Functional Characterization of the Human Platelet Glycoprotein V Gene Promoter: A Specific Marker of Late Megakaryocytic Differentiation

Abstract

Glycoprotein V (GPV), a subunit of the platelet GPIb-V-IX receptor for von Willebrand factor and thrombin, is specifically found in platelets and mature megakaryocytes. Studies of the GPV gene can therefore provide insight into the mechanisms governing megakaryocyte differentiation. The human GPV promoter was isolated, and elements important for its tissue specific transcriptional activity were localized using systematic DNase I protection and reporter deletion assays. A −1413/+25 fragment inserted into a luciferase reporter construct displayed promoter activity in Dami and HEL but not in K562, HL60, or HeLa cells. Progressive 5′ to 3′ deletion showed a putative enhancer region in the −1413/−903 segment that contained closely spaced GATA and Ets sites protected from DNase I digestion in Dami extracts. Regions similar to a GPIIb gene repressor were found at −816 and −610, with the first exhibiting repressor activity in Dami and HEL cells and the second protected from DNAse I. Deletions from −362 to −103, an area containing protected sites for Sp1, STAT, and GATA, induced a progressive decrease in activity. The −103/+1 fragment, bearing a proximal Ets footprinted site and a GATA/Ets tandem footprint, displayed 75% activity relative to the full-length promoter and retained cell specificity. In summary, this work defines several regions of the GPV gene promoter important for its activity. It contains megakaryocyte-specific signals, including erythro-megakaryocytic GATA, and Ets cis-acting elements, GPIIb-like repressor domains, and binding sites for ubiquitous factors such as Sp1, ETF, and STAT.