Upregulation of osteogenic factors induced by high-impact jumping suppresses adipogenesis in marrow but not adipogenic transcription factors in rat tibiae

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Jump training is a high-impact training regimen that increases bone volume in young bones. The aim of our study was to determine whether downregulation of adipogenesis that is associated with upregulation of osteogenesis is detected after jump training in growing rat tibiae. Four week-old rats were jump-trained for 1, 2, or 4 weeks for 5 days/week, and the height of jumping progressively increased to 35 cm.We performed morphometry to directly quantitate changes in bone volume and marrow adipocyte distribution in tibiae after the jump training.We also examined changes in expression of osteogenic and adipogenic transcription factor proteins and mRNAs after the jump training. Four weeks of jump training induced an increase in trabecular bone volume, which was associated with recruitment of runt-related transcription factor 2-expressing cells, as well as a decrease in marrow fat volume. However, peroxisome proliferator-activated receptor (PPAR) γ2 protein and mRNA expression levels did not change following high-impact jump training. The mRNA expression levels of the adipocyte differentiation genes CCAAT/enhancer-binding proteins (C/EBP) α, C/EBPβ, and C/EBPδ also showed no change during the training period in jump-trained rats. We suggest that levels of osteogenic factors that were upregulated by mechanical loading from high-impact jumping suppress adipogenesis in marrow rather than adipogenic transcription factors.