Many tissues are composed of an epithelium associated with a stroma that is comprised of ECM proteins, adipocytes, fibroblasts, endothelial, neuronal and immune cells. Fibroblasts, a major stromal cell type, produce many components of the ECM, as well as proteases that remodel it (1
). Additionally, fibroblast signaling is a critical determinant of epithelial and stromal cell fate during development and differentiation (2
), tissue homeostasis and wound healing (1
Stromal changes associated with malignant lesions are heterogeneous and range from tumor suppressing to tumor promoting. Tumors, sub-typed by marker expression in epithelial cells, can be further categorized by their stromal signature, which dictates good or poor outcome (3
). Fibroblasts within the stroma of malignant lesions, called carcinoma-associated fibroblasts (CAFs), differ from their counterparts in disease-free tissue (4
). CAFs can stimulate tumor progression of initiated non-tumorigenic epithelial cells, both in vitro and in vivo
, while normal fibroblasts cannot (5
). CAFs increase proliferation and decrease apoptosis in adjacent epithelial cells and promote angiogenesis by recruiting endothelial progenitor cells into tumors (5
). All other stromal components, including immune and endothelial cells, also participate in malignant progression (4
The stroma within, and immediately adjacent to, a malignant lesion may exhibit a range of histologic alterations, called desmoplasia. The alterations range from a predominantly cellular stroma, containing fibroblasts, vascular and immune cells with little ECM, to a dense tissue with a minimum of cells and a maximum content of matrix (4
). Additionally, tumor stroma exhibits a reduction in the size and number of adipocytes (8
). The most odious stromal changes include extreme ECM deposition and remodeling along with aberrant vasculature and fibroblast and immune cell infiltration. Classically, participation of the stroma has been viewed as a reactive process, where signals from malignant epithelial cells recruit and stimulate these stromal components.
Interestingly, some desmoplastic features are seen in cancer-free tissues of women at high risk for breast cancer, namely in the context of wound healing, radiation response, pregnancy-associated involution and high mammographic density (MD) (4
). MD is of particular interest since almost 1/3 of breast cancers are thought to be attributable to phenotypes associated with high breast density, MD being a strong risk factor with high prevalence (11
). MD is determined by the relative amounts of radiolucent material (fat) and radio-dense material (epithelial cells, fibroblasts and connective tissue) within the breast on a mammogram, either of which may occupy anywhere between 0 and 100% of the gland. Radio-dense areas exhibit several histological characteristics of stroma associated with malignant epithelial cells, specifically low adipocyte content and high ECM and stromal cell content (9
). Epidemiological studies suggest that women with high MD have a 4- to 6-fold increase in risk for invasive breast cancer compared to women with low MD (11
Studies comparing pairs of monozygotic and dizygotic twins attributed 53–67% of the variation in MD to heritable factors with the remaining portion modulated by environmental or physiological factors (16
). For example, post-menopausal hormone therapy increases MD in some women, while tamoxifen treatment decreases MD in both pre- and post-menopausal women and subsequently modulates breast cancer risk (19
). This modulation by exogenous factors provides an exciting opportunity for intervention and prevention.
Boyd and co-workers examined the molecular composition of low and high MD tissues (9
) and report higher levels of insulin like growth factor-I (IGF-I) and tissue inhibitor of metalloproteinase 3 (TIMP-3) in high MD tissue. Recent studies found additional changes including differences in estrogen and prostaglandin metabolism and TGFβ signaling (22
). While these studies begin to define the molecular constituents of MD that may explain the basis of high MD and its association with increased breast cancer risk, they are correlative and thus cannot demonstrate that these genes or proteins mechanistically modulate phenotypes of MD.
The observation of multiple desmoplastic phenotypes in tissues of high MD, in the absence of a tumor, strongly suggests that these stromal phenotypes are not simply part of a reactive response to existing tumor cells, but also represent a program involving multiple stromal components that may create a proactive milieu for the emergence or progression of cancer. Therefore, we hypothesized that the phenotypes observed in both high MD and tumor stroma may be controlled by a common molecular program and thus their stromal components would share phenotypic, molecular and functional characteristics.
Here we demonstrate that the level of expression of a single molecule, CD36, is necessary and sufficient to simultaneously control adipocyte content and matrix accumulation, two phenotypes shared by desmoplasia and high MD. CD36, a widely expressed transmembrane receptor, modulates cell type- and ligand-specific phenotypes including adipocyte differentiation, angiogenesis, apoptosis, TGFβ activation, cell-ECM interactions, and immune signaling (25
). We found that CD36 expression was negligible in multiple cell types of tumor stroma compared to surrounding histologically disease-free tissue. Strikingly, high MD tissues, devoid of any malignancy, also showed reduced CD36 levels in multiple cell types compared to low MD tissues. The low level of CD36 expression shared by high risk (but cancer-free) and malignant tissues suggests that it constitutes a causal and very early event in generating the distinctive characteristics of a tumor stroma.