Why the stroma matters in breast cancer: insights into breast cancer patient outcomes through the examination of stromal biomarkers

Abstract

Survival and recurrence rates in breast cancer are variable for common diagnoses, and therefore the biological underpinnings of the disease that determine those outcomes are yet to be fully understood. As a result, translational medicine is one of the fastest growing arenas of study in tumor biology. With advancements in genetic and imaging techniques, archived biopsies can be examined for purposes other than diagnosis. There is a great deal of evidence that points to the stroma as the major regulator of tumor progression following the initial stages of tumor formation, and the stroma may also contribute to risk factors determining tumor formation. Therefore, aspects of stromal biology are well-suited to be a focus for studies of patient outcome, where statistical differences in survival among patients provide evidence as to whether that stromal component is a signpost for tumor progression. In this review we summarize the latest research done where breast cancer patient survival was correlated with aspects of stromal biology, which have been put into four categories: reorganization of the extracellular matrix (ECM) to promote invasion, changes in the expression of stromal cell types, changes in stromal gene expression, and changes in cell biology signaling cascades to and from the stroma.

Figure 1. Tumor associated collagen signatures (TACS). Multiphoton image of an isolated ex vivo mouse breast tumor (MMTV-PyVT) containing distinct collagen organizational phenotypes. The weak autofluorescence signal present in the tumor epithelium is surrounded by the strong second harmonic generation (SHG) signal produced by collagen. This image contains all three of the archetypal tumor associated collagen signatures (TACS), as well as some brightly autofluorescent stromal cells. The first collagen signature (TACS-1) is defined as an increase in stromal collagen that retains a wavy appearance and is found throughout the tumor (arrow). The second signature (TACS-2) refers to the straightened appearance of individual collagen fibers, whereas the third signature (TACS-3) refers to the orientation of straightened collagen fibers. Specifically, TACS-3 is defined as collagen fibers oriented radially away from the tumor boundary (arrows). It has been shown that tumor cells will invade away from locations such as this along the straightened, perpendicularly aligned collagen fibers. Scale bar is 50 μm.

Figure 3. The TACS-3 phenotype isolates breast carcinoma for invasion. (A–C) A consequence of anchorage-independent growth of breast carcinoma is a co-mingling with the stromal matrix. Collagen can be observed to penetrate into the carcinoma cell mass, additionally some epithelial cells are disseminating away from the tumor, such infiltrations blur the tumor/stroma boundary, resulting in an irregular margin. The net effect is the isolation of cells into single-file columns of cells that have enhanced migration. Fibroblasts and other stromal cells are readily observable at these areas, and there is a great deal of evidence for their direct role in these activities. (D) An illustration depicting such a process, where the influences leading to such a phenotype will be detailed in Figure 4.

Figure 2. Aligned collagen is an avenue for invasion and metastasis. (A) H and E stained slide of human breast carcinoma. Disseminating cells can be observed migrating toward a nearby blood vessel. (B) Second harmonic generation (SHG) imaging of this same location reveals the underlying collagen matrix.

Figure 4. Stromal biomarkers that predict clinical outcome. An illustration of the biomarkers discussed in the manuscript text. The presence and influx of stromal cell types, alterations in gene expression, secreted factors and an altered collagen matrix are all depicted. The interplay of these features lead to a mechanism, aligned collagen, by which the stroma promotes tumor progression.