TR4 nuclear receptor functions as a fatty acid sensor to modulate CD36 expression and foam cell formation

Abstract

Testicular orphan nuclear receptor 4 (TR4) is an orphan member of the nuclear receptor superfamily with diverse physiological functions. Using TR4 knockout (TR4 ؊/؊ ) mice to study its function in cardiovascular diseases, we found reduced cluster of differentiation (CD)36 expression with reduced foam cell formation in TR4 ؊/؊ mice. Mechanistic dissection suggests that TR4 induces CD36 protein and mRNA expression via a transcriptional regulation. Interestingly, we found this TR4-mediated CD36 transactivation can be further enhanced by polyunsaturated fatty acids (PUFAs), such as omega-3 and -6 fatty acids, and their metabolites such as 15-hydroxyeico-satetraonic acid (15-HETE) and 13-hydroxy octa-deca dieonic acid (13-HODE) and thiazolidinedione (TZD)-rosiglitazone. Both electrophoretic mobility shift assays (EMSA) and chromatin immunoprecipitation (ChIP) assays demonstrate that TR4 binds to the TR4 response element located on the CD36 5 -promoter region for the induction of CD36 expression. Stably transfected TR4-siRNA or functional TR4 cDNA in the RAW264.7 macrophage cells resulted in either decreased or increased CD36 expression with decreased or increased foam cell formation. Restoring functional CD36 cDNA in the TR4 knockdown macrophage cells reversed the decreased foam cell formation. Together, these results reveal an important signaling pathway controlling CD36-mediated foam cell formation/cardiovascular diseases, and findings that TR4 transactivation can be activated via its ligands/activators, such as PUFA metabolites and TZD, may provide a platform to screen new drug(s) to battle the metabolism syndrome, diabetes, and cardiovascular diseases.A therosclerosis is a disease of both lipid disorder and chronic inflammation that results from interactions of modified lipoproteins, monocyte-derived macrophages, T cells, and cells from the vessel wall (1). Foam cell formation has a central role in the pathogenesis of atherosclerosis. Uptake of oxidized forms of low-density lipoproteins (oxLDL) induces macrophage maturation to form foam cells, which lead to the fatty streaks characteristic of early atherosclerosis. This uptake is mediated by specific macrophage scavenger receptors, including cluster of differentiation (CD)36 (2) and scavenger receptor A (SRA) (3), which recognize and internalize modified lipoproteins such as oxLDL. Among these 2 SRAs, CD36 accounts for a large proportion of oxLDL uptake by macrophages (4-7). Combined inhibition of SRA and CD36 blocks human and mouse foam cell formation in vitro, yet human subjects carrying CD36 mutation usually have a higher risk for atherosclerosis. CD36 roles in oxLDL-mediated foam cell formation in vitro are clear, but its roles in atherosclerosis in vivo are equivocal, which can be either atherogenic (6-8) or atheroprotective (9-11).Peroxisome proliferator-activated receptor (PPAR)␥ is one of the major CD36 upstream regulators in which PPAR␥/retinoid X receptor (RXR) can bind to DNA responsive elements on its 5Ј-promoter to modulate CD36 gene expression (12)....

Deficiency in TR4 nuclear receptor abrogates Gadd45a expression and increases cytotoxicity induced by ionizing radiation

“…The pSuperior.retro.puro. plasmids containing scramble small hairpin RNA (shRNA) and shRNA targeting TR4 were described in a previous publication [2]. …”

“…and Tsai et al . found that the polyunsaturated fatty acid metabolites 15-HETE, 13-HODE, and γ-linoleic acid are able to activate TR4 [2, 3]. This suggests that TR4 may be able to function as a fatty acid sensor upon activation via these ligands/activators.…”

Differential roles of PPARγ vs TR4 in prostate cancer and metabolic diseases

“…More importantly, from those extensive in vivo function studies, Xie et al (2009) discovered that the PUFA metabolites, 15-hydroxyeicosatetraenoic acid (15-HETE) and 13-hydroxyoctadecadienoic acid (13-HODE) as well as rosiglitazone, could function as ligands/activators to transactivate TR4 to a degree similar to PPARg. These unexpected results link these two different NRs and raise some very interesting questions.…”

Conjugated Linoleic Acid Negatively Regulates TR4 Activity in 3T3-L1 Adipocytes

“…Several reports have shown that trans-10, cis-12 CLA competitively inhibits effect of PPARγ agonist on PPARγ activity while it rather plays a role as a partial agonist of PPARγ in the absence of PPARγ agonist (Alibin et (Tsai et al, 2009). Moreover, Thiazolidinedione (TZD), a well-known PPARγ agonist, also promotes TR4 activity in the reporter gene assay (Xie et al, 2009). These previous reports strongly suggest that TR4 may function as a fatty acid sensor to modulate lipid homeostasis of our body (Xie et al, 2009).…”

“…Moreover, Thiazolidinedione (TZD), a well-known PPARγ agonist, also promotes TR4 activity in the reporter gene assay (Xie et al, 2009). These previous reports strongly suggest that TR4 may function as a fatty acid sensor to modulate lipid homeostasis of our body (Xie et al, 2009). Thus, we hypothesized that CLA, a dietary component of milk products and ruminant meats, may participate in lipid homeostasis in adipocytes in part by modulation of TR4 activity.…”