Abstract
The scavenger receptor FAT/CD36 contributes to the inflammation associated with diabetes, atherosclerosis, thrombosis, and Alzheimer disease. Underlying mechanisms include CD36 promotion of oxidative stress and its signaling to stress kinases. Here we document an additional mechanism for the role of CD36 in inflammation. CD36 regulates membrane calcium influx in response to endoplasmic reticulum (ER) stress, release of arachidonic acid (AA) from cellular membranes by cytoplasmic phospholipase A(2)α (cPLA(2)α) and contributes to the generation of proinflammatory eicosanoids. CHO cells stably expressing human CD36 released severalfold more AA and prostaglandin E(2) (PGE(2)), a major product of AA metabolism by cyclooxygenases, in response to thapsigargin-induced ER stress as compared with control cells. Calcium influx after ER calcium release resulted in phosphorylation of cPLA(2) and its translocation to membranes in a CD36-dependent manner. Peritoneal macrophages from CD36(-/-) mice exhibited diminished calcium transients and reduced AA release after thapsigargin or UTP treatment with decreased ERK1/2 and cPLA(2) phosphorylation. However, PGE(2) production was unexpectedly enhanced in CD36(-/-) macrophages, which probably resulted from a large induction of cyclooxygenase 2 mRNA and protein. The data demonstrate participation of CD36 in membrane calcium influx in response to ER stress or purinergic receptor stimulation resulting in AA liberation for PGE(2) formation. Collectively, these results identify a mechanism contributing to the pleiotropic proinflammatory effects of CD36 and suggest that its targeted inhibition may reduce the acute inflammatory response.
Highlights
Stress signaling pathways [1, 2]
Arachidonic Acid Release Is Facilitated by CD36—Chinese hamster ovary (CHO) cells have been used previously to dissect the pathways involved in arachidonic acid (AA) release and eicosanoid formation [51,52,53]
We demonstrate that CD36 participates in the regulation of cellular calcium influx in response to depletion of endoplasmic reticulum (ER) calcium stores triggered either via inhibition of SERCA or by purinergic receptor activation
Summary
Materials—Fura-2/AM and pyrrolidine (PYR) were purchased from Calbiochem, and racemic bromoenol lactone (BEL) was purchased from Cayman Chemical. After washing to remove unincorporated label, the cells were incubated with or without racemic BEL or the cPLA2␣-specific inhibitor PYR and stimulated for 5 min with 5 M thapsigargin or 100 M UTP. For CD36 trafficking after thapsigargin treatment, live cells were stained at 4 °C with CD36 antibody for 1 h, washed twice with cold PBS, and stained at 4 °C with secondary antibody Alexa 594 (Invitrogen) for 40 min. A discontinuous gradient of 33, 20, and 5% sucrose was overlaid above cell extracts in 40% sucrose This was followed by centrifugation (5 h, 47,000 rpm, ϳ146,698 ϫ g, 4 °C) in a TLS-55 rotor (Beckman Coulter, Brea, CA). Cells were extracted using lysis buffer, and labeled proteins were pulled down using a Dynabeads M-280 streptavidin kit (Invitrogen). Statistical Analysis—Statistical analyses were performed using the GraphPad Prism 4 t test (two-tailed), and differences were considered significant at p Յ 0.05
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