Maf1, a repressor of RNA polymerase III-dependent transcription, regulates bone mass

Maf1, a key repressor of RNA polymerase III-mediated transcription, has been shown to promote mesoderm formation in vitro. Here, we show for the first time that Maf1 plays a critical role in the regulation of osteoblast differentiation and bone mass. A high bone mass phenotype was noted in mice with global deletion of Maf1 (Maf1-/- mice), as well as paradoxically, in mice that overexpressed MAF1 in cells of the osteoblast lineage (Prx-Cre;LSL-Maf1 mice). Osteoblasts isolated from Maf1-/- mice unexpectedly showed reduced osteoblastogenesis ex vivo. Prx-Cre;LSL-Maf1 mice showed enhanced osteoblastogenesis concordant with their high bone mass phenotype. Thus, the high bone mass phenotype in Maf1-/- mice is likely due to the confounding effects of the global absence of MAF1 in Maf1-/- mice. Expectedly, MAF1 overexpression promoted osteoblast differentiation and shRNA-mediated Maf1 downregulation inhibited differentiation of ST2 cells, indicating an overall positive action of Maf1 on osteoblast formation. We also found that, in contrast to MAF1 overexpression, other perturbations that repress RNA pol III transcription, including Brf1 knockdown and the chemical inhibition of RNA pol III by ML-60218, paradoxically inhibited osteoblast differentiation. RNA-seq was used to determine the basis for these opposing actions. The three modalities used to perturb RNA pol III transcription resulted in distinct changes gene expression, indicating that this transcription process is highly sensitive and triggers diverse gene expression programs and phenotypic outcomes. Specifically, MAF1 overexpression in ST2 cells induced genes known to promote osteoblast differentiation. A subset of these genes was altered in an opposite manner with Brf1 downregulation or treatment with ML-60218, both of which also inhibit RNA pol III-mediated transcription. All these perturbations, however, enhanced adipogenesis in ST2 cell cultures. Furthermore, codon bias was observed in a subset of genes expressed during osteoblast differentiation. Together, these results reveal a novel role for Maf1 and RNA pol III-mediated transcription in osteoblast fate determination and differentiation and bone mass regulation.