Body mass influences cortical bone mass independent of leptin signaling

Abstract

Obesity in humans is associated with increased bone mass. Leptin, a hormone produced by fat cells, functions as a sentinel of energy balance, and may mediate the putative positive effects of body mass on bone. We performed studies in male C57Bl/6 wild type (WT) and leptin-deficient ob/ob mice to determine whether body mass gain induced by high fat intake increases bone mass and, if so, whether this requires central leptin signaling. The relationship between body mass and bone mass and architecture was evaluated in 9-week-old and 24-week-old WT mice fed a regular mouse diet. Femora and lumbar vertebrae were analyzed by micro computed tomography. In subsequent studies, slowly and rapidly growing ob/ob mice were injected in the hypothalamus with a recombinant adeno-associated virus containing the leptin gene (rAAV-lep) or a control vector, rAAV-GFP (green fluorescent protein). The mice were maintained on a regular control diet for 5 or 7 weeks and then subdivided into groups and either continued on the control diet or fed a high fat diet (45% of kcal from fat) for 8 weeks. In the WT mice, femoral and vertebral bone mass was positively correlated with body mass (Pearson's r=0.65-0.88 depending on endpoint). rAAV-lep therapy dramatically decreased body mass (-61%) but increased femur length. However, in the distal femur and lumbar vertebra, rAAV-lep therapy reduced cancellous bone volume/tissue volume, trabecular number and trabecular thickness, and increased trabecular spacing. The high fat diet increased body mass, irrespective of vector treatment. Total femur bone volume, length, cross-sectional volume, and cortical volume and thickness were increased in mice with increased body mass, independent of rAAV treatment. In the distal femur, increased body mass had no effect on cancellous architecture and there were no vector x body mass interactions. In WT mice, increased body mass resulted in increased (+33%) vertebral cancellous bone volume/tissue volume. Increased body mass had minimal independent effect on cancellous vertebral bone mass in ob/ob mice. Taken together, these findings suggest that increased body mass has a positive effect on femur cortical bone mass that is independent of leptin signaling.

Figure 1

Correlation between body mass and total femur bone volume (a and b) and body mass and total vertebra (LV3) bone volume (c and d) in 9 and 24-week-old male C57BL/6 mice, respectively, fed standard mouse chow (Experiment 1).

Figure 2

Effect of central leptin gene therapy and high fat diet on body mass in ob/ob mice administered either rAAV-GFP (control vector) or rAAV-lep icv (Experiment 2). All data are mean ± SE. NS, not significant, P>0.05.

Figure 3

Effect of central leptin gene therapy and increased body mass (BM) on femur length (a) total femur bone volume (b), midshaft femur cross-sectional volume (c), midshaft femur cortical volume (d), midshaft femur marrow volume (e), and midshaft femur cortical thickness (f) in ob/ob mice administered either rAAV-GFP (control vector) or rAAV-lep icv (Experiment 2). All data are mean ± SE. NS, not significant, P>0.05.

Figure 4

Effect of central leptin gene therapy and increased body mass (BM) on distal femur bone volume/tissue volume (a), trabecular number (b), trabecular thickness (c), and trabecular spacing (d) in ob/ob mice administered either rAAV-GFP (control vector) or rAAV-lep icv (Experiment 2). All data are mean ± SE. NS, not significant, P>0.05.

Figure 5

Effect of central leptin gene therapy and increased body mass (BM) on vertebral (LV3) bone volume/tissue volume (a), trabecular number (b), trabecular thickness (c), and trabecular spacing (d) in ob/ob mice administered either rAAV-GFP (control vector) or rAAV-lep icv (Experiment 2). All data are mean ± SE. *P<0.05. NS, not significant, P>0.05.