Mildly Oxidized LDL Induces Activation of Platelet-Derived Growth Factor β-Receptor Pathway

Abstract

Background-Mildly oxidized LDL (moxLDL) is thought to play a role in atherogenesis. MoxLDL induces derivatization of cell proteins and triggers a variety of intracellular signaling. We aimed to investigate whether moxLDL-induced protein derivatization may influence the activity of platelet-derived growth factor receptor ␤ (PDGFR␤), a tyrosine kinase receptor of major importance in vascular biology and atherogenesis. Methods and Results-In cultured rabbit arterial smooth muscle cells, moxLDL induces activation of the PDGFR␤ signaling pathway, as shown by PDGFR␤ tyrosine phosphorylation on Western blot and coimmunoprecipitation of SH2-containing proteins. The cellular events involved in the moxLDL-induced PDGFR␤ activation can be summarized as follows. Oxidized lipids from moxLDL trigger two phases of PDGFR␤ activation involving two separate mechanisms, as shown by experiments on cultured cells (in situ) and on immunopurified PDGFR␤ (in vitro): (1) the first phase may be mediated by 4-hydroxynonenal, which induces PDGFR␤ adduct formation and subsequent PDGFR␤ activation (antioxidant-insensitive step); (2) the second phase involves ceramide-mediated generation of H 2 O 2 (these steps being inhibited by tosylphenylalanylchloromethylketone, an inhibitor of ceramide formation, and by antioxidant BHT, exogenous catalase, or overexpressed human catalase). Because 4-hydroxynonenal-PDGFR␤ adducts are also detected in atherosclerotic aortas, it is suggested that this novel mechanism of moxLDL-induced PDGFR␤ activation may occur during atherogenesis. Conclusions-MoxLDL acts as a local autoparacrine mediator in the vascular wall, and PDGFR␤ acts as a sensor for both oxidized lipids and oxidative stress. This constitutes a novel mechanism of PDGFR␤ activation in atherosclerotic areas.

Ligand-independent trans-Activation of the Platelet-derived Growth Factor Receptor by Reactive Oxygen Species Requires Protein Kinase C-δ and c-Src

“…These results are in good agreement with a recent publication (38) showing that H 2 O 2 induced tyrosine phosphorylation of the PDGF␤ receptor as well. Also, our finding is further supported by the fact that PLC␥ is recruited to the PDGF␤ receptor after H 2 O 2 stimulation and because Tyr 1021 phosphorylation is known to provide a high affinity binding site for PLC␥ (19) Trans-activation of growth factor receptors such as EGF or PDGF receptor by stimuli that do not directly interact with the receptor is a current exciting topic of signal transduction research.…”

A Lipid Peroxidation-derived Inflammatory Mediator

“…The previous findings that (i) HNE triggers autophosphorylation of EGFR (47,48), (ii) HNE induces cell growth inhibition and apoptotic cell death by targeting EGFR (49), and (iii) HNE causes reduction of the kinase activity of v-Src (50) suggest that HNE, originating from oxidized LDL or generated inside the cells during oxidative stress, may react directly with tyrosine kinases. Indeed, oxidized LDL (but not native LDL) have been shown to induce the formation of HNE-EGFR and -PDGFR adducts, as evidenced by the presence in immunopurified EGFR and PDGFR of HNE protein epitopes reacting with anti-HNE protein antibodies and by the loss of free NH 2 groups (determined by the radiolabeled specific probe, [ 3 H] succinimidyl propionate) (47,51). This oxidized LDL-induced derivatization of EGFR and PDGFR by HNE is associated with activation of the intrinsic tyrosine kinase and with autophosphorylation of the receptors and the subsequent recruitment of SH2-containing proteins.…”

Self-regulatory role of 4-hydroxynonenal in signaling for stress-induced programmed cell death

“…Classically, the epidermal growth factor receptor (EGFR) is activated by binding to its ligand EGF. This interaction leads to dimerization of the receptor and activation of its intrinsic tyrosine kinase resulting in the transautophosphorylation of tyrosine residues and activation of downstream signaling events [64,[110][111][112]. Reaction of HNE with EGFR leads to its aggregation [111], which affects the EGFRmediated downstream signaling events in a manner somewhat similar to EGF [110][111][112].…”

“…This interaction leads to dimerization of the receptor and activation of its intrinsic tyrosine kinase resulting in the transautophosphorylation of tyrosine residues and activation of downstream signaling events [64,[110][111][112]. Reaction of HNE with EGFR leads to its aggregation [111], which affects the EGFRmediated downstream signaling events in a manner somewhat similar to EGF [110][111][112]. The role of the HNE in transducing the membrane receptor-mediated signaling is further confirmed by our recent studies in which we show that the overexpression of RalBP1 in response to the increased level of HNE leads to increased endocytosis of the EGF receptor.…”