Endocrine Abstracts

Introduction: White adipose tissue (WAT) and brown adipose tissue (BAT) are key regulators of systemic metabolic function. Bone marrow adipose tissue (BMAT) accounts for 10% of total adipose mass in healthy humans and therefore can be considered a third major adipose subtype. However, it is unclear if BMAT contributes to systemic energy homeostasis.

Objectives: Assess [¹⁸F]-Fluorodeoxyglucose (FDG) uptake into bone and the marrow cavity (MC) following: i) insulin treatment in mice, ii) acute and chronic cold (CC) exposure in mice, or iii) conditions of BAT activation in humans.

Methods: Objective 1: mice were fasted for 4h and then treated with insulin (0.75 IU/g) or saline (0.9%), then immediately with [¹⁸F]-FDG, and housed at room temperature (RT) for 1h before scanning. Objective 2: prior to [¹⁸F]-FDG administration mice were fasted for 4 h at RT (control) or 4 °C (acute or CC), with CC mice further housed at 4 °C for 72 h before fasting. Objective 3: human subjects were exposed to mild cold (16 °C) for 2 h before [¹⁸F]-FDG PET/CT scanning in i) participants who had received no medication or ii) three doses of prednisolone or placebo prior to attendance. All scans were analysed using PMOD software and measured activities of target tissues expressed as standard uptake values (SUV).

Results: The marrow cavity (MC) is the predominant site of [¹⁸F]-FDG skeletal uptake. Insulin stimulated [¹⁸F]-FDG uptake in the femur and the heart, as previously reported, but did not affect [¹⁸F]-FDG uptake in the bone or MC at other skeletal sites. Despite suggestions that MAT is BAT-like, we found that neither bone nor the MC was cold-responsive during acute cold exposure. However, CC exposure profoundly increased [¹⁸F]-FDG uptake in many of the tissues analysed. In cold-exposed humans, [¹⁸F]-FDG uptake in the MC of the humerus and clavicle was very high, occurring at 10 and 28% of the level in BAT. Despite acutely increasing BAT activity, glucocorticoids decreased [¹⁸F]-FDG uptake into the bone and had no effect of [¹⁸F]-FDG uptake into the MC.

Conclusion: Glucose uptake within the MC does not respond to insulin or to acute activators of BAT. However, the MC is significant site of basal glucose uptake in humans and mice, and contributes to increased glucose uptake following CC exposure. Thus, BMAT might play a role in systemic glucose clearance and thereby influence metabolic homeostasis.