although this impairment is not associated with a decrease in the total plasma amino acid concentration, EtOH may blunt the anabolic response to amino acids. To examine this hypothesis, rats were administered EtOH or saline (Sal) and 2.5 h thereafter were orally administered either leucine (Leu) or Sal. The gastrocnemius was removed 20 min later to assess protein synthesis and signaling components important in translational control of protein synthesis. Oral Leu increased muscle protein synthesis by the same magnitude in Sal- and EtOH-treated rats. However, the increase in the latter group was insufficient to overcome the suppressive effect of EtOH, and the rate of synthesis remained lower than that observed in rats from the Sal-Sal group. Leu markedly increased phosphorylation of Thr residues 36, 47, and 70 on 4E-binding protein (BP)1 in muscle from rats not receiving EtOH, and this response was associated with a redistribution of eukaryotic initiation factor (eIF) 4E from the inactive eIF4E. 4E-BP1 to the active eIF4E. eIF4G complex. In EtOH-treated rats, the Leu-induced phosphorylation of 4E-BP1 and changes in eIF4E availability were partially abrogated. EtOH also prevented the Leu-induced increase in phosphorylation of eIF4G, the serine/threonine protein kinase S6K1, and the ribosomal protein S6. Moreover, EtOH attenuated the Leu-induced phosphorylation of the mammalian target of rapamycin (mTOR). The ability of EtOH to blunt the anabolic effects of Leu could not be attributed to differences in the plasma concentrations of insulin, insulin-like growth factor I, or Leu. Finally, although EtOH increased the plasma corticosterone concentration, inhibition of glucocorticoid action by RU-486 was unable to prevent EtOH-induced defects in the ability of Leu to stimulate 4E-BP1, S6K1, and mTOR phosphorylation. Hence, ethanol produces a leucine resistance in skeletal muscle, as evidenced by the impaired phosphorylation of 4E-BP1, eIF4G, S6K1, and mTOR, that is independent of elevations in endogenous glucocorticoids.

Vascular endothelial growth factor induces protein synthesis in renal epithelial cells: a potential role in diabetic nephropathy

BACKGROUND

Vascular endothelial growth factor (VEGF) is an important determinant of ocular complications of diabetes. Its potential role in diabetic renal disease has not been extensively studied.

METHODS

We employed mice with streptozotocin-induced type 1 diabetes and db/db mice with type 2 diabetes to study the regulation of renal VEGF. Studies of VEGF regulation of protein synthesis were performed using proximal tubular epithelial (MCT) cells in culture.

RESULTS

A nearly three-fold increase of VEGF165 expression in the renal cortex was seen, coinciding with renal hypertrophy in mice with either type 1 or type 2 diabetes. VEGF increased de novo protein synthesis and induced significant hypertrophy in MCT cells. VEGF stimulation of protein synthesis was dependent on tyrosine phosphorylation of the type 2 VEGF receptor and phosphatidylinositol 3-kinase (PI 3-kinase) activity. Activity of Akt was increased two- to three-fold by VEGF. Expression of dominant-negative Akt showed that Akt activation was also needed for VEGF-induced protein synthesis and cell hypertrophy. As PI 3-kinase-Akt axis regulates initial events in protein translation, these events were examined in the context of VEGF regulation of protein synthesis. VEGF stimulated eukaryotic initiation factor 4E-binding protein (4E-BP1) phosphorylation, which was dependent on activation of PI 3-kinase and Akt. Stable transfection with 4E-BP1 Thr37,46-Ala37,46 mutant abolished the VEGF-induced de novo protein synthesis and cell hypertrophy.

CONCLUSION

VEGF augments protein synthesis and induces hypertrophy in MCT cells in a PI 3-kinase- and Akt-dependent manner. Phosphorylation of Thr37,46 in 4E-BP1 is required for VEGF-induced protein synthesis and hypertrophy in MCT cells. These data suggest a role for VEGF in the pathogenesis of diabetic renal disease.