Osteocrin, a Novel Bone-specific Secreted Protein That Modulates the Osteoblast Phenotype

Abstract

Although a number of secreted factors have been demonstrated to be bone regulators, none of these are unique to bone. Using a viral-based signal-trap strategy we have identified a novel gene we have termed "osteocrin." A 1280-bp mRNA encodes osteocrin producing a mature protein of 103 amino acids with a molecular mass of 11.4 kDa. Osteocrin shows no homology with any known gene except for two conserved sequence motifs reminiscent of dibasic cleavage sites found in peptide hormone precursors. Immunofluorescence and Western blot analysis confirmed the secretory nature of osteocrin. Two protein species were identified in the medium of cells overexpressing osteocrin, a full-length 11.4 kDa species and a processed ϳ5 kDa species. Mutation of the 76 KKKR 79 dibasic cleavage site abolished the appearance of this smaller osteocrin fragment. By in situ hybridization in mouse embryos, osteocrin was expressed specifically in Cbfa-1-positive, osteocalcin-negative osteoblasts. Immunohistochemistry on adult mouse bone showed osteocrin localization in osteoblasts and young osteocytes. By Northern blot analysis, osteocrin expression was only detected in bone, expression peaking just after birth and decreasing markedly with age. In primary osteoblastic cell cultures osteocrin expression coincided with matrix formation then decreased in very mature cultures. Treatment of cultures with 1,25-dihydroxyvitamin D 3 resulted in a rapid dose-dependent down-regulation of osteocrin expression, suggesting direct regulation. Chronic treatment of primary cultures with osteocrin-conditioned media inhibited mineralization and reduced osteocalcin and alkaline phosphatase expression. These results suggest that osteocrin represents a novel, unique vitamin D-regulated bone-specific protein that appears to act as a soluble osteoblast regulator.Bone is a dynamic tissue that is continually being modeled and remodeled through the coordinated actions of the bone forming osteoblasts and bone resorbing osteoclasts. Such remodeling is necessary to respond to the continually changing mechanical and regulatory demands placed upon the skeleton (1-4). The balance of the activity of the osteoblasts and osteoclasts is tightly regulated at both the systemic and local levels through the actions of a number of secreted molecules. Systemically, calciotropic hormones such as 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ) 1 and parathyroid hormone (PTH) can regulate both bone formation and resorption, acting on both osteoclastic and osteoblastic cell lineages (5-8), whereas 1,25(OH) 2 D 3 also acts via the intestine and kidney to regulate systemic calcium and phosphate availability (9, 10). Other hormones, such as growth hormone (11), estrogens (12), and thyroid hormone (13) also influence bone mass. More recently, it has been reported that the peptide hormone leptin exerts a strong systemic antiosteogenic effect when activating the sympathetic nervous system through a hypothalamic relay (14, 15).Remodeling occurs in discrete units, termed basic multicellular units, con...

c-MYC overexpression with loss of Ink4a/Arf transforms bone marrow stromal cells into osteosarcoma accompanied by loss of adipogenesis

“…Key genes for the regulation of embryonic organogenesis (Hoxa5, Hoxa9, Foxc1 and Foxf2) and the commitment of immature cells to multiple lineages (Pparg, Cebpa, Plxnd1, Egfl7, Mgp, Plxdc2, Ebf1 and Tcf3) were expressed at higher levels in AO cells than in AX cells, suggesting that AO cells have characteristics of immature mesenchymal cells (Wang and Reed, 1993;Luo et al, 1997;Wu et al, 1999;Rosen et al, 2000;Parker et al, 2004;Torres-Vazquez et al, 2004;Miller et al, 2007;Cole et al, 2008). The profile of AX cells, on the other hand, was indicative of both cells that are committed to the osteocyte lineage (Dlx3, Sp7, Dkk1, Ostn and Bmp3) and cells that are inclined to tumorigenesis in vivo (Cd276, Ptgs2, Vegfa, Epha2, Frzb and Sfrp1) (Kim et al, 1993;Daluiski et al, 2001;Dannenberg et al, 2001;Thomas et al, 2003;Lee et al, 2004 Zang et al, 2007;Brantley-Sieders et al, 2008;Engin et al, 2008;Li et al, 2008).…”

Musclin is an activity-stimulated myokine that enhances physical endurance

“…Two groups initially identified this peptide: one as bone-derived osteocrin (5) and the second as muscle-secreted musclin (4). Musclin mRNA expression has been linked to insulin-induced activation of protein kinase B (Akt) that phosphorylates forkhead box O1 transcription factor (FOXO1), causing it to be exported from the nucleus and thus releasing the musclin-encoding gene from transcriptional inhibition (4,6).…”

“…This study is focused on the recently discovered myokine musclin (4,5). Two groups initially identified this peptide: one as bone-derived osteocrin (5) and the second as muscle-secreted musclin (4).…”

L’ostéocrine, une nouvelle molécule de signalisation osseuse

“…Parmi ceux-ci, on compte plusieurs protéines sécrétées et leurs récepteurs respectifs dont les plus nuent d'être décelés, notamment l'ostéocrine, une protéine que notre groupe a découverte lors d'un criblage visant l'identification du sécrétome des cellules de l'os [1]. Le gène de l'ostéocrine a également été cloné à partir du muscle, et nommée muscline [2].…”

“…Le gène de l'ostéocrine a également été cloné à partir du muscle, et nommée muscline [2]. Le gène codant l'ostéocrine est exprimé presque exclu- NOUVELLES MAGAZINE sivement par les ostéoblastes [1] et les cellules musculaires [2][3][4]. L'ostéocrine est une protéine hautement conservée entre les espèces (Figure 1) et l'examen de sa structure primaire met en évidence des motifs rappelant ceux entrant dans la composition de plusieurs prohormones [5].…”