Ex vivo human cellular models to study adipocyte-induced transdifferentiation of osteoblasts

In human, an association between bone loss and increased marrow adipose tissue suggests that medullary adipocytes could play a role in osteoporosis by acting on neighboring bone-forming osteoblasts. Supporting this hypothesis, we previously showed, using coculture and conditioned medium models based on human bone marrow stromal cells of commercial origin, that factors secreted by adipocytes induced the transdifferentiation of osteoblasts towards an adipocyte-like phenotype. The aim of this study was to confirm the involvement of medullary adipocyte secretion products in the alteration of osteoblasts in pathophysiological conditions. To this end, two new cellular models were developed from human bone biopsies representing the aging skeleton. In the first model, outgrowth from trabecular bone fragments were used to isolate primary cells that displayed a specific osteoblast phenotype. We confirmed the transdifferentiation of these primary osteoblasts following their incubation with adipocyte conditioned medium as evidenced by the increase in the levels of adipogenic mRNA markers (PPARG, Leptin and HSD11B1, P < 0,001) and the decrease of osteogenic transcript (BGLAP, P < 0.05). The second model was based on primary adipocyte isolation through collagenase treatment followed by ceiling and 2D culture. Oil red O staining confirmed the isolation of fully-differentiated primary adipocytes from bone biopsies. Experiments performed with their conditioned medium validated what had been observed with primary osteoblasts: the effect of the adipocyte secretome on osteoblast fate. Overall, our results supported the relevance of these models to examine paracrine interactions between osteoblasts and adipocytes and confirmed the role played by adipocyte-secreted factors in osteoblast transdifferentiation.