Overexpression of ER and VDR is not sufficient to make ER-negative MDA-MB231 breast cancer cells responsive to 1α-hydroxyvitamin D5

1α-hydroxyvitamin D5 [1α(OH)D5] is an active vitamin D analog showing promising chemopreventive effect in breast carcinogenesis. We previously reported that estrogen receptor (ER)-positive breast cancer cells were sensitive, whereas ER-negative breast cancer cells were relatively resistant to their antiproliferative effects. In the present study, we used ER-negative MDA-MB231, ER-transfected MDA-MB231 (S30) and ER-positive BT474 cell lines to evaluate the possible association between ER status and cellular sensitivity to 1α(OH)D5 treatment. Our results demonstrate that ER expression in ER-negative breast cancer cells (S30) did not increase the sensitivity to 1α(OH)D5, whereas in ER-positive BT474 cells, the significant antiproliferative effect of 1α(OH)D5 was correlated with the downregulation of ER and progesterone receptor expression. Further analysis indicated that both MDA-MB231 and S30 cells express low vitamin D receptor (VDR) at transcriptional level and protein level. However, transfection of VDR failed to restore the sensitivity to 1α(OH)D5 in MDA-MB231 and S30 cells, although VDR direct target gene CYP24 was more responsive to 1α(OH)D5 treatment in MDA-MB231 and S30 cells overexpressing VDR. In addition, nuclear receptor cofactors NCoR1 and SRC1 that could potentially affect VDR action were also low in both MDA-MB231 and S30 cells in comparison with ER-positive, vitamin D-sensitive BT474 cells. These results suggest that in addition to the increased ER and VDR expression, the intact VDR signaling machinery as present in ER-positive, vitamin D-sensitive cells is essential for the antiproliferative action of vitamin D, whereas the direct VDR target genes such as CYP24 can remain responsive to augmented VDR expression.