Induction Of Runx2 Research Articles

JNK/ERK–AP-1/Runx2 induction “paves the way” to cartilage load-ignited chondroblastic differentiation

Chondro-osteogenesis and subsequently skeletal morphology are greatly influenced by mechanical loads. The exact mechanism(s) by which mechanical stimuli are transduced in chondrocytes remains obscure and appears to be equally complex with similar signal transducing systems. Here we investigated whether and to what extent the MAPK (JNK/ERK)-AP-1/Runx2 signaling pathways are engaged in this phenomenon, and assessed their involvement in the functional biology of articular cartilage. For this purpose, 14-day-old female Wistar rats were divided into 2 groups: the first group was fed hard diet (simulating physiologic temporomandibular joint (TMJ) loading), while the second group was fed soft diet (reduced TMJ loading). On day 21 (experiment initiation day - weaning day), biopsies from condyles of both groups were obtained after 6, 12 and 48 h of functional TMJ loading. Immunohistochemical methodology was employed to evaluate the expression levels of pc-Jun, c-Fos, JNK2, p-JNK, p-ERK and Runx2 due to alteration in functional load. Our data demsonstrate that the protein levels of all the aforementioned molecules were markedly increased in animals fed with the hard diet, throughout the experimental procedure. These results indicate that functional cartilage loading induces the AP-1 and Runx2 transcription factors through the JNK and ERK MAPK cascades. In as much as the above signaling mediators/effectors are considered to be crucial in the differentiation/maturation process of cartilage tissue, we pose that functional mechanical loading of condylar cartilage serves to "fine tune" chondroblastic differentiation/maturation.

BMP-2-induced Runx2 Expression Is Mediated by Dlx5, and TGF-β1 Opposes the BMP-2-induced Osteoblast Differentiation by Suppression of Dlx5 Expression

Intramuscular injection of BMP-2 induces ectopic bone formation in vivo. Similarly, BMP-2 treatment blocks myogenic differentiation and induces osteoblastic transdifferentiation of premyoblastic C2C12 cells. Previous reports suggested that BMP-2-stimulated Runx2 expression could play a pivotal role in transdifferentiation. However, increased Runx2 expression by TGF-beta 1 did not support osteoblast differentiation in vitro. These results indicate that the induction of Runx2 is not sufficient to explain the BMP-induced transdifferentiation. We found that Dlx5 is specifically expressed in osteogenic cells, and is specifically induced by BMP-2 or -4 signaling but not by other osteotrophic signals or other TGF-beta superfamily members. Cycloheximide treatment indicated that Dlx5 was immediately induced by BMP signaling, while Runx2 required de novo protein synthesis. In addition, blocking or overexpressing each transcription factor indicated that Dlx5 is an indispensable mediator of BMP-2-induced Runx2 expression but is not involved in TGF-beta 1-induced Runx2 expression. Moreover, TGF-beta 1 opposed BMP-2-induced osteogenic transdifferentiation through Dlx5 suppression by de novo induction of AP-1. Taken together, these results indicate that Dlx5 is an indispensable regulator of BMP-2-induced osteoblast differentiation as well as the counteraction point of the opposing TGF-beta 1 action.

Both the Smad and p38 MAPK pathways play a crucial role in Runx2 expression following induction by transforming growth factor-β and bone morphogenetic protein

The Runx family of transcription factors plays pivotal roles during normal development and in neoplasias. In mammals, Runx family genes are composed of Runx1 (Pebp2alphaB/Cbfa2/Aml1), Runx2 (Pebp2alphaA/Cbfa1/Aml3) and Runx3 (Pebp2alphaC/Cbfa3/Aml2). Runx1 and Runx3 are known to be involved in leukemogenesis and gastric carcinogenesis, respectively. Runx2, on the other hand, is a common target of transforming growth factor-beta1 (TGF-beta1) and bone morphogenetic protein-2 (BMP-2) and plays an essential role in osteoblast differentiation. Runx2 is induced by the receptor-activated Smad; Runx2 mediates the blockage of myogenic differentiation and induces osteoblast differentiation in C2C12 pluripotent mesenchymal precursor cells. However, Smad does not directly induce Runx2 expression; an additional step of de novo protein synthesis is required. Here we report that Smad-induced junB functions as an upstream activator of Runx2 expression. Furthermore, not only the Smad pathway but also the mitogen-activated protein kinase (MAPK) cascades are involved in the induction of Runx2 by TGF-beta1 and BMP-2. Our results demonstrate that following TGF-beta and BMP induction, both the Smad and p38 MAPK pathways converge at the Runx2 gene to control mesenchymal precursor cell differentiation.