Estradiol entry into endometrial cells in suspension.

Cells from a human endometrial adenocarcinoma cell line (HEC-50) were superfused with mixtures of [3H]E2 and [14C]E1 in order to estimate rates of entry and exit of E1 and E2 into and out of cells according to previously published procedures (J. steroid Biochem., 13 (1980) 1379). Proportionality between rates of entry and concentrations of E2 outside the cells, indicative of passive diffusion, was found at levels of E2 ranging from 1 to 100 ng/ml. Effects of albumin and of pure human sex steroid binding protein (SBP) on the rate of entry of E2 were also evaluated in parallel superfusions. In other single tracer experiments, [3H]E2 was used at concentrations as low as 100 pg/ml and the effects of plasma proteins on entry were evaluated by measuring steady-state concentrations of E2 and E1 in cells and superfusate. Results from these experiments indicate that albumin, and to a larger extent SBP, reduced the entry of E2 into HEC-50 cells. Similar results were obtained when CG-5 cells, a variant of the human breast cancer cell line MCF-7, were superfused with [3H]E2. Further experiments are needed, however, to determine the physiologic role of plasma estrogen binding proteins on the entry and metabolism of E1 and E2 into target cells.