The discovery of genes that are involved in mechanically stimulated bone formation as well as disuse induced bone loss will be critical for discovering the underlying mechanisms of Wolff's Law. In vitro
and in vivo
studies have been conducted to screen or evaluate genes responsive to mechanical loading and disuse/microgravity unloading [28
]. In this study we report the S14 gene, which plays an important role in lipogenesis, is up-regulated in the hindlimb disuse model, but has no response to anabolic mechanical stimulation.
It is well known that a decrease in bone volume associated with osteoporosis and age-related osteopenia is accompanied by increase in marrow adipose tissue [30
]. Indeed, an increase in marrow adipocytes is observed in conditions that lead to bone loss, such as ovariectomy [32
], immobilization [33
], or treatment with glucocorticoids [34
]. Our discovery of increased S14 gene expression in disuse rats is consistent with these observations. It is also known that obesity protects mammals from osteoporosis [17
]. Considering the important role S14 plays in lipogenesis, it is conceivable that it can play a significant role in this protective process. Recently it was reported that human spot 14 protein interacts with thyroid hormone receptor (TR) and suppresses the malic enzyme promoter activity enhanced by liganded TR [35
]. Considering the negative effects of thyroid hormone on bone remodeling [36
], it is reasonable to presume that S14 helps prevent bone resorption by reducing the negative effect of TR on the skeleton.
The mechanism that could account for the apparent reciprocal relationship between decreased bone density and increased fat formation is beginning to be understood. The bone marrow stroma is a complex system composed of mesenchymal cells (MSCs) which can replicate as undifferentiated cells, as well as differentiate into different lineages of mensenchymal tissues, including bone, cartilage, fat, muscle, and marrow stroma [38
]. Increasing evidence of transdifferentiation of these cells suggests a large degree of plasticity between osteoblasts and adipocytes [29
]. Although our result did not show significant reduction of S14 expression following anabolic mechanical stimulation, we could not eliminate the possibility that S14 may also play a role in the plasticity between osteoblastogenesis and adipogenesis. To support this speculation, we looked into the expression of S14 gene in fracture callus at different ages and time points using publicly available data (NCBI GEO GSE594) [42
]. S14 expression at early stages of fracture (3d, 1w, 2w) was reduced by at least 4-fold compared to intact (0d) and later stage fractures (4w, 6w).
It is unknown how disuse elevates the expression of S14. PPARγ (peroxisome proliferators-activated receptor γ) is considered as a master regulator of the adipocyte differentiation program and induces LXR (liver X receptor) [43
], which then stimulates SREBP-1 and S14 expression sequentially [45
]. It was previously reported that dietary PUFA (polyunsaturated fatty acids) suppresses fatty acid synthase transcription by decreasing nuclear SREBP-1 content and altering the binding of NF-Y to promoters of PUFA-response genes [48
]. Further, in vitro
experiments demonstrated that exposure of hMSC to 7d of modeled microgravity increases PPARγ expression [29
]. It is possible that during disuse (an in vivo
microgravity simulation), the PPARγ pathway can be activated and thus induce S14 expression. Thus, further investigations are needed to clearly define the molecular events involved in S14 expression and its involvement in the skeleton's adaptation to the absence of mechanical loading.