Abstract
We determined the effect of cell cycle position on the amount of dexamethasone that was specifically bound by mouse and human lymphoid cell lines. Cell lines that were either sensitive or resistant to growth inhibition by dexamethasone were compared. Exponentially growing cells were separated by centrifugal elutriation into fractions that corresponded to different positions in the cell cycle. The cell cycle phase distribution of each fraction was estimated by flow cytometry and autoradiography. The amount of dexamethasone bound per cell in each fraction was measured by a whole cell binding assay. In three dexamethasone-sensitive cell lines (two mouse and one human), we found that the amount of dexamethasone bound per cell increased 2–4-fold between G1 phase and S phase, and then decreased during G2/M phase. Results were the same when the amount of dexamethasone bound per milligram of cell protein was measured. Binding affinity was the same during G1 phase and S phase, but the proportion of bound dexamethasone that translocated to the nucleus was greater during S phase. In contrast, we found that the amount of dexamethasone bound per cell by three dexamethasone-resistant cell lines (two mouse and one human) did not increase during S phase. Our results indicate that cell cycle changes in dexamethasone binding are not simply related to changes in cell protein or cell volume during the cell cycle. An increase in dexamethasone binding during S phase may be required for dexamethasone to inhibit cell growth, and a failure of dexamethasone binding to increase during S phase might represent a new mechanism of dexamethasone resistance in lymphoid cells.
