Secretion of a cysteine proteinase from a hormone-independent cell population of cultured explants of murine mammary gland.

Cultured explants of mouse mammary gland were investigated for their capacity to secrete a cathepsin B-like cysteine proteinase. This enzyme had been shown previously to be secreted excessively from cultured explants of human breast tumors and spontaneous mouse mammary carcinomas. We now show that secretion is also observed from cultured explants of mammary gland. Lactating tissue and tissues obtained from mid- and late-pregnant mice were found to secrete the cysteine proteinase at very high rates, but secretion was also detectable from explants of virgin mammary glands and from tissues obtained from retired breeders. However, in all cases, it was found that secretion was greatest from explants maintained in hormone-free medium and did not depend on the maintenance of normal mammary gland function. Secretion was greatly reduced in the presence of the lactogenic hormone combination of insulin, prolactin, and hydrocortisone, and this suppression was found to be due to hydrocortisone. Insulin and prolactin, while resulting in better tissue maintenance in culture, had no effect on the secretion. Enzyme release was reversibly inhibited by cycloheximide and required the presence of viable tissue, ruling out the possibility that the accumulation of enzyme activity in the culture medium is due to dying cells. The presence of metabolically active cells in explants cultured in the absence of hormones was also demonstrated by the incorporation of radiolabeled precursors into protein and DNA. DNA synthesis in cultured explants of lactating tissue was not stimulated by the addition of hormones. Histological studies revealed that, while large areas of the explants showed severe degeneration after culture without added hormones, clusters of cells persisted which displayed a relatively high mitotic activity and which showed a lack of normal epithelial organization. These observations suggest the presence of a hormone-independent cell population in the mammary gland, which secretes the stable cysteine proteinase. The secretion process itself, however, is inhibited by corticosteroids.