CONTROL OF TGF-BETA RECEPTOR EXPRESSION IN BONE

Michael Centrella, Changhua Ji, Thomas L. McCarthy

Department of Surgery, Plastic Surgery Section, Yale University School of Medicine, 333 Cedar Street, PO Box 208041, New Haven, CT 06520-8041,

Received 12/1/97 Accepted 12/5/97

11. CBFa, TGF-BETA RECEPTOR I, AND BONE

The function of CBFa as a nuclear regulator initially seemed restricted to the expression of genes important to blood cell differentiation in higher organisms. Compatible with this, some leukemias correlate well with translocations in the CBFa or CBFb subunit genes. In two cases, breaks occur in the COOH-terminal transactivation region of the CBFa subunit. This perhaps allows strong binding by the intact Runt domain to the CBFb subunit and to DNA, even while it prevents effective transactivation at target gene promoter sites. Another translocation occurs at a site upstream of the Runt domain, and another within the CBFb subunit. While each effectively alters target gene expression in lymphoid cells, the molecular details of these events are not yet resolved (109,110). For example, new studies from Dr. Ito’s lab reveal that a splice variant of CBFa2, lacking a portion of the Runt domain, inhibits CBFa dependent T cell receptor promoter activity, myeloid cell proliferation and differentiation independently of its ability to bind DNA (111). Consistent with the widespread effects of CBFa-like transcription factors in Drosophila, other recent studies reveal the presence of these factors in muscle (112).

Work from Drs. Jane Lian and Gary Stein (University of Massachusetts Medical Center, Worcester), and Dr. Gerard Karsenty (MD Anderson Cancer Center, Houston) further reveal that CBFa may be an important regulator of osteocalcin expression by osteoblasts (113-118). One CBFa binding site may contribute as much as 75% to the activity of the osteocalcin promoter in osteoblast-like cells, perhaps accounting in part for the tissue-specific expression of this protein (116). Most importantly, after the osteocalcin studies and work from our lab with the TGF-betaRI promoter were reported (1996 meeting of the American Society for Bone and Mineral Research, abstract #56), Dr. Toshihisa Komori (Osaka University, Japan) produced mice in which the gene for the CBFa1 subunit was "knocked-out". Animals died very soon after birth and suffered severe osteogenic deformities, while cartilage formation appeared intact (119). Analogous results were seen by Dr. Michael J. Owen’s group (IRCF, London) (120). Mineralized skeletal elements consistent with osteoblast-dependent bone formation were not evident in these animals, and few if any cells with osteoblast morphology were apparent. Consistent with insertion, deletion or missense mutations in CBFa1 that occur in humans, these mice are considered models for the skeletal disorder cleidocranial dysplasia (119-121). However, genes directly effected by CBFa1, especially those important for skeletal development, are difficult to determine when the factor is absent or dysfunctional, and when osteoblasts are absent or hard to detect. Hormone dependent decreases in CBFa1 could also challenge skeletal integrity and more readily indicate important downstream targets. In this regard, our lab noted changes in CBFa1 expression with osteoblast differentiation (98), and that glucocorticoid rapidly and potently suppressed CBFa1 levels, CBFa1 binding to the TGF-betaRI promoter, TGF-betaRI promoter activity, TGF-betaRI mRNA and protein levels, and TGF-beta function in primary osteoblast cultures (122). We also find increases in CBFa1 expression by osteoblasts in response to BMP-2 (unpublished studies). These studies establish new molecular links in osteoblasts to CBFa1 with regard to important extracellular bone cell regulators and to a new genomic target, TGF-betaRI. These findings, changes in the expression of TGF-betaRI during osteoblast differentiation, and the importance of TGF-beta and TGF-betaRI for osteoblast activity suggest that our unique new observations with the TGF-betaRI promoter will significantly influence our understanding of TGF-beta biological activity in skeletal tissue, and of CBFa1-dependent control of bone cell function.