Bone marrow adipose tissue is an endocrine organ that contributes to increased circulating adiponectin during caloric restriction

Abstract

The adipocyte-derived hormone adiponectin promotes metabolic and cardiovascular health. Circulating adiponectin increases in lean states such as caloric restriction (CR), but the reasons for this paradox remain unclear. Unlike white adipose tissue (WAT), bone marrow adipose tissue (MAT) increases during CR, and both MAT and serum adiponectin increase in many other clinical conditions. Thus, we investigated whether MAT contributes to circulating adiponectin. We find that adiponectin secretion is greater from MAT than WAT. Notably, specific inhibition of MAT formation in mice results in decreased circulating adiponectin during CR despite unaltered adiponectin expression in WAT. Inhibiting MAT formation also alters skeletal muscle adaptation to CR, suggesting that MAT exerts systemic effects. Finally, we reveal that both MAT and serum adiponectin increase during cancer therapy in humans. These observations identify MAT as an endocrine organ that contributes significantly to increased serum adiponectin during CR and perhaps in other adverse states.

Figure 1. Relative expression and secretion of adiponectin is greater from MAT than from WAT

(A,B) iWAT, gWAT, pWAT, lumbar vertebrae (LV; for red marrow) and caudal vertebrae (CV; for MAT) were isolated from male C57BL/6J mice. (A) Micrographs of H&E-stained tissue sections. (B) Immunoblots of total protein lysates from tissues of three mice; ERK1/2 is a loading control. Phosphorylated forms of HSL and Perilipin A appear as multiple bands. (C-F) WAT, red marrow (RM) and MAT were isolated from New Zealand White rabbits. (C) Image of a bisected tibia showing the typical distribution of MAT. (D) Micrographs of H&E-stained tissue sections. (E) Immunoblots of total protein lysates from two rabbits, representative of four rabbits; α-tubulin is a loading control. (F) Immunoblots and silver stain of conditioned media from WAT and MAT explants from one rabbit, representative of seven rabbits. (G-I) Tibial MAT and scWAT were isolated from patients undergoing lower limb amputation. (G) Micrographs of H&E-stained tissue sections. (H) Immunoblots of total protein lysates of each tissue; -tubulin is a loading control. (I) Immunoblots and silver stain of conditioned media from explants of scWAT and MAT from patient 3. Lamin A/C was analyzed as an estimate of explant breakdown. Similar results were obtained for explants from patients 1 and 2 (Figure S1). In (F) and (I), silver staining was used to assess total protein content. Images in (A-B) and (C-D) are representative of ten mice or rabbits. For micrographs, scale bars = 200 μm. See also Figure S1.

Figure 2. Blocking MAT expansion directly prevents increased circulating adiponectin during CR

WT and Ocn-Wnt10b mice were fed ad libitum or a 30% CR diet from 9-15 weeks of age. (A) Masses of iWAT, gWAT and BAT in 15-week-old mice. (B,C) BM adiposity was assessed by osmium tetroxide staining of tibiae followed by μCT analysis. (B) Representative images of stained tibiae; scale bar = 1 mm. (C) MAT as % marrow volume (MV) was determined from μCT scans. (D) Analysis of HMW, MMW, LMW and total adiponectin in sera of 15-week-old mice. Immunoblots are from the same exposure of film. (E) Quantitation of serum adiponectin from (D). (F,G) qPCR analysis of Adipoq expression in iWAT, gWAT, or combined tibiae and femurs (Tib/Fem). (H-J) Total RNA and protein was isolated from quadriceps muscle. Expression of Pgc1a, Tfam and Acadm was determined by qPCR (H). Protein phosphorylation was determined by immunoblotting (I) and quantified by densitometry (J). CaMKII phosphorylation is a marker of Ca²⁺/calmodulin signaling. Data are mean ±SEM of the following numbers of mice: WT, n = 6; Wnt10b, n = 4; WT CR, n = 5; Wnt10b CR, n = 6. Similar results were observed in a second mouse cohort. For each diet group, significant differences between WT and Wnt10b mice are indicated by * (P <0.05), ** (P <0.01) or *** (P <0.001). Within each genotype, significant differences between ad libitum and CR diets are indicated by ## (P <0.01) or ### (P <0.001). See also Figure S2, Figure S3, and Table S1.

Figure 3. Both MAT and serum adiponectin increase during cancer therapy in humans

MAT, serum adiponectin and % body fat were assessed in patients undergoing radiotherapy for endometrial cancer or chemotherapy for ovarian cancer. (A) Representative MRI images of two patients before and at six months post-treatment. Arrows highlight increased vertebral MAT (signal fat fraction; sFF) post-treatment. (B) MAT was determined by water-fat MRI at the indicated time points. Data are mean ±SEM of 11-15 patients. (C) Total serum adiponectin concentrations were determined by ELISA and are mean ±SEM of 8-11 patients, with 11 patients assessed at baseline and 6 months, and 8 of these patients also assessed at 12 months. (D) Body fat %, as determined by DXA, shown as mean ±SEM of 11 patients. For (B-D), statistically significant differences between baseline and 6 or 12 months post-treatment are indicated by * (P < 0.05) or ** (P < 0.01).