Calcitriol is a positive effector of adipose differentiation in the OB 17 cell line: relationship with the adipogenic action of triiodothyronine.

In a previous report, we showed that physiological concentrations of calcitriol (1 alpha,25-(OH)2 vitamin D3 or VD), markedly stimulated the terminal adipose differentiation of Ob 17 preadipocytes cultured under standard conditions with fetal calf serum (FCS), and increased the differentiating effect of triiodothyronine (T3) reported as a necessary adipogenic factor in these cells. Here, we demonstrate, for the first time, that VD is an intrinsic strong adipogenic factor for the Ob 17 preadipocytes cultured in thyroid hormone-deprived medium (adipogenic concentrations: 0.025-0.25 nM in the presence of stripped FCS, 1-10 pM under serum-free conditions). VD action was potentiated by the coaddition of either T3, or arachidonic acid, two agents which also bear proper adipogenic properties. The efficient concentration ranges of other vitamin D3 metabolites suggest a mediation through the VD nuclear receptor (VDR). An expression of the VDR gene is here demonstrated in the Ob 17 cells, and evidence is given that VDR mRNA level increased during the differentiation process and that this increase is moderately amplified under long term treatment with adipogenic concentrations of VD. Our results strongly suggest that adipose differentiation is under the control of different closely related nuclear receptors acting at an early preadipocyte step and probably in an interchangeable manner depending on the availability of their respective ligands. The existence of an interplay between these receptors in exerting their adipogenic action is suggested.