The candidate proto-oncogene bcl-3 encodes a protein that shares structural features with IKB-~ and other proteins that bind to members of the Rel protein family. Here, we show that in contrast to the inhibitory activity of IKB-a, the bcl-3 gene product superactivates NF-KB p50 homodimer-mediated gene expression both in vivo and in vitro. BCL-3 protein can, as well, selectively associate with p50 homodimers in the presence of DNA containing a KB motif. These results strongly suggest that BCL-3 can act as a transcriptional coactivator, acting through DNA-bound pS0 homodimers.[Key Words: NF-KB; p50; IKB; transcription; coactivator]Received January 8, 1993; revised version accepted May 21, 1993.The candidate proto-oncogene bcl-3 was identified by the cloning of chromosomal breakpoints from chronic lymphocytic leukemia cells containing a t(14;19) translocation (Ohno et al. 1990). bcl-3 expression is activated by translocation without apparent alteration of the encoded protein structure. The deduced primary structure of BCL-3 contains seven repeats of an -30-amino-acid motif (ankyrin repeat) found in several other proteins (Nolan and Baltimore 1992). These include erythrocyte ankyrin, a cytoskeletal protein; yeast cell cycle regulatory proteins, such as cdclO and SWI6; and the transmembrane receptors notch, TAN1, and int-3.

Although Akt plays key roles in various cellular processes, the functions of Akt and Akt downstream signaling pathways in the cellular processes of skeletal development remain to be clarified. By analyzing transgenic embryos that expressed constitutively active Akt (myrAkt) or dominant-negative Akt in chondrocytes, we found that Akt positively regulated the four processes of chondrocyte maturation, chondrocyte proliferation, cartilage matrix production, and cell growth in skeletal development. As phosphorylation of GSK3beta, S6K, and FoxO3a was enhanced in the growth plates of myrAkt transgenic mice, we examined the Akt downstream signaling pathways by organ culture. The Akt-mTOR pathway was responsible for positive regulation of the four cellular processes. The Akt-FoxO pathway enhanced chondrocyte proliferation but inhibited chondrocyte maturation and cartilage matrix production, while the Akt-GSK3 pathway negatively regulated three of the cellular processes in limb skeletons but not in vertebrae due to less GSK3 expression in vertebrae. These findings indicate that Akt positively regulates the cellular processes of skeletal growth and endochondral ossification, that the Akt-mTOR, Akt-FoxO, and Akt-GSK3 pathways positively or negatively regulate the cellular processes, and that Akt exerts its function in skeletal development by tuning the three pathways in a manner dependent on the skeletal part.

Runx2 and Cbfbeta are essential for skeletal development during the embryonic stage. Runx2 has two isoforms with different N-termini. We examined the functions of the Runx2 isoforms and Cbfbeta in postnatal bone development. On luciferase and electrophoretic mobility shift assays, Runx2-I was less active than Runx2-II in the absence of Cbfb, but the two Runx2 isoforms had similar activity levels in the presence of Cbfb. We generated Runx2-I transgenic mice under the control of Col1a1 promoter and Runx2-I/Cbfb and Runx2-II/Cbfb double transgenic mice. Runx2-I transgenic mice showed less severe osteopenia and fragility than Runx2-II transgenic mice due to milder inhibition of both osteoblast maturation and transition to osteocytes, even though the former mice showed higher transgene expression. However, Runx2-I/Cbfb and Runx2-II/Cbfb double transgenic mice had enhanced inhibition of osteoblast maturation, resulting in similar severity of osteopenia and fragility, although the latter mice had less osteocytes. These findings indicate that (1) Runx2-II more strongly inhibits osteoblast maturation and transition to osteocytes than Runx2-I; (2) Cbfbeta regulates Runx2 function isoform-dependently; and (3) Runx2-I activity is highly dependent on Cbfbeta. These findings demonstrate that Runx2 isoforms exert their functions through at least partly different mechanisms and Cbfbeta regulates bone development by regulating Runx2 function isoform-dependently.

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