A mutational analysis of phosphatidylinositol-3-kinase activation by human colony-stimulating factor-1 receptor

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.

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

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

Regulation of mRNA translation in renal physiology and disease

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

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.

Downregulation of catalase by reactive oxygen species via PI 3 kinase/Akt signaling in mesangial cells

Reactive oxygen species (ROS) contribute to many glomerular diseases by targeting mesangial cells. ROS have been shown to regulate expression of many antioxidant enzymes including catalase. The mechanism by which the expression of catalase protein is regulated by ROS is not precisely known. Here we report that increased intracellular ROS level by hydrogen peroxide (H(2)O(2)) reduced the expression of catalase. H(2)O(2) increased phosphorylation of Akt kinase in a dose-dependent and sustained manner with a concomitant increase in the phosphorylation of FoxO1 transcription factor. Further analysis revealed that H(2)O(2) promoted rapid activation of phosphatidylinositol (PI) 3 kinase. The PI 3 kinase inhibitor Ly294002 and expression of tumor suppressor protein PTEN inhibited Akt kinase activity, resulting in the attenuation of FoxO1 phosphorylation and preventing the downregulating effect of H(2)O(2) on catalase protein level. Dominant negative Akt attenuated the inhibitory effect of H(2)O(2) on expression of catalase. Constitutively active FoxO1 increased the expression of catalase. However, dominant negative FoxO1 inhibited catalase protein level. Catalase transcription was reduced by H(2)O(2) treatment. Furthermore, expression of dominant negative Akt and constitutively active FoxO1 increased catalase transcription, respectively. These results demonstrate that ROS downregulate the expression of catalase in mesangial cells by PI 3 kinase/Akt signaling via FoxO1 as a target.

c-Src couples PI 3 kinase/Akt and MAPK signaling to PDGF-induced DNA synthesis in mesangial cells

Platelet-derived growth factor BB (PDGF) and PDGF receptor-beta (PDGFR) play critical roles in mesangial cell proliferation during embryonic development and in mesangioproliferative glomerulonephritis. We have shown previously that phosphatidylinositol (PI) 3 kinase/Akt and Erk1/2 mitogen-activated protein kinase (MAPK) contribute to PDGF-dependent proliferation of mesangial cells, but the mechanism by which these two enzyme cascades are activated by PDGFR signaling is not precisely known. We examined the role of c-Src tyrosine kinase in this process. PDGF increased phosphorylation of c-Src in a time-dependent manner indicating its activation. A pharmacologic inhibitor of c-Src, PP1, blocked PDGF-induced DNA synthesis with concomitant inhibition of c-Src phosphorylation. Immune-complex kinase assays of c-Src and PDGFR demonstrated inhibition of c-Src tyrosine kinase activity by PP1, without an effect on PDGFR tyrosine phosphorylation. Both PP1 and expression of dominant negative c-Src inhibited PDGF-induced PI 3 kinase, resulting in attenuation of Akt kinase activity. Expression of constitutively active c-Src increased Akt activity to the same extent as with PDGF. Constitutively active c-Src augmented PDGF-induced Akt activity, thus contributing to Akt signaling. Inhibition of c-Src tyrosine kinase blocked PDGF-stimulated MAPK activity and resulted in attenuation of c-fos gene transcription with concomitant prevention of Elk-1 transactivation. Furthermore, inhibition of c-Src increased p27(Kip1) cyclin kinase inhibitor, and attenuated PDGF-induced pRb phosphorylation and CDK2 activity. These data provide the first evidence in mesangial cells that PDGF-activated c-Src tyrosine kinase relays signals to PI 3 kinase/Akt and MAPK. Furthermore our results demonstrate that c-Src integrates signals into the nucleus to activate CDK2, which is required for DNA synthesis.

c-Fms tyrosine 559 is a major mediator of M-CSF-induced proliferation of primary macrophages

The molecular mechanisms by which binding of monocyte/macrophage colony-stimulating factor to its receptor c-Fms promotes replication in primary macrophages are incompletely understood, as all previous studies involved overexpression of receptor mutants in transformed cells not endogenously expressing the receptor. To address this issue we retrovirally expressed, in bone marrow-derived macrophages, a chimeric receptor containing a range of tyrosine to phenylalanine mutations in the c-Fms cytoplasmic tail. We measured incorporation of bromodeoxyuridine as a marker of proliferation and phosphorylation of ERKs, Akt, and the receptor itself. Our data indicate that tyrosine 559 is the major mediator of receptor activation and cell death, intracellular signaling, and cell proliferation and that the tyrosine residues at positions 697 and 807 play lesser roles in these events. Importantly, we find that activation of the ERK and Akt pathways is necessary but not sufficient for induction of macrophage proliferation. Using specific small molecule inhibitors we find that a combination of the Src family kinase, phosphatidylinositol 3-kinase/Akt, phospholipase C, and ERK pathways mediates macrophage proliferation in response to M-CSF.

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.

Colony-stimulating factor-1 requires PI3-kinase-mediated metabolism for proliferation and survival in myeloid cells

Colony-stimulating factor-1 (CSF-1) is essential for macrophage growth, differentiation and survival. Myeloid cells expressing a CSF-1 receptor mutant (DeltaKI) show markedly impaired CSF-1-mediated proliferation and survival, accompanied by absent signal transducers and activators of transcription 3 (Stat3) phosphorylation and reduced PI3-kinase/Akt activity. Restoring phosphatidylinositol 3-kinase (PI3-kinase) but not Stat3 signals reverses the mitogenic defect. CSF-1-induced proliferation and survival are sensitive to glycolytic inhibitors, 2-deoxyglucose and 3-bromopyruvate. Consistent with a critical role for PI3-kinase-regulated glycolysis, DeltaKI cells reconstituted with active PI3-kinase or Akt are hypersensitive to these inhibitors. CSF-1 upregulates hexokinase II (HKII) expression through PI3-kinase, and PI3-kinase transcriptionally activates the HKII promoter. Moreover, HKII overexpression partially restores mitogenicity. In contrast, Bcl-x(L) expression does not enhance long-term proliferation, although short-term cell death is suppressed in a glycolysis-independent manner. This study identifies robust PI3-kinase activation as essential for optimal CSF-1-mediated mitogenesis in myeloid cells, in part through regulation of HKII and support of glycolysis.

Green tea polyphenol epigallocatechin-3-gallate blocks PDGF-induced proliferation and migration of rat pancreatic stellate cells

AIM: To clarify the effects of epigallocatechin-3-gallate (EGCG) on the platelet-derived growth factor (PDGF)-BB-induced proliferation and migration of pancreatic stellate cells (PSCs).

METHODS: PSCs were isolated from rat pancreas tissue and used in their culture-activated, myofibroblast-like phenotype. Cell proliferation was assessed by measuring the incorporation of 5-bromo-2’-deoxyuridine. Cell migration was assessed using modified Boyden chambers. Cyclin D1, p21^Waf1, and p27^Kip1 expression and phosphorylation of PDGF β-receptor, extracellular signal-regulated kinase, and Akt were examined by Western blotting. Activation of phospha-tidylinositol 3-kinase was examined by kinase assay using phosphatidylinositol as a substrate. Cell cycle was assessed by flow cytometry after staining with propidium iodide.

RESULTS: EGCG at non-cytotoxic concentrations inhibited PDGF-induced proliferation and migration. This effect was associated with the inhibition of cell cycle progression beyond the G₁ phase, decreased cyclin D1 and increased p27^Kip1 expression. EGCG inhibited tyrosine phosphorylation of PDGF β-receptor and downstream activation of extracellular signal-regulated kinase and phosphatidylinositol 3-kinase/Akt pathways.

CONCLUSION: EGCG inhibited PDGF-BB-induced proliferation and migration of PSCs through the inhibition of PDGF-mediated signaling pathways.

EGF stimulates mesangial cell mitogenesis via PI3-kinase-mediated MAPK-dependent and AKT kinase-independent manner: involvement of c-fos and p27Kip1

Epidermal growth factor (EGF) is a potent mitogen for mesangial cells. The mechanism by which EGF induces DNA synthesis is not precisely understood. We investigated the role of phosphatidylinositol (PI)3-kinase in regulating mitogenesis. EGF increased PI3-kinase activity resulting in stimulation of PDK-1 and Akt kinase activities. Blocking of PI3-kinase activity using LY-294002 or adenoviral expression of PTEN, which dephosphorylates PI3,4,5-tris-phosphate and thus inactivates PI3-kinase signaling, significantly inhibits EGF-induced DNA synthesis. Expression of dominant-negative Akt kinase, however, had no effect on DNA synthesis. But it inhibited EGF-induced phosphorylation of FoxO3a transcription factor, thus demonstrating its functional consequences. These data indicate that EGF increases the DNA synthesis in a PI3-kinase-dependent but Akt-independent manner. In addition to activating PI3-kinase signaling, EGF increased Erk1/2 MAPK activity, leading to transcriptional activation of its nuclear target Elk-1 and resulting in c-fos expression. Inhibition of MAPK activity by MEK inhibitor U-0126 abolished EGF-induced DNA synthesis. Because EGF activates PI3-kinase, which also regulates DNA synthesis, the effect of PI3-kinase on MAPK activity was also examined. Inhibition of PI3-kinase signaling blocked EGF-induced MAPK activity as well as Elk-1-dependent reporter transcription and c-fos gene transcription. To further determine the mechanism of EGF-induced DNA synthesis, we investigated the effect of EGF on the cyclin-dependent kinase inhibitor p27(Kip1). EGF reduced the expression of p27(Kip1). Inhibition of PI3-kinase action or MAPK activity abolished the reduction in p27(Kip1) expression induced by EGF. These data provide the evidence that a linear signal transduction pathway involving PI3-kinase-dependent MAPK regulates EGF-induced DNA synthesis in mesangial cells by regulating c-fos and p27(Kip1) expression.

Insulin inhibits platelet-derived growth factor-induced cell proliferation

Cellular behavior can be considered to be the result of a very complex spatial and temporal integration of intracellular and extracellular signals. These signals arise from serum-soluble factors as well as from cell-substrate or cell-cell interactions. The current approach in mitogenesis studies is generally to analyze the effect of a single growth factor on serum-starved cells. In this context, a metabolic hormone such as insulin is found to be a mitogenic agent in many cellular types. In the present study, we have considered the effect of insulin stimulation in platelet-derived growth factor (PDGF)-activated NIH-3T3 and C2C12 cells. Our results show that insulin is able to inhibit strongly both NIH-3T3 and C2C12 cell growth induced by PDGF, one of the most powerful mitotic agents for these cell types. This inhibitory effect of insulin is due primarily to a premature down-regulation of the PDGF receptor. Thus, when NIH-3T3 or C2C12 cells are stimulated with both PDGF and insulin, we observe a decrease in PDGF receptor phosphorylation with respect to cells treated with PDGF alone. In particular, we find that costimulation with insulin leads to a reduced production of H2O2 with respect to cell stimulation with PDGF alone. The relative low concentration of H2O2 in PDGF/insulin-costimulated cell leads to a limited down-regulation of protein tyrosine phosphatases, and, consequently, to a reduced PDGF receptor phosphorylation efficiency. The latter is very likely to be responsible for the insulin-dependent inhibition of PDGF-receptor mitogenic signaling.

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.

Inactivation of platelet-derived growth factor receptor by the tumor suppressor PTEN provides a novel mechanism of action of the phosphatase

PTEN, mutated in a variety of human cancers, is a dual specificity protein phosphatase and also possesses D3-phosphoinositide phosphatase activity on phosphatidylinositol 3,4,5-tris-phosphate (PIP(3)), a product of phosphatidylinositol 3-kinase. This PIP(3) phosphatase activity of PTEN contributes to its tumor suppressor function by inhibition of Akt kinase, a direct target of PIP(3). We have recently shown that Akt regulates PDGF-induced DNA synthesis in mesangial cells. In this study, we demonstrate that expression of PTEN in mesangial cells inhibits PDGF-induced Akt activation leading to reduction in PDGF-induced DNA synthesis. As a potential mechanism, we show that PTEN inhibits PDGF-induced protein tyrosine phosphorylation with concomitant dephosphorylation and inactivation of tyrosine phosphorylated and activated PDGF receptor. Recombinant as well as immunopurified PTEN dephosphorylates autophosphorylated PDGF receptor in vitro. Expression of phosphatase deficient mutant of PTEN does not dephosphorylate PDGF-induced tyrosine phosphorylated PDGF receptor. Rather its expression increases tyrosine phosphorylation of PDGF receptor. Furthermore, expression of PTEN attenuated PDGF-induced signal transduction including phosphatidylinositol 3-kinase and Erk1/2 MAPK activities. Our data provide the first evidence that PTEN is physically associated with platelet-derived growth factor (PDGF) receptor and that PDGF causes its dissociation from the receptor. Finally, we show that both the C2 and tail domains of PTEN contribute to binding to the PDGF receptor. These data demonstrate a novel aspect of PTEN function where it acts as an effector for the PDGF receptor function and negatively regulates PDGF receptor activation.

Differential roles of signaling pathways for proliferation and migration of rat pancreatic stellate cells

Activated pancreatic stellate cells (PSCs) have recently been implicated in the pathogenesis of pancreatic fibrosis and inflammation. Accumulation of PSCs is a fundamental feature of pancreatic fibrosis, and platelet-derived growth factor (PDGF)-BB is the most potent mitogen for PSCs. But, the molecular mechanisms responsible for PDGF's actions in PSCs are largely unknown. In hepatic stellate cells, it has been established that activation of both phosphatidylinositol (PI) 3-kinase and extracellular-signal regulated kinase (ERK) pathways is required for PDGF-BB-induced proliferation and migration. The aim of this study was to elucidate the signaling pathways mediating PDGF-BB's actions in PSCs. PSCs were isolated from rat pancreas tissue and used in their culture-activated, myofibroblast-like phenotype. Culture-activated PSCs expressed PDGF alpha- and beta-receptors. PDGF-BB induced autophosphorylation of its receptor, followed by the activation of PI 3-kinase, Akt, and ERK pathways. Activation of PI 3-kinase was not required for PDGF-BB-induced ERK activation. PDGF-BB induced approximately five-fold increase in proliferation and chemotaxis of PSCs. Inhibition of ERK pathway with PD98059 completely blocked proliferation, whereas PD98059 had a modest inhibitory effect on cell migration (approximately 50%). On the other hand, inhibition of PI 3-kinase pathway with wortmannin or LY294002 almost completely inhibited migration, but did not affect proliferation of PSCs. In conclusion, our results suggest that ERK pathway regulates proliferation and migration in response to PDGF-BB, whereas PI 3-kinase mediates cellular migration, but not proliferation of PSCs.

New perspectives in PDGF receptor downregulation: the main role of phosphotyrosine phosphatases

Uncontrolled activation of receptor tyrosine kinases (RTKs) is implicated in the proliferation of cancerous cells, and deficiencies in RTKs results in pathological conditions such as developmental abnormalities and immunodeficiencies. Tight regulation of RTK cascades is therefore critical for eliciting an appropriate type and level of response to external stimuli. The aim of this work is to compare different RTK downregulation mechanisms, such as ligandinduced internalisation, ubiquitin-mediated proteolysis and dephosphorylation by protein phosphotyrosine phosphatase (PTPs). We choose platelet-derived growth factor receptor (PDGF-r) in NIH3T3 cells as a model of RTK. Our data suggest that PDGF-r internalisation could be mainly considered as a positive signaling system, as it is involved in MAPK activation rather than a downregulation of the mitotic signal. Inhibition of receptor ubiquitination does not result in regulation of PDGF-r tyrosine phosphorylation and does not lead to variation of intracellular signalling pathways. The overall PDGF-r protein degradation upon PDGF stimulation does not exceed 30-40% of the total receptor; thus the receptor remains functionally active for further stimulation. On the contrary, PTP-dependent dephosphorylation of the activated receptors appears to play a crucial role. In fact, inhibition of PTP upon PDGF stimulation results in upregulation of receptor phosphorylation level, of PI3K recruitment and activation and of cell cycle rate. On the contrary, PTP-dependent dephosphorylation does not affect the endosomic pool of activated receptor. Furthermore, we demonstrate that PDGF-r downregulation by means of PTP dephosphorylation is important for both short term (2 hours) and long-lasting (up to 8 hours) PDGF-r activation. Herein we propose a revisited model of PDGF-r downregulation in which PTPs dephosphorylation retains a major role, conferring on receptor internalisation a signal transduction function.

Signal transduction by the chemokine receptor CXCR3: activation of Ras/ERK, Src, and phosphatidylinositol 3-kinase/Akt controls cell migration and proliferation in human vascular pericytes

Hepatic stellate cells (HSC) and glomerular mesangial cells (MC) are tissue-specific pericytes involved in tissue repair, a process that is regulated by members of the chemokine family. In this study, we explored the signal transduction pathways activated by the chemokine receptor CXCR3 in vascular pericytes. In HSC, interaction of CXCR3 with its ligands resulted in increased chemotaxis and activation of the Ras/ERK cascade. Activation of CXCR3 also stimulated Src phosphorylation and kinase activity and increased the activity of phosphatidylinositol 3-kinase and its downstream pathway, Akt. The increase in ERK activity was inhibited by genistein and PP1, but not by wortmannin, indicating that Src activation is necessary for the activation of the Ras/ERK pathway by CXCR3. Inhibition of ERK activation resulted in a decreased chemotactic and mitogenic effect of CXCR3 ligands. In MC, which respond to CXCR3 ligands with increased DNA synthesis, CXCR3 activation resulted in a biphasic stimulation of ERK activation, a pattern similar to the one observed in HSC exposed to platelet-derived growth factor, indicating that this type of response is related to the stimulation of cell proliferation. These data characterize CXCR3 signaling in pericytes and clarify the relevance of downstream pathways in the modulation of different biologic responses.

Ligands of peroxisome proliferator-activated receptor gamma modulate profibrogenic and proinflammatory actions in hepatic stellate cells

BACKGROUND & AIMS

Proliferation and migration of hepatic stellate cells (HSCs) and expression of chemokines are involved in the pathogenesis of liver inflammation and fibrogenesis. Peroxisome proliferator-activated receptor (PPAR)-gamma is a receptor transcription factor that controls growth and differentiation in different tissues. We explored the effects of PPAR-gamma agonists on the biological actions of cultured human HSCs.

METHODS

HSCs were isolated from normal human liver tissue and used in their myofibroblast-like phenotype or immediately after isolation. Activation of PPAR-gamma was induced with 15-deoxy-Delta(12, 14)-prostaglandin J(2) or with troglitazone.

RESULTS

PPAR-gamma agonists dose-dependently inhibited HSC proliferation and chemotaxis induced by platelet-derived growth factor. This effect was independent of changes in postreceptor signaling or expression of c-fos and c-myc and was associated with inhibition of cell cycle progression beyond the G(1) phase. Activation of PPAR-gamma also resulted in a complete inhibition of the expression of monocyte chemotactic protein 1 at the gene and protein levels. Comparison of quiescent and culture-activated HSCs revealed a marked decrease in PPAR-gamma expression in activated cells.

CONCLUSIONS

Activation of PPAR-gamma modulates profibrogenic and proinflammatory actions in HSCs. Reduced PPAR-gamma expression may contribute to confer an activated phenotype to HSCs.

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

BACKGROUND

Platelet-derived growth factor (PDGF) isoforms act through two distinct cell surface alpha and beta receptors. Glomerular mesangial cells express both receptors. PDGF BB and AB are potent mitogens for glomerular mesangial cells, and PDGF BB stimulates cell migration in a phosphatidylinositol 3 (PI 3) kinase-dependent manner. In this study, we investigated the effect of PDGF AA on cell migration, PI 3 kinase and phospholipase C (PLC) activation, and the role of these two enzymes in mediating biological responses in these cells in response to all three isoforms.

METHODS

3H-thymidine incorporation and modified Boyden chamber assay were used to determine DNA synthesis and directed migration, respectively, in response to all three PDGF isoforms. Differential activation of alpha and beta receptors was studied by immunecomplex tyrosine kinase assay of corresponding receptor immunoprecipitates. PLC gamma 1 activity was determined by measuring total inositol phosphates in response to different PDGF isoforms. PI 3 kinase activity was determined in antiphosphotyrosine or PDGF receptor immunoprecipitates.

RESULTS

Both PDGF BB and AB resulted in stimulation of DNA synthesis and directed migration of mesangial cells. AA was neither chemotactic nor mitogenic. However, all three isoforms increased tyrosine phosphorylation of a 180 kD protein in antiphosphotyrosine immunoprecipitates, suggesting activation of respective receptors. Direct immunecomplex tyrosine kinase assay of alpha and beta receptors demonstrated significant activation of both of these receptors when cells are treated with PDGF BB or AB. PDGF AA increased tyrosine kinase activity of the alpha receptor but not the beta receptor. All three isoforms significantly stimulated the production of inositol phosphates with order of potency being BB > AB > AA. PDGF AA also dose dependently stimulated PI 3 kinase activity measured in antiphosphotyrosine immunoprecipitates of treated cells. A comparison of PI 3 kinase activity in antiphosphotyrosine immunoprecipitates from mesangial cells stimulated with three different PDGF isoforms showed significant activation of this enzyme with a decreasing order of activity: BB > AB > AA.

CONCLUSION

Taken together, these data demonstrate that all three isoforms of PDGF significantly stimulate PLC gamma 1 and PI 3 kinase, two enzymes necessary for both DNA synthesis and directed migration. However, activation of alpha receptor by PDGF AA with a subsequent increase in PLC and PI 3 kinase activities is not sufficient to induce these biological responses in mesangial cells. These data indicate that the extent of activation of signal transduction pathways may be a major determinant of the biological activity of different PDGF isoforms in mesangial cells.

Extracellular signal-regulated kinase activation differentially regulates platelet-derived growth factor's actions in hepatic stellate cells, and is induced by in vivo liver injury in the rat

Upon liver injury, hepatic stellate cells (HSC) show increased proliferation, motility, and extracellular matrix (ECM) production. The extracellular signal-regulated kinases (ERK) control different functions in a cell-specific manner. In this study, we evaluated the role of ERK activation in cultured HSC stimulated with platelet-derived growth factor (PDGF) and after induction of liver injury in vivo. HSC were isolated from normal human liver tissue, cultured on plastic, and used in their myofibroblast-like phenotype. In in vivo experiments, HSC were isolated from normal rats or at different time points after a single intragastric administration of CCl(4). Nontoxic concentrations of PD98059, a specific inhibitor of ERK activation, reduced PDGF-induced activation of ERK in a dose-dependent fashion. Suppression of ERK activation was associated with complete inhibition of HSC proliferation and with a 57% reduction in chemotaxis. In the presence of the ERK inhibitor, binding of the AP-1 complex and of STAT1 to the related regulatory elements was inhibited. The inhibition of the DNA binding activity of STAT1 was mediated by a reduction in PDGF-induced tyrosine phosphorylation. Expression of c-fos in response to PDGF was also reduced, but not suppressed, by treatment with PD98059. In HSC isolated from CCl(4)-treated rats, ERK activity increased as early as 6 hours following liver damage, and declined thereafter. The results of this study indicate that ERK activation regulates proliferation and chemotaxis of HSC, and modulates nuclear signaling. Acute liver damage in vivo leads to activation of ERK in HSC.

Activation of STAT1 alpha by phosphatase inhibitor vanadate in glomerular mesangial cells: involvement of tyrosine and serine phosphorylation

Vanadate is an insulinomimetic agent that has potent inhibitory effect on tyrosine phosphatases. We have recently demonstrated that low concentration of vanadate stimulates phosphotyrosine-dependent signal transduction pathways leading to gene expression and DNA synthesis in mesangial cells. To further examine the mechanisms by which vanadate activates mesangial cell, we studied its effect on signal transducer and activators of transcription (STAT). Incubation of lysates from vanadate-stimulated mesangial cells with a specific high affinity sis-inducible DNA element (SIE) resulted in the formation of protein-DNA complex. Supershift analysis using monoclonal antibody against STAT1 alpha showed its exclusive presence in the DNA-protein complex. Incubation of cell lysate with antiphosphotyrosine antibody or with excess phosphotyrosine caused decrease in binding of STAT1 alpha to SIE probe indicating that tyrosine phosphorylation and dimerization of this transcription factor are necessary for its activation. Immunoprecipitation followed by immunecomplex kinase assay showed increased tyrosine kinase activity of Janus kinase 2 (JAK2) in vanadate-treated mesangial cells. The addition of a monoclonal antiphosphoserine antibody to lysates from vanadate-treated mesangial cells results in supershift of protein-DNA complex indicating the presence of serine phosphorylated STAT1 alpha in this complex. Treatment of lystates from vanadated-stimulated mesangial cells with serine phosphatase PP2A causes inhibition of DNA-protein interaction. Collectively, our data indicate that at least one mechanism of activation of mesangial cells during vanadate treatment is increased activation of STAT1 alpha by both tyrosine and serine phosphorylation.

Mitogen-activated protein kinase (MAPK) regulates the expression of progelatinase B (MMP-9) in breast epithelial cells

Mitogen-activated protein kinases (MAPKs) play a major role in the mitogenic signal transduction pathway and are essential components of both growth and differentiation. Constitutive activation of the MAPK cascade is associated with the carcinogenesis and metastasis of human breast and renal cell carcinomas. The gelatinases B (MMP-9) and A (MMP-2) are 2 members of the matrix metalloproteinase (MMPs) family which are expressed in human cancers and thought to play a critical role in tumor cell invasion and metastasis. In a previous study, we have shown that EGF and amphiregulin upregulate MMP-9 in metastatic SKBR-3 cells but have no effect on MMP-2 secretion. We now investigated specific step(s) in EGF-induced signalling associated with regulation of cell proliferation and MMP-9 induction. EGF-induced signalling in SKBR-3 cells was blocked by relatively specific inhibitors either on ras (FPT inhibitor-1) or P13 kinase (Wortmannin) or by reduction in EGF-induced tyrosine kinase activity (RG 13022). Blocking these signalling pathways significantly inhibited of EGF-induced cell proliferation but only partially reduced in EGF-induced MMP-9 secretion. In contrast, when SKBR-3 cells were exposed to MEK inhibitor (PD 98059) or MAPK inhibitors (Apigenin or MAPK antisense phosphorothioate oligodeoxynucleotides), EGF-induced cell proliferation, MMP-9 induction and invasion through reconstituted basement membrane were significantly reduced. Our results suggest that interfering with MAPK activity may provide a novel means of controlling growth and invasiveness of tumors in which the signalling cascade is activated.

Bone morphogenetic protein-2 inhibits MAPK-dependent Elk-1 transactivation and DNA synthesis induced by EGF in mesangial cells

Bone morphogenetic protein-2 (BMP-2) is a member of the TGFbeta superfamily of growth and differentiation factors. We investigated the effect of BMP-2 on epidermal growth factor (EGF)-induced mitogenic signaling in kidney glomerular mesangial cells. BMP-2 dose-dependently inhibits EGF-induced DNA synthesis. Maximum effect was obtained at a concentration of 100 ng/ml. BMP-2 had no inhibitory effect on the EGF receptor (EGFR)-associated tyrosine kinase activity indicating that inhibition of DNA synthesis is due to regulation of post-receptor signaling event(s). EGF stimulates MAPK activity in mesangial cells in a time-dependent manner. Inhibition of MAPK by the MEK inhibitor PD098059 blocks EGF-induced DNA synthesis indicating the requirement of this enzyme activity in EGF-mediated mitogenic signaling. Furthermore, we show that exposure of mesangial cells to BMP-2 blocks EGF-induced MAPK activity which leads to phosphorylattion of Elk-1 transcription factor. Using a GAL-4 DNA binding-domain-Elk-1 transactivation domain fusion protein-based reporter assay, we demonstrate that BMP-2 inhibits EGF-induced Elk-1-mediated transcription. These data provide the first evidence that BMP-2 signaling in mesangial cells initiates a negative regulatory cross-talk with MAPK-based transcription to inhibit EGF-induced DNA synthesis.

Bone morphogenetic protein 2 inhibits platelet-derived growth factor-induced c-fos gene transcription and DNA synthesis in mesangial cells. Involvement of mitogen-activated protein kinase

Bone morphogenetic proteins (BMPs) play an important role in nephrogenesis. The biologic effect and mechanism of action of these proteins in the adult kidney has not yet been studied. We investigated the effect of BMP2, a member of these growth and differentiation factors, on mitogenic signal transduction pathways induced by platelet-derived growth factor (PDGF) in glomerular mesangial cells. PDGF is a growth and survival factor for these cells in vitro and in vivo. Incubation of mesangial cells with increasing concentrations of BMP2 inhibited PDGF-induced DNA synthesis in a dose-dependent manner with maximum inhibition at 250 ng/ml. Immune complex tyrosine kinase assay of PDGF receptor beta immunoprecipitates from lysates of mesangial cells treated with PDGF showed no inhibitory effect of BMP2 on PDGF receptor tyrosine phosphorylation. This indicates that the inhibition of DNA synthesis is likely due to postreceptor events. However, BMP2 significantly inhibited PDGF-stimulated mitogen-activated protein kinase (MAPK) activity that phosphorylates the Elk-1 transcription factor, a component of the ternary complex factor. Using a fusion protein-based reporter assay, we also show that BMP2 blocks PDGF-induced Elk-1-mediated transcription. Furthermore, we demonstrate that BMP2 inhibits PDGF-induced transcription of c-fos gene, a natural target of Elk-1 that normally forms a ternary complex that activates the serum response element of the c-fos gene. These data provide the first evidence that in mesangial cells, BMP2 signaling cross-talks with MAPK-based transcriptional events to inhibit PDGF-induced DNA synthesis. One target for this inhibition is the early response gene c-fos.

The Src and signal transducers and activators of transcription pathways as specific targets for low molecular weight phosphotyrosine-protein phosphatase in platelet-derived growth factor signaling

The low molecular weight phosphotyrosine-protein phosphatase (LMW-PTP) is a cytosolic phosphotyrosine-protein phosphatase specifically interacting with the activated platelet-derived growth factor (PDGF) receptor through its active site. Overexpression of the LMW-PTP results in modulation of PDGF-dependent mitogenesis. In this study we investigated the effects of this tyrosine phosphatase on the signaling pathways relevant for PDGF-dependent DNA synthesis. NIH 3T3 cells were stably transfected with active or dominant negative LMW-PTP. The effects of LMW-PTP were essentially restricted to the G1 phase of the cell cycle. Upon stimulation with PDGF, cells transfected with the dominant negative LMW-PTP showed an increased activation of Src, whereas the active LMW-PTP induced a reduced activation of this proto-oncogene. We observe that c-Src binding to PDGF receptor upon stimulation is prevented by overexpression of LMW-PTP. These effects were associated with parallel changes in myc expression. Moreover, wild-type and dominant negative LMW-PTP differentially regulated STAT1 and STAT3 activation and tyrosine phosphorylation, whereas they did not modify extracellular signal-regulated kinase activity. However, these modifications were associated with changes in fos expression despite the lack of any effect on extracellular signal-regulated kinase activation. Other independent pathways involved in PDGF-induced mitogenesis, such as phosphatidylinositol 3-kinase and phospholipase C-gamma1, were not affected by LMW-PTP. These data indicate that this phosphatase selectively interferes with the Src and the STATs pathways in PDGF downstream signaling. The resulting changes in myc and fos proto-oncogene expression are likely to mediate the modifications observed in the G1 phase of the cell cycle.

Effects of wortmannin and rapamycin on CSF-1-mediated responses in macrophages

There are differing views regarding the roles of phosphatidylinositol 3-kinases (PI3-kinases) and p70 S6 kinase (p70s6k) in growth factor-induced cellular responses. One approach that is widely employed to investigate these roles is to use the inhibitors, wortmannin and rapamycin, respectively. This approach is used here to study the responses in macrophages to colony stimulating factor-1 (CSF-1). Wortmannin (> or = 30 nM) and rapamycin (> or = 3 nM) both weakly inhibited CSF-1-stimulated DNA synthesis in murine bone marrow-derived macrophages (BMM), suggesting that there are PI3-kinase- and p70s6k-independent pathways required for the onset of S phase; interestingly the combination of the drugs gave dramatic suppression. Inhibition of DNA synthesis by rapamycin on the BMM was much less than that observed with the CSF-1-dependent cell line, BAC1.2F5. In BMM, wortmannin suppressed CSF-1-stimulated increase in p70s6k activity indicating that PI3-kinase activity may lie upstream. In contrast to some other growth factor/cell systems, no evidence was obtained using the inhibitors for the involvement of PI3-kinase or p70s6k in CSF-1-mediated induction of c-fos mRNA expression or Erk-1 activity; in addition, no evidence was found for an involvement in the CSF-1-mediated increase in cyclin D1 expression or STAT activation. The findings reinforce the need to study the signal transduction cascades relevant to each individual growth factor and preferably not in cell lines.

PI-3-kinase and MAPK regulate mesangial cell proliferation and migration in response to PDGF

Proliferation and migration are important biological responses of mesangial cells to injury. Platelet-derived growth factor (PDGF) is a prime candidate to mediate these responses in glomerular disease. PDGF and its receptor (PDGFR) are upregulated in the mesangium during glomerular injury. We have recently shown that PDGF activates phosphatidylinositol 3-kinase (PI-3-kinase) in cultured mesangial cells. The role of this enzyme and other more distal signaling pathways in regulating migration and proliferation of mesangial cells has not yet been addressed. In this study, we used two inhibitors of PI-3-kinase, wortmannin (WMN) and LY-294002, to investigate the role of this enzyme in these processes. Pretreatment of mesangial cells with WMN and LY-294002 dose-dependently inhibited PDGF-induced PI-3-kinase activity assayed in antiphosphotyrosine immunoprecipitates. WMN pretreatment also inhibited the PI-3-kinase activity associated with anti-PDGFR beta immunoprecipitates prepared from mesangial cells treated with PDGF. Pretreatment of the cells with different concentrations of WMN resulted in a dose-dependent inhibition of PDGF-induced DNA synthesis. Both WMN and LY-294002 inhibited PDGF-stimulated migration of mesangial cells in a dose-dependent manner. It has recently been shown that PI-3-kinase physically interacts with Ras protein. Because Ras is an upstream regulator of the kinase cascade leading to the activation of mitogen-activated protein kinase (MAPK), we determined whether activation of PI-3-kinase is necessary for activation of MAPK. Pretreatment of mesangial cells with WMN and LY-294002 significantly inhibited PDGF-induced MAPK activity as measured by immune complex kinase assay of MAPK immunoprecipitates. Furthermore, PD-098059, an inhibitor of MAPK-activating kinase inhibited PDGF-induced MAPK activity and resulted in significant reduction of mesangial cell migration in response to PDGF. These data indicate that MAPK is a downstream target of PI-3-kinase and that both these enzymes are involved in regulating proliferation and migration of mesangial cells.

LMW-PTP is a negative regulator of insulin-mediated mitotic and metabolic signalling

To understand the physiological role of low Mr weight phosphotyrosine protein phosphatase (LMW-PTP) in insulin mediated signaling, we established clonal cell lines overexpressing the dominant negative (C12S mutant) LMW-PTP (dnLMW-PTP) from NIH3T3 murine fibroblasts expressing insulin receptor. Upon insulin stimulation we observe an association between the dnLMW-PTP and the beta-subunit of the insulin receptor. This association is dependent on the tyrosine phosphorylation of the insulin receptor since it is not observed in unstimulated cells. Furthermore, in vitro binding experiments between dnLMW-PTP and the insulin receptor reveal that the interaction is mediated by the LMW-PTP catalytic site, as indicated by competition with orthovanadate. DnLMW-PTP overexpression influences both the mitogenic and the metabolic bioeffects of insulin. In particular, in cells overexpressing dnLMW-PTP we observe an increase in the glycogenosynthesis rate and in mitosis as indicated by glucose incorporation into glycogen and thymidine incorporation into DNA, respectively. Moreover, we studied the insulin mediated signal transduction pathways starting from insulin receptor, such as the Src kinase, the p21Ras/ERK, and the PI3K routes. Our findings are consistent with a specific regulation of mitogenesis by LMW-PTP through a pathway involving c-Src kinase but independent by both PI3K and ERK. These data strongly suggest that LMW-PTP acts as a negative regulator of both mitogenetic and metabolic insulin signalling.

Inhibition by pentoxifylline of extracellular signal-regulated kinase activation by platelet-derived growth factor in hepatic stellate cells

1. It has been proposed that pentoxifylline (PTF) acts an antifibrogenic agent by reducing the synthesis of extracellular matrix components, and this possibility has been confirmed in animal models of hepatic fibrosis. In this study the effects of PTF on the proliferation of extracellular matrix producing cells induced by platelet-derived growth factor (PDGF) were evaluated. The study was performed on hepatic stellate cells, currently indicated as the major source of extracellular matrix in fibrotic liver. 2. PTF caused a dose-dependent reduction of PDGF-induced mitogenesis with an IC50 of 170 microM, identical to the EC50 for the increase in intracellular cyclic AMP levels. Preincubation with PTF did not affect either PDGF-receptor autophosphorylation or phosphotidylinositol 3-kinase activity, whereas it markedly reduced PDGF-stimulated extracellular signal-regulated kinase (ERK) activity and ERK isoform phosphorylation. PTF also reduced PDGF-induced c-fos mRNA expression, which is dependent on activation of the RAS/ERK pathway. In addition, the PDGF-induced increase in cytsolic-free calcium was almost completely prevented by pretreating the cells with PTF. 3. The results of the present study indicate that PTF, in addition to its effect on collagen deposition and degradation, may exert an antifibrogenic effect by reducing the PDGF-induced proliferation of extracellular matrix producing cells. This effect appears to be mediated by a reduction of PDGF-stimulated ERK activity as well as of other intracellular signalling pathways such as the PDGF-induced elevation of cytosolic-free calcium.

Involvement of phosphatidylinositol 3-kinase in the activation of extracellular signal-regulated kinase by PDGF in hepatic stellate cells

Phosphatidylinositol 3-kinase (PI 3-K) is a lipid and protein kinase which associates with the activated platelet-derived growth factor (PDGF) receptor and other tyrosine kinases. We studied the effects of wortmannin, a selective inhibitor of PI 3-K, on the activation of extracellular-signal regulated kinase (ERK) by PDGF in cultured hepatic stellate cells, mesenchymal cells responsible for extracellular matrix synthesis within the liver. Incubation with 100 nM wortmannin, a dose which almost completely blocks PI 3-K, resulted in 50% reduction of ERK activity. Direct inhibition of ERK by wortmannin could not be considered responsible for this effect, since wortmannin did not inhibit ERK activity in vitro. Rather, inhibition of PI 3-K acts on the kinase cascade that leads to ERK activation, since PDGF-dependent phosphorylation of ERK was found to be reduced after incubation with wortmannin. Wortmannin also inhibited the increase in c-fos mRNA induced by PDGF, which is dependent on ERK activation. The results of this study show that in hepatic stellate cells PI 3-K is involved in ERK activation, although it is not necessary. These data indicate cross-talk between PI 3-K and the Ras/ERK pathway in PDGF-stimulated cells.

PDGF-mediated activation of phosphatidylinositol 3 kinase in human mesangial cells

Platelet-derived growth factor (PDGF) stimulates mitogenesis and exerts other biologic activities in glomerular mesangial cells. The precise mechanism of PDGF-induced mitogenesis in these cells is not clear. The activation of a signal transducing enzyme, phosphatidylinositol 3 kinase (PI 3 kinase) is associated with mitogenesis. Activation of PI 3 kinase results from stimulation of tyrosine kinase and G-protein-coupled classes of receptors. The synthesis of D3 phosphorylated inositides, the products of this enzymatic reaction, in non-nucleated cells such as blood platelets is dependent upon protein kinase C activation and G-proteins. We studied the activation of PI 3 kinase in response to PDGF in human glomerular mesangial cells. Using a PI 3 kinase 85 kD subunit specific antibody, we detected mesangial cell PI 3 kinase protein as 110 and 85 kD heterodimer. PDGF stimulated PI 3 kinase activity in antiphosphotyrosine immunoprecipitates in a dose-dependent manner showing maximum activation at 12 ng/ml. The antiphosphotyrosine associated PI 3 kinase activity showed biphasic kinetics with a fast peak within two minutes followed by a second peak at 10 minutes. Antiphosphotyrosine and PI 3 kinase immunoprecipitation studies indicated the association of the 85 kD PI 3 kinase subunit with PDGFR. Direct immunoprecipitation with PDGFR beta antibody showed the association of PI 3 kinase activity with the PDGF-receptor. The isoquinoline sulfonyl piperazine compound H7 at concentrations that inhibit PDGF-stimulated PKC activity had no effect on PDGF-stimulated PI 3 kinase activity in antiphospotyrosine immunoprecipitates. These data indicate that PI3 kinase activation is insensitive to PKC. Treatment of mesangial cells with pertussis toxin at concentrations that partially inhibited PDGF-induced DNA synthesis in human mesangial cells did not inhibit PDGF-induced PI 3 kinase activation. These data indicate that PDGF activates PI 3 kinase in mesangial cells and that pertussis toxin-sensitive G-proteins are not involved in PI 3 kinase activation. The data further dissociate activation of PI 3 kinase from mitogenesis in human mesangial cells.

Comparison of calcium-dependent conformational changes in the N-terminal SH2 domains of p85 and GAP defines distinct properties for SH2 domains

Src-homology region 2 (SH2) domains are stretches of about 100 amino acids which are found to be structurally conserved in a number of signaling molecules. These regions have been shown to bind with high affinity to phosphotyrosine residues within activated receptor tyrosine kinases. Here we report the bacterial expression and purification of individual N-terminal SH2 (NSH2) domains of phosphatidylinositol 3-kinase (PI-3K) binding subunit (p85) and Ras GTPase activating protein (GAP) in amounts suitable for structure-function studies. The p85NSH2 domain stains dark purple and absorbs around 620-640 nm with Stains-all, a dye known to bind to calcium binding proteins. This effect was not observed for the GAPNSH2 domain. Circular dichroism analysis of the N-terminal SH2 domain of these proteins shows that p85NSH2, but not GAPNSH2, undergoes a significant dose-dependent change in conformation in the presence of increasing calcium concentrations. Moreover, the conformational change of p85NSH2 induced by calcium could be replicated by addition of a phosphorylated hexapeptide (DYpMDMK) representing the alpha-PDGFR binding site for p85. Limited proteolysis studies showed a significant calcium-dependent increase in protection of p85NSH2 but not GAPNSH2 from degradation by subtilisin. Our results further indicate that holmium, a trivalent lanthanide ion, which has been previously shown to substitute for calcium, could also protect the p85NSH2 domain from proteolysis even at 10-fold lower concentrations. In vitro binding studies using purified preparations of activated alpha-PDGFR show that calcium did not affect the binding of GAPNSH2 domains to activated alpha-PDGFR.(ABSTRACT TRUNCATED AT 250 WORDS)

Haematopoietic colony stimulating factors CSF-1 and GM-CSF increase phosphatidylinositol 3-kinase activity in murine bone marrow-derived macrophages

The activity of phosphatidylinositol (PI) 3-kinase was examined in murine bone marrow-derived macrophages (BMM) stimulated with the haematopoietic growth factors colony stimulating factor-1 (CSF-1) and granulocyte/macrophage-CSF (GM-CSF). PI 3-kinase was immunoprecipitated from cell lysates using anti-phosphotyrosine antibody or an antibody directed against the 85K subunit of PI 3-kinase, and the activity assayed by the phosphorylation of PI in the presence of [gamma 32P]-ATP. The results demonstrate that CSF-1 increases the activity of PI 3-kinase, as compared to the non-stimulated control, in murine macrophages. Maximum activity was seen after 10 min of stimulation with CSF-1 at 3000-5000 U/ml. The dose-response of CSF-1 is consistent with other biochemical effects of CSF-1 seen in the BMM. GM-CSF also stimulated PI 3-kinase activity although to a lesser extent than CSF-1, correlating well with their degree of mitogenic activity on the BMM. Non-mitogenic macrophage activating agents, such as the phorbol myristate acetate, lipopolysaccharide, concanavalin A and formyl-methionyl-leucyl-phenylalanine, did not significantly increase the PI 3-kinase activity. Furthermore, CSF-1 failed to stimulate PI 3-kinase activity in resident peritoneal macrophages, a population of macrophages with poor proliferative capacity. These results suggest that the PI 3-kinase activity may be involved in the haemopoietic growth factor signalling pathways regulating macrophage growth.

Novel inositol containing phospholipids and phosphates: their synthesis and possible new roles in cellular signalling

Details of the widely employed PtdIns(4,5)P2 hydrolysis receptor-stimulated signalling pathway continue to be elucidated rapidly. However, it has recently become apparent that numerous other inositol lipids and phosphates are widespread and are likely to have important cellular functions. In this review, we focus particularly on three rapidly progressing areas: the synthesis and possible functions of 3-phosphorylated inositol lipids, particularly phosphatidylinositol 3,4,5-trisphosphate; the roles of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in coordinating intracellular Ca2+ mobilization and Ca2+ influx in stimulated cells; and the metabolism and possible functions of other inositol polyphosphates and of inositol polyphosphate pyrophosphates.

Decline of signal transduction by phospholipase C gamma 1 in IMR 90 human diploid fibroblasts at high population doubling levels

During cellular senescence in vitro, the cells do not respond mitogenically to serum growth factors at high population doubling levels. Phospholipase C activity in low PDL IMR 90 cells showed a 4.7-fold stimulation in response to 10% serum compared to 3.3-fold in high PDL cells when measured in whole cell extracts. Immunoaffinity purified tyrosine phosphorylated protein fraction showed a greater increase (5.2-fold) in phospholipase C activity in low PDL than high PDL cells (2.1-fold) in response to serum. Serum stimulated PLC gamma 1 activity was diminished in high PDL cells. Immunokinase assay of PLC gamma 1 immunoprecipitates from serum stimulated IMR 90 fibroblasts suggested that diminished enzymatic activity in high PDL cells is not due to less receptor coupled tyrosine phosphorylated PLC gamma 1 enzyme. Serum stimulated [3H]thymidine incorporation into DNA declined in parallel with the activity of PLC gamma 1, suggesting that its activation might play significant roles in this in vitro model for cellular senescence.

Phosphoinositide 3-kinase: a new effector in signal transduction?

Interest in phosphoinositide 3-kinase (PI 3-kinase) has been fuelled by its identification as a major phosphotyrosyl protein detected in cells following growth factor stimulation and oncogenic transformation. It is found complexed with activated growth factor receptors and non-receptor tyrosine kinases, thus suggesting that it participates in the signal transduction pathways initiated by the activation of tyrosine kinases. PI 3-kinase phosphorylates the 3-position in the inositol ring of the well known inositol phospholipids in vitro giving phosphatidylinositol 3-phosphate, phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate [PtdIns3P, PtdIns(3,4)P2 and PtdIns(3,4,5)P3], respectively. The cellular levels of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 rapidly increase in circumstances where PI 3-kinase becomes complexed with tyrosine kinases. Accumulation of the same lipids also occurs in platelets and neutrophils following stimulation of G-protein linked alpha-thrombin and chemotactic peptide receptors, respectively, leading to speculation that one or both of these lipids is a new second messenger whose function is not yet known. This review brings together recent information on the isolation, characterization and regulation of PI 3-kinase, the cellular occurrence of 3-phosphorylated inositol phospholipids and possible functions of the PI 3-kinase pathway in cell signalling.