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

The role of adipokines in chronic inflammation

Adipose tissue has traditionally been defined as connective tissue that stores excess calories in the form of triacylglycerol. However, the physiologic functions attributed to adipose tissue are expanding, and it is now well established that adipose tissue is an endocrine gland. Among the endocrine factors elaborated by adipose tissue are the adipokines; hormones, similar in structure to cytokines, produced by adipose tissue in response to changes in adipocyte triacylglycerol storage and local and systemic inflammation. They inform the host regarding long-term energy storage and have a profound influence on reproductive function, blood pressure regulation, energy homeostasis, the immune response, and many other physiologic processes. The adipokines possess pro- and anti-inflammatory properties and play a critical role in integrating systemic metabolism with immune function. In calorie restriction and starvation, proinflammatory adipokines decline and anti-inflammatory adipokines increase, which informs the host of energy deficits and contributes to the suppression of immune function. In individuals with normal metabolic status, there is a balance of pro- and anti-inflammatory adipokines. This balance shifts to favor proinflammatory mediators as adipose tissue expands during the development of obesity. As a consequence, the proinflammatory status of adipose tissue contributes to a chronic low-grade state of inflammation and metabolic disorders associated with obesity. These disturbances are associated with an increased risk of metabolic disease, type 2 diabetes, cardiovascular disease, and many other pathological conditions. This review focuses on the impact of energy homeostasis on the adipokines in immune function.

A T Cell View of the Bone Marrow

The majority of T cells present in the bone marrow (BM) represent an activated/memory phenotype and most of these, if not all, are circulating T cells. Their lodging in the BM keeps them activated, turning the BM microenvironment into a “memory reservoir.” This article will focus on how T cell activation in the BM results in both direct and indirect effects on the hematopoiesis. The hematopoietic stem cell niche will be presented, with its main components and organization, along with the role played by T lymphocytes in basal and pathologic conditions and their effect on the bone remodeling process. Also discussed herein will be how “normal” bone mass peak is achieved only in the presence of an intact adaptive immune system, with T and B cells playing critical roles in this process. Our main hypothesis is that the partnership between T cells and cells of the BM microenvironment orchestrates numerous processes regulating immunity, hematopoiesis, and bone remodeling.

Lipodystrophy and severe metabolic dysfunction in mice with adipose tissue-specific insulin receptor ablation

OBJECTIVE

Insulin signaling plays pivotal roles in the development and metabolism of many tissues and cell types. A previous study demonstrated that ablation of insulin receptor (IR) with aP2-Cre markedly reduced adipose tissues mass and protected mice from obesity. However, multiple studies have demonstrated widespread non-adipocyte recombination of floxed alleles in aP2-Cre mice. These findings underscore the need to re-evaluate the role of IR in adipocyte and systemic metabolism with a more adipose tissue-specific Cre mouse line.

METHODS

We generated and phenotyped a new adipose tissue-specific IR mouse model using the adipose tissue-specific Adipoq-Cre line.

RESULTS

Here we show that the Adipoq-Cre-mediated IR KO in mice leads to lipodystrophy and metabolic dysfunction, which is in stark contrast to the previous study. In contrast to white adipocytes, absence of insulin signaling does not affect development of marrow and brown adipocytes, but instead is required for lipid accumulation particularly for the marrow adipocytes. Lipodystrophic IR KO mice have profound insulin resistance, hyperglycemia, organomegaly, and impaired adipokine secretion.

CONCLUSIONS

Our results demonstrate differential roles for insulin signaling for white, brown, and marrow adipocyte development and metabolic regulation.

Bone marrow fat and the decline of B lymphopoiesis in rabbits

B lymphopoiesis is necessary to generate a diverse pool of naïve B cells that are able to respond to a broad spectrum of antigens during immune responses to pathogens and to vaccination. Rabbits have been utilized for many years to generate high affinity monoclonal and polyclonal antibodies. Specific antibodies generated in rabbits have greatly advanced scientific discoveries, but the unique qualities of rabbit B cell development have been underappreciated. Unlike in humans and mice, where B lymphopoiesis declines in mid to late life, B lymphopoiesis in rabbits arrests early in life, between 2 and 4 months of age. This review focuses on the early loss of B cell development in rabbits and the contribution of the bone marrow microenvironment to this process. We also propose directions for future research in this area, and discuss how the rabbit can be used as a model to understand the decline of B lymphopoiesis that occurs in humans late in life. Such studies will be important for developing therapeutics targeted to prevent and/or reverse declining B lymphopoiesis in the elderly, as well as boosting immunity and antibody responses after infection or vaccination.

The role of bone marrow adipocytes in bone metastasis

Adipocytes are a significant component of the bone marrow microenvironment. Although bone marrow adipocytes were first identified more than 100 years ago, it is only in recent years that an understanding of their complex physiological role is emerging. Bone marrow adipocytes act as local regulators of skeletal biology and homeostasis, with recent studies suggesting that marrow adipose tissue is metabolically active, and can function as an endocrine organ. As such, bone marrow adipocytes have the potential to interact with tumour cells, influencing both tumour growth and bone disease. This review discusses the current evidence for the role of bone marrow adipocytes in tumour growth within the bone marrow microenvironment and the development of the associated bone disease.

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Bone marrow adipocytes are a metabolically active source of lipids and adipokines.

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Marrow adipocytes increase with age, but their role in bone metastasis is ill-defined.

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Marrow adipocytes have tumour-promoting and -suppressive effects in bone metastasis.

Bone marrow adipose tissue: formation, function and regulation

The human body requires an uninterrupted supply of energy to maintain metabolic homeostasis and energy balance. To sustain energy balance, excess consumed calories are stored as glycogen, triglycerides and protein, allowing the body to continue to function in states of starvation and increased energy expenditure. Adipose tissue provides the largest natural store of excess calories as triglycerides and plays an important role as an endocrine organ in energy homeostasis and beyond. This short review is intended to detail the current knowledge of the formation and role of bone marrow adipose tissue (MAT), a largely ignored adipose depot, focussing on the role of MAT as an endocrine organ and highlighting the pharmacological agents that regulate MAT.

Acute knockdown of the insulin receptor or its substrates Irs1 and 2 in 3T3-L1 adipocytes suppresses adiponectin production

Loss of function of the insulin receptor (INSR) in humans produces severe insulin resistance. Unlike "common" insulin resistance, this is associated with elevated plasma levels of the insulin-sensitising, adipose-derived protein adiponectin. The underlying mechanism for this paradox is unclear, and it is at odds with the acute stimulation of adiponectin secretion reported on insulin treatment of cultured adipocytes. Given recent evidence for ligand-independent actions of the INSR, we used a lentiviral system to knock down Insr or its substrates Irs1 and Irs2 conditionally in 3T3-L1 murine preadipocytes/adipocytes to assess whether acute loss of their expression has different consequences to withdrawal of insulin. Efficient knockdown of either Insr or Irs1/2 was achieved by conditional shRNA expression, severely attenuating insulin-stimulated AKT phosphorylation and glucose uptake. Dual knockdown of Irs1 and Irs2 but not Insr in preadipocytes impaired differentiation to adipocytes. Acute knockdown of Insr or both Irs1 and Irs2 in adipocytes increased Adipoq mRNA expression but reduced adiponectin secretion, assessed by immunoassay. Knockdown sustained for 14 days also reduced immunoassay-detected adiponectin secretion, and moreover induced delipidation of the cells. These findings argue against a distinct effect of Insr deficiency to promote adiponectin secretion as the explanation for paradoxical insulin receptoropathy-related hyperadiponectinaemia.

Expansion of Bone Marrow Adipose Tissue During Caloric Restriction Is Associated With Increased Circulating Glucocorticoids and Not With Hypoleptinemia

Bone marrow adipose tissue (MAT) accounts for up to 70% of bone marrow volume in healthy adults and increases further in clinical conditions of altered skeletal or metabolic function. Perhaps most strikingly, and in stark contrast to white adipose tissue, MAT has been found to increase during caloric restriction (CR) in humans and many other species. Hypoleptinemia may drive MAT expansion during CR but this has not been demonstrated conclusively. Indeed, MAT formation and function are poorly understood; hence, the physiological and pathological roles of MAT remain elusive. We recently revealed that MAT contributes to hyperadiponectinemia and systemic adaptations to CR. To further these observations, we have now performed CR studies in rabbits to determine whether CR affects adiponectin production by MAT. Moderate or extensive CR decreased bone mass, white adipose tissue mass, and circulating leptin but, surprisingly, did not cause hyperadiponectinemia or MAT expansion. Although this unexpected finding limited our subsequent MAT characterization, it demonstrates that during CR, bone loss can occur independently of MAT expansion; increased MAT may be required for hyperadiponectinemia; and hypoleptinemia is not sufficient for MAT expansion. We further investigated this relationship in mice. In females, CR increased MAT without decreasing circulating leptin, suggesting that hypoleptinemia is also not necessary for MAT expansion. Finally, circulating glucocorticoids increased during CR in mice but not rabbits, suggesting that glucocorticoids might drive MAT expansion during CR. These observations provide insights into the causes and consequences of CR-associated MAT expansion, knowledge with potential relevance to health and disease.

Increased Bone Marrow Adiposity in a Context of Energy Deficit: The Tip of the Iceberg?

Elevated bone marrow adiposity (BMA) is defined as an increase in the proportion of the bone marrow (BM) cavity volume occupied by adipocytes. This can be caused by an increase in the size and/or number of adipocytes. BMA increases with age in a bone-site-specific manner. This increase may be linked to certain pathophysiological situations. Osteoporosis or compromised bone quality is frequently associated with high BMA. The involvement of BM adipocytes in bone loss may be due to commitment of mesenchymal stem cells to the adipogenic pathway rather than the osteogenic pathway. However, adipocytes may also act on their microenvironment by secreting factors with harmful effects for the bone health. Here, we review evidence that in a context of energy deficit (such as anorexia nervosa (AN) and restriction rodent models) bone alterations can occur in the absence of an increase in BMA. In severe cases, bone alterations are even associated with gelatinous BM transformation. The relationship between BMA and energy deficit and the potential regulators of this adiposity in this context are also discussed. On the basis of clinical studies and preliminary results on animal model, we propose that competition between differentiation into osteoblasts and differentiation into adipocytes might trigger bone loss at least in moderate-to-severe AN and in some calorie restriction models. Finally, some of the main questions resulting from this hypothesis are discussed.

New 3D-Culture Approaches to Study Interactions of Bone Marrow Adipocytes with Metastatic Prostate Cancer Cells

Adipocytes are a major component of the bone marrow that can critically affect metastatic progression in bone. Understanding how the marrow fat cells influence growth, behavior, and survival of tumor cells requires utilization of in vitro cell systems that can closely mimic the physiological microenvironment. Herein, we present two new three-dimensional (3D) culture approaches to study adipocyte-tumor cell interactions in vitro. The first is a transwell-based system composed of the marrow-derived adipocytes in 3D collagen I gels and reconstituted basement membrane-overlayed prostate tumor cell spheroids. Tumor cells cultured under these 3D conditions are continuously exposed to adipocyte-derived factors, and their response can be evaluated by morphological and immunohistochemical analyses. We show via immunofluorescence analysis of metabolism-associated proteins that under 3D conditions tumor cells have significantly different metabolic response to adipocytes than tumor cells grown in 2D culture. We also demonstrate that this model allows for incorporation of other cell types, such as bone marrow macrophages, and utilization of dye-quenched collagen substrates for examination of proteolysis-driven responses to adipocyte- and macrophage-derived factors. Our second 3D culture system is designed to study tumor cell invasion toward the adipocytes and the consequent interaction between the two cell types. In this model, marrow adipocytes are separated from the fluorescently labeled tumor cells by a layer of collagen I. At designated time points, adipocytes are stained with BODIPY and confocal z-stacks are taken through the depth of the entire culture to determine the distance traveled between the two cell types over time. We demonstrate that this system can be utilized to study effects of candidate factors on tumor invasion toward the adipocytes. We also show that immunohistochemical analyses can be performed to evaluate the impact of direct interaction of prostate tumor cells with adipocytes. Our models underline the importance of using the appropriate culture conditions to mimic physiological interactions between marrow adipocytes and metastatic tumor cells. These systems have a potential to be utilized for analyses of various factors that may be regulated by the adipocytes in bone. Their application likely extends beyond metastatic prostate cancer to other tumors that colonize the bone marrow microenvironment.

Exercise Regulation of Marrow Adipose Tissue

Despite association with low bone density and skeletal fractures, marrow adipose tissue (MAT) remains poorly understood. The marrow adipocyte originates from the mesenchymal stem cell (MSC) pool that also gives rise to osteoblasts, chondrocytes, and myocytes, among other cell types. To date, the presence of MAT has been attributed to preferential biasing of MSC into the adipocyte rather than osteoblast lineage, thus negatively impacting bone formation. Here, we focus on understanding the physiology of MAT in the setting of exercise, dietary interventions, and pharmacologic agents that alter fat metabolism. The beneficial effect of exercise on musculoskeletal strength is known: exercise induces bone formation, encourages growth of skeletally supportive tissues, inhibits bone resorption, and alters skeletal architecture through direct and indirect effects on a multiplicity of cells involved in skeletal adaptation. MAT is less well studied due to the lack of reproducible quantification techniques. In recent work, osmium-based 3D quantification shows a robust response of MAT to both dietary and exercise intervention in that MAT is elevated in response to high-fat diet and can be suppressed following daily exercise. Exercise-induced bone formation correlates with suppression of MAT, such that exercise effects might be due to either calorie expenditure from this depot or from mechanical biasing of MSC lineage away from fat and toward bone, or a combination thereof. Following treatment with the anti-diabetes drug rosiglitazone - a PPARγ-agonist known to increase MAT and fracture risk - mice demonstrate a fivefold higher femur MAT volume compared to the controls. In addition to preventing MAT accumulation in control mice, exercise intervention significantly lowers MAT accumulation in rosiglitazone-treated mice. Importantly, exercise induction of trabecular bone volume is unhindered by rosiglitazone. Thus, despite rosiglitazone augmentation of MAT, exercise significantly suppresses MAT volume and induces bone formation. That exercise can both suppress MAT volume and increase bone quantity, notwithstanding the skeletal harm induced by rosiglitazone, underscores exercise as a powerful regulator of bone remodeling, encouraging marrow stem cells toward the osteogenic lineage to fulfill an adaptive need for bone formation. Thus, exercise represents an effective strategy to mitigate the deleterious effects of overeating and iatrogenic etiologies on bone and fat.

Bone Marrow Adipose Tissue: A New Player in Cancer Metastasis to Bone

The bone marrow is a favored site for a number of cancers, including the hematological malignancy multiple myeloma, and metastasis of breast and prostate cancer. This specialized microenvironment is highly supportive, not only for tumor growth and survival but also for the development of an associated destructive cancer-induced bone disease. The interactions between tumor cells, osteoclasts and osteoblasts are well documented. By contrast, despite occupying a significant proportion of the bone marrow, the importance of bone marrow adipose tissue is only just emerging. The ability of bone marrow adipocytes to regulate skeletal biology and hematopoiesis, combined with their metabolic activity, endocrine functions, and proximity to tumor cells means that they are ideally placed to impact both tumor growth and bone disease. This review discusses the recent advances in our understanding of how marrow adipose tissue contributes to bone metastasis and cancer-induced bone disease.

Fatty Infiltration of Skeletal Muscle: Mechanisms and Comparisons with Bone Marrow Adiposity

Skeletal muscle and bone share common embryological origins from mesodermal cell populations and also display common growth trajectories early in life. Moreover, muscle and bone are both mechanoresponsive tissues, and the mass and strength of both tissues decline with age. The decline in muscle and bone strength that occurs with aging is accompanied in both cases by an accumulation of adipose tissue. In bone, adipocyte (AC) accumulation occurs in the marrow cavities of long bones and is known to increase with estrogen deficiency, mechanical unloading, and exposure to glucocorticoids. The factors leading to accumulation of intra- and intermuscular fat (myosteatosis) are less well understood, but recent evidence indicates that increases in intramuscular fat are associated with disuse, altered leptin signaling, sex steroid deficiency, and glucocorticoid treatment, factors that are also implicated in bone marrow adipogenesis. Importantly, accumulation of ACs in skeletal muscle and accumulation of intramyocellular lipid are linked to loss of muscle strength, reduced insulin sensitivity, and increased mortality among the elderly. Resistance exercise and whole body vibration can prevent fatty infiltration in skeletal muscle and also improve muscle strength. Therapeutic strategies to prevent myosteatosis may improve muscle function and reduce fall risk in the elderly, potentially impacting the incidence of bone fracture.

Skeletal integration of energy homeostasis: Translational implications

New evidence has recently emerged defining a close relationship between fat and bone metabolism. Adipose tissue is one of the largest organs in the body but its functions vary by location and origin. Adipocytes can act in an autocrine manner to regulate energy balance by sequestering triglycerides and then, depending on demand, releasing fatty acids through lipolysis for energy utilization, and in some cases through uncoupling protein 1 for generating heat. Adipose tissue can also act in an endocrine or paracrine manner by releasing adipokines that modulate the function of other organs. Bone is one of those target tissues, although recent evidence has emerged that the skeleton reciprocates by releasing its own factors that modulate adipose tissue and beta cells in the pancreas. Therefore, it is not surprising that these energy-modulating tissues are controlled by a central regulatory mechanism, primarily the sympathetic nervous system. Disruption in this complex regulatory circuit and its downstream tissues is manifested in a wide range of metabolic disorders, for which the most prevalent is type 2 diabetes mellitus. The aim of this review is to summarize our knowledge of common determinants in the bone and adipose function and the translational implications of recent work in this emerging field.

Hdac3 Deficiency Increases Marrow Adiposity and Induces Lipid Storage and Glucocorticoid Metabolism in Osteochondroprogenitor Cells

Bone loss and increased marrow adiposity are hallmarks of aging skeletons. Conditional deletion of histone deacetylase 3 (Hdac3) in murine osteochondroprogenitor cells causes osteopenia and increases marrow adiposity, even in young animals, but the origins of the increased adiposity are unclear. To explore this, bone marrow stromal cells (BMSCs) from Hdac3-depleted and control mice were cultured in osteogenic medium. Hdac3-deficient cultures accumulated lipid droplets in greater abundance than control cultures and expressed high levels of genes related to lipid storage (Fsp27/Cidec, Plin1) and glucocorticoid metabolism (Hsd11b1) despite normal levels of Pparγ2. Approximately 5% of the lipid containing cells in the wild-type cultures expressed the master osteoblast transcription factor Runx2, but this population was threefold greater in the Hdac3-depleted cultures. Adenoviral expression of Hdac3 restored normal gene expression, indicating that Hdac3 controls glucocorticoid activation and lipid storage within osteoblast lineage cells. HDAC3 expression was reduced in bone cells from postmenopausal as compared to young women, and in osteoblasts from aged as compared to younger mice. Moreover, phosphorylation of S424 in Hdac3, a posttranslational mark necessary for deacetylase activity, was suppressed in osseous cells from old mice. Thus, concurrent declines in transcription and phosphorylation combine to suppress Hdac3 activity in aging bone, and reduced Hdac3 activity in osteochondroprogenitor cells contributes to increased marrow adiposity associated with aging. © 2015 American Society for Bone and Mineral Research.

Changes in Skeletal Integrity and Marrow Adiposity during High-Fat Diet and after Weight Loss

The prevalence of obesity has continued to rise over the past three decades leading to significant increases in obesity-related medical care costs from metabolic and non-metabolic sequelae. It is now clear that expansion of body fat leads to an increase in inflammation with systemic effects on metabolism. In mouse models of diet-induced obesity, there is also an expansion of bone marrow adipocytes. However, the persistence of these changes after weight loss has not been well described. The objective of this study was to investigate the impact of high-fat diet (HFD) and subsequent weight loss on skeletal parameters in C57Bl6/J mice. Male mice were given a normal chow diet (ND) or 60% HFD at 6 weeks of age for 12, 16, or 20 weeks. A third group of mice was put on HFD for 12 weeks and then on ND for 8 weeks to mimic weight loss. After these dietary challenges, the tibia and femur were removed and analyzed by micro computed-tomography for bone morphology. Decalcification followed by osmium staining was used to assess bone marrow adiposity, and mechanical testing was performed to assess bone strength. After 12, 16, or 20 weeks of HFD, mice had significant weight gain relative to controls. Body mass returned to normal after weight loss. Marrow adipose tissue (MAT) volume in the tibia increased after 16 weeks of HFD and persisted in the 20-week HFD group. Weight loss prevented HFD-induced MAT expansion. Trabecular bone volume fraction, mineral content, and number were decreased after 12, 16, or 20 weeks of HFD, relative to ND controls, with only partial recovery after weight loss. Mechanical testing demonstrated decreased fracture resistance after 20 weeks of HFD. Loss of mechanical integrity did not recover after weight loss. Our study demonstrates that HFD causes long-term, persistent changes in bone quality, despite prevention of marrow adipose tissue accumulation, as demonstrated through changes in bone morphology and mechanical strength in a mouse model of diet-induced obesity and weight loss.

Marrow Adipose Tissue Expansion Coincides with Insulin Resistance in MAGP1-Deficient Mice

Marrow adipose tissue (MAT) is an endocrine organ with the potential to influence skeletal remodeling and hematopoiesis. Pathologic MAT expansion has been studied in the context of severe metabolic challenge, including caloric restriction, high fat diet feeding, and leptin deficiency. However, the rapid change in peripheral fat and glucose metabolism associated with these models impedes our ability to examine which metabolic parameters precede or coincide with MAT expansion. Microfibril-associated glycoprotein-1 (MAGP1) is a matricellular protein that influences cellular processes by tethering signaling molecules to extracellular matrix structures. MAGP1-deficient (Mfap2 (-/-)) mice display a progressive excess adiposity phenotype, which precedes insulin resistance and occurs without changes in caloric intake or ambulation. Mfap2 (-/-) mice were, therefore, used as a model to associate parameters of metabolic disease, bone remodeling, and hematopoiesis with MAT expansion. Marrow adiposity was normal in Mfap2 (-/-) mice until 6 months of age; however, by 10 months, marrow fat volume had increased fivefold relative to wild-type control at the same age. Increased gonadal fat pad mass and hyperglycemia were detectable in Mfap2 (-/-) mice by 2 months, but peaked by 6 months. The development of insulin resistance coincided with MAT expansion. Longitudinal characterization of bone mass demonstrated a disconnection in MAT volume and bone volume. Specifically, Mfap2 (-/-) mice had reduced trabecular bone volume by 2 months, but this phenotype did not progress with age or MAT expansion. Interestingly, MAT expansion in the 10-month-old Mfap2 (-/-) mice was associated with modest alterations in basal hematopoiesis, including a shift from granulopoiesis to B lymphopoiesis. Together, these findings indicate MAT expansion is coincident with insulin resistance, but not excess peripheral adiposity or hyperglycemia in Mfap2 (-/-) mice; and substantial MAT accumulation does not necessitate a proportional decrease in either bone mass or bone marrow cellularity.

Bone Marrow Adipose Tissue: To Be or Not To Be a Typical Adipose Tissue?

Bone marrow adipose tissue (BMAT) emerges as a distinct fat depot whose importance has been proved in the bone-fat interaction. Indeed, it is well recognized that adipokines and free fatty acids released by adipocytes can directly or indirectly interfere with cells of bone remodeling or hematopoiesis. In pathological states, such as osteoporosis, each of adipose tissues - subcutaneous white adipose tissue (WAT), visceral WAT, brown adipose tissue (BAT), and BMAT - is differently associated with bone mineral density (BMD) variations. However, compared with the other fat depots, BMAT displays striking features that makes it a substantial actor in bone alterations. BMAT quantity is well associated with BMD loss in aging, menopause, and other metabolic conditions, such as anorexia nervosa. Consequently, BMAT is sensed as a relevant marker of a compromised bone integrity. However, analyses of BMAT development in metabolic diseases (obesity and diabetes) are scarce and should be, thus, more systematically addressed to better apprehend the bone modifications in that pathophysiological contexts. Moreover, bone marrow (BM) adipogenesis occurs throughout the whole life at different rates. Following an ordered spatiotemporal expansion, BMAT has turned to be a heterogeneous fat depot whose adipocytes diverge in their phenotype and their response to stimuli according to their location in bone and BM. In vitro, in vivo, and clinical studies point to a detrimental role of BM adipocytes (BMAs) throughout the release of paracrine factors that modulate osteoblast and/or osteoclast formation and function. However, the anatomical dissemination and the difficulties to access BMAs still hamper our understanding of the relative contribution of BMAT secretions compared with those of peripheral adipose tissues. A further characterization of the phenotype and the functional regulation of BMAs are ever more required. Based on currently available data and comparison with other fat tissues, this review addresses the originality of the BMAT with regard to its development, anatomy, metabolic properties, and response to physiological cues.

Hypothalamic Leptin Gene Therapy Reduces Bone Marrow Adiposity in ob/ob Mice Fed Regular and High-Fat Diets

Low bone mass is often associated with elevated bone marrow adiposity. Since osteoblasts and adipocytes are derived from the same mesenchymal stem cell (MSC) progenitor, adipocyte formation may increase at the expense of osteoblast formation. Leptin is an adipocyte-derived hormone known to regulate energy and bone metabolism. Leptin deficiency and high-fat diet-induced obesity are associated with increased marrow adipose tissue (MAT) and reduced bone formation. Short-duration studies suggest that leptin treatment reduces MAT and increases bone formation in leptin-deficient ob/ob mice fed a regular diet. Here, we determined the long-duration impact of increased hypothalamic leptin on marrow adipocytes and osteoblasts in ob/ob mice following recombinant adeno-associated virus (rAAV) gene therapy. Eight- to 10-week-old male ob/ob mice were randomized into four groups: (1) untreated, (2) rAAV-Lep, (3) rAAV-green fluorescent protein (rAAV-GFP), or (4) pair-fed to rAAV-Lep. For vector administration, mice were injected intracerebroventricularly with either rAAV-leptin gene therapy (rAAV-Lep) or rAAV-GFP (9 × 10(7) particles) and maintained for 30 weeks. In a second study, the impact of increased hypothalamic leptin levels on MAT was determined in mice fed high-fat diets; ob/ob mice were randomized into two groups and treated with either rAAV-Lep or rAAV-GFP. At 7 weeks post-vector administration, half the mice in each group were switched to a high-fat diet for 8 weeks. Wild-type (WT) controls included age-matched mice fed regular or high-fat diet. High-fat diet resulted in a threefold increase in MAT in WT mice, whereas MAT was increased by leptin deficiency up to 50-fold. Hypothalamic leptin gene therapy increased osteoblast perimeter and osteoclast perimeter with minor change in cancellous bone architecture. The gene therapy decreased MAT levels in ob/ob mice fed regular or high-fat diet to values similar to WT mice fed regular diet. These findings suggest that leptin plays an important role in regulating the differentiation of MSCs to adipocytes and osteoblasts, a process that may be dysregulated by high-fat diet. However, the results also illustrate that reducing MAT by increasing leptin levels does not necessarily result in increased bone mass.

The Bone Marrow-Derived Stromal Cells: Commitment and Regulation of Adipogenesis

Bone marrow (BM) microenvironment represents an important compartment of bone that regulates bone homeostasis and the balance between bone formation and bone resorption depending on the physiological needs of the organism. Abnormalities of BM microenvironmental dynamics can lead to metabolic bone diseases. BM stromal cells (also known as skeletal or mesenchymal stem cells) [bone marrow stromal stem cell (BMSC)] are multipotent stem cells located within BM stroma and give rise to osteoblasts and adipocytes. However, cellular and molecular mechanisms of BMSC lineage commitment to adipocytic lineage and regulation of BM adipocyte formation are not fully understood. In this review, we will discuss recent findings pertaining to identification and characterization of adipocyte progenitor cells in BM and the regulation of differentiation into mature adipocytes. We have also emphasized the clinical relevance of these findings.

Increased Circulating Adiponectin in Response to Thiazolidinediones: Investigating the Role of Bone Marrow Adipose Tissue

BACKGROUND

Bone marrow adipose tissue (MAT) contributes to increased circulating adiponectin, an insulin-sensitizing hormone, during caloric restriction (CR), but whether this occurs in other contexts remains unknown. The antidiabetic thiazolidinediones (TZDs) also promote MAT expansion and hyperadiponectinemia, even without increasing adiponectin expression in white adipose tissue (WAT).

OBJECTIVES

To test the hypothesis that MAT expansion contributes to TZD-associated hyperadiponectinemia, we investigated the effects of rosiglitazone, a prototypical TZD, in wild-type (WT) or Ocn-Wnt10b mice. The latter resist MAT expansion during CR, leading us to postulate that they would also resist this effect of rosiglitazone.

DESIGN

Male and female WT or Ocn-Wnt10b mice (C57BL/6J) were treated with or without rosiglitazone for 2, 4, or 8 weeks, up to 30 weeks of age. MAT content was assessed by osmium tetroxide staining and adipocyte marker expression. Circulating adiponectin was determined by ELISA.

RESULTS

In WT mice, rosiglitazone caused hyperadiponectinemia and MAT expansion. Compared to WT mice, Ocn-Wnt10b mice had significantly less MAT in distal tibiae and sometimes in proximal tibiae; however, interpretation was complicated by the leakage of osmium tetroxide from ruptures in some tibiae, highlighting an important technical consideration for osmium-based MAT analysis. Despite decreased MAT in Ocn-Wnt10b mice, circulating adiponectin was generally similar between WT and Ocn-Wnt10b mice; however, in females receiving rosiglitazone for 4 weeks, hyperadiponectinemia was significantly blunted in Ocn-Wnt10b compared to WT mice. Notably, this was also the only group in which tibial adiponectin expression was lower than in WT mice, suggesting a close association between MAT adiponectin production and circulating adiponectin. However, rosiglitazone significantly increased adiponectin protein expression in WAT, suggesting that WAT contributes to hyperadiponectinemia in this context. Finally, rosiglitazone upregulated uncoupling protein 1 in brown adipose tissue (BAT), but this protein was undetectable in tibiae, suggesting that MAT is unlikely to share thermogenic properties of BAT.

CONCLUSION

TZD-induced hyperadiponectinemia is closely associated with increased adiponectin production in MAT but is not prevented by the partial loss of MAT that occurs in Ocn-Wnt10b mice. Thus, more robust loss-of-MAT models are required for future studies to better establish MAT's elusive functions, both on an endocrine level and beyond.

Signaling Interplay between Bone Marrow Adipose Tissue and Multiple Myeloma cells

In the year 2000, Hanahan and Weinberg (1) defined the six Hallmarks of Cancer as: self-sufficiency in growth signals, evasion of apoptosis, insensitivity to antigrowth mechanisms, tissue invasion and metastasis, limitless replicative potential, and sustained angiogenesis. Eleven years later, two new Hallmarks were added to the list (avoiding immune destruction and reprograming energy metabolism) and two new tumor characteristics (tumor-promoting inflammation and genome instability and mutation) (2). In multiple myeloma (MM), a destructive cancer of the plasma cell that grows predominantly in the bone marrow (BM), it is clear that all these hallmarks and characteristics are in play, contributing to tumor initiation, drug resistance, disease progression, and relapse. Bone marrow adipose tissue (BMAT) is a newly recognized contributor to MM oncogenesis and disease progression, potentially affecting MM cell metabolism, immune action, inflammation, and influences on angiogenesis. In this review, we discuss the confirmed and hypothetical contributions of BMAT to MM development and disease progression. BMAT has been understudied due to technical challenges and a previous lack of appreciation for the endocrine function of this tissue. In this review, we define the dynamic, responsive, metabolically active BM adipocyte. We then describe how BMAT influences MM in terms of: lipids/metabolism, hypoxia/angiogenesis, paracrine or endocrine signaling, and bone disease. We then discuss the connection between BMAT and systemic inflammation and potential treatments to inhibit the feedback loops between BM adipocytes and MM cells that support MM progression. We aim for researchers to use this review to guide and help prioritize their experiments to develop better treatments or a cure for cancers, such as MM, that associate with and may depend on BMAT.

Adipocytes exert lipotoxic effects on osteoblast through activating hypoxia signaling pathway in vitro

This study aimed to investigate the effect of adipocytes on osteoblastic bone formation in vitro. The differentiated and undifferentiated 3T3-L1 cells were co-cultured with primary calvarial osteoblasts. At 48 h, proliferated osteoblasts decreased significantly after co-culture with differentiated preadipocytes as compared to those co-cultured with undifferentiated preadipocytes; at 7 days, the expressions of bone formation-related genes decreased in osteoblasts co-cultured with differentiated preadipocytes; at 14 days, osteoblasts mineralization also decreased significantly after co-culture with differentiated preadipocytes. To further determine whether the decreased proliferation and mineralization were related to the hypoxia signaling pathway, the expressions of hypoxia related-genes were detected. Results showed the expressions of these genes significantly increased after co-culture with differentiated preadipocytes. NF-κB and IL-6 expressions were also up-regulated in osteoblasts co-cultured with differentiated preadipocytes. Osteoblasts from Hif1α(f/f) mice showed increased proliferation and mineralization after co-culture with adipocytes transfected with adenoviral-cre, accompanied by up-regulated expressions of bone formation-related genes and down-regulated expressions of NF-κB and IL-6. These results demonstrated that adipocytes exert a negative effect on the proliferation and mineralization of osteoblasts via up-regulating the expressions of hypoxia-related genes, and NF-κB and IL-6 may impair the osteoblastic bone formation.

Navigating the bone marrow niche: translational insights and cancer-driven dysfunction

The bone marrow niche consists of stem and progenitor cells destined to become mature cells such as haematopoietic elements, osteoblasts or adipocytes. Marrow cells, influenced by endocrine, paracrine and autocrine factors, ultimately function as a unit to regulate bone remodelling and haematopoiesis. Current evidence highlights that the bone marrow niche is not merely an anatomic compartment; rather, it integrates the physiology of two distinct organ systems, the skeleton and the marrow. The niche has a hypoxic microenvironment that maintains quiescent haematopoietic stem cells (HSCs) and supports glycolytic metabolism. In response to biochemical cues and under the influence of neural, hormonal, and biochemical factors, marrow stromal elements, such as mesenchymal stromal cells (MSCs), differentiate into mature, functioning cells. However, disruption of the niche can affect cellular differentiation, resulting in disorders ranging from osteoporosis to malignancy. In this Review, we propose that the niche reflects the vitality of two tissues - bone and blood - by providing a unique environment for stem and stromal cells to flourish while simultaneously preventing disproportionate proliferation, malignant transformation or loss of the multipotent progenitors required for healing, functional immunity and growth throughout an organism's lifetime. Through a fuller understanding of the complexity of the niche in physiologic and pathologic states, the successful development of more-effective therapeutic approaches to target the niche and its cellular components for the treatment of rheumatic, endocrine, neoplastic and metabolic diseases becomes achievable.

Adipose tissue dysfunction and its effects on tumor metabolism

Growing by an alarming rate in the Western world, obesity has become a condition associated with a multitude of diseases such as diabetes, metabolic syndrome and various cancers. Generally viewed as an abnormal accumulation of hypertrophied adipocytes, obesity is also a poor prognostic factor for recurrence and chemoresistance in cancer patients. With more than two-thirds of the adult population in the United States considered clinically overweight or obese, it is critical that the relationship between obesity and cancer is further emphasized and elucidated. Adipocytes are highly metabolically active cells, which, through release of adipokines and cytokines and activation of endocrine and paracrine pathways, affect processes in neighboring and distant cells, altering their normal homeostasis. This work will examine specifically how adipocyte-derived factors regulate the cellular metabolism of malignant cells within the tumor niche. Briefly, tumor cells undergo metabolic pressure towards a more glycolytic and hypoxic state through a variety of metabolic regulators and signaling pathways, i.e., phosphoinositol-3 kinase (PI3K), hypoxia-inducible factor-1 alpha (HIF-1α), and c-MYC signaling. Enhanced glycolysis and high lactate production are hallmarks of tumor progression largely because of a process known as the Warburg effect. Herein, we review the latest literature pertaining to the body of work on the interactions between adipose and tumor cells, and underlining the changes in cancer cell metabolism that have been targeted by the currently available treatments.

The past 10 years-new hormones, new functions, new endocrine organs

Since the publication of the first issue of this journal in November 2005, our understanding of the endocrine system has evolved, with the identification of novel hormones and novel endocrine roles for previously identified molecules. Here, we have asked six of our Advisory Board Members to comment on how these insights have led to the recognition that many organs and tissues that were not widely considered part of the classic endocrine system in the past have important endocrine functions.

Green tea supplementation benefits body composition and improves bone properties in obese female rats fed with high-fat diet and caloric restricted diet

This study investigated the effects of green tea polyphenols (GTP) supplementation on body composition, bone properties, and serum markers in obese rats fed a high-fat diet (HFD) or a caloric restricted diet (CRD). Forty-eight female rats were fed an HFD ad libitum for 4 months, and then either continued on the HFD or the CRD with or without 0.5% GTP in water. Body composition, bone efficacy, and serum markers were measured. We hypothesized that GTP supplementation would improve body composition, mitigate bone loss, and restore bone microstructure in obese animals fed either HFD or CRD. CRD lowered percent fat mass; bone mass and trabecular number of tibia, femur and lumbar vertebrae; femoral strength; trabecular and cortical thickness of tibia; insulin-like growth factor-I and leptin. CRD also increased percent fat-free mass; trabecular separation of tibia and femur; eroded surface of tibia; bone formation rate and erosion rate at tibia shaft; and adiponectin. GTP supplementation increased femoral mass and strength (P = .026), trabecular thickness (P = .012) and number (P = .019), and cortical thickness of tibia (P < .001), and decreased trabecular separation (P = .021), formation rate (P < .001), and eroded surface (P < .001) at proximal tibia, and insulin-like growth factor-I and leptin. There were significant interactions (diet type × GTP) on osteoblast surface/bone surface, mineral apposition rate at periosteal and endocortical bones, periosteal bone formation rate, and trabecular thickness at femur and lumbar vertebrate (P < .05). This study demonstrates that GTP supplementation for 4 months benefited body composition and improved bone microstructure and strength in obese rats fed with HFD or HFD followed by CRD diet.

The Role of Bone Marrow Fat in Skeletal Health: Usefulness and Perspectives for Clinicians

CONTEXT

There is growing interest in the relationship between bone marrow fat (BMF), bone mineral density (BMD), and fractures. Moreover, BMF might be influenced by metabolic diseases associated with bone loss and fractures, such as type 2 diabetes mellitus (T2DM), anorexia nervosa (AN), and obesity.

METHODS

The primary-source literature for this review was acquired using a PubMed search for articles published between January 2000 and April 2015. Search terms included BMF, BMD, fractures, T2DM, AN, and obesity. The titles and abstracts of all articles were reviewed for relevant subjects.

RESULTS

Magnetic resonance imaging, with or without spectroscopy, was used to noninvasively quantify BMF in humans. A negative relationship was found between BMD and BMF in both healthy and osteopenic/osteoporotic populations. Data are lacking on the relationship between BMF and fractures. Studies in populations of individuals with metabolic diseases such as T2DM, AN, and obesity have shown BMF abnormalities.

CONCLUSIONS

We conclude that most human data demonstrate an inverse relationship between BMF and BMD, but data on the relationship with fractures are inconsistent and need further study. In daily practice, the usefulness for clinicians of assessing BMF using magnetic resonance imaging is still limited. However, the perspectives are exciting, particularly in terms of improving the diagnosis and management of osteoporosis.

Adipogenesis is under surveillance of Hsp90 and the high molecular weight Immunophilin FKBP51

Adipose tissue plays a central role in the control of energy balance as well as in the maintenance of metabolic homeostasis. It was not until recently that the first evidences of the role of heat shock protein (Hsp) 90 and high molecular weight immunophilin FKBP51 have been described in the process of adipocyte differentiation. Recent reports describe their role in the regulation of PPARγ, a key transcription factor in the control of adipogenesis and the maintenance of the adipocyte phenotype. In addition, novel roles have been uncovered for FKBP51 in the organization of the architecture of the nucleus through its participation in the reorganization of the nuclear lamina. Therefore, the aim of this review is to integrate and discuss the recent advances in the field, with special emphasis on the roles of Hsp90 and FKBP51 in the process of adipocyte differentiation.

Region-specific variation in the properties of skeletal adipocytes reveals regulated and constitutive marrow adipose tissues

Marrow adipose tissue (MAT) accumulates in diverse clinical conditions but remains poorly understood. Here we show region-specific variation in MAT adipocyte development, regulation, size, lipid composition, gene expression and genetic determinants. Early MAT formation in mice is conserved, whereas later development is strain dependent. Proximal, but not distal tibial, MAT is lost with 21-day cold exposure. Rat MAT adipocytes from distal sites have an increased proportion of monounsaturated fatty acids and expression of Scd1/Scd2, Cebpa and Cebpb. Humans also have increased distal marrow fat unsaturation. We define proximal 'regulated' MAT (rMAT) as single adipocytes interspersed with active haematopoiesis, whereas distal 'constitutive' MAT (cMAT) has low haematopoiesis, contains larger adipocytes, develops earlier and remains preserved upon systemic challenges. Loss of rMAT occurs in mice with congenital generalized lipodystrophy type 4, whereas both rMAT and cMAT are preserved in mice with congenital generalized lipodystrophy type 3. Consideration of these MAT subpopulations may be important for future studies linking MAT to bone biology, haematopoiesis and whole-body metabolism.

Adiponectin Enhances Cold-Induced Browning of Subcutaneous Adipose Tissue via Promoting M2 Macrophage Proliferation

Adiponectin is an abundant adipokine with pleiotropic protective effects against a cluster of obesity-related cardiometabolic disorders. However, its role in adaptive thermogenesis has scarcely been explored. Here we showed that chronic cold exposure led to a markedly elevated production of adiponectin in adipocytes of subcutaneous white adipose tissue (scWAT), which in turn bound to M2 macrophages in the stromal vascular fraction. Chronic cold exposure-induced accumulation of M2 macrophages, activation of beige cells, and thermogenic program were markedly impaired in scWAT of adiponectin knockout (ADN KO) mice, whereas these impairments were reversed by replenishment with adiponectin. Mechanistically, adiponectin was recruited to the cell surface of M2 macrophages via its binding partner T-cadherin and promoted the cell proliferation by activation of Akt, consequently leading to beige cell activation. These findings uncover adiponectin as a key efferent signal for cold-induced adaptive thermogenesis by mediating the crosstalk between adipocytes and M2 macrophages in scWAT.

Negative Skeletal Effects of Locally Produced Adiponectin

Epidemiological studies show that high circulating levels of adiponectin are associated with low bone mineral density. The effect of adiponectin on skeletal homeostasis, on osteoblasts in particular, remains controversial. We investigated this issue using mice with adipocyte-specific over-expression of adiponectin (AdTg). MicroCT and histomorphometric analysis revealed decreases (15%) in fractional bone volume in AdTg mice at the proximal tibia with no changes at the distal femur. Cortical bone thickness at mid-shafts of the tibia and at the tibiofibular junction was reduced (3–4%) in AdTg mice. Dynamic histomorphometry at the proximal tibia in AdTg mice revealed inhibition of bone formation. AdTg mice had increased numbers of adipocytes in close proximity to trabecular bone in the tibia, associated with increased adiponectin levels in tibial marrow. Treatment of BMSCs with adiponectin after initiation of osteoblastic differentiation resulted in reduced mineralized colony formation and reduced expression of mRNA of osteoblastic genes, osterix (70%), Runx2 (52%), alkaline phosphatase (72%), Col1 (74%), and osteocalcin (81%). Adiponectin treatment of differentiating osteoblasts increased expression of the osteoblast genes PPARγ (32%) and C/ebpα (55%) and increased adipocyte colony formation. These data suggest a model in which locally produced adiponectin plays a negative role in regulating skeletal homeostasis through inhibition of bone formation and by promoting an adipogenic phenotype.

The Relationship of Fat Distribution and Insulin Resistance with Lumbar Spine Bone Mass in Women

Bone marrow harbors a significant amount of body adipose tissue (BMAT). While BMAT might be a source of energy for bone modeling and remodeling, its increment can also represent impairment of osteoblast differentiation. The relationship between BMAT, bone mass and insulin sensitivity is only partially understood and seems to depend on the circumstances. The present study was designed to assess the association of BMAT with bone mineral density in the lumbar spine as well as with visceral adipose tissue, intrahepatic lipids, HOMA-IR, and serum levels of insulin and glucose. This cross-sectional clinical investigation included 31 non-diabetic women, but 11 had a pre-diabetes status. Dual X-ray energy absorptiometry was used to measure bone mineral density and magnetic resonance imaging was used to assess fat deposition in BMAT, visceral adipose tissue and liver. Our results suggest that in non-diabetic, there is an inverse relationship between bone mineral density in lumbar spine and BMAT and a trend persists after adjustment for weight, age, BMI and height. While there is a positive association between visceral adipose tissue and intrahepatic lipids with serum insulin levels, there is no association between BMAT and serum levels of insulin. Conversely, a positive relationship was observed between BMAT and serum glucose levels, whereas this association was not observed with other fat deposits. These relationships did not apply after adjustment for body weight, BMI, height and age. The present study shows that in a group of predominantly non-obese women the association between insulin resistance and BMAT is not an early event, as occurs with visceral adipose tissue and intrahepatic lipids. On the other hand, BMAT has a negative relationship with bone mineral density. Taken together, the results support the view that bone has a complex and non-linear relationship with energy metabolism.

Changes in vertebral bone marrow fat and bone mass after gastric bypass surgery: A pilot study

Bone marrow fat may serve a metabolic role distinct from other fat depots, and it may be altered by metabolic conditions including diabetes. Caloric restriction paradoxically increases marrow fat in mice, and women with anorexia nervosa have high marrow fat. The longitudinal effect of weight loss on marrow fat in humans is unknown. We hypothesized that marrow fat increases after Roux-en-Y gastric bypass (RYGB) surgery, as total body fat decreases. In a pilot study of 11 morbidly obese women (6 diabetic, 5 nondiabetic), we measured vertebral marrow fat content (percentage fat fraction) before and 6 months after RYGB using magnetic resonance spectroscopy. Total body fat mass declined in all participants (mean ± SD decline 19.1 ± 6.1 kg or 36.5% ± 10.9%, p<0.001). Areal bone mineral density (BMD) decreased by 5.2% ± 3.5% and 4.1% ± 2.6% at the femoral neck and total hip, respectively, and volumetric BMD decreased at the spine by 7.4% ± 2.8% (p<0.001 for all). Effects of RYGB on marrow fat differed by diabetes status (adjusted p=0.04). There was little mean change in marrow fat in nondiabetic women (mean +0.9%, 95% CI -10.0 to +11.7%, p=0.84). In contrast, marrow fat decreased in diabetic women (-7.5%, 95% CI -15.2 to +0.1%, p=0.05). Changes in total body fat mass and marrow fat were inversely correlated among nondiabetic (r=-0.96, p=0.01) but not diabetic (r=0.52, p=0.29) participants. In conclusion, among those without diabetes, marrow fat is maintained on average after RYGB, despite dramatic declines in overall fat mass. Among those with diabetes, RYGB may reduce marrow fat. Thus, future studies of marrow fat should take diabetes status into account. Marrow fat may have unique metabolic behavior compared with other fat depots.

Transcriptional control and hormonal response of thermogenic fat

Obesity and its associated metabolic diseases present a major public health problem around the world. The discovery that thermogenic fat is active in adult humans has sparked a renewal of interest in the study of its development and function and in the feasibility of using modulators of thermogenesis to work against obesity. In recent years, it has been shown that there are at least two distinct types of thermogenic fat cells: brown and beige fat. In this review, we discuss the transcriptional mediators of thermogenesis and the signaling molecules that regulate thermogenic cells. We also review the effects of thermogenic fat activation on whole-body metabolic parameters and evaluate the increasing evidence that activating thermogenesis in humans can be a viable method of ameliorating obesity. In these discussions, we highlight targets that can potentially be stimulated or modified in anti-obesity treatments.

The brown fat secretome: metabolic functions beyond thermogenesis

Brown fat is highly active in fuel oxidation and dissipates chemical energy through uncoupling protein (UCP)1-mediated heat production. Activation of brown fat leads to increased energy expenditure, reduced adiposity, and lower plasma glucose and lipid levels, thus contributing to better homeostasis. Uncoupled respiration and thermogenesis have been considered to be responsible for the metabolic benefits of brown adipose tissue. Recent studies have demonstrated that brown adipocytes also secrete factors that act locally and systemically to influence fuel and energy metabolism. This review discusses the evidence supporting a thermogenesis-independent role of brown fat, particularly through its release of secreted factors, and their implications in physiology and therapeutic development.

The use of nano-computed tomography to enhance musculoskeletal research

Advances in computed tomography (CT) imaging are opening new avenues toward more precise characterization and quantification of connective tissue microarchitecture. In the last two decades, micro-computed tomography (microCT) has significantly augmented destructive methods for the 3D micro-analysis of tissue structure, primarily in the bone research field. Recently, microCT has been employed in combination with contrast agents to generate contrast-enhanced images of soft tissues that are otherwise difficult to visualize due to their native radiodensity. More recent advances in CT technology have enabled ultra-high resolution imaging by utilizing a more powerful nano-focused X-ray source, such as that found in nano-computed tomography (nanoCT) systems. NanoCT imaging has facilitated the expansion of musculoskeletal research by reducing acquisition time and significantly expanding the range of samples that can be imaged in terms of size, age and tissue-type (bone, muscle, tendon, cartilage, vessels and adipose tissue). We present the application and early results of nanoCT imaging in various tissue types and how this ultra-high resolution imaging modality is capable of characterizing microstructures at levels of details previously not possible. Contrast-enhanced imaging techniques to enable soft-tissue visualization and characterization are also outlined.

A phase I feasibility study of multi-modality imaging assessing rapid expansion of marrow fat and decreased bone mineral density in cancer patients

PURPOSE

Cancer survivors are at an increased risk for fractures, but lack of effective and economical biomarkers limits quantitative assessments of marrow fat (MF), bone mineral density (BMD) and their relation in response to cytotoxic cancer treatment. We report dual energy CT (DECT) imaging, commonly used for cancer diagnosis, treatment and surveillance, as a novel biomarker of MF and BMD.

METHODS

We validated DECT in pre-clinical and phase I clinical trials and verified with water-fat MRI (WF-MRI), quantitative CT (QCT) and dual-energy X-ray absorptiometry (DXA). Basis material composition framework was validated using water and small-chain alcohols simulating different components of bone marrow. Histologic validation was achieved by measuring percent adipocyte in the cadaver vertebrae and compared with DECT and WF-MRI. For a phase I trial, sixteen patients with gynecologic malignancies (treated with oophorectomy, radiotherapy or chemotherapy) underwent DECT, QCT, WF-MRI and DXA before and 12months after treatment. BMD and MF percent and distribution were quantified in the lumbar vertebrae and the right femoral neck.

RESULTS

Measured precision (3mg/cm(3)) was sufficient to distinguish test solutions. Adiposity in cadaver bone histology was highly correlated with MF measured using DECT and WF-MRI (r=0.80 and 0.77, respectively). In the clinical trial, DECT showed high overall correlation (r=0.77, 95% CI: 0.69, 0.83) with WF-MRI. MF increased significantly after treatment (p<0.002). Chemotherapy and radiation caused greater increases in MF than oophorectomy (p<0.032). L4 BMD decreased 14% by DECT, 20% by QCT, but only 5% by DXA (p<0.002 for all). At baseline, we observed a statistically significant inverse association between MF and BMD which was dramatically attenuated after treatment.

CONCLUSION

Our study demonstrated that DECT, similar to WF-MRI, can accurately measure marrow adiposity. Both imaging modalities show rapid increase in MF following cancer treatment. Our results suggest that MF and BMD cannot be used interchangeably to monitor skeletal health following cancer therapy.

Adipocyte-derived players in hematologic tumors: useful novel targets?

INTRODUCTION

Adipocytes and their products play essential roles in tumor establishment and progression. As the main cellular component of the bone marrow, adipocytes may contribute to the development of hematologic tumors.

AREAS COVERED

This review summarizes experimental data on adipocytes and their interaction with various cancer cells. Special focus is set on the interactions of bone marrow adipocytes and normal and transformed cells of the hematopoietic system such as myeloma and leukemia cells. Current in vitro and in vivo data are summarized and the potential of novel therapeutic targets is critically discussed.

EXPERT OPINION

Targeting lipid metabolism of cancer cells and adipocytes in combination with standard therapeutics might open novel therapeutic avenues in these cancer entities. Adipocyte-derived products such as free fatty acids and specific adipokines such as adiponectin may be vital anti-cancer targets in hematologic malignancies. However, available data on lipid metabolism is currently mostly referring to peripheral fat cell/cancer cell interactions and results need to be evaluated specifically for the bone marrow niche.