Positive and negative regulation of chondrogenesis by splice variants of PEBP2 alpha A/CBF alpha 1 in clonal mouse EC cells, ATDC5

The alpha A type of the alpha subunit of the polyomavirus enhancer binding protein 2 (PEBP2 alpha A), also called the core binding factor alpha 1 (CBF alpha 1) or til-1, plays crucial roles in osteogenesis. Little is known, h owever, about the function of PEBP2 alpha A in chondrogenesis. Here, we exa mined the role of PEBP2 alpha A in chondrogenesis of clonal mouse embryonal carcinoma cells, ATDC5, which are committed as chondroprogenitors. We foun d that as ATDC5 cells condensed and formed cartilaginous nodules, PEBP2 alp ha A increased, and the level was maintained throughout the process of chon drocytic maturation. When an established dominant negative form of PEBP2 al pha A was introduced in undifferentiated ATDC5 cells, the cellular condensa tion and the subsequent processes were inhibited. This inhibition was overc ome with BMP-4 treatment, which increased the endogenous expression of PEBP 2 alpha A. Thus, the process of chondrogenesis is regulated by the lever of PEBP2 alpha A activity. Along with the wild-type PEBP2 alpha A, a splice V ariant form, til-1 G2, is naturally expressed in ATDC5 cells. In luciferase reporter assays, til-1 G2 not only exhibited a limited ability to transact ivate the osteocalcin promoter but also inhibited the activity achieved by the wild-type PEBP2 alpha A. When til-1 G2 was overexpressed by stable tran sfection in undifferentiated ATDC5 cells, it inhibited the progression of c hondrogenesis. Therefore, we conclude that PEBP2 alpha A acts as a positive regulator of chondrogenesis, and that this positive effect may be finely t uned by the opposing effect of the til-l G2 isoform. J. Cell. Physiol. 181: 169-178, 1999. (C) 1999 Wiley-Liss, Inc.