Expression of pregnancy-specific genes in preneoplastic mouse mammary tissues from virgin mice

Abstract

Experimentally induced breast cancer is often preceded by the appearance of preneoplastic lesions which possess the attributes of hyperplastic normal tissue. These lesions can be isolated and carried as stably transplantable outgrowth lines which continue to morphologically resemble differentiating mammary tissue (Medina, D. Methods Cancer Res., 7: 3-53, 1973). We established seven serially transplantable hyperplastic alveolar nodule (HAN) outgrowth lines from virgin mouse mammary tissues following induction by mouse mammary tumor virus, dimethylbenz(alpha)-anthracene, and/or pituitary isografts. The expression of mammary differentiation-specific casein genes was measured in these hyperplastic outgrowths by immunocytochemistry, specific radioimmune precipitation, and blot hybridization of total RNA. All seven HAN outgrowth lines were immunologically positive for casein both in situ and upon explant culture. Unlike explants from normal virgin mouse mammary gland, exposure to insulin, hydrocortisone, and prolactin induced an increase in casein synthesis in HAN explant cultures which was independent of DNA synthesis. [35S]Methionine-labeled polypeptides synthesized in explant cultures of HAN outgrowths freshly isolated from virgin hosts were analyzed by radioimmune precipitation and gel electrophoresis. This analysis demonstrated that all major species of casein, alpha (Mr 46,000), beta (Mr 27,000), and gamma (Mr 25,000), were constitutively (i.e., in the absence of lactogenic stimuli) expressed in these preneoplastic alveolar mammary outgrowths. In support of this observation, RNA homologous to beta- and alpha-casein cDNA probes was often detectable in total RNA preparations from freshly isolated fragments of HAN outgrowths. A second mammary differentiation specific gene product, alpha-lactalbumin, was also detected in HAN outgrowths both in situ and following explant culture. Enzymatically active alpha-lactalbumin was present in extracts of freshly isolated HAN outgrowth tissues and was detectable in these same tissues by immunoperoxidase. In general, alpha-lactalbumin synthesis was increased during explant culture in the presence of lactogenic hormones; however, in contrast to casein synthesis, insulin-hydrocortisone-prolactin-induced increase in alpha-lactalbumin production in vitro was occasionally dependent upon DNA synthesis as it is in explants from normal virgin mouse mammary tissue.(ABSTRACT TRUNCATED AT 400 WORDS)