Maspin alters the carcinoma proteome

SERPINB5 promotes colorectal cancer invasion and migration by promoting EMT and angiogenesis via the TNF-α/NF-κB pathway

This study aimed to investigate the role of SERPINB5 in colorectal cancer (CRC). We established knockdown and overexpression models of SERPINB5 in CRC cells and conducted bioinformatics analysis to assess the clinicopathological significance of SERPINB5 expression in CRC patients. Human CRC cells were transfected with LV-SERPINB5 and sh-SERPINB5 lentivirus for subsequent functional and mechanistic studies. Results showed that high SERPINB5 expression correlated positively with CEA levels, N stage and lymphatic infiltration, while displaying a negative correlation with progression-free survival. Overexpression of SERPINB5 in CRC cells upregulated the expression of TNF-α, p-NF-κB/p65, N-cadherin, MMP2 and MMP9, accompanied by decreased E-cadherin expression. In addition, SERPINB5 overexpression enhanced the migration, invasion, and proliferation of CRC cells. Furthermore, overexpression of SERPINB5 in CRC cells increased VEGFA expression, and the conditioned medium from SERPINB5-overexpressing CRC cells promoted tube formation of HUVECs. Conversely, overexpression of SERPINB5 in HUVECs decreased VEGFA expression and inhibited tube formation. Notably, these changes in CRC cells were reversed by QNZ, a specific inhibitor of the TNF-α/NF-κB pathway. In summary, our findings revealed that high SERPINB5 expression correlated with poor progression-free survival in CRC patients. Moreover, SERPINB5 could induce EMT and angiogenesis by activating the TNF-α/NF-κB pathway, thereby promoting the invasion and migration of CRC cells.

DNMT2/TRDMT1 gene knockout compromises doxorubicin-induced unfolded protein response and sensitizes cancer cells to ER stress-induced apoptosis

The acidic, hypoxic and nutrient-deprived tumor microenvironment may induce endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) may exert an important cytoprotective role by promoting folding of newly synthesized proteins and cancer cell survival. The lack of DNMT2/TRDMT1 methyltransferase-mediated C38 tRNA methylation compromises translational fidelity that may result in the accumulation of misfolded and aggregated proteins leading to proteotoxic stress-related cell death. In the present study, DNMT2/TRDMT1 gene knockout-mediated effects were investigated during doxorubicin (DOX)-induced ER stress and PERK-, IRE1- and ATF6-orchestrated UPR in four genetically different cellular models of cancer (breast and cervical cancer, osteosarcoma and glioblastoma cells). Upon DOX stimulation, DNMT2/TRDMT1 gene knockout impaired PERK activation and modulated NSUN and 5-methylcytosine RNA-based responses and microRNA profiles. The lack of DNMT2/TRDMT1 gene in DOX-treated four cancer cell lines resulted in decreased levels of four microRNAs, namely, miR-23a-3p, miR-93-5p, miR-125a-5p and miR-191-5p involved in the regulation of several pathways such as ubiquitin-mediated proteolysis, amino acid degradation and translational misregulation in cancer. We conclude that DNMT2/TRDMT1 gene knockout, at least in selected cellular cancer models, affects adaptive responses associated with protein homeostasis networks that during prolonged ER stress may result in increased sensitivity to apoptotic cell death.

Zinc oxide nanoparticles enhance expression of maspin in human breast cancer cells

Toxic and apoptotic impacts of zinc oxide nanoparticle (ZNP) on different cancer cells have been reported. Maspin (a mammary serine protease inhibitor) as a tumor suppressor gene can inhibit tumor growth and metastasis. The expression of maspin is modulated by p53, Bcl-2 family genes, and estrogen receptor α (ER-α). This study aimed to assess the ZNP effects on maspin expression in MCF-7 cells (a breast cancer cell). Experimental groups (ZNP5, ZNP10, and ZNP20) received 5, 10, and 20 μM/mL ZNP for 48 h, respectively. 17-β-estradiol (E2) was used to evaluate the role of ER-α in the anticancer impact of ZNP. Cell viability, Annexin V, migration assay, gene expression, and western blotting methods were applied to evaluate ZNP effects on the MCF-7 cells. ZNP at the concentrations of 10 and 20 μM/mL could significantly decrease the viability and migration rate, and significantly increase apoptosis percentage in the MCF-7 cells. ZNP significantly enhanced mRNA expression and protein level of maspin in MCF-7 cells in a concentration-dependent way. ZNP concentration-dependently elevated mRNA expression and protein level of p53 and Bax while reduced the expression of Bcl-2 and ER-α. E2 promoted cancer cell growth by enhancing survival and migration rates. E2 treatment reduced mRNA expression and protein level of maspin and p53, and elevated Bcl-2 expression. ZNP considerably changed these events induced by E2 in the MCF-7 cells. It is concluded that the maspin overexpression is one of the toxic mechanisms of the ZNP on the ER-α-positive breast cancer cells, and can suppress the migration of these cells.

Effects of Noninhibitory Serpin Maspin on the Actin Cytoskeleton: A Quantitative Image Modeling Approach

Recent developments in quantitative image analysis allow us to interrogate confocal microscopy images to answer biological questions. Clumped and layered cell nuclei and cytoplasm in confocal images challenges the ability to identify subcellular compartments. To date, there is no perfect image analysis method to identify cytoskeletal changes in confocal images. Here, we present a multidisciplinary study where an image analysis model was developed to allow quantitative measurements of changes in the cytoskeleton of cells with different maspin exposure. Maspin, a noninhibitory serpin influences cell migration, adhesion, invasion, proliferation, and apoptosis in ways that are consistent with its identification as a tumor metastasis suppressor. Using different cell types, we tested the hypothesis that reduction in cell migration by maspin would be reflected in the architecture of the actin cytoskeleton. A hybrid marker-controlled watershed segmentation technique was used to segment the nuclei, cytoplasm, and ruffling regions before measuring cytoskeletal changes. This was informed by immunohistochemical staining of cells transfected stably or transiently with maspin proteins, or with added bioactive peptides or protein. Image analysis results showed that the effects of maspin were mirrored by effects on cell architecture, in a way that could be described quantitatively.

Role of maspin in cancer

Maspin (mammary serine protease inhibitor), is a member of the serine protease inhibitor/non-inhibitor superfamily. Its expression is down-regulated in breast, prostate, gastric and melanoma cancers but over-expressed in pancreatic, gallbladder, colorectal, and thyroid cancers suggesting that maspin may play different activities in different cell types. However, maspin expression seems to be correlated with better prognosis in prostate, bladder, lung, gastric, colorectal, head and neck, thyroid and melanoma cancer. In breast and ovarian cancer maspin significance is associated with its subcellular localization: nucleus maspin expression correlates with a good prognosis, whilst in pancreatic cancer it predicts a poor prognosis. Since tumor metastasis requires the detachment and invasion of tumor cells through the basement membrane and stroma, a selectively increased adhesion by the presence of maspin may contribute to the inhibition of tumor metastasis. Furthermore the different position of maspin inside the cell or its epigenetic modifications may explain the different behavior of the expression of maspin between tumors. The expression of maspin might be useful as a prognostic and possibly predictive factor for patients with particular types of cancer and data can guide physicians in selecting therapy. Its expression in circulating tumor cells especially in breast cancer, could be also useful in clinical practice along with other factors, such as age, comorbidities, blood examinations in order to select the best therapy to be carried out. Focusing on the malignancies in which maspin showed a positive prognostic value, therapeutic approaches studied so far aimed to re-activate a dormant tumor suppressor gene by designed transcription factors, to hit the system that inhibits the expression of maspin, to identify natural substances that can determine the activation and the expression of maspin or possible “molecules binds” to introduce maspin in cancer cell and gene therapy capable of up-regulating the maspin in an attempt to reduce primarily the risk of metastasis.

Further studies in these directions are necessary to better define the therapeutic implication of maspin.

An emerging role for the nuclear localization of maspin in the suppression of tumor progression and metastasis

Maspin, a member of the serpin family of serine protease inhibitors, was originally identified as a tumor suppressor that is expressed in normal mammary epithelial cells but is reduced or absent in breast carcinomas. Early enthusiasm for maspin as a biomarker for disease progression has been tempered by clinical data that associates maspin with favourable outcomes in some studies and poor prognosis in others. Here, we review all of the published clinical studies for maspin in breast and ovarian cancers and propose that the apparent discordance between clinical reports is a consequence of differential cellular distribution of maspin. Indeed, it was thought that an extracellular pool of maspin possessed tumor suppressor activity, acting by inhibiting migration and increasing cell adhesion. Recent evidence from our group and others indicates, however, that the nuclear localization of maspin in cancer cells is necessary for its tumor suppressor activity. We provide additional data here to demonstrate that nuclear-localized maspin binds to chromatin and is required to effectively prevent cells from metastasizing. Our knowledge of other serpins that localize to the nucleus should help to inform future studies of nuclear maspin. Elucidation of the molecular mechanisms regulating the localization and activities of maspin should pave the way for the development of improved diagnostics and therapies for cancer.

Nuclear localization of maspin is essential for its inhibition of tumor growth and metastasis

Maspin (mammary serine protease inhibitor or SerpinB5) acts as a tumor suppressor when overexpressed in aggressive cancer cell lines. However, its role in human cancer is controversial. Maspin expression has been associated with a poor prognosis in some studies, whereas in others, with favorable outcome. The clinical data suggest, however, that nuclear-localized maspin is associated with improved survival. We hypothesized that the tumor suppressor activity of maspin may require nuclear localization, and that the discordance between clinical and experimental reports is a consequence of the variable subcellular distribution of maspin. Furthermore, we surmized that nuclear maspin could function as a tumor suppressor through the regulation of genes involved in tumor growth and invasion. Maspin or maspin fused to a nuclear export signal were expressed in metastatic human breast and epidermoid carcinoma cell lines. We found that pan-cellular localized maspin inhibited in vivo tumor growth and metastasis when assessed in xenograft chicken embryo and murine mammary fat pad injection models. However, when maspin was excluded from the nucleus via a nuclear exclusion signal, it no longer functioned as a metastasis suppressor. Using chromatin immunoprecipitation, we show that nuclear maspin was enriched at the promoter of colony-stimulating factor-1 (CSF-1) and associated with diminished levels of CSF-1 mRNA. Our findings demonstrate that the nuclear localization of maspin is required for its tumor and metastasis suppressor functions in vivo, and suggest that its mechanism of action involves, in part, direct association of maspin with target genes.

Reactivation of MASPIN in non-small cell lung carcinoma (NSCLC) cells by artificial transcription factors (ATFs)

Tumor suppressor genes have antiproliferative and antimetastatic functions, and thus, they negatively affect tumor progression. Reactivating specific tumor suppressor genes would offer an important therapeutic strategy to block tumor progression. Mammary Serine Protease Inhibitor (MASPIN) is a tumor suppressor gene that is not mutated or rearranged in tumor cells, but is silenced during metastatic progression by transcriptional and epigenetic mechanisms. In this work, we have investigated the ability of Artificial Transcription Factors (ATFs) to reactivate MASPIN expression and to reduce tumor growth and metastatic dissemination in Non-Small Cell Lung Carcinoma (NSCLC) cell lines carrying a hypermethylated MASPIN promoter. We found that the ATFs linked to transactivator domains were able to demethylate the MASPIN promoter. Consistently, we observed that co-treatment of ATF-transduced cells with methyltransferase inhibitors enhanced MASPIN expression as well as induction of tumor cell apoptosis. In addition to tumor suppressive functions, restoration of endogenous MASPIN expression was accompanied by inhibition of metastatic dissemination in nude mice. ATF-mediated reactivation of MASPIN lead to changes in cell motility and to induction of E-CADHERIN. These data suggest that ATFs are able to reprogram aggressive lung tumor cells towards a more epithelial, differentiated phenotype, and thus, represent novel therapeutic agents for metastatic lung cancers.

Cancer proteomics by quantitative shotgun proteomics

A major scientific challenge at the present time for cancer research is the determination of the underlying biological basis for cancer development. It is further complicated by the heterogeneity of cancer's origin. Understanding the molecular basis of cancer requires studying the dynamic and spatial interactions among proteins in cells, signaling events among cancer cells, and interactions between the cancer cells and the tumor microenvironment. Recently, it has been proposed that large-scale protein expression analysis of cancer cell proteomes promises to be valuable for investigating mechanisms of cancer transformation. Advances in mass spectrometry technologies and bioinformatics tools provide a tremendous opportunity to qualitatively and quantitatively interrogate dynamic protein-protein interactions and differential regulation of cellular signaling pathways associated with tumor development. In this review, progress in shotgun proteomics technologies for examining the molecular basis of cancer development will be presented and discussed.

Nuclear localization of mammary serine protease inhibitor (MASPIN): is its impact on the prognosis in laryngeal carcinoma due to a proapoptotic effect?

PURPOSE

Cell death by apoptosis is considered a regulator of cell number; cancer cells are defective in their response to apoptosis. Many potential markers of apoptosis are under study: M30 immunoreactivity is confined to the cytoplasm of apoptotic epithelial cells and is expressed during early apoptosis. Mammary serine protease inhibitor (MASPIN), a suppressor of tumor growth, seems to be involved in the induction of tumour cell apoptosis. The aim of our preliminary study was to investigate, for the first time, the relations between MASPIN subcellular pattern of expression, nuclear MASPIN expression, M30 expression, and prognosis in laryngeal carcinoma.

MATERIALS AND METHODS

Subcellular pattern of distribution of MASPIN and nuclear MASPIN expression were immunohistochemically determined in 66 consecutive cases of laryngeal carcinoma. M30 expression in correspondent carcinoma fields was also calculated.

RESULTS

M30 expression was significantly higher in the group of laryngeal carcinomas with MASPIN nuclear localization (P = .024). Our investigation found a reduced carcinoma recurrence rate in the group of patients with MASPIN nuclear localization (P value = .0086). The log-rank test showed a significantly longer disease-free interval in patients with nuclear MASPIN localization (P = .029).

CONCLUSIONS

These preliminary results support the hypothesis of an apoptosis-sensitizing effect of nuclear MASPIN in laryngeal carcinoma with the potential perspective of a clinical use of the tumour suppressive proapoptotic function of MASPIN.

IRF6 in development and disease: a mediator of quiescence and differentiation

Post utero development of the mammary gland is a complex developmental process characterized by states of rapid cell proliferation (branching morphogenesis) followed by functional differentiation (lactation) and the consequent apoptosis (involution) of the secretory mammary epithelial cell. This process is cyclical, such that involution returns the mammary gland to a near-virgin-like state capable of responding to morphogenic cues with each consecutive pregnancy. Importantly, many of the regulatory processes which oversee mammary gland development are corrupted or otherwise compromised during the development of breast cancer. For example, Interferon Regulatory Factor 6 (IRF6) is a novel protein with growth inhibitory properties that was initially identified in mammary epithelial cells through its interaction with maspin, a known tumor suppressor in normal breast tissue. Recent findings from our laboratory suggest that IRF6 functions synergistically with maspin to regulate mammary epithelial cell differentiation by acting on the cell cycle. This perspective focuses on the possible involvement of IRF6 in promoting differentiation by regulating exit from the cell cycle and entry into the G(0) phase of cellular quiescence, and how these new findings shed light on normal mammary gland development and the initiation and progression of breast cancer.

Interferon regulatory factor 6 promotes cell cycle arrest and is regulated by the proteasome in a cell cycle-dependent manner

Interferon regulatory factor 6 (IRF6) is a novel and unique member of the IRF family of transcription factors. IRF6 has not been linked to the regulatory pathways or functions associated with other IRF family members, and the regulation and function of IRF6 remain unknown. We recently identified a protein interaction between IRF6 and the tumor suppressor maspin. To gain insight into the biological significance of the maspin-IRF6 interaction, we examined the regulation and function of IRF6 in relation to maspin in normal mammary epithelial cells. Our results demonstrate that in quiescent cells, IRF6 exists primarily in a nonphosphorylated state. However, cellular proliferation leads to rapid IRF6 phosphorylation, resulting in proteasome-dependent IRF6 degradation. These data are supported in situ by the increased expression of IRF6 in quiescent, differentiated lobuloalveolar cells of the lactating mammary gland compared to its expression in proliferating ductal and glandular epithelial cells during pregnancy. Furthermore, the reexpression of IRF6 in breast cancer cells results in cell cycle arrest, and the presence of maspin augments this response. These data support a model in which IRF6, in collaboration with maspin, promotes mammary epithelial cell differentiation by facilitating entry into the G(0) phase of the cell cycle.

Paradoxical expression of maspin in gastric carcinomas: correlation with carcinogenesis and progression

Maspin, a serine protease inhibitor related to the serpin family, can suppress invasion and metastasis of malignancies. To clarify the role of maspin in the genesis and progression of gastric carcinomas, its expression pattern and level were studied by immunohistochemistry on tissue microarrays containing gastric carcinoma (n = 237), normal gastric mucosa (n = 23), intestinal metaplasia (n = 38), and adenoma (n = 42); and the findings were compared with clinicopathological parameters. Furthermore, maspin expression in the gastric carcinoma cell lines (HCG-27, MKN28, and MKN45) was examined by immunohistochemistry and Western blotting. We found that cytoplasmic and nuclear maspin expression paralleled each other (P < .05) and decreased from intestinal metaplasia, adenoma, and carcinoma to normal gastric mucosa (P < .05). A significant positive association was noted with depth of invasion, lymphatic invasion, lymph node metastasis, and TNM stage (P < .05) but not with sex or Lauren's classification (P > .05). Univariate and multivariate analyses indicated that expression of maspin correlated negatively with cumulative patient survival in gastric carcinoma (P < .05) but was not an independent factor in the prognosis. The 2 independent factors, depth of invasion and lymphatic invasion, influenced the relation between nuclear maspin expression and survival, whereas only depth of invasion correlated with cytoplasmic maspin. Our study indicated that maspin expression experiences upregulation in gastric precancerous lesions and then slight downregulation with malignant transformation. High expression may paradoxically promote invasion and metastasis of gastric carcinomas and could be considered a good marker for the pathobiological behaviors of gastric carcinomas.

Maspin augments proteasome inhibitor-induced apoptosis in prostate cancer cells

Proteasome inhibitors are known to induce apoptosis in a variety of cancer cells. On the other hand, maspin, a non-inhibitory serine protease inhibitor, is shown to sensitize cancer cells to therapeutic agents that induce apoptosis. We examined the consequence of maspin expression in prostate cancer cells targeted for treatment with various proteasome inhibitors. We observed that proteasome inhibitors induced apoptosis more effectively in maspin transfected human prostate cancer DU145 cells than in control cells. Interestingly, increased apoptosis in these cells was associated with a significant induction of maspin expression. MG-132, a proteasome inhibitor, induced endogenous and ectopic [cytomegalovirus promoter (CMV)-driven] maspin expression, and maspin siRNA attenuated MG-132-induced apoptosis. Proteasome inhibitor-induced maspin expression was inhibited by actinomycin D (Act D) and cyclohexamide (CHX), and by the inhibitors of p38MAPK, but not ERK1/2 or NF-kappaB. Electrophoretic mobility-shift assay (EMSA) and promoter-reporter activity analyses suggested that p38MAPK activated transcription factor AP-1 is responsible for proteasome inhibitor-induced maspin expression. Taken together, these observations demonstrate that proteasome inhibitors induce maspin expression by activating p38MAPK pathway, and that maspin thus expressed, in turn, augments proteasome inhibitor-induced apoptosis in prostate cancer cells. Our results suggest that gene therapy involving ectopic maspin expression may dramatically improve the efficacy of proteasome inhibitors for the treatment of prostate cancer.

Loss of maspin expression contributes to a more invasive potential in malignant melanoma

Deregulation of protease expression and activity is known to play an important role in tumour progression of malignant melanoma. The serpin maspin, a tumour suppressor in breast and prostate cancer was described as an inhibitor of cell migration and inducer of cell adhesion between the basement membrane and extracellular matrix resulting in inhibition of tumour metastasis. In contrast, overexpression of maspin is correlated with poor prognosis in other cancers. However, little is known about expression, regulation and function of maspin in malignant melanoma. In this study, we found loss of maspin expression in malignant melanoma cells compared with normal human epidermal melanocytes, which was analysed by quantitative real-time PCR, Western blot analysis, immunohistochemistry and microarray. For functional studies, melanoma cell clones stably transfected with a maspin expression vector were tested for changes in proliferation, migration and invasion. Although we could not see differences in proliferation and migration, we detected strongly reduced invasive capacity in the melanoma cell clones in which maspin is re-expressed compared with control. Reduced invasive potential was also detected in three different melanoma cell lines transiently transfected with a maspin expression vector. Furthermore, exogenously added maspin alone was sufficient to reduce invasion in MelIm significantly, indicating that maspin directly inhibits invasion on the cell surface. In summary, we believe that maspin is a tumour suppressor in malignant melanoma.

Adaptation of energy metabolism in breast cancer brain metastases

Brain metastases are among the most feared complications in breast cancer, as no therapy exists that prevents or eliminates breast cancer spreading to the brain. New therapeutic strategies depend on specific knowledge of tumor cell properties that allow breast cancer cell growth within the brain tissue. To provide information in this direction, we established a human breast cancer cell model for brain metastasis based on circulating tumor cells from a breast cancer patient and variants of these cells derived from bone or brain lesions in immunodeficient mice. The brain-derived cells showed an increased potential for brain metastasis in vivo and exhibited a unique protein expression profile identified by large-scale proteomic analysis. This protein profile is consistent with either a selection of predisposed cells or bioenergetic adaptation of the tumor cells to the unique energy metabolism of the brain. Increased expression of enzymes involved in glycolysis, tricarboxylic acid cycle, and oxidative phosphorylation pathways suggests that the brain metastatic cells derive energy from glucose oxidation. The cells further showed enhanced activation of the pentose phosphate pathway and the glutathione system, which can minimize production of reactive oxygen species resulting from an enhanced oxidative metabolism. These changes promoted resistance of brain metastatic cells to drugs that affect the cellular redox balance. Importantly, the metabolic alterations are associated with strongly enhanced tumor cell survival and proliferation in the brain microenvironment. Thus, our data support the hypothesis that predisposition or adaptation of the tumor cell energy metabolism is a key element in breast cancer brain metastasis, and raise the possibility of targeting the functional differentiation in breast cancer brain lesions as a novel therapeutic strategy.

Maspin controls mammary tumor cell migration through inhibiting Rac1 and Cdc42, but not the RhoA GTPase

Rac1 and Cdc42 are members of the Rho family of small GTPases that play essential roles in diverse cellular functions, including cell migration. The activities of these Rho family proteins are controlled by growth factor receptor activation and cell-ECM interactions. Here, we show that maspin, a well-documented tumor suppressor gene, also controls cell motility through inhibiting Rac1/Cdc42 activity. Using the GST-PAK and GST-Rho binding protein pull-down assays for GTP-bound Rac1, Cdc42, and RhoA, we showed that treatment of MDA-MB-231 tumor cells with recombinant maspin for a short time period significantly inhibited the activity of Rac1 and Cdc42, but not RhoA. The reactive site loop (RSL) within maspin protein is the functional domain involved in the inhibition. Maspin mutants with the RSL deleted or a point mutation in the RSL region lost their inhibitory activity. We further examined the ability of maspin to inhibit Rac1- and Cdc42-mediated signaling pathways and transcription factors. Treatment of MDA-MB-231 cells with maspin led to the inhibition of JNK kinase activity as assayed by immuno-kinase assays. In addition, the AP-1 transcription activity downstream of JNK kinase pathway was also reduced. Together, we have identified Rac1 and Cdc42 as the downstream targets that mediate the inhibition of mammary tumor cell migration by maspin.

Maspin: the new frontier

Maspin (mammary serine protease inhibitor) was identified in 1994 by subtractive hybridization analysis of normal mammary tissue and breast cancer cell lines. Subsequently, emerging evidence portrays maspin as a multifaceted protein, interacting with diverse group of intercellular and extracellular proteins, regulating cell adhesion, motility, apoptosis, and angiogenesis and critically involved in mammary gland development. The tissue-specific expression of maspin is epigenetically controlled, and aberrant methylation of maspin promoter is closely associated with maspin gene silencing. Identification of new tissue sites expressing maspin and novel maspin-binding partners has expanded the horizon for maspin research and promises maspin-based therapeutic approaches for combating cancer. This perspective briefly outlines the past and present strides in deciphering this unique molecule and speculates on new frontiers in maspin research and prospects of maspin as a diagnostic/prognostic indicator in cancer.

Maspin and tumor metastasis

For most cancer cell types, the acquisition of metastatic activity leads to clinically incurable disease. Improvements in surgery and radiotherapy, and the development of new chemotherapeutic agents or their use in new combinations, have, so far, only incrementally improved patient survival. Despite the obvious importance of metastasis, the process remains incompletely characterized at the molecular and biochemical levels. Tumor metastasis is a complex process and requires multiple cellular functions over time. From cellular invasion, extravasation from the primary tumor, intravasation to the secondary organs, to successful colonization, tumor cells utilize many cellular or biochemical mechanisms to complete the metastatic spread. During the process of metastasis, there are consistent changes in gene expression. Studies of genes that are reduced or silenced have yielded surprising insights into in vivo mechanisms of regulating tumor metastasis. This review describes a tumor suppressor gene, Maspin, which is often silenced in cancer cells and exhibits suppressing activity against tumor growth and metastasis. Maspin has been shown to be involved in processes that are important to both tumor growth and metastasis such as cell invasion, angiogenesis, and more recently apoptosis. Hence, many efforts have been devoted to deciphering the molecular mechanism of maspin. While some insights have come from the protease inhibitory effect of maspin, more perceptive results on how maspin may function in suppressing tumor metastasis have come from studies of gene manipulation, protein interactions and global protein profiling.

Biological functions of maspin

Maspin (Mammary Serine Protease Inhibitor) was first reported in 1994 as a serpin with tumor suppressive properties. Maspin was initially isolated through subtractive hybridization and differential display analysis as a 42-kDa protein that is expressed in normal mammary epithelial cells but reduced or absent in breast carcinomas (Zou et al., 1994). Further research led to maspin's characterization as a class II tumor suppressor based on its ability to inhibit cell invasion, promote apoptosis, and inhibit angiogenesis (Sheng et al., 1996; Zhang et al., 2000b; Jiang et al., 2002). Since then, efforts have been made to characterize maspin's tumor suppressive mechanisms. In particular, researchers have studied maspin localization, the regulation of maspin expression, and more recently, maspin protein interactions. By elucidating these mechanisms, researchers are beginning to understand the complex, pleiotropic nature of maspin and the pathways through which maspin exerts its tumor suppressive properties. These new findings not only further enhance our understanding of cancer biology but also provide an avenue to develop maspin's potential as a diagnostic marker for cancer progression, and as a potentially powerful therapeutic agent in the fight against breast cancer.