Mammary carcinogenesis is preceded by altered epithelial cell turnover in transforming growth factor-alpha and c-myc transgenic mice

In-silico insights on the prognostic potential of immune cell infiltration patterns in the breast lobular epithelium

Scattered inflammatory cells are commonly observed in mammary gland tissue, most likely in response to normal cell turnover by proliferation and apoptosis, or as part of immunosurveillance. In contrast, lymphocytic lobulitis (LLO) is a recurrent inflammation pattern, characterized by lymphoid cells infiltrating lobular structures, that has been associated with increased familial breast cancer risk and immune responses to clinically manifest cancer. The mechanisms and pathogenic implications related to the inflammatory microenvironment in breast tissue are still poorly understood. Currently, the definition of inflammation is mainly descriptive, not allowing a clear distinction of LLO from physiological immunological responses and its role in oncogenesis remains unclear. To gain insights into the prognostic potential of inflammation, we developed an agent-based model of immune and epithelial cell interactions in breast lobular epithelium. Physiological parameters were calibrated from breast tissue samples of women who underwent reduction mammoplasty due to orthopedic or cosmetic reasons. The model allowed to investigate the impact of menstrual cycle length and hormone status on inflammatory responses to cell turnover in the breast tissue. Our findings suggested that the immunological context, defined by the immune cell density, functional orientation and spatial distribution, contains prognostic information previously not captured by conventional diagnostic approaches.

Mouse model for probing tumor suppressor activity of protein phosphatase 2A in diverse signaling pathways

Evidence that protein phosphatase 2A (PP2A) is a tumor suppressor in humans came from the discovery of mutations in the genes encoding the Aα and Aβ subunits of the PP2A trimeric holoenzymes, Aα-B-C and Aβ-B-C. One point mutation, Aα-E64D, was found in a human lung carcinoma. It renders Aα specifically defective in binding regulatory B' subunits. Recently, we reported a knock-in mouse expressing Aα-E64D and an Aα knockout mouse. The mutant mice showed a 50-60% increase in the incidence of lung cancer induced by benzopyrene. Importantly, PP2A's tumor suppressor activity depended on p53. These data provide the first direct evidence that PP2A is a tumor suppressor in mice. In addition, they suggest that PP2A is a tumor suppressor in humans. Here, we report that PP2A functions as a tumor suppressor in mice that develop lung cancer triggered by oncogenic K-ras. We discuss whether PP2A may function as a tumor suppressor in diverse tissues, with emphasis on endometrial and ovarian carcinomas, in which Aα mutations were detected at a high frequency. We propose suitable mouse models for examining whether PP2A functions as tumor suppressor in major growth-stimulatory signaling pathways, and we discuss the prospect of using the PP2A activator FTY720 as a drug against malignancies that are driven by these pathways.

Reviewing once more the c-myc and Ras collaboration: converging at the cyclin D1-CDK4 complex and challenging basic concepts of cancer biology

The c-myc is a proto-oncogene that manifests aberrant expression at high frequencies in most types of human cancer. C-myc gene amplifications are often observed in various cancers as well. Ample studies have also proved that c-myc has a potent oncogenicity, which can be further enhanced by collaborations with other oncogenes such as Bcl-2 and activated Ras. Studies on the collaborations of c-myc with Ras or other genes in oncogenicity have established several basic concepts and have disclosed their underlying mechanisms of tumor biology, including "immortalization" and "transformation". In many cases, these collaborations may converge at the cyclin D1-CDK4 complex. In the meantime, however, many results from studies on the c-myc, Ras and cyclin D1-CDK4 also challenge these basic concepts of tumor biology and suggest to us that the immortalized status of cells should be emphasized. Stricter criteria and definitions for a malignantly transformed status and a benign status of cells in culture also need to be established to facilitate our study of the mechanisms for tumor formation and to better link up in vitro data with animal results and eventually with human cancer pathology.

Genetic manipulation of individual somatic mammary cells in vivo reveals a master role of STAT5a in inducing alveolar fate commitment and lactogenesis even in the absence of ovarian hormones

Assessing the molecular control of development and cell fate in individual cells in the intact mammary epithelium has not been possible to date. By exploiting an intraductal retrovirus (RCAS)-mediated gene delivery method to introduce a marker gene, we found that ductal epithelial cells are turned over with a half time of approximately 1month in adult virgin mice. However, following RCAS-mediated introduction of a constitutively activated STAT5a (caSTAT5a), caSTAT5a-activated ductal epithelial cells expand and replace other cells in the epithelium, eventually forming a mammary gland resembling that in a late pregnant mouse, suggesting that STAT5a activation alone is sufficient to mediate pregnancy-induced mammary cell expansion, alveolar cell fate commitment, and lactogenesis. Furthermore, such caSTAT5a-induced alveolar differentiation does not require ovarian functions, although caSTAT5a-induced cell proliferation is partly reduced in ovariectomized mice. In conclusion, in this first report of studying the developmental role of a gene in a few cells in a normally developed virgin mammary ductal tree, STAT5a activation causes alveolar fate commitment and lactogenesis, and with the help of ovarian hormones, drives alveolar expansion.

Short-term prophylactic tamoxifen reduces the incidence of antiestrogen-resistant/estrogen receptor-positive/progesterone receptor-negative mammary tumors

Although many estrogen receptor-positive (ER+) breast cancers are effectively treated with selective estrogen receptor modulators and down-regulators (SERM/SERD), some are highly resistant. Resistance is more likely if primary cancers are devoid of progesterone receptors (PR-) or have high levels of growth factor activity. In this study, a transgenic mouse line that expresses transforming growth factor-alpha (NRL-TGFalpha mice) and that develops ER+/PR- mammary tumors was used to assess the possible effects of (a) therapeutic delivery of the SERM, tamoxifen, or SERD, ICI I82,780 (ICI), on the growth of established tumors and (b) short-term prophylactic tamoxifen administration on the initial development of new mammary tumors. To determine the therapeutic effects of tamoxifen and ICI on the growth of established tumors, mice were exposed to 3 weeks of drug treatment. Neither drug influenced tumor growth or glandular pathology. To determine if early prophylactic tamoxifen could alter tumorigenesis, a 60-day tamoxifen treatment was initiated in 8-week-old mice. Compared with placebo-treated mice, tamoxifen reduced tumor incidence by 50% and significantly decreased the degree of mammary hyperplasia. Prophylactic tamoxifen also significantly extended the life span of tumor-free mice. These data show that in this mouse model, established ER+/PR- mammary tumors are resistant to SERM/SERD treatment but the development of new mammary tumors can be prevented by an early course of tamoxifen. This study validates the utility of NRL-TGFalpha mice for (a) identifying candidate biomarkers of efficacious tamoxifen chemoprevention and (b) modeling the evolution of tamoxifen resistance.

Roles of transforming growth factor-alpha in mammary development and disease

Transforming growth factor-alpha (TGFalpha) is a member of the epidermal growth factor (EGF) family. Expression of TGFalpha is highly regulated in response to exogenous cellular signals including cytokines and other growth factors. The growth factor has been found to be indispensable for proper development of many tissues and organs. TGFalpha has also been implicated in numerous disease states including forms of breast cancer. This minireview summarizes the basic biology of TGFalpha and its actions during normal and pathogenic development of the mammary epithelium.