15-Deoxy Δ^12,14-prostaglandin J₂ suppresses transcription by promoter 3 of the human thromboxane A₂ receptor gene through peroxisome proliferator-activated receptor γ in human erythroleukemia cells

In humans, thromboxane (TX) A₂ signals through two receptor isoforms, thromboxane receptor (TP)α and TPβ, which are transcriptionally regulated by distinct promoters, Prm1 and Prm3, respectively, within the single TP gene. The aim of the current study was to investigate the ability of the endogenous peroxisome proliferator-activated receptor (PPAR)γ ligand 15-deoxy-Δ^12,14-prostaglandin J ₂ (15d-PGJ₂) to regulate expression of the human TP gene and to ascertain its potential effects on the individual TPα and TPβ isoforms. 15d-PGJ₂ suppressed Prm3 transcriptional activity and TPβ mRNA expression in the platelet progenitor megakaryocytic human erythroleukemia (HEL) 92.1.7 cell line but had no effect on Prm1 or Prm2 activity or on TPα mRNA expression. 15d-PGJ₂ also resulted in reductions in the overall level of TP protein expression and TP-mediated intracellular calcium mobilization in HEL cells. 15d-PGJ₂ suppression of Prm3 transcriptional activity and TPβ mRNA expression was found to occur through a novel mechanism involving direct binding of PPARγ-retinoic acid X receptor (RXR) heterodimers to a PPARγ response element (PPRE) composed of two imperfect hexameric direct repeat (DR) sequences centred at -159 and -148, respectively, spaced by five nucleotides (DRS). These data provide direct evidence for the role of PPARγ in the regulation of human TP gene expression within the vasculature and point to further critical differences in the modes of transcriptional regulation of TPα and TPβ in humans. Moreover, these data highlight a further link between enhanced risk of cardiovascular disease in diabetes mellitus associated with increased synthesis and action of thromboxane A₂ (TXA₂).