Thyroid hormone, vitamin D and retinoid receptor expression and signalling in primary cultures of rat osteoblastic and immortalised osteosarcoma cells

3,5,3'-Tri-iodothyronine (T-3), 1 alpha,25(OH)2-vitamin D-3 (D-3) and retinoids activate related nuclear receptors which interact by heterodimerisation to regulate gene expression. Actions of each hormone are discrete and may be specified by changes in the relative concentrations of their receptors (T3R, vitamin D receptor (VDR), retinoic acid receptor (RAR), retinoid X receptor (RXR)). T-3, D-3 and retinoids are essential for skeletal development and maintenance and we have previously shown complex interactions amongst their signalling pathways in osteosarcoma cells. In these studies we demonstrate that similar T3R, VDR, RAR and RXR proteins are co-expressed in both osteoblast lineage cell primary cultures and osteosarcoma cells by Western blotting. We investigated whether hormone interactions in bone result from changes in receptor stoichiometry. Cells were treated with combinations of T-3, D-3, 9-cis retinoic acid (9-cis RA) and all-trans retinoic acid (RA) that are known from previous studies to produce complex cell specific responses. No alteration in expression of any receptor protein was seen in response to any hormone combination in three phenotypically distinct osteosarcoma cell lines. Thus, in contrast to studies of overexpressed receptors in vitro, changes in the physiological concentrations of endogenous T3R, VDR, RAR and RXR do not specify discrete hormone actions in osteoblastic cells. Other unidentified factors are likely to modulate hormone action in these bone cells.