Exercise Decreases Marrow Adipose Tissue Through ß-Oxidation in Obese Running Mice

Abstract

The relationship between marrow adipose tissue (MAT) and bone health is poorly understood. We used running exercise to ask whether obesity-associated MAT can be attenuated via exercise and whether this correlates with gains in bone quantity and quality. C57BL/6 mice were divided into diet-induced obesity (DIO, n = 14) versus low-fat diet (LFD, n = 14). After 3 months, 16-week-old mice were allocated to an exercise intervention (LFD-E, DIO-E) or a control group (LFD, DIO) for 6 weeks (4 groups, n = 7/group). Marrow adipocyte area was 44% higher with obesity (p < 0.0001) and after exercise 33% lower in LFD (p < 0.0001) and 39% lower in DIO (p < 0.0001). In LFD, exercise did not affect adipocyte number; however, in DIO, the adipocyte number was 56% lower (p < 0.0001). MAT was 44% higher in DIO measured by osmium-μCT, whereas exercise associated with reduced MAT (-23% in LFD, -48% in DIO, p < 0.05). MAT was additionally quantified by 9.4TMRI, and correlated with osmium-µCT (r = 0.645; p < 0.01). Consistent with higher lipid beta oxidation, perilipin 3 (PLIN3) rose with exercise in tibial mRNA (+92% in LFD, +60% in DIO, p < 0.05). Tibial µCT-derived trabecular bone volume (BV/TV) was not influenced by DIO but responded to exercise with an increase of 19% (p < 0.001). DIO was associated with higher cortical periosteal and endosteal volumes of 15% (p = 0.012) and 35% (p < 0.01), respectively, but Ct.Ar/Tt.Ar was lower by 2.4% (p < 0.05). There was a trend for higher stiffness (N/m) in DIO, and exercise augmented this further. In conclusion, obesity associated with increases in marrow lipid-measured by osmium-μCT and MRI-and partially due to an increase in adipocyte size, suggesting increased lipid uptake into preexisting adipocytes. Exercise associated with smaller adipocytes and less bone lipid, likely invoking increased ß-oxidation and basal lipolysis as evidenced by higher levels of PLIN3. © 2017 American Society for Bone and Mineral Research.

Keywords: BONE; EXERCISE; MARROW ADIPOSE TISSUE; MEDICAL IMAGE ANALYSIS; OBESITY.

Fig. 1

Exercise reduces fat mass. C57BL/6 mice were fed either a control (LFD) or high-fat diet (DIO). After 3 months on each diet, 16-week-old mice were divided into runners (LFD-E, DIO-E) versus non-runners (LFD, DIO). (A) Final weights of each group. (B) Average running distance. (C) Total body fat mass and (D) lean mass as measured by MRI. (E) Weight of perigonadal fat pad. Results are expressed as mean±SEM. a, Significant main effect due to diet by 2- way ANOVA; b, significant main effect due to exercise by 2-way ANOVA. Interaction value was significant (p<0.01) for fat mass by 2-way ANOVA. Significance for between-group comparisons: *p<0.05; **p<0.01; ***p<0.001; ****p<0.0001.

Fig. 2

Exercise lowers MAT volume in both lean and obese mice. (A) Visualization of osmium stain by µCT in sagittal (left) and coronal (right) planes of the femur; n = 7/group, each image represents 7 images superimposed on each other. (B) Quantification of the bone volume and fat volume normalized to bone as measured by µCT. (C) Visualization of MAT measured by MRI; n = 6 per group, each image represents 6 images superimposed on each other. (D) Quantification of marrow fat volume by MRI normalized to bone volume. (E) Correlation of osmium-µCT quantification method of quantification to MRI method. Interaction values by 2-way ANOVA were non-significant. Between-group significance expressed per Fig. 1 legend.

Fig. 3

DIO increase in marrow adipocyte size is reversed by exercise. Adipocyte size was assessed in high-power field images of the femoral metaphysis. (A) Representative images for each group. (B) Number of adipocytes per square millimeter. (C, D) Area of adipocytes represented as both mean±SEM and as Gaussian fit histogram (LFD, n = 1101; LFD-E, n = 886; DIO, n = 563; DIO-E, n = 793 adipocytes); interaction value was solely significant (p<0.05) for adipocyte number by 2-way ANOVA. Between-group significance expressed per Fig. 1 legend.

Fig. 4

Exercise increases trabecular bone thickness more significantly in obese than lean mice. Bone microarchitecture parameters analyzed by µCT. (A) Trabecular bone parameters, including bone volume fraction (BV/TV), trabecular thickness (Tb.Th), trabecular number (Tb.N), and trabecular spacing (Tb. Sp), were measured in the tibial proximal metaphysis via µCT. (B) Cortical bone parameters, including marrow cavity area, cortical bone area fraction, cortical thickness, cortical porosity, endosteal volume, and periosteal volume, were measured in the tibial mid-diaphysis; interaction values by 2-way ANOVA were non-significant. Between-group significance expressed per Fig. 1 legend.

Fig. 5

Obesity-induced biomechanical changes are reversed with exercise. Biomechanical testing performed by µCT and 4-point bending. (A) Moment of inertia values (Ixx, Iyy, and polar moment of inertia) measured by µCT. (B) Bone elasticity (Young’s modulus) and stiffness as measured by 4-point bending; interaction values by 2-way ANOVA were non-significant. a, Significant main effect due to diet by 2-way ANOVA; b, significant main effect due to exercise by 2-way ANOVA; c, d trend due to diet and exercise, respectively. Between-group significance expressed per Fig. 1 legend.

Fig. 6

Marrow fat gene expression supports its use as a local energy depot. Whole tibia mRNA expression analysis was performed for white fat markers and the lipid droplet associated perilipin family (n = 6/ group). Interaction value was solely significant (p<0.05) for PLIN1 by 2- way ANOVA. a, Significant main effect due to diet by 2-way ANOVA; b, significant main effect due to exercise by 2-way ANOVA; ● trend; between-group significance expressed per Fig. 1 legend.

Fig. 7

Both obesity and exercise impact biomechanical properties of bone. Schematic of biomechanical and microarchitectural effects of diet and exercise on bone.