Nephronectin promotes osteoblast differentiation via the epidermal growth factor-like repeats.

We found that nephronectin was significantly down-regulated by TGF-beta1. To determine the function of nephronectin in osteogenesis, we generated various constructs to produce stable MC3T3-E1 cell lines, expressing and secreting nephronectin protein, including full-length (Npnt), lacking EGF-like repeats (Np-MAM), and lacking RGD and MAM domains (Np-EGF). We demonstrated that nephronectin promotes differentiation during osteoblast differentiation and the EGF-like repeats were essential. Lack of these repeats resulted in inhibiting the change in morphology. Over-expression of nephronectin results in earlier formation of bone nodules than the vector control. ERK activation is essential for nephronectin-induced osteoblast differentiation.