Bone-Fat Interaction

SYNOPSIS

Marrow adipose tissue (MAT) is a recently identified endocrine organ capable of modulating a host of responses. Given its intimate proximity to the bone microenvironment, the impact marrow adipocytes exert on bone has attracted much interest and scientific inquiry. While many questions and controversies still remain relative to marrow adipocytes, multiple conditions/ disease states in which alterations occur in MAT have provided clues about their function. In general, the consensus in the field is that MAT is inversely associated with bone density and quality. Although further investigation is warranted, MAT has clearly been demonstrated as an active dynamic depot that contributes to bone turnover and overall metabolic homeostasis.

Introduction

Osteoporosis and low bone mass (i.e., osteopenia) are major public health concerns affecting a staggering 54 million in the U.S.¹ Moreover, as the nation’s demographic continues to shift towards an older population these statistics are projected to continue to rise.² Approximately 2 million osteoporotic-related fracture occur each year, costing the nation $17 billion per year.² In addition to the financial burden, osteoporosis-related fractures often lead to multiple comorbidities (i.e., hypertension, deficiency anemias, fluid and electrolyte imbalance)³, and patients frequently experience diminished quality of life due to immobility, pain, and isolation.⁴ While therapeutic treatment options have significantly aided in the management of osteoporosis, some patients still experience undesirable, adverse side-effects,^5-7 and therefore, continued development of refined options is necessary. As this quest continues, it is imperative to gain further insight in to the cellular and molecular responses occurring within the bone niche.

Bone is an incredibly dynamic tissue that undergoes continuous remodeling involving bone resorbing osteoclasts, bone forming osteoblasts, and mechanical sensing osteocytes. While much of bone biology has focused on these primary cell types, the bone marrow compartment also provides a unique environment in which communication between various cells can directly and indirectly impact the bone. One such cell population that has attracted much attention and scientific inquiry in the past decade are marrow adipocytes, often referred to as marrow adipose tissue (MAT) and/ or yellow adipose tissue (YAT). These adipocytes can be found interspersed throughout the marrow compartment. Recently, two “types” of MAT, constitutive MAT (cMAT) and regulated MAT (rMAT), have been described based on their (1) cellular morphology, (2) region specificity, and (3) fatty acid composition.⁸ Both in human and mouse tissues cMAT is described as containing large adipocytes localized at the distal tibia, and primarily composed of unsaturated lipids.⁸ Conversely, rMAT is mainly found in the proximal tibia, interspersed with active hematopoiesis, and composed of saturated lipids.⁸ Although our understanding of MAT has advanced significantly in the past decade, many questions still remain. It is therefore the aim of this review is to provide the most current opinions relative to MAT and bone, while providing a brief overview of clinical scenarios in which MAT is altered.

Current Controversies and Fundamental Questions

Lineage of Bone Marrow Adipocytes

Unlike peripheral adipocytes or white adipose tissue (WAT), which are primarily derived from mesenchymal stem cells (MSC) through vascular infiltration,^{9, 10} the definitive lineage of marrow adipocytes remains largely unknown and controversial. For example, while these cells have classic adipocyte functions and pathology by their hallmark ability to store lipids, bone marrow adipocytes express the osteoprogenitor marker osterix, encoded by the Sp7 gene.¹¹ Given the expression of Sp7, one theory, is that the development of marrow adipocytes results from a shift in allocation of MSCs from the osteoblast lineage towards the adipocyte lineage, subsequently decreasing bone formation.^12-14 Another possibility is that the marrow adipocyte could arise from bone lining cells, poorly characterized flat fibroblastic cells that express some markers of the osteogenic lineage (e.g. Sp7).

In addition to demonstrating features characteristic of WAT and Sp7, marrow adipocytes also exhibit some brown adipose tissue (BAT) transcriptional markers and target genes (i.e., Prdm16, FoxC2, Pgc1α, Dio2, β3AR, and Ucp1).¹⁵ Some literature also describes fibroblast adventitial reticular cells of the venous sinusoids accumulating lipids to “convert” to adipocytes. Under these circumstances, marrow adipocytes are presumed to primarily function as space-fillers in the marrow cavity for inactive or reduced numbers of hematopoietic cells,^{16, 17}. Another, more recent discovery completely shifts the idea that bone marrow adipocytes exclusively develop from MSCs pools and suggests they may arise from hematopoietic stem cells (HSCs). These data demonstrate that HSCs have the ability to hone to non-tissue resident fat depots, differentiate to adipocytes, and undergo de novo generation.^{9, 18} Moreover, the identification of differential bone marrow adipose depots (i.e., cMAT and rMAT) has given rise to the possibility that adipocytes within the marrow space are a heterogeneous population, derived from multiple sources. Nonetheless, the controversy surrounding the origin of bone marrow adipocytes underscores the complexity of these unique cells and further investigation is warranted.

Bone Marrow Adipocyte Function

Aside from the lineage tracing of marrow adipocytes, the next fundamental question that arises is that of MAT function. Our understanding of marrow adipocytes now extends well beyond their historical role as passive, “space-filling” support for the hematopoietic microenvironment. While marrow adipocytes have a defined function as regulators of hematopoietic activity,¹⁹ evidence also suggests MAT impacts systemic metabolism as well as bone turnover. Given their adipose pathology and biology, bone marrow adipocytes store fatty acids, and therefore, can impact global metabolism either by clearance of circulating fats and/or by their mobilization. Additionally, the recent identification of MAT as an endocrine organ capable of producing hormones such as adiponectin and leptin strongly suggests bone marrow adipocytes regulate systemic metabolism.^{20, 21} While the impact of marrow adipocytes on bone appears to be complex, evidence also indicates that an inverse relationship exists between MAT and skeletal mass (Table 1). As described in the previous section, one theory could involve the “see-saw” effect between osteoblasts and adipocytes, however, it is likely more complicated than proposed. The predominant localization of marrow adipocytes to the trabecular compartment of bone suggests their direct interaction facilitates bone turnover.⁸ Although many questions still remain, clinical scenarios in which MAT is altered has allowed investigators to gain further insight into how this novel organ impacts bone and fracture risk.

Table 1.

Relationship between marrow adiposity, bone mineral density, and fracture risk in various clinical conditions.

	Marrow Adiposity	Bone Mineral Density	Fracture Risk	References
Anorexia Nervosa	↑	↓	↑	22, 23, 25
Type 1 Diabetes Mellitus	Inconclusive	↓	↑	35, 42, 47
Type 2 Diabetes Mellitus	Inconclusive	No Change or ↑	↑	55, 77, 78
Aging	↑	↓	↑	1, 57, 79
Gonadal Deficiency	↑	↓	↑	61, 62
Unloading	↑	↓	↑	66, 68

Clinical Scenarios of Altered MAT and their Bone Phenotype

Anorexia Nervosa

Anorexia nervosa is a prevalent psychiatric disorder characterized by extreme self-imposed starvation as well as the subsequent weight loss and depletion of energy stores. A striking health consequence of anorexia nervosa is an ~7-fold increase in fracture risk which is predominantly due to significant bone loss and decreased bone turnover.^22-24 Another, somewhat paradoxical feature of anorexia nervosa is that despite the lack of peripheral fat, the bone medullary space experiences a dramatic but reversible increase in MAT.^{25, 26} Histological and pathological evaluation of the bone marrow cavity is characterized by hypoplasia accompanied by the accumulation of adipocytes and a pink gelatinous material.²⁷ This gelatinous material tends to surround adipocytes and is thought to be a result of fat atrophy during severe starvation. The progression of these changes within the marrow space appears to primarily depend on body weight loss or weight gain with treatment.²⁸ As a cautionary note, serous changes occurring in the bone marrow of patients with anorexia nervosa can often mask stress fracture during routine MRI.²⁹ Anorexia nervosa patients also experience decreases in serum leptin,^{30, 31} insulin-like growth factor (IGF)-1³¹ as well as increases in adiponectin²⁰ and preadipocyte factor (Pref)-1³². Whether the relationship of these various biomarkers is directly associated with MAT and their precise mechanisms of action remain to be further elucidated in patients with anorexia nervosa, however, they all have documented effects on bone metabolism. Furthermore, some of the bone loss, and the excess marrow adiposity is reversible, when eating resumes normally and weight is restored.

Type 1 Diabetes Mellitus

Type 1 diabetes mellitus (T1DM) is another condition which patients experience dramatic bone loss and increased risk of fracture,^33-35 however, the MAT phenotype appears to be slightly more ambiguous. Mouse models of T1DM (i.e., streptozotocin or STZ-induced) have been characterized as an appropriate model based on their (1) ablation of β-cells and subsequent attenuation of insulin production; (2) decreased body weight; (3) hyperglycemia; and (4) significant bone loss.³⁶ Additionally, these animal models have consistently demonstrated increased marrow adiposity.^36-40 Despite these observations in animal models, assessment of MAT in T1DM patients has yielded less impressive results. For example, T1DM patients with severe sensory polyneuropathy revealed a slight shift in T1 waves of routine MRI, indicative of marrow fat, in the tibia compared to matched controls.⁴¹ However, the author’s noted that while this signaling shift was significant it was not overtly abnormal.⁴¹ Somewhat consistent with this finding, Slade et al.,⁴² reported no differences in marrow adiposity from any site tested (e.g., vertebrae, femur epiphysis, femur metaphysis, and tibia metaphysis) between control and T1DM patients. An unexpected and key finding to this study was that serum lipids (e.g., cholesterol, cholesterol/HDL ratio, LDL, and triglycerides) demonstrated a strong relationship with marrow adiposity, not duration of T1DM or HbA_1C.⁴² Taken together, these studies reveal that further clinical studies are warranted to fully understand whether MAT is altered clinically during T1DM.

Obesity and Type 2 Diabetes Mellitus

One of the most striking health consequences related to the prevalence of obesity (body mass index or BMI ≥30 kg/m² in adults) has been the staggering increase in cases of type 2 diabetes (T2DM), and while not all type 2 diabetics are overweight or obese, the majority of the cases occur in this population.⁴³ Understanding the bone phenotype in obese individuals and patients with T2DM has been extremely complex and outside of the scope of the current review, however, the current stance within the field is that type 2 diabetics experience an increased risk of fracture, independent of BMD.^44-52 In one study it appears that visceral fat in otherwise healthy women correlated with vertebral marrow adiposity, however bone density was not reported.⁵³ In addition to marrow visceral fat, bone marrow fat content was also shown to be associated with intramyocellular and intrahepatic lipids, as well as serum cholesterol and triglycerides, both of which are elevated during obesity.⁵⁴ One logical explanation of increased MAT during obesity would be that cells within the marrow compartment are exposed to or come in contact with more fatty acids, and therefore, readily accumulate and store these substrates. Given the intimate relationship between obesity and T2DM, similar changes in MAT have also been documented. Interestingly, one study noted that while total lipids were not different between type 2 diabetics and control subjects, patients with T2DM had higher saturated fat in the marrow compartment.⁵⁵ Moreover, T2DM patients with previous fracture had the highest saturated fatty acids and lowest unsaturated fatty acids.⁵⁵ These data suggest that bone marrow fat composition may serve as a novel tool to assess fracture risk within T2DM patients. It is also interesting to note that weight-loss from roux-un-Y gastric bypass reduces MAT after 6 months only in diabetic patients and not non-diabetic group.⁵⁶ Although the implications of obesity and T2DM on bone health remains somewhat controversial, these data further implicate the role of MAT as a regulator of systemic metabolism.

Aging and Gonadal-Deficiency

Age-related changes in bone (i.e., decreased BMD, increased risk of fracture) are often thought of as the most classic alterations occurring in the skeleton. Aside from affecting an enormous portion of the population, age-related and gonadal deficiency osteoporosis have a strong historical presence in the bone literature In 2000, Schellinger and colleagues⁵⁷ used proton MR to determine that fat content within the vertebra (L2) increased with age, and, interestingly, men had higher fat content than women. These results were independently corroborated shortly thereafter.⁵⁸ In addition to the age-associated increase in bone marrow adipocytes and decrease in BMD, bone formation rates have also been inversely correlated to MAT.⁵⁹ These data provide evidence that as the adipocyte portion increased within the bone marrow cavity, osteoblast activity decreased. Moreover, the composition of marrow fat in age-related osteoporosis appears to be preferentially composed of unsaturated fatty acids.⁶⁰

As aging progresses in women, gonadal deficiency or menopause is a natural physiological consequence. As such, the inverse relationship between BMD and MAT also exists in postmenopausal women.⁶¹ The changes in marrow adiposity are most evident in the axial skeleton and it has been reported that the increase in MAT can occur quickly after withdrawal of estrogen or decrease rapidly in response to exogenous estrogen.⁶² Furthermore, the fatty acid composition of MAT in postmenopausal women also follows a similar profile demonstrating lower saturated fatty acids and increased monounsaturated fatty acids, particularly in participants with previous fracture.⁶³ This is particularly interesting given the opposite observation in T2DM. It is also important to note that the treatment of postmenopausal osteoporosis with the bisphosphonates risendronate and zoledronate not only significantly reduces the risk of fracture, but can also decrease MAT.^{64, 65} Collectively, these data clearly demonstrate that an inverse relationship exists between bone density and MAT in age-related and postmenopausal osteoporosis.

Disuse and Unloading of the Skeleton

Mechanical loading of the skeleton is crucial to overall bone health and quality. This is namely demonstrated in the severe loss of bone experienced by astronauts and cosmonauts during spaceflight as well as in bedridden patients.⁶⁶ Likewise, loading of the skeleton by gravity and weight-bearing exercises has shown to increase BMD and decrease fracture risk.⁶⁷ While clinical data remains relatively scarce due to the uniqueness and vulnerability of the primary populations affected by disuse, Trudel et al.,⁶⁸ demonstrated that 60 days of bedrest increased fat accumulation in vertebral bone marrow. The authors go on to describe that the elevated MAT remained even after a year of recovery (reambulation or normal physical activity).⁶⁸ It should also be noted this increase in bone marrow adiposity has also been documented in rodent models of disuse (i.e., hindlimb suspension)⁶⁹ and during microgravity/ spaceflight. Additionally, when mechanical stress or loading is introduced to bone in the form of exercise, it appears to decrease MAT while increasing BMD.^{70, 71} While more research is needed to fully elucidate how mechanical stress, or the lack thereof, effect MAT, the decrease of BMD appears to be accompanied by an increase in marrow adiposity.

Other Clinical Scenarios

The most extensively studied clinical diseases and conditions that alterations in have been studied MAT are outlined above, however, other research alludes to unique scenarios in the clinic that MAT is also altered. For example, bone loss and increased fracture risk is associated with alcoholism, it has also been demonstrated in rodent models that alcohol consumption dramatically increases vertebral fat accumulation.⁷² Treatment options for various conditions and disease states can also impact marrow adiposity and bone. One such example of this is from radiation exposure used to treat various cancers. Exposure of the bone marrow compartment to radiation is sometimes targeted as in Hodgkin’s lymphoma, but can also occur secondary due to the proximity of the targeted area, as in many pelvic cancers. None-the-less, radiation treatment causes direct damage to red and white blood cells often eradicating blood-related cancers, however, the repopulation of the marrow compartment is accomplished by adipocyte replacement.^73-75 Perhaps consequently to this massive expansion of MAT, the risk fracture to the irradiated area is often 3-times that of non-irradiated populations.⁷⁶ While this is by no means an all-inclusive list of clinical scenarios in which MAT is altered, it is imperative to continue to collect this data to gain further insight into how this novel tissue can impact multiple disease states. The advent of techniques such as MRI spectroscopy in larger clinical trials is likely to provide greater insight into the pathophysiology of MAT.

Future Considerations/ Summary

Marrow adipose tissue is an active dynamic depot that contributes to overall metabolic homeostasis. Understanding both its function and origin will go a long way in terms of determining how marrow adipocytes sense energy needs in the organism and how these cells respond to environmental and nutrient stress. Importantly, defining if these adipocytes are unique in their fatty acid transport and in their adipokine secretion should help to clarify the role of MAT in bone acquisition and maintenance.

KEY POINTS.

• In general, an inverse relationship exists between marrow fat and bone density.

• Multiple diseases associated with increased fracture risk also present with increased marrow adipose tissue.

• Composition of marrow adipose tissue differs between anatomical sites.

• Exact stem cell lineage and precise function of marrow adipocytes remains controversial.