A lymphoid enhancer binding factor (Lef) 1 isoform regulates osteoblast maturation

The canonical Wnt signaling pathway has emerged as an important regulator of bone formation, regeneration and repair. Studies over the past decade have demonstrated that activation of canonical Wnt signaling generally promotes osteoblast proliferation and enhances bone mass, while suppression of Wnt signaling results in bone loss. Gaining a better understanding of Wnt signaling in the context of skeletal metabolism is important because current skeletal regeneration and repair treatments have limitations, anti-resorptive therapies have unknown long-term health consequences and the demand for therapies is rising as our population ages. Osteoporosis is a growing healthcare challenge that affects about 44 million Americans. Cancer survival rates drop drastically in patients suffering from bone metastases. A complete, molecular understanding of the canonical Wnt signaling cascade in the context of skeletal biology will promote the development of new therapies for the treatment of osteoporosis, skeletal cancer metastasis, and other skeletal diseases that result in uncoupling of bone formation and resorption.;Lymphoid Enhancer Binding Factor (Lef) 1 is a transcription factor in the canonical Wnt/Lrp5/6/beta-catenin signaling cascade, which regulates osteoblast differentiation, bone density and skeletal strength. In this thesis, I describe the expression and function of an alternative Lef1 isoform in osseous cells. Lef1DeltaN is a naturally occurring isoform driven by a promoter (p2) within the intron between exons 3 and 4 of Lef1. Lef1DeltaN is induced during late osteoblast differentiation. This is opposite to the expression pattern of the full-length Lef1 protein, which as we previously showed, decreases during differentiation. We showed that the Lef1DeltaN p2 promoter is active in osteoblasts and Runx2 positively regulates its activity. BMP2 also promotes Lef1DeltaN expression, whereas Wnt3a represses it. Lef1DeltaN overexpression in differentiating osteoblasts induced osteocalcin and type 1 collagen expression, which suggests Lef1DeltaN is a crucial regulator of terminal differentiation in osseous cells. We found Lef1DeltaN interacts with beta-catenin to activate a Lef1-responsive promoter and stimulate the transcription of genes involved in late osteoblast differentiation. We mapped the region of Lef1DeltaN that associates with beta-catenin to an element within the first 61 amino acids of Lef1DeltaN and showed this region was required to induce type 1 collagen and osteocalcin expression during osteoblast maturation. Taken together, Lef1DeltaN interacts with beta-catenin to regulate terminal differentiation in osseous cells.