Molecular and functional characterization of human bone marrow adipocytes

Abstract

Adipocytes are a cell population largely located in the human bone marrow cavity. In this specific microenvironment where adipocytes can interact with a variety of different cells, the role of fat is mainly unknown. To our knowledge, this report is the first to characterize mature adipocytes isolated from human bone marrow (BM-A) molecularly and functionally to better understand their roles into the hematopoietic microenvironment. Healthy BM-A were isolated after collagenase digestion and filtration. We studied the morphology of BM-A, their gene expression and immunophenotypic profile and their functional ability in the hematopoietic microenvironment, comparing them with adipocytes derived from adipose tissue (AT-A). BM-A showed a unilocular lipid morphology similar to AT-A and did not lose their morphology in culture; they showed a comparable pattern of stem cell-surface antigens to AT-A. In line with these observations, molecular data showed that BM-A expressed some embryonic stem cells genes, such as Oct4, KLf4, c-myc, Gata4, Tbx1, and Sox17, whereas they did not express the stem cell markers Sox2 and Nanog. Moreover, BM-A had long telomeres that were similar to bone marrow mesenchymal stem cells. Notably, BM-A supported the survival and differentiation of hematopoietic stem cells in long-term cultures. These results showed that BM-A are stromal cells with a gene expression pattern that distinguished them from AT-A. BM-A showed stem cell properties through their hematopoietic supporting function, which was certainly linked to their role in the maintenance of the bone marrow microenvironment. Depending on specific demands, BM-A may acquire different functions based on their local environment.

Plasticity of human dedifferentiated adipocytes toward endothelial cells

“…Moreover, in line with these results, the dedifferentiated fat cells displayed long telomeres (11.5 6 0.7 kb) [37]. Thus, telomere length and the absence of karyotype alterations demonstrated that these cells were not susceptible to malignant transformation.…”

Marrow Adipose Tissue: Trimming the Fat

“…This also prompts the question – are all MAT-appearing cells within the bone marrow actually adipocytes? Expression of key adipogenic transcription factors Pparg , Cebpa , and Cebpb in isolated MAT cells from healthy animals supports this identity [5,43]. However, the storage of marrow lipid in non-MAT populations, especially in states of disease, has not been thoroughly explored.…”

“…MAT is also intimately associated with the blood-forming marrow. Primary human MAT adipocytes, purified from the iliac crest, have the ability to support differentiation of CD34+ hematopoietic progenitor cells in vitro [43]. However, in vivo evidence in mice suggests that MAT adipocytes are predominantly negative regulators of the hematopoietic microenvironment, inducing both quiescence and loss of progenitor cells [15].…”

“…In mice, in vitro BMSC-derived adipocytes also produce CXCL1 and CXCL2 [49]. Ex vivo primary human MAT adipocytes from the iliac crest also secrete detectable levels of IL-6 and G-CSF, in addition to IL-8, after 7-days of ceiling culture [43]. It has been proposed that secretion of cytokines by MAT stimulates local differentiation and activation of osteoclasts [49].…”

Bone Marrow Adipose Tissue and Skeletal Health

“…However, this may be related more to the nature rather than the quantity of marrow adipose. For example, during puberty and with fracture repair processes, marrow fat exhibits a brown adipocyte-like phenotype characterized by the expression of brown adipocyte transcription factors (e.g., PR domain containing 16 (Prdm16) and Forkhead Box C2 (FoxC2)) and marker genes (e.g., PGC1 α , deiodinase 2 (Dio2), beta-3-adrenergic receptor (β3AR), and uncoupling protein 1 (UCP1)) [1, 4, 63, 64]. It is generally believed that marrow adipocytes exhibiting brown adipocyte-like phenotype contribute to a microenvironment favorable for osteogenesis by providing the necessary energy balance, adaptive thermogenesis, and/or by releasing pro-osteogenic factors such as insulin-like growth factor 1 (IGF1) and leptin [1, 4, 64, 65].…”