WNT Genes and Breast Cancer. In: Bowcock AM, editor. Breast Cancer. Totowa: Humana Press; 1999. pp. 181-98. [DOI: 10.1007/978-1-59259-456-6_8]

Wnt Signaling Pathway in Mammary Gland Development and Carcinogenesis

The signaling pathway mediated by Wingless-type (Wnt) proteins is highly conserved in evolution. This pivotal pathway is known to regulate cell fate decisions, cell proliferation, morphology, migration, apoptosis, differentiation and stem cell self-renewal. It currently includes the canonical or Wnt/beta-catenin pathway in which Wnt proteins bind to 'frizzled' receptors, which leads to downstream activation of gene transcription by beta-catenin. Second, the noncanonical or beta-catenin-independent pathways are now known to be mediated by three possible mechanisms: (1) the Wnt/Ca(2+) pathway, (2) the Wnt/G protein signaling pathway, and (3) the Wnt/PCP or planar cell polarity pathway. Wnt signaling is implicated at several stages of mammary gland growth and differentiation, and possibly in the involution of mammary gland following lactation. Recent evidence suggests the role of Wnt signaling in human breast cancer involves elevated levels of nuclear and/or cytoplasmic beta-catenin using immunohistochemistry, overexpression or downregulation of specific Wnt proteins, overexpression of CKII and sFRP4, downregulation of WIF-1 and sFRP1, as well as amplification of DVL-1. Further research is required to determine how Wnt signaling is involved in the development of different histological types of breast cancer and whether it promotes the viability of cancer stem cells or not.

Impact of p53 loss on reversal and recurrence of conditional Wnt-induced tumorigenesis

Aberrant activation of Wnt signaling is oncogenic and has been implicated in a variety of human cancers. We have developed a doxycycline-inducible Wnt1 transgenic mouse model to determine the dependence of established mammary adenocarcinomas on continued Wnt signaling. Using this model we show that targeted down-regulation of the Wnt pathway results in the rapid disappearance of essentially all Wnt-initiated invasive primary tumors as well as pulmonary metastases. Tumor regression does not require p53 and occurs even in highly aneuploid tumors. However, despite the dependence of primary mammary tumors and metastases on continued Wnt signaling and the dispensability of p53 for tumor regression, we find that a substantial fraction of tumors progress to a Wnt-independent state and that p53 suppresses this process. Specifically, loss of one p53 allele dramatically facilitates the progression of mammary tumors to a Wnt1-independent state both by impairing the regression of primary tumors following doxycycline withdrawal and by promoting the recurrence of fully regressed tumors in the absence of doxycycline. Thus, although p53 itself is dispensable for tumor regression, it nevertheless plays a critical role in the suppression of tumor recurrence. Our findings demonstrate that although even advanced stages of epithelial malignancy remain dependent upon continued Wnt signaling for maintenance and growth, loss of p53 facilitates tumor escape and the acquisition of oncogene independence.

Genetically engineered rodent models of mammary gland carcinogenesis: an overview

Breast cancer is a multifactorial disease that develops as a result of interactions among genetic, environmental, and hormonal factors. Multiple genetic derangements are believed to be involved in the pathogenesis of breast cancer, including the inactivation of tumor suppressor genes and/or the disregulation of proto-oncogenes. Age, hormones, and environmental factors further influence these genetic derangements. Spontaneous and chemically induced animal models of breast cancer have been limited in their usefulness. The advent of targeted gene mutations has allowed for a more specific exploration of the pathogenesis of breast cancer by creating mouse models that mimic single or multiple gene alterations found in human mammary tumors. The genes targeted in these models include mouse mammary tumor integration sites and genes that encode for growth regulators, signal transduction proteins, cell cycle proteins, and cell matrix proteinases. In this review, I summarize tumor morphology and the relevance of each model to the pathogenesis and progression of human breast cancer. These models have great potential for elucidating the multistep process of mammary gland carcinogenesis and for contributing to the identification of novel therapeutic targets.

Wnt signaling in breast cancer: have we come full circle?

Since the original identification of Wnt1 as a mammary oncogene in mouse mammary tumor virus infected mice, questions have been asked about its relevance to human breast cancer. Wnt1 is now known to be one of a large family of Wnt genes encoding structurally similar secreted signaling proteins, several of which are functionally redundant. The principal intracellular signaling pathway activated by these proteins has been elucidated in recent years. Components of this pathway include proto-oncogene products, such as beta-catenin, and tumor suppressor proteins such as APC. Although WNT1 itself has not been implicated in human breast neoplasms, it has been reported that other WNT genes are sometimes overexpressed in human breast cancer and there is growing evidence that downstream components of the Wnt signaling pathway are activated in a significant proportion of breast tumors.