Interplay between desmoglein2 and hypoxia controls metastasis in breast cancer

Metastasis is the major cause of cancer death. An increased level of circulating tumor cells (CTCs), metastatic cancer cells that have intravasated into the circulatory system, is particularly associated with colonization of distant organs and poor prognosis. However, the key factors required for tumor cell dissemination and colonization remain elusive. We found that high expression of desmoglein2 (DSG2), a component of desmosome-mediated intercellular adhesion complexes, promoted tumor growth, increased the prevalence of CTC clusters, and facilitated distant organ colonization. The dynamic regulation of DSG2 by hypoxia was key to this process, as down-regulation of DSG2 in hypoxic regions of primary tumors led to elevated epithelial−mesenchymal transition (EMT) gene expression, allowing cells to detach from the primary tumor and undergo intravasation. Subsequent derepression of DSG2 after intravasation and release of hypoxic stress was associated with an increased ability to colonize distant organs. This dynamic regulation of DSG2 was mediated by Hypoxia-Induced Factor1α (HIF1α). In contrast to its more widely observed function to promote expression of hypoxia-inducible genes, HIF1α repressed DSG2 by recruitment of the polycomb repressive complex 2 components, EZH2 and SUZ12, to the DSG2 promoter in hypoxic cells. Consistent with our experimental data, DSG2 expression level correlated with poor prognosis and recurrence risk in breast cancer patients. Together, these results demonstrated the importance of DSG2 expression in metastasis and revealed a mechanism by which hypoxia drives metastasis.

Breast cancer is the most common cancer of women worldwide. With early detection and advances in therapeutic strategies, the 5-y relative survival rate for all stages combined is higher than 90% (according to the Surveillance, Epidemiology, and End Results (SEER) database maintained by the US National Cancer Institute). However, metastasis is now the major cause of breast cancer death (1), as the survival rate for women with metastasized breast cancers remains below 30%. Thus, identification of key factors in breast tumorigenesis and metastasis is important to identify therapeutic targets and strategies to improve prognosis.Metastasis is a complex process involving tumor cell intrinsic alterations and extrinsic interaction with the microenvironment to select for highly aggressive cancer cells. Hypoxia, a key microenvironmental factor in solid tumors, activates hypoxic signaling to increase plasticity and promote epithelial−mesenchymal transition (EMT) to drive the first step of metastasis (2). High plasticity allows cancer cells to disseminate from the primary site and intravasate into the circulatory or lymphatic system. Most of these circulating tumor cells (CTCs) die in circulation, and only a small fraction of CTCs are able to survive and eventually colonize distant organs (3, 4). Recent evidence has shown that CTC numbers can be used as an independent predictor for survival in patients with metastatic cancers (5–7). Further improvement in CTC detection methods led to identification of CTC clusters and the finding that clustered CTCs exhibit epithelial/mesenchymal hybrid (partial EMT) phenotype which allows them to move collectively (8). Collective movement makes these cancer cells more apoptosis resistant, more capable of avoiding immune surveillance, and better able to colonize distant organs. Importantly, CTC clustering ability has been positively correlated with poor clinical outcome (9–12). While a few factors that promote CTC cluster formation have been identified (9, 13–15), the key mechanisms that allow CTC clusters to survive in the vascular system and allow them to more efficiently metastasize than unclustered CTCs remain elusive.Cell adhesion proteins play critical roles in intercellular contacts and epithelial tissue dynamics. Deregulation of cell adhesion molecules contributes to tumor metastasis (16, 17). Among cell adhesion molecules, desmosomes are of particular interest for cancer biology. Desmosomes form patch-like adhesion structures that mark the intercellular midline and connect to the intermediate filament cytoskeleton to maintain cell−cell adhesion and tissue integrity (18). The desmosome is a protein complex containing two transmembrane proteins, desmocollin (DSC1 to DSC3) and desmoglein (DSG1 to DSG4), as well as adaptor proteins, plakoglobin and desmoplakin, that bind intermediate filaments (19). Among the human DSGs, DSG1 and DSG3 expression is mainly restricted to stratified squamous epithelia (20). DSG2 is the most ubiquitously expressed isoform, including mammary tissue, and is a key factor for cell aggregation and oncogenic function in lung and prostate cancers (20–23). However, whether DSG2 is involved in CTC clustering and metastasis remains unknown.We found that tumor growth and colonization were promoted by DSG2 expression at both the primary site and distant organ. High DSG2 expression in the primary tumor was associated with increased prevalence of CTC clusters. However, HIF1α-mediated suppression of DSG2 under hypoxia was required for cancer cell invasion and migration. Once in the vascular system, the cancer cells were released from hypoxic stress, and DSG2 expression was derepressed. This DSG2 reactivation was essential for CTCs to colonize distant organs. Consistent with these experimental observations, clinical data indicated that breast cancer patients whose tumors expressed DSG2 (DSG2 positive) had worse prognosis and higher recurrence risk than those with DSG2-negative tumors. Together, these results show that dynamic changes of DSG2 expression are required for breast tumorigenesis, CTC clustering, invasion, and metastasis. Our data also identify regulatory mechanisms underlying DSG2 repression and derepression during specific stages of breast cancer progression.