Matrix Metalloproteinases and Breast Cancer

3.1. Catalytic Activity of MMPs

MMPs are catalytic enzymes, which are synthesized as inactive prozymogenes. Catalytic activity depends on the zinc ions at the catalytic active site. Interaction between a cysteine-sulphydryl group in propeptide domain and zinc ion bound to the catalytic domain, keeps them inactive. They are activated by proteinase activity on prodomain that depend on the presence of the zinc ion at the catalytic active site (1, 5, 7). Activation of MMPs usually occurs outside the cells by other activated MMPs or serine proteinase. But MMP-11, MMP-28, and MT-MMPs can also be activated by intracellular furin-like serine proteinase before being localized at the cell surface (7). Observations from mammary glands-epithelial cells have demonstrated some interaction among PEX domains and integrins or other cell surface receptors, which might mediate MMP activation; This process recruits soluble MMP in specific site of proteolysis in extracellular matrix (ECM) or regulates MMP endocytosis and turnover (8). In an experimental murin mammary carcinoma model, binding of MMP-9 to the hyaluronan receptor CD44 mediated catalytic activation of transforming growth factor-β (TGF-β) and resulted in tumor progression and angiogenesis (9). Membrane type-1 MMP (MT1-MMP) PEX domain docking to CD44 leads to localization of MT1-MMP at the leading edge of migration cells and facilities their migration (10, 11).

3.2. Polymorphism of MMPs in Breast Cancer

Breast cancer is the most common female cancer in the world. Although a high rate of breast cancer has been seen in Western communities, there is a dramatic increase in breast cancer incidence among Iranian women as well (6). Based on numerous clinical and epidemiological studies, certain MMPs and ADAMS play a major role in breast cancer. They can be regarded as predictive biomarkers of primary diagnosis and prognosis of breast cancer and have predictive value for evaluation of disease, tumor recurrence, invading of tumor cells to other sites, and therapeutic outcomes (2). A systematic review that was carried out on MMP-1, -2, -3, -9 polymorphisms in breast cancer concluded that there is no significance association between polymorphism of MMP-1, -2, -3, -9 and breast cancer (12).

3.3. Role of MMPs in Initiation of Breast Cancer

Although MMPs roles were regarded as exclusively significant in promotion, invasion, and metastasis of cancer cells, recent evidence suggests that MMPs participate in several stages of cancer progression altering 6 principle processes of cell physiology, including self-supporting growth signals, resistance to growth inhibitory signals, insensitivity to apoptosis, over replication, angiogenesis, and invasion to other tissues and metastasis (5, 13).

MMPs are normally secreted by two major sources; cancerous cells, which produce MMP-7 and tumor stromal cells, which secrete MMP-2 and MMP-9. MMPs of the second group are ultimately recruited to cancer cell membranes. In fact, MMP-2 is produced by stromal cells of breast tumor while MMP-9 is found in both stromal and cancer cell membranes (5). It is noteworthy that upregulation of MMPs in breast cancer is not a result of gene amplification or mutations. Perhaps increase in MMP expression is due to transcriptional changes rather than loss of genes that encoding the MMPs mRNA. So this state probably results from mutations on oncogene or tumor suppressors that leads to activation of oncogenes and disruption of tumor suppressors (5). Most common genetic mutations that contribute to breast cancer include c-oncogenes such as C-erbB-2, C-myc, Ras, and tumor suppressor genes such as p53 and RB. A mutation in p53 results in upregulation of all downstream classical oncogenes and transcription of MMP-1 and MMP-13 (1, 14, 15). However, some of c-oncogenes by modulating expression of MMPs contribute to tumorigenesis. For example, transfection of C-erbB-2 or C-Ras to non-invasive MCF-7, breast cancer cell lines, which leads to upregulation of MMP-2 expression (16). Whereas, transfection of PEA-3 to MCF-7 cells results in MMP-9 overproduction (17).

3.4. MMPs and Breast Cancer Cell Growth

Animal studies carried out about MMPs roles in breast cancer have clarified that overexpression of WAP-MMP-3 in transgenic mice leads to mammary hyperplasia and cancer (18, 19). Injection of MMP-11 to MCF-7 cells increases tumor cells (1). However, other studies revealed a decrease in mammary carcinogenesis and tumor cell survival and growth following the very intervention. Expression of MMP-7 in mouse mammary tumor virus (MMTV) (MMTV longterminal receptor promoter enhancer) and MMTV-Her2/neu in transgenic mice promoted premalignant hyperplastic nodules and increased mammary carcinogenesis (20-22). Similar to MMTV-MMP-7, MMTV-MMP-14 expression resulted in mammary hyperplasia and cancer in mice (23), whereas administration of WAP-TIMP-1 and albumin-TIMP-1 in mice reduced mammary neoplasia (24). Possible underlying mechanisms through which MMPs contribute to cancer initiation of breast include: a) releasing some growth factors such as TGF-α (2), b) increasing the bioavailability of growth factors by degrading the binding proteins; for example, cleavage of insulin like growth factor binding proteins (IGF-BPs) by MMPs, consequently releases IGFs or degradation of perlecan by MMPs results in high levels of fibroblast growth factor (FGFs) (25-27), c) regulating growth signals through integrins, d) inducing cell survival and inhibition of cancer cell apoptosis, especially by MMP-11, MMP-9, and partly MMP-7 by means of generating pro-survival proteins such as IGFs, pro heparin-binding EGF-like growth factor (proHB-EGF) and ErbB4 receptor tyrosine kinase. Although MMP-9 and MMP-11 promote cell survival and decrease apoptosis, they induce cell death during development (28-30), e) activation of growth factor receptors; for example, MMP-2 and MMP-9 by degrading ectodomain of FGF receptor1 result in binding of FGF to its receptor (1).

3.5. Role of MMPs in Developing of Breast Cancer and Metastasis

Escape of tumor cells into neighboring tissues, invasion ability, and metastasis to distant sites are contributing factors in aggressiveness of tumors (31, 32). Consequently, facilitation of cancer cell proliferation and growth by misregulation of MMPs contributes to tumor development and metastasis through multiple mechanisms, which are modulated by proteolytic function of MMPs. Mostimportant ways through which MMPs are involved in tumor invasion include angiogenesis, epithelial-mesenchymal transition, inflammation and immune responses to cancer, and metastasis.

Degradation of ECM facilitates movement of cells. In other words, proteolytic actions of MMPs might lead to uncovering of the crypt sites and presenting novel activity fragment that can modulate tumor development such as migration and angiogenesis (5, 33).

3.6. MMPs Associated With Angiogenesis

Formation of new vessels from existing neighboring blood vessels is a necessary step for tumor progression and growth of a tumor size to approximately 2 mm (1, 2). Angiogenesis consists of 4 main steps: a) remodeling of the surrounding ECM of blood vessels; b) invasion of the surrounding cells by angiogenic signals in endothelial cells; c) migration of endothelial cells as a result of proliferation signals activation, which leads to generating of a column form; d) organization into 3D structure and new capillary (1). MMPs roles in angiogenesis are complex and conflicting. Generally MMPs can promote angiogenesis by two different mechanisms: first, by cleaving ECM and invading endothelial cells; for example, degrading of collagen type 1 is essential for endothelial cell migration and capillary formation (5, 34); and second, stimulation of some growth factors released from ECM sources, which has a major role in promotion or maintenance of the angiogenic phenotypes such as vascular endothelial growth factor (VEGF) and bFGF (34). Some studies have indicated that MMPs, which directly regulate the switch of angiogenesis, including MMP-2, -9, -14 and probably MMP-19, -1 and -13 (34, 35). MMP-2 expression in tumor cells develops angiogenesis by remodeling of collagen type 4 and generating active site to allow for binding of molecules to αvβ3 (36). MMP-9 can regulate and increase bioavailability of pro-angiogenic agents like VEGF (37). MMP-14 acts by cleaving the fibrin matrix that surrounds the vessels, thereby causes more invasion of endothelial cells (38). In the same way, both MMP-1 and MMP-3 can degrade endothelial derived perlecan and stimulate bFGF secretion (34). There is some evidence that indicates MMPs might have inhibitory actions against angiogenesis. Thereby MMP roles are critical, which can negatively regulate the vascular growth (2). For example, MMP-2, -3, -7, -9 and -12 by degrading plasminogen produce angiostatin that potentially inhibits angiogenesis by reducing endothelial cells proliferation. Also MMP-3, -9, -12, and -13 can produce endostatin, which is produced as a breakdown remnant of the basement-membrane collagen type 8 (39-42). Endostatin potentially reduces angiogenesis by means of inhibition of cell proliferation and invasion. Moreover, MMP-12 by degrading the cell membrane bound urokinase-type plasminogen activator receptor can inhibit endothelial cell invasion and angiogenesis (43).

3.7. MMPs and Epithelial-Mesenchymal Transition (EMT)

EMT is a process that occurs during the development of tissues and organs (8). EMT is defined as events such as formation of specific epithelial cells, degradation of the basement membrane, entering of the separated cells, disruption of epithelial tissue constitution, and development of a novel mesenchymal form (44). MMPs have been demonstrated to be important factors in epithelial-mesenchymal transition process in breast cancer progression through 3 mechanisms: a) secretion of MMPs from the surroundings of the tumor leads to induction of EMT in epithelial cells, b) cancer cells express and secrete more MMPs followed by EMT, which in turn increases aggressiveness and metastasis of cells, c) over production of MMPs results in generation of stromal-like cells which induce tumor initiation (4).

3.8. MMPs and Metastasis

Various studies extensively surveyed the MMPs roles in metastasis. Maintaining the stroma integrity and their adhesion together are contributing factors for inhibition of metastasis and invasion of cancerous cells. Due to degrading activity of MMPs during the metastasis in mammary glands, these barriers are disrupted and consequently tumor cells cross the basement membranes and spread to the surrounding stroma. Then, they can invade to blood or lymphatic vessels and be colonized (5). Generally metastasis is modulated by expression of tissue inhibitors of metalloproteinase (TIMPs) and MMPs; over production of TIMPs reduces and MMP-2, -3, -9, -13 and -14 increases metastasis by cleaving collagen type 1 and matrigel (45-51). Only a limited number of cell lines and animal studies have been performed thus far, investigating the role of MMPs in breast cancer metastasis (52). Aggressive manner in MDA-MB-435 human breast cancer cell line was decreased by administration of TIMP-4 into them (53). Also in transgenic models of breast cancer cells, injection of MDA-MB-231 into nude mice by overproduction of TIMP-2 resulted in decreased osteolytic lesions (54). Similar to other steps of breast cancer progression, MMPs promote metastasis through degradation of ECM. Most common MMPs, which modulate invasion of breast cancer, including MMP-2 and MMP-9. These MMPs mediate the invasion by generating collagen 4 of ECM (1). MMP-2 activity leads to disruption of laminin-5, which can trigger cancer cell mobility and migration (55, 56). Moreover, MMP-9 binds to CD44 presented in the cell surface. This localization of MMP-9 is essential for tumor invasion and angiogenesis (5). The most common site, that breast cancer cells invade is bone; this results in disturbing the balance between bone remodeling and formation, and ultimately leading to net bone degradation. Such degradation is the main effect of breast cancer metastasis that is mediated by osteoclasts resorption activity that is catalyzed by MMPs (57).