TIEG1 null mouse-derived osteoblasts are defective in mineralization and in support of osteoclast differentiation in vitro

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 OBs' 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.