Meta-Analysis of Ocy-454 Showed Interrupted Osteocyte Maturation in Spaceflight Affects SOST Expression and Hypoxic Response

<b>Background/Objectives</b>: Changes in sclerostin expression regulated by SOST in osteocytes during spaceflight may be associated with bone loss; however, the underlying mechanisms remain unclear. The aim of this study was to clarify the relationship between SOST expression and bone loss by identifying the gene expression differences between osteocytes with high and low SOST expressions. <b>Methods</b>: We used the NASA GeneLab Database OSD-324, which is a microarray of data about the Ocy454 osteocytic cell line cultured for 2, 4, and 6 days during spaceflight, and the GSE102958 microarray in the Gene Expression Omnibus. We also analyzed the characteristics of SOST gene expression in osteocytes during spaceflight using merged datasets. <b>Results</b>: The findings of Gene Set Enrichment Analysis (GSEA) revealed that five gene sets related with H3K27me3 significantly upregulated with NES > 2.0 during spaceflight compared with ground controls. We also found 77 and 617 differentially expressed genes (DEGs) in flight 6d vs. low and high SOST expression, respectively. We used the transcriptional regulatory relationships unraveled by the sentence-based text-mining (TRRUST) database to determine the most significant upstream transcription factor (TF) of genes downregulated in osteocytes during spaceflight compared with those expressing abundant SOST. We detected that TF Sp7 is the most significant, with FDR < 0.01. Moreover, the GSEA findings indicated that the hypoxic pathway is prolonged in osteocytes during spaceflight compared to those at ground level. <b>Conclusions</b>: The gene expression profiles of osteocytes during spaceflight and in comparatively immature osteocytes with low SOST expression were similar. Furthermore, early osteocyte maturation inhibited by downregulated Sp7 during spaceflight extended the hypoxic response.