Bone marrow adipose tissue (BMAT) comprises 10% of total fat mass in healthy humans, and evidence suggests that BMAT mass increases with age, in osteoporosis and in obesity. Interestingly, bone marrow adipocytes do not decrease in size under conditions of caloric restriction. A study published in Cell Reports now shows that the lipid metabolism profile of bone marrow adipocytes is distinct from that of subcutaneous adipocytes.

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To examine the characteristics of human bone marrow adipocytes, BMAT and subcutaneous adipose tissue (SCAT) were obtained from patients undergoing hip replacement surgery. Samples from the different depots were paired from each patient. Adipocytes in BMAT had similar morphological properties to white adipocytes in SCAT; both cell types contain a large, lipid filled droplet.

Notably, proteomic analysis of bone marrow and subcutaneous adipocytes showed differences in the expression of proteins related to lipid metabolism pathways. Specifically, bone marrow adipocytes were enriched in pathways for cholesterol metabolism, arachidonic metabolism and sphingolipid signalling and show decreases in lipolytic enzymes. The lipid profiles of both cell types were also examined, showing enrichment of diverse lipid species, for example cholesterol and monoacylglycerols.

Functional characterization assays were performed on bone marrow adipocytes, comparing them to subcutaneous adipocytes from the same patient. Interestingly, bone marrow adipocytes show a defect in lipolysis, which is associated with a substantial downregulation in monoacylglycerol lipase, a key enzyme involved in lipolysis. This finding was not recapitulated in primary bone marrow-derived mesenchymal stromal cells (BM-MSCs) differentiated to become adipocytes in vitro. Owing to this discrepancy, the study authors suggest that future experiments with BM-MSCs be interpreted with caution.

bone marrow adipocytes show a defect in lipolysis

In summary, bone marrow adipocytes show distinct lipid metabolism to subcutaneous adipocytes, with decreased lipolysis and enriched cholesterol metabolism. As such, bone marrow adipocytes preserve their triglyceride stores and might be speculated not to release free fatty acids. The precise physiological functions of this cell type, however, remain to be elucidated.