Why Does Breast Cancer Often Spread to the Lung?

“Emerging evidence supports an intriguing host-tumor relationship that drives lung metastasis of breast cancer.”

Breast cancer is the most commonly diagnosed cancer among women and the second leading cause of cancer-related deaths in women. In 2014, it is estimated that there will be over 230,000 new cases of breast cancer diagnosed in the USA alone [1]. When breast cancer is confined to the breast tissue or regions slightly beyond the breast, traditional methods of treatment (surgery, radiation) are fairly successful, with overall 5-year survival rates of 98.6% for localized breast cancer and 84.4% for regional breast cancer [1,2]. However, the majority of deaths associated with breast cancer are not due to the initial tumor but rather to metastatic disease, the process by which cancer spreads from an initial site of development to distant sites throughout the body. Following metastasis, overall 5-year survival declines considerably to 24.3%, representing a major problem in breast cancer patients. This substantial drop in survival is largely due to the ineffectiveness of current treatments in the metastatic setting [1,2].

Metastasis follows a series of complex and coordinated steps, ultimately leading to tissue invasion and establishment of secondary tumors [3]. The prototypical cascade begins with the invasion and dissemination of a primary tumor through the vasculature or lymphatics, survival within the circulation, extravasation into a new tissue and establishment and maintenance of secondary metastases [2,4]. Although most cancers have the potential to spread to multiple parts of the body, they usually prefer certain sites in relation to others. This ‘organ tropism’ is seen in breast cancer, with common clinical sites of metastasis including lung, bone, liver, brain and lymph nodes [5], with the lung being one of the most common and deadly sites [2]. Although the physiological and molecular determinants of breast cancer lung metastasis remain poorly understood, emerging evidence supports an intriguing host–tumor relationship that drives this process.

Many theories have been proposed to explain organ-specific metastasis; however, only two have really withstood the test of time. Stephen Pagets seed and soil hypothesis, originally developed in 1889, proposed that metastasis occurs as a result of favorable interactions between the cancer cells themselves (seed) and their specific organ microenvironment (soil) [6–8]. Thus, only when the seed and soil are compatible will metastasis occur. In 1929, James Ewing challenged this theory, postulating that metastatic spread occurs strictly as a result of mechanical factors inherent to the circulatory system, and that preferential metastasis can be accounted for by circulatory patterns alone [6,8]. It is likely that these theories are not mutually exclusive when applied to lung-specific breast cancer metastasis, but rather interplay between the two exists physiologically. Breast cancer cells that escape from the primary tumor invade the local vasculature, are taken to the heart via the venous circulation and subsequently delivered to the lungs. Within the lungs, tumor cells may mechanically arrest in the first capillary beds they encounter, after which successive initiation, growth and maintenance of these secondary tumors within the lung is influenced, at least in part, by specific lung-derived signals/factors. So what makes the lung such an attractive destination or ‘soil’ for metastatic breast cancer cells?

Several studies have demonstrated that specific molecular characteristics and/or genetic signatures of breast cancer cells are associated with lung-specific metastasis. Clinically, gene expression profiling and immunohistochemical markers are used to classify breast cancers into one of four molecular subtypes: luminal A/B, HER-2-enriched or basal-like (triple negative) [9,10]. Although most research has focused on associating subtypes with overall survival, a 2010 clinical study examining the metastatic patterns of different breast cancer subtypes concluded that certain subtypes were associated with distinct patterns of spread. Using 15-year cumulative incidence rates according to metastatic site for breast cancer patients, it was observed that more aggressive subtypes such as HER-2-positive and triple-negative tumors demonstrated a high rate of metastasis to the lung, whereas less aggressive subtypes tend to preferentially metastasize to the bone [10]. This suggests that a disparity exists in the metastatic behavior of breast cancer subtypes and supports the notion that genetic differences in breast cancer cells may be associated with specific sites of metastasis such as the lung.

These clinical observations are supported by elegant experimental studies carried out by Joan Massagué and colleagues, who set out to identify defined gene signatures linked to lung-specific breast cancer metastasis. Reasoning that genomic instability within the tumor cell population results in distinct genetic variants with differential metastatic capabilities better adapted to certain sites, they generated a ‘lung-seeking’ variant of the aggressive human triple-negative breast cancer cell line MDA-MB-231 via multiple rounds of in vivo selection of lung metastases in a mouse model. They then performed transcriptome microarray analyses on both the parental and lung-seeking breast cancer cell populations [11]. Their work revealed a lung-specific gene set that was fundamentally distinct from a previously identified bone-specific metastasis gene signature [3,11]. Similar to the association between molecular subtypes and patterns of metastasis, many of the genes identified in the lung-specific set code for extracellular and receptor proteins including growth and survival factors (epiregulin – a HER receptor ligand), adhesion receptors (ROBO1), chemokines (CXCL1), secreted proteases (MMP1) [11] and transcriptional regulator proteins (ID1, ID3) [3,12,13]. Importantly, this lung-specific gene expression signature is clinically correlated with lung metastasis when assessed in breast cancer patients [11]. However, it still remains unclear as to when these cells acquire this specific gene signature during metastatic progression, that is, whether it is simply due to inherent breast cancer cell genetics and/or the result of exposure to the lung microenvironment during metastatic development.

Growing evidence indicates that metastasis is a complex process involving molecular coordination between cancer cells and various microenvironmental factors. While the studies discussed above provide valuable knowledge concerning the contribution of cancer cells (seeds) to lung tropism, they offer little insight about the influence that the lung microenvironment (soil) has on breast cancer metastasis. The fact that many of the genes identified previously in the lung-specific gene signature code for extracellular and secretory proteins suggests an association between breast cancer cells and their microenvironment. It is becoming increasingly clear that certain microenvironments can profoundly influence tumor cell biology, including establishment in secondary sites, growth and proliferation, angiogenesis and chemotherapy resistance [12]. For example, previous studies using prostate and colon cancer models have shown that the expression level of many metastasis-associated genes can be induced by certain factors present in specific metastatic environments, suggesting an important role of the organ microenvironment in influencing tissue-specific metastasis [14]. Chemokines – a family of small-secreted signaling proteins – have been implicated in lung-specific patterns of breast cancer metastasis. Breast cancer cells have been shown to preferentially express the functionally active CXCR4 chemokine receptor, involved in cell migration and tissue invasion. Coupled with the preferential expression of its ligand (CXCL12) in the lung, this indicates that chemokine – ligand interactions may be important in directly mediating lung-specific patterns of breast cancer metastasis [15].

Work done by our group has led to the establishment of a comprehensive ex vivo mouse model system for studying the influence of soluble organ microenvironments on breast cancer cell behavior. The model system is composed of organ-conditioned media, representing soluble factors secreted by organs clinically involved in breast cancer metastasis, including the lung. Using this model, we showed that four different human breast cancer cell lines displayed organ-specific patterns of migration and proliferation that corresponded to their metastatic behavior in vivo [16]. Detailed investigation of lung-derived soluble factors revealed over 70 different factors, including chemokines, cytokines, growth factors and soluble extracellular matrix components. Importantly, many of these proteins are associated with various steps of the metastatic cascade, including migration, adhesion and tumor growth. Moreover, a particular set of these soluble factors interact with breast cancer cell-derived mediators of lung-specific metastasis previously identified by Massagué's group including epiregulin (a member of the epidermal growth factor family), osteopontin (a chemoattractant and matrix protein involved in cell adhesion, migration and invasion) and urokinase-type plasminogen activator (secreted protease involved in tumor invasion) [11,16]. In addition to the soluble lung microenvironment, ongoing work by our group is currently investigating the role of the insoluble lung matrix on breast cancer cell behavior. The insoluble lung microenvironment represents a significant portion of the total lung tissue area and therefore cannot be overlooked [17,18], particularly since it likely provides important tumor cell–matrix interactions that promote cell growth and proliferation and that cannot be accurately reproduced in vitro [19]. Taken together, these findings support an intimate association between breast cancer cells and their specific organ–microenvironments during metastasis, demonstrating the critical role of molecular cross-talk between seed and soil in driving the deadly process of metastasis.

The studies described above provide a foundation for definitively characterizing (and interfering with) factors involved in promoting breast cancer metastasis to the lung. Furthermore, complementary studies examining the relationship between the soluble and insoluble lung microenvironment will provide an understanding of how the two interact with breast cancer cells to promote and establish viable lung metastases. Looking forward, effective translation of this knowledge from the research lab into the clinic will allow for the future development of novel targeted therapies to limit the extent of lung metastases in breast cancer patients.

Acknowledgements

The authors thank members of their laboratory and their collaborators for their research work and helpful discussions.