ErbB2 is a receptor tyrosine kinase whose activity in normal cells depends on dimerization with another ligand-binding ErbB receptor. In contrast, amplification of c-erbB2 in tumors results in dramatic overexpression and constitutive activation of the receptor. Breast cancer cells overexpressing ErbB2 depend on its activity for proliferation, because treatment of these cells with ErbB2-specific antagonistic antibodies or kinase inhibitors blocks tumor cells in the G 1 phase of the cell cycle. Intriguingly, loss of ErbB2 signaling is accompanied by a decrease in the phosphotyrosine content of ErbB3. On the basis of these results, it has been proposed that ErbB3 might be a partner for ErbB2 in promoting cellular transformation. To test this hypothesis and directly examine the role of the ''kinase dead'' ErbB3, we specifically ablated its expression with a designer transcription factor (E3). By infection of ErbB2-overexpressing breast cancer cells with a retrovirus expressing E3, we show that ErbB3 is an essential partner in the transformation process. Loss of functional ErbB2 or ErbB3 has similar effects on cell proliferation and cell cycle regulators. Furthermore, expression of constitutively active protein kinase B rescues the proliferative block induced as a consequence of loss of ErbB2 or ErbB3 signaling. These results demonstrate that ErbB2 overexpression and activity alone are insufficient to promote breast tumor cell division. Furthermore, we identify ErbB3's role, which is to couple active ErbB2 to the phosphatidylinositol 3-kinase͞protein kinase B pathway. Thus, the ErbB2͞ErbB3 dimer functions as an oncogenic unit to drive breast tumor cell proliferation.T he family of ErbB receptor tyrosine kinases includes four members: epidermal growth factor (EGF) receptor͞ErbB1, ErbB2, ErbB3, and ErbB4. Binding of peptides of the EGFrelated growth factor family to the extracellular domain of ErbB receptors results in the formation of homo-and heterodimers. Ligand binding induces the intrinsic receptor kinase activity, ultimately leading to stimulation of intracellular signaling cascades (1, 2). The physiological role of ErbB2, in the context of ErbB ligand signaling, is to serve as a coreceptor (3, 4). In fact, ErbB2 appears to be the preferred partner of the other ligandbound ErbBs (5, 6). The importance of heterodimer-mediated signaling in normal development is obvious from studies in genetically modified mice. This is particularly true for ErbB2͞ ErbB3 and ErbB2͞ErbB4 heterodimers. Loss of ErbB2 or ErbB3 has a similar impact on neuronal development (7), whereas loss of ErbB2 or ErbB4 has major effects on heart development (8, 9).A wealth of clinical data has demonstrated that ErbB receptor tyrosine kinases, in particular ErbB1 and ErbB2, have roles in human cancer development, thus making them attractive targets for cancer therapies (10-13). ErbB2 overexpression, generally attributable to gene amplification, occurs in 25-30% of breast cancer and correlates with shorter time to relapse and lower overall survival (1...

The regulation of plasminogen activation involves genes for two plasminogen activators (tissue type and urokinase type), two specific inhibitors (type 1 and type 2), and a membrane-anchored urokinase-type plasminogen-activator-specific receptor. This system plays an important role in various biological processes involving extracellular proteolysis. Recent studies have revealed that the system, through interplay with integrins and the extracellular matrix protein vitronectin, is also involved in the regulation of cell migration and proliferation in a manner independent of proteolytic activity. The genes are expressed in many different cell types and their expression is under the control of diverse extracellular signals. Gene expression reflects the levels of the corresponding mRNA, which should be the net result of synthesis and degradation. Thus, modulation of mRNA stability is an important factor in overall regulation. This review summarizes current understanding of the biology and regulation of genes involved in plasminogen activation at different levels.

Overexpression of fibroblast growth factor receptor (FGFR) tyrosine kinases has been found in many human breast cancers and has been associated with poor patient prognosis. In order to understand the mechanism by which FGFR mediates breast cancer cell proliferation, we used a low molecular weight compound, PD173074, that selectively inhibits FGFR tyrosine kinase activity and autophosphorylation. This potential anticancer agent caused a G1 growth arrest of MDA-MB-415, MDA-MB-453 and SUM 52 breast cancer cells. Our analyses revealed that FGFR signaling links to the cell cycle machinery via D-type cyclins. PD173074-mediated inhibition of FGFR activity caused downregulation of cyclin D1 and cyclin D2 expression, inhibition of cyclin D/cdk4 activity and, as a consequence, reduction of pRB phosphorylation. Retroviral-mediated ectopic expression of cyclin D1 prevented pRB hypophosphorylation and the cell cycle G1 block in PD173074-treated cells, suggesting a central role for D cyclins in proliferation of FGFR-driven breast cancer cells. The repression of FGFR activity caused downregulation of MAPK in MDA-MB-415 and MDA-MB-453 cells. In SUM 52 cells, both MAPK and PI3K signaling pathways were suppressed. In conclusion, results shown here describe a mechanism by which FGFR promotes proliferation of breast cancer cells.

Alterations in ErbB2 or fibroblast growth factor receptor-4 (FGFR-4) expression and activity occur in a significant fraction of breast cancers. Because signaling molecules and pathways cooperate to drive cancer progression, simultaneous targeting of multiple pathways is an appealing therapeutic strategy. With this in mind, we examined breast tumor cells for their sensitivity to the ErbB2 and FGFR inhibitors, PKI166 and PD173074, respectively. Simultaneous blocking of ErbB2 and FGFR-4 in MDA-MB-453 tumor cells had a stronger antiproliferative effect than treatment with individual inhibitors. Examination of cell cycle regulators revealed a novel translation-mediated mechanism whereby ErbB2 and FGFR-4 cooperate to regulate cyclin D1 levels. Our results showed that FGFR-4 and ErbB2 via the MAPK and the phosphatidylinositol 3-kinase/protein kinase B pathways, respectively, both contribute to the maintenance of constitutive activity of the mammalian target of rapamycin translational pathway. Dual inhibition of these receptors strongly blocked S6 kinase 1 (S6K1) activity and cyclin D1 translation, as attested by a decrease in cyclin D1 mRNA association with polysomes. Ectopic expression of active protein kinase B or active S6K1 abrogated the dual inhibitor-mediated down-regulation of cyclin D1 expression, demonstrating the importance of these FGFR-4/ErbB2 signaling targets in regulating cyclin D1 translation. S6K1 has the central role in this process, since small interfering RNA-targeted S6K1 depletion led to a decrease in cellular S6K1 activity and, as a consequence, repression of cyclin D1 expression. Thus, we propose a novel mechanism for controlling cyclin D1 expression downstream of combined activity of ErbB2 and FGFR-4 that involves S6K1-mediated translation.

Expression of genes of the plasminogen activator (PA) system declines at the G 0 /G 1 -S-phase boundary of the cell cycle. We found that overexpression of E2F1-3, which acts mainly in late G 1 , inhibits promoter activity and endogenous expression of the urokinase-type PA (uPA) and PA inhibitor 1 (PAI-1) genes. This effect is dose dependent and conserved in evolution. Mutation analysis indicated that both the DNA-binding and transactivation domains of E2F1 are necessary for this regulation. Interestingly, an E2F1 mutant lacking the pRBbinding region strongly repressed the uPA and PAI-1 promoters. An E2F-mediated negative effect was also observed in pRB and p107/p130 knockout cell lines. This is the first report that E2F can act as a repressor independently of pocket proteins. Mutation of AP-1 elements in the uPA promoter abrogated E2F-mediated transcriptional inhibition, suggesting the involvement of AP-1 in this regulation. Results shown here identify E2F as an important component of transcriptional control of the PA system and thus provide new insights into mechanisms of cellular proliferation.Extracellular proteolysis, especially that mediated by the plasminogen activator (PA) system, plays an important role in various physiological and pathological processes, such as angiogenesis, wound healing, inflammation, and tumor metastasis (1, 27). PAs, urokinase-type PA (uPA) and tissue-type PA (tPA), are secreted serine proteases that convert the ubiquitous zymogen plasminogen to plasmin. This trypsin-like protease degrades a wide range of substrates, including various extracellular matrix proteins, such as fibronectin, vitronectin, and fibrin. Of the two PAs, uPA is considered to be engaged more in cell-associated proteolysis due to the presence of a cell surface-associated uPA receptor (uPAR). The activities of both uPA and tPA are negatively regulated by the binding of PA inhibitor 1 (PAI-1) and PAI-2, which are members of the serine protease inhibitor superfamily. Interestingly, both uPA and PAI-1 are highly expressed in various metastatic tumors, suggesting that controlled proteolysis is important for metastasis (1).The PA system may also have a significant role in cell cycle progression, where cells undergo detachment from neighboring cells and the extracellular matrix. It has been reported that PAI-1 and uPA mRNAs are rapidly induced soon after exposure of growth-arrested cells to serum-containing medium and that this expression declines prior to DNA synthesis in the G 1 -to-S transition phase (21,55,66). This pattern of expression appears also in the second cell cycle of synchronized cells, suggesting that the regulation is cell cycle dependent. Induction of the transcription of these genes in early-to-mid-G 1 phase is thought to be mediated through AP-1-and c-mycresponsive elements present in their promoters (33, 55). However, the suppression mechanism acting on the transcription of these genes in late G 1 has not been elucidated.One of the key regulators of cell cycle events at the boundary of the G 0 /G 1...

Three di erent novel BRCA1 mutations, ®ve independent cases of the same 12 bp insertion-duplication in intron-20 and two novel rare BRCA1 sequence variants were identi®ed among 122 Polish women with positive, in most cases moderate family history of breast and/or ovarian cancer, 80 controls and 34 unselected breast cancer tissue specimens. All mutations and variants were germline. The 4153 delA frameshift mutation, the Tyr105Cys missense mutation and two cases of the alteration in intron-20 were found in the group of healthy women with positive family history. Two other cases of the intronic insertion were found in unselected controls. Their carriers had no family history of breast or ovarian cancer but other cancers occurred in their families. The 1782 Trp/STOP nonsense mutation and one case of the insertion in intron-20 were ®rst found in tissue specimens of breast cancer patient and breast/ovarian cancer patient, respectively. Their carriers also had no family history of breast or ovarian cancer. The distribution of the insertion in intron-20 in analysed groups and results of RT ± PCR experiments suggest a less prominent role for this variant considered earlier a splicing mutation. This study shows also, that more population-oriented research is needed, involving women with less profound or even no family history of breast and ovarian cancer, to better understand the role and signi®cance of di erent BRCA1 variants and mutations.Keywords: BRCA1; mutations and variants; SSCPheteroduplex analysis; Polish women; positive family history IntroductionApproximately one in ten women in western countries will develop breast cancer during their lifetime . In most cases the disease is sporadic; however, some 5 ± 10% of women with breast cancer inherit increased susceptibility to the disease (Claus et al., 1991). Within this fraction of breast cancer the germline mutations in BRCA1 (Miki et al., 1994) and BRCA2 genes most frequently form a favorable genetic background for the disease. Also at increased risk are the carriers of one abnormal allele of the ataxia telangiectasia gene (Swift et al., 1991), the carriers of rare minisatelite allele at the HRAS1 locus (Krontiris et al., 1993) and the members of Li Fraumeni families with germline mutation in the p53 gene (Srivastava et al., 1990).The BRCA1 gene located on chromosome 17q21 (Hall et al., 1990) is also linked to hereditary ovarian cancer (Narod et al., 1991). More than 400 BRCA1 sequence variants have been identi®ed thus far and deposited in Breast Cancer Information Core Database (Friend et al., 1995) which now contains about 150 di erent cancer predisposing mutations. Most of this knowledge comes from studies of breast and breastovarian cancer families. Therefore the known mutations are probably biased towards those with more severe e ects on phenotype. It is expected that less expressive mutations will be identi®ed when studies expand and include women with less profound family history and women from the general population. It is also very likely that the inciden...

Gene expression of the plasminogen activation system is cell-cycle dependent. Previously, we showed that ectopic expression of E2F1 repressed the plasminogen activator inhibitor type 1 (PAI-1) promoter in a manner dependent on the presence of DNA-binding and transactivation domains of E2F1 but independent of binding to pocket-binding proteins, suggesting a novel mechanism for E2F-mediated negative gene regulation [Koziczak, M., Krek, W. & Nagamine, Y. (2000) Mol. Cell. Biol. 20, 2014-2022. However, it remains to be seen whether endogenous E2F can exert a similar effect. We report here that down-regulation of PAI-1 gene expression correlates with an increase in endogenous E2F activity. When cells were treated with a cdk2/4-specific inhibitor, which maintains E2F in an inactive state, the decline of serum-induced PAI-1 mRNA levels was suppressed. In mutant U2OS cells expressing a temperature-sensitive retinoblastoma protein (pRB), a shift to a permissive temperature induced PAI-1 mRNA expression. In U2OS cells stably expressing an E2F1-estrogen receptor chimeric protein that could be activated by tamoxifen, PAI-1 gene transcription was markedly reduced by tamoxifen even in the presence of cycloheximide. These results all indicate that endogenous E2F can directly repress the PAI-1 gene. DNase I hypersensitive-site analysis of the PAI-1 promoter suggested the involvement of conformation changes in chromatin structure of the PAI-1 promoter. 5 0 deletion analysis of the PAI-1 promoter showed that multiple sites were responsible for the E2F negative regulation, some of which were promoter dependent. Interestingly, one of these sites is a p53-binding element.Keywords: cell cycle; E2F; gene expression; p53; PAI-1.Plasminogen activator inhibitor type 1 (PAI-1) is one of the components of the plasminogen activation (PA) system. This system plays an important role in various physiological and pathological processes where degradation of the extracellular matrix (ECM) is required, such as angiogenesis, wound healing, inflammation and tumour metastasis (reviewed in [1,2]). The PA system also includes the urokinase-type plasminogen activator (uPA), tissue-type plasminogen activator (tPA), and the uPA receptor (uPAR). The activators convert proteolytically inactive plasminogen into active plasmin, that, in turn, can degrade ECM components. uPA-catalysed plasminogen activation is augmented by binding of uPA to a surface-associated uPAR [3]. PAI-1 can inhibit the proteolytic activity of PA and thus prevent ECM degradation [4]. Binding of PAI-1 to receptor-bound uPA results in targeting of this complex to endocytosis receptors and rapid clearance from the cell surface [5]. The PA system can also influence cellular activities without involving proteolytic activity, e.g. via uPAR-mediated cell attachment to and movement of the ECM protein vitronectin. Here, uPA and PAI-1 play positive and negative roles, respectively, in these processes [6,7]. PAI-1 also disrupts uPA/uPAR complex binding to integrins and through this action blocks uP...

The alkylating agent N-methyl-N-nitro-N-nitrosoguanidine (MNNG) is a widely spread environmental carcinogen that causes DNA lesions leading to cell killing. MNNG can also induce a cell-protective response by inducing the expression of DNA repair/transcription-related genes. We recently demonstrated that urokinase-type plasminogen activator, an extracellular protease to which no DNA repair functions have been assigned, was induced by MNNG. Here, we show that the physiological inhibitor of urokinase-type plasminogen activator, PAI-1, is also induced by MNNG in a p53-dependent fashion, because MNNG induced PAI-1 in p53-expressing cells but not in p53؊/؊ cells. MNNG induced p53 phosphorylation at serine 15, resulting in stabilization of the p53 protein, and this phosphorylation event was central for p53-dependent PAI-1 transcription. Finally, we showed that PAI-1 transcriptional induction by MNNG required a p53-responsive element located at ؊136 base pairs in the PAI-1 promoter, because specific mutation of this site abrogated the induction. Because PAI-1 is a prognostic factor in many metastatic cancers, being involved in the control of tumor invasiveness, our finding that a genotoxic agent induces the PAI-1 gene via p53 adds a new feature to the role of the tumor-suppressor p53 protein.Our results also suggest the possibility that genotoxic agents contribute to tumor metastasis by inducing PAI-1 without involving genetic modification.