C - Michigan Technological University

osteoblast activity. c-fos is a proto-oncogene in the AP-1 family of transcription factors. Itis essential in the regulation of bone development (for review see: [295]) and for theanabolic actions of PTH on bone [296, 297]. Its expression was expected to decrease inosteoblasts cultured in hibernating bear serum. Osteopontin (OPN), collagen I (ColI),and OCN are essential cell matrix proteins produced by osteoblasts during the formationof bone (for review see: [298]). These genes were expected to decrease in osteoblastscultured in hibernating bear serum, which would suggest that a serum-borne factor isresponsible for the decreased bone formation observed in hibernating bears [18, 20].The fourth hypothesis of this study was that expression of pro-resorptive geneswould decrease in osteoblasts cultured in serum from hibernating bears. Under catabolicstimulation (e.g. reduced mechanical loading), osteoblasts increase expression of thepro-resorptive cytokine receptor activator for NFκ-B ligand (RANKL) and decreaseexpression of its decoy receptor OPG. An increased RANKL to OPG ratio promotesosteoclast activation followed by the resorption of bone (for review see: [299]). Boneresorption is decreased during hibernation in bears [18, 20]; therefore it was expectedthat the expression of RANKL would decrease and OPG would increase in osteoblastscultured in serum from hibernating bears (compared to cells in active serum).Macrophage colony stimulating factor (M-CSF) is a pro-resorptive cytokine expressed bya variety of bone cells, including osteoblasts. It promotes the proliferation of osteoclastprecursors (for review see: [300]). Osteoblast expression of M-CSF was expected todecrease in cells cultured in serum from hibernating bears.Finally, the gene expressions of three cell-surface receptors were quantified inosteoblasts cultured in seasonal bear serum. The PTH receptor (PTH1R) was expectedto increase in osteoblasts cultured in hibernating bear serum. This would suggest that afactor in hibernating bear serum increases PTH signaling in osteoblasts even though thecirculating levels of PTH remain homeostatic in these serum samples (reported inChapter 4). An increase in PTH signaling prevents bone loss during disuse in rats [56],and may help protect hibernating bears from bone loss. ADRβ2, the adrenergic receptorwhich binds NE, was expected to decrease in osteoblasts cultured in the serum ofhibernating bears since beta-blockers inhibit bone loss due to disuse [217, 220]. Asobserved in Chapter 3, although there is no seasonal variation in serum NE of black92