VITAMIN-D RECEPTOR-BINDING AND BIOLOGICAL EFFECTS OF CHOLECALCIFEROL ANALOGS IN RAT-THYROID CELLS

The cholecalciferol analogues '(E)-enyl]-9,10-secopregna-5(Z),7(E),10( 19)-triene (calcipotriol, MC 903), y-pentoxy]-9,10-secopregna-5(Z),7(E ),10(19)-triene (KH 1060) and ene-1'-yl]-9,10-secopregna-5(Z),7(E),10( 19)-triene (EB 1089) have been modified in the side chain to increase their effects on cell differentiation and proliferation and to reduce the risk of inducing hypercalcemia. The effects of these analogues wer e tested on FRTL-5 cells, a strain of continuously growing and well-di fferentiated rat thyroid cells. FRTL-5 cells express a normal vitamin D receptor (VDR), and 1,25-(OH)(2)D-3 potently attenuates the thyrotro pin (TSH) stimulated production of the intracellular signalling molecu le 3',5'-cyclic adenosine monophosphate (cAMP), iodide uptake and cell growth of these cells. These effects were also induced by the choleca lciferol analogues after 4 days of incubation. KH 1060 was the biologi cally most potent of the analogues and, compared to KH 1060, the IC50 values were 1.2-, 2.7- and 14-fold higher when 1,25-(OH)(2)D-3, EB 108 9 and MC 903, respectively, were used for the displacement of receptor bound [H-3]1,25-(OH)(2)D-3. As indicated by their VDR binding, 1,25-( OH)(2)D-3 and EB 1089 were equipotent inhibitors of the TSH stimulated adenylyl cyclase activity, iodide uptake and FRTL-5 cell growth. The analogue MC 903 was the second most potent inhibitor of cell growth in spite of expressing the lowest affinity for the VDR and the weakest i nhibition of TSH-stimulated adenylyl cyclase activity and iodide uptak e. In conclusion, the biological effects of these cholecalciferol anal ogues in rat thyroid FRTL-5 cells seem to be mainly determined by thei r binding affinity for the VDR, although non-genomic effects can not b e excluded.