TIEG and estrogen modulate SOST expression in the murine skeleton

Abstract

TIEG knockout (KO) mice exhibit a female-specific osteopenic phenotype and altered expression of TIEG in humans is associated with osteoporosis. Gene expression profiling studies identified sclerostin as one of the most highly up-regulated transcripts in the long bones of TIEG KO mice relative to WT littermates suggesting that TIEG may regulate SOST expression. TIEG was shown to substantially suppress SOST promoter activity and the regulatory elements through which TIEG functions were identified using promoter deletion and chromatin immunoprecipitation assays. Knockdown of TIEG in IDG-SW3 osteocyte cells using shRNA and CRISPR-Cas9 technology resulted in increased SOST expression and delayed mineralization, mimicking the results obtained from TIEG KO mouse bones. Given that TIEG is an estrogen regulated gene, and as changes in the hormonal milieu affect SOST expression, we performed ovariectomy (OVX) and estrogen replacement therapy (ERT) studies in WT and TIEG KO mice followed by miRNA and mRNA sequencing of cortical and trabecular compartments of femurs. SOST expression levels were considerably higher in cortical bone compared to trabecular bone. In cortical bone, SOST expression was increased following OVX only in WT mice and was suppressed following ERT in both genotypes. In contrast, SOST expression in trabecular bone was decreased following OVX and significantly increased following ERT. Interestingly, a number of miRNAs that are predicted to target sclerostin exhibited inverse expression levels in response to OVX and ERT. These data implicate important roles for TIEG and estrogen-regulated miRNAs in modulating SOST expression in bone.

Sclerostin antibody treatment rescues the osteopenic bone phenotype of TGFβ inducible early gene‐1 knockout female mice

“…Further, estrogen replacement therapy results in reduced circulating sclerostin (Modder et al, 2011). Similarly, TIEG KO mice have elevated sclerostin levels (Subramaniam et al, 2018) and exhibit a female‐specific low bone mass phenotype (Bensamoun et al, 2006a; Subramaniam et al, 2018), implicating the role of sex steroids in regulating bone mass in the TIEG KO mouse model. Previous work has demonstrated that TIEG expression is regulated by estrogen (Hawse et al, 2008b) and that loss of this gene attenuates the impact of OVX and estrogen replacement therapy (Hawse et al, 2014).…”

“…Elevated circulating levels of sclerostin protein were also found in the serum of TIEG KO animals (Subramaniam et al, 2018). Further, TIEG was shown to directly suppress the activity of the SOST promoter (Subramaniam et al, 2018).…”

“…Loss of TIEG expression was also shown to impair nuclear localization of β‐catenin resulting in suppression of canonical Wnt pathway activity (Subramaniam et al, 2017a). Furthermore, increased expression of sclerostin messenger RNA was detected by RNAseq and RT‐PCR in the cortical shells of long bones isolated from female TIEG KO mice compared with WT littermates (Subramaniam, Pitel, Bruinsma, Monroe, & Hawse, 2018). Elevated circulating levels of sclerostin protein were also found in the serum of TIEG KO animals (Subramaniam et al, 2018).…”

“…Furthermore, increased expression of sclerostin messenger RNA was detected by RNAseq and RT‐PCR in the cortical shells of long bones isolated from female TIEG KO mice compared with WT littermates (Subramaniam, Pitel, Bruinsma, Monroe, & Hawse, 2018). Elevated circulating levels of sclerostin protein were also found in the serum of TIEG KO animals (Subramaniam et al, 2018). Further, TIEG was shown to directly suppress the activity of the SOST promoter (Subramaniam et al, 2018).…”

“…Given that female TIEG KO mice exhibit a low bone mass phenotype (Bensamoun et al, 2006a; Hawse et al, 2008) and have elevated levels of sclerostin (Subramaniam et al, 2018), we anticipated that Scl‐Ab treatment would positively influence bone mass in this model. In this study, we provide evidence that Scl‐Ab therapy increases bone mass in female TIEG KO as well as WT animals.…”