Henrik ten Freyhaus

Phosphatidylinositol 3-Kinase-dependent Membrane Recruitment of Rac-1 and p47phox Is Critical for α-Platelet-derived Growth Factor Receptor-induced Production of Reactive Oxygen Species

Platelet-derived growth factor (PDGF) plays a critical role in the pathogenesis of proliferative diseases. NAD(P)H oxidase (Nox)-derived reactive oxygen species (ROS) are essential for signal transduction by growth factor receptors. Here we investigated the dependence of PDGF-AA-induced ROS production on the cytosolic Nox subunits Rac-1 and p47phox, and we systematically evaluated the signal relay mechanisms by which the αPDGF receptor (αPDGFR) induces ROS liberation. Stimulation of the αPDGFR led to a time-dependent increase of intracellular ROS levels in fibroblasts. Pharmacological inhibitor experiments and enzyme activity assays disclosed Nox as the source of ROS. αPDGFR activation is rapidly followed by the translocation of p47phox and Rac-1 from the cytosol to the cell membrane. Experiments performed in p47phox(-/-) cells and inhibition of Rac-1 or overexpression of dominant-negative Rac revealed that these Nox subunits are required for PDGF-dependent Nox activation and ROS liberation. To evaluate the signaling pathway mediating PDGF-AA-dependent ROS production, we investigated Ph cells expressing mutant αPDGFRs that lack specific binding sites for αPDGFR-associated signaling molecules (Src, phosphatidylinositol 3-kinase (PI3K), phospholipase Cγ, and SHP-2). Lack of PI3K signaling (but not Src, phospholipase Cγ, or SHP-2) completely abolished PDGF-dependent p47phox and Rac-1 translocation, increase of Nox activity, and ROS production. Conversely, a mutant αPDGFR able to activate only PI3K was sufficient to mediate these subcellular events. Furthermore, the catalytic PI3K subunit p110α (but not p110β) was identified as the crucial isoform that elicits αPDGFR-mediated production of ROS. Finally, bromodeoxyuridine incorporation and chemotaxis assays revealed that the lack of ROS liberation blunted PDGF-AA-dependent chemotaxis but not cell cycle progression. We conclude that PI3K/p110α mediates growth factor-dependent ROS production by recruiting p47phox and Rac-1 to the cell membrane, thereby assembling the active Nox complex. ROS are required for PDGF-AA-dependent chemotaxis but not proliferation.

Imatinib mesylate for the treatment of pulmonary arterial hypertension.

Introduction: Despite recent advances, pulmonary arterial hypertension (PAH) remains a devastating disease which harbors a poor prognosis. Novel therapeutic approaches directly targeting pulmonary vascular remodeling are warranted. Areas covered: This review delineates the current limitations in the management of PAH and focuses on a novel, anti-proliferative therapeutic concept. It will help readers understand the mechanisms of receptor tyrosine kinase signaling, with a special focus on platelet-derived growth factor (PDGF) receptors and their role in the pathobiology of PAH. Furthermore, it provides a comprehensive summary regarding the rationale, efficacy and safety of the tyrosine kinase inhibitor imatinib mesylate, which potently inhibits the PDGF receptor, as an additional treatment option in PAH. Expert opinion: PDGF is a potent mitogen for pulmonary vascular smooth muscle cells and represents an important mediator of pulmonary vascular remodeling. Imatinib mesylate, a compound that inhibits the Bcr-Abl kinase and was developed for the treatment of chronic myeloid leukemia, also targets PDGF receptors. Both experimental and clinical data indicate that it reverses the vascular remodeling process even when it is fully established. Results from Phase II and III clinical trials suggest potent and prolonged efficacy in patients with severe PAH (i.e., pulmonary vascular resistance > 800 dynes*s*cm-5). Future studies should evaluate the long-term clinical efficacy and safety of imatinib, including patients with less impaired hemodynamics. Based on the current knowledge, this compound is likely to become an additional treatment option for patients with PAH and has the potential to at least partially correct the pathology of the disease.

Significant improvement of right ventricular function by imatinib mesylate in scleroderma-associated pulmonary arterial hypertension

Sirs: Pulmonary arterial hypertension (PAH) is a devastating disease that harbours a poor prognosis, particularly in patients with scleroderma-associated PAH. Although modern therapies such as endothelin receptor antagonists (ERA), phosphodiesterase type-5 inhibitors (PDE5i) and prostanoids have improved the clinical situation and outcome of affected patients [5], the current medical treatment of PAH is not satisfactory. Recent experimental data suggest that platelet-derived growth factor (PDGF) plays a pivotal role in the pathobiology of PAH by initiating and maintaining the underlying pulmonary vascular remodelling [9]. Consistently, inhibition of PDGF receptor (PDGFR) signalling by the tyrosine kinase inhibitor imatinib mesylate was recently shown to reverse PAH in animal models and to improve the clinical situation in selected patients [3, 7, 9, 10]. However, imatinib was also shown to exert significant cardiotoxicity in animals and humans through its inhibitory effect on the non-receptor Abelson tyrosine kinase (c-Abl) [4]. Further analyses of clinical studies revealed that congestive heart failure is a rare event in patients receiving imatinib therapy, but occurs more frequently in patients with pre-existing cardiac conditions [1]. The latter fact may be of particular significance in patients with PAH and impaired right ventricular function since the right ventricle responds particularly sensitive to hemodynamic and/or cardiotoxic impairment. In addition, patients with scleroderma-associated PAH harbour a lower right ventricular contractility as compared to those with idiopathic PAH [6].

Hypoxia enhances platelet-derived growth factor signaling in the pulmonary vasculature by down-regulation of protein tyrosine phosphatases.

RATIONALE Platelet-derived growth factor (PDGF) plays a pivotal role in the pathobiology of pulmonary hypertension (PH) because it promotes pulmonary vascular remodeling. PH is frequently associated with pulmonary hypoxia. OBJECTIVES To investigate whether hypoxia alters PDGF β receptor (βPDGFR) signaling in the pulmonary vasculature. METHODS The impact of chronic hypoxia on signal transduction by the βPDGFR was measured in human pulmonary arterial smooth muscle cells (hPASMC) in vitro, and in mice with hypoxia-induced PH in vivo. MEASUREMENTS AND MAIN RESULTS Chronic hypoxia significantly enhanced PDGF-BB-dependent proliferation and chemotaxis of hPASMC. Pharmacologic inhibition of PI3 kinase (PI3K) and PLCγ abrogated these events under both normoxia and hypoxia. Although hypoxia did not affect βPDGFR expression, it increased the ligand-induced tyrosine phosphorylation of the receptor, particularly at binding sites for PI3K (Y751) and PLCγ (Y1021). The activated βPDGFR is dephosphorylated by protein tyrosine phosphatases (PTPs). Interestingly, hypoxia decreased expression of numerous PTPs (T cell PTP, density-enhanced phosphatase-1, PTP1B, and SH2 domain-containing phosphatase-2), resulting in reduced PTP activity. Hypoxia-inducible factor (HIF)-1α is involved in this regulation of gene expression, because hypoxia-induced βPDGFR hyperphosphorylation and PTP down-regulation were abolished by HIF-1α siRNA and by the HIF-1α inhibitor 2-methoxyestradiol. βPDGFR hyperphosphorylation and PTP down-regulation were also present in vivo in mice with chronic hypoxia-induced PH. CONCLUSIONS Hypoxia reduces expression and activity of βPDGFR-antagonizing PTPs in a HIF-1α-dependent manner, thereby enhancing receptor activation and proliferation and chemotaxis of hPASMC. Because hyperphosphorylation of the βPDGFR and down-regulation of PTPs occur in vivo, this mechanism likely has significant impact on the development and progression of PH and other hypoxia-associated diseases.

The NAD(P)H Oxidase Inhibitor Apocynin Improves Endothelial NO/Superoxide Balance and Lowers Effectively Blood Pressure in Spontaneously Hypertensive Rats: Comparison to Calcium Channel Blockade

The vascular NAD(P)H oxidase contributes to endothelial dysfunction and high blood pressure in the spontaneously hypertensive rat by enhancing superoxide production. We investigated the effects of apocynin, a NAD(P)H oxidase inhibitor, on blood pressure and vascular radical and nitric oxide formation in SHR and compared its effects to the calcium channel blocker nifedipine. Apocynin (over four weeks) lowered systolic blood pressure significantly and as effectively as nifedipine. Both apocynin and nifedipine significantly reduced superoxide production. In parallel, vascular nitric oxide production and ecNOS activity was significantly increased by apocynin treatment. Therefore, apocynin may be an effective antihypertensive drug in essential hypertension.

Ferric carboxymaltose improves exercise capacity and quality of life in patients with pulmonary arterial hypertension and iron deficiency: A pilot study☆

BACKGROUND Pulmonary arterial hypertension (PAH) is a progressive condition harboring a poor prognosis. Iron deficiency in PAH correlates with disease severity and mortality. While replacement therapy may be beneficial, dietary iron absorption is impaired in PAH patients by hepcidin, a key regulatory protein of iron homoeostasis. We therefore assessed the therapeutic potential and safety of intravenous iron supplementation in patients with PAH and iron deficiency. METHODS 20 patients with PAH and iron deficiency, who were on stable targeted PAH therapy, received a single infusion of ≤1000 mg ferric carboxymaltose. All patients were assessed at baseline and two months after iron treatment. Exercise capacity was evaluated based on the 6-minute-walking distance (6MWD), and quality of life (QoL) was assessed by the SF-36 questionnaire (100 point scale). The effects were compared to 20 matched patients with stable PAH without iron deficiency who did not receive ferric carboxymaltose. RESULTS In iron deficient patients, iron supplementation led to a marked improvement of iron status (serum iron 5.7±0.4 to 11.1±1.1 μmol/L, ferritin 29.3±6.3 to 145.2±25.4 μg/L, transferrin saturation 7.5±0.7 to 19.3±2.3%, all p≤0.001). Iron-deficient patients receiving ferric carboxymaltose showed a significant increase of the 6MWD from 346.5±28.3 to 374.0±25.5 m (p=0.007), whereas no significant changes were found in the control group not receiving iron supplementation (6MWD 389.9±25.3 to 379.6±26.2 m; n.s.), resulting in a net increase in the 6MWD of 37.8m (p=0.003). This was associated with an improvement in QoL (SF-36 score from 44.3±3.7 to 50.6±3.6; p=0.01). Only minimal side-effects were reported. CONCLUSIONS These data indicate that parenteral iron supplementation with ferric carboxymaltose significantly improves exercise capacity and QoL and is well tolerated in patients with PAH and iron deficiency, and when administered in addition to targeted PAH therapies. Our results provide proof of concept for further studies evaluating the potential of iron as an adjunct in PAH treatment on a larger scale.

Disruption of Platelet-Derived Growth Factor–Dependent Phosphatidylinositol 3-Kinase and Phospholipase Cγ 1 Activity Abolishes Vascular Smooth Muscle Cell Proliferation and Migration and Attenuates Neointima Formation In Vivo

OBJECTIVES We tested the hypothesis whether selective blunting of platelet-derived growth factor (PDGF)-dependent vascular smooth muscle cell (VSMC) proliferation and migration is sufficient to prevent neointima formation after vascular injury. BACKGROUND To prevent neointima formation and stent thrombosis after coronary interventions, it is essential to inhibit VSMC proliferation and migration without harming endothelial cell function. The role of PDGF-a potent mitogen and chemoattractant for VSMC that does not affect endothelial cells-for neointima formation remains controversial. METHODS To decipher the signaling pathways that control PDGF beta receptor (βPDGFR)-driven VSMC proliferation and migration, we characterized 2 panels of chimeric CSF1R/βPDGFR mutants in which the binding sites for βPDGFR-associated signaling molecules (Src, phosphatidylinositol 3-kinase [PI3K], GTPase activating protein of ras, SHP-2, phospholipase Cγ 1 [PLCγ]) were individually mutated. Based on in vitro results, the importance of PDGF-initiated signals for neointima formation was investigated in genetically modified mice. RESULTS Our results indicate that the chemotactic response to PDGF requires the activation of Src, PI3K, and PLCγ, whereas PDGF-dependent cell cycle progression is exclusively mediated by PI3K and PLCγ. These 2 signaling molecules contribute to signal relay of the βPDGFR by differentially regulating cyclin D1 and p27(kip1). Blunting of βPDGFR-induced PI3K and PLCγ signaling by a combination mutant (F3) completely abolished the mitogenic and chemotactic response to PDGF. Disruption of PDGF-dependent PI3K and PLCγ signaling in mice expressing the F3 receptor led to a profound reduction of neointima formation after balloon injury. CONCLUSIONS Signaling by the activated βPDGFR, particularly through PI3K and PLCγ, is crucial for neointima formation after vascular injury. Disruption of these specific signaling pathways is sufficient to attenuate pathogenic processes such as vascular remodeling in vivo.

Pulmonary hypertension due to left heart disease: Updated Recommendations of the Cologne Consensus Conference 2011

The 2009 European Guidelines on Diagnosis and Treatment of Pulmonary Hypertension (PH) have been adopted for Germany. While the guidelines contain detailed recommendations regarding pulmonary arterial hypertension (PAH), they contain only a relatively short paragraph on other, much more frequent forms of PH including PH owing to left heart disease. The guidelines point out that the drugs currently used to treat patients with PAH (prostanoids, endothelin receptor antagonists and phosphodiesterase type 5 inhibitors) have not been sufficiently investigated in other forms of PH. However, despite the lack of respective efficacy data an uncritical use of targeted PAH drugs in patients with PH associated with left heart disease is currently observed at an increasing rate. This development is a matter of concern. On the other hand, PH is a frequent problem that is highly relevant for morbidity and mortality in patients with left heart disease. It that sense, the practical implementation of the European Guidelines in Germany requires the consideration of several specific issues and already existing novel data. This requires a detailed commentary to the guidelines, and in some aspects an update already appears necessary. In June 2010, a Consensus Conference organized by the PH working groups of the German Society of Cardiology (DGK), the German Society of Respiratory Medicine (DGP) and the German Society of Pediatric Cardiology (DGPK) was held in Cologne, Germany. This conference aimed to solve practical and controversial issues surrounding the implementation of the European Guidelines in Germany. To this end, a number of working groups was initiated, one of which was specifically dedicated to PH due to left heart disease. This commentary describes in detail the results and recommendations of the working group which were last updated in October 2011.

Genetic Ablation of PDGF-Dependent Signaling Pathways Abolishes Vascular Remodeling and Experimental Pulmonary Hypertension

Objective— Despite modern therapies, pulmonary arterial hypertension (PAH) harbors a high mortality. Vascular remodeling is a hallmark of the disease. Recent clinical studies revealed that antiremodeling approaches with tyrosine–kinase inhibitors such as imatinib are effective, but its applicability is limited by significant side effects. Although imatinib has multiple targets, expression analyses support a role for platelet-derived growth factor (PDGF) in the pathobiology of the disease. However, its precise role and downstream signaling events have not been established. Approach and Results— Patients with PAH exhibit enhanced expression and phosphorylation of &bgr; PDGF receptor (&bgr;PDGFR) in remodeled pulmonary arterioles, particularly at the binding sites for phophatidyl-inositol-3-kinase and PLC&ggr; at tyrosine residues 751 and 1021, respectively. These signaling molecules were identified as critical downstream mediators of &bgr;PDGFR-mediated proliferation and migration of pulmonary arterial smooth muscle cells. We, therefore, investigated mice expressing a mutated &bgr;PDGFR that is unable to recruit phophatidyl-inositol-3-kinase and PLC&ggr; (&bgr;PDGFRF3/F3). PDGF-dependent Erk1/2 and Akt phosphorylation, cyclin D1 induction, and proliferation, migration, and protection against apoptosis were abolished in &bgr;PDGFRF3/F3 pulmonary arterial smooth muscle cells. On exposure to chronic hypoxia, vascular remodeling of pulmonary arteries was blunted in &bgr;PDGFRF3/F3 mice compared with wild-type littermates. These alterations led to protection from hypoxia-induced PAH and right ventricular hypertrophy. Conclusions— By means of a genetic approach, our data provide definite evidence that the activated &bgr;PDGFR is a key contributor to pulmonary vascular remodeling and PAH. Selective disruption of PDGF-dependent phophatidyl-inositol-3-kinase and PLC&ggr; activity is sufficient to abolish these pathogenic responses in vivo, identifying these signaling events as valuable targets for antiremodeling strategies in PAH.

PI3-kinase/Akt-dependent antiapoptotic signaling by the PDGF α receptor is negatively regulated by Src family kinases

Regulation of growth factor dependent cell survival is crucial for development and disease progression. Here, we report a novel function of Src kinases as a negative regulator of platelet‐derived growth factor (PDGF) dependent cell survival. We characterized a series of PDGF α receptor (PDGFRA) mutants, which lack the binding sites for Src, phosphatidylinositol 3′‐kinase (PI3K), SHP‐2 or phospholipase C‐γ. We found that PDGFRA‐dependent cell survival was mainly mediated through activation of PI3K, and was negatively regulated by Src. Characterization of the downstream signaling events revealed that PI3K activates the protein kinase Akt, which in turn phosphorylates and thus inactivates proapoptotic Forkhead transcription factors. Src phosphorylates the ubiquitin‐ligase c‐Cbl, which is required for degradation of the activated receptor. Consequently, overexpression of c‐Cbl prevented PDGFRA‐mediated cell survival, whereas it did not affect this response, when Src was unable to associate with the receptor. This novel function of Src in antiapoptotic signaling introduces Src kinases as an interesting therapeutic target in apoptosis related diseases.