Identification of modified forms of human glucocorticoid receptors during the cell cycle

Glucocorticoids induce alkaline phosphatase activity in HeLa S3 cells only during the Late G1 and S phases of the cell cycle. In order to determine why these cells respond to glucocorticoids only during these phases, we have determined the physicochemical characteristics of the glucocorticoid receptor (GR)1 in synchronized HeLa S3 cells. GR isolated from HeLa S3 cells in the Late G1 and S phases elute from DEAE cellulose as a single, major peak at a competing [KCl] of 0.15 M (Form I). In contrast, GR isolated from cells in the G2 and Early G1 phases show a biphasic elution pattern; a more highly acidic GR form elutes at a [KCl] or 0.25 M (Form II). Similar elution patterns are observed when GR is chromatographed on hydroxylapatite (HAP). In these studies a [K2HPO4] of 0.125 M elutes the major GR form in Late G1 and S phase cells, whereas a [K2HPO4] of 0.15 M elutes a second binding species from G2 and Early G1 phase cells. GR sediments through 5-20% sucrose density gradients as an approximately 4.6S species under 0.15 M KCl buffer conditions in all phases of the cell cycle; thus the altered receptor charge is not accompanied by a change in receptor size or shape. These data suggest that HeLa S3 cell GR are susceptible to modification during the cell cycle. Conversion of GR to acidic forms may relate to the receptors' ability to transmit hormonal signals.