Sp1 and KLF15 regulate basal transcription of the human LRP5 gene

In order to characterize the transcriptional regulation of human LRP5 gene, we cloned the 5' flanking region and evaluated its transcriptional activity in a luciferase reporter system. We demonstrated that both KLF15 and Sp1 binding sites between -72 bp and -53 bp contribute to the transcriptional activation of human LRP5 promoter. Chromatin immunoprecipitation assay demonstrated that the ubiquitous transcription factors KLF15 and Sp1 bind to this region. Using Drosophila SL2 cells, we showed that KLF15 and Sp1 trans-activated the LRP5 promoter in a manner dependent on the presence of Sp1-binding and KLF15-binding motifs.Both KLF15 and Sp1 binding sites contribute to the basal activity of human LRP5 promoter. This study provides the first insight into the mechanisms by which transcription of human LRP5 gene is regulated.The LRP5 gene, located on human chromosome 11q13, contains 23 exons encoding a 1615 amino acid single-pass transmembrane receptor that belongs to the low density lipoprotein (LDL) receptor superfamily. LRP5 is highly conserved between species. There is 95% identity between human and mouse LRP5 protein[1], and 40% identity with the Drosophila orthologous gene, Arrow [2]. Additionally, LRP5 bears 71% amino acid identity with LRP6[3]. LRP5 contains an extracellular domain, a membrane-spanning domain, and an intracellular domain. The extracellular domain, located at the N-terminus, consists of four EGF-receptor-like cysteine-rich repeats with associated YWTD spacer domains and three LDL receptor-like ligand binding domains[1]. LRP5 is expressed in multiple adult and embryonic tissues, including bone, macrophages, fat, brain, heart, liver, skin and pancreas, with strongest expression occurring in the liver[1]. In bone, it is expressed by osteoblasts of the endosteal and trabecular bone surfaces but not by osteoclasts[4,5].Shortly after the discovery of LRP5, multiple lines of evidence indicate that the LRP5 plays an important role in bone formation throug