PTHrP(1‐34)‐mediated repression of the PHEX gene in osteoblastic cells involves the transcriptional repressor E4BP4

Abstract

PHosphate-regulating gene with homology to Endopeptidase on the X chromosome (PHEX) has been identified as the gene mutated in X-linked hypophosphatemia (XLH) syndrome, the most prevalent form of rickets in humans. The predominant expression of PHEX in bones and teeth, and the defective mineralization of these tissues in XLH patients indicate that PHEX is an important regulator of mineralization. Parathyroid hormone (PTH) and PTH-related protein (PTHrP) are known to regulate the expression of numerous genes in osteoblastic cells through activation of the protein kinase A pathway, including repression of PHEX. PTH also activates the transcriptional repressor E4BP4 through the same pathway, suggesting that PTH or PTHrP-mediated repression of PHEX expression could involve E4BP4. To evaluate this possibility, we treated UMR-106 osteoblastic cells with PTHrP(1-34), and used RT-PCR and immunoblotting to analyze PHEX and E4BP4 expression. E4BP4 mRNA and protein levels were rapidly increased in cells treated with PTHrP(1-34), with a concomitant decrease in PHEX expression. This downregulation of PHEX could be reproduced by overexpression of E4BP4. Moreover, PTHrP(1-34)-mediated PHEX repression was blocked when cells were transfected with a siRNA targeting E4BP4 mRNA. Finally, DNA pull-down and luciferase assays showed that two E4BP4 response elements located in PHEX promoter were functional. These results underline the important role of E4BP4 in osteoblastic cells and further define the repression mechanism of PHEX gene by PTHrP(1-34).

New Frontiers for the NFIL3 bZIP Transcription Factor in Cancer, Metabolism and Beyond

“…In hepatocytes Nfil3 represses Fgf21 (Fibroblast Growth Factor 21) expression in a histone methyltransferase G9a-dependent manner 31 . Nfil3 represses many additional genes including Period 2 (Per2), Arntl (Aryl Hydrocarbon Receptor Nuclear Translocator-Like), Usp2-45 (Ubiquitin-specific protease 2-45), BGL-GS (also known as BCL2- like 14 ), Cox2 (Cyclooxygenase 2), Runx2 (Runt-related transcription factor 2), Phex (Phosphate Regulating Endopeptidase Homolog), Ptgs2 (Prostaglandin-endoperoxide synthase 2 , also known as Cox2 ), Pgr (Progesterone receptor) , and Areg (amphiregulin) , which are regulated in diverse tissues ranging from pancreatic beta cells to osteoblasts 21,32-36 .…”

“…Cyclooxygenase 2 (Cox2) is thought be one of the Nfil3 repressed genes downstream of PTH in mouse osteoblasts 21 . Additional Nfil3 targets in murine osteoblasts include Runx2, Osterix, and Phex 21,35,36 . These studies provide important clues about Nfil3 signal transduction in osteoblasts and place its action downstream of PKA.…”

PTH and the Regulation of Mesenchymal Cells within the Bone Marrow Niche

“…E4-binding protein 4 (E4BP4) has been pinpointed as a potential mediator that plays a part in the catabolic activities of PTH and PTHrP on bone. It was one of the cAMP-PKA pathway targets in response to PTH, and it exerts its effects by downregulating the expression of pivotal genes such as phosphate regulating endopeptidase X-Linked (PHEX), RUNX2, and OSX—each of which is integral to the differentiation and functional activity of osteoblasts [ 221 ].…”