CK2 is the key molecular switch regulating C2C12 cell differentiation into osteoblasts and adipocytes
Date
2011
Authors
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Publisher
University of Delaware
Abstract
Age related osteoporosis is a medical condition in which bones become weak and can break easily. The quality of patients’ life becomes significantly less than average. Existing treatments are costly and not efficient. The majority of the existing drugs target osteoclasts maturation or activity (bisphosphonates and Selective Estrogen Receptor Modulators [SERMS]), but bone remodeling is needed to maintain body homeostasis, and these drugs were reported to cause side effects. Parathyroid hormone (PTH) is the only treatment available that increases bone turnover. Additional treatment which would increase bone mass are in desperate need. Bone Morphogenetic Protein 2 (BMP2) treatment was shown to increase bone mass and is a promising potential treatment for age related osteoporosis. BMP2 has multiple effects on the stem cell differentiation driving Mesenchymal Stem Cells (MSCs) into adipocytes and osteoblasts by a mechanism that is not fully understood. Osteoporotic patients have increased number of adipocytes in their bone marrow, but a decreased number of osteoblasts. BMP2 signaling is a complex process and begins with BMP2 binding to BMP Receptor Type Ia (BMPRIa) and BMP Receptor Type II (BMPRII). Upon ligand binding, BMPRII phosphorylates BMPRIa. Activated BMPRIa initiates Smad-dependent and Smad-independent signaling including p38, JNK, ERK, PI3K, and NF-kB pathways. BMP receptors are found in caveolae and Clathrin Coated Pits (CCPs) and the current dogma of BMP2-induced signaling states that CCPs are needed for Smad signaling and Smad-independent signals are delivered via cholesterol enriched membrane domain. There is some evidence that CCPs might be inhibitory to Smad signaling. In the present study it was shown that BMP2-induced initial BMPRIa and Smad phosphorylation takes place in caveolae, revising the current dogma. Membrane localization of BMPRIa and Smad phosphorylation is an important step for designing future therapeutic applications as well as understanding BMP2 induced signaling. Due to its ability to cause differentiation into osteoblasts and adipocytes, BMP2 treatment could potentially cause a problem for its usage as a treatment for age related osteoporosis. In order to design more specific treatments for targeting osteogenesis, the mechanism of differentiation needs to be fully understood. The present study was designed to address the differences in the BMP2-induced signaling resolving in adipogenesis and osteogenesis. Here, we report that BMP2-induced osteogenesis signals via Smad4 and mTor-AKT pathway, while adipogenesis signals via Smad4, MEK, NF-kB and p38 pathways.
Additionally, it was found that Caspase1 activity causes adipocyte differentiation, but inhibits osteoblast differentiation. Finally, CK2 was identified to be a molecular switch directing C2C12 differentiation depending on the specific site of interaction with BMPRIa. Blockage of CK2.1 and CK2.2 interaction sites with specific peptides led to adipogenesis and blockage of CK2.3 interaction site resulted in osteogenesis. Moreover, CK2.3 peptide, in addition to resulting in differentiation into osteoblasts, inhibited adipocyte differentiation. It was also shown to redirect mature adipocytes into osteoblasts. CK2.3 had the greatest potential to be used in treatment of age related osteoporosis due to its specific action directed towards osteoblasts. Caspase1 inhibitors also present interest for clinical applications, and need further exploration.