Chemically induced rat mammary tumor treated with tamoxifen showed decreased expression of cyclin D1, cyclin E, and p21(Cip1)

Potent inhibition of rhabdoid tumor cells by combination of flavopiridol and 4OH-tamoxifen

BACKGROUND

Rhabdoid Tumors (RTs) are highly aggressive pediatric malignancies with poor prognosis. There are currently no standard or effective treatments for RTs in part because treatments are not designed to specifically target these tumors. Our previous studies indicated that targeting the cyclin/cdk pathway is a novel therapeutic strategy for RTs and that a pan-cdk inhibitor, flavopiridol, inhibits RT growth. Since the toxicities and narrow window of activity associated with flavopiridol may limit its clinical use, we tested the effect of combining flavopiridol with 4-hydroxy-Tamoxifen (4OH-Tam) in order to reduce the concentration of flavopiridol needed for inhibition of RTs.

METHODS

The effects of flavopiridol, 4OH-Tam, and their combination on RT cell cycle regulation and apoptosis were assessed by: i) cell survival assays, ii) FACS analysis, iii) caspase activity assays, and iv) immunoblot analysis. Furthermore, the role of p53 in flavopiridol- and 4OH-Tam-mediated induction of cell cycle arrest and apoptosis was characterized using RNA interference (siRNA) analysis. The effect of p53 on flavopiridol-mediated induction of caspases 2, 3, 8 and 9 was also determined.

RESULTS

We found that the combination of flavopiridol and 4OH-Tam potently inhibited the growth of RT cells. Low nanomolar concentrations of flavopiridol induced G₂ arrest, which was correlated to down-modulation of cyclin B1 and up-regulation of p53. Addition of 4OH-Tam did not affect flavopiridol-mediated G₂ arrest, but enhanced caspase 3,7-mediated apoptosis induced by the drug. Abrogation of p53 by siRNA abolished flavopiridol-induced G₂ arrest, but enhanced flavopiridol- (but not 4OH-Tam-) mediated apoptosis, by enhancing caspase 2 and 3 activities.

CONCLUSIONS

Combining flavopiridol with 4OH-Tam potently inhibited the growth of RT cells by increasing the ability of either drug alone to induce caspases 2 and 3 thereby causing apoptosis. The potency of flavopiridol was enhanced by abrogation of p53. Our results warrant further studies investigating the combinatorial effects of flavopiridol and 4OH-Tam as a novel therapeutic strategy for RTs and other tumors that have been shown to respond to flavopiridol.

Indole-3-Carbinol Selectively Uncouples Expression and Activity of Estrogen Receptor Subtypes in Human Breast Cancer Cells

Estrogen-responsive breast cancer cells, such as MCF7 and T47D cells, express both estrogen receptor (ER)-α (ERα) and ERβ. Indole-3-carbinol (I3C) strongly down-regulated ERα protein and transcript levels, without altering the level of ERβ protein, in both cell lines. In cells transfected with the ERα promoter linked to a luciferase gene reporter, I3C ablated ERα promoter activity. Propyl pyrazole triol (PPT) is a highly selective ERα agonist, whereas, 17β-estradiol activates both ERα and ERβ. I3C treatment inhibited the PPT- and 17β-estradiol-induced proliferation of breast cancer cells, disrupted the PPT and 17β-estradiol stimulation of estrogen response element (ERE)-driven reporter plasmid activity as well as of endogenous progesterone receptor transcripts. Using an in vitro ERE binding assay, I3C was shown to inhibit the level of functional ERα and stimulated the level of ERE binding ERβ even though the protein levels of this receptor remained constant. In ERα−/ERβ+ MDA-MB-231 breast cancer cells, I3C treatment stimulated a 6-fold increase in binding of ERβ to the ERE. I3C also induced ERE- and activator protein 1-driven reporter plasmid activities in the absence of an ER agonist, suggesting that ERβ is activated in indole-treated cells. Taken together, our results demonstrate that the expression and function of ERα and ERβ can be uncoupled by I3C with a key cellular consequence being a significantly higher ERβ:ERα ratio that is generally highly associated with antiproliferative status of human breast cancer cells.

Targeting cyclin D1, a downstream effector of INI1/hSNF5, in rhabdoid tumors

Rhabdoid tumors (RTs) are aggressive and currently incurable pediatric malignancies. INI1/hSNF5 is a tumor suppressor biallelically inactivated in RTs. Our previous studies have indicated that cyclin D1 is a key downstream target of INI1/hSNF5 and genesis and/or survival of RTs in vivo is critically dependent on the presence of cyclin D1. In this report, we have tested the hypothesis that therapeutic targeting of cyclin D1 is an effective means of treating RTs. We found that RNA interference of cyclin D1 in rhabdoid cells was sufficient to induce G1 arrest and apoptosis. Furthermore, we found that pharmacological intervention with low micromolar concentrations of N-(4-hydroxyphenyl)retinamide (4-HPR), which downmodulates cyclin D1, induced G1 arrest and apoptosis in rhabdoid cell lines. 4-HPR in combination with 4-hydroxy-tamoxifen (4OH-Tam), synergistically inhibited survival as well as anchorage-dependent and -independent growth of rhabdoid cells and caused synergistic induction of cell cycle arrest and apoptosis. 4-HPR and tamoxifen exhibited synergistic growth inhibition of RTs in xenograft models in vivo. The effects of combination of drugs were correlated to the depletion of cyclin D1 levels both in in vitro and in vivo tumor models. These results demonstrate that 4-HPR and tamoxifen are effective chemotherapeutic agents for RTs. We propose that downmodulation of cyclin D1 is a novel and effective therapeutic strategy for RTs.

Cyclin D1 expression is dependent on estrogen receptor function in tamoxifen-resistant breast cancer cells

The development of resistance to tamoxifen, the most common antiestrogen used in the treatment of breast cancer, is a frequent and severe clinical problem. Tamoxifen-resistant tumors are still capable of responding to other hormonal therapies such as those that downregulate estrogen receptor expression. Mechanisms leading to acquisition of tamoxifen-resistant but hormone-sensitive growth are not completely understood. In tamoxifen-sensitive breast cancer cells, tamoxifen inhibits, whereas estrogen induces, expression of cyclin D1, a key cell cycle regulatory protein. Ectopic expression of cyclin D1 can lead to antiestrogen resistance. Thus, to determine whether cyclin D1 is involved in the growth of tamoxifen-resistant cells, we developed several tamoxifen-resistant variants from MCF-7 cells. These variants grow in the absence of estrogen or in the presence of tamoxifen, but their growth is inhibited by estrogen receptor downregulators. We show here that cyclin D1 expression is maintained at comparable levels in all tamoxifen-resistant variants, whereas pS2, another estrogen-regulated protein, is not. The addition of physiological levels of estrogen further stimulates cyclin D1 expression and proliferation. In contrast, treatment with estrogen receptor downregulators decreases cyclin D1 expression and proliferation. Thus, changes in cyclin D1 expression upon second-line hormonal therapy may predict hormonal sensitivity of tamoxifen-resistant tumors. These studies suggest that estrogen receptor mediates cyclin D1 expression and growth of tamoxifen-resistant tumors.

Cell proliferation, apoptosis, and expression of cyclin D1 and cyclin E as potential biomarkers in tamoxifen-treated mammary tumors

Tamoxifen has been widely used for treatment, and more recently, for the prevention of breast cancer. Since breast carcinomas are composed of heterogeneous populations of estrogen receptor-positive (ER+) cells, we hypothesized that tamoxifen may suppress tumor growth by differentially affecting cell proliferation and apoptosis. ER+ mammary tumors were induced in Sprague-Dawley rats by N-methyl-N-nitrosourea (MNU) and when they became palpable, the animals were treated for 5, 10, or 20 days with tamoxifen, 1.0 mg/kg body weight. Tamoxifen induced a time-dependent decrease in proliferating (BrdU-labeled) cells, arrested the cells in G1/0 phase, and differentially decreased the cyclin E and cyclin D1 expression at mRNA and protein levels. In the same tumors, apoptotic cells increased during the first 10 days of treatment, but their number remained unchanged with extension of the treatment to 20 days. Thus, we provide data that tamoxifen may differentially affect cell proliferation and apoptosis in mammary tumors and that the expression levels of cyclin D1 and cyclin E might also be considered potential intermediate biomarkers of response of mammary tumors to tamoxifen and possibly to other selective estrogen receptor modulators (SERMs).

Expression of cyclin D1/2 in the lungs of strain A/J mice fed chemopreventive agents

Male strain A mice were fed a diet containing chemopreventive agents. After 1 and 3 weeks on the diets, lung nuclear fractions were examined for expression of cyclin D1/2 with western blot analysis. In animals fed a diet containing a mixture of myoinositol and dexamethasone, a treatment found previously to be effective in preventing the development of tobacco smoke-induced lung tumors in A/J mice, cyclin D1/2 expression was reduced to 30-40% of control levels. A similar decrease in cyclin D1/2 expression was found when animals were fed either myoinositol or dexamethasone alone. Paradoxically, tobacco smoke by itself had a similar effect on cyclin D1/2 expression. On the other hand, several agents that had been previously found not to be effective against tobacco smoke carcinogenesis [phenethyl isothiocyanate, 1,4-phenylenebis(methylene)selenoisocyanate, N-acetylcysteine, acetylsalicylic acid, D-limonene and beta carotene] did not decrease cyclin D1/2 expression after 1 or 3 weeks of feeding. It was concluded that expression of cyclin D1/2 might be a potentially useful marker in the identification of chemopreventive agents for tobacco smoke and could be of some help in the evaluation of their effects.