TIEG1 Null Mouse-Derived Osteoblasts Are Defective in Mineralization and in Support of Osteoclast Differentiation In Vitro

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Abstract
Transforming growth factor β-inducible early gene 1 (TIEG1) is a member of the Krüppel-like transcription factor family. To understand the physiological role of TIEG1, we generated TIEG−/− (null) mice and found that the TIEG−/− mice had increased osteoblast numbers with no increased bone formation parameters. However, when calvarial osteoblasts (OBs) were isolated from neonatal TIEG−/− and TIEG+/+ mice and cultured in vitro, the TIEG−/− cells displayed reduced expression of important OB differentiation markers. When the OBs were differentiated in vitro by treatment with bone morphogenic protein 2, the OBs from TIEG+/+ calvaria displayed several mineralized nodules in culture, whereas those from TIEG−/− mice showed no nodules. To characterize the OBs9 ability to support osteoclast differentiation, the OBs from TIEG+/+ and TIEG−/− mice were cultured with marrow and spleen cells from TIEG+/+ mice. Significantly fewer osteoclasts developed when TIEG−/− OBs were used to support osteoclast differentiation than when TIEG+/+ OBs were used. Examination of gene expression in the TIEG−/− OBs revealed decreased RANKL and increased OPG expression compared to TIEG+/+ OBs. The addition of RANKL to these cocultures only partially restored the ability of TIEG−/− OBs to support osteoclast differentiation, whereas M-CSF alone or combined with RANKL had no additional effect on osteoclast differentiation. We conclude from these data that TIEG1 expression in OBs is critical for both osteoblast-mediated mineralization and osteoblast support of osteoclast differentiation.

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