Marrow fat metabolism is linked to the systemic energy metabolism

Abstract

Recent advances in understanding the role of bone in the systemic regulation of energy metabolism indicate that bone marrow cells, adipocytes and osteoblasts, are involved in this process. Marrow adipocytes store significant quantities of fat and produce adipokines, leptin and adiponectin, which are known for their role in the regulation of energy metabolism, whereas osteoblasts produce osteocalcin, a bone-specific hormone that has a potential to regulate insulin production in the pancreas and adiponectin production in fat tissue. Both osteoblasts and marrow adipocytes express insulin receptor and respond to insulin-sensitizing anti-diabetic TZDs in a manner, which tightly links bone with the energy metabolism system. Metabolic profile of marrow fat resembles that of both, white and brown fat, which is reflected by its plasticity in acquiring different functions including maintenance of bone micro-environment. Marrow fat responds to physiologic and pathologic changes in energy metabolism status by changing volume and metabolic activity. This review summarizes available information on the metabolic function of marrow fat and provides hypothesis that this fat depot may acquire multiple roles depending on the local and perhaps systemic demands. These functions may include a role in bone energy maintenance and endocrine activities to serve osteogenesis during bone remodeling and bone healing.

Figure 1

Contribution of factors regulating systemic energy metabolism on bone cell development and bone remodeling. The role of β-adrenergic signaling on osteoblast and osteoclasts development, and metabolic function of adipocytes highlighted by bold letters and referenced. Abbreviations: FFA – free fatty acids, PGJ2 – prostaglandin J2, GC – glucocorticoids.

Figure 2

mCT renderings of murine femur (A) and tibia (B) representing longitudinal cross section of mineralized bone tissue (left panel) and fat in the same bone after decalcification and staining with osmium tetroxide (right panel).