Cyclin D1 is required for transformation by activated Neu and is induced through an E2F-dependent signaling pathway

NC381, a novel anticancer agent, arrests the cell cycle in G0-G1 and inhibits lung tumor cell growth in vitro and in vivo

Although clotrimazole (CLT), an antifungal drug, inhibits tumor cell proliferation and angiogenesis, its clinical application is hampered by significant hepatotoxicity due to the presence of an imidazole moiety. In our attempts to develop CLT analogs that are devoid of imidazole and are as efficacious as CLT, one pharmacophore designated NC381 was generated and shown to inhibit tumor cell growth via a mechanism similar to that of CLT. In vitro, treatment of NCI-H460 nonsmall cell lung cancer (NSCLC) cells with NC381 inhibited growth in a time-dependent manner. Flow cytometric analysis demonstrated that the decrease in cell growth was associated with inhibition of cell cycle progression at the G(1)-S phase transition, resulting in G(0)-G(1) arrest. There was a concomitant inhibition of cyclin D1 expression and subsequent reduction in the formation of the cyclin D1-CDK4 complex. Consistent with a decrease in the cyclin D1-CDK4 complex, NC381 treatment resulted in significant inhibition of pRb phosphorylation. There also were changes in the activity of cell cycle-related proteins, including p16(Ink4) and p27(Kip1). Together, these results are consistent with a model in which NC381 arrests cell cycle progression via inhibition of the pathway that promotes exit from the G(1) phase of the cell cycle. Furthermore, the clinical applicability of NC381 was evaluated in an in vivo murine xenograft model of human NSCLC (NCI-H460). NC381 treatment resulted in significant inhibition of tumor growth. Given the poor prognosis and the limited treatment options available, the present results underscore the potential of NC381 in the treatment of human NSCLC.

Autocrine-mediated ErbB-2 kinase activation of STAT3 is required for growth factor independence of pancreatic cancer cell lines

Pancreatic ductal adenocarcinoma (PDAC) cell lines, MIA PaCa-2, and UK Pan-1, were used to investigate the role of ErbB2 in PDAC oncogenesis. Both these cell lines exhibit exogenous growth factor-independent proliferation that was attributed to the production of autocrine growth factors and/or overexpression of growth factor receptors. The exogenous growth factor-independent phenotype displayed by these PDAC cell lines was dependent on ErbB2 kinase activity since treatment of cells with tyrphostin AG879 prevented serum-free media (SFM) induction of cell proliferation. We determined that ErbB2 kinase contributed to aberrant cell cycle regulation in PDAC through the induction of cyclin D1 levels and the suppression of p21(Cip1) and p27(Kip1). Inhibition of ErbB2 kinase led to cell cycle arrest marked by an increased association of p27(Kip1) with cdk2 and reduced levels of phosphorylated pRb. We further observed constitutive STAT3 activation in the PDAC cell lines and an increase in STAT3 activation upon stimulating quiescent cells with SFM. Inhibitors of ErbB2 kinase blocked STAT3 activation, whereas inhibition of EGFR kinase led to a slight reduction of STAT3 activation. STAT3 was coimmunoprecipitated with ErbB2. SFM stimulation caused an increase in the association of ErbB2 and STAT3, which was blocked by inhibition of ErbB2 kinase. Expression of a STAT3 dominant negative prevented SFM-stimulated cell proliferation of MIA PaCa-2 cells, suggesting that activation of STAT3 by ErbB2 is required for a growth factor-independent phenotype of these cells. Consistent with this observation in PDAC cell lines, we found that most PDAC tumor specimens (10 of 11) showed constitutive activation of STAT3 and that ErbB2 was readily detected in most of these tumors (nine of 11). We believe that these findings indicate a novel mechanism of oncogenesis in PDAC and may suggest future therapeutic strategies in the treatment of PDAC.

E2F decoy oligodeoxynucleotides effectively inhibit growth of human tumor cells

Abnormal cell proliferation, largely dependent upon deregulation of cell-cycle regulatory proteins, is an important feature of several forms of human cancer. The transcription factor, E2F, plays a critical role in the trans-activation of several genes involved in cell-cycle regulation, thereby regulating cell growth. We have demonstrated that E2F decoy oligodeoxynucleotides (ODNs) with a circular dumbbell structure (CD-E2F decoy) corresponding to E2F binding sites effectively inhibit cell proliferation of primary cultured cells. Here we found that the E2F decoy ODNs inhibited serum-induced promoter activity of E2F-dependent genes in a sequence-specific manner in a RB-positive human osteosarcoma, U2OS, as well as in a RB-negative human cervical carcinoma, C33A. This E2F decoy ODN strongly inhibited gene expression of endogenous E2F1 and PCNA and proliferation of these cancer cells. Our results suggest that this decoy ODN strategy could represent a powerful investigative and potentially therapeutic strategy in the prevention and treatment of cancer.

DACH1 inhibits transforming growth factor-beta signaling through binding Smad4

The vertebrate homologues of Drosophila dachsund, DACH1 and DACH2, have been implicated as important regulatory genes in development. DACH1 plays a role in retinal and pituitary precursor cell proliferation and DACH2 plays a specific role in myogenesis. DACH proteins contain a domain (DS domain) that is conserved with the proto-oncogenes Ski and Sno. Since the Ski/Sno proto-oncogenes repress AP-1 and SMAD signaling, we hypothesized that DACH1 might play a similar cellular function. Herein, DACH1 was found to be expressed in breast cancer cell lines and to inhibit transforming growth factor-beta (TGF-beta)-induced apoptosis. DACH1 repressed TGF-beta induction of AP-1 and Smad signaling in gene reporter assays and repressed endogenous TGF-beta-responsive genes by microarray analyses. DACH1 bound to endogenous NCoR and Smad4 in cultured cells and DACH1 co-localized with NCoR in nuclear dotlike structures. NCoR enhanced DACH1 repression, and the repression of TGF-beta-induced AP-1 or Smad signaling by DACH1 required the DACH1 DS domain. The DS domain of DACH was sufficient for NCoR binding at a Smad4-binding site. Smad4 was required for DACH1 repression of Smad signaling. In Smad4 null HTB-134 cells, DACH1 inhibited the activation of SBE-4 reporter activity induced by Smad2 or Smad3 only in the presence of Smad4. DACH1 participates in the negative regulation of TGF-beta signaling by interacting with NCoR and Smad4.

Molecular evolutionary patterns in breast cancer

Geno-phenotypic patterns of pre-invasive and invasive lobular breast cancers and infiltrating ductal carcinomas of low, intermediate, and high grade are reviewed. One of the main differences between lobular breast cancers and ductal carcinomas is the presence of inactivating E-cadherin gene mutations in lobular breast cancers. In many other respects, lobular breast cancers and low-grade ductal carcinomas exhibit similar geno-phenotypic profiles. The development of p53 dysfunction may be a hallmark of infiltrating ductal cancers of intermediate and high grade. Sequential Her-2/neu and ras abnormalities define a subset of aggressive high-grade tumors, and the development of Rb dysfunction may define a separate subset of aggressive ductal cancers. Based on these observations, a branching molecular evolutionary model for the development and progression of breast cancer is proposed.

Cyclin D1 repression of peroxisome proliferator-activated receptor gamma expression and transactivation

The cyclin D1 gene is overexpressed in human breast cancers and is required for oncogene-induced tumorigenesis. Peroxisome proliferator-activated receptor gamma (PPAR gamma) is a nuclear receptor selectively activated by ligands of the thiazolidinedione class. PPAR gamma induces hepatic steatosis, and liganded PPAR gamma promotes adipocyte differentiation. Herein, cyclin D1 inhibited ligand-induced PPAR gamma function, transactivation, expression, and promoter activity. PPAR gamma transactivation induced by the ligand BRL49653 was inhibited by cyclin D1 through a pRB- and cdk-independent mechanism, requiring a region predicted to form an helix-loop-helix (HLH) structure. The cyclin D1 HLH region was also required for repression of the PPAR gamma ligand-binding domain linked to a heterologous DNA binding domain. Adipocyte differentiation by PPAR gamma-specific ligands (BRL49653, troglitazone) was enhanced in cyclin D1(-/-) fibroblasts and reversed by retroviral expression of cyclin D1. Homozygous deletion of the cyclin D1 gene, enhanced expression by PPAR gamma ligands of PPAR gamma and PPAR gamma-responsive genes, and cyclin D1(-/-) mice exhibit hepatic steatosis. Finally, reduction of cyclin D1 abundance in vivo using ponasterone-inducible cyclin D1 antisense transgenic mice, increased expression of PPAR gamma in vivo. The inhibition of PPAR gamma function by cyclin D1 is a new mechanism of signal transduction cross talk between PPAR gamma ligands and mitogenic signals that induce cyclin D1.

Activation of cyclin D1 expression by the ERK5 cascade

Transcriptional activation of the cyclin D1 gene is a key step in cell proliferation. Accordingly, cyclin D1 overexpression is frequently an early step in neoplastic transformation, particularly in mammary epithelium. Numerous studies have linked elevated cyclin D1 promoter activity to a sustained activation of the ERK1/2 cascade. Here we show that the ERK5 cascade, a distinct mitogen-induced MAPK pathway, can also drive cyclin D1 expression. In CCL39 cells, serum induces a strong, prolonged peak of ERK1/2 and ERK5 phosphorylation, and subsequently elevates cyclin D1 mRNA and protein levels. Overexpression of constitutively active MEK5 and wt ERK5 induces a cyclin D1 reporter gene (D1 -973-luciferase) at least as well as constitutively active MEK1. Activation is blocked by kinase-dead mutants of ERK5 and ERK2, respectively. Mutation of the CRE at -50 in the cyclin D1 promoter decreases activation by the ERK5 but not the ERK1/2 cascade. Importantly, expression of kinase-dead ERK5 diminishes endogenous cyclin D1 protein induction by serum in CCL39 cells and the breast cancer cell lines MCF-7 and HS579. These data identify the cyclin D1 gene as a novel target of the ERK5 cascade, an observation with important implications in cancers involving cyclin D1 deregulation.

Molecular targets as therapeutic strategies in the management of breast cancer

Although the molecular and genetic determinants of most sporadic breast cancers remain unknown, increasing understanding of molecular and genetic events affecting breast carcinogenesis has provided information about the potential roles of specific biomarkers in tumour development and spread. It is now recognised that mutations of some tumour suppressor genes appear to play important early roles in the formation of some breast cancers. In addition, alterations in proto-oncogenes may contribute to the development of some breast cancers. The study of breast tumour biology at the molecular level has led to the development of targeted drug design, which provides a variety of agents targeted at specific molecules for the prevention, diagnosis and treatment of breast cancer. This review will describe the recognised molecular targets in breast cancer.

Interrogating mouse mammary cancer models: insights from gene expression profiling

Numerous mouse models for mammary cancer have been developed and characterized based upon their biological, molecular, and histopathological features. In an effort to dissect the molecular anatomy of such models and compare their gene expression profiles to those of human breast cancer, six models representing various oncogenic pathways have been investigated using cDNA microarray technology. Results of these analyses are presented and discussed in the context of technological challenges presented by analyzing data on such a large scale. Further expression profiling coupled with emerging proteomic technologies will more completely define and distinguish mouse models of mammary cancer from each other and provide a comprehensive basis for comparing such models with the human disease they are intended to represent.

Gene expression phenotypic models that predict the activity of oncogenic pathways

High-density DNA microarrays measure expression of large numbers of genes in one assay. The ability to find underlying structure in complex gene expression data sets and rigorously test association of that structure with biological conditions is essential to developing multi-faceted views of the gene activity that defines cellular phenotype. We sought to connect features of gene expression data with biological hypotheses by integrating 'metagene' patterns from DNA microarray experiments in the characterization and prediction of oncogenic phenotypes. We applied these techniques to the analysis of regulatory pathways controlled by the genes HRAS (Harvey rat sarcoma viral oncogene homolog), MYC (myelocytomatosis viral oncogene homolog) and E2F1, E2F2 and E2F3 (encoding E2F transcription factors 1, 2 and 3, respectively). The phenotypic models accurately predict the activity of these pathways in the context of normal cell proliferation. Moreover, the metagene models trained with gene expression patterns evoked by ectopic production of Myc or Ras proteins in primary tissue culture cells properly predict the activity of in vivo tumor models that result from deregulation of the MYC or HRAS pathways. We conclude that these gene expression phenotypes have the potential to characterize the complex genetic alterations that typify the neoplastic state, whether in vitro or in vivo, in a way that truly reflects the complexity of the regulatory pathways that are affected.

Dual mechanisms for lysophosphatidic acid stimulation of human ovarian carcinoma cells

BACKGROUND

Lysophosphatidic acid (LPA), at concentrations present in ascitic fluid, indirectly stimulates the growth of malignant ovarian tumors by increasing the expression of vascular endothelial growth factor (VEGF) in ovarian cancer cells. We investigated whether LPA could also directly promote ovarian tumor growth by increasing the level of cyclin D1, a key G1-phase checkpoint regulator, which thereby increases cell proliferation.

METHODS

Expression of cyclin D1 and LPA receptors (EDG4 and EDG7) was determined in six ovarian cancer cell lines (including OVCAR-3 cells) and immortalized ovarian surface epithelial cells (IOSE-29). Cyclin D1 promoter activity was measured in LPA-treated OVCAR-3 cells cotransfected with cyclin D1 promoter-driven luciferase constructs and cDNA expression plasmids for IkappaBalphaM (a nuclear factor kappaB [NFkappaB] super-repressor).

RESULTS

Four of six cancer cell lines, including OVCAR-3, overexpressed cyclin D1 protein relative to levels in IOSE-29 cells. LPA treatment increased cyclin D1 protein in a dose- and time-dependent manner in OVCAR-3 cells but not in IOSE-29 cells. LPA stimulated cyclin D1 promoter activity (3.0-fold, 95% confidence interval [CI] = 2.7-fold to 3.3-fold). Mutation of the NFkappaB-binding site in the cyclin D1 promoter to block NFkappaB binding and expression of IkappaBalphaM, which binds NFkappaB and inhibits its binding to the promoter, markedly diminished LPA stimulation of cyclin D1 promoter activity (activity stimulated only 1.4-fold, 95% CI = 1.1-fold to 1.7-fold, and 0.7-fold, 95% CI = 0.6-fold to 0.8-fold, respectively). EDG4 was overexpressed in all cancer cell lines studied relative to that in IOSE-29 cells, but EDG7 was overexpressed in only two lines.

CONCLUSIONS

Dual mechanisms are probably involved in LPA stimulation of ovarian tumor growth in vivo. In addition to the previously characterized indirect mechanism that increases angiogenesis via VEGF, LPA may directly increase the level of cyclin D1 in ovarian cancer cells, increasing their proliferation.

EGFR, HER-2/neu, cyclin D1, p21 and p53 in correlation to cell proliferation and steroid hormone receptor status in ductal carcinoma in situ of the breast

Abnormalities in G1/S transition in cell cultures have been attributed to alterations in ErbB (erythroblastic leukaemia viral [v-erb-b] oncogene homologue, avian) signalling, cyclin D1 overexpression or disturbance of the p21(WAF1) (p21)-mediated cell cycle arrest induced by p53. To investigate the significance of these mechanisms on an early stage of human breast tumour growth, we studied the expression of EGFR (ErbB1), HER-2/neu (ErbB2), cyclin D1, p21 and p53 as well as oestrogen (ER) and progesterone receptor (PgR) in paraffin sections of 45 ductal carcinoma in situ (DCIS) by immunohistochemistry. Cell proliferation was assessed by immunohistochemical quantification of Ki-67. Five cases with cyclin D1 overexpression were analysed by FISH for CCND1 amplification. Increased proliferative activity was observed in 46% of DCIS. It was correlated with the expression of EGFR and HER-2/neu (p < 0.05), but neither with cyclin D1 and p21 overexpression nor with p53 accumulation. ErbB positive status was associated with p21 overexpression (p < 0.05). In addition we found a correlation between the overexpression of p21 and cyclin D1 restricted to ErbB-positive cases (p = 0.013). ErbB-negative tumours with increased proliferative activity were ER and cyclin D1 positive. No CCND1 amplification was detected in the analysed cases. In conclusion, our data support that EGFR and HER-2/neu play an important role in cell cycle control in DCIS. p21 appears to be a potential mediator of ErbB signalling. We propose that cyclin D1 could be indirectly induced by ErbB signalling through p21. Besides, ER-mediated upregulation of cyclin D1 seems to be a possible mechanism of maintaining cell proliferation in DCIS in case of EGFR- and HER-2/neu-negativity.

Roles of mitogen-activated protein kinase and phosphoinositide 3'-kinase in ErbB2/ErbB3 coreceptor-mediated heregulin signaling

ErbB2/HER2 and ErbB3/HER3, two members of the ErbB/HER family, together constitute a heregulin coreceptor complex that elicits a potent mitogenic and transforming signal. Among known intracellular effectors of the ErbB2/ErbB3 heregulin coreceptor are mitogen-activated protein kinase (MAPK) and phosphoinositide (PI) 3-kinase. Activation of the distinct MAPK and PI 3-kinase signaling pathways by the ErbB2/ErbB3 coreceptor in response to heregulin and their relative contributions to the mitogenic and transformation potentials of the activated coreceptor were investigated here. To this end, cDNAs encoding the wild-type ErbB3 protein (ErbB3-WT) and ErbB3 proteins with amino acid substitutions in either the Shc-binding site (ErbB3-Y1325F), the six putative PI 3-kinase-binding sites (ErbB3-6F), or both (ErbB3-7F) were generated and expressed in NIH-3T3 cells to form functional ErbB2/ErbB3 heregulin coreceptors. While the coreceptor incorporating ErbB3-WT activated both the MAPK and the PI 3-kinase signaling pathways, those incorporating ErbB3-Y1325F or ErbB3-6F activated either PI 3-kinase or MAPK, respectively. The ErbB2/ErbB3-7F coreceptor activated neither. Elimination of either signaling pathway lowered basal and eliminated heregulin-dependent expression of cyclin D1, which was in each case accompanied by an attenuated mitogenic response. Selective elimination of the PI 3-kinase pathway severely impaired the ability of heregulin to transform cells expressing the coreceptor, whereas attenuation of the MAPK pathway had a lesser effect. Thus, while both pathways contributed in a roughly additive manner to the mitogenic response elicited by the activated ErbB2/ErbB3 coreceptor, the PI 3-kinase pathway predominated in the induction of cellular transformation.

The prognostic impact of hormone receptors and c-erbB-2 in pregnancy-associated breast cancer and their correlation with BRCA1 and cell cycle modulators

A population-based series of 122 patients with pregnancy-associated breast carcinomas was histologically revised and the relationship between hormone receptors, c-erbB-2, BRCA1, p27, cyclin E, and cyclin D1 was studied. The 5-year overall survival was 41%; 70% had tumor size >20 mm; 72% had metastasized to regional lymph nodes; 95% were histologic grade II or III; 66% and 75% were negative for estrogen and progesterone receptor, respectively; and c-erbB-2 expression was high (44%). BRCA1 expression was reduced in 33% of the cases. The expression of p27, cyclin D1, and cyclin E was low, 11%, 9%, and 16%, respectively. Cyclin D1 was positively associated with the hormone receptors (p< or =0.01). In multivariate analysis, lymph node status, progesterone receptor, and c-erbB-2 were significant prognostic factors. In subdividing the group according to lymph node status, c-erbB-2 and progesterone receptor retained a prognostic significance in the node positive group only. In conclusion, pregnancy-associated breast carcinomas are aggressive tumors, with low expression of hormone receptors, BRCA1, p27, and cyclin E and D1, and high expression of c-erbB-2.

Loss of caveolin-1 gene expression accelerates the development of dysplastic mammary lesions in tumor-prone transgenic mice

Caveolin-1 is the principal structural component of caveolae microdomains, which represent a subcompartment of the plasma membrane. Several independent lines of evidence support the notion that caveolin-1 functions as a suppressor of cell transformation. For example, the human CAV-1 gene maps to a suspected tumor suppressor locus (D7S522/7q31.1) that is frequently deleted in a number of carcinomas, including breast cancers. In addition, up to 16% of human breast cancers harbor a dominant-negative mutation, P132L, in the CAV-1 gene. Despite these genetic associations, the tumor suppressor role of caveolin-1 still remains controversial. To directly assess the in vivo transformation suppressor activity of the caveolin-1 gene, we interbred Cav-1 (-/-) null mice with tumor-prone transgenic mice (MMTV-PyMT) that normally develop multifocal dysplastic lesions throughout the entire mammary tree. Herein, we show that loss of caveolin-1 gene expression dramatically accelerates the development of these multifocal dysplastic mammary lesions. At 3 wk of age, loss of caveolin-1 resulted in an approximately twofold increase in the number of lesions (foci per gland; 3.3 +/- 1.0 vs. 7.0 +/- 1.2) and an approximately five- to sixfold increase in the total area occupied by these lesions. Similar results were obtained at 4 wk of age. However, complete loss of caveolin-1 was required to accelerate the appearance of these dysplastic mammary lesions, because Cav-1 (+/-) heterozygous mice did not show any increases in foci development. We also show that loss of caveolin-1 increases the extent and the histological grade of these mammary lesions and facilitates the development of papillary projections in the mammary ducts. Finally, we demonstrate that cyclin D1 expression levels are dramatically elevated in Cav-1 (-/-) null mammary lesions, consistent with the accelerated appearance and growth of these dysplastic foci. This is the first in vivo demonstration that caveolin-1 can function as a transformation suppressor gene.

Cyclin alterations in giant cell tumor of bone

Cyclins play an important role in regulating the passage of dividing cells through critical checkpoints in the cell cycle. Because alterations of several cyclins, especially cyclin D1, have been implicated in the development of many human neoplasms, we examined 32 cases of giant cell tumor of long bones for cyclin D1 gene amplification and protein overexpression using differential polymerase chain reaction and immunohistochemistry, respectively. In addition, the expression of cyclin D3, cyclin B1, and the proliferation-associated antigen Ki-67 (MIB-1) was assessed immunohistochemically. Low-level cyclin D1 gene amplification was detected in 61% of giant cell tumor cases. All tumors showed cyclin D1, cyclin D3, cyclin B1, and Ki-67 (MIB-1) staining; however, the distribution was very characteristic. Cyclin D1 protein expression was seen predominantly in the nuclei of the giant cells, with occasional mononuclear cells staining. There was no correlation between cyclin D1 gene amplification and protein overexpression. Cyclin D3 staining showed a similar distribution, with 88% of cases showing protein overexpression. Cyclin D1 and/or D3 staining in the giant cells was never associated with staining for either cyclin B1 or Ki-67 (MIB-1), as the expression of the latter two proteins was restricted to the mononuclear cells. Cyclin B1 overexpression was seen in 44% of cases. Ki-67 (MIB-1) staining was present in all cases, and between 10 to 50% of the mononuclear cells were positive. These results suggest that alterations in cyclin D1 and/or D3 might play a role in the pathogenesis of giant cell tumor of bone.

IKKalpha regulates mitogenic signaling through transcriptional induction of cyclin D1 via Tcf

The Wnt/beta-catenin/Tcf and IkappaB/NF-kappaB cascades are independent pathways involved in cell cycle control, cellular differentiation, and inflammation. Constitutive Wnt/beta-catenin signaling occurs in certain cancers from mutation of components of the pathway and from activating growth factor receptors, including RON and MET. The resulting accumulation of cytoplasmic and nuclear beta-catenin interacts with the Tcf/LEF transcription factors to induce target genes. The IkappaB kinase complex (IKK) that phosphorylates IkappaB contains IKKalpha, IKKbeta, and IKKgamma. Here we show that the cyclin D1 gene functions as a point of convergence between the Wnt/beta-catenin and IkappaB pathways in mitogenic signaling. Mitogenic induction of G(1)-S phase progression and cyclin D1 expression was PI3K dependent, and cyclin D1(-/-) cells showed reduced PI3K-dependent S-phase entry. PI3K-dependent induction of cyclin D1 was blocked by inhibitors of PI3K/Akt/IkappaB/IKKalpha or beta-catenin signaling. A single Tcf site in the cyclin D1 promoter was required for induction by PI3K or IKKalpha. In IKKalpha(-/-) cells, mitogen-induced DNA synthesis, and expression of Tcf-responsive genes was reduced. Reintroduction of IKKalpha restored normal mitogen induction of cyclin D1 through a Tcf site. In IKKalpha(-/-) cells, beta-catenin phosphorylation was decreased and purified IKKalpha was sufficient for phosphorylation of beta-catenin through its N-terminus in vitro. Because IKKalpha but not IKKbeta induced cyclin D1 expression through Tcf activity, these studies indicate that the relative levels of IKKalpha and IKKbeta may alter their substrate and signaling specificities to regulate mitogen-induced DNA synthesis through distinct mechanisms.

Inflammatory breast cancer: relationship between growth factor signaling and motility in aggressive cancers

A variety of phenotypic characteristics are required for a cancer cell to successfully complete the metastatic cascade. Acquisition of a motile and invasive phenotype is one requirement for a cell to become metastatically competent. The Rho (Ras homology) GTPases are a subfamily of small GTP-binding proteins, which are related to the Ras oncogene. All aspects of cellular motility and invasion are controlled by the Rho GTPases and are closely linked to signals from the extracellular environment, particularly in response to growth factors. Dysregulation of Rho activation through aberrant growth-factor signaling, loss of function of key Rho-regulatory proteins or overexpression of Rho mRNA could result in increased Rho activity and cellular motility. Therefore, the importance of the Rho GTPases in the progression of aggressive cancers, is becoming more appreciated.

The prolyl isomerase Pin1 in breast development and cancer

The prolyl isomerase Pin1 specifically isomerizes certain phosphorylated Ser/Thr-Pro bonds and thereby regulates various cellular processes. Pin1 is a target of several oncogenic pathways and is overexpressed in human breast cancer. Its overexpression can lead to upregulation of cyclin D1 and transformation of breast epithelial cells in collaboration with the oncogenic pathways. In contrast, inhibition of Pin1 can suppress the transformation of breast epithelial cells. In addition, Pin1 knockout in mice prevents massive proliferation of breast epithelial cells during pregnancy. Pin1 plays a pivotal role in breast development and may be a promising new anticancer target.

Curcumin-induced suppression of cell proliferation correlates with down-regulation of cyclin D1 expression and CDK4-mediated retinoblastoma protein phosphorylation

Cyclin D1 is a proto-oncogene that is overexpressed in many cancers including breast and prostate. It plays a role in cell proliferation through activation of cyclin-dependent kinases. Curcumin, a diferuloylmethane, is a chemopreventive agent known to inhibit the proliferation of several breast and prostate cancer cell lines. It is possible that the effect of curcumin is mediated through the regulation of cyclin D1. In the present report we show that inhibition of the proliferation of various prostate, breast and squamous cell carcinoma cell lines by curcumin correlated with the down-regulation of the expression of cyclin D1 protein. In comparison, the down-regulation by curcumin of cyclin D2 and cyclin D3 was found only in selective cell lines. The suppression of cyclin D1 by curcumin led to inhibition of CDK4-mediated phosphorylation of retinoblastoma protein. We found that curcumin-induced down-regulation of cyclin D1 was inhibited by lactacystin, an inhibitor of 26S proteosome, suggesting that curcumin represses cyclin D1 expression by promoting proteolysis. We found that curcumin also down-regulated mRNA expression, thus suggesting transcriptional regulation. Curcumin also inhibited the activity of the cyclin D1 promoter-dependent reporter gene expression. Overall our results suggest that curcumin down-regulates cyclin D1 expression through activation of both transcriptional and post-transcriptional mechanisms, and this may contribute to the antiproliferative effects of curcumin against various cell types.

ErbB-2-induced mammary tumor growth: the role of cyclin D1 and p27Kip1

The neu (c-erbB-2, HER2) proto-oncogene encodes a receptor tyrosine kinase that is a member of an important growth factor receptor family which includes the epidermal growth factor receptor (EGFR, ErbB1), ErbB3 and ErbB4. The neu is found over-expressed in 20-30% of human breast tumors. The c-erbB-2 is sufficient for the induction of mammary tumorigenesis in transgenic mice and the pathology of these mammary tumors strongly resembles human breast cancer. Murine transgenic models engineered to recapitulate human breast cancer provide an excellent and straightforward approach to dissect the molecular mechanisms governing the onset and progression of this disease. The molecular mechanisms by which ErbB-2 transforms cells involves direct effects on components of the cell-cycle regulatory apparatus. Recent studies have demonstrated a key role for components of the cell-cycle, in particular cyclin D1 and p27Kip1 (p27) in the onset and progression of ErbB-2-induced murine mammary tumorigenesis. Such studies have provided further impetus to therapeutics targeting these cell-cycle proteins.

Regulation of airway smooth muscle cyclin D1 transcription by protein kinase C-delta

The precise mechanism by which protein kinase C-delta (PKCdelta) inhibits cell cycle progression is not known. We investigated the regulation of cyclin D1 transcription by PKCdelta in primary bovine airway smooth muscle cells. Overexpression of the active catalytic subunit of PKCdelta attenuated platelet-derived growth factor (PDGF)-mediated transcription from the cyclin D1 promoter, whereas overexpression of a dominant-negative PKCdelta increased promoter activity. A PKCdelta-specific pseudosubstrate increased cyclin D1 protein abundance. To determine the transcriptional mechanism by which PKCdelta negatively regulates cyclin D1 expression, we transiently transfected cells with cDNAs encoding cyclin D1 promoter 5' deletions and site mutations in the context of a -66 promoter fragment. We found that the -57 to -52 CRE/ATF2 site functions as a basal level and PDGF enhancer, whereas the -39 to -30 nuclear factor-kappaB site functions as a basal level suppressor. Further, PDGF and PKCdelta responsiveness of the cyclin D1 promoter was maintained following 5' deletion to the Ets-containing -22 minimal promoter. Finally, using electrophoretic mobility gel shift and reporter assays, we determined that PKCdelta inhibits CRE/ATF2 binding and transactivation, activates nuclear factor-kappaB binding and transactivation, and attenuates Ets transactivation. These data suggest that PKCdelta attenuates cyclin D1 promoter activity via the regulation of three distinct cis-acting regulatory elements.

PIN1 is an E2F target gene essential for Neu/Ras-induced transformation of mammary epithelial cells

Oncogenes Neu/HER2/ErbB2 and Ras can induce mammary tumorigenesis via upregulation of cyclin D1. One major regulatory mechanism in these oncogenic signaling pathways is phosphorylation of serines or threonines preceding proline (pSer/Thr-Pro). Interestingly, the pSer/Thr-Pro motifs in proteins exist in two completely distinct cis and trans conformations, whose conversion is catalyzed specifically by the essential prolyl isomerase Pin1. By isomerizing pSer/Thr-Pro bonds, Pin1 can regulate the conformation and function of certain phosphorylated proteins. We have previously shown that Pin1 is overexpressed in breast tumors and positively regulates cyclin D1 by transcriptional activation and posttranslational stabilization. Moreover, in Pin1 knockout mice, mammary epithelial cells fail to undergo massive proliferation during pregnancy, as is the case in cyclin D1 null mice. These results indicate that Pin1 is upregulated in breast cancer and may be involved in mammary tumors. However, the mechanism of Pin1 overexpression in cancer and its significance in cell transformation remain largely unknown. Here we demonstrate that PIN1 expression is mediated by the transcription factor E2F and enhanced by c-Neu and Ha-Ras via E2F. Furthermore, overexpression of Pin1 not only confers transforming properties on mammary epithelial cells but also enhances the transformed phenotypes of Neu/Ras-transformed mammary epithelial cells. In contrast, inhibition of Pin1 suppresses Neu- and Ras-induced transformed phenotypes, which can be fully rescued by overexpression of a constitutively active cyclin D1 mutant that is refractory to the Pin1 inhibition. Thus, Pin1 is an E2F target gene that is essential for the Neu/Ras-induced transformation of mammary epithelial cells through activation of cyclin D1.

Oncogenic Ras/Her-2 mediate hyperproliferation of polarized epithelial cells in 3D cultures and rapid tumor growth via the PI3K pathway

Carcinogenesis by oncogenic Ras and Her-2 involves enhanced proliferation of epithelial cells in vivo. However, hyperproliferation induced by these oncogenes, or their downstream pathways in vitro has mainly been studied in cultured, fibroblastic cell lines. Here, we demonstrate that oncogenic Ha-Ras or constitutively active Her-2 cause increased proliferation and cyclin D1 upregulation in fully polarized, mammary epithelial cells (EpH4), if cultivated as organotypic structures in three-dimensional collagen/matrigel matrices. Under standard culture conditions, however, these oncogenes failed to induce hyperproliferation. Using both specific low molecular weight inhibitors and Ras-effector-specific mutants, we dissected signaling pathways downstream of oncogenic Ras (PI3K, Mek1/MAPK) with respect to (i) hyperproliferation in collagen gels and tumorigenesis in mice and (ii) epithelial/mesenchymal transition (EMT). We show that the Ras-activated PI3K pathway is required to induce rapid tumor growth and enhanced proliferation of EpH4 cells in collagen gels, but fails to cause EMT in vitro and in vivo. On the other hand, Ras-dependent activation of the Mek1/MAPK pathway in EpH4 cells (previously shown to cause EMT and metastasis) did not induce hyperproliferation in collagen gels and caused only slow tumor growth. Our data thus indicate that Ras-dependent signaling through the PI3K- and MAPK pathways fulfil distinct, but complementary functions during carcinogenesis.

Distinct roles for phosphoinositide 3-kinase, mitogen-activated protein kinase and p38 MAPK in mediating cell cycle progression of breast cancer cells

Addition of the ErbB-ligand, Heregulinbeta1 (HRG), to breast tumour-derived T47D cells promotes D-cyclin expression, p21(cip1) synthesis, cyclin-dependent kinase (CDK) activation through re-distribution of p27(kip1) and DNA synthesis. In contrast EGF has no effect on T47D cell cycle progression. By comparing these two ligands and the use of specific inhibitors for phosphatidylinositol-3 kinase (PI3K), mitogen-activated protein kinase (MAPK) and p38MAPK, we have identified several molecular mechanisms required for ErbB receptor-mediated proliferation. The PI3K, MAPK and p38MAPK pathways each displayed distinct activation profiles in response to either HRG or EGF, with obvious differences in both the intensity and duration of signal output. Through inhibition of each of these pathways it is apparent that each pathway is necessary, yet insufficient alone, to stimulate proliferation. Each pathway regulates distinct subsets of essential cell cycle regulators and integration of these signal networks is required for the timely expression of these components, which culminates in cell cycle progression. Significantly, the mechanisms controlling ligand-stimulated proliferation through ErbB2 are strikingly similar to the mechanisms through which overexpressed, constitutively activated, ErbB2 orchestrates uncontrolled proliferation in cancer cells. This suggests that downstream effectors of ErbB receptors represent good therapeutic targets for breast cancer.

Acetylation in hormone signaling and the cell cycle

The last decade has seen a substantial change in thinking about the role of acetylation in regulating diverse cellular processes. The correlation between histone acetylation and gene transcription has been known for many years. The cloning and biochemical characterization of the enzymes that regulate this post-translational modification has led to an understanding of the diverse role histone acetyltransferases (HATs) play in cellular function. Histone acetylases modify histones, transcription factors, co-activators, nuclear transport proteins, structural proteins and components of the cell cycle. This review focuses on the role of histone acetylases in coordinating hormone signaling and the cell cycle. Transition through the cell cycle is regulated by a family of protein kinase holoenzymes, the cyclin-dependent kinases (Cdks) and their heterodimeric cyclin partners. Recent studies have identified important cross-talk between the cell cycle regulatory apparatus and proteins regulating histone acetylation. The evidence for a dynamic interplay between components regulating the cell cycle and acetylation of target substrates provides an important new level of complexity in the mechanisms governing hormone signaling.

Initiating oncogenic event determines gene-expression patterns of human breast cancer models

Molecular expression profiling of tumors initiated by transgenic overexpression of c-myc, c-neu, c-ha-ras, polyoma middle T antigen (PyMT) or simian virus 40 T/t antigen (T-ag) targeted to the mouse mammary gland have identified both common and oncogene-specific events associated with tumor formation and progression. The tumors shared great similarities in their gene-expression profiles as compared with the normal mammary gland with an induction of cell-cycle regulators, metabolic regulators, zinc finger proteins, and protein tyrosine phosphatases, along with the suppression of some protein tyrosine kinases. Selection and hierarchical clustering of the most variant genes, however, resulted in separating the mouse models into three groups with distinct oncogene-specific patterns of gene expression. Such an identification of targets specified by particular oncogenes may facilitate development of lesion-specific therapeutics and preclinical testing. Moreover, similarities in gene expression between human breast cancers and the mouse models have been identified, thus providing an important component for the validation of transgenic mammary cancer models.

Androgen receptor acetylation governs trans activation and MEKK1-induced apoptosis without affecting in vitro sumoylation and trans-repression function

The androgen receptor (AR) is a nuclear hormone receptor superfamily member that conveys both trans repression and ligand-dependent trans-activation function. Activation of the AR by dihydrotestosterone (DHT) regulates diverse physiological functions including secondary sexual differentiation in the male and the induction of apoptosis by the JNK kinase, MEKK1. The AR is posttranslationally modified on lysine residues by acetylation and sumoylation. The histone acetylases p300 and P/CAF directly acetylate the AR in vitro at a conserved KLKK motif. To determine the functional properties governed by AR acetylation, point mutations of the KLKK motif that abrogated acetylation were engineered and examined in vitro and in vivo. The AR acetylation site point mutants showed wild-type trans repression of NF-kappa B, AP-1, and Sp1 activity; wild-type sumoylation in vitro; wild-type ligand binding; and ligand-induced conformational changes. However, acetylation-deficient AR mutants were selectively defective in DHT-induced trans activation of androgen-responsive reporter genes and coactivation by SRC1, Ubc9, TIP60, and p300. The AR acetylation site mutant showed 10-fold increased binding of the N-CoR corepressor compared with the AR wild type in the presence of ligand. Furthermore, histone deacetylase 1 (HDAC1) bound the AR both in vivo and in cultured cells and HDAC1 binding to the AR was disengaged in a DHT-dependent manner. MEKK1 induced AR-dependent apoptosis in prostate cancer cells. The AR acetylation mutant was defective in MEKK1-induced apoptosis, suggesting that the conserved AR acetylation site contributes to a pathway governing prostate cancer cellular survival. As AR lysine residue mutations that abrogate acetylation correlate with enhanced binding of the N-CoR repressor in cultured cells, the conserved AR motif may directly or indirectly regulate ligand-dependent corepressor disengagement and, thereby, ligand-dependent trans activation.

ErbB2/Neu-induced, cyclin D1-dependent transformation is accelerated in p27-haploinsufficient mammary epithelial cells but impaired in p27-null cells

ErbB2/Neu destabilizes the cyclin-dependent kinase (Cdk) inhibitor p27 and increases expression of cyclin D1. Therefore, we studied the roles of p27 and cyclin D1 in ErbB2-mediated mammary epithelial cell transformation. Overexpression of ErbB2 or cyclin D1 in p27(+/-) primary murine mammary epithelial cells resulted in increased proliferation, cyclin D1 nuclear localization, and colony formation in soft agar compared to those in p27(+/+) cells. In contrast, ErbB2- or cyclin D1-overexpressing p27(-/-) cells displayed reduced proliferation, anchorage-independent growth, Cdk4 activity, cyclin D1 expression, and cyclin D1 nuclear localization compared to wild-type cells. A cyclin D1 mutation in its nuclear export sequence (T286A) partially rescued nuclear localization of cyclin D1 in p27(-/-) cells but did not increase proliferation or Cdk4 kinase activity. Overexpression of E2F1, however, increased proliferation to the same degree in p27(+/+), p27(+/-), and p27(-/-) cells. Mammary glands from MMTV (mouse mammary tumor virus)-neu/p27(+/-) mice exhibited alveolar hyperplasia, enhanced proliferation, decreased apoptosis, and accelerated tumor formation compared to MMTV-neu/p27(+/+) glands. However, MMTV-neu/p27(-/-) glands showed decreased proliferation, cyclin D1 expression, and Cdk4 activity, as well as markedly prolonged tumor latency, compared to MMTV-neu/p27(+/+) glands. These results suggest that p27(+/-) mammary epithelium may be more susceptible to oncogene-induced tumorigenesis, whereas p27-null glands, due to severely impaired cyclin D1/Cdk4 function, are more resistant to transformation.

Recent advances in inducible expression in transgenic mice

In order to accurately analyze gene function in transgenic mice, as well as to generate credible murine models of human diseases, the ability to regulate temporal- and spatial-specific expression of target genes is absolutely critical. Pioneering work in inducible transgenics, begun in the 1980s and continuing to the present, has led to the development of a variety of different inducible systems dedicated to this goal, the shared basis of which is the accurate conditional expression of a given transgene. Recent advances in inducible transgene expression in mice are discussed.

Convergence of mitogenic and DNA damage signaling in the G1 phase of the cell cycle

Research into the molecular basis of cancer has a central tenet. Cancer arises from genetic alterations that disconnect growth and differentiation signaling pathways from the machinery that regulates cellular proliferation. In multi-cellular eukaryotes, proliferation is regulated by external signals, such as the availability of growth factors and nutrients and by internal signals, such as those sensing cellular integrity. Cellular stress created either by lack of mitogens or damage to cellular components, such as DNA, stimulates responses that enforce temporal or permanent withdrawal from the cell cycle. Although these stress responses stem from different sources and activate distinct pathways, they converge on the same components of the cell cycle machinery in the G1 phase of the cell cycle. This review will highlight and compare aspects of the G1 arrest in response to stress generated either by lack of mitogens or damage to DNA.

Cyclin D-dependent kinases, INK4 inhibitors and cancer

The Cyclin D-Cdk4,6/INK4/Rb/E2F pathway plays a key role in controlling cell growth by integrating multiple mitogenic and antimitogenic stimuli. The components of this pathway are gene families with a high level of structural and functional redundancy and are expressed in an overlapping fashion in most tissues and cell types. Using classical transgenic technology as well as gene-targeting in ES cells, a series of mouse models have been developed to study the in vivo function of individual components of this pathway in both normal homeostasis and tumor development. These models have proven to be useful to define specific as well as redundant roles among members of these cell cycle regulatory gene families. This pathway is deregulated in the vast majority of human tumors by genetic and epigenetic alterations that target at least some of its key members such as Cyclin D1, Cdk4, INK4a and INK4b, pRb etc. As a consequence, some of these molecules are currently being considered as targets for cancer therapy, and several novel molecules, such as Cdk inhibitors, are under development as potential anti-cancer drugs.

Rho regulates p21(CIP1), cyclin D1, and checkpoint control in mammary epithelial cells

The small GTPase Rho is important for cell cycle progression and Ras transformation in fibroblasts. However, it is unclear whether Rho is needed for proliferation in other cell types, and its targets in promoting normal cell cycle progression are unknown. Here, we demonstrate that Rho is required for G1 to S progression in MCF10A mammary epithelial cells, both in response to EGF and in response to oncogenic Ras. We describe two effects of Rho, the repression of p21(CIP1) and the induction of cyclin D1, that may underlie its role in promoting S phase entry. The Rho inhibitor, C3 exotransferase, induced p21(CIP1) both in EGF-stimulated and V12Ras-expressing cells. In addition, C3 blocked EGF-stimulated cyclin D1 promoter activity whereas V14RhoA induced the cyclin D1 promoter and cooperated with V12Ras in cyclin D1 induction. Finally, a high proportion of cells co-expressing V14RhoA and V12Ras displayed lobulated, polyploid nuclei that were actively synthesizing DNA. Our results demonstrate that Rho plays a fundamental role in promoting Ras-dependent S phase entry in mammary epithelial cells, whether in response to normal or oncogenic signaling, and indicate that in cells expressing oncogenic Ras, the activation of Rho diminishes p21(CIP1) expression, increases cyclin D1 promoter activity, and uncouples DNA synthesis from mitosis.

Cyclin A transcriptional suppression is the major mechanism mediating homocysteine-induced endothelial cell growth inhibition

Previously, it was reported that homocysteine (Hcy) specifically inhibits the growth of endothelial cells (ECs), suppresses Ras/mitogen-activated protein (MAP) signaling, and arrests cell growth at the G1/S transition of the cell cycle. The present study investigated the molecular mechanisms underlying this cell-cycle effect. Results showed that clinically relevant concentrations (50 μM) of Hcy significantly inhibited the expression of cyclin A messenger RNA (mRNA) in ECs in a dose- and time-dependent manner. G1/S-associated molecules that might account for this block were not changed, because Hcy did not affect mRNA and protein expression of cyclin D1 and cyclin E. Cyclin D1- and E-associated kinase activities were unchanged. In contrast, cyclin A–associated kinase activity and CDK2 kinase activity were markedly suppressed. Nuclear run-on assay demonstrated that Hcy decreased the transcription rate of the cyclin A gene but had no effect on the half-life of cyclin A mRNA. In transient transfection experiments, Hcy significantly inhibited cyclin A promoter activity in endothelial cells, but not in vascular smooth muscle cells. Finally, adenovirus-transduced cyclin A expression restored EC growth inhibition and overcame the S phase block imposed by Hcy. Taken together, these findings indicate that cyclin A is a critical functional target of Hcy-mediated EC growth inhibition.

Cyclin A transcriptional suppression is the major mechanism mediating homocysteine-induced endothelial cell growth inhibition

Previously, it was reported that homocysteine (Hcy) specifically inhibits the growth of endothelial cells (ECs), suppresses Ras/mitogen-activated protein (MAP) signaling, and arrests cell growth at the G1/S transition of the cell cycle. The present study investigated the molecular mechanisms underlying this cell-cycle effect. Results showed that clinically relevant concentrations (50 μM) of Hcy significantly inhibited the expression of cyclin A messenger RNA (mRNA) in ECs in a dose- and time-dependent manner. G1/S-associated molecules that might account for this block were not changed, because Hcy did not affect mRNA and protein expression of cyclin D1 and cyclin E. Cyclin D1- and E-associated kinase activities were unchanged. In contrast, cyclin A–associated kinase activity and CDK2 kinase activity were markedly suppressed. Nuclear run-on assay demonstrated that Hcy decreased the transcription rate of the cyclin A gene but had no effect on the half-life of cyclin A mRNA. In transient transfection experiments, Hcy significantly inhibited cyclin A promoter activity in endothelial cells, but not in vascular smooth muscle cells. Finally, adenovirus-transduced cyclin A expression restored EC growth inhibition and overcame the S phase block imposed by Hcy. Taken together, these findings indicate that cyclin A is a critical functional target of Hcy-mediated EC growth inhibition.

Cyclin A transcriptional suppression is the major mechanism mediating homocysteine-induced endothelial cell growth inhibition

Previously, it was reported that homocysteine (Hcy) specifically inhibits the growth of endothelial cells (ECs), suppresses Ras/mitogen-activated protein (MAP) signaling, and arrests cell growth at the G(1)/S transition of the cell cycle. The present study investigated the molecular mechanisms underlying this cell-cycle effect. Results showed that clinically relevant concentrations (50 microM) of Hcy significantly inhibited the expression of cyclin A messenger RNA (mRNA) in ECs in a dose- and time-dependent manner. G(1)/S-associated molecules that might account for this block were not changed, because Hcy did not affect mRNA and protein expression of cyclin D1 and cyclin E. Cyclin D1- and E-associated kinase activities were unchanged. In contrast, cyclin A-associated kinase activity and CDK2 kinase activity were markedly suppressed. Nuclear run-on assay demonstrated that Hcy decreased the transcription rate of the cyclin A gene but had no effect on the half-life of cyclin A mRNA. In transient transfection experiments, Hcy significantly inhibited cyclin A promoter activity in endothelial cells, but not in vascular smooth muscle cells. Finally, adenovirus-transduced cyclin A expression restored EC growth inhibition and overcame the S phase block imposed by Hcy. Taken together, these findings indicate that cyclin A is a critical functional target of Hcy-mediated EC growth inhibition.

Suppression of Neu-induced mammary tumor growth in cyclin D1 deficient mice is compensated for by cyclin E

Amplification and/or overexpression of the receptor tyrosine kinase HER2/Neu and the cell cycle regulatory gene cyclin D1 are frequently associated with human breast cancer. We studied the functional significance of cyclin D1 in Neu-induced mammary oncogenesis by developing mice overexpressing either wild-type or mutant Neu in a cyclin D1 deficient background. The absence of cyclin D1 suppresses mammary tumor formation induced by the wild-type or activated mutant form of Neu, which promote multi- and single-step progression of tumorigenesis, respectively. These data indicate that cyclin D1 is preferentially required for Neu-mediated signal transduction pathways in mammary oncogenesis. Significantly, 35% of mutant Neu/cyclin D1(-/-) mice regained mammary tumor potential due to compensation by cyclin E. Thus, shared targets of cyclins D1 and E are important in modulating Neu function in mammary tumorigenesis. Our results imply that the combinatorial inhibition of cyclins D1 and E might be useful in the treatment of malignancies induced by Neu.

Functional roles of Akt signaling in mouse skin tumorigenesis

The mouse skin carcinogenesis protocol is a unique model for understanding the molecular events leading to oncogenic transformation. Mutations in the Ha-ras gene, and the presence of functional cyclin D1 and the EGF receptor, have proven to be important in this system. However, the signal transduction pathways connecting these elements during mouse skin carcinogenesis are poorly understood. This paper studies the relevance of the Akt and ERK pathways in the different stages of chemically induced mouse skin tumors. Akt activity increases throughout the entire process, and its early activation is detected prior to increased cyclin D1 expression. ERK activity rises only during the later stages of malignant conversion. The observed early increase in Akt activity appears to be due to raised PI-3K activity. Other factors acting on Akt such as ILK activation and decreased PTEN phosphatase activity appear to be involved at the conversion stage. To further confirm the involvement of Akt in this process, PB keratinocytes were transfected with Akt and subsequently injected into nude mice. The expression of Akt accelerates tumorigenesis and contributes to increased malignancy of these keratinocytes as demonstrated by the rate of appearance, the growth and the histological characteristics of the tumors. Collectively, these data provide evidence that Akt activation is one of the key elements during the different steps of mouse skin tumorigenesis.

The putative oncoprotein Bcl-3 induces cyclin D1 to stimulate G(1) transition

Bcl-3 is a distinctive member of the IkappaB family of NF-kappaB inhibitors because it can function to coactivate transcription. A potential involvement of Bcl-3 in oncogenesis is highlighted by the fact that it was cloned due to its location at a breakpoint junction in some cases of human B-cell chronic lymphocytic leukemia and that it is highly expressed in human breast tumor tissue. To analyze the effects of Bcl-3 dysregulation in breast epithelial cells, we created stable immortalized human breast epithelial cell lines either expressing Bcl-3 or carrying the corresponding vector control plasmid. Analysis of the Bcl-3-expressing cells suggests that these cells have a shortened G(1) phase of the cell cycle as well as a significant increase in hyperphosphorylation of the retinoblastoma protein. Additionally, the cyclin D1 gene was found to be highly expressed in these cells. Upon further analysis, Bcl-3, acting as a coactivator with NF-kappaB p52 homodimers, was demonstrated to directly activate the cyclin D1 promoter through an NF-kappaB binding site. Therefore, our results demonstrate that dysregulated expression of Bcl-3 potentiates the G(1) transition of the cell cycle by stimulating the transcription of the cyclin D1 gene in human breast epithelial cells.

Protein kinase C isoforms involved in the transcriptional activation of cyclin D1 by transforming Ha-Ras

Transcriptional activation of the cyclin D1 by oncogenic Ras appears to be mediated by several pathways leading to the activation of multiple transcription factors which interact with distinct elements of the cyclin D1 promoter. The present investigations revealed that cyclin D1 induction by transforming Ha-Ras is MEK- and Rac-dependent and requires the PKC isotypes epsilon, lambda, and zeta, but not cPKC-alpha. This conclusion is based on observations indicating that cyclin D1 induction by transforming Ha-Ras was depressed in a dose-dependent manner by PD98059, a selective inhibitor of the mitogen-activated kinase kinase MEK-1, demonstrating that Ha-Ras employs extracellular signal-regulated kinases (ERKs) for signal transmission to the cyclin D1 promoter. Evidence is presented that PKC isotypes epsilon and zeta, but not lambda are required for the Ras-mediated activation of ERKs. Expression of kinase-defective, dominant negative (DN) mutants of nPKC-epsilon or aPKC-zeta inhibit ERK activation by constitutively active Raf-1. Phosphorylation within the TEY motif and subsequent activation of ERKs by constitutively active MEK-1 was significantly inhibited by DN aPKC-zeta, indicating that aPKC-zeta functions downstream of MEK-1 in the pathway leading to cyclin D1 induction. In contrast, TEY phosphorylation induced by constitutively active MEK-1 was not effected by nPKC-epsilon, suggesting another position for this kinase within the cascade investigated. Transformation by oncogenic Ras requires activation of several Ras effector pathways which may be PKC-dependent and converge on the cyclin D1 promoter. Therefore, we investigated a role for PKC isotypes in the Ras-Rac-mediated transcriptional regulation of cyclin D1. We have been able to reveal that cyclin D1 induction by oncogenic Ha-Ras is Rac-dependent and requires the PKC isotypes epsilon, lambda, and zeta, but not cPKC-alpha. Evidence is presented that aPKC-lambda acts upstream of Rac, between Ras and Rac, whereas the PKC isotypes epsilon and zeta act downstream of Rac and are required for the activation of ERKs.

Ultraviolet radiation alters the phosphorylation of RNA polymerase II large subunit and accelerates its proteasome-dependent degradation

It has been shown that ultraviolet (UV) radiation induces the ubiquitination of the large subunit of RNA polymerase II (RNAP II-LS) as well as its proteasomal degradation. Studies in mammalian cells have indicated that highly phosphorylated forms of RNAP II-LS are preferentially ubiquitinated, but studies in Saccharomyces cerevisiae have provided evidence that unphosphorylated RNAP II-LS is an equally suitable substrate. In the present study, an antibody (ARNA-3) that recognizes all forms of RNAP II-LS, regardless of the phosphorylation status of its C-terminal domain (CTD), was utilized to evaluate the degradation of total cellular RNAP II-LS in human fibroblasts under basal conditions or after UV-C (10J/m(2)) irradiation. It was found that UV radiation rapidly shifted the phosphorylation profile of RNAP II-LS from a mixture of dephosphorylated and phosphorylated forms to entirely more phosphorylated forms. This shift in phosphorylation status was not blocked by pharmacologic inhibition of either the ERK or p38 pathways, both of which have been implicated in the cellular UV response. In addition to shifting the phosphorylation profile, UV radiation led to net degradation of total RNAP II-LS. UV-induced degradation of RNAP II-LS was also greatly reduced in the presence of the transcriptional and CTD kinase inhibitor DRB. Using a panel of protease inhibitors, it was shown that the bulk of UV-induced degradation is proteasome-dependent. However, the UV-induced loss of hypophosphorylated RNAP II-LS was proteasome-independent. Lastly, UV radiation induced a similar shift to all hyperphosphorylated RNAP II-LS in Cockayne syndrome (CS) cells of complementation groups A or B (CSA or CSB) when compared to appropriate controls. The UV-induced degradation rates of RNAP II-LS were not significantly altered when comparing CSA or CSB to repair competent control cells. The implications for the cellular UV response are discussed.

Cyclin D1 overexpression and p53 inactivation immortalize primary oral keratinocytes by a telomerase-independent mechanism

The immortalization of human cells is a critical step in multistep carcinogenesis. Oral-esophageal carcinomas, a model system to investigate molecular mechanisms underlying squamous carcinogenesis, frequently involve cyclin D1 overexpression and inactivation of the p53 tumor suppressor. Therefore, our goal was to establish the functional role of cyclin D1 overexpression and p53 inactivation in the immortalization of primary human oral squamous epithelial cells (keratinocytes) as an important step toward malignant transformation. Cyclin D1 overexpression alone was found to induce extension of the replicative life span of normal oral keratinocytes, whereas the combination of cyclin D1 overexpression and p53 inactivation led to their immortalization. This study also demonstrates that immortalization of oral keratinocytes can be independent of telomerase activation, involving an alternative pathway of telomere maintenance (ALT).

Induction of cyclin D1 transcription and CDK2 activity by Notch(ic): implication for cell cycle disruption in transformation by Notch(ic)

Notch genes encode a family of transmembrane proteins that are involved in many cellular processes such as differentiation, proliferation, and apoptosis. Although it is well established that all four Notch genes can act as oncogenes, the mechanism by which Notch proteins transform cells remains unknown. Previously, we have shown that transformation of RKE cells can be conditionally induced by hormone activation of Notch(ic)-estrogen receptor (ER) chimeras. Using this inducible system, we show that Notch(ic) activates transcription of the cyclin D1 gene with rapid kinetics. Transcriptional activation of cyclin D1 is independent from serum-derived growth factors and de novo synthesis of secondary transcriptional activators. Moreover, hormone activation of Notch(ic)-ER proteins induces CDK2 activity in the absence of serum. Upregulation of cyclin D1 and activation of CDK2 by Notch(ic) result in the promotion of S-phase entry. These data demonstrate the first evidence that Notch(ic) proteins can directly regulate factors involved in cell cycle control and affect cellular proliferation. Furthermore, nontransforming Notch(ic) proteins do not induce cyclin D1 expression, indicating that the mechanism of transformation involves cell cycle deregulation through constitutive expression of cyclin D1. Finally, we have identified a CSL [stands for CBF1, Su(H), and Lag-1] binding site within the human and rat cyclin D1 promoters, suggesting that Notch(ic) proteins activate cyclin D1 transcription through a CSL-dependent pathway.

Estrogen regulation of cyclin D1 gene expression in ZR-75 breast cancer cells involves multiple enhancer elements

Cyclin D1 gene expression is induced by 17beta-estradiol (E2) in human breast cancer cells and is important for progression of cells through the G(1) phase of the cell cycle. The mechanism of activation of cyclin D1 is mitogen- and cell context-dependent, and this study describes the role of multiple promoter elements required for induction of cyclin D1 by E2 in estrogen receptor (ER)-positive ZR-75 breast cancer cells. Transcriptional activation of cyclin D1 by E2 was dependent, in part, on a proximal cAMP-response element at -66, and this was linked to induction of protein kinase A-dependent pathways. These results contrasted to a recent report showing that induction of cyclin D1 by E2 in ER-positive MCF-7 and HeLa cells was due to up-regulation of c-jun and subsequent interaction of c-Jun-ATF-2 with the CRE. Moreover, further examination of the proximal region of the cyclin D1 promoter showed that three GC-rich Sp1-binding sites at -143 to -110 were also E2-responsive, and interaction of ERalpha and Sp1 proteins at these sites was confirmed by electromobility shift and chromatin immunoprecipitation assays. Thus, induction of cyclin D1 by E2 in ZR-75 cells is regulated through nuclear ERalpha/Sp1 and epigenetic protein kinase A activation pathways, and our results suggest that this mechanism may be cell context-dependent even among ER-positive breast cancer cell lines.

Sp1 and krüppel-like factor family of transcription factors in cell growth regulation and cancer

The Sp/KLF family contains at least twenty identified members which include Sp1-4 and numerous krüppel-like factors. Members of the family bind with varying affinities to sequences designated as 'Sp1 sites' (e.g., GC-boxes, CACCC-boxes, and basic transcription elements). Family members have different transcriptional properties and can modulate each other's activity by a variety of mechanisms. Since cells can express multiple family members, Sp/KLF factors are likely to make up a transcriptional network through which gene expression can be fine-tuned. 'Sp1 site'-dependent transcription can be growth-regulated, and the activity, expression, and/or post-translational modification of multiple family members is altered with cell growth. Furthermore, Sp/KLF factors are involved in many growth-related signal transduction pathways and their overexpression can have positive or negative effects on proliferation. In addition to growth control, Sp/KLF factors have been implicated in apoptosis and angiogenesis; thus, the family is involved in several aspects of tumorigenesis. Consistent with a role in cancer, Sp/KLF factors interact with oncogenes and tumor suppressors, they can be oncogenic themselves, and altered expression of family members has been detected in tumors. Effects of changes in Sp/KLF factors are context-dependent and can appear contradictory. Since these factors act within a network, this diversity of effects may arise from differences in the expression profile of family members in various cells. Thus, it is likely that the properties of the overall network of Sp/KLF factors play a determining role in regulation of cell growth and tumor progression.