Role of ATF7‐TAF12 interactions in the vitamin D response hypersensitivity of osteoclast precursors in Paget's disease

ABSTRACT Osteoclast (OCL) precursors from many Paget's disease (PD) patients express measles virus nucleocapsid protein (MVNP) and are hypersensitive to 1,25‐dihydroxyvitamin D3 (1,25‐(OH)2D3; also know as calcitriol). The increased 1,25‐(OH)2D3 sensitivity is mediated by transcription initiation factor TFIID subunit 12 (TAF12), a coactivator of the vitamin D receptor (VDR), which is present at much higher levels in MVNP‐expressing OCL precursors than normals. These results suggest that TAF12 plays an important role in the abnormal OCL activity in PD. However, the molecular mechanisms underlying both 1,25‐(OH)2D3's effects on OCL formation and the contribution of TAF12 to these effects in both normals and PD patients are unclear. Inhibition of TAF12 with a specific TAF12 antisense construct decreased OCL formation and OCL precursors' sensitivity to 1,25‐(OH)2D3 in PD patient bone marrow samples. Further, OCL precursors from transgenic mice in which TAF12 expression was targeted to the OCL lineage (tartrate‐resistant acid phosphatase [TRAP]‐TAF12 mice), formed OCLs at very low levels of 1,25‐(OH)2D3, although the OCLs failed to exhibit other hallmarks of PD OCLs, including receptor activator of NF‐κB ligand (RANKL) hypersensitivity and hypermultinucleation. Chromatin immunoprecipitation (ChIP) analysis of OCL precursors using an anti‐TAF12 antibody demonstrated that TAF12 binds the 24‐hydroxylase (CYP24A1) promoter, which contains two functional vitamin D response elements (VDREs), in the presence of 1,25‐(OH)2D3. Because TAF12 directly interacts with the cyclic adenosine monophosphate–dependent activating transcription factor 7 (ATF7) and potentiates ATF7‐induced transcriptional activation of ATF7‐driven genes in other cell types, we determined whether TAF12 is a functional partner of ATF7 in OCL precursors. Immunoprecipitation of lysates from either wild‐type (WT) or MVNP‐expressing OCL with an anti‐TAF12 antibody, followed by blotting with an anti‐ATF7 antibody, or vice versa, showed that TAF12 and ATF7 physically interact in OCLs. Knockdown of ATF7 in MVNP‐expressing cells decreased cytochrome P450, family 24, subfamily A, polypeptide 1 (CYP24A1) induction by 1,25‐(OH)2D3, as well as TAF12 binding to the CYP24A1 promoter. These results show that ATF7 interacts with TAF12 and contributes to the hypersensitivity of OCL precursors to 1,25‐(OH)2D3 in PD.