5-Aza-4'-thio-2'-deoxycytidine, a New Orally Bioavailable Nontoxic "Best-in-Class": DNA Methyltransferase 1-Depleting Agent in Clinical Development.

DNA methyltransferase (DNMT) 1 is an enzyme that functions as a maintenance methyltransferase during DNA replication, and depletion of this enzyme from cells is considered to be a rational goal in DNA methylation-dependent disorders. Two DNMT1-depleting agents 5-aza-2'-deoxycytidine (aza-dCyd, decitabine) and 5-aza-cytidine (aza-Cyd, azacitidine) are currently used for the treatment of myelodysplastic syndromes and acute myeloid leukemia and have also been investigated for nononcology indications, such as sickle cell disease. However, these agents have several off-target activities leading to significant toxicities that limit dosing and duration of treatment. Development of more selective inhibitors of DNMT1 could therefore afford treatment of long durations at effective doses. We have discovered that 5-aza-4'-thio-2'-deoxycytidine (aza-T-dCyd) is as effective as aza-dCyd in depleting DNMT1 in mouse tumor models but with markedly low toxicity. In this review we describe the preclinical studies that led to the development of aza-T-dCyd as a superior DNMT1-depleting agent with respect to aza-dCyd and will describe its pharmacology, metabolism, and mechanism of action. In an effort to understand why aza-T-dCyd is a more selective DNMT1 depleting agent than aza-dCyd, we will also compare and contrast the activities of these two agents. SIGNIFICANCE STATEMENT: Aza-T-dCyd is a potent DNMT1-depleting agent. Although similar in structure to decitabine (aza-dCyd), its metabolism and mechanism of action is different than that of aza-dCyd, resulting in less off-target activity and less toxicity. The larger therapeutic index of aza-T-dCyd (DNMT1 depletion vs. toxicity) in mice suggests that it would be a better clinical candidate to selectively deplete DNMT1 from target cells and determine whether or not depletion of DNMT1 is an effective target for various diseases.

Cks1 proteasomal turnover is a predominant mode of regulation in breast cancer cells: role of key tyrosines and lysines.

Constitutive levels of Cks1 protein are very high in mammary carcinoma tissue and in breast tumor cell lines. However, despite being transcribed at relatively high levels, Cks1 protein is very low in normal mammary tissue. Also, basal Cks1 is barely detectable in primary human mammary epithelial cells (HMECs). Epoximicin, a proteasome inhibitor, induced detectable endogenous Cks1 in HMECs, and upregulated it above the basal level in MCF-7 breast cancer cells. Interestingly, transiently transfected Cks1 is remarkably unstable and accumulates only upon proteasomal blockade in multiple cell lines even when driven by the strong CMV promoter-enhancer. We examined the stability of site-directed Cks1 mutants in order to identify the structural determinants of its turnover in cancer cells. Since protein turnover is regulated by phosphorylation, and phosphoproteomic studies reveal phosphorylated tyrosines in Cks1, we replaced its five conserved tyrosines (Y) with phenylalanine (F), both individually and in combinations. We find that like wild-type, all transiently transfected mutant Cks1 vectors, even when driven by the CMV promoter-enhancer, expressed detectable protein only in cells treated with epoximicin. However, turnover of the Y8F, Y12F and Y19F Cks1 mutants was more rapid than that of wild-type, Y7F and Y57F. Since lysines are modified by ubiquitination or acetylation we also examined the consequences of lysine to arginine (K-R) substitutions on Cks1 proteasomal turnover. We found that the individual mutations K4R, K26R, K30R, and K34R slowed Cks1 turnover, while the K79R mutation or the combined mutation K75-76-78-79R increased turnover. Taken together, regulation of Cks1 protein stability is crucially dependent on specific tyrosine and lysine residues which are potential sites for post-translational modifications.

Cks1 proteasomal degradation is induced by inhibiting Hsp90-mediated chaperoning in cancer cells.

PURPOSE: Cks1, a conformationally heterogenous 9 kDa protein, is markedly overexpressed in cancer cells and contributes to tumor development. Cks1 is an essential component of the SCF-Skp2 ubiquitin ligase complex that targets the Cdk inhibitors p27(Kip1) and p21(Cip1). Cks1 is known to interact with the Hsp90-Cdc37 chaperone machinery, although whether this facilitates its conformational maturation and stability is not known. To test whether abrogating the chaperone function of Hsp90 could destabilize Cks1, we examined the effects of treating different cancer cell lines with the benzoquinone ansamycin 17-allylamino geldanamycin (17-AAG), a compound that selectively binds Hsp90 and potently inhibits its ATP-dependent chaperone activity. METHODS: The effect of Hsp90 inhibition using 17-AAG on Cks1 protein and associated cell cycle proteins including Skp2, p27(Kip1), p21(Cip1), and Cdk1 in cancer cells was determined by Western blotting. Ubiquitination analysis was carried out by transfecting cells with an HA-ubiquitin plasmid and specifically immunoprecipitating Cks1 to examine polyubiquitinated species. Flow cytometry was utilized to examine the effects of Hsp90 inhibition on cell cycle profiles. RESULTS: Here, we demonstrate for the first time that inhibition of Hsp90 utilizing 17-AAG destabilizes Cks1 in cancer cells by promoting its ubiquitination and proteasomal degradation. 17-AAG-induced Cks1 depletion was accompanied by concomitant decreases in Skp2 and Cdk1. 17-AAG treatment also induced G2/M accumulation in MCF-7 breast carcinoma cells, and G1 accumulation in the colon carcinoma lines HCT116 and SW620. CONCLUSIONS: We conclude that perturbing the Hsp90 pathway could provide a useful therapeutic strategy in tumors driven by Cks1 overexpression.

Novel DNA methyltransferase-1 (DNMT1) depleting anticancer nucleosides, 4'-thio-2'-deoxycytidine and 5-aza-4'-thio-2'-deoxycytidine.

PURPOSE: Currently approved DNA hypomethylating nucleosides elicit their effects in part by depleting DNA methyltransferase I (DNMT1). However, their low response rates and adverse effects continue to drive the discovery of newer DNMT1 depleting agents. Herein, we identified two novel 2'-deoxycytidine (dCyd) analogs, 4'-thio-2'-deoxycytidine (T-dCyd) and 5-aza-4'-thio-2'-deoxycytidine (aza-T-dCyd) that potently deplete DNMT1 in both in vitro and in vivo models of cancer and concomitantly inhibit tumor growth. METHODS: DNMT1 protein levels in in vitro and in vivo cancer models were determined by Western blotting and antitumor efficacy was evaluated using xenografts. Effects on CpG methylation were evaluated using methylation-specific PCR. T-dCyd metabolism was evaluated using radiolabeled substrate. RESULTS: T-dCyd markedly depleted DNMT1 in CCRF-CEM and KG1a leukemia and NCI-H23 lung carcinoma cell lines, while it was ineffective in the HCT-116 colon or IGROV-1 ovarian tumor lines. On the other hand, aza-T-dCyd potently depleted DNMT1 in all of these lines indicating that dCyd analogs with minor structural dissimilarities induce different DNMT1 turnover mechanisms. Although T-dCyd was deaminated to 4'-thio-2'-deoxyuridine, very little was converted to 4'-thio-thymidine nucleotides, suggesting that inhibition of thymidylate synthase would be minimal with 4'-thio dCyd analogs. Both T-dCyd and aza-T-dCyd also depleted DNMT1 in human tumor xenografts and markedly reduced in vivo tumor growth. Interestingly, the selectivity index of aza-T-dCyd was at least tenfold greater than that of decitabine. CONCLUSIONS: Collectively, these data show that 4'-thio modified dCyd analogs, such as T-dCyd or aza-T-dCyd, could be a new source of clinically effective DNMT1 depleting anticancer compounds with less toxicity.

Kinomic exploration of temozolomide and radiation resistance in Glioblastoma multiforme xenolines.

BACKGROUND AND PURPOSE: Glioblastoma multiforme (GBM) represents the most common and deadly primary brain malignancy, particularly due to temozolomide (TMZ) and radiation (RT) resistance. To better understand resistance mechanisms, we examined global kinase activity (kinomic profiling) in both treatment sensitive and resistant human GBM patient-derived xenografts (PDX or "xenolines"). MATERIALS AND METHODS: Thirteen orthotopically-implanted xenolines were examined including 8 with known RT sensitivity/resistance, while 5 TMZ resistant xenolines were generated through serial TMZ treatment in vivo. Tumors were harvested, prepared as total protein lysates, and kinomically analyzed on a PamStation®12 high-throughput microarray platform with subsequent upstream kinase prediction and network modeling. RESULTS: Kinomic profiles indicated elevated tyrosine kinase activity associated with the radiation resistance phenotype, including FAK and FGFR1. Furthermore, network modeling showed VEGFR1/2 and c-Raf hubs could be involved. Analysis of acquired TMZ resistance revealed more kinomic variability among TMZ resistant tumors. Two of the five tumors displayed significantly altered kinase activity in the TMZ resistant xenolines and network modeling indicated PKC, JAK1, PI3K, CDK2, and VEGFR as potential mediators of this resistance. CONCLUSION: GBM xenolines provide a phenotypic model for GBM drug response and resistance that when paired with kinomic profiling identified targetable pathways to inherent (radiation) or acquired (TMZ) resistance.

Cks1: Structure, Emerging Roles and Implications in Multiple Cancers.

Deregulation of the cell cycle results in loss of normal control mechanisms that prevent aberrant cell proliferation and cancer progression. Regulation of the cell cycle is a highly complex process with many layers of control. One of these mechanisms involves timely degradation of CDK inhibitors (CKIs) like p27Kip1 by the ubiquitin proteasomal system (UPS). Cks1 is a 9 kDa protein which is frequently overexpressed in different tumor subtypes, and has pleiotropic roles in cell cycle progression, many of which remain to be fully characterized. One well characterized molecular role of Cks1 is that of an essential adaptor that regulates p27Kip1 abundance by facilitating its interaction with the SCF-Skp2 E3 ligase which appends ubiquitin to p27Kip1 and targets it for degradation through the UPS. In addition, emerging research has uncovered p27Kip1-independent roles of Cks1 which have provided crucial insights into how it may be involved in cancer progression. We review here the structural features of Cks1 and their functional implications, and also some recently identified Cks1 roles and their involvement in breast and other cancers.

Phase 2 trial of clofarabine in combination with etoposide and cyclophosphamide in pediatric patients with refractory or relapsed acute lymphoblastic leukemia.

The outcomes in children with refractory/relapsed (R/R) acute lymphoblastic leukemia (ALL) are dismal. The efficacy and safety of intravenous clofarabine 40 mg/m(2) per day, cyclophosphamide 440 mg/m(2) per day, and etoposide 100 mg/m(2) per day for 5 consecutive days in pediatric patients with R/R ALL was evaluated in this phase 2 study. The primary endpoint was overall response rate (complete remission [CR] plus CR without platelet recovery [CRp]). Among the 25 patients (median age, 14 years; pre-B cell ALL, 84%; ≥ 2 prior regimens: 84%; refractory to previous regimen: 60%), the overall response rate was 44% (7 CR, 4 CRp) with a 67.3-week median duration or remission censored at last follow-up. Most patients proceeded to alternative therapy, and 10 patients (40%) received hematopoietic stem cell transplantation. Six patients (24%) died because of treatment-related adverse events associated with infection, hepatotoxicity, and/or multiorgan failure. The study protocol was amended to exclude patients with prior hematopoietic stem cell transplantation after 4 of the first 8 patients developed severe hepatotoxicity suggestive of veno-occlusive disease. No additional cases of veno-occlusive disease occurred. The regimen offered encouraging response rates and sustained remission in R/R patients. Future investigation should include exploration of patient selection, dosing, and supportive care. This trial was registered at www.clinicaltrials.gov as #NCT00315705.

Mesd is a universal inhibitor of Wnt coreceptors LRP5 and LRP6 and blocks Wnt/beta-catenin signaling in cancer cells.

Mesd is a specialized chaperone for low-density lipoprotein receptor-related protein 5 (LRP5) and LRP6. In our previous studies, we found that Mesd binds to mature LRP6 on the cell surface and blocks the binding of Wnt antagonist Dickkopf-1 (Dkk1) to LRP6. Herein, we demonstrate that Mesd also binds to LRP5 with a high affinity and is a universal inhibitor of LRP5 and LRP6 ligands. Mesd not only blocks binding of Wnt antagonists Dkk1 and Sclerostin to LRP5 and LRP6 but also inhibits Wnt3A and Rspondin1-induced Wnt/beta-catenin signaling in LRP5- and LRP6-expressing cells. We also found that Mesd, Dkk1, and Sclerostin compete with one another for binding to LRP5 and LRP6 at the cell surface. More importantly, we demonstrated that Mesd is able to suppress LRP6 phosphorylation and Wnt/beta-catenin signaling in prostate cancer PC-3 cells and inhibits PC-3 cell proliferation. Our results indicate that recombinant Mesd protein is a useful tool for studying Wnt/beta-catenin signaling on the cell surface and has a potential therapeutic role in Wnt-dependent cancers.

High Cks1 expression in transgenic and carcinogen-initiated mammary tumors is not always accompanied by reduction in p27Kip1.

Cks1 plays an essential role in SCFSkp2-mediated ubiquitination, and consequently turnover, of the cdk2 inhibitor and tumor supressor p27Kip1. High Cks1 expression is associated with aggressive breast tumors and correlates with low p27Kip1 levels in some cases, although it is also an independent prognostic marker for survival, and provides predictive information in addition to that provided by p27Kip1 alone. In this report we demonstrate that Cks1 protein and mRNA are elevated to very high levels in mammary tumors initiated by erbB2, c-myc and polyoma middle-T (PyMT) in transgenic mice, whereas Cks1 protein is hardly detectable in the normal mammary epithelium. Cks1 is also highly upregulated in rat mammary tumors initiated by methylnitrosourea (MNU). Despite high levels of Cks1 expression, p27Kip1 levels were not reduced, and were in fact slightly higher in mammary tumors initiated by erbB2, PyMT and MNU. In contrast mammary tumors from MMTV-c-myc mice did exhibit low p27Kip1 and higher levels of Skp2. Together, these data suggest that deregulated Cks1 expression might play roles in oncogene and carcinogen-initiated mammary tumorigenesis independent of p27Kip1 turnover in certain tumors. Stable overexpression of Cks1 in human breast carcinoma MCF-7 cells did not significantly reduce p27Kip1 expression, although it conferred resistance to Faslodex (ICI 182780)-mediated inhibition of colony outgrowth in these cells. In contrast, Cks1-depleted MCF-7 cells formed fewer colonies in estrogen-containing medium. Therefore, our studies also suggest that Cks1 levels regulate the responsiveness of ER+ breast cancers to estrogens and anti-estrogens.

Vascular endothelial growth factor reduces tamoxifen efficacy and promotes metastatic colonization and desmoplasia in breast tumors.

Clinical studies have shown that decreased tamoxifen effectiveness correlates with elevated levels of vascular endothelial growth factor (VEGF)-A(165) in biopsy samples of breast cancers. To investigate the mechanisms underlying tamoxifen resistance and metastasis, we engineered the estrogen receptor (ER)-positive MCF-7 human breast cancer cell line to express VEGF to clinically relevant levels in a doxycycline-regulated manner. Induction of VEGF expression in orthotopically implanted xenografts that were initially tamoxifen responsive and noninvasive resulted in tamoxifen-resistant tumor growth and metastasis to the lungs. Lung metastases were also observed in a VEGF-dependent manner following tail vein injection of tumor cells. At both primary and metastatic sites, VEGF-overexpressing tumors exhibited extensive fibroblastic stromal content, a clinical feature called desmoplasia. VEGF-induced metastatic colonies were surrounded by densely packed stromal cells before detectable angiogenesis, suggesting that VEGF is involved in the initiation of desmoplasia. Because expression of VEGF receptors R1 and R2 was undetectable in these tumor cells, the observed VEGF effects on reduction of tamoxifen efficacy and metastatic colonization are most likely mediated by paracrine signaling that enhances tumor/stromal cell interactions and increases the level of desmoplasia. This study reveals new roles for VEGF in breast cancer progression and suggests that combination of antiestrogens and VEGF inhibitors may prolong tamoxifen sensitivity and prevent metastasis in patients with ER-positive tumors.

Cks1 regulates cdk1 expression: a novel role during mitotic entry in breast cancer cells.

Cks1, a small protein whose expression is strongly associated with aggressive breast tumors, is a component of cyclin-cdk complexes, as well as the SCF(Skp2) ubiquitin ligase. In these studies, we explored its roles in estrogen receptor-positive breast tumor cells. When exposed to the antiestrogen ICI 182780, these cells accumulate in G(1) by reducing the expression of Cks1, and increasing the levels of p130/Rb2, a cdk2 inhibitor and SCF(Skp2) target. Heregulin beta1 or estradiol abrogate antiestrogen effects by increasing Cks1 expression, down-regulating p130/Rb2 and inducing S phase entry. Depletion of Cks1 in these cells by RNA interference concomitantly decreased Skp2 and up-regulated p130/Rb2 and another SCF(Skp2) target, p27(Kip1). Remarkably, however, Cks1-depleted cells not only exhibit slowed G(1) progression, but also accumulate in G(2)-M due to blocked mitotic entry. Notably, we show that cdk1 expression, which is crucial for M phase entry, is drastically diminished by Cks1 depletion, and that restoration of cdk1 reduces G(2)-M accumulation in Cks1-depleted cells. cdk1 reduction in Cks1-depleted cells is a consequence of a marked decrease in its mRNA and not due to alteration in its proteolytic turnover. Both heregulin beta1 and estradiol could neither restore cdk1 nor sustain cycling in Cks1-depleted cells, although classical estrogen receptor function remained unaltered. Cks1 depletion also decreased Skp2 in human mammary epithelial cells without altering cell cycle progression. Thus, the indispensability of Cks1 to the breast cancer cell cycle, versus its redundancy in normal cells, suggests that Cks1 abrogation could be an effective interventional strategy in breast cancer.

MEK ablation in MCF-7 cells blocks DNA synthesis induced by serum, but not by estradiol or growth factors.

The role of mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK) signaling in estrogen receptor positive (ER(+)) MCF-7 breast carcinoma cells is not well understood. We depleted MEK by cotransfection of MEK1 and MEK2 siRNA duplexes in a MCF-7 derived line (MCF-7/ lacZ, ML-20) and determined its effect on serum, 17beta-estradiol (E(2)), and growth factor induced DNA synthesis. MEK knockdown did not decrease fetal bovine serum-induced DNA synthesis in ML-20 cells although it did inhibit DNA synthesis induced by estrogen-stripped calf serum (CCS) suggesting that MEK activation plays an important role in growth signaling induced by serum components other than estrogen. Consistent with this notion, MEK knockdown only modestly decreased DNA synthesis induced by E(2)-supplemented CCS medium in ML-20 cells. Similarly, MEK knockdown only caused moderate decreases in DNA synthesis induced by fibroblast growth factor-1 (FGF-1) or heregulin-beta1 (HRGbeta1) in this media. Also, there were only minimal effects of MEK knockdown in cells treated with growth factor-supplemented serum-free medium. Although MEK depletion inhibited ERK1/2 phosphorylation induced by CCS in these cells, that induced by growth factor supplemented CCS media was relatively unaffected. Similarly, ERK1/2 phosphorylation induced by growth factor-supplemented serum-free media was also relatively unaffected by MEK depletion. These results suggest that pathways regulating DNA synthesis induced by serum in MCF-7 cells are significantly more dependent on constitutive MEK levels than that induced by E(2) or growth factors.

siRNA mediated knockdown of fibroblast growth factor receptors 1 or 3 inhibits FGF-induced anchorage-independent clonogenicity but does not affect MAPK activation.

Supplementation with exogenous growth factors such as fibroblast growth factors (FGFs) is essential for anchorage-independent growth of the SW-13 human adrenal adenocarcinoma cell line. We have found that SW-13 cells express mRNAs for FGFRs 1, 3, and 4, but not FGFR2. To assess the roles of individual FGFRs, in anchorage-independent growth, we determined the effects of down-regulation of each FGFR on FGF2- and FGF4-mediated soft agar colony formation in these cells. Using RNAi strategies we found that knockdown of either FGFR1 or FGFR3 leads to inhibition of FGF2- or FGF4-induced soft agar clonogenicity without affecting that induced by heregulin beta1. However, this inhibition is independent of ERK1/2 activation as levels of FGF-induced phospho-ERK 1/2 remain unchanged upon knockdown of either FGFR1 or FGFR3. Conversely, RNAi-mediated knockdown of FGFR4 appeared to have no significant effect on either FGF2- or FGF4-induced anchorage-independent colony formation, or ERK1/2 phosphorylation. These results suggest that constitutive levels of both FGFR1 and FGFR3, but not FGFR4 are essential for FGF-stimulated anchorage-independent growth of SW-13 cells.

c-Abl-independent p73 stabilization during gemcitabine- or 4'-thio-beta-D-arabinofuranosylcytosine-induced apoptosis in wild-type and p53-null colorectal cancer cells.

Nucleoside anticancer drugs like gemcitabine (2'-deoxy-2',2'-difluorocytidine) are potent inducers of p53, and ectopic expression of wild-type p53 sensitizes cells to these agents. However, it is also known that nucleosides are efficient activators of apoptosis in tumor cells that do not express a functional p53. To clarify this issue, we examined the effects of gemcitabine and 4'-thio-beta-d-arabinofuranosylcytosine (T-ara-C) on p73, a structural and functional homologue of p53, whose activation could also account for nucleoside-induced apoptosis because no functionally significant mutations of p73 have been reported in cancers. Acute treatment of HCT 116 colon carcinoma cells with gemcitabine or T-ara-C induced marked cytotoxicity and cleavage of caspase-3 and poly(ADP-ribose) polymerase. T-ara-C and gemcitabine markedly induced p53 accumulation as well as increased levels of phospho-p53 (Ser15/Ser20/Ser46) and induced its binding to a consensus p53 response element. Despite robust activation of p53 by T-ara-C and gemcitabine, we found that wild-type and p53-/- HCT 116 cells exhibited almost equivalent sensitivity towards these nucleosides. Examination of p73 revealed that T-ara-C and gemcitabine markedly increased p73 protein levels and p73 DNA-binding activities in both p53-/- and wild-type cells. Furthermore, T-ara-C- and gemcitabine-induced increases in p73 levels occur due to a decrease in p73 protein turnover. RNA interference studies show that nucleoside-induced p73 increases are independent of c-Abl, a nucleoside-activated kinase recently implicated in p73 stabilization. HCT 116 lines, wherein the downstream p53/p73 targets Bax and PUMA (p53 up-regulated modulator of apoptosis) were deleted, were less sensitive to T-ara-C and gemcitabine. Together, these studies indicate that c-Abl-independent p73 stabilization pathways could account for the p53-independent mechanisms in nucleoside-induced apoptosis.

Breast fibroblasts modulate epithelial cell proliferation in three-dimensional in vitro co-culture.

BACKGROUND: Stromal fibroblasts associated with in situ and invasive breast carcinoma differ phenotypically from fibroblasts associated with normal breast epithelium, and these alterations in carcinoma-associated fibroblasts (CAF) may promote breast carcinogenesis and cancer progression. A better understanding of the changes that occur in fibroblasts during carcinogenesis and their influence on epithelial cell growth and behavior could lead to novel strategies for the prevention and treatment of breast cancer. To this end, the effect of CAF and normal breast-associated fibroblasts (NAF) on the growth of epithelial cells representative of pre-neoplastic breast disease was assessed. METHODS: NAF and CAF were grown with the nontumorigenic MCF10A epithelial cells and their more transformed, tumorigenic derivative, MCF10AT cells, in direct three-dimensional co-cultures on basement membrane material. The proliferation and apoptosis of MCF10A cells and MCF10AT cells were assessed by 5-bromo-2'-deoxyuridine labeling and TUNEL assay, respectively. Additionally, NAF and CAF were compared for expression of insulin-like growth factor II as a potential mediator of their effects on epithelial cell growth, by ELISA and by quantitative, real-time PCR. RESULTS: In relatively low numbers, both NAF and CAF suppressed proliferation of MCF10A cells. However, only NAF and not CAF significantly inhibited proliferation of the more transformed MCF10AT cells. The degree of growth inhibition varied among NAF or CAF from different individuals. In greater numbers, NAF and CAF have less inhibitory effect on epithelial cell growth. The rate of epithelial cell apoptosis was not affected by NAF or CAF. Mean insulin-like growth factor II levels were not significantly different in NAF versus CAF and did not correlate with the fibroblast effect on epithelial cell proliferation. CONCLUSION: Both NAF and CAF have the ability to inhibit the growth of pre-cancerous breast epithelial cells. NAF have greater inhibitory capacity than CAF, suggesting that the ability of fibroblasts to inhibit epithelial cell proliferation is lost during breast carcinogenesis. Furthermore, as the degree of transformation of the epithelial cells increased they became resistant to the growth-inhibitory effects of CAF. Insulin-like growth factor II could not be implicated as a contributor to this differential effect of NAF and CAF on epithelial cell growth.

Prolonged extracellular signal-regulated kinase 1/2 activation during fibroblast growth factor 1- or heregulin beta1-induced antiestrogen-resistant growth of breast cancer cells is resistant to mitogen-activated protein/extracellular regulated kinase kinase inhibitors.

Increased growth factor receptor signaling is implicated in antiestrogen-resistant breast tumors suggesting that abrogation of such signaling could restore or prolong sensitivity to antihormonal agents. Activation of the mitogen-activated protein/extracellular regulated kinase kinase (MEK)-extracellular regulated kinase (ERK)1/2 cascade is a common component of such pathways. We investigated the ability of the MEK activation inhibitor U0126 to block the increased growth of estrogen receptor-positive MCF-7 breast cancer cells caused by fibroblast growth factor 1 (FGF-1), heregulin beta1 (HRGbeta1), and epidermal growth factor (EGF) in the presence of the pure antiestrogen ICI 182780 (Faslodex; fulvestrant). We found that either FGF-1 or HRGbeta1 but not EGF substantially reduced the inhibitory effects of U0126 on growth and ERK1/2 activation, including the combined inhibitory effects of U0126 and ICI 182780. FGF-1 and HRGbeta1 also reduced the inhibition of ERK1/2 phosphorylation by the MEK inhibitors PD98059 and PD184161. Interestingly, a transiently transfected dominant-negative MEK1 completely abrogated activation of a coexpressed green fluorescent protein-ERK2 reporter by all three of the factors. Despite a short-lived activation of Ras and Raf-1 by all three of the growth factors, both FGF-1 and HRGbeta1, unlike EGF, induced a prolonged activation of MEK and ERK1/2 in these cells. Thus, activation of FGF-1- and HRGbeta1-specific signaling causes MEK-dependent prolonged activation of ERK1/2, which is incompletely susceptible to known MEK inhibitors. We also demonstrate that the cytosolic phospholipase A2 inhibitor arachidonyl trifluoro methyl ketone and the pan PKC inhibitor bisindolymaleimide abrogated U0126-resistant phosphorylation of ERK1/2 induced by HRGbeta1 but not by FGF-1. Phosphorylation of ERK5 by all three of the factors was also resistant to U0126 suggesting that its activation is not sufficient to overturn growth inhibition due to diminished ERK1/2 activation. Therefore, therapy combining antiestrogens and MEK inhibitors may be ineffective in some antiestrogen-resistant estrogen receptor-positive breast cancers.

Homogeneity and long-term stability of tetracycline-regulated gene expression with low basal activity by using the rtTA2S-M2 transactivator and insulator-flanked reporter vectors.

Inducible expression of tetracycline responsive element (TRE)-regulated genes in nearly all cells in a stable clone has generally been problematic, especially in long-term culture. Heterogeneity of tet-inducible expression is generally attributed to the instability of the original tet-transactivators tTA and rtTA. These transactivators have cryptic splice sites, prokaryotic codons and full VP16 domains, all of which contribute to their instability. Moreover, they also require high concentrations of Doxycycline (Dox). The 5 amino acid substitutions in the rtTA variant rtTA2S-M2 confer exquisite sensitivity to Dox. Moreover, humanized codons, removal of cryptic splice sites and minimal VP16 domains in rtTA2S-M2 results in its being better tolerated within cells. However, the ability of this modified transactivator to maintain homogeneous inducibility in long-term culture has not been examined. We demonstrate that rtTA2S-M2 expressing clones exhibit functional transactivator activity for over 7 months in culture. Furthermore, rtTA2S-M2 expressing clones with chromosomally integrated copies of a TRE-green fluorescent protein (GFP) reporter also exhibited homogeneous inducibility in long-term culture. Importantly, the inherent reduced toxicity and improved stability of rtTA2S-M2 obviates the need to continuously select for its message, once clones with functional transactivator are isolated. The use of rtTA2S-M2 did not, however, preclude clones with stably integrated TRE-reporter from exhibiting leakiness. However, inclusion of flanking double copies of a 'minimal core element' of the chicken beta-globin gene insulator, instead of the 1.4 kb region, in the TRE-reporter was sufficient to markedly reduce the frequency of clones with high basal expression. Inclusion of the insulator core also did not affect the maximal expression levels of the inducible gene, which typically equaled or exceeded that observed with the strong constitutive CMV promoter. Finally, with this system homogeneous inducibility was observed rapidly and with low doses of Dox.