No abstract

Adipocytes were identified in human bone marrow more than a century ago, yet until recently little has been known about their origin, development, function or interactions with other cells in the bone marrow. Little functional significance has been attributed to these cells, a paradigm that still persists today. However, we now know that marrow adipose tissue increases with age and in response to a variety of physiologic induction signals. Bone marrow adipocytes have recently been shown to influence other cell populations within the marrow and can affect whole body metabolism by the secretion of a defined set of adipokines. Recent research shows that marrow adipocytes are distinct from white, brown and beige adipocytes, indicating that the bone marrow is a distinct adipose depot. This review will highlight recent data regarding these areas and the interactions of marrow adipose tissue (MAT) with cells within and outside of the bone marrow.

Adipocytes reside in discrete, well-defined depots throughout the body. In addition to mature adipocytes, white adipose tissue depots are composed of many cell types, including macrophages, endothelial cells, fibroblasts, and stromal cells, which together are referred to as the stromal vascular fraction (SVF). The SVF also contains adipocyte progenitors that give rise to mature adipocytes in those depots. Marrow adipose tissue (MAT) or marrow fat has long been known to be present in bone marrow (BM) but its origin, development, and function remain largely unknown. Clinically, increased MAT is associated with age, metabolic diseases, drug treatment, and marrow recovery in children receiving radiation and chemotherapy. In contrast to the other depots, MAT is unevenly distributed in the BM of long bones. Conventional quantitation relies on sectioning of the bone to overcome issues with distribution but is time-consuming, resource intensive, inconsistent between laboratories and may be unreliable as it may miss changes in MAT volume. Thus, the inability to quantitate MAT in a rapid, systematic, and reproducible manner has hampered a full understanding of its development and function. In this chapter, we describe a new technique that couples histochemical staining of lipid using osmium tetroxide with microcomputerized tomography to visualize and quantitate MAT within the medullary canal in three dimensions. Imaging of osmium staining provides a high-resolution map of existing and developing MAT in the BM. Because this method is simple, reproducible, and quantitative, we expect it will become a useful tool for the precise characterization of MAT.

Summary White adipose tissue (WAT) morphology characterized by hypertrophy (i.e. fewer but larger adipocytes) associates with increased adipose inflammation, lipolysis, insulin resistance and risk of diabetes. However, the causal relationships and the mechanisms controlling WAT morphology are unclear. Herein, we identified EBF1 as an adipocyte-expressed transcription factor with decreased expression/activity in WAT hypertrophy. In human adipocytes, the regulatory targets of EBF1 were enriched for genes controlling lipolysis and adipocyte morphology/differentiation and in both humans and murine models, reduced EBF1 levels associated with increased lipolysis and adipose hypertrophy. Although EBF1 did not affect adipose inflammation, TNFα reduced EBF1 gene expression. High fat diet-intervention in Ebf1+/− mice resulted in more pronounced WAT hypertrophy and attenuated insulin sensitivity compared with wild-type littermate controls. We conclude that EBF1 is an important regulator of adipose morphology and fat cell lipolysis and may constitute a link between WAT inflammation, altered lipid metabolism, adipose hypertrophy and insulin resistance.

Receptor activator of nuclear factor-kappaB ligand (RANKL) is essential for osteoclast differentiation, and hormones and cytokines that stimulate bone resorption increase RANKL expression in stromal/osteoblastic cells. We have previously shown that PTH and 1,25-dihydroxyvitamin D(3) control murine RANKL gene expression in vitro, in part, via an evolutionarily conserved transcriptional enhancer, designated the distal control region (DCR), located 76 kb upstream from the transcription start site. Herein we describe the phenotype of mice lacking this enhancer. Deletion of the DCR reduced PTH and 1,25-dihydroxyvitamin D(3) stimulation of RANKL mRNA and osteoclast formation in primary bone marrow cultures as well as stimulation of RANKL mRNA in bone. DCR deletion also reduced basal RANKL mRNA levels in bone, thymus, and spleen. Moreover, mice lacking the DCR exhibited increased bone mass and strength. The increase in bone mass was due to reduced osteoclast and osteoblast formation leading to a low rate of bone remodeling similar to that observed in humans and mice with hypoparathyroidism. These findings demonstrate that hormonal control of RANKL expression via the DCR is a critical determinant of the rate of bone remodeling.