Runx2 is a common target of transforming growth factor beta1 and bone morphogenetic protein 2, and cooperation between Runx2 and Smad5 induces osteoblast-specific gene expression in the pluripotent mesenchymal precursor cell line C2C12

Divergence and convergence of TGF-beta/BMP signaling

The transforming growth factor-beta (TGF-beta) superfamily includes more than 30 members which have a broad array of biological activities. TGF-beta superfamily ligands bind to type II and type I serine/threonine kinase receptors and transduce signals via Smad proteins. Receptor-regulated Smads (R-Smads) can be classified into two subclasses, i.e. those activated by activin and TGF-beta signaling pathways (AR-Smads), and those activated by bone morphogenetic protein (BMP) pathways (BR-Smads). The numbers of type II and type I receptors and Smad proteins are limited. Thus, signaling of the TGF-beta superfamily converges at the receptor and Smad levels. In the intracellular signaling pathways, Smads interact with various partner proteins and thereby exhibit a wide variety of biological activities. Moreover, signaling by Smads is modulated by various other signaling pathways allowing TGF-beta superfamily ligands to elicit diverse effects on target cells. Perturbations of the TGF-beta/BMP signaling pathways result in various clinical disorders including cancers, vascular diseases, and bone disorders.

Differential expression patterns of Runx2 isoforms in cranial suture morphogenesis

Runx2 (previously known as Cbfal/Pebp2alphaA/AML3), a key transcription factor in osteoblast differentiation, has at least two different isoforms using alternative promoters, which suggests that the isoforms might be expressed differentially. Haploinsufficiency of the Runx2 gene is associated with cleidocranial dysplasia (CCD), the main phenotype of which is inadequate development of calvaria. In spite of the biological relevance, Runx2 gene expression patterns in developing calvaria has not been explored previously, and toward this aim we developed three probes: pRunx2, which comprises the common coding sequence of Runx2 and hybridizes with all isoforms; pPebp2alphaA, which specifically hybridizes with the isoform transcribed with the proximal promoter; and pOsf2, which hybridizes with the isoform transcribed with the distal promoter. These probes were hybridized with tissue sections of mouse calvaria taken at various time points in development. Runx2 expression was localized to the critical area of cranial suture closure, being found in parietal bones, osteogenic fronts, and sutural mesenchyme. Pebp2alphaA and Osf2 showed tissue-specific expression patterns. The sites of Pebp2alphaA expression were almost identical to that of pRunx2 hybridization but expression was most intense in the sutural mesenchyme, where undifferentiated mesenchymal cells reside. The Osf2 isoform was strongly expressed in the osteogenic fronts, as well as in developing parietal bones, where osteopontin (OP) and osteocalcin (OC) also were expressed. However, in contrast to Pebp2alphaA, Osf2 expression did not occur in sutural mesenchyme. Pebp2alphaA also was expressed prominently in primordial cartilage that is found under the sutural mesenchyme and is not destined to be mineralized. Thus, Osf2 isoforms contribute to events later in osteoblast differentiation whereas the Pebp2alphaA isoform participates in a wide variety of cellular activities ranging from early stages of osteoblast differentiation to the final differentiation of osteoblasts.

TGF-beta-induced repression of CBFA1 by Smad3 decreases cbfa1 and osteocalcin expression and inhibits osteoblast differentiation

Transforming growth factor-beta (TGF-beta), a secreted factor present at high levels in bone, inhibits osteoblast differentiation in culture; yet, the mechanism of this inhibition remains unclear. We studied the effects of TGF-beta and its effectors, the Smads, on the expression and function of the osteoblast transcription factor CBFA1. TGF-beta inhibited the expression of the cbfa1 and osteocalcin genes, whose expression is controlled by CBFA1 in osteoblast-like cell lines. This inhibition was mediated by Smad3, which interacts physically with CBFA1 and represses its transcriptional activity at the CBFA1-binding OSE2 promoter sequence. The repression of CBFA1 function by Smad3 contrasts with previous observations that Smads function as transcription activators. This repression occurred in mesenchymal but not epithelial cells, and depended on the promoter sequence. Smad3-mediated repression of CBFA1 provides a central regulatory mechanism for the inhibition of osteoblast differentiation by TGF-beta, since it inhibits both cbfa1 transcription and transcriptional activation of osteoblast differentiation genes by CBFA1. Altering Smad3 signaling influenced osteoblast differentiation in the presence or absence of TGF-beta, implicating Smad3/TGF-beta-mediated repression in autocrine regulation of osteoblast differentiation.

Incadronate and etidronate accelerate phosphate-primed mineralization of MC4 cells via ERK1/2-Cbfa1 signaling pathway in a Ras-independent manner: further involvement of mevalonate-pathway blockade for incadronate

Two types of bisphosphonates (BPs), incadronate (INC) and etidronate (ETI) accelerated phosphate (Pi)-primed mineralization of MC4 cells in a subnanomolar dose range. Intracellular signaling pathways involved were examined. 1) The effect of INC but not ETI was partially suppressed by two mevalonate (MVA) pathway metabolites, farnesylpyrophosphate (FPP) and geranylgeranylpyrophosphate (GGPP). 2) The BP-like accelerating effect was produced by statins and also by Toxin B, a Rho GTPases-specific inhibitor. 3) INC induced Cbfa1-nuclear localization within hours; and in an in vivo experiment using ovariectomized mice, its 3 weeks dosing exhibited the same effect in tibial extracts. 4) BPs promoted luciferase expression in murine p1.3-osteocalcin gene 2-luc and p6-osteoblast specific element 2-luc transfected cells, just as MVA, FPP and GGPP did independently and additively to INC. 5) BPs activated extracellular signal-regulated kinase (ERK1/2) in a Ras-independent manner within 5 min, and Pi was found to sensitize MC4 cells to BPs. MVA and its metabolites also activated ERKs but in a Ras-dependent manner and additively to INC. Ras dependency was determined using N17Ras-transfected cells. A MEK (MAP kinase-ERK kinase)-specific inhibitor PD98059 alone partly and with FPP completely blocked INC-induced mineralization. The results suggest that BPs act on Pi-sensitized MC4 cells to accelerate mineralization via nonRas-MEK-ERK1/2-Cbfa1 transactivation pathway and INC additionally acts by inhibiting the MVA pathway.