DJ: Cytoplasmic sequestration and functional repression of p53 in the mammary epithelium is reversed by hormonal treatment

ABSTRACT Proper function of the p53 tumor suppressor gene is critical for inhibiting tumor development in a broad spectrum of tissues. Although the mammary gland is highly susceptible to tumor formation, the functional status of p53 in the normal tissue had not been investigated. Therefore, expression, localization, and activity of p53 were examined in normal mammary tissues. High levels of p53 protein were found expressed in the cytoplasm of the ductal epithelium of the quiescent mammary gland. Ionizing radiation failed to recruit p53 to the nucleus, and p53-dependent responses were minimal. However, transient hormonal stimulation resulted in nuclear accumulation of p53, an induction of p21/WAF1, and a 5-fold increase in apoptosis after ionizing radiation. Therefore, the functional state of wild-type p53 in the mammary epithelium can be regulated by hormonal stimuli

Investigating the Genetic Circuitry of Mastermind in Drosophila, a Notch Signal Effector

Notch signaling regulates multiple developmental processes and is implicated in various human diseases. Through use of the Notch transcriptional co-activator mastermind, we conducted a screen for Notch signal modifiers using the Exelixis collection of insertional mutations, which affects ∼50% of the Drosophila genome, recovering 160 genes never before associated with Notch, extending the previous roster of genes that interact functionally with the Notch pathway and mastermind. As the molecular identity for most recovered genes is known, gene ontology (GO) analysis was applied, grouping genes according to functional classifications. We identify novel Notch-associated GO categories, uncover nodes of integration between Notch and other signaling pathways, and unveil groups of modifiers that suggest the existence of Notch-independent mastermind functions, including a conserved ability to regulate Wnt signaling

Development of Spontaneous Mammary Tumors in BALB/c p53 Heterozygous Mice : A Model for Li-Fraumeni Syndrome

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 p53null 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