Pbx1 Represses Osteoblastogenesis by Blocking Hoxa10-Mediated Recruitment of Chromatin Remodeling Factors

Abstract

Abdominal-class homeodomain-containing (Hox) factors form multimeric complexes with TALE-class homeodomain proteins (Pbx, Meis) to regulate tissue morphogenesis and skeletal development. Here we have established that Pbx1 negatively regulates Hoxa10-mediated gene transcription in mesenchymal cells and identified components of a Pbx1 complex associated with genes in osteoblasts. Expression of Pbx1 impaired osteogenic commitment of C3H10T1/2 multipotent cells and differentiation of MC3T3-E1 preosteoblasts. Conversely, targeted depletion of Pbx1 by short hairpin RNA (shRNA) increased expression of osteoblastrelated genes. Studies using wild-type and mutated osteocalcin and Bsp promoters revealed that Pbx1 acts through a Pbx-binding site that is required to attenuate gene activation by Hoxa10. Chromatin-associated Pbx1 and Hoxa10 were present at osteoblast-related gene promoters preceding gene expression, but only Hoxa10 was associated with these promoters during transcription. Our results show that Pbx1 is associated with histone deacetylases normally linked with chromatin inactivation. Loss of Pbx1 from osteoblast promoters in differentiated osteoblasts was associated with increased histone acetylation and CBP/p300 recruitment, as well as decreased H3K9 methylation. We propose that Pbx1 plays a central role in attenuating the ability of Hoxa10 to activate osteoblast-related genes in order to establish temporal regulation of gene expression during osteogenesis.

Pbx1 dependent control of VMC differentiation kinetics underlies gross renal vascular patterning

“…Several transcription factors (TFs) are known to activate or repress Pdgfrb in cell culture systems; however, the biological function of Pdgfrb regulation by these TFs in vivo remains elusive (Ballagi et al, 1995;Uramoto et al, 2004;Jin et al, 2008;Weissmueller et al, 2014). In the present study, we show Koss et al, 2012) and the osteoblast-related gene Osx (Sp7 -Mouse Genome Informatics) (Gordon et al, 2010). Notably, recent reports suggest the ability of Pbx to interact with and recruit co-repressors, including Hdac1 (part of the Sin3/NuRD/Co-REST regulatory complex) and Hdac3 (part of the NCoR/SMRT complex), which facilitate chromatin condensation by histone 3 deacetylation and nucleosome positioning (Asahara et al, 1999;Chariot et al, 1999;Saleh et al, 2000;Choe et al, 2009;Gordon et al, 2010).…”

A unique stylopod patterning mechanism by Shox2 controlled osteogenesis

“…7P). It was reported previously that excessive expression of Pbx (Gordon et al, 2010) or Meis (Mercader et al, 2009) inhibited osteogenesis. Shox2 thus probably functions to antagonize the inhibitory effect of Meis and Pbx for stylopodial skeletogenesis.…”

The Murine Pbx1-d Lupus Susceptibility Allele Accelerates Mesenchymal Stem Cell Differentiation and Impairs Their Immunosuppressive Function

“…Pbx1 is an essential gene to maintain stem cell functions, as shown both for hematopoietic stem cells and MSCs (12,13). MSCs serve as a reservoir for tissue regeneration through their multipotent differentiation (14) as well as a regulator of tissue and immune homeostasis (15).…”

“…1A) that we previously used for CD4 + T cells (4). The main isoform expressed by B6 MSCs was Pbx1-b, which has been shown by others to be the major isoform expressed in normal MSCs (13), whereas the main isoform expressed by Sle1a1 MSCs was Pbx1-d (Fig. 1B).…”

“…PBX1-d was found more frequently in SLE patients than in healthy controls, and its expression corresponded to a decreased naive/central memory CD4 + T cell ratio, which is characteristic of SLE T cells (4). Prior to this newly discovered role in regulating T cell functions, full-length Pbx1-b was known to be necessary to block lineage-specific differentiation and maintains self-renewal of hematopoietic stem cells (12) as well as mesenchymal stem cells (MSCs) (13). We have shown using an MSC line that the overexpression of Pbx1-d was functionally equivalent to knocking down Pbx1-b, resulting in an accelerated differentiation into osteoblasts (11).…”