prostaglandin-d2 and Leukemia--Erythroblastic--Acute

The human thromboxane (TX)A2 receptor (TP) gene encodes two TP isoforms, TPalpha and TPbeta, that are regulated by distinct promoters designated promoter Prm1 and Prm3, respectively. Previous studies established that 15d-Delta12,14-prostaglandin J2 (15d-PGJ2) selectively inhibits Prm3 activity and TPbeta expression through a peroxisome proliferator-activated receptor (PPAR)gamma mechanism without affecting Prm1 activity or TPalpha expression in human megakaryocytic erythroleukemia (HEL) 92.1.7 cells. Herein, we investigated the effect of synthetic thiazolidinedione (TZD) PPARgamma ligands rosiglitazone and troglitazone on TP gene expression in HEL cells. Like 15d-PGJ2, both TZDs suppressed Prm3 activity, TPbeta mRNA expression and TP-mediated calcium mobilization without affecting Prm1 or TPalpha mRNA expression. However, unlike 15d-PGJ2, both TZDs mediated their PPARgamma-dependent effects through trans-repression of an activator protein-1 (AP-1) element, a site previously found to be critical for basal Prm3 activity. These data provide further evidence for the role of PPARgamma in regulating the human TP gene; they highlight further differences in TPalpha and TPbeta expression/regulation and point to essential differences between natural and synthetic PPARgamma agonists in mediating those effects.

Topics: Cell Line; Cell Line, Tumor; Chromans; Genes, Reporter; Humans; Leukemia, Erythroblastic, Acute; Luciferases; PPAR gamma; Promoter Regions, Genetic; Prostaglandin D2; Protein Isoforms; Receptors, Thromboxane A2, Prostaglandin H2; Retinoid X Receptor alpha; RNA, Messenger; Rosiglitazone; Thiazolidinediones; Transcription Factor AP-1; Transcription, Genetic; Troglitazone

In humans, thromboxane (TX) A2 signals through two receptor isoforms, thromboxane receptor (TP)alpha and TPbeta, 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)gamma ligand 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) to regulate expression of the human TP gene and to ascertain its potential effects on the individual TPalpha and TPbeta isoforms. 15d-PGJ2 suppressed Prm3 transcriptional activity and TPbeta 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 TPalpha 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 TPbeta mRNA expression was found to occur through a novel mechanism involving direct binding of PPARgamma-retinoic acid X receptor (RXR) heterodimers to a PPARgamma response element (PPRE) composed of two imperfect hexameric direct repeat (DR) sequences centred at -159 and -148, respectively, spaced by five nucleotides (DR5). These data provide direct evidence for the role of PPARgamma in the regulation of human TP gene expression within the vasculature and point to further critical differences in the modes of transcriptional regulation of TPalpha and TPbeta 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).

Topics: Binding Sites; Cell Line, Tumor; Gene Expression Regulation; Humans; Leukemia, Erythroblastic, Acute; Ligands; PPAR gamma; Promoter Regions, Genetic; Prostaglandin D2; Protein Isoforms; Receptors, Thromboxane A2, Prostaglandin H2; Response Elements; RNA, Messenger; Transcription, Genetic