Calcitriol down-modulates the 3,5,3'-triiodothyronine (T-3) receptors and affects, in a biphasic manner, the T-3-dependent adipose differentiation of Ob 17 preadipocytes

In previous reports, we showed that T-3 is required for terminal differentiation of the murine Ob 17 preadipocytes, and that it partially down-modulates the abundance of its own nuclear receptor sites (T(3)R). We also reported that a profound depletion of the T(3)R was produced by all-trans-retinoic acid at concentrations that inhibit adipose differentiation. Here, we report that calcitriol (VD), which activates a nuclear receptor (VDR) closely related to the T(3)R and retinoid receptors, also markedly affects nuclear T-3 binding and T-3-induced differentiation of Ob 17 cells. Within a nearly physiological concentration range (0.1-2.5 nM), calcitriol profoundly down-modulated T(3)R abundance without altering the affinity for T-3. The T(3)R depletion was a fast event, sustained under VD and reversed within 48 h of VD withdrawal. The order of efficient concentration ranges of VD and analogs suggests an involvement of the VDR. The T(3)R-depleting effect of VD was observed at every stage of adipose differentiation and was additive to the depleting effect of T-3. Within the 0.1-2.5 nM VD concentration range, the c-erbA alpha and -alpha 1 messenger RNA levels (only c-erbA alpha gene products were detected in these cells) were poorly decreased; VD also did not alter a protein band specifically detected with specific anti-c-erbA alpha 1 antibodies in Western blots of nuclear extracts. VD accelerated the T(3)R disappearance rate; the results suggest that this would probably involve sequestration, rather than degradation events. Interestingly, calcitriol added to the culture medium of Ob 17 preadipocytes markedly influenced the adipose differentiation, exerting a clear-cut stimulation at levels of 0.25 nM. Both effects were observed provided that VD was added within an early critical period of the differentiation process, as we previously reported for T-3. The stimulations caused by low concentrations of VD and 1.5 nM T-3 were additive. Increasing the VD concentration produced a progressive attenuation, then a suppression, of the stimulating effect of T-3. Comparative analyses of VD-related changes in adipose differentiation and T(3)R abundance suggest that a correlation may exist between optimal differentiation and a partial depletion of the T(3)R play a role in the differentiation of Ob 17 preadipocytes. Moreover, the results suggest that there may be a T-3 receptor site concentration optimal for efficient differentiation. A regulation of this concentration involves ligands of other closely related receptors and, thus, probably the interplays that exist between these receptors.