Bone Abstracts

We sought to elucidate the effects of dietary caloric alterations on bone marrow adiposity and the effects of β-adrenergic signaling on marrow stromal cells’ adipogenic differentiation. Male 6-week-old C57BL/6 mice were assigned into three groups: an ad-libitum fed control diet (CON; 10 kcal% fat), a high calorie diet (HIGH; 60 kcal% fat) and a low calorie diet (LOW; 30% kcal restriction vs CON diet). In each diet group, mice were treated with vehicle (VEH: DI water) or propranolol, a β-adrenergic receptor antagonist (BB; 0.5 g/l in drinking water). Over 12 weeks, the number of adipocytes in the bone marrow area significantly increased in LOWVEH and HIGHVEH mice compared with CONVEH mice. Propranolol significantly mitigated the increased number of adipocytes in the bone marrow area seen in both LOWVEH and HIGHVEH mice. Isopreterenol, a major β-adrenergic receptor (βAR) agonist, increased while propranolol suppressed lipid droplet accumulation and adipogenic marker gene expressions, including adiponectin, adipocyte protein 2, pparγ, in 3T3L1 preadipocytes and bone marrow stromal cells (bMSCs). Next, we performed two compartment co-cultures of 3T3L1 preadipocytes and MC3T3E1 osteoblasts to elucidate the role of sympathetic nervous system (SNS) in osteoblasts mediated regulation of marrow adipogenesis. Isopreterenol led to an increment of adipogenic marker gene expressions while propranolol mitigated isoproterenol induced increase of adipogenic gene expressions in 3T3L1 cells co-cultured with osteoblasts. Levels of adipogenic marker gene expressions decreased when 3T3L1 cells were co-cultured in the presence of conditioned media from MC3T3E1 cells. However, the decreased adipogenic marker gene expressions were elevated when 3T3L1 cells cultured with conditioned media from the isopreterenol treated MC3T3E1 cells. Collectively, these data suggest that SNS activation through βAR signaling is critical for the regulation of marrow adipogenesis and the SNS regulation of marrow adipogenesis occurs in part via the osteoblasts.