UC Berkeley

Central to our understanding of breast cancer – a disease of dysregulation, is first defining how the normal human breast functions in the context of its microenvironment. Breast architecture is a complex interaction of structure and function directed towards maintenance of a relatively stable equilibrium – a homeostasis, in the performance of specific physiological processes. Cells are not cell-autonomous, they interact not only with other cells but also their microenvironment – one that they continually shape through a complex feedback system of dynamic and reciprocal signaling. We argue that it is through this framework that we can start to address one of the central challenges in breast cancer research to understand the molecular mechanisms driving progression from benign to invasive.

As molecular profiling of Ductal Carcinoma In Situ (DCIS) and Invasive Ductal Carcinoma (IDC) have shown no robust genetic differences between the cancers, it has been argued that invasiveness is not a property acquired by the tumor, but rather a functional consequence of the loss of tumor-suppressive properties of the underlying supporting cells, specifically the myoepithelial cells that lie basally to the tumor cell-of-origin. Myoepithelial cells have been shown to be key regulators of normal tissue architecture, and dysregulation in their function can lead to loss of apico-basal polarity of luminal cells and changes in the deposition of basement membrane components (Gudjonsson, T., et al. 2002). Furthermore, in a xenograft model of DCIS, normal myoepithelial cells have been shown to have tumor-­suppressive properties, able to delay both tumor growth and acquisition of an invasive phenotype (Hu, M., et al. 2008). On the other hand, a transcriptomic profiling study comparing different cell populations of the breast between normal, DCIS and IDC shows that the earliest genetic changes occurring between normal and DCIS tissue arise in the myoepithelial lineage – largely through the aberrant upregulation of genes for secreted factors including matrix-remodeling enzymes, tumor-promoting cytokines and signaling factors (Allinen, M., et al. 2004). Thus, myoepithelial cells undergo a switch from tumor-suppressive to tumor-promoting behavior from normal to the earliest stage of cancer.

When do these changes in the myoepithelial lineage begin to occur and what are the driving forces behind these changes? As changes in normal mammary gland architecture and its cellular and molecular composition are already apparent in aged breast tissue, including loss of lineage specificity in the epithelial lineages (Garbe, J.C., et al. 2012) (Benz, C.C. 2008), we propose that the underlying changes in the normal function of myoepithelial cells start to occur as early as aging, and play a key role in the processes underlying age-related susceptibility to breast cancer. Furthermore, we hypothesize that changes in the microenvironment that accompany aging and tumor progression drive these changes in the myoepithelial lineage through the process of “dynamic reciprocity,” where dynamic and reciprocal signaling between the altered myoepithelial microenvironment and the myoepithelial cells lead to further disruption of proper function of the lineage.

To address these questions, we have (1) undertaken comprehensive transcriptomic profiling studies of myoepithelial cells in the context of normal primary breast tissue, and of an established human mammary epithelial cell (HMEC) culture system that is able to maintain the transcriptional, biochemical, and phenotypic features of lineage and chronological age (Garbe, J.C., et al. 2012); and (2) designed relatively simple yet biologically-relevant culture systems that allow for functional studies of myoepithelial cells in “young” vs. “aged” systems, and in “normal-like” vs. “tumor-like” microenvironments to illustrate how aberrations in normal myoepithelial function can lead to the changes we observe in aging and cancer.

Our work to develop analytical methodologies that aim to move gene-level and cell-level analysis into a systems-level approach have generated a complex profile of the myoepithelial lineage: (1) Our transcriptomic profiling of primary myoepithelial cells, along with the other major cell populations of the breast, has elucidated major characteristics of myoepithelial cells including upregulated lineage-specific genes and their associated pathways, as well as potential cell-cell interactions and co-regulatory networks between myoepithelial cells and other cell populations in the breast, and their contribution to the maintenance of the tissue microenvironment; (2) Our transcriptomic profiling of myoepithelial cells isolated from HMEC derived from a cohort of young pre-menopausal and older post-menopausal women have identified key pathways, regulatory networks and transcription factors dysregulated in the myoepithelial lineage during aging. While our work to design biologically-relevant cell culture systems has revealed key insights into the phenotypic changes observed in aging and cancer: (1) our “young” and “aged” co-culture system illustrates the ability of myoepithelial cells to confer an age-specific phenotype to luminal cells; and (2) our “normal-like” and “tumor-like” culture system demonstrates the ability of extracellular matrix (ECM) proteins to differentially regulate the myoepithelial secretome, with “tumor-like” ECM inducing myoepithelial secretion of angiogenic, cancer-associated factors, and matrix remodeling enzymes that mimic their aberrant secretory behavior in DCIS.

One the standing challenges of breast cancer research is to understand the underlying molecular mechanisms driving progression of breast cancer from benign to invasive. While progress has been made to identify tumor suppressor genes and oncogenes, as well as genetic alterations that associate with certain cancer subtypes, the correlation of these genotypic-phenotypic events is not fully understood. This poses a problem not only in understanding the biology of cancer, but equally significant, in developing reliable prognostic markers of progression and recurrence, as well as designing treatment options for women. Our current understanding of progression implicates myoepithelial cells as a major driver regulating transition to invasiveness. Thus, our understanding of how normal myoepithelial cells function, and how they become dysregulated in biologically-relevant model systems of aged and tumor-like microenvironments can provide insight into the molecular mechanisms which underlie the loss of normal function of the myoepithelial lineage that drive cancer progression.