Activation of PLC and PI 3 kinase by PDGF receptor alpha is not sufficient for mitogenesis and migration in mesangial cells

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.

Affinity binding-mediated fluorometric protein assay based on the use of target-triggered DNA assembling probes and aptamers labelled with upconversion nanoparticles: application to the determination of platelet derived growth factor-BB

The target-triggered DNA assembling probe is presented for highly selective protein detection. Target-triggered DNA assembling is used in an amplification strategy based on affinity binding for identification and determination of proteins in general. Specifically, it was applied to the platelet derived growth factor-BB (PDGF-BB). A hairpin DNA (H-DNA) probe was designed containing (a) an aptamer domain for protein recognition and (b) a blocked DNAzyme domain for DNAzyme cleavage. An assistant DNA (A-DNA) probe containing aptamer and complementary domains was also employed to recognize protein and to induce DNA assembly. Once H-DNA and A-DNA recognize the same protein, H-DNA and A-DNA are in close proximity to each other. This induces DNA assembling for protein-triggered complex (Protein-Complex) with free DNAzyme domains. The free DNAzymes trigger the circular cleavage of molecular beacons for amplified signals. The assay is performed by fluorometry at an excitation wavelength of 980 nm and by collecting fluorescence at 545 nm. The platelet derived growth factor-BB (PDGF-BB) was accurately identified and selectively determined by this assay with a 22 pM detection limit (using the 3σ criterion). The responses for PDGF-BB is nearly 6-fold higher than for PDGF-AB, and 16-fold higher than PDGF-AA. This upconversion assay avoids any interference by the autofluorescence of biological fluids. Graphical abstractSchematic representation of the principle of the target-triggered DNA assembling probes mediated amplification strategy based on affinity binding for PDGF-BB. The UCNP probe is used for the quantitation of PDGF-BB with high selectivity.

PDGFRα expression as a novel therapeutic marker in well-differentiated neuroendocrine tumors

AIMS

To evaluate the biological significance of dense vascular networks associated with low-grade NENs, we assessed the impact of PDGFRα tissue expression in 77 GEP/NEN patients, associating PDGFRα expression with the morphological characterization in low-grade tumors.

METHODS AND RESULTS

Paraffin-embedded specimens of 77 GEP- NEN tissues, collected from January 2006 to March 2018, were evaluated for PDGFRα tissue expression and correlations with clinicopathological characteristics. PDGFRα tissue expression was significantly correlated with grade and the NEN growth pattern (p < 0.001) but not with gender, primary site or lymph nodes metastatic status. PDGFRα staining was mainly localized in the vascular pole of the neuroendocrine cells and in Enterochromaffin (EC) cells. In particular PDGFRα tissue expression was significantly more expressed in G2 (p < 0.001) than G1 and G3 cases (p 0.004; p < 0.0002;) and correlated with an insular growth pattern. PDGFRα tissue expression was associated with the Ki67 index and we found a significant negative trend of association with the Ki67 proliferation index (P < 0.001): thus PDGFRα expression is referred to morphological and not to proliferative data.

CONCLUSIONS

PDGFRα represents an effective target for new anti-angiogenic treatment in WD- GEP-NENs, in particular in G2 cases, and in G3 cases only when there is a mixed insular-acinar pattern. In this context, it is important to carefully delineate those tumors that might better respond to this type of treatment alone or in combination. Further investigation of the relationship between PD-L1 and PDGFRa is warranted, and may contribute to optimize the therapeutic approach in patients with GEP-NENs.

Tyrosines-740/751 of PDGFRβ contribute to the activation of Akt/Hif1α/TGFβ nexus to drive high glucose-induced glomerular mesangial cell hypertrophy

Glomerular mesangial cell hypertrophy contributes to the complications of diabetic nephropathy. The mechanism by which high glucose induces mesangial cell hypertrophy is poorly understood. Here we explored the role of the platelet-derived growth factor receptor-β (PDGFRβ) tyrosine kinase in driving the high glucose-induced mesangial cell hypertrophy. We show that high glucose stimulates the association of the PDGFRβ with PI 3 kinase leading to tyrosine phosphorylation of the latter. High glucose-induced Akt kinase activation was also dependent upon PDGFRβ and its tyrosine phosphorylation at 740/751 residues. Inhibition of PDGFRβ activity, its downregulation and expression of its phospho-deficient (Y740/751F) mutant inhibited mesangial cell hypertrophy by high glucose. Interestingly, expression of constitutively active Akt reversed this inhibition, indicating a role of Akt kinase downstream of PDGFRβ phosphorylation in this process. The transcription factor Hif1α is a target of Akt kinase. siRNAs against Hif1α inhibited the high glucose-induced mesangial cell hypertrophy. In contrast, increased expression of Hif1α induced hypertrophy similar to high glucose. We found that inhibition of PDGFRβ and expression of PDGFRβ Y740/751F mutant significantly inhibited the high glucose-induced expression of Hif1α. Importantly, expression of Hif1α countered the inhibition of mesangial cell hypertrophy induced by siPDGFRβ or PDGFRβ Y740/751F mutant. Finally, we show that high glucose-stimulated PDGFRβ tyrosine phosphorylation at 740/751 residues and the tyrosine kinase activity of the receptor regulate the transforming growth factor-β (TGFβ) expression by Hif1α. Thus we define the cell surface PDGFRβ as a major link between high glucose and its effectors Hif1α and TGFβ for induction of diabetic mesangial cell hypertrophy.

PDGF receptor-β uses Akt/mTORC1 signaling node to promote high glucose-induced renal proximal tubular cell collagen I (α2) expression

Increased expression of PDGF receptor-β (PDGFRβ) has been shown in renal proximal tubules in mice with diabetes. The core molecular network used by high glucose to induce proximal tubular epithelial cell collagen I (α2) expression is poorly understood. We hypothesized that activation of PDGFRβ by high glucose increases collagen I (α2) production via the Akt/mTORC1 signaling pathway in proximal tubular epithelial cells. Using biochemical and molecular biological techniques, we investigated this hypothesis. We show that high glucose increases activating phosphorylation of the PDGFRβ, resulting in phosphorylation of phosphatidylinositol 3-kinase. A specific inhibitor, JNJ-10198409, and small interfering RNAs targeting PDGFRβ blocked this phosphorylation without having any effect on MEK/Erk1/2 activation. We also found that PDGFRβ regulates high glucose-induced Akt activation, its targets tuberin and PRAS40 phosphorylation, and finally, mTORC1 activation. Furthermore, inhibition of PDGFRβ suppressed high glucose-induced expression of collagen I (α2) in proximal tubular cells. Importantly, expression of constitutively active Akt or mTORC1 reversed these processes. As a mechanism, we found that JNJ and PDGFRβ knockdown inhibited high glucose-stimulated Hif1α expression. Furthermore, overexpression of Hif1α restored expression of collagen I (α2) that was inhibited by PDGFRβ knockdown in high glucose-stimulated cells. Finally, we show increased phosphorylation of PDGFRβ and its association with Akt/mTORC1 activation, Hif1α expression, and elevated collagen I (α2) levels in the renal cortex of mice with diabetes. Our results identify PDGFRβ as a driver in activating Akt/mTORC1 nexus for high glucose-mediated expression of collagen I (α2) in proximal tubular epithelial cells, which contributes to tubulointerstitial fibrosis in diabetic nephropathy.

A positive feedback loop involving Erk5 and Akt turns on mesangial cell proliferation in response to PDGF

Platelet-derived growth factor BB and its receptor (PDGFRβ) play a pivotal role in the development of renal glomerular mesangial cells. Their roles in increased mesangial cell proliferation during mesangioproliferative glomerulonephritis have long been noted, but the operating logic of signaling mechanisms regulating these changes remains poorly understood. We examined the role of a recently identified MAPK, Erk5, in this process. PDGF increased the activating phosphorylation of Erk5 and tyrosine phosphorylation of proteins in a time-dependent manner. A pharmacologic inhibitor of Erk5, XMD8-92, abrogated PDGF-induced DNA synthesis and mesangial cell proliferation. Similarly, expression of dominant negative Erk5 or siRNAs against Erk5 blocked PDGF-stimulated DNA synthesis and proliferation. Inhibition of Erk5 attenuated expression of cyclin D1 mRNA and protein, resulting in suppression of CDK4-mediated phosphorylation of the tumor suppressor protein pRb. Expression of cyclin D1 or CDK4 prevented the dominant negative Erk5- or siErk5-mediated inhibition of DNA synthesis and mesangial cell proliferation induced by PDGF. We have previously shown that phosphatidylinositol 3-kinase (PI3-kinase) contributes to PDGF-induced proliferation of mesangial cells. Inhibition of PI3-kinase blocked PDGF-induced phosphorylation of Erk5. Since PI3-kinase acts through Akt, we determined the role of Erk5 on Akt phosphorylation. XMD8-92, dominant negative Erk5, and siErk5 inhibited phosphorylation of Akt by PDGF. Interestingly, we found inhibition of PDGF-induced Erk5 phosphorylation by a pharmacological inhibitor of Akt kinase and kinase dead Akt in mesangial cells. Thus our data unfold the presence of a positive feedback microcircuit between Erk5 and Akt downstream of PI3-kinase nodal point for PDGF-induced mesangial cell proliferation.

Src tyrosine kinase mediates platelet-derived growth factor BB-induced and redox-dependent migration in metanephric mesenchymal cells

The adult kidney is derived from the interaction between the metanephric blastema and the ureteric bud. Platelet-derived growth factor (PDGF) receptor β is essential for the development of the mature glomerular tuft, as mice deficient for this receptor lack mesangial cells. This study investigated the role of Src tyrosine kinase in PDGF-mediated reactive oxygen species (ROS) generation and migration of metanephric mesenchymal cells (MMCs). Cultured embryonic MMCs from wild-type and PDGF receptor-deficient embryos were established. Migration was determined via wound-healing assay. Unlike PDGF AA, PDGF BB-induced greater migration in MMCs with respect to control. This was abrogated by neutralizing an antibody to PDGF BB. Phosphatidylinositol 3-kinase (PI3K) inhibitors suppressed PDGF BB-induced migration. Conversely, mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK) inhibitors had no effect. Src inhibitors inhibited PDGF-induced cell migration, PI3K activity, and Akt phosphorylation. Adenoviral dominant negative Src (AD DN Src) abrogated PDGF BB-induced Akt phosphorylation. Hydrogen peroxide stimulated cell migration. PDGF BB-induced wound closure was inhibited by the antioxidants N-acetyl-l-cysteine, tiron, and the flavoprotein inhibitor diphenyleneiodonium. These cells express the NADPH oxidase homolog Nox4. Inhibiting Nox4 with antisense oligonucleotides or small interfering RNA (siRNA) suppressed PDGF-induced wound closure. Inhibition of Src with siRNA reduced PDGF BB-induced ROS generation as assessed by 2',7'-dichlorodihydrofluorescein diacetate fluorescence. Furthermore, PDGF BB-stimulated ROS generation and migration were similarly suppressed by Ad DN Src. In MMCs, PDGF BB-induced migration is mediated by PI3K and Src in a redox-dependent manner involving Nox4. Src may be upstream to PI3K and Nox4.

Growth arrest-specific protein 1 is a novel endogenous inhibitor of glomerular cell activation and proliferation

Growth arrest-specific protein-1 (GAS1) is a GPI-anchored protein which is highly expressed in embryonic mouse fibroblasts and inhibits their proliferation. Glomerular mesangial cells release soluble GAS1 protein into the supernatant in vitro. Growth arrest led to GAS1 overexpression and increased release. Secretion involved disintegrin and metalloproteinase 10 and 17 as signified by inhibition experiments. Recombinant soluble GAS1 protein inhibited the proliferation of mesangial cells. Conversely, the induction of mesangial cell proliferation by PDGF-BB or -DD led to downregulation of GAS1 mRNA. Specific ligands of the PDGF α-receptor, PDGF-AA and -CC, had no effect. The GAS1 protein was localized in podocytes in kidneys from healthy rats. During the time course of mesangioproliferative glomerulonephritis in anti-Thy1.1-treated rats, glomerular GAS1 expression decreased prior to the onset of mesangial cell proliferation and increased at later stages during glomerular recovery. Finally, a plasmid expressing soluble GAS1 fused to an Fc fragment was systemically overexpressed in rats with mesangioproliferative glomerulonephritis. This ameliorated renal damage was indicated by decreased albuminuria and serum creatinine. Gas1/Fc-transfected rats also exhibited a reduction of the glomerular mesangial cell activation and proliferation. Thus, GAS1 is a novel endogenous inhibitor of glomerular mesangial cell proliferation and may be a novel therapeutic target in mesangioproliferative glomerular diseases.

The platelet-derived growth factor system in renal disease: an emerging role of endogenous inhibitors

The platelet-derived growth factor (PDGF) family consists of four isoforms which are secreted as homodimers (PDGF-AA, PDGF-BB, PDGF-CC and PDGF-DD) or heterodimers (PDGF-AB), and two receptor chains (PDGFR-α and -β). All members of the PDGF system are constitutively or inducibly expressed in renal cells and are involved in the regulation of cell proliferation and migration, the accumulation of extracellular matrix proteins and the secretion of pro- and anti-inflammatory mediators. Particular roles have been identified in mediating mesangioproliferative changes, renal interstitial fibrosis and glomerular angiogenesis. Different endogenous inhibitors of PDGF-induced biological responses exist which affect the activation/deactivation of PDGF isoforms, the activity of the PDGFRs, or which block downstream signaling pathways of the autophosphorylated PDGFRs. The novel endogenous inhibitor nephroblastoma overexpressed gene (NOV, CCN3) reduces PDGF-induced cell proliferation and is downregulated by PDGF isoforms itself. Among all identified inhibitors only few "true" PDGF antagonists have been identified. A better understanding of these inhibitors may aid in the design of novel therapeutic approaches to PDGF-mediated diseases.

PDGF-induced proliferation in human arterial and venous smooth muscle cells: molecular basis for differential effects of PDGF isoforms

Platelet-derived growth factor (PDGF) has been implicated in the pathogenesis of arterial atherosclerosis and venous neointimal hyperplasia. We examined the effects of PDGF isoforms on smooth muscle cells (SMCs) from arterial and venous origins in order to further understand the differential responsiveness of these vasculatures to proliferative stimuli. Serum-starved human arterial and venous SMCs exhibited very different proliferative responses to PDGF isoforms. Whereas, proliferation of arterial SMCs was strongly stimulated by PDGF-AA, venous SMCs showed no proliferative response to PDGF-AA, but instead demonstrated a significantly greater proliferative response to PDGF-BB than arterial SMCs. Part of this difference could be attributed to differences in PDGF receptors expression. There was a 2.5-fold higher (P < 0.05) density of PDGF receptor-α (PDGF-Rα) and a 6.6-fold lower (P < 0.05) density of PDGF-Rβ expressed on arterial compared to venous SMCs. Concomitant with an increased proliferative response to PDGF-AA in arterial SMCs was a marked PDGF-Rα activation, enhanced phosphorylation of ERK1/2 and Akt, a transient activation of c-Jun NH2-terminal kinase (JNK), and a significant reduction in expression of the cell-cycle inhibitor p27(kip1). This pattern of signaling pathway changes was not observed in venous SMCs. No phosphorylation of PDGF-Rα was detected after venous SMC exposure to PDGF-AA, but there was enhanced phosphorylation of ERK1/2 and Akt in venous SMCs, similar to that seen in the arterial SMCs. PDGF-BB stimulation of venous SMC resulted in PDGF-Rβ activation as well as transactivation of epidermal growth factor receptor (EGF-R); transactivation of EGF-R was not observed in arterial SMCs. These results may provide an explanation for the differential susceptibility to proliferative vascular diseases of arteries and veins.

PDGF receptor-{beta} modulates metanephric mesenchyme chemotaxis induced by PDGF AA

PDGF B chain or PDGF receptor (PDGFR)-beta-deficient (-/-) mice lack mesangial cells. To study responses of alpha- and beta-receptor activation to PDGF ligands, metanephric mesenchymal cells (MMCs) were established from embryonic day E11.5 wild-type (+/+) and -/- mouse embryos. PDGF BB stimulated cell migration in +/+ cells, whereas PDGF AA did not. Conversely, PDGF AA was chemotactic for -/- MMCs. The mechanism by which PDGFR-beta inhibited AA-induced migration was investigated. PDGF BB, but not PDGF AA, increased intracellular Ca(2+) and the production of reactive oxygen species (ROS) in +/+ cells. Transfection of -/- MMCs with the wild-type beta-receptor restored cell migration and ROS generation in response to PDGF BB and inhibited AA-induced migration. Inhibition of Ca(2+) signaling facilitated PDGF AA-induced chemotaxis in the wild-type cells. The antioxidant N-acetyl-l-cysteine (NAC) or the NADPH oxidase inhibitor diphenyleneiodonium (DPI) abolished the BB-induced increase in intracellular Ca(2+) concentration, suggesting that ROS act as upstream mediators of Ca(2+) in suppressing PDGF AA-induced migration. These data indicate that ROS and Ca(2+) generated by active PDGFR-beta play an essential role in suppressing PDGF AA-induced migration in +/+ MMCs. During kidney development, PDGFR beta-mediated ROS generation and Ca(2+) influx suppress PDGF AA-induced chemotaxis in metanephric mesenchyme.

PDGFR-A is a therapeutic target in alveolar rhabdomyosarcoma

Alveolar rhabdomyosarcoma is an aggressive skeletal muscle cancer of childhood. Our initial studies of rhabdomyosarcoma gene expression for patients enrolled in a national clinical trial suggested that platelet-derived growth factor receptor A (PDGFR-A) may be a mediator of disease progression and metastasis. Using our conditional mouse tumor models that authentically recapitulate the primary mutations and metastatic progression of alveolar rhabdomyosarcomas in humans, we found by immunoblotting and immunokinase assays that PDGFR-A and its downstream effectors, mitogen-activated protein kinase and Akt, were highly activated in both primary and metastatic tumors. Inhibition of PDGFR-A by RNA interference, small molecule inhibitor or neutralizing antibody had a dramatic effect on tumor cell growth both in vitro and in vivo, although resistance evolved in one-third of tumors. These results establish proof-of-principal for PDGFR-A as a therapeutic target in alveolar rhabdomyosarcoma.

Resveratrol inhibits PDGF receptor mitogenic signaling in mesangial cells: role of PTP1B

Mesangioproliferative glomerulonephritis is associated with overactive PDGF receptor signal transduction. We show that the phytoalexin resveratrol dose dependently inhibits PDGF-induced DNA synthesis in mesangial cells with an IC(50) of 10 microM without inducing apoptosis. Remarkably, the increased SIRT1 deacetylase activity induced by resveratrol was not necessary for this inhibitory effect. Resveratrol significantly blocked PDGF-stimulated c-Src and Akt kinase activation, resulting in reduced cyclin D1 expression and attenuated pRb phosphorylation and cyclin-dependent kinase-2 (CDK2) activity. Furthermore, resveratrol inhibited PDGFR phosphorylation at the PI 3 kinase and Grb-2 binding sites tyrosine-751 and tyrosine-716, respectively. This deficiency in PDGFR phosphorylation resulted in significant inhibition of PI 3 kinase and Erk1/2 MAPK activity. Interestingly, resveratrol increased the activity of protein tyrosine phosphatase PTP1B, which dephosphorylates PDGF-stimulated phosphorylation at tyrosine-751 and tyrosine-716 on PDGFR with concomitant reduction in Akt and Erk1/2 kinase activity. PTP1B significantly inhibited PDGF-induced DNA synthesis without inducing apoptosis. These results for the first time provide evidence that the stilbene resveratrol targets PTP1B to inhibit PDGFR mitogenic signaling.

CCN3 is a novel endogenous PDGF-regulated inhibitor of glomerular cell proliferation

CCN proteins affect cell proliferation, migration, attachment, and differentiation. We identified CCN3 as a suppressed gene following platelet-derived growth factor (PDGF)-BB or -DD stimulation in a cDNA-array analysis of mesangial cells. In vitro growth-arrested mesangial cells overexpressed and secreted CCN3, whereas the addition of the recombinant protein inhibited cell growth. Induction of mesangial cell proliferation by PDGF-BB or the specific PDGF beta-receptor ligand PDGF-DD led to downregulation of CCN3 mRNA, confirming the array study. Specific PDGF alpha-receptor ligands had no effect. CCN3 protein was found in arterial smooth muscle cells, the medullary interstitium, and occasional podocytes in the healthy rat kidney. Glomerular CCN3 was low prior to mesangial proliferation but increased as glomerular cell proliferation subsided during mesangioproliferative glomerulonephritis (GN). Inhibition of PDGF-B in mesangioproliferative disease led to overexpression of glomerular CCN3 mRNA. CCN3 localized mostly to podocytes in human glomeruli, but this expression varied widely in different human glomerulonephritides. Glomerular cell proliferation negatively correlated with CCN3 expression in necrotizing GN. Our study identifies CCN3 as an endogenous inhibitor of mesangial cell growth and a modulator of PDGF-induced mitogenesis.

Mitogenic signaling via platelet-derived growth factor beta in metanephric mesenchymal cells

Mice deficient in either platelet-derived growth factor (PDGF) B chain or PDGF receptor (PDGFR) beta lack mesangial cells. PDGF stimulates proliferation and migration of metanephric mesenchymal cells, from which mesangial cells are derived. Binding of PDGF to PDGFR-beta induces autophosphorylation at specific tyrosine residues and activates various effector proteins, including phosphatidylinositol-3-kinase (PI3-K). This study explored the role of PI 3-K and reactive oxygen species (ROS) in PDGF-mediated signaling using cells established from wild-type and PDGFR-beta -/- metanephric blastemas at 11.5 days post-conception. PDGF-induced effects that were dependent on PI3-K activation were determined using PDGFR-beta -/- cells made to express "add-back" mutant PDGFR-beta capable of binding PI3-K. We found that PDGF is mitogenic for mesenchymal cells expressing PDGFR-beta, and PI3-K is an important regulator of PDGF-induced DNA synthesis. Activation of ERK1/2 is partially dependent on PI3-K, and both the PI3-K and MEK-ERK1/2 pathways contribute to PI3-K-dependent mitogenesis. In addition, PDGF-induced DNA synthesis in wild-type cells was found to be dependent on ROS that are generated downstream of PI3-K activation. Using antisense oligonucleotides and small interfering RNA, we determined that the NAD(P)H oxidase Nox4 produces these ROS that activate Akt and the MEK-ERK1/2 mitogenic cascade. In conclusion, the present study demonstrates Nox4 involvement in PDGF-induced DNA synthesis in metanephric mesenchymal cells and provides the first evidence that PDGF-induced PI3-K activity enhances production of ROS by Nox4.

Platelet-derived growth factor reorganizes the actin cytoskeleton through 3-phosphoinositide-dependent and 3-phosphoinositide-independent mechanisms in human mesangial cells

BACKGROUND

Platelet-derived growth factor (PDGF) is a potent activator of mesangial cell proliferation and migration. Although phosphoinositide 3-kinase (PI3K) enzymes are important downstream targets of the PDGF receptor, the contribution made by their 3-phosphoinositide products in the reorganization of actin cytoskeleton and focal adhesions has been questioned.

METHODS AND RESULTS

Pharmacological inhibition of the PI3K activity blocks PDGF-induced migration of human primary mesangial cells using an in vitro scrape wound healing assay. Acute (<10 min) inhibition of the PI3K activity did not alter the effect of PDGF on either stress fibre dissolution or reorganization of focal adhesions. However, at later times (>30 min), PDGF-stimulated responses were inhibited. In contrast, PDGF-stimulated membrane ruffling remained insensitive to PI3K inhibitors throughout. Inhibition of protein kinase C and Erk also attenuated PDGF-stimulated mesangial cell migration; however, neither signaling pathway was responsible for the initial effects on filamentous actin and focal adhesions.

CONCLUSIONS

We propose that following PDGF stimulation of mesangial cells, reorganization of the actin cytoskeleton occurs in a biphasic manner. The mechanism responsible for mesangial cell migration that occurs immediately following PDGF stimulation may serve to 'prime' for the subsequent 3-phosphoinositide-, protein-kinase-C-, and Erk-dependent migration.

Biological responses to PDGF-BB versus PDGF-DD in human mesangial cells

Platelet-derived growth factor (PDGF)-BB and PDGF-DD mediate mesangial cell proliferation in vitro and in vivo. While PDGF-BB is a ligand for the PDGF alpha- and beta-receptor chains, PDGF-DD binds more selectively to the beta-chain, suggesting potential differences in the biological activities. Signal transduction and regulation of gene expression induced by PDGF-BB and -DD were compared in primary human mesangial cells (HMCs), which expressed PDGF alpha- and beta-receptor subunits. The growth factor concentrations used were chosen based on their equipotency in inducing HMCs proliferation and binding to the betabeta-receptor. Both growth factors, albeit at different concentrations induced phosphorylation and activation of extracellular signal-regulated kinase 1 (ERK1) and ERK2. In addition, PDGFs led to the phosphorylation and activation of signal transducers and activators of transcription 1 (STAT1) and STAT3. HMCs proliferation induced by either PDGF-BB or -DD could be blocked by signal transduction inhibitors of the mitogen-activated protein kinase-, Janus kinase (JAK)/STAT-, or phosphatidyl-inositol 3-kinase pathways. Using a gene chip array and subsequent verification by real-time reverse transcriptase (RT)-polymerase chain reaction, we found that in HMC genes for matrix metalloproteinase 13 (MMP-13) and MMP-14 and, to a low extent, cytochrome B5 and cathepsin L were exclusively regulated by PDGF-BB, whereas no exclusive gene regulation was detected by PDGF-DD. However, at the protein level, both MMP-13 and -14 were equally induced by PDGF-BB and -DD. PDGF-BB and -DD effect similar biological responses in HMCs albeit at different potencies. Rare apparently differential gene regulation did not result in different protein expression, suggesting that in HMCs both PDGFs exert their biological activity almost exclusively via the PDGF beta-receptor.

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.

Regulation of PDGF and its receptors in fibrotic diseases

Platelet-derived growth factor (PDGF) isoforms play a major role in stimulating the replication, survival, and migration of myofibroblasts during the pathogenesis of fibrotic diseases. During fibrogenesis, PDGF is secreted by a variety of cell types as a response to injury, and many pro-inflammatory cytokines mediate their mitogenic effects via the autocrine release of PDGF. PDGF action is determined by the relative expression of PDGF alpha-receptors (PDGFRalpha) and beta-receptors (PDGFRbeta) on the surface of myofibroblasts. These receptors are induced during fibrogenesis, thereby amplifying biological responses to PDGF isoforms. PDGF action is also modulated by extracellular binding proteins and matrix molecules. This review summarizes the literature on the role of PDGF and its receptors in the development of fibrosis in a variety of organ systems, including lung, liver, kidney, and skin.

Physiology and gene expression characteristics of carcinogen-initiated and tumor-transformed glial progenitor cells derived from the CNS of methylnitrosourea (MNU)-treated Sprague-Dawley rats

Glial progenitors from the brain of normal adult Sprague-Dawley rats were compared to their initiated and malignant counterparts that were isolated from apparently normal brains of animals exposed to methylnitrosourea (MNU). Fibroblast growth factor-2 (FGF-2) or platelet-derived growth factor (PDGF)-A or -B induced differentiation of normal progenitors to a pro-astrocytic or oligodendrocytic morphology, respectively, whereas the combination of these factors resulted in their terminal differentiation to oligodendrocytes and senescence. In contrast, initiated progenitors did not exit the cell cycle when stimulated with PDGF and/or FGF-2. cDNA oligoarray analysis and RT-PCR verification showed an early upregulation/ induction of growth factor/receptors, PDGF-A, PDGFR-beta, IGFR-1, IGF-1 and -2, IL-6, MEGF-5, FRAG-1, IRS-2, HSPG, and FGFR-1, followed by a late increase in the expression IGFBP-6, PDGF-alpha, FGFR-4A, c/ERB-A, and FGFR-4, 2, and 1 during the tumorigenic progression. Western blot analyses demonstrated that MNU exposure caused progressive reduction of p21 protein levels, an increase of Rb phosphorylation, activation of AKT and CDK2, and upregulation of FGF receptors. Double immunofluorescence labeling showed progressive increase in nuclear colocalization of FGFR1, 2, and 4, which peaked in malignant lines. It is postulated that transition of normal rat glial progenitors to an initiated state is driven by IGF-1 and 2, IL-6, and the upregulation of the receptors PDGFR-beta and FGFR-1, 2, and 4. Deregulation of the cell cycle in this state involves reduction of p21 protein, concomitant upregulation of CDC2, and an increase in Rb phosphorylation that favors expression and nuclear translocation of FGFR-4 and FRAG-1 and 2. These events are associated with progressive activation of AKT and RAS. Malignant transformation is enhanced by near elimination of p21 and PC3, induction of AP-1 (upregulation of JUN-B, c-JUN, FRA-1), activation of the NF-kB pro-survival pathway, and inhibition of the TGF-beta pro-apoptotic pathway possibly in response to changes in the expression of nerve growth factor (NGF) I-A and NGFI-B. These data demonstrate that the events leading to malignancy in the rat brain in response to MNU treatment are to a great extent similar to those described for secondary glial malignancies in humans.

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

TGFbeta stimulates expression of fibronectin in various cells, including mesangial cells. The mechanism by which TGFbeta exerts this effect is not fully understood. We investigated the involvement of tyrosine phosphorylation and the phosphatidylinositol (PI) 3 kinase/Akt signalling pathway in this process. TGFbeta increased tyrosine phosphorylation, resulting in activation of PI 3 kinase in mesangial cells. Inhibition of tyrosine phosphorylation blocked TGFbeta-induced fibronectin expression. Inhibition of PI 3 kinase activity also prevented fibronectin expression induced by TGFbeta. Furthermore, expression of constitutively active PI 3 kinase by adenovirus-mediated gene transfer increased fibronectin expression similar to TGFbeta. TGFbeta stimulated Akt serine threonine kinase in a tyrosine kinase- and PI 3 kinase-dependent manner. Inhibition of TGFbeta-induced Akt activity by adenovirus-mediated expression of a dominant-negative mutant of Akt abolished expression of fibronectin. Dominant-negative PI 3 kinase or dominant-negative Akt inhibited TGFbeta-induced fibronectin transcription. In contrast, and similarly to TGFbeta, expression of constitutively active PI 3 kinase or constitutively active Akt increased transcription of fibronectin, confirming a prominent role of these kinases in expression of fibronectin. These data provide the evidence that activation of TGFbeta receptor serine threonine kinase stimulates the PI 3 kinase/Akt pathway in a tyrosine phosphorylation-dependent manner and define a role for the same signal transduction pathway in TGFbeta-induced fibronectin expression.

Phosphatidylinositol 3-kinase regulates bone morphogenetic protein-2 (BMP-2)-induced myocyte enhancer factor 2A-dependent transcription of BMP-2 gene in cardiomyocyte precursor cells

The growth and differentiation factor bone morphogenetic protein-2 (BMP-2) regulates cardiac development during vertebrate embryogenesis. In cardiac precursor cells, BMP-2 has recently been shown to induce expression of cardiac transcription factors, including myocyte enhancer factor 2A (MEF-2A). The specific signal transduction mechanism by which BMP-2 regulates these actions is not known. We investigated the role of phosphatidylinositol (PI) 3-kinase in regulating these processes in cardiomyocyte precursor CL6 cells. BMP-2 increased PI 3-kinase activity in these cells in a time-dependent manner, resulting in increased expression of sarcomeric myosin heavy chain (MHC) and MEF-2A. Inhibition of PI 3-kinase abolished these actions of BMP-2, indicating the involvement of PI 3-kinase in these processes. Furthermore, BMP-2 stimulated specific protein.DNA complex formation when an MEF-2 DNA recognition element was used as probe. Antibody supershift assay confirmed the presence of MEF-2A in this protein.DNA complex. Inhibition of PI 3-kinase activity completely prevented the MEF-2A.DNA complex formation. BMP-2 also increased transcription of a reporter gene driven by an MEF-2-specific DNA element in a PI 3-kinase-dependent manner. Ectopic expression of MEF-2A increased BMP-2 transcription to the same extent induced by BMP-2, indicating that MEF-2A may participate in BMP-2 autoregulation in CL6 cells. Expression of dominant negative PI 3-kinase completely abolished BMP-2-induced as well as MEF-2A-mediated BMP-2 transcription. Furthermore expression of MEF-2A increased MHC expression in a PI 3-kinase-dependent manner. Together these data provide the first evidence that BMP-2-induced PI 3-kinase signaling regulates MEF-2A expression and define a mechanism of MEF-2A-dependent BMP-2 transcription.

BMP-2 regulates cardiomyocyte contractility in a phosphatidylinositol 3 kinase-dependent manner

Bone morphogenetic protein-2 (BMP-2) regulates development of heart during vertebrate embryogenesis. In vitro BMP-2 induces differentiation of precardiac cells into mature cardiomyocytes by inducing the expression of cardiac-specific genes. However, the role of BMP-2 and its signaling in other cardiac functions have not been studied. We examined the action of phosphatidylinositol (PI) 3 kinase in isolated adult rat cardiomyocytes. Incubation of rat ventricular cardiomyocytes with BMP-2 increased the PI 3 kinase activity. Ly294002, a pharmacological inhibitor of PI 3 kinase, blocked BMP-2-induced PI 3 kinase activity completely. To investigate the contractility of isolated cardiomyocytes, fractional shortening was examined. BMP-2 significantly increased the percent fractional shortening of the cardiomyocytes. Inhibition of PI 3 kinase activity completely abolished this action of BMP-2. These data indicate that PI 3 kinase regulates BMP-2-induced myocyte contractility. To further confirm this observation, we used adenovirus-mediated gene transfer to express a constitutively active myristoylated catalytic subunit of PI 3 kinase in rat cardiomyocytes. Infection of cardiomyocytes with the adenovirus vector increased the expression of constitutively active PI 3 kinase within 24 h. Expression of constitutively active PI 3 kinase significantly increased cardiomyocyte contractility. Together, these data show for the first time that the growth and differentiation factor, BMP-2, stimulates cardiomyocyte contractility. Also we provide the first evidence that BMP-2-induced PI 3 kinase activity regulates this cardiomyocyte function.

Requirement of BMP-2-induced phosphatidylinositol 3-kinase and Akt serine/threonine kinase in osteoblast differentiation and Smad-dependent BMP-2 gene transcription

The mechanism by which bone morphogenetic protein-2 (BMP-2) induces osteoblast differentiation is not precisely known. We investigated the involvement of the phosphatidylinositol (PI) 3-kinase/Akt signal transduction pathway in modulation of this process. BMP-2 stimulated PI 3-kinase activity in osteogenic cells. Inhibition of PI 3-kinase activity with the specific inhibitor Ly-294002 prevented BMP-2-induced alkaline phosphatase, an early marker of osteoblast differentiation. Expression of dominant-negative PI 3-kinase also abolished osteoblastic induction of alkaline phosphatase in response to BMP-2, confirming the involvement of this lipid kinase in this process. BMP-2 stimulated Akt serine/threonine kinase activity in a PI 3-kinase-dependent manner in osteoblast precursor cells. Inhibition of Akt activity by a dominant-negative mutant of Akt blocked BMP-2-induced osteoblastic alkaline phosphatase activity. BMP-2 stimulates its own expression during osteoblast differentiation. Expression of dominant-negative PI 3-kinase or dominant-negative Akt inhibited BMP-2-induced BMP-2 transcription. Because all the known biological activities of BMP-2 are mediated by transcription via BMP-specific Smad proteins, we investigated the involvement of PI 3-kinase in Smad-dependent BMP-2 transcription. Smad5 stimulated BMP-2 transcription independent of addition of the ligand. Dominant-negative PI 3-kinase or dominant-negative Akt inhibited Smad5-dependent transcription of BMP-2. Furthermore dominant-negative Akt inhibited translocation of BMP-specific Smads into nucleus. Together these data provide the first evidence that activation of BMP receptor serine/threonine kinase stimulates the PI 3 kinase/Akt pathway and define a role for this signal transduction pathway in BMP-specific Smad function during osteoblast differentiation.

Effect of platelet-derived growth factor isoforms in rat metanephric mesenchymal cells

Platelet-derived growth factor (PDGF) B-chain or PDGF beta-receptor-deficient mice lack mesangial cells. To explore potential mechanisms for failure of PDGF A-chain to rescue mesangial cell phenotype, we investigated the biological effects and signaling pathways of PDGF AA and PDGF BB in metanephric mesenchymal (MM) cells isolated from rat kidney. PDGF AA caused modest cell migration but had no effect on DNA synthesis, unlike PDGF BB, which potently stimulated migration and DNA synthesis. PDGF AA and PDGF BB significantly increased the activities of phosphatidylinositol 3-kinase (PI 3-K) and mitogen-activated protein kinase (MAPK). PDGF BB was more potent than PDGF AA in activating PI 3-K or MAPK in these cells. Pretreatment of MM cells with the MAPK kinase (MEK) inhibitor PD-098059 abrogated PDGF BB-induced DNA synthesis, whereas the PI 3-K inhibitor wortmannin had a very modest inhibitory effect on DNA synthesis (approximately Delta20%). On the other hand, wortmannin completely blocked PDGF AA- and PDGF BB-induced migration, whereas PD-098059 had a modest inhibitory effect on cell migration. These data demonstrate that activation of MAPK is necessary for the mitogenic effect of PDGF BB, whereas PI 3-K is required for the chemotactic effect of PDGF AA and PDGF BB. Although PDGF AA stimulates PI 3-K and MAPK activity, it is not mitogenic and only modestly chemotactic. Collectively, the data may have implications related to the failure of PDGF AA to rescue mesangial cell phenotype in PDGF B-chain or PDGF-beta-receptor deficiency.

Akt serine threonine kinase regulates platelet-derived growth factor-induced DNA synthesis in glomerular mesangial cells: regulation of c-fos AND p27(kip1) gene expression

Proliferation of mesangial cells requires platelet-derived growth factor receptor beta (PDGFR)-mediated signal transduction. We have previously shown that activation of phosphatidylinositol (PI) 3-kinase is necessary for PDGFR-induced DNA synthesis in these cells. The mechanism by which PI 3-kinase stimulates DNA synthesis is not known. One target of PI 3-kinase, Akt serine threonine kinase, regulates survival of many cells by inhibiting the actions of certain proapoptotic proteins. In this study, we investigated the role of Akt in PDGF-induced DNA synthesis in mesangial cells. PDGF increased Akt serine threonine kinase activity in a time- and PI 3-kinase-dependent manner. Expression of dominant negative Akt by adenovirus-mediated gene transfer blocked PDGF-induced activation of endogenous Akt in mesangial cells, resulting in complete inhibition of DNA synthesis. On the other hand, inhibition of MAPK attenuated PDGF-induced DNA synthesis only partially. Inhibition of Akt also attenuated PDGF-induced c-fos gene transcription, with concomitant inhibition of Elk-1-dependent transcription, indicating positive regulation of this early response gene by Akt. To further determine the role of Akt in PDGF-induced DNA synthesis, we investigated its effect on cyclin-dependent kinase 2 (CDK2). PDGF stimulated CDK2 activity in mesangial cells and decreased the level of p27(kip1) cyclin kinase inhibitor protein. Expression of dominant negative Akt increased p27(kip1) protein and resulted in inhibition of CDK2 activity. The increase in p27(kip1) expression in response to Akt kinase inhibition was due to increased transcription of the p27(kip1) gene. p27(kip1) transcription similarly was decreased by expression of constitutively active Akt kinase in mesangial cells. These data provide the first evidence that Akt kinase regulates PDGF-induced DNA synthesis by regulating CDK2 activity and define Akt-mediated inhibition of transcription of p27(kip1) as one of the mechanisms for PDGF-induced DNA synthesis in mesangial cells.

Ceramide blocks PDGF-induced DNA synthesis in mesangial cells via inhibition of Akt kinase in the absence of apoptosis

The mechanism of action of ceramide in glomerular mesangial cells has not been studied. We investigated the effect of C2 ceramide on the mitogenic signal transduction pathways induced by PDGF in mesangial cells. Increasing concentrations of C2 ceramide inhibited PDGF-induced DNA synthesis in a dose-dependent manner with maximum inhibition at 15 microM. This inhibition of DNA synthesis was associated with attenuation of PDGF-induced early response gene c-fos transcription. PDGF receptor beta immunecomplex kinase assay showed no inhibitory effect of C2 ceramide on PDGF receptor tyrosine kinase activity. We have recently shown that the mitogenic effect of PDGF is mediated by the enzyme phosphatidylinositol (PI) 3 kinase in mesangial cells. C2 ceramide had no effect on PDGF-induced PDGFR-associated PI 3 kinase activity. These data indicate that inhibitory effect of C2 on PDGF-induced DNA synthesis is likely due to post-receptor and post-PI 3 kinase events. To address the mechanism of C2-mediated inhibition of DNA synthesis, we investigated the downstream target of PI 3 kinase, Akt. PDGF time-dependently increased Akt kinase activity in a PI 3 kinase-dependent manner. Incubation of mesangial cells with C2 ceramide inhibited PDGF-induced Akt activity. Akt kinase inhibits apoptosis of cells via phosphorylation of multiple proapoptotic proteins. However, inhibition of Akt activity by C2 ceramide did not induce apoptosis in mesangial cells. These data provide the first evidence that in mesangial cells, ceramide cross-talks with PI 3 kinase-dependent Akt kinase to inhibit PDGF-induced DNA synthesis without inducing apoptosis.

Increased effect of interferon gamma on PDGF-induced c-fos gene transcription in glomerular mesangial cells: differential effect of the transcriptional coactivator CBP on STAT1alpha activation

We have previously shown that interferon gamma (IFNgamma) synergistically increases PDGF-induced DNA synthesis in mesangial cells. To examine the mechanism, we studied its effect on PDGF-induced c-fos gene transcription using a reporter mesangial cell in which firefly luciferase gene is driven by c-fos promoter. IFNgamma significantly enhanced PDGF-induced c-fos transcription. We have shown previously that PDGF-induced c-fos transcription in mesangial cells is mediated by the ternary complex factor Elk-1. Using a GAL-4 DNA binding-domain-Elk-1 transactivation domain fusion protein-based reporter assay we showed that the increased effect of IFNgamma was not mediated by Elk-1 transactivation. Gel mobility shift assay of lysates of mesangial cells treated with a combination of IFNgamma and PDGF using sis-inducible DNA element (SIE) showed increased STAT1alpha-SIE complex formation as compared to the PDGF alone. To investigate the transcriptional consequences of this observation, stable reporter mesangial cells in which luciferase gene is driven by four copies of SIE was used. IFNgamma and PDGF in combination significantly increased SIE-dependent transcription as compared to PDGF or IFNgamma alone. Using an antibody in the gel mobility shift assay we showed that the PDGF-induced SIE-STAT1alpha complex recruited the transcriptional coactivator CBP. However, the STAT1alpha-SIE complex formed in the presence of IFNgamma and PDGF did not contain CBP. Taken together, our data provide the first evidence that the synergistic effect of IFNgamma on PDGF-induced DNA synthesis may be the result of increased c-fos gene transcription via SIE. This effect occurs in the presence of increased activation of STAT1alpha without recruitment of the transcriptional coactivator CBP.