Antagonistic Effects of Oxidized Low Density Lipoprotein and α-Tocopherol on CD36 Scavenger Receptor Expression in Monocytes

Abstract

Vitamin E deficiency increases expression of the CD36 scavenger receptor, suggesting specific molecular mechanisms and signaling pathways modulated by ␣-tocopherol. We show here that ␣-tocopherol down-regulated CD36 expression (mRNA and protein) in oxidized low density lipoprotein (oxLDL)-stimulated THP-1 monocytes, but not in unstimulated cells. Furthermore, ␣-tocopherol treatment of monocytes led to reduction of fluorescent oxLDL-3,3-dioctadecyloxacarbocyanine perchlorate binding and uptake. Protein kinase C (PKC) appears not to be involved because neither activation of PKC by phorbol 12-myristate 13-acetate nor inhibition by PKC412 was affected by ␣-tocopherol. However, ␣-tocopherol could partially prevent CD36 induction after stimulation with a specific agonist of peroxisome proliferator-activated receptor-␥ (PPAR␥; troglitazone), indicating that this pathway is susceptible to ␣-tocopherol action. Phosphorylation of protein kinase B (PKB) at Ser 473 was increased by oxLDL, and ␣-tocopherol could prevent this event. Expression of PKB stimulated the CD36 promoter as well as a PPAR␥ element-driven reporter gene, whereas an inactive PKB mutant had no effect. Moreover, coexpression of PPAR␥ and PKB led to additive induction of CD36 expression. Altogether, our results support the existence of PKB/PPAR␥ signaling pathways that mediate CD36 expression in response to oxLDL. The activation of CD36 expression by PKB suggests that both lipid biosynthesis and fatty acid uptake are stimulated by PKB.In many cell types, oxidized low density lipoproteins (oxLDL) 2 modulate cellular processes such as apoptosis, adhesion, migration, gene expression, and the induction of signal transduction cascades (1). Exposure of monocytes to oxLDL may alter gene expression and signaling, making them more susceptible to the following pro-atherogenic stimuli. The migration of monocytes into the intima and the conversion of monocytes/macrophages into foam cells represent initial steps in atherosclerosis. Current strategies to prevent atherosclerosis are aimed either at lowering the cholesterol load of lipoproteins or at reducing oxidative stress.Vitamin E is a redox-active natural compound that can act, depending on the conditions, as a pro-or antioxidant on low density lipoproteins (LDL) in vitro and in vivo (2-5). The major form of vitamin E in human plasma is ␣-tocopherol, and reduced plasma levels of ␣-tocopherol, such as in vitamin E-deficient mice, increase the incidence of atherosclerosis (6). Animal and cell culture studies strongly suggest that vitamin E can prevent atherosclerosis; however, the anti-atherogenic effects in clinical trials are still controversial (7-10). ␣-Tocopherol in lipoproteins (mainly LDL) and also in the subendothelial space has been assumed to play a central role in reducing atherosclerosis by preventing lipid peroxidation and consequent lesion development. Nevertheless, since many compounds exist that can interfere with the oxidation of LDL without being equally effective, alternative modes of action have bee...

Modulation of Proteasome Activity by Vitamin E in THP‐1 Monocytes

“…Several studies have shown the central role of CD36 in the development of atherosclerotic plaques by increasing the cellular uptake of modified LDL (9,10,19,21). CD36 expression is often increased at the atherosclerotic lesion by triggers such as oxLDL via PKB and PPARg activation (21). Interestingly, oxLDL has also been shown to inhibit the proteasome (63) and increased levels of ubiquitinated proteins and decreased proteasome activity were detected in unstable atherosclerotic plaques (12,64).…”

“…Extracts were prepared as previously described (21,22). Briefly, THP-1 cells were harvested by centrifugation, washed two times with PBS, and taken up in 100 ml sterile distilled water containing 1 mM DTT.…”

12-O-tetradecanoylphorbol-1, 3-acetate induces the negative regulation of protein kinase B by protein kinase Cα during gastric cancer cell apoptosis

“…One possibility is that the binding of PKC to PKB may protect Thr308 but not Ser473 from being phosphorylated by PDK1. Accumulated evidence has shown that the regulation of PKB phosphorylation by PKCs, whether negatively or positively, mainly occurs at Ser473 [11,22,[36][37][38]. A putative PDK2 is believed to phosphorylate PKB at Ser473 [39].…”

Rôles de PPAR et de p21WAF1/CIP1dans la différenciation monocyte/macrophage

“…Plus récemment, Chawla et al [37] ont démontré que les agonistes de PPAR ont des effets antiinflammatoires indépendants de ce facteur de transcription. Même si le rôle précis de PPAR dans l'inflammation reste encore à élucider, il est indubitable que ce facteur de transcription a une place centrale dans le métabolisme lipidique et l'activation du macrophage via la régulation du récepteur scavenger CD36, la capture des LDL (low density lipoproteins) oxydées et sa capacité intrinsèque de se lier à des acides gras poly-insaturés ou oxydés [38]. Il est particulièrement intéressant de noter ici que le promoteur du gène p21 WAF1/CIP1 possède justement des sites putatifs de réponse aux PPAR et il a d'ailleurs déjà été clairement montré que le promoteur de p21 WAF1/CIP1 est sensible à l'acide rétinoïque (RXR-RAR) ainsi qu'à la vitamine D3 [6,7] (Figure 3).…”