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Recent advancements in the field of cancer have established that the process of metastasis is organ-specific with tumor cell dissemination occurring in the very early stages of disease. Pre-metastatic niches are actively remodeled and transformed by both primary tumor specific factors and by influences from the extracellular matrix.Although improvements in cancer therapies have significantly improved outcomes in patients with early stage disease, the risk of recurrence and relapse leading to mortality remains high. Recent studies have emerged highlighting the influence of dormant tumor cells and exosomes as key players in cancer relapse. In this review we discuss the critical mediators of tumor progression and their link to cancer dormancy, while also exploring possible therapeutics for targeting relapse., (Copyright © 2021. Published by Elsevier Ltd.)

The normal mammary gland and invasive breast tumors are both complex 'organs' composed of multiple cell types as well as extracellular matrix in three-dimensional (3D) space. Conventionally, both normal and malignant breast cells are studied in vitro as two-dimensional monolayers of epithelial cells, which results in the loss of structure and tissue function. Many laboratories are now investigating regulation of signaling function in the normal mammary gland using 3D cultures. However, it is also important to assay malignant breast cells ex vivo in a physiologically relevant environment to more closely mimic tumor architecture, signal transduction regulation and tumor behavior in vivo. Here we present the potential of these 3D models for drug testing, target validation and guidance of patient selection for clinical trials. We also argue that in order to get full insight into the biology of the normal and malignant breast, and to create in vivo-like models for therapeutic approaches in humans, we need to continue to create more complex heterotypic models to approach the full context the cells encounter in the human body.

In order to understand why cancer develops as well as predict the outcome of pharmacological treatments, we need to model the structure and function of organs in culture so that our experimental manipulations occur under physiological contexts. This review traces the history of the development of a prototypic example, the three-dimensional (3D) model of the mammary gland acinus. We briefly describe the considerable information available on both normal mammary gland function and breast cancer generated by the current model and present future challenges that will require an increase in its complexity. We propose the need for engineered tissues that faithfully recapitulate their native structures to allow a greater understanding of tissue function, dysfunction, and potential therapeutic intervention.

That cancer development is a multistep process, driven in large part by genetic change, is well established. However, it is becoming increasingly clear that, prior to its emergence, the tumorigenic phenotype must overcome the suppressive effects of the surrounding microenvironment. Because the microenvironment is tissue-specific, cancer in each organ must develop unique strategies to overcome these normal epigenetic suppressors. Surprisingly, the induction of glandularity during the earliest stages of ovarian carcinoma development produces a microenvironment that has much in common with the normal mammary gland. This phenotypic convergence may explain why similar genetic and epigenetic changes appear to play a role in breast and ovarian tumor progression., (Copyright 2002 Published by Elsevier Science Ltd.)

Many cancer investigations have focussed on the eradication of the cancer cell itself and in doing so, overlook the inherent complexity and heterogeneity of solid tumors. Here, we argue that, in many cases, it is the altered communication within the tumor, rather than mutations per se, that is the defining characteristic of cancer. As a result, tumorigenesis can be indirectly initiated by environmental or inherited factors that affect the stromal cells. We propose that anticancer research might be more effective if aimed at eradicating the cause of abnormality rather than just treating the end result., (Copyright 2001 Academic Press.)

Normal mammary homeostasis, and by implication tumorigenesis, are dependent upon the dynamic interplay between epithelial cells, stromal components and the extracellular matrix. To study the evolution of human breast cancer, a functionally relevant cell culture model is required which recognizes the complexity of the mammary gland's microenvironment. The development of an appropriate breast epithelial cancer cell model will be dependent on the ability to recreate the 'normal' and 'neoplastic' tissue microenvironment in culture. Towards this goal, a 3-dimensional extracellular matrix (ECM) assay, employing a reconstituted basement membrane, has been developed which allows for the rapid and accurate discrimination of normal and neoplastic cells when cultured. To investigate stromal/epithelial cell interactions, we have developed a tumor environment assay which essentially mirrors the tumor microenvironment histologically. The use of a novel, near diploid, human breast epithelial cell line, HMT-3522, which has transformed spontaneously with passage in culture, together with these 3-dimensional culture assays is expected to provide meaningful markers of initiation and progression.

An intact basement membrane (BM) is essential for the proper function, differentiation and morphology of many epithelial cells. The disruption or loss of this BM occurs during normal development as well as in the disease state. To examine the importance of BM during mammary gland development in vivo, we generated transgenic mice that inappropriately express autoactivating isoforms of the matrix metalloproteinase stromelysin-1. The mammary glands from these mice are both functionally and morphologically altered throughout development. We have now documented a dramatic incidence of breast tumors in several independent lines of these mice. These data suggest that overexpression of stromelysin-1 and disruption of the BM may be a key step in the multi-step process of breast cancer.

It has recently been established that the extracellular matrix is required for normal functional differentiation of mammary epithelia not only in culture, but also in vivo. The mechanisms by which extracellular matrix affects differentiation, as well as the nature of extracellular matrix constituents which have major impacts on mammary gland function, have only now begun to be dissected. The intricate variety of extracellular matrix-mediated events and the remarkable degree of plasticity of extracellular matrix structure and composition at virtually all times during ontogeny, make such studies difficult. Similarly, during carcinogenesis, the extracellular matrix undergoes gross alterations, the consequences of which are not yet precisely understood. Nevertheless, an increasing amount of data suggests that the extracellular matrix and extracellular matrix-receptors might participate in the control of most, if not all, of the successive stages of breast tumors, from appearance to progression and metastasis.