The only known homeostatic regulator of fat mass is the leptin system. We hypothesized that there is a second homeostat regulating body weight with an impact on fat mass. In this study we have added and removed weight loads from experimental animals and measured the effects on the biological body weight. The results demonstrate that there is a body weight homeostat that regulates fat mass independently of leptin. As the body weight-reducing effect of increased loading was dependent on osteocytes, we propose that there is a sensor for body weight in the long bones of the lower extremities acting as “body scales.” This is part of a body weight homeostat, “gravitostat,” that keeps body weight and body fat mass constant.

Subjects spending much time sitting have increased risk of obesity but the mechanism for the antiobesity effect of standing is unknown. We hypothesized that there is a homeostatic regulation of body weight. We demonstrate that increased loading of rodents, achieved using capsules with different weights implanted in the abdomen or s.c. on the back, reversibly decreases the biological body weight via reduced food intake. Importantly, loading relieves diet-induced obesity and improves glucose tolerance. The identified homeostat for body weight regulates body fat mass independently of fat-derived leptin, revealing two independent negative feedback systems for fat mass regulation. It is known that osteocytes can sense changes in bone strain. In this study, the body weight-reducing effect of increased loading was lost in mice depleted of osteocytes. We propose that increased body weight activates a sensor dependent on osteocytes of the weight-bearing bones. This induces an afferent signal, which reduces body weight. These findings demonstrate a leptin-independent body weight homeostat (“gravitostat”) that regulates fat mass.

Epidemiologic studies demonstrate that subjects spending much time sitting have increased risk of obesity, diabetes, and cardiovascular diseases. There is even epidemiologic evidence for an association between sitting time and overall mortality (1, 2). The mechanism for the antiobesity effect of standing is essentially unknown. It is probable that part of the effect of high sitting time on cardiometabolic phenotypes is caused by the associated low degree of exercise. However, the results of some articles demonstrate that the association of a sedentary behavior, as reflected by much sitting time, with the metabolic syndrome, is independent of physical activity (3, 4). We hypothesized that there is a homeostat (5) in the lower extremities regulating body weight with an impact on fat mass. Such a homeostat would (together with leptin) ensure sufficient whole body energy depots but still protect land-living animals from becoming too heavy. A prerequisite for such homeostatic regulation of body weight is that the integration center, which may be in the brain, receives afferent information from a body weight sensor. Thereafter, the integration center may adjust the body weight by acting on an effector (6).

Results

Leptin-Independent Body Weight Sensing for Fat Mass Homeostasis. A prerequisite for homeostatic feedback regulation of energy depots in body fat tissue (energy balance) is that energy regulating parts of the brain receive information from fat tissue about its size. The fat-derived hormone leptin, discovered by Friedman and coworkers over 20 y ago, is so far the only known such afferent homeostatic factor (7⇓–9). In the present study, we next investigated the interactions between increased loading, a stimuli normally reflecting increased body fat mass, and leptin. The loading-induced decrease in body weight was seen in leptin-deficient obese (Ob/Ob) mice (Fig. 2A), in the same way as in wild-type mice (Fig. 1B). In addition, the combined effect of increased loading and leptin treatment was studied in wild-type mice. Leptin was given to loaded and control mice on days 11–15 after implantation of capsules. It was found that leptin treatment suppressed body weight (Fig. 2B) and body fat (Fig. 2C) to a similar extent in loaded and control mice, while none of the treatments affected muscle mass (Fig. 2D). Thus, the loading-induced homeostatic regulation of body weight was independent of the well-established fat mass reducing effect of leptin, revealing two independent negative feedback systems for fat mass homeostasis. Fig. 2. Leptin-independent body weight sensing for fat mass homeostasis. (A) The effect of increased loading on the change in body weight in leptin deficient Ob/Ob mice (control n = 7 and load n = 10). The effect of combined loading and leptin treatment (1.5 µg/g BW twice daily) on (B) changes in biological body weight, (C) fat mass, and (D) muscle mass in mice (n = 10). Data are expressed as mean ± SEM *P < 0.05. Since the body weight reducing effect of increased loading was caused by reduced food intake, we analyzed the expression of appetite regulating genes in the hypothalamus. Increased loading augmented the expression of the obesity promoting neuropeptides AgRP and NPY (Fig. S1J). These two peptides are expressed by essentially the same neurons in the arcuate nucleus of the hypothalamus and their expression is suppressed by leptin. Therefore, the increase in AgRP and NPY expression is likely to be a failed compensatory mechanism induced by low fat mass and low serum leptin in the mice exposed to increased loading (Fig. 1 D and E and Fig. S1J), consistent with a leptin-independent mechanism for increased loading to reduce body weight.

The Suppression of Body Weight and Fat Mass by Loading Is Dependent on Osteocytes. It is known that osteocytes can sense dynamic short-term high-impact bone loading for local bone adaptation (10⇓–12). We therefore postulated that chronic static moderately increased bone loading, induced by increased body weight, also activates osteocytes, and thereby reduces fat mass via a systemic signal. To determine the role of osteocytes for the suppression of body weight by increased loading, we established an osteocyte depleted transgenic mouse model using diphtheria toxin-driven cell depletion specifically of DMP1 positive osteocytes (Fig. S2). The normal suppression of body weight by increased loading observed in mice with intact osteocytes (Fig. 3A) was lost in osteocyte-depleted mice (Fig. 3B). Increased loading decreased the weight of WAT (Fig. 3C) and serum leptin levels (Fig. 3D) in mice with intact osteocytes but not in osteocyte-depleted mice, while there was no significant differences in the skeletal muscle weight between the groups (Fig. 3E). These findings demonstrate that the suppression of body weight by loading is dependent on osteocytes. We propose that increased body weight activates a sensor dependent on the osteocytes of the weight-bearing bones. This induces an afferent signal to reduce food intake (Fig. 3F). Fig. 3. The suppression of body weight and fat mass by loading is dependent on osteocytes. Effect of increased loading on change in biological body weight in (A) control female mice with intact osteocytes (control n = 11 and load n = 12) and in (B) osteocyte-depleted (OCyD) female mice (n = 9). The effect of loading of control mice with intact osteocytes and OCyD mice on (C) fat mass, (D) serum leptin levels, and (E) muscle mass, as measured 21 d after initiation of loading. Data are expressed as mean ± SEM *P < 0.05. (F) Hypothesis for homeostatic regulation of body fat mass by two different signal systems. The first previously known pathway is fat-derived leptin in circulation acting on the brain to decrease food intake and fat mass. The second mechanism is that increased fat mass is counteracted by the body weight homeostat (gravitostat). Increased body weight activates a sensor dependent on the osteocytes of the weight-bearing bones. This induces an afferent signal to reduce food intake. A growing body of data indicates that the skeleton is an endocrine organ that regulates energy and glucose metabolism through, at least in part, the release of the bone-derived hormone osteocalcin (13, 14). We therefore hypothesized that the homeostatic regulation of body weight and fat mass by osteocytes in response to changes in body weight may be mediated by osteocalcin, or another known bone-derived circulating factor. To investigate this hypothesis, we determined the effect of increased loading for 6 d on the bone expression and circulating levels of four bone-derived candidate factors (sclerostin, osteocalcin, FGF23, and lipocalin 2) that might mediate this effect (Fig. S3). As previously shown for dynamic short-term high-impact loading, static moderately increased loading for 6 d reduced the bone expression of Sost (the gene coding for the bone mass suppressing factor sclerostin) (Fig. S3A). However, serum levels of sclerostin were not affected by increased loading (Fig. S3B). Although osteocalcin is a crucial regulator of energy metabolism in rodents, no effect of increased loading on the expression of osteocalcin in bone (Fig. S3C) or on circulating levels of total (Fig. S3D), carboxylated (Fig. S3E), or undercarboxylated (Fig. S3F) osteocalcin was observed. Furthermore, serum testosterone levels, known to be regulated by bone-derived osteocalcin and to regulate fat mass, were not affected by loading (15) (Fig. S3G). FGF23 is an osteocyte-derived endocrine acting factor. Increased loading did not significantly alter FGF23 expression in bone (Fig. S3H) or serum FGF23 levels (Fig. S3I). It was recently demonstrated that bone-derived lipocalin 2 suppresses appetite via a MC4R-dependent pathway (16). Increased loading did not significantly alter lipocalin 2 mRNA levels in bone (Fig. S3J) or serum lipocalin 2 levels in mice (Fig. S3K). These findings do not support that any of the four main bone-derived circulating candidate factors mediate the effect of the osteocyte-dependent body weight sensing mechanism.