α‐Tocopheryl Phosphate Induces VEGF Expression via CD36/PI3Kγ in THP‐1 Monocytes

Abstract

The CD36 scavenger receptor binds several ligands and mediates ligand uptake and ligand-dependent signal transduction and gene expression, events that may involve CD36 internalization. Here we show that CD36 internalization in THP-1 monocytes is triggered by α-tocopherol (αT) and more strongly by α-tocopheryl phosphate (αTP) and EPC-K1, a phosphate diester of αTP and L-ascorbic acid. αTP-triggered CD36 internalization is prevented by the specific covalent inhibitor of selective lipid transport by CD36, sulfo-N-succinimidyl oleate (SSO). Moreover, SSO inhibited the CD36-mediated uptake of 14C-labelled αTP suggesting that αTP binding and internalization of CD36 is involved in cellular αTP uptake, whereas the uptake of αT was less affected. Similar to that, inhibition of selective lipid transport of the SR-BI scavenger receptor resulted mainly in reduction of αTP and not αT uptake. In contrast, uptake of αT was mainly inhibited by Dynasore, an inhibitor of clathrin-mediated endocytosis, suggesting that the differential regulatory effects of αTP and αT on signaling may be influenced by their different routes of uptake. Interestingly, αTP and EPC-K1 also reduced the neutral lipid content of THP-1 cells and the phagocytosis of fluorescent Staphylococcus aureus bioparticles. Moreover, induction of the vascular endothelial growth factor (VEGF) promoter activity by αTP occurred via CD36/PI3Kγ/Akt, as it could be inhibited by specific inhibitors of this pathway (SSO, Wortmannin, AS-605240). These results suggest that αTP activates PI3Kγ/Akt signaling leading to VEGF expression in monocytes after binding to and/or transport by CD36, a receptor known to modulate angiogenesis in response to amyloid beta, oxLDL, and thrombospondin. J. Cell. Biochem. 118: 1855-1867, 2017. © 2017 Wiley Periodicals, Inc.

Vitamin E: Regulatory Role on Signal Transduction

“…In stimulated human neutrophils, PKC and tyrosine phosphorylation were decreased by αTS because of activation of a tyrosine phosphatase (288). Regulatory effects on both cAMP levels and tyrosine phosphorylation were also observed after treatment of monocytes with αTP, but any involvement of CD36-mediated signaling remains to be further investigated (42,152).…”

“…Whereas translocation of αTTP to the plasma membrane and vitamin E secretion appears to be specific for αT and to involve binding to phosphatidylinositol 4,5‐bisphosphate [PI(4,5)P 2 ] and phosphatidylinositol 3,4‐bisphosphate [PI(3,4)P 2 ] , the intracellular transport of the eight vitamin E analogues and their distribution to organelles appears not to involve αTTP, and either occurs passively in association with membranes and vesicles or involve the above‐mentioned LTP that are also involved in signaling . In addition to the LTP relevant for αT transport, the cellular levels and activity of αTP are influenced by the organic anion transporters , the scavenger receptors (CD36, SR‐BI) , and intracellular phosphatases/esterases . Although there is still limited information available on the mechanisms by which vitamin E modulates specifically its own cellular levels in plasma and tissue, as described in the following some of the proteins involved in the uptake, transport, distribution, and metabolism respond to the levels of vitamin E to maintain the vitamin E levels within a physiological range (Fig.…”

“…Whereas translocation of αTTP to the plasma membrane and vitamin E secretion appears to be specific for αT and to involve binding to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P 2 ] and phosphatidylinositol 3,4-bisphosphate [PI(3,4)P 2 ] (37, 46), the intracellular transport of the eight vitamin E analogues and their distribution to organelles appears not to involve αTTP, and either occurs passively in association with membranes and vesicles or involve the above-mentioned LTP that are also involved in signaling (40,47,48). In addition to the LTP relevant for αT transport, the cellular levels and activity of αTP are influenced by the organic anion transporters (49), the scavenger receptors (CD36, SR-BI) (42), and intracellular phosphatases/esterases (50)(51)(52)(53)(54).…”

“…Interestingly, the reduction of CD36 expression and cell surface exposition by αT and more so by αTP in monocytes/macrophages and in T/G VSMC (that were close to their replicative limit) may play a role in the modulation of cellular senescence ( Fig. 4) (42,79,80,117). As first observed using microarrays, CD36 mRNA expression is induced in senescent cells as member of a number of lipid-regulating genes possibly leading to accumulation of ceramides and triacylglycerols with long and very long unsaturated fatty acyl side chains (259,260).…”

“…Whereas at high concentrations vitamin E can be absorbed by passive diffusion and bulk endocytosis, a certain degree of competition between hydrophobic vitamins observed in intestinal epithelial cells suggests common and overlapping mechanisms of uptake involving specific transporters as it occurs also with fatty acids . Vitamin E is distributed throughout the body located mostly within lipoproteins (VLDL, LDL, HDL, and chylomicrons) and albumin/afamin from where it is delivered to cells in tissues after hydrolysis of triacylglycerides (TG) by lipoprotein lipase, and selective transport by specific transporters at the membrane such as the LDL receptor (LDLr), the scavenger receptors/fatty acids transporters (CD36, SR‐BI), or the lipid transport proteins (LTP) such as the phospholipid transfer proteins (PLTP), the Nieman‐Pick‐C1/like 1 (NPC1L1), and the three tocopherol associated proteins (TAP1, TAP2, TAP3 or SEC14L2, SEC14L3, SEC14L4, respectively) . Most of these transport proteins and receptors do not distinguish between the eight major natural analogues of vitamin E (α‐, β‐, γ‐, δ‐ tocopherols, and tocotrienols); the selectivity for mainly the alpha form of vitamin E, RRR ‐alpha‐tocopherol (αT), occurs in subsequent steps by means of selective retention, protection and incorporation into VLDL by the liver α‐tocopherol transfer protein (αTTP), secretion by ATP‐binding cassette transporters A1 and G1 (ABCA1, ABCG1), and by preventing its metabolism that occurs with the other analogues of vitamin E (βT, γT, and δT), of excess αT and of all four tocotrienols (αTT, βTT, γTT, and δTT) by cytochrome P450 enzymes (CYP3A, CYP4F2) and their subsequent elimination in urine as water‐soluble vitamin E metabolites .…”