Modulation of cell cycle control by vitamin D₃ and its analogue, EB1089, in human breast cancer cells

Abstract

Examination of a panel of ER positive breast cancer cell lines showed that they were differentially growth inhibited by vitamin D₃ and its analogue EB1089. EB1089 treatment of the breast cancer cell lines MCF-7 E, BT20, T47D, and ZR75 demonstrated a correlation between a reduction in Cdk2 kinase activity towards phosphorylation of histone H1 and a decrease in DNA synthesis, while no modulation of Cdk2 activity was observed in the vitamin D₃ and EB1089 resistant cell line MCF-7 L. This was accompanied by a time dependent decrease in the percentage of S phase cells in the responsive lines. Characterization of the expression levels of Cdk2 and its related cell cycle proteins in MCF-7 E cells showed that after EB1089 treatment, there was a concentration and time dependent up-regulation of p21 as well as a decrease in cyclin A proteins. Paradoxically, cyclin E levels were increased as a function of treatment. Analysis of cyclin-Cdk2-Cdki complex formation showed that in EB1089 treated MCF-7 E cells, Cdk2, cyclin A and cyclin E immunoprecipitates contained an increased abundance of p21. In contrast to MCF-7 E cells, increases in both p21 and p27 as well as their complex formation with Cdk2 were observed in BT20 and ZR75 cells. These findings indicate that up-regulation of p21 as well as p27 in some cell types may account for the inactivation of Cdk2 activity and a G₁ block of the cell cycle following EB1089 treatment.