The functioning of signaling pathways in breast cancer-associated lung metastasis.
Breast cancer is the most common form of cancer in women. Breast cancer has a heterogeneous etiology. Genetic and environmental factors contribute to the pathogenesis and progression of breast cancer. Various genes as proliferation and nuclear factors have been identified in breast cancer. Therefore, the genetic component of patients is important in determining disease behavior, response to anticancer therapeutics, and patient survival. Prognosis of breast cancer is associated with potential metastatic properties of primary breast tumors. Metastasis is the leading cause of death in patients with breast cancer. Therefore, it is important to understand the mechanisms underlying the development of distant metastases to specific regions and has clinical value. Metastasis shows an organ-specific spread pattern and occurs with a series of complex and multistep events associated with each other, such as angiogenesis, invasion, migration-motility, extravasation, and proliferation. Breast cancer often metastasizes to the bone, liver, brain, and lungs. Metastasis may develop years after successful primary treatment. The metastatic process will become clear, as information about molecules and genes associated with metastases increases, and this is extremely important for cancer treatment.
- breast cancer
Breast cancer, which is one of the most common malignant diseases of women worldwide, is a heterogeneous disease with unknown pathogenesis. Genetic and environmental factors contribute to the pathogenesis and progression of breast cancer. Although an improvement has recently been detected in the diagnosis and treatment of breast cancer compared with other cancers, its contribution to survival was inadequate.
Breast cancer-associated death or survival is associated with the potential metastatic features of the primary breast tumors. Metastatic disease is the leading cause of death in breast cancer patients. Distant metastasis develops in ~20–30% of the early-stage breast cancer patients. Approximately 90% of deaths result from the complications due to recurrent or metastatic diseases. Therefore, it is very important to understand the underlying mechanisms in the development of distant metastases to specific regions. Metastases may show an organ-specific dissemination pattern. Metastasis may develop years after successful primary treatment. Metastasis frequently develops in the bone, liver, brain, and lungs in breast cancer.
Identification of the molecules, and genes associated with metastasis, and clarification of the contribution of these molecules to metastatic process are important for the treatment of cancer. Metastasis is the dissemination of the cancer cells from their primary region to different tissues and organs in the body. Metastasis develops with a series of complex and multistep chains of events such as angiogenesis, invasion, migration-motility, extravasation, and proliferation.
The anomalies of different genes as
Different breast cancer cellular subtypes in primary breast cancer tissue metastasize in relation to their target organ. The route of metastasis is generated with the interaction of this different subtype cells and microenvironment of the tumor and with the organ they will locate, and this is named as “organotrophic metastasis.”
Understanding the molecular mechanism of organotrophic metastasis is very important for biological indicator prediction, developing the innovative therapeutic strategies, and for improving the survival. Development of metastasis in distant regions is associated with the interaction between the tumor cells and host microenvironment. Before the initiation of tumor dissemination, the host microenvironment is modified to support the tumoral growth, in other words to create a pre-metastatic niche (PMN). PMN is organized with the factors secreted from tumor cell with the changes in the host cell metabolism and microenvironment. In addition, tumor cells also interact with the extracellular matrix (ECM) of the host tissue to facilitate metastasis.
Generally, breast cancer is classified as in situ carcinoma and invasive carcinoma in a simple way, and most breast cancers are invasive. More than 80% of invasive breast cancers may be investigated in two different subgroups as invasive ductal carcinoma (IDC), and some breast cancers may be investigated as invasive lobular carcinoma (ILC). Organ preference of metastasis in ILC and IDC is significantly distinct. Invasive ductal carcinomas do metastasis to the lungs, distant lymphatic glands, and central nervous system (CNS); however, ILC is known to do threefold higher metastasis to the peritone, gastrointestinal system, and ovaries .
Breast cancer has a tendency to do metastasis on the bone, liver, lung, and distant lymphatic glands. The most common metastasis type is the bone metastases detected in 70% of metastatic breast cancer patients . The second most common metastasis region was the liver with ~30%, and the brain was reported as the third most common metastasis region with a rate of 10–30% .
The most common metastatic region in all subtypes except basal-like tumors is the bone. Luminal B, HER2+/ER/PR+ and HER2+/ER/PR, tumors do more metastasis to the brain, liver, lungs, and bone than the luminal A tumors. Basal-like tumors do higher rates of the brain, lungs, and distant lymphatic node metastasis; however, the liver and bone metastases are less frequently detected in basal-like tumors . Although triple negative breast cancer (TNBC) tumors show a metastatic ratio similar to non-basal tumors, TNBC tumors have less liver metastasis than the non-basal tumors .
Some molecules may have different roles in different metastasis regions in accordance with their content. Although transforming growth factor beta (
Breast cancer cells are detected to highly express the chemokine receptors
2. Metastasis-associated signal transduction pathways and genes
2.1 p38/MAPK pathway
p38/MAPK signal transduction pathway increases the breast carcinoma vascularization and growth by promoting the expression and accumulation of pro-tumorigenic factors.
The inactivation of the p38/MAPK signaling pathway was provided by the expression of the kinase-inactive mutant (dn-p38) of p38/MAPK14 in metastatic breast cancer cells in the studies, and with the deterioration of the tumor p38/MAPK signal, the development of breast cancer and metastasis ability was shown to decrease in breast carcinoma xenografts . The conducted kinase-inactive mutant significantly decreased the dn-p38, tumor blood vessel density, and lumen dimensions. p38 controls the expression of the pro-angiogenic extracellular factors such as matrix protein fibronectin, cytokine, vascular endothelial growth factor A (
Tumor microenvironment (TME) is an important factor in cancer progression, recurrence, and response to treatment. TME blood vessels consist of stromal cells (fibroblasts, adipocytes) and infiltrating immune cells. Myeloid cells stimulate the tumor vascularization and metastasis by secreting metalloproteinase
The deterioration of p38/MAPK signal causes no decrease in the expression of
p38/MAPK induces the expression of pro-angiogenic cytokines that include
p38/MAPK affects the development and metastasis of breast cancer by changing the tumor microenvironment of p38/MAPK signal. The inactivation of p38/MAPK signal in breast cancer cells decreases the growth of tumor xenografts and metastasis. Tumoral and stromal cells in breast TME stimulate the cytokine-mediated p38/MAPK signal which increases the expression of the pro-angiogenic and pro-invasive factors such as
2.2 Tumor endothelial marker 8 (TEM8)
Tumor endothelial marker (
The destruction of
TEM8 expressed by cancer cells causes the development of angiogenesis by affecting the cancer cell proliferation and endothelial cell migration.
2.3 APOBEC3B gene
Another important molecule in the development of metastatic potential of breast cancer is
3. Metastasis of breast cancer to different organs
3.1 Lymph node metastasis
Lymph node metastasis shows that distant metastasis risk is higher. The absence of lymph node metastases is associated with lower metastasis risk; however, the presence of more than four lymph node metastases is the precursor that distant metastasis risk is significantly higher. Distant tumor metastasis develops through axillary lymphoid nodes (ALD) and blood circulation. Therefore, lymph nodes are used as an indicator of the metastasis ability of tumor cells. There is an association between the tumor size and the rate of lymph node metastasis.
CCN proteins which have oncogenic functions in breast cancer mainly consist of
3.2 Bone metastasis
The common cause of morbidity and mortality in most advanced stage breast cancer patients is the development of osteolytic bone metastasis. The most frequently detected area of metastasis in metastatic breast cancer is the bone and constitutes 70% of the metastases. Most bone metastases detected in breast cancer are associated with osteolytic-type metastatic lesions owing to the osteoclast-mediated bone resorption. Although all subtypes of breast cancer have a tendency of bone metastases, luminal subtype tumors develop higher bone metastases (80.5%) than the basal-like (41.7%) and HER2+ tumors (55.6%) .
Tumor cells demonstrate different reactions in accordance with the environment in the new organ such as gene expression, growth ability, and response to treatment. Therefore, any of the breast cancer cell reaching to the bone may promote the excessive growth in molecular interaction with osteoblasts and osteoclasts. The molecules produced by cancer cells or with the parathyroid hormone-associated protein in the bone microenvironment and converting growth factor β (
Some cancer cells in the primary tumor accumulate additional genetic changes which lead to bone metastasis. This causes invasion and colonization of tumor cells to the bone matrix. The destruction of the bone matrix with tumor cells facilitates the metastasis by the TGF and metastasis genes responding to TGF causing the increase of
Figure 3 demonstrates the functioning between the
Primary breast tumor develops with the accumulation of oncogenic mutations from normal breast epithelium. The increased expression of gene classes that facilitate metastasis to different organs among tumor cells enables the invasion of the bone matrix, colonization of metastatic tumor cell, and destruction of the bone matrix .
CCN protein family consists of six members as
One of the overexpressed genes in bone-specific metastasis is the
3.3 Liver metastasis
The liver is the most common metastatic region for cancers and represents the second organ where breast cancer metastasis occurs. The development of liver metastasis in breast cancer patients is associated with Wnt signal and Ki67 signal independent of beta-catenin and an indicator of poor prognosis.
Metastasis is a multistep procedure which is responsible for most cancer-associated deaths and is affected by both cell-cell or cell-matrix interactions and tumor microenvironment (vascularization, etc.).
Clinically, low oxygen level (hypoxia) is known to be associated with metastasis . Lysyl oxidase (
3.4 Brain metastasis
Brain/CNS (central nervous system) metastasis develops in 10–30% of metastatic breast cancer patients. Brain metastasis (BM) is detected as a complication that generally develops in the late stages of disease. Brain metastases develop after systemic emergence of metastases in the lungs, liver, and bone . Two main primary tumors that do metastasis to the brain are lung and breast adenocarcinomas . Brain metastases are associated with neurological disorders by affecting both the cognitive and sensory functions in addition to their association with highly poor prognosis.
Breast cancer is the most common cancer type where brain metastasis develops after lung metastasis. Lung and breast cancer-associated brain metastasis is more frequently detected than the primary brain tumors. Brain metastasis incidence has gradually been increasing in breast cancer patients. Due to the development of systemic therapies, many breast cancer patients live longer, but still in a way brain metastases may develop. Various factors were described for increased brain metastasis risk in breast cancer patients. These factors may be reported as early age, poorly differentiated tumor histology (high grade), hormone receptor negativity, and metastasis in more than four lymph nodes. These factors were associated with the brain metastasis risk . HER2-positive and TNBC patients have a higher risk of brain metastasis than the luminal-type breast cancer patients. Brain metastasis is detected in 30–40% of HER2-positive and triple negative breast cancer patients . Brain metastasis in lung cancer generally develops within 2 years after the diagnosis of primary lung cancer, and brain metastasis in breast cancer is generally associated with the metastatic stage of the disease and develops 10 years after the primary diagnosis and after a successful treatment. However, brain metastasis in triple negative breast cancer patients develops in earlier periods. The development of brain metastasis in breast cancer was detected to be associated with Wnt, Notch, and EGFR pathways .
HER2 amplifications and mutations were frequently demonstrated in breast cancer and in breast cancers with brain metastasis . There are no target-specific treatment options in the clinical practice generally in breast cancers that carry
Brain metastasis is a multistep procedure with migration, intravasation, circulation, adhesion, extravasation, and brain microenvironment. Particularly the blood-brain barrier (BBB) is highly selective in the entrance of tumor cells and therapeutics to the brain microenvironment. In compliance with that, the cells to make a metastatic lesion in the brain have a specific clonal origin. This shows that a brain metastasis shared the common abnormalities with a metastasis ancestor cell, and the further abnormalities could only be present in only brain metastatic subclones. More frequent detection of
3.5 Lung metastasis
Luminal breast tumors have the tendency to do metastasis to the bone; however, basal-like breast tumors mainly do metastasis to the lungs. The genes that are effective in the emergence of lung metastasis are generally associated with poor prognosis . An epidermal growth factor receptor-ligand epiregulin (
Other genes except
The decrease of the expression of a tumor suppressor gene
A Notch signaling pathway receptor
Wnt/β-catenin signaling has a significant role in the embryonic induction and tumorigenesis of the breast gland . The nuclear localization and overexpression of β-catenin are an indicator of Wnt/β-catenin signal activation. Various clinical and laboratory studies showed that the abnormal activation of Wnt/β-catenin signaling was associated with poor prognosis in breast cancer patients and mainly increased in triple negative cancer subtype . In addition, the Wnt-helper receptor
|Notch pathway||Wnt pathway||Hedgehog pathway|
|Uncontrolled growth||The self-renewal of breast cancer stem cells|
|The self-renewal of breast cancer stem cells||EMT|
|Formation of lung niches|
|Development of lung metastasis|
Hedgehog (Hg) signaling pathway has a significant role in the development of ducts of the breast. In addition, Hg regulates the breast cancer stem cells and has a significant role in cancerogenesis . Hg proteins regulate the breast cancer cell migration. Hg, Notch, and Wnt signaling pathways demonstrate joint behavior in tumor development and metastasis in cancer. These signaling pathways have significant roles in the development of breast cancer and lung metastasis.
Breast cancer is characterized with a separate metastatic pattern including the regional lymph nodes, bone marrow, lung, and liver. Chemokines are a group of small-molecular-weight protein which bind to chemokine receptors attached to G protein. Chemokines have a significant role in various pathological conditions such as cell migration, development, and inflammation. Binding of chemokines to receptors causes a structural change which activates the signaling pathways and promotes the migration. Chemokine and chemokine receptors have a critical role in identification of metastatic targets of tumor cells. Chemokines are divided into two groups in accordance with their functions as inflammatory chemokines and homeostatic chemokines. Inflammatory chemokines are induced by inflammation, and homeostatic chemokines are structurally expressed and have a role in homeostatic immune regulation .
Chemokines have a significant role in the progression of cancers  and have functions in tumoral growth, aging, angiogenesis epithelial-mesenchymal transition, and metastasis. The expression of chemokines and their receptors changes in malignity and then causes abnormal chemokine receptor signaling. This change stems from the inactivation of the tumor-suppressive genes or from the structural activation of oncogenes that have a role in the regulation of chemokines .
The in vivo inactivation of
Tumor cell migration and metastasis have various similarities with the leukocyte trafficking that are regulated by chemokines and their receptors. Cell trafficking-associated ligands
Extracellular matrix (ECM) proteins tenascin-C (TNC), periostin (POSTN), and versican (VCAN) are highly important molecules in the formation of metastasis and have a critical role in the formation of breast cancer colonization in the lung tissue that has a tendency for metastasis. Tenascin-C, which is normally produced by fibroblasts, is also secreted by breast cancer stem cells. This abnormal expression of tenascin-C by breast cancer stem cells forms a niche in lung colonization and creates a metastasis-initiating effect. Periostin is a stromal factor that may bind to Wnt ligands and is effective in breast cancer metastasis .
Cancer-associated fibroblasts (
Because the lungs have a unique histological feature, cancer cell meets with high interstitial fluid pressure and thus supports the
As conclusion, the expression changes in these genes in breast cancer cells may be detected in bone, lung, brain, liver, and lymph node metastases. The studies revealed that there were important differences in metastatic behavior between breast cancer subtypes (Table 2). Therefore, the treatment of metastatic breast cancer must be performed by targeting the organ with metastasis, and the development of target molecules will form the future treatment protocols.
|Molecular subtypes of breast cancer||TNBC|
|Molecular pathways and genes|
Luminal B, HER2+/ER/PR+ and HER2+/ER/PR, tumors do more metastasis to the brain, liver, lung, and bone than the luminal A tumors. Basal-like tumors do higher rates of brain and lung metastases. As demonstrated in Table 2, breast cancer cells do metastasis to the lung through triple negative breast cancer, basal, luminal B, HER2 molecular subtypes, the genes activated by growth factor receptors, matrix metalloproteinases, and the pathways of
Individualized target-specific appropriate treatment methods will be developed for metastatic breast cancer owing to the knowledge of the association of genes with each other that cause metastasis and the follow-up of the pathways where these genes gained function. There is an association between genomic differences and various gene expressions that cause poor prognosis in breast cancer. The gene expression profiles of primary tumors must be compared and associated with metastasis for describing and clarifying the tumor factors of metastatic breast cancer. The better understanding of the functioning of these genes will help to develop specific therapeutic approaches for metastatic breast cancer.
The molecules and genes on the pathways will be used in the diagnosis, prognosis and treatment response of metastatic breast cancer in the future. These effective molecules will be used as a tumor-specific indicator, and also detected in different biological materials like tissue, saliva, blood, serum, and urine in metastatic breast cancer. In addition, these genes may be used as therapeutic targets. The inactivation of these genes by inhibition or with biological antibodies through apoptosis is significantly important to resolve the tumor and metastasis. Different therapeutic strategies will be developed, and these molecules will be used in individualized treatment for inhibiting the tumor metastasis considering the associations between these genes, and chemokines, and integrins. The breast cancer molecular subtypes will be treated, and a progress will be enabled in the treatment of metastatic breast cancer with the development of molecular drugs which inhibit the active pathways or eliminate the pathway transition of the genes effective in metastatic breast cancer.
The authors thank Kadriye Yilmaz from the Department of Foreign Languages at the University of Istanbul for their language corrections.
Conflict of interest
The authors declare that they have no conflict of interests.