BIG-3, a novel WD-40 repeat protein, is expressed in the developing growth plate and accelerates chondrocyte differentiation in vitro.

Among the local signaling pathways that regulate the sequential steps of chondrocyte differentiation is the bone morphogenetic protein (BMP) signaling pathway. We have identified a novel gene, named BIG-3 (BMP-2-induced gene 3 kb) that is expressed in a BMP-regulated fashion in the prechondroblastic cell line MLB13MYC clone 17. BIG-3 is also expressed in proliferating and hypertrophic chondrocytes in the developing growth plate in vivo. We undertook studies to address whether BIG-3 played a functional role in chondrocyte differentiation, using mouse clonal chondrogenic ATDC5 cells. BIG-3 protein levels increased during ITS (insulin, transferrin, sodium selenite)-induced ATDC5 differentiation and in response to BMP-2 treatment. To determine whether stable expression of BIG-3 could alter the program of chondrocytic differentiation, ATDC5 cells were stably transfected with the full-length coding region of BIG-3 (ATDC5-BIG-3) or with the empty vector (ATDC5-EV). Accelerated matrix proteoglycan synthesis was observed in the pooled ATDC5-BIG-3 clones. Alkaline phosphatase and osteopontin mRNA levels were also increased in ATDC5-BIG-3 clones compared with ATDC5-EV clones. Stable expression of BIG-3 also accelerated mineralized matrix formation in both the presence and absence of ITS. These findings, which demonstrate that BIG-3 accelerates chondrocyte differentiation in vitro, combined with the observation that BIG-3 is expressed in the growth plate during embryonic development, suggest that this novel protein is likely to play an in vivo regulatory role in the developing growth plate.