CommentaryTissue architecture: the ultimate regulator of breast epithelial function
Introduction
A problem in developmental biology that continues to take center stage is how higher organisms generate diverse tissues and organs given the same cellular genotype. In cell and tumor biology, the key question is not the production of form, but its preservation: how do tissues and organs maintain homeostasis, and how do cells within tissues lose or overcome these controls in cancer? Undoubtedly, mechanisms that maintain tissue specificity should share features with those employed to drive formation of the tissues. However, they are unlikely to be identical. At a simplistic level, developmental pathways may be thought of as a series of extremely rapid short-term events. Each new step depends on what came before, and the outcome is the organism itself at birth. All organs, with a few notable exceptions, such as the mammary gland and the brain, ‘arrive’ together and are complete when the organism is born. In mice and humans, these events occur in a mere 21 days and 9 months respectively. The stability of the differentiated state and the homeostasis of the organism, on the other hand, will last 40–110 times longer. How does the organism achieve this feat? How are tissues maintained? These questions also relate fundamentally to how tissues become malignant and, although not discussed here, to aging.
While there is much literature on differentiation — loosely defined as the gain of a single or a series of functions — we know much less about the forces and the pathways that maintain organ morphology and function as a unit. This may be partly because it is difficult to study a tissue as a unit in vivo and there are few techniques that allow maintenance of organs in vitro long enough and in such a way as to make cell and molecular biology experiments possible. Techniques for culturing cells in three-dimensional gels (3D) as a surrogate for tissues, however, have been steadily improving (for a recent review of current models, see [1]) and the method is now used by several laboratories.
In this commentary we discuss the following: first, how our laboratory came to develop a model of the mammary gland acinus; second, what this model has told us about mechanisms that govern tissue specificity and malignancy; and third, possible directions for future studies. We summarize the evidence for the central role of ECM signaling in the maintenance of mammary function in culture and (more briefly) its role in tumorigenesis. This is followed by a discussion of the role that tissue architecture and tissue polarity (as opposed to cell polarity) may play in these processes.
In an elegantly written and reasoned essay [2], Kirschner et al. coined the new science of developmental biology ‘molecular vitalism’. They framed new concepts for self-organization as well as schemes for information flow in biological organization. Rao et al. [3••] reviewed and elaborated on differential-equation-based models of biochemical reaction networks and intracellular noise, with emphasis on bacteria and phage. Similarly, Hartwell et al. [4] discussed the synergy between experiment and theory in elucidating ‘modules’ — collections of interacting molecules — and in unraveling how these modules collaborate to perform cellular functions such as signal transduction. We believe that many of these ideas will also be applicable to the maintenance of tissue specificity. As much as we agree with Kirschner et al. [2] regarding the limitations of the machine analogy to biological systems, we conclude with thoughts on how we may proceed to model the complex tissue networks that govern breast tissue architecture. We suggest that our understanding of the structure and function of breast tissue would benefit from examining recent techniques for modeling large complex networks such as the World Wide Web and the Internet backbone among others 5., 6.••.
Section snippets
What constitutes a unit of function in metazoa?
Single cells are units of function for the single-celled organism. The following instructive question may be asked: what is meant by a unit of function in higher organisms? The hierarchical nature of biological form and function argues for an operational definition, one that depends upon context and desired outcome. Thus, single non-malignant mammary cells are ‘functional’ in that if they can attach to a substratum, they can proliferate, or at least survive and metabolize for a substantial
The mammary acinus as an experimental organism
To systematically explore the mechanisms behind tissue specificity in an epithelial model system, we chose to study the mammary gland, and more specifically the mammary acinus, as an experimental ‘organism’ (see Figure 1). The mammary gland is one of very few organs in which substantial development occurs only after an animal is born; it also undergoes cycles of growth, differentiation, apoptosis, regression and remodeling during the lifetime of the organism. As such the mammary gland is a
The importance of laminin, polarity and myoepithelial cells
The importance of laminin 1, β1-integrin and other ECM receptors in mammary gland function has been amply demonstrated both in culture and in vivo in the last decade 24., 34., 35., 36., 37.. However, if mammary epithelial cells can form functional acini in the presence of a laminin-rich gel in culture, what then is the role of the myoepithelial cells which surround the luminal epithelial cells in vivo (see Figure 1)? Luminal cells embedded in 3D collagen-I express different surface integrins
Normal and malignant breast cells can be distinguished in 3D BM
One characteristic of epithelial cells in tissue culture plastic is that, unlike with fibroblasts, it is not always easy to distinguish normal from malignant cells because they often grow at similar rates and are morphologically also similar. Together with Ole Petersen’s laboratory, we developed a versatile assay to rapidly distinguish normal and malignant human breast cells in 3D BM in a defined medium [49] by modifying the rodent assay discussed above (see Figure 2); for recent detailed
Restoration of tissue architecture can trump the malignant phenotype of breast cancer cells
Examination of surface receptors of the human breast cell progression series mentioned above (HMT3522) 53., 54. indicated that several integrins and growth-factor-receptor pathways were in ‘overdrive’, leading to imbalanced signaling. Correcting β1 integrin and EGFR activities and/or inhibiting related signaling pathways (MAP kinase and PI3 kinase) could revert the malignant phenotype despite the malignant genotype (see Figure 5; 59., 60., 61.). Re-expression of several molecules that are
Restoration of form as a means of deciphering how form is maintained: modeling breast tissue architecture
In an effort to interpret our mammary-gland-specific results and to synthesize a conceptual framework for subsequent modeling, we have begun to seek inspiration from other disciplines with the expectation that any parallels that emerge can guide our future thinking. We briefly outline how a mechanism based on stochastic variation [3••] that is believed to be important in cell fate and guidance 71., 72. may be relevant to the mammary gland acinus. In the literature, the nature of the
Conclusions
The efforts to model an acinus of the mammary gland was rooted in the early studies of cell and developmental biologists in the late 1960s and early 1970s, and yet it has taken a few decades to amass enough data for its utility to be recognized. It is now ready for broader use, and several laboratories in addition to ours are using the 3D BM model of breast acini to generate tissue-relevant data 66., 75.•, 76.•. Models of skin, kidney, liver, and other tissues have also been attempted with
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
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of special interest
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of outstanding interest
Acknowledgements
The work described in this chapter was supported by funds from the US Department of Energy, Office of Biological and Environmental Research (DE-AC0376 SF00098), the National Cancer Institute (CA64786-02 and CA57621), and by an Innovator Award from the US Department of Defense Breast Cancer Research Program (DAMD17-02-1-0438 to MJB). ISM was supported by funds from the California Breast Cancer Research Program and by a grant from the Lawrence Berkeley National Laboratory LDRD Program. AR was
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