TGF-β1 and WISP-1/CCN-4 can regulate each other's activity to cooperatively control osteoblast function

Wnt-induced secreted protein-1 (WISP-1), like other members of the CCN family, is expressed in skeletal tissues. Its mechanism of action remains unknown. Expression of WISP-1 was analyzed in human bone marrow stroma cells (hBMSC) by RT-PCR. We identified two major transcripts corresponding to those of full-length WISP-1, and of the splice variant WISP-1 va which lacks a putative BMP/TGF-β binding site. To investigate the function of WISP-1 in bone, hBMSC cultures were treated with recombinant human (rh)WISP-1 and analyzed for proliferation and osteogenic differentiation. WISP-1 treatment increased both BrdU incorporation and alkaline phosphatase (AP) activity. Considering the known functional synergy found between the TGF-β super-family and members of the CCN family, we next tested the effect of WISP-1 on TGF-β1 activity. We found that rhWISP-1 could reduce rhTGF-β1 induced BrdU incorporation. Similarly, rhTGF-β1 inhibited rhWISP-1 induction of AP activity. To explore functional differences between the WISP-1 variants, WISP-1 or WISP-1 va were transfected into hBMSC. Both variants could strongly induce BrdU incorporation. However, there were no effects of either variant on AP activity without an additional osteogenic stimulus such as TGF-β1. Taken together our results suggest a functional relationship between WISP-1 and TGF-β1. To further define this relationship we analyzed the effect of WISP-1 on TGF-β signaling. rhWISP-1 significantly reduced TGF-β1 induced phosphorylation of Smad-2. Our data indicates that full-length WISP-1 and its variant WISP-1va are modulators of proliferation and osteogenic differentiation, and may be novel regulators of TGF-β1 signaling in osteoblast-like cells.