Human platelets express functional retinoid X receptors. (A) Western blot analysis showing positive controls for RXR isoforms (\#945;, \#946;, and \#947;) in HUVECs, and the expression of RXR\#945; and RXR\#946; in human platelet-rich plasma (PRP), washed platelets (WPs), megakaryoblast cell line Meg-01 (Meg), and the cytosol (Cyt) and membrane (Memb) fractions of PRP. In addition, a positive control peptide for RXR\#945; (RXR\#945;⁺) was also included. (B) Typical aggregometer traces showing the changes in light transmittance seen as platelets aggregate (in sequence from left to right) to ADP (4 \#956;M; left panel), 9cRA (10\#956;M) given 3 minutes prior to ADP; to collagen (1 \#956;M), 9cRA (10 \#956;M) given 3 minutes prior to collagen; and to the TXA₂ mimetic U46619 (1 \#956;M) and 9cRA (10\#956;M) given 3 minutes prior to U46619. (C) RXR ligands 9cRA (\#9632;), and methoprene acid (MA; \#9679;), but not the retinoic acid receptor ligand all-trans retinoic acid (ATRA; \#9633;) inhibit ADP-induced platelet aggregation. Figure represents the mean ± SEM changes in percent of ADP aggregation. (D) 9cRA (1-20 \#956;M; 3-minute pretreatment) inhibits ADP-induced (2 or 4 \#956;M; \#9632;) and U46619-induced (1 \#956;M; \#9830;), but not collagen-induced (1 \#956;M; \#9675;) platelet aggregation in a concentration-dependent manner. Figure represents the mean ± SEM changes in agonist-induced aggregation (titrated to approximately 85% of maximum). (E) 9cRA (1-20 \#956;M; 3-minute pretreatment) more potently inhibits ADP-induced (2 or 4 \#956;M; \#9632;) and collagen-induced (1 \#956;M; \#9675;) platelet TXA₂ secretion. Released TXA₂ was measured in PRP at the end of aggregation assays (15 minutes) by using an assay for its stable metabolite, TXB₂. Figure represents the mean ± SEM TXA₂ release. Data represents result from at least 4 separate donors.