A role for collagen XXIII in cancer cell adhesion, anchorage-independence and metastasis.

Collagen XXIII is a transmembrane collagen previously shown to be upregulated in metastatic prostate cancer that has been used as a tissue and fluid biomarker for non-small cell lung cancer and prostate cancer. To determine whether collagen XXIII facilitates cancer cell metastasis in vivo and to establish a function for collagen XXIII in cancer progression, collagen XXIII knockdown cells were examined for alterations in in vivo metastasis as well as in vitro cell adhesion. In experimental and spontaneous xenograft models of metastasis, H460 cells expressing collagen XXIII shRNA formed fewer lung metastases than control cells. Loss of collagen XXIII in H460 cells also impaired cell adhesion, anchorage-independent growth and cell seeding to the lung, but did not affect cell proliferation. Corroborating a role for collagen XXIII in cell adhesion, overexpression of collagen XXIII in H1299 cells, which do not express endogenous collagen XXIII, enhanced cell adhesion. Consequent reduction in OB-cadherin, alpha-catenin, gamma-catenin, beta-catenin, vimentin and galectin-3 protein expression was also observed in response to loss of collagen XXIII. This study suggests a potential role for collagen XXIII in mediating metastasis by facilitating cell-cell and cell-matrix adhesion as well as anchorage-independent cell growth.

miR-135a contributes to paclitaxel resistance in tumor cells both in vitro and in vivo.

Cancer cell resistance to paclitaxel continues to be a major clinical problem. In this study, we utilized microRNA (miRNA) arrays to screen for differentially expressed miRNAs in paclitaxel-resistant cell lines established in vitro. We observed concordant upregulation of miR-135a in paclitaxel-resistant cell lines representing three human malignancies. Subsequently, the role of miRNA-135a was evaluated in an in vivo model of paclitaxel resistance. In this model, mice were inoculated subcutaneously with a non-small cell lung carcinoma cell line and treated with paclitaxel for a prolonged period. In paclitaxel-resistant cell lines, established either in vitro or in vivo, blockage of miR-135a sensitized resistant cell lines to paclitaxel-induced cell death. We further demonstrated a correlation between paclitaxel response and miR-135a expression in paclitaxel-resistant subclones that were established in vivo. The paclitaxel-resistant phenotype of these subclones was maintained upon retransplantation in new mice, as shown by decreased tumor response upon paclitaxel treatment compared with controls. Upregulation of miR-135a was associated with reduced expression of the adenomatous polyposis coli gene (APC). APC knockdown increased paclitaxel resistance in parental cell lines. Our results indicate that paclitaxel resistance is associated with upregulation of miR-135a, both in vitro and in vivo, and is in part determined by miR-135a-mediated downregulation of APC.

Antizyme, a mediator of ubiquitin-independent proteasomal degradation and its inhibitor localize to centrosomes and modulate centriole amplification.

The potential tumor suppressor antizyme and its endogenous inhibitor (antizyme inhibitor, AZI) have been implicated in the ubiquitin-independent proteasomal degradation of proteins involved in cell proliferation as well as in the regulation of polyamine levels. We show here that both antizyme and AZI concentrate at centrosomes and that antizyme preferentially associates with the maternal centriole. Interestingly, alterations in the levels of these proteins have opposing effects on centrosomes. Depletion of antizyme in various cell lines and primary cells leads to centrosome overduplication, whereas overexpression of antizyme reduces numerical centrosome abnormalities. Conversely, silencing of the antizyme inhibitor, AZI, results in a decrease of numerical centrosome abnormalities, whereas overexpression of AZI leads to centrosome overduplication. We further show that the numerical centrosome abnormalities are due to daughter centriole amplification. In summary, our results demonstrate that alterations in the antizyme/AZI balance cause numerical centrosomal defects and suggest a role for ubiquitin-independent proteasomal degradation in centrosome duplication.

Motility and invasion are differentially modulated by Rho family GTPases.

Cell migration in vivo often requires invasion through tissue matrices. Currently little is known regarding the signaling pathways that regulate cell invasion through three-dimensional matrices. The small GTPases Cdc42, Rac and Rho are key regulators of actin cytoskeletal and adhesive structures. We now show that expression of dominant negative forms of either Cdc42, Rac or Rho inhibited PDGF-BB-stimulated Rat1 fibroblast invasion into 3D collagen matrices, indicating that the activity of each of these GTPases is necessary for cell invasion. In contrast, only Rac activation was required for PDGF-BB-stimulated locomotion across a planar substrate in the Boyden chamber. Interestingly, PDGF-induced invasion was also strongly inhibited by expression of constitutively active forms of Cdc42 or Rho, and to a lesser extent by constitutively active Rac. We also show that constitutively active V12-Rac significantly stimulated basal Rat1 fibroblast invasion, independent of PI-3-kinase activity, and that this effect was suppressed by the effector mutant V12/H40-Rac. These results suggest that cellular invasion may require an optimal level of activation of Cdc42, Rho and Rac, and that migration and invasion are differentially modulated by Rho family GTPases.

C-terminal variations in beta-thymosin family members specify functional differences in actin-binding properties.

Mammalian cells express several isoforms of beta-thymosin, a major actin monomer sequestering factor, including thymosins beta4, beta10, and beta15. Differences in actin-binding properties of different beta-thymosin family members have not been investigated. We find that thymosin beta15 binds actin with a 2.4-fold higher affinity than does thymosin beta4. Mutational analysis was performed to determine the amino acid differences in thymosin beta15 that specify its increased actin-affinity. Previous work with thymosin beta4 identified an alpha-helical domain, as well as a conserved central motif, as crucial for actin binding. Mutational analysis confirms that these domains are also vital for actin binding in thymosin beta15, but that differences in these domains are not responsible for the variation in actin-binding properties between thymosins beta4 and beta15. Truncation of the unique C-terminal residues in thymosin beta15 inhibits actin binding, suggesting that this domain also has an important role in mediating actin-binding affinity. Replacement of the 10 C-terminal amino acids of thymosin beta15 with those of thymosin beta4 did, however, reduce the actin-binding affinity of the hybrid relative to thymosin beta15. Similarly, replacement of the thymosin beta4 C-terminal amino acids with those of thymosin beta15 led to increased actin binding. We conclude that functional differences between closely related beta-thymosin family members are, in part, specified by the C-terminal variability between these isoforms. Such differences may have consequences for situations where beta-thymosins are differentially expressed as in embryonic development and in cancer.

Molecular cloning and structural characterization of the rat thymosin beta15 gene.

Thymosin beta15 is expressed in metastatic variants of Dunning rat prostatic carcinoma but not in a nonmetastatic variant. It is also upregulated in malignant human prostate cancers and was shown to be a predictive marker for patient outcome in prostate cancer. To explore the molecular mechanism of transcriptional regulation of thymosin beta15, we isolated and characterized the rat thymosin beta15 gene. The gene appears to exist as a single copy in the rat genome and to comprise three exons distributed over 2kb. The transcription start site was defined by primer extension analysis at 30 bp upstream of the translation start site. Sequence analysis of the 5'-flanking region of the transcription start site revealed properties consistent with promoter activity. The promoter region is GC rich and contains numerous consensus transcription factor binding sites as well as a GC box, but no TATA box. The transcriptional activity of the 5'-flanking region was analyzed by transient transfection of rat prostate cancer cells with firefly luciferase-encoding gene expression vector constructs. The isolated 5' region showed significant promoter activity. The identification of the thymosin beta15 promoter could aid our understanding of the regulation of this gene and its enhanced expression in human cancer.

Interaction of the Ras-related protein associated with diabetes rad and the putative tumor metastasis suppressor NM23 provides a novel mechanism of GTPase regulation.

Rad is the prototypic member of a new class of Ras-related GTPases. Purification of the GTPase-activating protein (GAP) for Rad revealed nm23, a putative tumor metastasis suppressor and a development gene in Drosophila. Antibodies against nm23 depleted Rad-GAP activity from human skeletal muscle cytosol, and bacterially expressed nm23 reconstituted the activity. The GAP activity of nm23 was specific for Rad, was absent with the S105N putative dominant negative mutant of Rad, and was reduced with mutations of nm23. In the presence of ATP, GDP.Rad was also reconverted to GTP.Rad by the nucleoside diphosphate (NDP) kinase activity of nm23. Simultaneously, Rad regulated nm23 by enhancing its NDP kinase activity and decreasing its autophosphorylation. Melanoma cells transfected with wild-type Rad, but not the S105N-Rad, showed enhanced DNA synthesis in response to serum; this effect was lost with coexpression of nm23. Thus, the interaction of nm23 and Rad provides a potential novel mechanism for bidirectional, bimolecular regulation in which nm23 stimulates both GTP hydrolysis and GTP loading of Rad whereas Rad regulates activity of nm23. This interaction may play important roles in the effects of Rad on glucose metabolism and the effects of nm23 on tumor metastasis and developmental regulation.

Sensitivity to polyamine-induced growth arrest correlates with antizyme induction in prostate carcinoma cells.

High local polyamine concentrations may suppress cell growth of primary prostatic carcinomas. When cell growth assays were conducted in defined serum-free medium, spermine inhibited the growth of poorly metastatic rat prostate carcinoma cells, causing cell cycle arrest in the G1 phase. In contrast, highly metastatic prostate carcinoma cells were resistant to the growth inhibitory activity of spermine. Ornithine decarboxylase antizyme levels, measured by Western blotting, were elevated 1-2 h after spermine treatment of spermine-sensitive cells but not spermine-resistant cells. Spermine uptake was similar in both the sensitive and resistant cell lines. These results suggest that failure to induce antizyme correlates with spermine resistance in prostate carcinoma.

Thymosin beta15 expression in tumor cell lines with varying metastatic potential.

We previously isolated thymosin beta15 from highly metastatic Dunning rat prostatic carcinoma cells. Immunohistochemical study of human prostate cancer specimens revealed a general correlation between Gleason grade and thymosin beta15 expression, with high-grade (more malignant) tumors showing increased staining compared to low-grade tumors. To determine whether thymosin beta15 may be differentially expressed in cancer cells with different metastatic potential other than in the prostatic carcinoma cells, we examined thymosin beta15 mRNA levels in tumor cell lines from different species. We also examined thymosin beta15 protein levels in human breast cancer samples. Thymosin beta15 was upregulated in the highly metastatic mouse lung and human breast cancer cell lines in comparison to the nonmetastatic counterparts. Immunohistochemical staining showed the evidence of upregulation of thymosin beta15 in malignant human breast carcinomas as compared to benign breast tumors. The expression of thymosin beta15 was correlated with the metastatic potential of the mouse lung carcinoma and human breast carcinoma cells in addition to the prostatic carcinomas. Thymosin beta15 may be a useful marker to predict metastatic potential of certain human cancers.

Angiogenesis and tumor metastasis.

Angiogenesis, the recruitment of new blood vessels, is an essential component of the metastatic pathway. These vessels provide the principal route by which tumor cells exit the primary tumor site and enter the circulation. For many tumors, the vascular density can provide a prognostic indicator of metastatic potential, with the highly vascular primary tumors having a higher incidence of metastasis than poorly vascular tumors. Tumor angiogenesis is regulated by the production of angiogenic stimulators including members of the fibroblast growth factor and vascular endothelial growth factor families. In addition, tumors may activate angiogenic inhibitors such as angiostatin and endostatin that can modulate angiogenesis both at the primary site and at downstream sites of metastasis. The potential use of these and other natural and synthetic angiogenic inhibitors as anticancer drugs is currently under intense investigation. Such agents may have reduced toxicity and be less likely to generate drug resistance than conventional cytotoxic drugs. Clinical trials are now underway to develop optimum treatment strategies for antiangiogenic agents.

The role of cell motility in prostate cancer.

Cell motility is a critical determinant of prostate cancer metastasis. The current review discusses the role for cell motility in metastatic dissemination, the evidence that prostate cancer metastasis is dependent on increased cell motility and describes the molecules whose expression has been shown to correlate with the increased motility that accompanies prostate cancer progression. These include receptors for growth factors and cytokines that regulate cell motility as well as intracellular proteins that interact with actin or that regulate signal transduction associated with cell motility. Motility related modulators include both positive regulators of cell movement that are upregulated during tumor progression and suppressors of cell movement that are down-regulated during progression. Because altered expression of such genes may determine the metastatic potential of any particular prostate tumor, we conclude that the appearance or disappearance of motility-related molecules could be used to aid in the diagnosis and prognosis of human prostate cancer.

Localization and quantitation of expression of the cell motility-related protein thymosin beta15 in human breast tissue.

Thymosin beta15 is a newly discovered 5300-Da protein that binds actin monomers and inhibits actin polymerization and might thus increase cellular motility. Thymosin beta15 is upregulated at both the mRNA and protein levels in prostate cell lines in a manner directly related to their capacity to metastasize. We hypothesize that because this protein is upregulated in cells with a propensity to metastasize, it might be a useful prognostic marker in breast cancer. Because this is a newly described protein, neither the subcellular localization of thymosin beta15 or its expression in breast cancer has been examined. We describe the use of an affinity-purified polyclonal antibody to show that within breast epithelium, thymosin beta15 is localized diffusely throughout the cytoplasm and that thymosin beta15 is upregulated in malignant (compared with benign) breast tissue. In contrast to the prostate model, thymosin beta15 is upregulated in nonmetastatic breast cancer and even ductal carcinoma in situ (compared with benign breast tissue), and, consequently, it might represent a potential early marker for breast malignancy. Additional studies are needed to evaluate the precise role and prognostic value of thymosin beta15 in breast cancer.

Isolation of a mouse cDNA encoding mSTI1, a stress-inducible protein containing the TPR motif.

We report the isolation and sequencing of the complete 2079-bp cDNA fragment encoding mSTI1, a murine stress-inducible protein. The predicted ORF encodes a protein of 543 amino acids (aa) and Mr 62,582. The predicted protein has significant homology to stress-inducible proteins from humans (IEF SSP 3521), soybean (GMSTI), yeast (STI1) and a parasite, Leishmania donovani (LSIP). All of these proteins contain 34-aa repeat motifs, termed tetratricopeptide repeats (TPRs), that are proposed to be involved in intra- and intermolecular protein interactions. mSTI1 has ten potential TPR motifs, a putative nuclear localization signal (NLS), six potential phosphorylation sites for casein kinase II and a central proline-rich region. Western analysis detected a protein of approx. 63 kDa in all the major mouse organs and in mouse, monkey and human cell lines.

Stress-inducible, murine protein mSTI1. Characterization of binding domains for heat shock proteins and in vitro phosphorylation by different kinases.

We have recently isolated the cDNA for the murine homologue of the stress-inducible phosphoprotein STI1 (also known as IEF SSP 3521 or p60). STI1 was previously shown to be 2-fold up-regulated in MRC-5 fibroblasts upon viral transformation and to exist in a macromolecular complex with heat shock proteins of the HSP 70 and 90 families. By peptide-sequencing we have identified the two heat shock proteins that bind to murine STI1 (mSTI1) as HSC 70 and HSP 84/86. We describe two separate binding regions within mSTI1 for the two heat shock proteins. In the presence of cell extracts, the N-terminal region of mSTI1 binds preferentially to HSC 70, whereas the C-terminal portion of the molecule promotes the binding of HSP 84/86. Heat treatment caused a strong induction of mSTI1 message without affecting the steady-state level of the protein significantly. In addition, heat treatment led to changes in the isoform-composition of mSTI1. pp70(s6k), pp90(rsk), and mitogen-activated protein kinase-activated protein kinase 2 were tested as possible STI1 kinases in vitro using recombinant mSTI1 as a substrate: only pp90(rsk) was able to phosphorylate recombinant mSTI1. In vitro kinase assays using casein kinase II suggest serine 189 to be a likely phosphorylation site in mSTI1.

Platelet-derived growth factor and fibronectin-stimulated migration are differentially regulated by the Rac and extracellular signal-regulated kinase pathways.

Directed cell migration is essential for a variety of important biological processes ranging from development and angiogenesis to metastasis. Ras plays a pivotal role in the signaling cascade that governs chemotaxis of fibroblasts toward platelet-derived growth factor-BB (PDGF-BB). Ras activates multiple downstream pathways, which include the extracellular signal-regulated kinase (ERK), Rac, and Ral signaling cascades. We therefore investigated the role of the Rac and ERK pathways in cell migration. We showed that migration of fibroblasts toward PDGF-BB is inhibited by expression of dominant negative Asn-17 Rac1. Blocking of the ERK pathway by either expression of dominant negative Ala-218/Ala-222-mitogen-activated protein kinase kinase (A218/A222-MEK1) or by a MEK-specific inhibitor did not inhibit migration toward PDGF-BB. In contrast, migration toward soluble fibronectin was suppressed by inhibition of the ERK pathway but not by Asn-17 Rac1 expression. These results indicate that directed cell migration mediated by different receptor classes in response to different ligands differentially utilizes the Rac and ERK pathways and suggest that Rac might play a critical role in pathological processes such as angiogenesis and metastasis.

Thymosin beta 15: a novel regulator of tumor cell motility upregulated in metastatic prostate cancer.

The Dunning rat prostatic carcinoma is a model system where cell motility closely correlates with the metastatic phenotype. We have identified a novel gene, upregulated in the highly motile and metastatic Dunning cancer cell lines, that represents a new member of the thymosin-beta family, thymosin beta 15. Transfection of antisense thymosin beta 15 constructs into rat prostatic carcinoma cells demonstrates that this molecule positively regulates cell motility, a critical component of the metastatic pathway. Thymosin beta 15 levels are elevated in human prostate cancer and correlate positively with the Gleason tumor grade. Thymosin beta 15 may represent a potential new biochemical marker for human prostate cancer progression.

Differential endothelial migration and proliferation to basic fibroblast growth factor and vascular endothelial growth factor.

Neovascularization is a feature of a variety of pathological processes. We compared the characteristics of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) on migration and proliferation of human umbilical vein endothelium (HUVEC). Both VEGF and bFGF induced endothelial cell migration at similar concentrations (1/2 max. VEGF = approximately 1.0 ng/ml, bFGF = approximately 5.0 ng/ml). However, VEGF-stimulated migration was two-fold greater than bFGF at 1 and 10 ng/ml (p < 0.05). In contrast, bFGF induced proliferation four-fold more effectively than VEGF (1/2 max. 1 ng/ml and 1.4 ng/ml respectively). Checkerboard migration assays for bFGF showed a predominantly chemokinetic pattern, whereas VEGF was predominantly chemotactic. VEGF and bFGF were not synergistic in monolayer proliferation and migration assays. Three angiogenesis inhibitors, alpha-interferon, TNP-470, and platelet factor-4, inhibited VEGF and bFGF induced cell migration. These results indicate that VEGF and bFGF are chemoattractants that stimulate endothelial migration by different mechanisms and that both can be inhibited by known angiogenesis inhibitors.