The orphan nuclear estrogen receptor-related receptor alpha (ERR alpha) isexpressed throughout osteoblast differentiation and regulates bone formation in vitro

The orphan nuclear estrogen receptor-related receptor alpha (ERR alpha), is expressed by many cell types, but is very highly expressed by osteoblastic cells in which it transactivates at least one osteoblast-associated gene, osteopontin. To study the putative involvement of ERR alpha in bone, we fir st assessed its expression in rat calvaria (RC) in vivo and in RC cells in vitro. ERR alpha mRNA and protein were expressed at all developmental stage s from early osteoprogenitors to bone-forming osteoblasts, but protein was most abundant in mature cuboidal osteoblasts. To assess a functional role f or ERR alpha in osteoblast differentiation and bone formation, we blocked i ts expression by antisense oligonucleotides in either proliferating or diff erentiating RC cell cultures and found inhibition of cell growth and a prol iferation-independent inhibition of differentiation. On the other hand, ERR alpha overexpression in RC cells increased differentiation and maturation of progenitors to mature bone-forming cells. Our findings show that ERR alp ha is highly expressed throughout the osteoblast developmental sequence and plays a physiological role in differentiation and bone formation at both p roliferation and differentiation stages. In addition, we found that manipul ation of receptor levels in the absence of known ligand is a fruitful appro ach for functional analysis of this orphan receptor and identification of p otential target genes.