Tyrosine phosphorylation-dependent PI 3 kinase/Akt signal transduction regulates TGFbeta-induced fibronectin expression in mesangial cells

Isolation and target identification of anti-renal fibrosis compounds from Cordyceps militaris

Four undescribed compounds including one aromatic glucoside derivative, cordyceglycoside A (1), one new isoleucine derivative inner salt, cordycepisosalt A (2), a rare four-membered lactam, cinerealactam B (3), and one sesquiterpene derivative, cordycepsetp A (4), together with six known compounds were isolated from Cordyceps militaris. The structures including absolute configurations of these new compounds, were unambiguously elucidated by spectroscopic data analysis and single crystal X-ray diffraction. Biological evaluation of compounds 1-4 showed that 3 displays anti-renal fibrotic activities in TGF-β1 induced NRK-52e cells. Furthermore, DARTS coupled with LC-MS/MS analysis was used to identify candidate target proteins for 3. Subsequently, C1qbp knockdown using siRNA allowed us to validate the target protein of 3.

Mitoquinone Alleviates Donation after Cardiac Death Kidney Injury during Hypothermic Machine Perfusion in Rat Model

Transplanted organs are subjected to harmful conditions through stopping blood flow, hypothermic storage of the graft, and subsequent reperfusion. In particular, kidneys donated from patients after cardiac arrest (DCD) are classified as more vulnerable to ischemia-reperfusion injury (IRI). Hypothermic machine perfusion is proposed as a solution for better kidney storage before transplantation, and it is a good platform for additional graft treatment. Antioxidants have gained interest in regenerative medicine due to their ability to scavenge reactive oxygen species (ROS), which play a key role in IRI. We evaluated the effect of Mitoquinone (MitoQ), a strong mitochondria-targeted antioxidant, administered directly to the perfusing buffer. Rat kidneys were isolated, randomly classified into one of the following groups, donation after brainstem death (DBD), DCD, and DCD with MitoQ, and perfused for 22 hours with a hypothermic machine perfusion system. Subsequently, we detected levels of kidney injury (KIM-1) and oxidative stress (ROS/RNS, cytochrome C oxidase, and mitochondrial integrity) markers. We compared the activation of the apoptosis pathway (caspase 3 and 9), the concentration of phosphorylated Akt (pAkt), and the pAkt/total Akt ratio. MitoQ reduces KIM-1 concentration, total ROS/RNS, and the level of caspases. We observed a decrease in pAkt and the pAkt/total Akt ratio after drug administration. The length of warm ischemia time negatively impacts the graft condition. However, MitoQ added to the perfusing system as an 'on pump' therapy mitigates injury to the kidney before transplantation by inhibiting apoptosis and reducing ROS/RNS levels. We propose MitoQ as a potential drug for DCD graft preconditioning.

TGFβ instructs mTORC2 to activate PKCβII for increased TWIST1 expression in proximal tubular epithelial cell injury

The plasticity of proximal tubular epithelial cells in response to TGFβ contributes to the expression of TWIST1 to drive renal fibrosis. The mechanism of TWIST1 expression is not known. We show that both PI3 kinase and its target mTORC2 increase TGFβ-induced TWIST1 expression. TGFβ enhances phosphorylation on Ser-660 in the protein kinase C βII (PKCβII) hydrophobic motif site. Remarkably, phosphorylation-deficient PKCβIIS660A, kinase-dead PKCβII, and PKCβII knockdown blocked TWIST1 expression by TGFβ. Inhibition of TWIST1 arrested TGFβ-induced tubular cell hypertrophy and the expression of fibronectin, collagen I (α2), and α-smooth muscle actin. By contrast, TWIST1 overexpression induced these pathologies. Interestingly, the inhibition of PKCβII reduced these phenomena, which were countered by the expression of TWIST1. These results provide the first evidence for the involvement of the mTORC2-PKCβII axis in TWIST1 expression to promote tubular cell pathology.

Oncoprotein DJ-1 interacts with mTOR complexes to effect transcription factor Hif1α-dependent expression of collagen I (α2) during renal fibrosis

Proximal tubular epithelial cells respond to transforming growth factor β (TGFβ) to synthesize collagen I (α2) during renal fibrosis. The oncoprotein DJ-1 has previously been shown to promote tumorigenesis and prevent apoptosis of dopaminergic neurons; however, its role in fibrosis signaling is unclear. Here, we show TGFβ-stimulation increased expression of DJ-1, which promoted noncanonical mTORC1 and mTORC2 activities. We show DJ-1 augmented the phosphorylation/activation of PKCβII, a direct substrate of mTORC2. In addition, coimmunoprecipitation experiments revealed association of DJ-1 with Raptor and Rictor, exclusive subunits of mTORC1 and mTORC2, respectively, as well as with mTOR kinase. Interestingly, siRNAs against DJ-1 blocked TGFβ-stimulated expression of collagen I (α2), while expression of DJ-1 increased expression of this protein. In addition, expression of dominant negative PKCβII and siRNAs against PKCβII significantly inhibited TGFβ-induced collagen I (α2) expression. In fact, constitutively active PKCβII abrogated the effect of siRNAs against DJ-1, suggesting a role of PKCβII downstream of this oncoprotein. Moreover, we demonstrate expression of collagen I (α2) stimulated by DJ-1 and its target PKCβII is dependent on the transcription factor hypoxia-inducible factor 1α (Hif1α). Finally, we show in the renal cortex of diabetic rats that increased TGFβ was associated with enhanced expression of DJ-1 and activation of mTOR and PKCβII, concomitant with increased Hif1α and collagen I (α2). Overall, we identified that DJ-1 affects TGFβ-induced expression of collagen I (α2) via an mTOR-, PKCβII-, and Hif1α-dependent mechanism to regulate renal fibrosis.

TGFβ acts through PDGFRβ to activate mTORC1 via the Akt/PRAS40 axis and causes glomerular mesangial cell hypertrophy and matrix protein expression

Interaction of transforming growth factor-β (TGFβ)-induced canonical signaling with the noncanonical kinase cascades regulates glomerular hypertrophy and matrix protein deposition, which are early features of glomerulosclerosis. However, the specific target downstream of the TGFβ receptor involved in the noncanonical signaling is unknown. Here, we show that TGFβ increased the catalytic loop phosphorylation of platelet-derived growth factor receptor β (PDGFRβ), a receptor tyrosine kinase expressed abundantly in glomerular mesangial cells. TGFβ increased phosphorylation of the PI 3-kinase-interacting Tyr-751 residue of PDGFRβ, thus activating Akt. Inhibition of PDGFRβ using a pharmacological inhibitor and siRNAs blocked TGFβ-stimulated phosphorylation of proline-rich Akt substrate of 40 kDa (PRAS40), an intrinsic inhibitory component of mTORC1, and prevented activation of mTORC1 in the absence of any effect on Smad 2/3 phosphorylation. Expression of constitutively active myristoylated Akt reversed the siPDGFRβ-mediated inhibition of mTORC1 activity; however, co-expression of the phospho-deficient mutant of PRAS40 inhibited the effect of myristoylated Akt, suggesting a definitive role of PRAS40 phosphorylation in mTORC1 activation downstream of PDGFRβ in mesangial cells. Additionally, we demonstrate that PDGFRβ-initiated phosphorylation of PRAS40 is required for TGFβ-induced mesangial cell hypertrophy and fibronectin and collagen I (α2) production. Increased activating phosphorylation of PDGFRβ is also associated with enhanced TGFβ expression and mTORC1 activation in the kidney cortex and glomeruli of diabetic mice and rats, respectively. Thus, pursuing TGFβ noncanonical signaling, we identified how TGFβ receptor I achieves mTORC1 activation through PDGFRβ-mediated Akt/PRAS40 phosphorylation to spur mesangial cell hypertrophy and matrix protein accumulation. These findings provide support for targeting PDGFRβ in TGFβ-driven renal fibrosis.

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.

Aldosterone promotes renal interstitial fibrosis via the AIF‑1/AKT/mTOR signaling pathway

A number of studies have shown that aldosterone serves an important role in promoting renal interstitial fibrosis, although the specific mechanism remains to be elucidated. A previous study revealed that the fibrotic effect of aldosterone was associated with the expression of allograft inflammatory factor 1 (AIF‑1) in RAW264.7 macrophage cells, in a time‑ and concentration‑dependent manner. However, the exact mechanism through which aldosterone promotes renal interstitial fibrosis remains unknown. In the present study, the effects of aldosterone on renal inflammatory cell infiltration, collagen deposition and the expression levels of AIF‑1, phosphatidylinositol 3‑kinase (PI3K), AKT serine/threonine kinase (AKT), mammalian target of rapamycin (mTOR), the oxidative stress factor NADPH oxidase 2 (NOX2) and nuclear transcription factor erythroid‑related factor 2 (Nrf2) were assessed in normal rats, rats treated with aldosterone, rats treated with aldosterone and spironolactone and those treated with spironolactone only (used as the control). The effect of aldosterone on these factors was also investigated in the renal interstitium of unilateral ureteral obstruction (UUO) rats. Additionally, the AIF‑1 gene was overexpressed and knocked down in macrophage RAW264.7 cells, and the effects of aldosterone on PI3K, AKT, mTOR, NOX2 and Nrf2 were subsequently investigated. The results showed that aldosterone promoted inflammatory cell infiltration, collagen deposition and the expression of AIF‑1, PI3K, AKT, mTOR and NOX2, but inhibited the expression of Nrf2. In the UUO rats, aldosterone also promoted renal interstitial inflammatory cell infiltration, collagen deposition and the expression of AIF‑1, NOX2, PI3K, AKT and mTOR, whereas the expression of Nrf2 was downregulated by aldosterone compared with that in the UUO‑only group; the influence of aldosterone was counteracted by spironolactone in the normal and UUO rats. In vitro, aldosterone upregulated the expression levels of AKT, mTOR, NOX2 and Nrf2 in RAW264.7 cells compared with those in untreated cells. Suppressing the expression of AIF‑1 inhibited the effects of aldosterone, whereas the overexpression of AIF‑1 enhanced these effects in RAW264.7 cells. These findings indicated that aldosterone promoted renal interstitial fibrosis by upregulating the expression of AIF‑1 and that the specific mechanism may involve AKT/mTOR and oxidative stress signaling.

Erlotinib attenuates the progression of chronic kidney disease in rats with remnant kidney

Background

Increasing evidence indicates that epidermal growth factor receptor (EGFR) has a pathogenic role in renal fibrosis. Currently no effective treatment can completely halt the progression of chronic kidney disease (CKD). This study was undertaken to investigate the renoprotective effects of erlotinib, a tyrosine kinase inhibitor that can block EGFR activity in the progression of CKD and the mechanisms involved.

Methods

Sprague Dawley rats with 5/6 nephrectomy were administered either erlotinib or vehicle from 2 weeks after surgery and for a period of 8 weeks. Blood pressure, proteinuria and serum creatinine were measured periodically. Renal morphological investigations were performed at sacrifice. In vitro, we used normal human mesangial cells (NHMCs) and human proximal tubular cells to investigate the inhibitory effects of erlotinib on renal fibrosis-associated signaling pathways by western blotting.

Results

Erlotinib treatment significantly blunted the progression of CKD as evidenced by reduced levels of serum creatinine, proteinuria and renal cortical profibrogenic genes and scores of glomerulosclerosis and tubulointerstitial damage. Tubulointerstitial macrophage infiltration and multiple pro-inflammatory cytokine gene expression levels were also attenuated by erlotinib treatment. In vitro, heparin-binding epidermal growth factor-like growth factor-induced Akt and extracellular-regulated kinase (ERK) 1/2 activation in normal human mesangial cells and human proximal tubular cells was inhibited by pretreatment with erlotinib.

Conclusions

EGFR blocking by erlotinib protected against renal fibrosis in 5/6 nephrectomized rats via inhibition of Akt and ERK 1/2 signaling pathways, which are associated with renal fibrosis. Erlotinib also has anti-inflammatory properties, which may contribute to its renoprotective effects. Erlotinib represents a potential novel therapeutic strategy for the treatment of CKD.

High glucose increases miR-214 to power a feedback loop involving PTEN and the Akt/mTORC1 signaling axis

The mechanism of PTEN repression by high glucose in diabetic nephropathy is not known. Using proximal tubular cells, we show that inhibition of PI3 kinase/Akt and their inactive enzymes prevents high glucose-induced PTEN downregulation. Similarly, rapamycin (Rapa) and shRaptor block suppression of PTEN by high glucose. In contrast, the constitutive activation of Akt and mechanistic target of rapamycin (mTOR)C1 decrease the expression of PTEN, similarly to high glucose. Remarkably, PI3 kinase/Akt/mTORC1 inhibition significantly attenuates high glucose-stimulated increase in miR-214, which targets PTEN, while constitutively active Akt/mTORC1 increases miR-214. Furthermore, anti-miR-214 and mTORC1 inhibition block high glucose-induced hypertrophy and fibronectin expression. These results reveal the first evidence for the presence of a high glucose-forced positive feedback conduit between the three-layered kinase cascade and miR-214/ PTEN in tubular cell injury.

Deacetylation of S6 kinase promotes high glucose-induced glomerular mesangial cell hypertrophy and matrix protein accumulation

S6 kinase acts as a driver for renal hypertrophy and matrix accumulation, two key pathologic signatures of diabetic nephropathy. As a post-translational modification, S6 kinase undergoes acetylation at the C terminus. The role of this acetylation to regulate kidney glomerular cell hypertrophy and matrix expansion is not known. In mesangial cells, high glucose decreased the acetylation and enhanced phosphorylation of S6 kinase and its substrates rps6 and eEF2 kinase that lead to dephosphorylation of eEF2. To determine the mechanism of S6 kinase deacetylation, we found that trichostatin A, a pan-histone deacetylase (HDAC) inhibitor, blocked all high glucose-induced effects. Furthermore, high glucose increased the expression and association of HDAC1 with S6 kinase. HDAC1 decreased the acetylation of S6 kinase and mimicked the effects of high glucose, resulting in mesangial cell hypertrophy and expression of fibronectin and collagen I (α2). In contrast, siRNA against HDAC1 inhibited these effects by high glucose. A C-terminal acetylation-mimetic mutant of S6 kinase suppressed high glucose-stimulated phosphorylation of S6 kinase, rps6 and eEF2 kinase, and inhibited the dephosphorylation of eEF2. Also, the acetylation mimetic attenuated the mesangial cell hypertrophy and fibronectin and collagen I (α2) expression. Conversely, an S6 kinase acetylation-deficient mutant induced all the above effects of high glucose. Finally, in the renal glomeruli of diabetic rats, the acetylation of S6 kinase was significantly reduced concomitant with increased HDAC1 and S6 kinase activity. In aggregate, our data uncovered a previously unrecognized role of S6 kinase deacetylation in high glucose-induced mesangial cell hypertrophy and matrix protein expression.

Mesangial Cells and Renal Fibrosis

The main cellular constituents in glomerular mesangium are mesangial cells, which account for approximately 30-40% of the total cells in the glomerulus. Together with the mesangial matrix, mesangial cells form the glomerular basement membrane (GBM) in the glomerulus, whose main function is to perform the filtration. Under the pathologic conditions, mesangial cells are activated, leading to hyperproliferation and excess extracellular matrix (ECM). Moreover, mesangial cells also secrete several kinds of inflammatory cytokines, adhesion molecules, chemokines, and enzymes, all of which participate in the process of renal glomerular fibrosis. During the past years, researchers have revealed the roles of mesangial cells and the associated signal pathways involved in renal fibrosis. In this section, we will discuss how mesangial cells are activated and its contributions to renal fibrosis, as well as the molecular mechanisms and novel anti-fibrotic agents. Full understanding of the contributions of mesangial cells to renal fibrosis will benefit the clinical drug developing.

Akt2 causes TGFβ-induced deptor downregulation facilitating mTOR to drive podocyte hypertrophy and matrix protein expression

TGFβ promotes podocyte hypertrophy and expression of matrix proteins in fibrotic kidney diseases such as diabetic nephropathy. Both mTORC1 and mTORC2 are hyperactive in response to TGFβ in various renal diseases. Deptor is a component of mTOR complexes and a constitutive inhibitor of their activities. We identified that deptor downregulation by TGFβ maintains hyperactive mTOR in podocytes. To unravel the mechanism, we found that TGFβ -initiated noncanonical signaling controls deptor inhibition. Pharmacological inhibitor of PI 3 kinase, Ly 294002 and pan Akt kinase inhibitor MK 2206 prevented the TGFβ induced downregulation of deptor, resulting in suppression of both mTORC1 and mTORC2 activities. However, specific isoform of Akt involved in this process is not known. We identified Akt2 as predominant isoform expressed in kidney cortex, glomeruli and podocytes. TGFβ time-dependently increased the activating phosphorylation of Akt2. Expression of dominant negative PI 3 kinase and its signaling inhibitor PTEN blocked Akt2 phosphorylation by TGFβ. Inhibition of Akt2 using a phospho-deficient mutant that inactivates its kinase activity, as well as siRNA against the kinase markedly diminished TGFβ -mediated deptor suppression, its association with mTOR and activation of mTORC1 and mTORC2. Importantly, inhibition of Akt2 blocked TGFβ -induced podocyte hypertrophy and expression of the matrix protein fibronectin. This inhibition was reversed by the downregulation of deptor. Interestingly, we detected increased phosphorylation of Akt2 concomitant with TGFβ expression in the kidneys of diabetic rats. Thus, our data identify previously unrecognized Akt2 kinase as a driver of TGFβ induced deptor downregulation and sustained mTORC1 and mTORC2 activation. Furthermore, we provide the first evidence that deptor downstream of Akt2 contributes to podocyte hypertrophy and matrix protein expression found in glomerulosclerosis in different renal diseases.

Deletion of β1-integrin in collecting duct principal cells leads to tubular injury and renal medullary fibrosis

The renal collecting duct (CD) contains two major cell types, intercalated (ICs) and principal cells (PCs). A previous report showed that deletion of β1-integrin in the entire renal CD causes defective CD morphogenesis resulting in kidney dysfunction. However, subsequent deletion of β1-integrin specifically in ICs and PCs, respectively, did not cause any morphological defects in the CDs. The discrepancy between these studies prompts us to reinvestigate the role of β1-integrin in CD cells, specifically in the PCs. We conditionally deleted β1-integrin in mouse CD PCs using a specific aquaporin-2 (AQP2) promoter Cre-LoxP system. The resulting mutant mice, β-1^f/fAQP2-Cre+, had lower body weight, failed to thrive, and died around 8-12 wk. Their CD tubules were dilated, and some of them contained cellular debris. Increased apoptosis and proliferation of PCs were observed in the dilated CDs. Trichrome staining and electron microscopy revealed the presence of peritubular and interstitial fibrosis that is associated with increased production of extracellular matrix proteins including collagen type IV and fibronectin, as detected by immunoblotting. Further analysis revealed a significantly increased expression of transforming growth factor-β (TGF-β)-induced protein, fibronectin, and TGF-β receptor-1 mRNAs and concomitantly increased phosphorylation of SMAD-2 that indicates the activation of the TGF-β signaling pathway. Therefore, our data reveal that normal expression of β1-integrin in PCs is a critical determinant of CD structural and functional integrity and further support the previously reported critical role of β1-integrin in the development and/or maintenance of the CD structure and function.

Lentiviral Vector-Mediated FoxO1 Overexpression Inhibits Extracellular Matrix Protein Secretion Under High Glucose Conditions in Mesangial Cells

Diabetic nephropathy is characterized by inordinate secretion of extracellular matrix (ECM) proteins from mesangial cells (MCs), which is tightly associated with excessive activation of TGF-β signaling. The forkhead transcription factor O1 (FoxO1) protects mesangial cells from hyperglycemia-induced oxidative stress, which may be involved in ameliorating the redundant secretion of ECM proteins under high glucose conditions. Here, we reported that high glucose elevated the level of p-Akt to attenuate endogenous FoxO1 bioactivities in MCs, accompanied with decreases in the mRNA expressions of catalase (CAT) and superoxide dismutase 2 (SOD2). Meanwhile, the expressions of major ECM proteins-FN and Col I-increased under high glucose conditions, in consistent with the activation of TGF-β/Smad signaling. By contrast, overexpression of nucleus-localized FoxO1 (insensitive to Akt phosphorylation) directly up-regulated the expressions of anti-oxidative enzymes, accompanied with inactivation of TGF-β/Smad3 pathway, as well as decreases of extracellular matrix proteins. Moreover, similar to those MCs overexpressed of nucleus-localized FoxO1 in high glucose conditions, MCs with down-regulation of FoxO1 by small interference-RNA under normal glucose conditions showed increased FN level and activated TGF-β/Smad3 pathway. Our findings link the anti-oxidative activity of FoxO1 and the TGF-β-induced secretion of ECM proteins, indicating the novel role of FoxO1 in protecting MCs under high glucose conditions.

Regulatory mechanisms of TGF-β1-induced fibrogenesis of human alveolar epithelial cells

Pulmonary fibrosis is characterized by an extensive activation of fibrogenic cells and deposition of extracellular matrix (ECM). Transforming growth factor (TGF)‐β1 plays a pivotal role in the pathogenesis of pulmonary fibrosis, probably through the epithelial‐ to‐mesenchymal transition (EMT) and ECM production. The present study investigates potential mechanism by which TGF‐β1 induces EMT and ECM production in the fibrogenesis of human lung epithelial cells during pulmonary fibrosis. The expression of EMT phenotype and other proteins relevant to fibrogenesis were measured and the cell bio‐behaviours were assessed using Cell‐IQ Alive Image Monitoring System. We found that TGF‐β1‐induced EMT was accompanied with increased collagen I deposition, which may be involved in the regulation of connective tissue growth factor (CTGF) and phosphoinositide 3‐kinase (PI3K) signalling pathway. Treatment with PI3K inhibitors significantly attenuated the TGF‐β1‐ induced EMT, CTGF expression and collagen I synthesis in lung epithelial cells. The interference of CTGF expression impaired the basal and TGF‐β1‐stimulated collagen I deposition, but did not affect the process of EMT. Our data indicate that the signal pathway of TGF‐β1/PI3K/CTGF plays an important role in the fibrogenesis of human lung epithelial cells, which may be a novel therapeutic approach to prevent and treat pulmonary fibrosis.

Regulatory roles of microRNA-21 in fibrosis through interaction with diverse pathways (Review)

MicroRNA-21 (miR-21) is a small, non-coding RNA which can regulate gene expression at the post‑transcriptional level. While the fibrogenic process is vital in tissue repair, proliferation and transition of fibrogenic cells combined with an imbalance of secretion and degradation of the extracellular matrix results in excessive tissue remodeling and fibrosis. Recent studies have indicated that miR‑21 is overexpressed during fibrosis and can regulate the fibrogenic process in a variety of organs and tissues via diverse pathways. The present review summarized the significant roles of miR-21 in fibrosis and discussed the underlying key pathways.

Cellular and Molecular Mechanisms of Chronic Kidney Disease with Diabetes Mellitus and Cardiovascular Diseases as Its Comorbidities

Chronic kidney disease (CKD), diabetes mellitus (DM), and cardiovascular diseases (CVD) are complex disorders of partly unknown genesis and mostly known progression factors. CVD and DM are the risk factors of CKD and are strongly intertwined since DM can lead to both CKD and/or CVD, and CVD can lead to kidney disease. In recent years, our knowledge of CKD, DM, and CVD has been expanded and several important experimental, clinical, and epidemiological associations have been reported. The tight cellular and molecular interactions between the renal, diabetic, and cardiovascular systems in acute or chronic disease settings are becoming increasingly evident. However, the (patho-) physiological basis of the interactions of CKD, DM, and CVD with involvement of multiple endogenous and environmental factors is highly complex and our knowledge is still at its infancy. Not only single pathways and mediators of progression of these diseases have to be considered in these processes but also the mutual interactions of these factors are essential. The recent advances in proteomics and integrative analysis technologies have allowed rapid progress in analyzing complex disorders and clearly show the opportunity for new efficient and specific therapies. More than a dozen pathways have been identified so far, including hyperactivity of the renin–angiotensin (RAS)–aldosterone system, osmotic sodium retention, endothelial dysfunction, dyslipidemia, RAS/RAF/extracellular-signal-regulated kinase pathway, modification of the purinergic system, phosphatidylinositol 3-kinase (PI 3-kinase)-dependent signaling pathways, and inflammation, all leading to histomorphological alterations of the kidney and vessels of diabetic and non-diabetic patients. Since a better understanding of the common cellular and molecular mechanisms of these diseases may be a key to successful identification of new therapeutic targets, we review in this paper the current literature about cellular and molecular mechanisms of CKD.

Activation of glycogen synthase kinase 3β ameliorates diabetes-induced kidney injury

Increase in protein synthesis contributes to kidney hypertrophy and matrix protein accumulation in diabetes. We have previously shown that high glucose-induced matrix protein synthesis is associated with inactivation of glycogen synthase kinase 3β (GSK3β) in renal cells and in the kidneys of diabetic mice. We tested whether activation of GSK3β by sodium nitroprusside (SNP) mitigates kidney injury in diabetes. Studies in kidney-proximal tubular epithelial cells showed that SNP abrogated high glucose-induced laminin increment by stimulating GSK3β and inhibiting Akt, mTORC1, and events in mRNA translation regulated by mTORC1 and ERK. NONOate, an NO donor, also activated GSK3β, indicating that NO may mediate SNP stimulation of GSK3β. SNP administered for 3 weeks to mice with streptozotocin-induced type 1 diabetes ameliorated kidney hypertrophy, accumulation of matrix proteins, and albuminuria without changing blood glucose levels. Signaling studies showed that diabetes caused inactivation of GSK3β by activation of Src, Pyk2, Akt, and ERK; GSK3β inhibition activated mTORC1 and downstream events in mRNA translation in the kidney cortex. These reactions were abrogated by SNP. We conclude that activation of GSK3β by SNP ameliorates kidney injury induced by diabetes.

TGFβ-induced deptor suppression recruits mTORC1 and not mTORC2 to enhance collagen I (α2) gene expression

Enhanced TGFβ activity contributes to the accumulation of matrix proteins including collagen I (α2) by proximal tubular epithelial cells in progressive kidney disease. Although TGFβ rapidly activates its canonical Smad signaling pathway, it also recruits noncanonical pathway involving mTOR kinase to regulate renal matrix expansion. The mechanism by which chronic TGFβ treatment maintains increased mTOR activity to induce the matrix protein collagen I (α2) expression is not known. Deptor is an mTOR interacting protein that suppresses mTOR activity in both mTORC1 and mTORC2. In proximal tubular epithelial cells, TGFβ reduced deptor levels in a time-dependent manner with concomitant increase in both mTORC1 and mTORC2 activities. Expression of deptor abrogated activity of mTORC1 and mTORC2, resulting in inhibition of collagen I (α2) mRNA and protein expression via transcriptional mechanism. In contrast, neutralization of endogenous deptor by shRNAs increased activity of both mTOR complexes and expression of collagen I (α2) similar to TGFβ treatment. Importantly, downregulation of deptor by TGFβ increased the expression of Hif1α by increasing translation of its mRNA. TGFβ-induced deptor downregulation promotes Hif1α binding to its cognate hypoxia responsive element in the collagen I (α2) gene to control its protein expression via direct transcriptional mechanism. Interestingly, knockdown of raptor to specifically block mTORC1 activity significantly inhibited expression of collagen I (α2) and Hif1α while inhibition of rictor to prevent selectively mTORC2 activation did not have any effect. Critically, our data provide evidence for the requirement of TGFβ-activated mTORC1 only by deptor downregulation, which dominates upon the bystander mTORC2 activity for enhanced expression of collagen I (α2). Our results also suggest the presence of a safeguard mechanism involving deptor-mediated suppression of mTORC1 activity against developing TGFβ-induced renal fibrosis.

High glucose forces a positive feedback loop connecting Akt kinase and FoxO1 transcription factor to activate mTORC1 kinase for mesangial cell hypertrophy and matrix protein expression

High glucose-induced Akt acts as a signaling hub for mesangial cell hypertrophy and matrix expansion, which are recognized as cardinal signatures for the development of diabetic nephropathy. How mesangial cells sustain the activated state of Akt is not clearly understood. Here we show Akt-dependent phosphorylation of the transcription factor FoxO1 by high glucose. Phosphorylation-deficient, constitutively active FoxO1 inhibited the high glucose-induced phosphorylation of Akt to suppress the phosphorylation/inactivation of PRAS40 and mTORC1 activity. In contrast, dominant negative FoxO1 increased the phosphorylation of Akt, resulting in increased mTORC1 activity similar to high glucose treatment. Notably, FoxO1 regulates high glucose-induced protein synthesis, hypertrophy, and expression of fibronectin and PAI-1. High glucose paves the way for complications of diabetic nephropathy through the production of reactive oxygen species (ROS). We considered whether the FoxO1 target antioxidant enzyme catalase contributes to sustained activation of Akt. High glucose-inactivated FoxO1 decreases the expression of catalase to increase the production of ROS. Moreover, we show that catalase blocks high glucose-stimulated Akt phosphorylation to attenuate the inactivation of FoxO1 and PRAS40, resulting in the inhibition of mTORC1 and mesangial cell hypertrophy and fibronectin and PAI-1 expression. Finally, using kidney cortices from type 1 diabetic OVE26 mice, we show that increased FoxO1 phosphorylation is associated with decreased catalase expression and increased fibronectin and PAI-1 expression. Together, our results provide the first evidence for the presence of a positive feedback loop for the sustained activation of Akt involving inactivated FoxO1 and a decrease in catalase expression, leading to increased ROS and mesangial cell hypertrophy and matrix protein expression.

RhoA/Rho kinase mediates TGF-β1-induced kidney myofibroblast activation through Poldip2/Nox4-derived reactive oxygen species

The small G proteins Rac1 and RhoA regulate actin cytoskeleton, cell shape, adhesion, migration, and proliferation. Recent studies in our laboratory have shown that NADPH oxidase Nox4-derived ROS are involved in transforming growth factor (TGF)-β1-induced rat kidney myofibroblast differentiation assessed by the acquisition of an α-smooth muscle actin (α-SMA) phenotype and expression of an alternatively spliced fibronectin variant (Fn-EIIIA). Rac1 and RhoA are essential in signaling by some Nox homologs, but their role as effectors of Nox4 in kidney myofibroblast differentiation is not known. In the present study, we explored a link among Rac1 and RhoA and Nox4-dependent ROS generation in TGF-β1-induced kidney myofibroblast activation. TGF-β1 stimulated an increase in Nox4 protein expression, NADPH oxidase activity, and abundant α-SMA and Fn-EIIIA expression. RhoA but not Rac1 was involved in TGF-β1 induction of Nox4 signaling of kidney myofibroblast activation. TGF-β1 stimulated active RhoA-GTP and increased Rho kinase (ROCK). Inhibition of RhoA with small interfering RNA and ROCK using Y-27632 significantly reduced TGF-β1-induced stimulation of Nox4 protein, NADPH oxidase activity, and α-SMA and Fn-EIIIA expression. Treatment with diphenyleneiodonium, an inhibitor of NADPH oxidase, did not decrease RhoA activation but inhibited TGF-β1-induced α-SMA and Fn-EIIIA expression, indicating that RhoA is upstream of ROS generation. RhoA/ROCK also regulated polymerase (DNA-directed) δ-interacting protein 2 (Poldip2), a newly discovered Nox4 enhancer protein. Collectively, these data indicate that RhoA/ROCK is upstream of Poldip2-dependent Nox4 regulation and ROS production and induces redox signaling of kidney myofibroblast activation and may broader implications in the pathophysiology of renal fibrosis.

Potential role of Akt signaling in chronic kidney disease

Renal fibrosis, particularly tubulointerstitial fibrosis, is the common final outcome of almost all chronic kidney diseases. However, the mechanisms involved in the development of renal fibrosis are poorly understood. The Akt (also known as protein kinase B, PKB) family is serine/threonine protein kinases that play critical roles in regulating growth, proliferation, survival, metabolism and other cellular activities. Cytokines, high-glucose medium, transforming growth factor-β1 or advanced glycation end-products activate Akt in different renal cells. Increased Akt activation has been found in experimental tubulointerstitial fibrosis. In addition, Akt activation is also an important node in diverse signaling cascades involved in kidney damage. These data give evidence for a role for Akt in renal fibrosis, but no reviews are available on the role of Akt in the process. Thus, our aim is to review the role of Akt activation and signaling in renal fibrosis.

Requirement for active glycogen synthase kinase-3β in TGF-β1 upregulation of connective tissue growth factor (CCN2/CTGF) levels in human gingival fibroblasts

Connective tissue growth factor (CCN2/CTGF) mediates transforming growth factor-β (TGF-β)-induced fibrosis. Drug-induced gingival overgrowth is tissue specific. Here the role of the phosphoinositol 3-kinase (PI3K) pathway in mediating TGF-β1-stimulated CCN2/CTGF expression in primary human adult gingival fibroblasts and human adult lung fibroblasts was compared. Data indicate that PI3K inhibitors attenuate upregulation of TGF-β1-induced CCN2/CTGF expression in human gingival fibroblasts independent of reducing JNK MAP kinase activation. Pharmacologic inhibitors and small interfering (si)RNA-mediated knockdown studies indicate that calcium-dependent isoforms and an atypical isoform of protein kinase C (PKC-δ) do not mediate TGF-β1-stimulated CCN2/CTGF expression in gingival fibroblasts. As glycogen synthase kinase-3β (GSK-3β) can undergo phosphorylation by the PI3K/pathway, the effects of GSK-3β inhibitor kenpaullone and siRNA knockdown were investigated. Data in gingival fibroblasts indicate that kenpaullone attenuates TGF-β1-mediated CCN2/CTGF expression. Activation of the Wnt canonical pathways with Wnt3a, which inhibits GSK-3β, similarly inhibits TGF-β1-stimulated CCN2/CTGF expression. In contrast, inhibition of GSK-3β by Wnt3a does not inhibit, but modestly stimulates, CCN2/CTGF levels in primary human adult lung fibroblasts and is β-catenin dependent, consistent with previous studies performed in other cell models. These data identify a novel pathway in gingival fibroblasts in which inhibition of GSK-3β attenuates CCN2/CTGF expression. In adult lung fibroblasts inhibition of GSK-3β modestly stimulates TGF-β1-regulated CCN2/CTGF expression. These studies have potential clinical relevance to the tissue specificity of drug-induced gingival overgrowth.

TGFβ-induced PI 3 kinase-dependent Mnk-1 activation is necessary for Ser-209 phosphorylation of eIF4E and mesangial cell hypertrophy

Transforming growth factorβ (TGFβ)-induced canonical signal transduction is involved in glomerular mesangial cell hypertrophy; however, the role played by the noncanonical TGFβ signaling remains largely unexplored. TGFβ time-dependently stimulated eIF4E phosphorylation at Ser-209 concomitant with enhanced phosphorylation of Erk1/2 (extracellular signal regulated kinase1/2) and MEK (mitogen-activated and extracellular signal-regulated kinase kinase) in mesangial cells. Inhibition of Erk1/2 by MEK inhibitor or by expression of dominant negative Erk2 blocked eIF4E phosphorylation, resulting in attenuation of TGFβ-induced protein synthesis and mesangial cell hypertrophy. Expression of constitutively active (CA) MEK was sufficient to induce protein synthesis and hypertrophy similar to those induced by TGFβ. Pharmacological or dominant negative inhibition of phosphatidylinositol (PI) 3 kinase decreased MEK/Erk1/2 phosphorylation leading to suppression of eIF4E phosphorylation. Inducible phosphorylation of eIF4E at Ser-209 is mediated by Mnk-1 (mitogen-activated protein kinase signal-integrating kinase-1). Both PI 3 kinase and Erk1/2 promoted phosphorylation of Mnk-1 in response to TGFβ. Dominant negative Mnk-1 significantly inhibited TGFβ-stimulated protein synthesis and hypertrophy. Interestingly, inhibition of mTORC1 activity, which blocks dissociation of eIF4E-4EBP-1 complex, decreased TGFβ-stimulated phosphorylation of eIF4E without any effect on Mnk-1 phosphorylation. Furthermore, mutant eIF4E S209D, which mimics phosphorylated eIF4E, promoted protein synthesis and hypertrophy similar to TGFβ. These results were confirmed using phosphorylation deficient mutant of eIF4E. Together our results highlight a significant role of dissociation of 4EBP-1-eIF4E complex for Mnk-1-mediated phosphorylation of eIF4E. Moreover, we conclude that TGFβ-induced noncanonical signaling circuit involving PI 3 kinase-dependent Mnk-1-mediated phosphorylation of eIF4E at Ser-209 is required to facilitate mesangial cell hypertrophy.

FOG2 protein down-regulation by transforming growth factor-β1-induced microRNA-200b/c leads to Akt kinase activation and glomerular mesangial hypertrophy related to diabetic nephropathy

Glomerular hypertrophy is a hallmark of diabetic nephropathy. Akt kinase activated by transforming growth factor-β1 (TGF-β) plays an important role in glomerular mesangial hypertrophy. However, the mechanisms of Akt activation by TGF-β are not fully understood. Recently, miR-200 and its target FOG2 were reported to regulate the activity of phosphatidylinositol 3-kinase (the upstream activator of Akt) in insulin signaling. Here, we show that TGF-β activates Akt in glomerular mesangial cells by inducing miR-200b and miR-200c, both of which target FOG2, an inhibitor of phosphatidylinositol 3-kinase activation. FOG2 expression was reduced in the glomeruli of diabetic mice as well as TGF-β-treated mouse mesangial cells (MMC). FOG2 knockdown by siRNAs in MMC activated Akt and increased the protein content/cell ratio suggesting hypertrophy. A significant increase of miR-200b/c levels was detected in diabetic mouse glomeruli and TGF-β-treated MMC. Transfection of MMC with miR-200b/c mimics significantly decreased the expression of FOG2. Conversely, miR-200b/c inhibitors attenuated TGF-β-induced decrease in FOG2 expression. Furthermore, miR-200b/c mimics increased the protein content/cell ratio, whereas miR-200b/c inhibitors abrogated the TGF-β-induced increase in protein content/cell. In addition, down-regulation of FOG2 by miR-200b/c could activate not only Akt but also ERK, which was also through PI3K activation. These data suggest a new mechanism for TGF-β-induced Akt activation through FOG2 down-regulation by miR-200b/c, which can lead to glomerular mesangial hypertrophy in the progression of diabetic nephropathy.

TGF-β induces acetylation of chromatin and of Ets-1 to alleviate repression of miR-192 in diabetic nephropathy

MicroRNAs (miRNAs), such as miR-192, mediate the actions of transforming growth factor-β1 (TGF-β) related to the pathogenesis of diabetic kidney diseases. We found that the biphasic induction of miR-192 expression by TGF-β in mouse renal glomerular mesangial cells initially involved the Smad transcription factors, followed by sustained expression that was promoted by acetylation of the transcription factor Ets-1 and of histone H3 by the acetyltransferase p300, which was activated by the serine and threonine kinase Akt. In mesangial cells from Ets-1-deficient mice or in cells in which Ets-1 was knocked down, basal amounts of miR-192 were higher than those in control cells, but sustained induction of miR-192 by TGF-β was attenuated. Furthermore, inhibition of Akt or ectopic expression of dominant-negative histone acetyltransferases decreased p300-mediated acetylation and Ets-1 dissociation from the miR-192 promoter and prevented miR-192 expression in response to TGF-β. Activation of Akt and p300 and acetylation of Ets-1 and histone H3 were increased in glomeruli from diabetic db/db mice compared to nondiabetic db/+ mice, suggesting that this pathway may contribute to diabetic nephropathy. These findings provide insight into the regulation of miRNAs through signaling-mediated changes in transcription factor activity and in epigenetic histone acetylation under normal and disease states.

Over-production of nitric oxide by oxidative stress-induced activation of the TGF-β1/PI3K/Akt pathway in mesangial cells cultured in high glucose

AIM

To investigate whether NO over-production in rat mesangial cells cultured in high glucose (HG) is related to activation of the TGF-β1/PI3K/Akt pathway.

METHODS

Rat mesangial cells line (HBZY-1) was exposed to HG (24.44 mmol/L) or H2O2 (10 μmol/L) for 16 h. NO release was quantified using the Griess assay. The TGF-β1 level was measured using ELISA. The protein expression of p-Akt, t-Akt, Bim, and iNOS was examined by Western blotting. The mRNA levels of TGF-β1 and Bim were measured using RT-PCR. The cell proliferation rate was estimated using a BrdU incorporation assay.

RESULTS

Treatment of the cells with HG, H2O2, or TGF-β1 (5 ng/mL) significantly increased the NO level that was substantially inhibited by co-treatment with the NADPH oxidase inhibitor diphenylene iodonium (DPI), TGF-β1 inhibitor SB431542, or PI3K inhibitor LY294002. Both HG and H2O2 significantly increased the protein and mRNA levels of TGF-β1 in the cells, and HG-induced increases of TGF-β1 protein and mRNA were blocked by co-treatment with DPI. Furthermore, the treatment with HG or H2O2 significantly increased the expression of phosphorylated Akt and iNOS and cell proliferation rate, which was blocked by co-treatment with DPI, SB431542, or LY294002. Moreover, the treatment with HG or H2O2 significantly inhibited Bim protein and mRNA expression, which was reversed by co-treatment with DPI, SB431542, or LY294002.

CONCLUSION

The results demonstrate that high glucose causes oxidative stress and NO over-production in rat mesangial cells in vitro via decreasing Bim and increasing iNOS, which are at least partially mediated by the TGF-β1/PI3K/Akt pathway.

Transforming growth factor β integrates Smad 3 to mechanistic target of rapamycin complexes to arrest deptor abundance for glomerular mesangial cell hypertrophy

In many renal diseases, transforming growth factor β (TGFβ)-stimulated canonical Smad 3 and noncanonical mechanistic target of rapamycin (mTOR) promote increased protein synthesis and mesangial cell hypertrophy. The cellular underpinnings involving these signaling molecules to regulate mesangial cell hypertrophy are not fully understood. Deptor has recently been identified as an mTOR interacting protein and functions as an endogenous inhibitor of the kinase activity for both TORC1 and TORC2. Prolonged incubation of mesangial cells with TGFβ reduced the levels of deptor concomitant with an increase in TORC1 and TORC2 activity. Sustained TGFβ activation was required to inhibit association of deptor with mTOR, whereas rapid activation had no effect. Using the mTOR inhibitor PP242, we found that TGFβ-induced both early and sustained activation of TORC1 and TORC2 was necessary for deptor suppression. PP242-induced reversal of deptor suppression by TGFβ was associated with a significant inhibition of TGFβ-stimulated protein synthesis and hypertrophy. Interestingly, expression of siRNA against Smad 3 or Smad 7, which blocks TGFβ receptor-specific Smad 3 signaling, prevented TGFβ-induced suppression of deptor abundance and TORC1/2 activities. Furthermore, overexpression of Smad 3 decreased deptor expression similar to TGFβ stimulation concomitant with increased TORC1 and TORC2 activities. Finally, knockdown of deptor reversed Smad 7-mediated inhibition of protein synthesis and mesangial cell hypertrophy induced by TGFβ. These data reveal the requirement of both early and late activation of mTOR for TGFβ-induced protein synthesis. Our results support that TGFβ-stimulated Smad 3 acts as a key node to instill a feedback loop between deptor down-regulation and TORC1/2 activation in driving mesangial cell hypertrophy.

Molecular events in matrix protein metabolism in the aging kidney

We explored molecular events associated with aging-induced matrix changes in the kidney. C57BL6 mice were studied in youth, middle age, and old age. Albuminuria and serum cystatin C level (an index of glomerular filtration) increased with aging. Renal hypertrophy was evident in middle-aged and old mice and was associated with glomerulomegaly and increase in mesangial fraction occupied by extracellular matrix. Content of collagen types I and III and fibronectin was increased with aging; increment in their mRNA varied with the phase of aging. The content of ZEB1 and ZEB2, collagen type I transcription inhibitors, and their binding to the collagen type Iα2 promoter by ChIP assay also showed age-phase-specific changes. Lack of increase in mRNA and data from polysome assay suggested decreased degradation as a potential mechanism for kidney collagen type I accumulation in the middle-aged mice. These changes occurred with increment in TGFβ mRNA and protein and activation of its SMAD3 pathway; SMAD3 binding to the collagen type Iα2 promoter was also increased. TGFβ-regulated microRNAs (miRs) exhibited selective regulation. The renal cortical content of miR-21 and miR-200c, but not miR-192, miR-200a, or miR-200b, was increased with aging. Increased miR-21 and miR-200c contents were associated with reduced expression of their targets, Sprouty-1 and ZEB2, respectively. These data show that aging is associated with complex molecular events in the kidney that are already evident in the middle age and progress to old age. Age-phase-specific regulation of matrix protein synthesis occurs and involves matrix protein-specific transcriptional and post-transcriptional mechanisms.

Snail1 controls TGF-β responsiveness and differentiation of mesenchymal stem cells

The Snail1 transcriptional repressor plays a key role in triggering epithelial to mesenchymal transition. Although Snail1 is widely expressed in early development, in adult animals it is limited to a subset of mesenchymal cells where it has a largely unknown function. Using a mouse model with inducible depletion of Snail1, here we demonstrate that Snail1 is required to maintain mesenchymal stem cells (MSCs). This effect is associated to the responsiveness to TGF-β1 which shows a strong Snail1 dependence. Snail1-depletion in conditional knock-out adult animals causes a significant decrease in the number of bone marrow-derived MSCs. In culture, Snail1-deficient MSCs prematurely differentiate to osteoblasts or adipocytes and, in contrast to controls, are resistant to the TGF-β1-induced differentiation block. These results demonstrate a new role for Snail1 in TGF-β response and MSC maintenance.

TGFβ-stimulated microRNA-21 utilizes PTEN to orchestrate AKT/mTORC1 signaling for mesangial cell hypertrophy and matrix expansion

Transforming growth factor-β (TGFβ) promotes glomerular hypertrophy and matrix expansion, leading to glomerulosclerosis. MicroRNAs are well suited to promote fibrosis because they can repress gene expression, which negatively regulate the fibrotic process. Recent cellular and animal studies have revealed enhanced expression of microRNA, miR-21, in renal cells in response to TGFβ. Specific miR-21 targets downstream of TGFβ receptor activation that control cell hypertrophy and matrix protein expression have not been studied. Using 3′UTR-driven luciferase reporter, we identified the tumor suppressor protein PTEN as a target of TGFβ-stimulated miR-21 in glomerular mesangial cells. Expression of miR-21 Sponge, which quenches endogenous miR-21 levels, reversed TGFβ-induced suppression of PTEN. Additionally, miR-21 Sponge inhibited TGFβ-stimulated phosphorylation of Akt kinase, resulting in attenuation of phosphorylation of its substrate GSK3β. Tuberin and PRAS40, two other Akt substrates, and endogenous inhibitors of mTORC1, regulate mesangial cell hypertrophy. Neutralization of endogenous miR-21 abrogated TGFβ-stimulated phosphorylation of tuberin and PRAS40, leading to inhibition of phosphorylation of S6 kinase, mTOR and 4EBP-1. Moreover, downregulation of miR-21 significantly suppressed TGFβ-induced protein synthesis and hypertrophy, which were reversed by siRNA-targeted inhibition of PTEN expression. Similarly, expression of constitutively active Akt kinase reversed the miR-21 Sponge-mediated inhibition of TGFβ-induced protein synthesis and hypertrophy. Furthermore, expression of constitutively active mTORC1 prevented the miR-21 Sponge-induced suppression of mesangial cell protein synthesis and hypertrophy by TGFβ. Finally, we show that miR-21 Sponge inhibited TGFβ-stimulated fibronectin and collagen expression. Suppression of PTEN expression and expression of both constitutively active Akt kinase and mTORC1 independently reversed this miR-21-mediated inhibition of TGFβ-induced fibronectin and collagen expression. Our results uncover an essential role of TGFβ-induced expression of miR-21, which targets PTEN to initiate a non-canonical signaling circuit involving Akt/mTORC1 axis for mesangial cell hypertrophy and matrix protein synthesis.

MicroRNA circuits in transforming growth factor-β actions and diabetic nephropathy

Diabetes is associated with significantly increased rates of kidney disease or diabetic nephropathy (DN), a severe microvascular complication that can lead to end-stage renal disease. End-stage renal disease needs to be treated by dialysis or kidney transplantation and also is associated with cardiovascular disease and macrovascular complications. Therefore, effective renal protection is critical to reduce the rates of mortality associated with diabetes. Although key signal transduction and gene regulation mechanisms have been identified and several drugs are currently in clinical use, the rates of DN are still escalating, suggesting the imperative need to identify new biomarkers and drug targets. The recent discovery of microRNAs (miRNAs) and their cellular functions provide an opportunity to fill these critical gaps. Because miRNAs can modulate the actions of key factors involved in DN such as transforming growth factor-β, they could be novel targets for the treatment of DN. This review covers the recent studies on the roles of miRNAs and miRNA circuits in transforming growth factor-β actions and in DN.

Transforming growth factor beta-induced reactivation of Epstein-Barr virus involves multiple Smad-binding elements cooperatively activating expression of the latent-lytic switch BZLF1 gene

Transforming growth factor β (TGF-β) physiologically induces Epstein-Barr virus (EBV) lytic infection by activating the expression of EBV's latent-lytic switch BZLF1 gene. Liang et al. (J. Biol. Chem. 277:23345-23357, 2002) previously identified a Smad-binding element (SBE) within the BZLF1 promoter, Zp; however, it accounts for only 20 to 30% of TGF-β-mediated activation of transcription from Zp. Here, we identified additional factors responsible for the rest of this activation. The incubation of EBV-positive MutuI cells with a TGF-β neutralizing antibody or inhibitors of the TGF-β type I receptor (TβRI) or Smad3 eliminated the TGF-β-induced reactivation of EBV. The coexpression of Smad2, Smad3, and Smad4 together with a constitutively active form of TβRI induced 15- to 25-fold transcription from Zp in gastric carcinoma AGS cells. By electrophoretic mobility shift assays, we identified four additional Smad-binding elements, named SBE2 to SBE5. Substitution mutations in individual SBEs reduced Smad-mediated activation of Zp by 20 to 60%; together, these mutations essentially eliminated it. Chromatin immunoprecipitation assays confirmed that Smad4 newly bound the Zp region of the EBV genome following the incubation of MutuI cells with TGF-β. SBE2 overlaps the ZEB-binding ZV silencing element of Zp. Depending upon posttranslational modifications, Smad4 either competed with ZEB1 for binding or formed a complex with ZEB1 on the Zp ZV element in a cell-free assay system. In transiently transfected cells, exogenously expressed ZEB1 inhibited Smad-mediated transcriptional activation from Zp. We conclude that TGF-β induces EBV lytic reactivation via the canonical Smad pathway by activating BZLF1 gene expression through multiple SBEs acting in concert.

MicroRNA-21 orchestrates high glucose-induced signals to TOR complex 1, resulting in renal cell pathology in diabetes

Hyperglycemia induces a wide array of signaling pathways in the kidney that lead to hypertrophy and matrix expansion, eventually culminating in progressive kidney failure. High glucose-induced reduction of the tumor suppressor protein phosphatase and tensin homolog deleted in chromosome 10 (PTEN) contributes to renal cell hypertrophy and matrix expansion. We identified microRNA-21 (miR-21) as the molecular link between high glucose and PTEN suppression. Renal cortices from OVE26 type 1 diabetic mice showed significantly elevated levels of miR-21 associated with reduced PTEN and increased fibronectin content. In renal mesangial cells, high glucose increased the expression of miR-21, which targeted the 3'-UTR of PTEN mRNA to inhibit PTEN protein expression. Overexpression of miR-21 mimicked the action of high glucose, which included a reduction in PTEN expression and a concomitant increase in Akt phosphorylation. In contrast, expression of miR-21 Sponge, to inhibit endogenous miR-21, prevented down-regulation of PTEN and phosphorylation of Akt induced by high glucose. Interestingly, high glucose-stimulated miR-21 inactivated PRAS40, a negative regulator of TORC1. Finally, miR-21 enhanced high glucose-induced TORC1 activity, resulting in renal cell hypertrophy and fibronectin expression. Thus, our results identify a previously unrecognized function of miR-21 that is the reciprocal regulation of PTEN levels and Akt/TORC1 activity that mediate critical pathologic features of diabetic kidney disease.

Glucose-lowering effect of DLBS3233 is mediated through phosphorylation of tyrosine and upregulation of PPARγ and GLUT4 expression

Background:

DLBS3233 is a standardized extract combination containing Lagerstroemia speciosa and Cinnamomum burmannii. The effect of DLBS3233 on glucose uptake, adiponectin secretion, and insulin signaling was examined in this study.

Methods:

3T3 Swiss albino preadipocytes and adipocytes were used to investigate gene expression detected using the reverse transcription polymerase chain reaction method. Immunoblotting assay and in vitro glucose uptake assay were also carried out in the experiment.

Results:

DLBS3233 was seen to increase phosphorylation at the tyrosine residue of the insulin receptor substrate. DLBS3233 was also found to enhance the expression of genes associated with increased insulin signaling and sensitivity, such as peroxisome proliferator-activated receptor gamma, phosphatidylinositol-3 kinase, Akt, and glucose transporter 4. In addition, glucose transporter 4 protein levels were seen to increase as a result of DLBS3233 administration. The combination of extracts also increased glucose uptake and adiponectin secretion, and decreased resistin secretion significantly relative to control cells. Moreover, DLBS3233 administered to insulin-resistant Wistar rats showed an ability to control blood sugar, insulin levels, and other lipoproteins, including high-density lipoprotein, low-density lipoprotein, triglycerides, and total cholesterol.

Conclusion:

DLBS3233, as a combination of herbal extracts, holds promise in the treatment of type 2 diabetes, and possibly also in prevention of the disease.

Myofibroblast differentiation during fibrosis: role of NAD(P)H oxidases

Progression of fibrosis involves interstitial hypercellularity, matrix accumulation, and atrophy of epithelial structures, resulting in loss of normal function and ultimately organ failure. There is common agreement that the fibroblast/myofibroblast is the cell type most responsible for interstitial matrix accumulation and consequent structural deformations associated with fibrosis. During wound healing and progressive fibrotic events, fibroblasts transform into myofibroblasts acquiring smooth muscle features, most notably the expression of alpha-smooth muscle actin and synthesis of mesenchymal cell-related matrix proteins. In renal disease, glomerular mesangial cells also acquire a myofibroblast phenotype and synthesize the same matrix proteins. The origin of interstitial myofibroblasts during fibrosis is a matter of debate, where the cells are proposed to derive from resident fibroblasts, pericytes, perivascular adventitial, epithelial, and/or endothelial sources. Regardless of the origin of the cells, transforming growth factor-beta1 (TGF-β1) is the principal growth factor responsible for myofibroblast differentiation to a profibrotic phenotype and exerts its effects via Smad signaling pathways involving mitogen-activated protein kinase and Akt/protein kinase B. Additionally, reactive oxygen species (ROS) have important roles in progression of fibrosis. ROS are derived from a variety of enzyme sources, of which the nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase family has been identified as a major source of superoxide and hydrogen peroxide generation in the cardiovasculature and kidney during health and disease. Recent evidence indicates that the NAD(P)H oxidase homolog Nox4 is most accountable for ROS-induced fibroblast and mesangial cell activation, where it has an essential role in TGF-β1 signaling of fibroblast activation and differentiation into a profibrotic myofibroblast phenotype and matrix production. Information on the role of ROS in mesangial cell and fibroblast signaling is incomplete, and further research on myofibroblast differentiation during fibrosis is warranted.

Reactive oxygen species derived from Nox4 mediate BMP2 gene transcription and osteoblast differentiation

BMP-2 (bone morphogenetic protein-2) promotes differentiation of osteoblast precursor cells to mature osteoblasts that form healthy bone. In the present study, we demonstrate a novel mechanism of BMP-2-induced osteoblast differentiation. The antioxidant NAC (N-acetyl-L-cysteine) and the flavoprotein enzyme NAD(P)H oxidase inhibitor DPI (diphenyleneiodonium) prevented BMP-2-stimulated alkaline phosphatase expression and mineralized bone nodule formation in mouse 2T3 pre-osteoblasts. BMP-2 elicited a rapid generation of ROS (reactive oxygen species) concomitant with increased activation of NAD(P)H oxidase. NAC and DPI inhibited BMP-2-induced ROS production and NAD(P)H oxidase activity respectively. NAD(P)H oxidases display structurally similar catalytic subunits (Nox1-5) with differential expression in various cells. We demonstrate that 2T3 pre-osteoblasts predominantly express the Nox4 isotype of NAD(P)H oxidase. To extend this finding, we tested the functional effects of Nox4. Adenovirus-mediated expression of dominant-negative Nox4 inhibited BMP-2-induced alkaline phosphatase expression. BMP-2 promotes expression of BMP-2 for maintenance of the osteoblast phenotype. NAC and DPI significantly blocked BMP-2-stimulated expression of BMP2 mRNA and protein due to a decrease in BMP2 gene transcription. Dominant-negative Nox4 also mimicked this effect of NAC and DPI. Our results provide the first evidence for a new signalling pathway linking BMP-2-stimulated Nox4-derived physiological ROS to BMP-2 expression and osteoblast differentiation.

Fish oil prevents breast cancer cell metastasis to bone

The data derived from epidemiological and animal models confirm a beneficial effect of fish oil (rich in ω-3 polyunsaturated fatty acids) in the amelioration of tumor growth and progression, including breast cancer. The breast cancer patients often develop bone metastasis evidenced by osteolytic lesions, leading to severe pain and bone fracture. Using a mouse model of MDA-MB-231 human breast cancer cell metastasis to bone, here we show that fish oil diet enriched in DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid) prevents the formation of osteolytic lesions in bone, indicating suppression of cancer cell metastasis to bone. These results are supported by our data showing both DHA and EPA significantly attenuate the migration/invasion of MDA-MB-231 breast cancer cells in culture. The mechanism that limits breast cancer cells to selective metastasis to bone remains hitherto unexplored. Aberrant increased expression of CD44 is associated with generation of cancer stem cells, which contribute to metastasis of breast cancer cells. We demonstrate that DHA and EPA significantly inhibit the expression of CD44 protein and mRNA by a transcriptional mechanism. Furthermore, we show markedly reduced levels of CD44 mRNA and protein in the tumors of mice, which were fed fish oil diet than those in control diet. Our data provide the first evidence for a salutary effect of fish oil on breast cancer metastasis to bone. Our results identify a novel function of the fish oil active components, DHA and EPA, which target the cell-intrinsic pro-metastatic molecule CD44 to inhibit migration/invasion.

TGFβ enforces activation of eukaryotic elongation factor-2 (eEF2) via inactivation of eEF2 kinase by p90 ribosomal S6 kinase (p90Rsk) to induce mesangial cell hypertrophy

eEF2 phosphorylation is under tight control to maintain mRNA translation elongation. We report that TGFβ activates eEF2 by decreasing eEF2 phosphorylation and simultaneously increasing eEF2 kinase phosphorylation. Remarkably, inhibition of Erk1/2 blocked the TGFβ-induced dephosphorylation and phosphorylation of eEF2 and eEF2 kinase. TGFβ increased phosphorylation of p90Rsk in an Erk1/2-dependent manner. Inactive p90Rsk reversed TGFβ-inhibited phosphorylation of eEF2 and suppressed eEF2 kinase activity. Finally, inactive p90Rsk significantly attenuated TGFβ-induced protein synthesis and hypertrophy of mesangial cells. These results present the first evidence that TGFβ utilizes the two layered kinase module Erk/p90Rsk to activate eEF2 for increased protein synthesis during cellular hypertrophy.

MicroRNAs and their role in progressive kidney diseases

MicroRNAs (miRs) are a family of short non-coding RNAs. These endogenously produced factors have been shown to play important roles in gene regulation. The discovery of miRs has greatly expanded our knowledge of gene regulation at the posttranscriptional level. miRs inhibit target gene expression by blocking protein translation or by inducing mRNA degradation and therefore have the potential to modulate physiologic and pathologic processes. The imperative need to determine their cellular targets and disease relevance has sparked an unprecedented explosion of research in the miR field. Recent findings have revealed critical functions for specific miRs in cellular events such as proliferation, differentiation, development, and immune responses and in the regulation of genes relevant to human diseases. Of particular interest to renal researchers are recent reports that key miRs are highly expressed in the kidney and can act as effectors of TGF-beta actions and high glucose in diabetic kidney disease. Moreover, podocyte-specific deletion of Dicer, a key enzyme involved in miR biogenesis, led to proteinuria and severe renal dysfunction in mice. Hence, studies aimed at determining the in vitro and in vivo functions of miRs in the kidney could determine their value as therapeutic targets for progressive renal glomerular and tubular diseases. Translational approaches could be facilitated by the development of effective inhibitors of specific miRs and methods for optimal delivery of anti-miRs to the kidney. The major goal of this review is to highlight key functions of these miRs and their relationships to human diseases, with special emphasis on diabetic kidney disease.

Phosphatidylinositol 3-kinase/Akt pathway targets acetylation of Smad3 through Smad3/CREB-binding protein interaction: contribution to transforming growth factor beta1-induced Epstein-Barr virus reactivation

Epstein-Barr virus, a ubiquitous human herpesvirus, is associated with the development of carcinomas and lymphomas. We previously showed that transforming growth factor beta1 (TGF-beta1) mediated the virus to enter the lytic cycle, which is triggered by expression of Z Epstein-Barr virus replication activator (ZEBRA), through the ERK 1/2 MAPK signaling pathway. We report here that Akt, activated downstream from ERK 1/2, was required for TGF-beta1-induced ZEBRA expression and enabled Smad3, a mediator of TGF-beta1 signaling, to be acetylated by direct interaction with the co-activator CREB-binding protein and then to regulate TGF-beta1-induced ZEBRA expression.

TGF-beta activates Akt kinase through a microRNA-dependent amplifying circuit targeting PTEN

Akt kinase is activated by transforming growth factor-beta1 (TGF-β) in diabetic kidneys and plays important roles in fibrosis, hypertrophy and cell survival in glomerular mesangial cells (MC)1–11. However, the mechanisms of Akt activation by TGF-β are not fully understood. Here we show that TGF-β activates Akt in MC by inducing microRNA-216a (miR-216a) and miR-217, both of which target phosphatase and tensin homologue (PTEN). Both these miRs are located within the second intron of a non-coding RNA (RP23-298H6.1-001). The RP23 promoter was activated by TGF-β and also by miR-192 via E-box-regulated mechanisms as shown previously3. Akt activation by these miRs also led to MC survival and hypertrophy similar to TGF-β. These studies reveal a mechanism of Akt activation via PTEN downregulation by two miRs regulated by upstream miR-192 and TGF-β. Due to the diversity of PTEN function12, 13, this miR amplifying circuit may play key roles not only in kidney disorders, but also other diseases.

High glucose activates PKC-zeta and NADPH oxidase through autocrine TGF-beta1 signaling in mesangial cells

Conversion of normally quiescent mesangial cells into extracellular matrix-overproducing myofibroblasts in response to high ambient glucose and transforming growth factor (TGF)-beta(1) is central to the pathogenesis of diabetic nephropathy. Previously, we reported that mesangial cells respond to high glucose by generating reactive oxygen species (ROS) from NADPH oxidase dependent on protein kinase C (PKC) -zeta activation. We investigated the role of TGF-beta(1) in this action of high glucose on primary rat mesangial cells within 1-48 h. Both high glucose and exogenous TGF-beta(1) stimulated PKC-zeta kinase activity, as measured by an immune complex kinase assay and immunofluorescence confocal cellular imaging. In high glucose, Akt Ser473 phosphorylation appeared within 1 h and Smad2/3 nuclear translocation was prevented with neutralizing TGF-beta(1) antibodies. Neutralizing TGF-beta(1) antibodies, or a TGF-beta receptor kinase inhibitor (LY364947), or a phosphatidylinositol 3,4,5-trisphosphate (PI3) kinase inhibitor (wortmannin), prevented PKC-zeta activation by high glucose. TGF-beta(1) also stimulated cellular membrane translocation of PKC-alpha, -beta(1), -delta, and -epsilon, similar to high glucose. High glucose and TGF-beta(1) enhanced ROS generation by mesangial cell NADPH oxidase, as detected by 2,7-dichlorofluorescein immunofluorescence. This response was abrogated by neutralizing TGF-beta(1) antibodies, LY364947, or a specific PKC-zeta pseudosubstrate peptide inhibitor. Expression of constitutively active PKC-zeta in normal glucose caused upregulation of p22(phox), a likely mechanism of NADPH oxidase activation. We conclude that very early responses of mesangial cells to high glucose include autocrine TGF-beta(1) stimulation of PKC isozymes including PI3 kinase activation of PKC-zeta and consequent generation of ROS by NADPH oxidase.

Akt kinase targets association of CBP with SMAD 3 to regulate TGFbeta-induced expression of plasminogen activator inhibitor-1

Transforming growth factor-beta (TGFbeta) controls expression of plasminogen activator inhibitor type 1 (PAI-1), which regulates degradation of extracellular matrix proteins in fibrotic diseases. The TGFbeta receptor-specific Smad 3 has been implicated in the PAI-1 expression. The mechanism by which non-Smad signaling contributes to this process is not known. We studied the cross-talk between Smad 3 and PI 3 kinase/Akt signaling in TGFbeta-induced PAI-1 expression in renal mesangial cells. Inhibition of PI 3 kinase and Akt kinase blocked TGFbeta- and Smad 3-mediated expression of PAI-1. In contrast, constitutively active PI 3 kinase and Akt kinase increased PAI-1 expression, similar to TGFbeta. Inhibition of PI 3 kinase and Akt kinase had no effect on TGFbeta-induced Smad 3 phosphorylation and its translocation to the nucleus. Notably, inhibition of PI 3 kinase-dependent Akt kinase abrogated TGFbeta-induced PAI-1 transcription, without affecting binding of Smad 3 to the PAI-1 Smad binding DNA element. However, PI 3 kinase inhibition and dominant negative Akt kinase antagonized the association of the transcriptional coactivator CBP with Smad 3 in response to TGFbeta, resulting in inhibition of Smad 3 acetylation. Together our findings identify TGFbeta-induced PI 3 kinase/Akt signaling as a critical regulator of Smad 3-CBP interaction and Smad 3 acetylation, which cause increased PAI-1 expression.

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.

TGFbeta intercepts nuclear glycogen synthase kinase 3beta to inhibit PDGF-induced DNA synthesis in mesangial cells

Here, we demonstrate a mechanism of TGFbeta-mediated inhibition of PDGF-induced DNA synthesis in mesangial cells. TGFbeta significantly inhibited nuclear Akt phosphorylation without any effect on PDGF-stimulated phosphorylation of PDGFR at PI 3 kinase binding site (Tyr-751). Remarkably, TGFbeta inhibited cyclin D1 and cyclin E expression with concomitant decrease in CDK2 activity induced by PDGF. More importantly, we demonstrate that TGFbeta significantly abolished Akt-mediated serine-9 phosphorylation of glycogen synthase kinase 3beta (GSK3beta), thus prevented its inactivation. Expression of inactive GSK3betaK85R mutant increased cyclin D1 expression and DNA synthesis similar to PDGF. These results provide the first evidence that TGFbeta intercepts Akt kinase activity in the nucleus to block inactivation of GSK3beta, leading to attenuation of PDGF-induced CDK2 activity and DNA synthesis.

Role of the Akt/FoxO3a pathway in TGF-beta1-mediated mesangial cell dysfunction: a novel mechanism related to diabetic kidney disease

Diabetic nephropathy (DN) is characterized by mesangial cell (MC) expansion and accumulation of extracellular matrix proteins. TGF-beta is increased in MC under diabetic conditions and in DN and activates key signaling pathways, including the phosphoinositide-3-kinase/Akt (PI3K/Akt) pathway. FoxO transcription factors play roles in cell survival and oxidative stress and are negatively regulated by Akt-mediated phosphorylation. We tested whether phosphorylation-mediated inactivation of FoxO3a by TGF-beta can mediate MC survival and oxidative stress. TGF-beta treatment significantly increased levels of p-Akt (activation) and p-FoxO3a (inactivation) in cultured MC. This FoxO3a inactivation was accompanied by significant decreases in the expression of two key FoxO3a target genes, the proapoptotic Bim and antioxidant manganese superoxide dismutase in MC. TGF-beta treatment triggered the nuclear exclusion of FoxO3a, significantly inhibited FoxO3a transcriptional activity, and markedly protected MC from apoptosis. A PI3K inhibitor blocked these TGF-beta effects. It is interesting that p-Akt and p-FoxO3A levels also were increased in renal cortical tissues from rats and mice at 2 wk after the induction of diabetes by streptozotocin, thus demonstrating in vivo significance. In summary, TGF-beta and diabetes can increase FoxO3a phosphorylation and transcriptional inactivation via PI3K/Akt. These new results suggest that Akt/FoxO pathway regulation may be a novel mechanism by which TGF-beta can induce unopposed MC survival and oxidant stress in early DN, thereby accelerating renal disease.

Arachidonic acid-dependent activation of a p22(phox)-based NAD(P)H oxidase mediates angiotensin II-induced mesangial cell protein synthesis and fibronectin expression via Akt/PKB

Angiotensin II (Ang II) induces protein synthesis and hypertrophy through arachidonic acid (AA)- and redoxdependent activation of the serine-threonine kinase Akt/PKB in mesangial cells (MCs). The role of NAD(P)H oxidase component p22( phox ) was explored in this signaling pathway and in Ang II-induced expression of the extracellular matrix protein fibronectin. Ang II causes activation of Akt/PKB and induces fibronectin protein expression, effects abrogated by phospholipase A(2) inhibition and mimicked by AA. Ang II and AAalso elicited an increase in fibronectin expression that was reduced with a dominant negative mutant of Akt/PKB. Exposure of the cells to hydrogen peroxide stimulates Akt/PKB activity and fibronectin synthesis. The antioxidant N-acetylcysteine abolished Ang II- and AA-induced Akt/PKB activation and fibronectin expression. Western blot analysis revealed high levels of p22( phox ) in MCs. Antisense (AS) but not sense oligonucleotides for p22( phox ) prevented ROS generation in response to Ang II and AA. AS p22( phox ) inhibited Ang II- or AA-induced Akt/PKB as well as protein synthesis and fibronectin expression. These data provide the first evidence, in MCs, of activation by AAof a p22( phox )-based NAD(P)H oxidase and subsequent generation of ROS. Moreover, this pathway mediates the effect of Ang II on Akt/PKB-induced protein synthesis and fibronectin expression.

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.

Nox4 NAD(P)H oxidase mediates hypertrophy and fibronectin expression in the diabetic kidney

Renal hypertrophy and extracellular matrix accumulation are early features of diabetic nephropathy. We investigated the role of the NAD(P)H oxidase Nox4 in generation of reactive oxygen species (ROS), hypertrophy, and fibronectin expression in a rat model of type 1 diabetes induced by streptozotocin. Phosphorothioated antisense (AS) or sense oligonucleotides for Nox4 were administered for 2 weeks with an osmotic minipump 72 h after streptozotocin treatment. Nox4 protein expression was increased in diabetic kidney cortex compared with non-diabetic controls and was down-regulated in AS-treated animals. AS oligonucleotides inhibited NADPH-dependent ROS generation in renal cortical and glomerular homogenates. ROS generation by intact isolated glomeruli from diabetic animals was increased compared with glomeruli isolated from AS-treated animals. AS treatment reduced whole kidney and glomerular hypertrophy. Moreover, the increased expression of fibronectin protein was markedly reduced in renal cortex including glomeruli of AS-treated diabetic rats. Akt/protein kinase B and ERK1/2, two protein kinases critical for cell growth and hypertrophy, were activated in diabetes, and AS treatment almost abolished their activation. In cultured mesangial cells, high glucose increased NADPH oxidase activity and fibronectin expression, effects that were prevented in cells transfected with AS oligonucleotides. These data establish a role for Nox4 as the major source of ROS in the kidneys during early stages of diabetes and establish that Nox4-derived ROS mediate renal hypertrophy and increased fibronectin expression.