The role of lymphocyte enhancer-binding factor 1 (Lef1) in osteoblast differentiation

Bone disease affects more than 10 million Americans. In light of this, it has become clear that research into the molecular mechanisms of bone development and maintenance must be a priority. Many recent studies involving bone development have revolved around components of the Wnt signaling cascade. Increased Wnt signaling is associated with increased bone formation. Though some research indicates that Wnt signaling induces osteoblast differentiation, we and others suggest that Wnt signaling increases the pool of osteoblast progenitors in order to enhance bone formation.; Lef1 is a nuclear effector of the Wnt signaling pathway. In order to define the role of Lef1 in osteoblast biology, we studied the effect of Lef1 on the osteocalcin promoter. Lef1 inhibits Runx2-mediated transcription of the osteocalcin promoter, but does not require direct DNA binding to the osteocalcin promoter to repress its transcription. To determine the role of Lef1 in osteoblast maturation, we stably expressed Lef1 in preosteoblasts. Preosteoblasts constitutively expressing Lef1 expressed differentiation markers later and at lower levels and failed to mineralize. Conversely, when we expressed a short hairpin RNA (shRNA) to reduce Lef1 expression, preosteoblast differentiation and mineralization was accelerated. These results indicate that Lef1 is a suppressor of osteoblast differentiation.; To identify some of the genes affected by Lef1, we used gene chip analysis on Lef-suppressed (L2) cells. One gene that was strongly down regulated was alpha 1 (XI) collagen (Col11a1). Lef1 activates the COL11A1 promoter in a concentration dependent manner. Mutation of any or all of four identified Lef1 binding sites did not affect the responsiveness of the COL11A1 promoter to Lef1. Lef1 lackin its DNA binding domain could not activate the COL11A1 promoter. These data suggest that Lef1 activates the COL11A1 promoter either through binding to a cryptic Lef1 DNA binding site or that it interacts with the promoter indirectly via interaction with another protein through its DNA binding domain.; This research outlines a role for Lef1 in modulating osteoblastic gene expression during osteoblast differentiation and is consistent with the hypothesis that Wnt signaling inhibits osteoblast maturation in favor of promoting the expansion of the osteoblast progenitor pool.