A 51K progestin-regulated protein secreted by human endometrial cells in primary culture

We studied the effect of a progestin (R5020) on proteins released by stromal and epithelial endometrial cells in primary culture. Stromal and epithelial cells was isolated by collagenase and hyaluronidase digestion of endometrial tissue. The synthesis of proteins released into the cell culture medium was assayed by [35S]methionine incorporation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by fluorography. The relative proportions of individual proteins secreted by stromal and epithelial cells varied. However, 29K, 128K, and 150K proteins were more abundant in media from epithelial cell cultures, whereas 60K and 70K proteins were more abundant in media from stromal cell cultures. More proteins were secreted by cells obtained in the luteal phase than by those obtained in the proliferative phase. R5020 consistently stimulated the synthesis of a minor protein of 51,000 mol wt secreted by both epithelial and stromal cells. Physiological concentrations of dexamethasone, dihydrotestosterone, or estradiol did not stimulate synthesis of the 51K protein. The effect of R5020 was concentration dependent; maximal synthesis occurred with 10 nM R5020 and 4 days of treatment. This 51K protein is different from the estrogen-regulated 52K protein of breast cancers. These results indicate that cultured endometrial cells can synthesize and release a variety of proteins in vitro. One of them, the 51K protein, is a marker of responsiveness to progestin.