p21激活激酶在肿瘤中的作用及其靶向治疗

p21激活激酶(p21-activated kinases,Paks)是一类丝氨酸/苏氨酸激酶,在进化上高度保守.Paks可被多种上游信号特别是小G蛋白Rho家族的Rac和Cdc42激活,对多种重要的信号通路及细胞功能进行调控.Paks在多种肿瘤中异常表达,参与细胞骨架重构、细胞运动、细胞增殖、分化、凋亡、有丝分裂及血管生成等,在肿瘤的发生、发展中起重要作用.因此,开展Paks靶向治疗的研究,可为肿瘤治疗提供新的思路.     

Targeting group II PAKs in cancer and metastasis

The p21 activated kinases (PAKs) play an essential role in cell signaling and control a variety of cellular functions including cell motility, survival, angiogenesis and mitosis. PAKs are important regulators in growth factor signaling, cytoskeletal reorganization and growth factor-mediated cell migration. Overexpression of PAKs has been detected in many cancers and linked to increased migration potential, anchorage independent growth and metastasis. Six isoforms of PAKs are expressed in human and based on their regulatory properties they have been classified into group I (PAK1–3) and group II (PAK4–6). Besides the well studied group I family, members of the group II PAKs also emerged as interesting targets for the development of new inhibitors for cancer therapy. The availability of high resolution crystal structures for all group II PAKs and their fundamentally different regulatory properties when compared with group I enzymes has opened new opportunities for rational drug designing strategies. In this review, we summarize the results of recent advances of the function of group II PAKs in tumorigenesis and metastasis as well as opportunities for exploring the unique catalytic domain dynamics of this protein family for the design of group II PAK specific inhibitors.     

The insulin-like growth factor system and cancer

The insulin-like growth factor (IGF) family of ligands, binding proteins and receptors is an important growth factor system involved in both the development of the organism and the maintenance of normal function of many cells of the body. The system also has powerful anti-apoptotic effects. More recently, evidence has accrued to demonstrate that the IGFs play an important role in cancer. Individuals with serum IGF-II levels in the upper quartile of the normal range (and IGF binding protein-3 levels in the lower quartiles) have a relative risk for developing breast, prostate, colon and lung cancer. IGF-II is commonly expressed by tumor cells and may act as an autocrine growth factor; occasionally even reaching target tissues and causing tumor-induced hypoglycemia. The IGF-I receptor is commonly (though not always) overexpressed in many cancers, and many recent studies have identified new signaling pathways emanating from the IGF-I receptor that affect cancer cell proliferation, adhesion, migration and cell death; functions that are critical for cancer cell survival and metastases. In this review, many aspects of the IGF system and its relationship to cancer will be discussed.     

Role of Eph/ephrin tyrosine kinase in malignant glioma

Accumulating evidence has revealed that the tyrosine kinases play a major role in glioma proliferation and invasion. The largest family of tyrosine kinases, the Eph family, and its ligands, the ephrins, are frequently overexpressed in glioma, suggesting important roles for their bidirectional signals in glioma pathobiology. Ephs bind to cell surface-associated ephrin ligands on neighboring cells and have many biological functions during embryonic development of the central nervous system, including axon mapping, cell migration, and angiogenesis. Recent findings suggest that Eph/ephrin signaling affects glioma cell growth, migration, and invasion in vitro and in vivo. However, their roles in glioma seem complex, because both tumor growth promoter and suppressor potentials have been ascribed to Ephs and ephrins. Here, we review recent advances in research on the role of Eph/ephrin signaling in glioma and suggest that the Eph/ephrin system could be a potential target of glioma therapy.     

Multifunctional roles of insulin-like growth factor binding protein 5 in breast cancer

The insulin-like growth factor axis, which has been shown to protect cells from apoptosis, plays an essential role in normal cell physiology and in cancer development. The family of insulin-like growth factor binding proteins (IGFBPs) has been shown to have a diverse spectrum of functions in cell growth, death, motility, and tissue remodeling. Among the six IGFBP family members, IGFBP-5 has recently been shown to play an important role in the biology of breast cancer, especially in breast cancer metastasis; however, the exact mechanisms of action remain obscure and sometimes paradoxical. An in-depth understanding of IGFBP-5 would shed light on its potential role as a target for breast cancer therapeutics.     

Protein tyrosine kinases in malignant melanoma

Protein tyrosyl phosphorylation is an essential component in intracellular signalling, with diverse and crucial functions including mediation of cell proliferation, survival, death, differentiation, migration and attachment. It is regulated by the balance between the activities of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases. A number of PTKs are encoded by proto-oncogenes or viral oncogenes, and are thus strongly implicated in cancer. While a role for PTKs in human melanoma is less firmly established, human melanomas or melanoma cells have been reported to contain more tyrosine phosphate than normal melanocytes, and some receptor PTKs (EPH-A2/ ECK and EPH-B3) are overexpressed in over 90% of melanoma cell lines. Other specific PTKs are also frequently overexpressed, including KDR and fibroblast growth factor receptor-4 (FGF-R4), while, interestingly, yet others, such as KIT and FES, are consistently downregulated in melanoma cell lines. All of these differentially expressed PTKs are candidates for gene products important in melanoma development. In addition, PTKs expressed in significant amounts in both benign and malignant melanocytes, such as insulin-like growth factor-1 receptor (IGF1-R), FGF-R1, HER2/NEU and FAK, are likely to play a role in melanoma genesis and progression.     

An essential role of Pak1 phosphorylation of SHARP in Notch signaling

The p21-activated kinases (Paks), an evolutionarily conserved family of serine/threonine kinases, play an important role in cytoskeletal reorganization in mammalian cells. The Notch signaling pathway plays an important role in the determination of cell fate/differentiation in a number of organs. Notch signaling is a complex process, and the mechanism by which Notch regulates multiple cellular processes is intriguing. The expression of both Notch and Pak1 has been shown to be deregulated in several human cancers. Using yeast two-hybrid screening, we identified SHARP, one of the Notch signaling components, as a Pak1-interacting protein. We found that SHARP is a physiologic interacting substrate of Pak1, and that this interaction enhances SHARP-mediated repression of Notch target genes. Pak1 phosphorylation sites in SHARP were mapped to Ser3486 and Thr3568 within the SHARP repression domain. Mutation of Pak1 phosphorylation sites in SHARP, inhibition of Pak1 functions by a Pak1-autoinhibitory fragment (amino acids 83-149), or expression of Pak1-specific siRNA interfered with SHARP-mediated repression of Notch target reporter gene activation. These results demonstrate that Pak1-SHARP interaction plays an essential role in enhancing the corepressor functions of SHARP, thereby modulating Notch signaling in human cancer cells.     

TGFβ‐induced epithelial‐to‐mesenchymal transition in prostate cancer cells is mediated via TRPM7 expression

Growth factors, such as the transforming growth factor beta (TGFβ), play an important role in promoting metastasis of prostate cancer, thus understanding how TGFβ could induce prostate cancer cell migration may enable us to develop targeted strategies for treatment of advanced metastatic prostate cancer. To more clearly define the mechanism(s) involved in prostate cancer cell migration, we undertook a series of studies utilizing non‐malignant prostate epithelial cells RWPE1 and prostate cancer DU145 and PC3 cells. Our studies show that increased cell migration was observed in prostate cancer cells, which was mediated through epithelial‐to‐mesenchymal transition (EMT). Importantly, addition of Mg²⁺, but not Ca²⁺, increased cell migration. Furthermore, TRPM7 expression, which functions as an Mg²⁺ influx channel, was also increased in prostate cancer cells. Inhibition of TRPM7 currents by 2‐APB, significantly blocked cell migration in both DU145 and PC3 cells. Addition of growth factor TGFβ showed a further increase in cell migration, which was again blocked by the addition of 2‐APB. Importantly, TGFβ addition also significantly increased TRPM7 expression and function, and silencing of TRPM7 negated TGFβ‐induced cell migration along with a decrease in EMT markers showing loss of cell adhesion. Furthermore, resveratrol, which decreases prostate cancer cell migration, inhibited TRPM7 expression and function including TGFβ‐induced cell migration and activation of TRPM7 function. Together, these results suggest that Mg²⁺ influx via TRPM7 promotes cell migration by inducing EMT in prostate cancer cells and resveratrol negatively modulates TRPM7 function thereby inhibiting prostate cancer metastasis.     

Fyn

Fyn is 59‐kDa member of the Src family of kinases that is historically associated with T‐cell and neuronal signaling in development and normal cellular physiology. Whereas Src has been heavily studied in cancer, less attention has been traditionally awarded to the other Src kinases such as Fyn. Our group has shown that Fyn is particularly upregulated in prostate cancer in contrast to the alternative members of the Src family. This suggests that it may mediate several important processes attributed to Src kinases in prostate cancer and other malignancies. These functions include not only cellular growth and proliferation but also morphogenesis and cellular motility. Together, these suggest a role for Fyn in both progression and metastasis. As several agents in clinical development affect Fyn activation, understanding the role that Fyn plays in cancer is of great importance in oncology. Cancer 2010. © 2010 American Cancer Society.     

Hepatocyte growth-factor and its receptor C-met regulate both cell-growth and invasion of human pancreatic-cancer

Hepatocyte growth factor (HGF) has an important role not only in liver regeneration buy also in the development of cancer metastasis. It has been known that HGF and its receptor/c-MET are overexpressed in human pancreatic cancer in vivo, compared with the normal pancreas. To examine the propensity of pancreatic cancer to metastasize and its association with poor prognosis, we studied the effects of HGF and c-MET on pancreatic cancer cell growth and invasion in vitro. Dose-dependently, HGF promoted both the growth and invasiveness of pancreatic cancer cells that expressed c-MET; as a chemoattractant, the high gradient of HGF determined the direction of the invasiveness of the cells. No stimulant effect, however, was observed in cancer cells that did not express c-MET. These results suggest that HGF and c-MET may play important roles in human pancreatic cancer cell growth and invasion-metastatic potential.     

Differential expression and phosphorylation of Pak1 and Pak2 in ovarian cancer: effects on prognosis and cell invasion

Ovarian cancer is a gynecological malignancy with high mortality. Therefore, the identification of novel prognostic and therapeutic targets is important. p21‐activated kinases (Paks) are involved in cytoskeleton reorganization. This study investigated the clinical significance of total and phosphorylated (p) Pak1 and Pak2 as well as their functional roles in ovarian cancer. Expressions of Pak1, p‐Pak1 Thr²¹², Pak2 and p‐Pak2 Ser²⁰ in ovarian normal and cancerous cell lines as well as in clinical samples of ovarian tumors were evaluated. The effects of Pak1 and Pak2 on ovarian cancer cell functions were determined. Pak1, p‐Pak1 and p‐Pak2 were overexpressed in ovarian cancer cell lines, and clinical samples of ovarian cancers were compared with benign ovarian lesions/inclusion cysts. Similar Pak2 expression levels were observed among normal and cancerous cell lines and clinical samples. After multiple testing correction, high Pak1 and nuclear p‐Pak1 expression in ovarian cancers was significantly associated with histological type and tumor grade, respectively. Pak1 and p‐Pak1 expression was associated with poor overall and disease‐free survival. Pak1 was an independent prognostic factor. Knockdown of Pak1 and Pak2 in ovarian cancer cell lines reduced cell migration and invasion but did not affect cell proliferation and apoptosis. Knockdown of Pak1 also reduced p38 activation and downregulated vascular endothelial growth factor. Conversely, ectopic Pak1 overexpression enhanced ovarian cancer cell migration and invasion in a kinase‐dependent manner, along with increased p38 activation. Our findings suggest that Pak1, p‐Pak1 and p‐Pak2 play important roles in ovarian carcinogenesis. Pak1 and p‐Pak1 may be potential prognostic markers and therapeutic molecular targets in ovarian cancer.     

Signaling multiplex of the epidermal growth factor receptor

Cells must be able to not only internalize extracellular signals, but also decipher these messages that ultimately result in a variety of cell-specific outcomes. Physiologically, transmembrane receptor tyrosine kinases play an important role in normal development. Pathologically, these receptors are implicated in deregulation of growth displayed by cancer cells. Over the past 30 years, the mechanistic understanding of cell signaling through receptor tyrosine kinases has become quite sophisticated. This review focuses primarily on the epidermal growth factor receptor and cetuximab, the only monoclonal antibody approved for targeting this receptor.     

Signaling multiplex of the epidermal growth factor receptor

Cells must be able to not only internalize extracellular signals, but also decipher these messages that ultimately result in a variety of cell-specific outcomes. Physiologically, transmembrane receptor tyrosine kinases play an important role in normal development. Pathologically, these receptors are implicated in deregulation of growth displayed by cancer cells. Over the past 30 years, the mechanistic understanding of cell signaling through receptor tyrosine kinases has become quite sophisticated. This review focuses primarily on the epidermal growth factor receptor and cetuximab, the only monoclonal antibody approved for targeting this receptor.     

Syk: a new player in the field of breast cancer

Breast tumor development and progression are thought to occur through a complex, multistep process, including oncogene activation (eg HER2/neu) and mutation or loss of tumor suppressor genes (eg p53). Determining the function of genetic alterations in breast carcinoma tumorigenesis and metastasis has been the focus of intensive research efforts for several decades. One group of proteins that play a critical role in breast cancer cell signaling pathways are tyrosine kinases. Overexpression of the tyrosine kinase HER2/neu is observed in many human breast cancers and is positively correlated with enhanced tumorigenesis. Recently, another tyrosine kinase, Syk, has been implicated as an important inhibitor of breast cancer cell growth and metastasis. This recent finding was unexpected, since Syk function has been predominantly linked to hematopoietic cell signaling, and is discussed further in this commentary.     

Raf kinase inhibitor protein: a prostate cancer metastasis suppressor gene

Defining the mechanisms that confer metastatic ability on cancer cells is an important goal towards prevention of metastasis. A gene array screen between a non-metastatic prostate cancer cell and its metastatic derivative line revealed decreased expression of Raf kinase inhibitor protein (RKIP) in the metastatic cell line. This finding is consistent with the possibility that loss of RKIP is associated with metastasis. RKIP is expressed in many tissues including brain, lung, and liver. RKIP blocks Raf-induced phosphorylation of MEK. In addition to its modulation of Raf signaling, RKIP modulates both G-protein signaling and NF-kappaB activity. The impact that RKIP has on multiple signaling pathways grants it the ability to play a role in several cellular functions including membrane biosynthesis, spermatogenesis, and neural signaling. Novel cellular functions for RKIP continue to be identified, several of which contribute to cancer biology. For example, RKIP promotes apoptosis of cancer cells, which suggests that loss of RKIP in cancer will protect cancer cells against cell death. Additionally, restoration of RKIP expression ina metastatic prostate cancer cell line does not effect primary tumor growth, but it does inhibit prostate cancer metastasis. These parameters identify RKIP as a metastasis suppressor gene, which suggest that it or proteins it interacts with are putative molecular targets to control metastasis. These findings are supported by the observation that RKIP expression is decreased in metastases of prostate cancer patients, compared to normal prostate or the primary prostate tumor. In this review, RKIP biology and its role in cancer will be described.     

The roles of TGF-β signaling in carcinogenesis and breast cancer metastasis

Transforming growth factor-β (TGF-β) ligand is a multifunctional growth factor that regulates various cell behavior, such as cell proliferation, differentiation, migration, and apoptosis. Because TGF-β is a potent growth inhibitor, abnormalities in TGF-β signaling result in carcinogenesis. In addition to tumor suppressor function, TGF-β acts as an oncogenic factor. In particular, TGF-β signaling plays an important role during metastasis of breast cancer. Recently, epithelial-mesenchymal transition (EMT) has been shown to confer malignant properties such as cell motility and invasiveness to cancer cells and plays crucial roles during cancer metastasis. Moreover, breast stem-like cells exhibit EMT properties. Because TGF-β is a potent regulator of EMT as well as cell stemness, TGF-β signaling might play a crucial role in the regulation of breast cancer stem cells.     

Crosstalk between EGFR and TrkB enhances ovarian cancer cell migration and proliferation

Ovarian cancer remains the leading cause of fatality among all gynecologic cancers, although promising therapies are in the making. It has been speculated that metastasis is critical for ovarian cancer, and yet the molecular mechanisms of metastasis in ovarian cancer are poorly understood. Growth factors have been proven to play important roles in cell migration associated with metastasis, and inhibition of growth factor receptors and their distinct cell signaling pathways has been intensively studied, and yet the uncovered interaction or crosstalk among various growth factor receptors complicates this otherwise promising approach. We investigated the crosstalk between EGFR and TrkB, both of which have been known to be important in cell survival and migration in response to EGF and BDNF. Our results showed that both EGF and BDNF induced cell migration and cell proliferation in cultured human ovarian cancer cells (Caov3 cell line). EGF and BDNF transactivated TrkB and EGFR respectively, and activated downstream cell survival components such as Akt. EGFR and TrkB kinase inhibitors inhibited EGF- and BDNF-induced TrkB and EGFR activation and Akt phosphorylation, and cell proliferation and migration. Using EGFR knockout cells, we further demonstrated that EGFR is required for EGF-induced cell migration. Collectively, our data indicate that EGFR and TrkB crosstalk each other in response to EGF and BDNF, leading to cell survival pathway activation in ovarian cancer cells. Our data suggest that a combination of inhibitors of both receptors with cell survival pathway inhibitors would provide a better outcome in the clinical treatment of ovarian cancer.     

Matrix metalloproteinase-1 promotes prostate tumor growth and metastasis

Cell migration and invasion are critical events during the progression to metastasis. Matrix metalloproteinase-1 (MMP-1) is involved in the progression of human malignancies, but the precise role of MMP-1 in tumor invasion and metastasis remains unclear. In the present study, we investigated the role of MMP-1 in tumor cell invasion and metastasis by overexpressing MMP-1 in prostate cancer cells. Overexpression of MMP-1 in prostate cancer cells increases cell invasion and migration as measured by modified transwell assays. Furthermore, the results from a bioluminescence tumor/metastasis model showed that the overexpression of MMP-1 significantly induces prostate tumor growth and the incidence of lung metastasis. We observed that this increase in tumor growth correlates with an increase in tumor angiogenesis. In addition, we assessed the importance of MMP-1 expression in cell invasion and migration by inhibiting MMP-1 activity with specific inhibitor and antibodies. Blockade of MMP-1 activity inhibited prostate cancer cell migration and invasion in vitro. Treatment of mice with an MMP-1 specific inhibitor significantly decreased prostate tumor growth and incidence of lung metastasis in vivo. Collectively, our findings suggest that MMP-1 plays an important role in prostate cancer progression during the invasive and metastatic stages of the disease.     

Interleukin-8 confers androgen-independent growth and migration of LNCaP: differential effects of tyrosine kinases Src and FAK

Interleukin-8 (IL-8), a chemokine implicated in the metastasis and angiogenesis of a variety of cancers, has been reported to be overexpressed in prostate cancer. In this study, we ascribe a new role for IL-8 in prostate cancer progression using LNCaP cells. We demonstrate that IL-8 activates the androgen receptor and confers androgen-independent growth, while serving as a potent chemotactic factor. Our evaluation of the possible signal pathways involved in androgen-independence and cell migration shows that the tyrosine kinases Src and FAK (focal adhesion kinase) are involved in IL-8-induced signaling. Pharmacological and genetic inhibitors of Src and FAK interfere with IL-8-induced cell migration, while only the Src inhibitor was able to repress androgen-independent growth. This suggests that both growth and migration depend on the activity of Src, whereas cell migration also requires the activation of FAK. Our evidence that IL-8-induced androgen-independent growth is, at least in part, due to androgen receptor activation includes (1) an inhibitor of androgen receptor activity diminishes cell growth; (2) androgen receptor transactivation potential is augmented by IL-8 and (3) androgen receptor is recruited to the promoter of prostate specific antigen (PSA) upon IL-8 treatment, based on chromatin immunoprecipitation experiments. Taken together, our data suggest that in addition to its role in metastasis and angiogenesis, IL-8 may also serve as a facilitator for androgen-independent transition of prostate cancers. To our knowledge, this is the first report about the tyrosine kinase signals and androgen receptor activation induced by IL-8 in prostate cancer cells. The observation that IL-8 mediates its growth and chemotactic effects via Src and FAK suggests the potential use for tyrosine kinase inhibitors at early stage of prostate cancer development.