1,25-DIHYDROXYVITAMIN-D3 STIMULATES GROWTH AND INHIBITS CALCITONIN SECRETION IN A HUMAN C-CELL CARCINOMA CELL-LINE

1,25-Dihydroxyvitamin D3 (1,25D3), calcitonin (CT) and parathyroid hor mone are the major calcium-regulating hormones. In addition, 1,25D3 ha s been reported to be a modulator of cell growth and differentiation i n many tissues. Recently, a suppressive effect of 1,25D3 on CT secreti on and synthesis in C cells Was demonstrated in vivo and also in vitro , but there are no data about its effects on thyroid C cell growth. We investigated the effects of 1,25D3 on basal and stimulated CT secreti on and on [H-3]-thymidine incorporation, using a human medullary thyro id carcinoma cell line (TT cells). After a 4-day exposure to 1,25D3, T T cells showed a dose-dependent inhibition of basal CT secretion (64% of the value for the control group at 100 nM 1,25D3). Calcium (3 mM) p lus K+ (50 mM) greatly increased CT secretion in both the control and vitamin D-treated groups. However, in the cells preincubated with 1,25 D3 the stimulated CT levels were less than observed in controls. A dos e-dependent increase in [H-3]-thymidine incorporation (200% of the val ue for the control group at 100 nM 1,25D3) and in cell number (150% of the value for the control group at 100 nM 1,25D3 after 72 h) was obse rved in the groups treated with 1,25D3. 24,25D3 had no effect on CT se cretion or cell growth compared to the control group. These data show that 1,25D3 decreased basal and Ca2+-stimulated CT secretion, a specia lized function of these cells, and stimulated their growth. Hence, in contrast to its effects on other cell lines, 1,125D3 appears to induce a dedifferentiation on TT cells.