Although alterations in the p53 tumor suppressor gene are detected frequently in human breast cancers, mammary tumors are observed infrequently in p53 null mice. This has led to the suggestion that absence of p53 alone is not sucient for induction of mammary tumors. However, early death of p53 null mice from thymic lymphomas may obscure tumor phenotypes that would develop later. Therefore, p53null mammary epithelium was transplanted into cleared mammary fat pads of wild type p53 BALB/c hosts to allow long-term analysis of mammary tumor phenotypes. Five treatments were compared for their eects on tumor incidence in hosts bearing transplants of p53 null vs p53 wt mammary transplants for each treatment group were 62% vs 0%, 100% vs 0%, 68% vs 0%, 60% vs 4% and 91% vs 14%, respectively. The mammary tumors that developed in the p53 null mammary epithelium were all adenocarcinomas and were frequently aneuploid. These data demonstrate that the absence of p53 is sucient to cause development of mammary tumors and that hormonal stimulation enhances the tumorigenicity of p53 null mammary epithelium to a greater extent than DMBA exposure alone. This model provides an in situ approach to examine the molecular basis for the role of p53 in the regulation of mammary tumorigenesis.

Breast cancer is the most frequent tumor type among women in the United States and in individuals with Li-Fraumeni syndrome. The p53 tumor suppressor gene is altered in a large proportion of both spontaneous breast malignancies and Li-Fraumeni breast cancers. This suggests that loss of p53 can accelerate breast tumorigenesis, yet p53-deficient mice rarely develop mammary tumors. To evaluate the effect of p53 loss on mammary tumor formation, the p53(null) allele was back-crossed onto the BALB/c genetic background. Median survival was 15.4 weeks for BALB/c-p53(-/-) mice compared to 54 weeks for BALB/c-p53(+/-) mice. Sarcomas and lymphomas were the most frequent tumor types in BALB/c-p53(-/-) mice, whereas 55% of the female BALB/c-p53(+/-) mice developed mammary carcinomas. The mammary tumors were highly aneuploid, frequently lost the remaining wild-type p53 allele, but rarely lost BRCA1. Although mammary tumors were rarely detected in BALB/c-p53(-/-) female mice, when glands from BALB/c-p53(-/-) mice were transplanted into wild-type BALB/c hosts, 75% developed mammary tumors. The high rate of mammary tumor development in the BALB/c background, but not C57Bl/6 or 129/Sv, suggests a genetic predisposition toward mammary tumorigenesis. Therefore, the BALB/c-p53(+/-) mice provide a unique model for the study of breast cancer in Li-Fraumeni syndrome. These results demonstrate the critical role that the p53 tumor suppressor gene plays in preventing tumorigenesis in the mammary gland.

In mammals, weaning of neonates and subsequent milk stasis initiates removal of the secretory epithelium of the mammary gland by apoptosis. The p53 tumor suppressor gene is induced rapidly following weaning of neonates, but its role in the process of involution has not been de®ned. Therefore, experiments were performed to identify the cell types in which the p53 gene is expressed during involution and determine the consequences of its absence in BALB/c-p53 null mice. Both p53 mRNA and protein were detected in the mammary epithelium within 48 h following weaning and resulted in an eightfold increase in levels of p21 WAF1 mRNA. Induction of p21 WAF1mRNA was absent in BALB/c-p53 null mice, and therefore, was shown to be p53-dependent. The BALB/cp53 null mice exhibited delayed involution of the mammary epithelium, as measured by 60% greater epithelial area compared to BALB/c-p53 wt mice through 5 days postweaning. The delay was transient with no dierences being apparent at 7 days post-weaning. Expression of the stromal protease stromelysin-1 was unaected by the absence of p53 suggesting that stromal responses were intact. These data demonstrate that p53 participates in the ®rst stage of involution initiated by the epithelium itself, but does not aect the second phase during which stromal proteases are induced.

DNA damage normally induces p53 activity, but responses to ionizing radiation in the mammary epithelium vary among developmental stages. The following studies examined the hormones and growth factors that regulate radiation-responsiveness of p53 in mouse mammary epithelium. Immunoreactive p21/WAF1 and TUNEL staining were used as indicators of p53 activity following exposure to ionizing radiation. In ovariectomized mice, radiation-induced accumulation of p21/WAF1 was minimal in the mammary epithelial cells (o1%). Systemic injections of estrogen and progesterone (E þ P) for 72 h were necessary to recover maximal expression of p21/ WAF1 following ionizing radiation (55%). The effects of E þ P on radiation-induced p21/WAF1 were p53-dependent as responses were absent in Trp53 À/À mice. Though hormonal treatments stimulated increases in the proportion of cycling cells (PCNA-positive), this was not directly correlated with p53 activity. Whole organ cultures were used to determine whether E þ P act directly upon the mammary gland. Treatment with E þ P was sufficient to render p53 responsive to radiation, but TGF-b-neutralizing antibodies blocked responsiveness. In the absence of E þ P, TGF-b1 alone did not alter p53 activity. These results demonstrate that estrogen and progesterone together with TGF-b signaling are necessary for maintenance of p53 activity in the mammary epithelium.

Regulation and functions of the p53 tumor suppressor gene have been studied extensively with respect to its critical role in maintaining the stability of genomic DNA following genotoxic insults. However, p53 is also induced by physiologic stimuli resulting in cell cycle arrest and apoptosis. In other situations, the activity of p53 must be repressed to prevent inappropriate removal of cells. The mammary gland provides a valuable system in which to study the mechanisms by which the expression and biological responses to p53 can be regulated under a variety of physiological circumstances. The pro-apoptotic role of p53 in the secretory mammary epithelium may be especially relevant to lactation in livestock. We have utilized p53-deficient mice to establish the molecular targets of p53 in the mammary gland and biological consequences when it is absent. The p21/WAF1 gene (Cdkn1a) is a transcriptional target gene of the p53 protein that responds to elevated levels of p53 during milk stasis providing an endogenous reporter of p53 activity. Abrogation of p53 resulted in delayed involution of the mammary epithelium, demonstrating the physiological role of p53 in regulating involution. Though delayed, stromal proteases were induced in the mammary gland by 5 d postweaning, providing a p53-independent mechanism that resulted in removal of the residual secretory epithelium. These processes can be interrupted by treatment with hydrocortisone. These data establish p53 as a physiological regulator of involution that acts to rapidly initiate apoptosis in the secretory epithelium in response to stress signals, but also indicate the presence of compensatory pathways to effect involution. Additional mechanisms involving intracellular stress signaling pathways (e.g., Stat3) and stromal-mediated pathways have been identified and, together with p53 pathways, may be used to identify animals with greater persistency of lactation.