Wnt/β catenin signaling activates expression of the bone-related transcription factor RUNX2 in select human osteosarcoma cell types

Abstract

Osteosarcoma is the most common malignant bone tumor in children and adolescents. Metastasis and poor responsiveness to chemotherapy in osteosarcoma correlates with over-expression of the runt-related transcription factor RUNX2, which normally plays a key role in osteogenic lineage commitment, osteoblast differentiation, and bone formation. Furthermore, WNT/-catenin signaling is over-activated in osteosarcoma and promotes tumor progression. Importantly, the WNT/-catenin pathway normally activates RUNX2 gene expression during osteogenic lineage commitment. Therefore, we examined whether the WNT/-catenin pathway controls the tumor-related elevation of RUNX2 expression in osteosarcoma. We analyzed protein levels and nuclear localization of -catenin and RUNX2 in a panel of human osteosarcoma cell lines (SAOS, MG63, U2OS, HOS, G292, and 143B). In all six cell lines, -catenin and RUNX2 are expressed to different degrees and localized in the nucleus and/or cytoplasm. SAOS cells have the highest levels of RUNX2 protein that is localized in the nucleus, while MG63 cells have the lowest RUNX2 levels which is mostly localized in the cytoplasm. Levels of -catenin and RUNX2 protein are enhanced in HOS, G292, and 143B cells after treatment with the GSK3 inhibitor SB216763. Furthermore, small interfering RNA (siRNA)-mediated depletion of -catenin inhibits RUNX2 expression in G292 cells. Thus, WNT/-catenin activation is required for RUNX2 expression in at least some osteosarcoma cell types, where RUNX2 is known to promote expression of metastasis related genes.