CHARACTERIZATION OF THE VITAMIN-D RECEPTOR FROM THE CACO-2 HUMAN COLON-CARCINOMA CELL-LINE - EFFECT OF CELLULAR-DIFFERENTIATION

The human colon carcinoma cell line, Caco-2, is the only intestinal cell line to spontaneously differentiate in culture to a population exhibiting structural and biochemical characteristics of mature enterocytes. We conducted studies to establish the presence of the vitamin D receptor (VDR), determine changes in VDR concentration and affinity with differentiation and determine whether 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3) mediates a functional response in this cell line. We found that Caco-2 cells possess a specific 1,25(OH)2D3 binding protein similar to the mammalian VDR. It has an equilibrium dissociation constant (Kd) of 0.72 nm, binds vitamin D analogues in order of their biological activities in vivo (1,25(OH)2D3 > 25(OH)D3 > 24,25(OH)2D3), sediments as a single peak on sucrose density gradients at 3.7 S, and is eluted from a DNA-cellulose column by 0.16 m KCl. The maximum number of binding sites was 2.6-fold greater in the differentiated cell (Day 15) compared to the preconfluent, undifferentiated (Day 4) cell (23 fmol/ mg protein vs 56 fmol/mg protein). Cell growth was reduced 59% when exposed to 10-7 m 1,25(OH)2D3 for 8 days. Alkaline phosphatase activity significantly increased in cultures incubated with 10-8 m 1,25(OH)2D3 for up to 4 days when treatment was started in both undifferentiated cells (Day 5) and differentiated cells (Day 11). These findings suggest that the VDR present in undifferentiated and differentiated Caco-2 cells is functional. Caco-2 cells provide a unique in vitro model to study vitamin D-regulated functions in differentiated mammalian enterocytes. © 1991.