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Reconstitution of Runx2/Cbfa1‐null cells identifies a requirement for BMP2 signaling through a Runx2 functional domain during osteoblast differentiation

2006; Wiley; Volume: 100; Issue: 2 Linguagem: Inglês

10.1002/jcb.21039

1097-4644

Jong‐Sup Bae, Soraya Gutiérrez, Radhika Narla, Jitesh Pratap, Rajitha Devados, André J. van Wijnen, Janet L. Stein, Gary S. Stein, Jane B. Lian, Amjad Javed,

Bone and Dental Protein Studies

Abstract The Runx2/Cbfa1 transcription factor is a scaffolding protein that promotes osteoblast differentiation; however, the specific Runx2‐functional domains required for induction of the osteogenic lineage remain to be identified. We approached this question using a TERT‐immortalized cell line derived from calvaria of Runx2‐null mice by reconstituting the osteogenic activity with wild‐type and deletion mutants of Runx2. The presence or absence of osteogenic media (β‐glycerol phosphate and ascorbic acid) and/or with BMP2 did not stimulate osteoblastic gene expression in the Runx2‐null cells. However, cells infected with wild‐type Runx2 adenovirus showed a robust temporal increase in the expression of osteoblast marker genes and were competent to respond to BMP2. Early markers (i.e., collagen type‐1, alkaline phosphatase) were induced (four‐ to eightfold) at Days 4 and 8 of culture. Genes representing mature osteoblasts (e.g., Runx2, osteopontin, bone sialoprotein, osteocalcin) were temporally expressed and induced from 18‐ to 36‐fold at Days 8 and 12. Interestingly, TGFβ and Vitamin D‐mediated transcription of osteoblast genes (except for osteopontin) required the presence of Runx2. Runx2 lacking the C‐terminal 96 amino acids (Runx2 Δ432) showed a pattern of gene expression similar to wild‐type protein, demonstrating the Groucho interaction and part of the activation domain are dispensable for Runx2 osteogenic activity. Upon further deletion of the Runx2 C‐terminus containing the nuclear matrix targeting signal and Smad‐interacting domain (Δ391), we find none of the osteoblast markers are expressed. Therefore, the Runx2 391‐432 domain is essential for execution of the BMP2 osteogenic signal. J. Cell. Biochem. 100: 434–449, 2007. © 2006 Wiley‐Liss, Inc.