HMG-CoA reductase inhibitors (statins) as anticancer drugs (review)

Apart from their lipid lowering activity, HMG-CoA reductase inhibitors (statins) impair numerous cellular functions associated with metastasis, e.g. gene expression, angiogenesis, cell adhesion, cell motility and invasiveness. Furthermore, statins have impact on apoptotic cell death and modulate cellular susceptibility to cell killing by anticancer drugs and ionizing radiation. Part of the effects provoked by statins are due to the inhibition of the prenylation of low molecular weight GTPases, in particular Ras and Rho, which play key roles in signaling evoked by stimulation of cell surface receptors. C-terminal lipid modification of Ras/Rho GTPases is essential for their correct intracellular localization and function. By depletion of the cellular pool of isoprene precursor molecules, statins reduce the level of membrane-bound active Ras/Rho proteins, thereby impairing corresponding functions. Since broad clinical experience already exists for statins, their incorporation into established tumor-therapeutic regimens would be realizable in a rather short period of time. Here, data available at present arguing for the usefulness of statins in anticancer therapy are summarized and discussed.     

Ras superfamily monomeric G proteins in carcinoma cell motility

With over 100 members in humans, the Ras superfamily is a diverse group of monomeric G proteins participating in many cellular processes. Members of the Ras, Rho, and Arf families have been shown to regulate cell motility in fibroblasts and epithelial cells. Ras and Rho family members are also widely involved in human tumorigenesis, either through activating mutations or by overexpression. In this review, tools for studying carcinoma cell migration are discussed and evidence for regulation of carcinoma cell motility by specific Ras superfamily members is summarized. Novel emerging mechanisms of migration in carcinoma cells involving RhoC and Ral are also discussed.     

Integrin-mediated function of Rab GTPases in cancer progression

The RAS (rat sarcoma) superfamily of small GTPases is broadly subdivided into five groups: Ras, Rho, Rab, Ran, and Arf. Rab family proteins are important in regulating signal transduction and cellular processes such as differentiation, proliferation, vesicle transport, nuclear assembly, and cytoskeleton formation. However, some Rab proteins have been reported to be necessary for the adhesion and migration of cancer cells. Although Ras and Rho family members have been strongly implicated in cancer progression, knowledge of Rabs action in this regard is limited. Some reports have also linked Rab GTPases with cancer cell migration and invasiveness. This review discusses the implications of the involvement of Rabs in malignant transformation and cancer therapy through integrin-mediated signaling events, with particular emphasis on breast cancer.     

Identification of H-Ras,RhoA, Rac1 and Cdc42 responsive genes

The superfamily of small GTP-binding proteins has expanded dramatically in recent years. The Ras family has long been associated with signaling pathways contributing to normal and aberrant cell growth, while Rho-related protein function is to integrate extracellular signals with specific targets regulating cell morphology, cell aggregation, tissue polarity, cell motility and cytokinesis. Recent findings suggest that certain Rho proteins, including RhoA, Rac1 and Cdc42, can also play a role in signal transduction to the nucleus and cell growth control. However, the nature of the genes regulated by Ras and Rho GTPases, as well as their contribution to their numerous biological effects is still largely unknown. To approach these questions, we investigated the global gene expression pattern induced by activated forms of H-Ras, RhoA, Rac1 and Cdc42 using cDNA microarrays comprising 19 117 unique elements. Using this approach, we identified 1184 genes that were up- or downregulated by at least twofold. Hierarchical cluster analysis revealed the existence of patterns of gene regulation both unique and common to H-Ras V12, RhoA QL, Rac1 QL and Cdc42 QL activation. For example, H-Ras V12 upregulated osteopontin and Akt 1, and H-Ras and RhoA stimulated cyclin G1, cyclin-dependent kinase 8, cyclin A2 and HMGI-C, while Rac1 QL and Cdc42 QL upregulated extracellular matrix and cell adhesion proteins such as alpha-actinin 4, procollagen type I and V and neuropilin. Furthermore, H-Ras V12 downregulated by >eightfold 52 genes compared to only three genes by RhoA QL, Rac1 QL and Cdc42 QL. These results provide key information to begin unraveling the complexity of the molecular mechanisms underlying the transforming potential of Ras and Rho proteins, as well as the numerous morphological and cell cycle effects induced by these small GTPases.64     

Rho, Rac, Pak and angiogenesis: old roles and newly identified responsibilities in endothelial cells

Angiogenesis-the develoment of microvasculature-requires, in part, directed endothelial cell motility and responsiveness to external signals. Several of the proteins, which modulate and/or direct endothelial cell motility and morphology in angiogenesis are the Rho GTPases (Rho, Rac, and Cdc42) and Pak (a downstream effector of Rac and Cdc42). Previously, overexpression and activation of Rho GTPases and Pak had been implicated in the development of cancer, through their roles in cancer cell transformation, stimulation of proliferation, inhibition of apoptosis, and migration. Yet regardless of the transformed status of cells within a tumor, without a blood supply most tumors cannot grow larger than 1-2 mm. The blood supply in tumors is provided by capillaries formed of endothelial cells in a process called angiogenesis. Consequently, there is enormous interest in the role of the wild type endothelial cells-and the signaling mechanisms required to support angiogenesis and subsequent growth of metastatic and aggressive cancers. Recent work has begun to uncover the roles of the Rho GTPases and Pak in the regulation of normal endothelial cell function. This review will discuss the current literature regarding the roles of Rho and Rac, and the Rac effector-Pak, in endothelial cells, and we will propose new avenues of research for interaction of the AGC kinase-PKG, with the Rho GTPases and Pak in the cell motility and cell morphology of endothelial cells.     

Prenyl-binding domains: potential targets for Ras inhibitors and anti-cancer drugs

Ras and Rho GTPases are prominent participants in malignant transformation. They possess an essential prenyl group (farnesyl or geranylgeranyl) that endows them with membrane-tethering ability and functional specificity. Accumulating evidence suggests that prenyl groups are involved primarily in lipid-protein interactions, and recent experiments point to prenyl-binding hydrophobic pockets in proteins regulating Ras and Rho in normal cells and cancer cells. This review presents the evidence for such prenyl-binding domains as significant players in the control of Ras-like GTPases, and the emerging concept of prenyl-binding domains as potential targets for Ras inhibitors and anti-cancer drugs.     

Inverted signaling hierarchy between RAS and RAC in T-lymphocytes

In order to generate coherent biological responses to extracellular stimuli, cells have established synergistic and antagonistic crosstalk between pathways with similar or opposing functions, respectively. Two routes cooperating in the generation of mitogenic and cytoskeletal functions are those induced by Ras and Rho/Rac GTPases. In these signaling interactions, Rho/Rac proteins have been always placed in a downstream position respect to Ras in all cell systems analysed so far. In this report, we describe that such signaling hierarchy does not apply to T-lymphocytes. Thus, we show that both Rac1 GDP/GTP exchange factors such as Vav and constitutively active versions of Rac1 can promote the effective stimulation of the Ras pathway in T-lymphocytes. The molecular link for this new type of pathway interconnectivity is RasGRP1, a diacylglycerol-dependent GDP/GTP exchange factor for Ras that translocates to the plasma membrane in a Vav- and Rac1-dependent manner. The effect of the Vav/Rac1 pathway on the Ras pathway is highly dependent on the activity of phospholipase C-gamma, the key cellular supplier of intracellular diacylglycerol. Signaling experiments suggest that this crosstalk represents a signaling strategy used by the T-cell receptor to promote robust biological responses of both the Rac/Rho and Ras pathways upon antigen engagement.     

Molecular Basis for Rho GTPase Signaling Specificity

There is now considerable evidence for the involvement of aberrant Rho GTPase activation in breast cancer development. Like Ras, Rho GTPases function as signaling nodes regulated by diverse extracellular stimuli. Rho GTPase activation is facilitated by multiple regulatory proteins, in particular guanine nucleotide exchange factors (GEFs) such as Dbl family proteins. Activated Rho GTPases in turn interact with and regulate a spectrum of functionally diverse downstream effectors, initiating a network of cytoplasmic and nuclear signaling cascades. Thus, Rho GTPases represent points of signaling convergence as well as relay switches that disseminate signaling divergence. In this review, we highlight issues relating to the structural basis by which Dbl family GEFs facilitate signaling convergence and Rho GTPase activation, and how Rho GTPases promote signal dissemination through downstream effectors.     

Rho GTPases in human carcinogenesis: a tale of excess

Rho GTPases are proteins that in response to diverse stimuli control key signaling and structural aspects of the cell. Although early studies had proposed a role for Rho GTPases in cellular transformation, this effect was underestimated by the fact that no genetic mutations affecting Rho-encoding genes was found in human tumors. However, in recent years a high incidence of overexpression of different members of the family of Rho GTPases in human tumors has been detected which is leading to a great interest in the cellular effects elicited by these oncoproteins. As well, the characterization of downstream effectors and upstream regulators of Rho GTPases provides crucial clues on the specific cellular effects that permit aberrant cellular growth and tumorigenesis. A direct link between the functions of some of these signaling elements and regulation of the cell cycle, cytoskeletal rearrangements and cell adhesion has been observed in distinct types of human tumors. Provided this information, a number of drugs that affect Rho signaling at different levels have been described with promisingin vivo antitumoral activity. In this review, the current evidence of dysregulation of Rho signaling in human tumors is assembled.     

Rho蛋白在肿瘤发生发展中的作用

Rho蛋白是参与细胞内信号转导的重要蛋白,Rho家族成员及其各自的已知下游效应分子参与调节细胞的增殖、基因表达,同时是改变细胞骨架组装、调控细胞迁移进而参与肿瘤发生发展的关键因子,具有潜在而重要的临床应用价值。     

Rho signaling, ROCK and mDia1, in transformation, metastasis and invasion

The Rho subgroup of the Rho GTPases consisting of RhoA, RhoB and RhoC induces a specific type of actin cytoskeleton and carry out a variety of functions in the cell. mDia and ROCK are downstream effectors of Rho mediating Rho action on the actin cytoskeleton; mDia produces actin filaments by nucleation and polymerization and ROCK activate myosin to cross-link them for induction of actomyosin bundles and contractility. mDia is potentially linked to Rac activation and membrane ruffle formation through c-Src-induced phosphorylation of focal adhesion proteins, and ROCK antagonizes this mDia action. Thus, cell morphogenesis, adhesion, and motility can be determined by the balance between mDia and ROCK activities. Though they are not oncogenes by themselves, overexpression of RhoA and RhoC are often found in clinical cancers, and RhoC has been repeatedly identified as a gene associated with metastasis. The Rho-ROCK pathway is implicated in Ras-mediated transformation, the amoeboid movement of tumor cells in the three-dimensional matrix, and transmigration of tumor cells through the mesothelial monolayer. On the other hand, the Rho-mDia1 pathway is implicated in Src-mediated remodeling of focal adhesions and migration of tumor cells. There is also an indication that the Rho pathway other than ROCK is involved in Src-mediated induction of podosome and regulation of matrix metalloproteases. Thus, Rho mediates various phenotypes of malignant transformation by Ras and Src through its effectors, ROCK and mDia.     

Regulation of cancer cell motility through actin reorganization

Cell migration is a critical step in tumor invasion and metastasis, and regulation of this process will lead to appropriate therapies for treating cancer. Cancer cells migrate in various ways, according to cell type and degree of differentiation. The different types of cell migration are regulated by different mechanisms. Reorganization of the actin cytoskeleton is the primary mechanism of cell motility and is essential for most types of cell migration. Actin reorganization is regulated by Rho family small GTPases such as Rho, Rac, and Cdc42. These small GTPases transmit extracellular chemotactic signals to downstream effectors. Of these downstream effectors, Wiskott-Aldrich syndrome protein (WASP) family proteins are key regulators of cell migration. Activated WASP family proteins induce the formation of protrusive membrane structures involved in cell migration and degradation of the extracellular matrix. Inhibition of Rho family small GTPase signaling suppresses the migration and invasion of cancer cells. Thus, control of cell migration via the actin cytoskeleton provides the possibility of regulating cancer cell invasion and metastasis.     

Fragile histidine triad-mediated tumor suppression of lung cancer by targeting multiple components of the Ras/Rho GTPase molecular switch

The fragile histidine triad (FHIT) gene has been shown to function as a tumor suppressor gene in vitro and in vivo. However, the mechanism of its action is still largely unknown. To elucidate the molecular mechanism and biological pathway in FHIT-mediated tumor suppression, we used a complementary gene and protein expression profiling with DNA microarray and ProteinChip technologies to quantitatively monitor cellular changes in gene and protein expression and discover the molecular targets of FHIT in non-small cell lung carcinoma (NSCLC) cells. The Ras/Rho signaling pathway was identified as one of the unique biological pathways associated with FHIT activity. A significantly down-regulated expression of genes and proteins of multiple key components in the Ras/Rho GTPases molecular switch, including Ran, Rab, Rac, Rap, and Ral, was observed on gene and protein expression profiles and further validated by Western blot analysis. Ectopic activation of FHIT in FHIT-deficient H1299 cells also significantly reduced the invasive potential of tumor cells by down-regulating expression of RhoC, a potential marker of tumor cell invasion and metastases. A simultaneous knockdown of the expression of several key Ras/Rho signaling molecules using gene-specific small interfering RNAs (RHO-siRNA) targeting selected Rab11, Rac1, and Rap1 genes significantly inhibited tumor cell growth and induced apoptosis in NSCLC cells in vitro, and a local injection of RHO-siRNAs complexed with N-[1-(2,3-dioleoyloxyl)propyl]-N,N,N-trimethylammoniummethyl sulfate:cholesterol nanoparticles inhibited tumor growth in A549 tumor xenografts in mice, mimicking the AdFHIT-mediated tumor-suppressing effect. These results suggest a new role of FHIT in down-regulating the Ras/Rho GTPase-associated oncogenic signaling pathway.