Loss of growth inhibitory effects of retinoic acid in human breast cancer cells following long-term exposure to retinoic acid

Paraben Toxicology

Parabens now being formally declared as the American Contact Dermatitis Society (non)allergen of the year, the allergologic concerns regarding parabens raised during the past century are no longer a significant issue. The more recent toxicological concerns regarding parabens are more imposing, stemming from the gravity of the noncutaneous adverse health effects for which they have been scrutinized for the past 20 years. These include endocrine activity, carcinogenesis, infertility, spermatogenesis, adipogenesis, perinatal exposure impact, and nonallergologic cutaneous, psychologic, and ecologic effects. To assert that parabens are safe for use as currently used in the cosmetics, food, and pharmaceutical industries, all toxicological end points must be addressed. We seek to achieve perspective through this exercise: perspective for the professional assessing systemic risk of parabens by all routes of exposure. The data reviewed in this article strive to provide a balanced perspective for the consumer hopefully to allay concerns regarding the safety of parabens and facilitate an informed decision-making process. Based on currently available scientific information, claims that parabens are involved in the genesis or propagation of these controversial and important health problems are premature. Haste to remove parabens from consumer products could result in their substitution with alternative, less proven, and potentially unsafe alternatives, especially given the compelling data supporting the lack of significant dermal toxicity of this important group of preservatives.

Proteomics to Predict Loss of RXR-γ During Progression of Epithelial Ovarian Cancer

Retinoid and rexinoid receptors are known to regulate key processes during development, differentiation, and cell death in vertebrates. However, their contributions to progression of malignant disease remain largely elusive although it is realized that transformed cancer cells, which essentially evade apoptosis, may display altered molecular expressions or functions associated with retinoid signaling. Here, using a progression model of ovarian cancer, we describe a proteomics-based approach including experimental procedures toward identification and validation of altered protein profiles during transformation. Effectively, this specifies loss of RXR-γ during progression of epithelial ovarian cancer.

Sumoylation of TCF21 downregulates the transcriptional activity of estrogen receptor-alpha

Aberrant estrogen receptor-α (ERα) signaling is recognized as a major contributor to the development of breast cancer. However, the molecular mechanism underlying the regulation of ERα in breast cancer is still inconclusive. In this study, we showed that the transcription factor 21 (TCF21) interacted with ERα, and repressed its transcriptional activity in a HDACs-dependent manner. We also showed that TCF21 could be sumoylated by the small ubiquitin-like modifier SUMO1, and this modification could be reversed by SENP1. Sumoylation of TCF21 occurred at lysine residue 24 (K24). Substitution of K24 with arginine resulted in complete abolishment of sumoylation. Sumoylation stabilized TCF21, but did not affect its subcellular localization. Sumoylation of TCF21 also enhanced its interaction with HDAC1/2 without affecting its interaction with ERα. Moreover, sumoylation of TCF21 promoted its repression of ERα transcriptional activity, and increased the recruitment of HDAC1/2 to the pS2 promoter. Consistent with these observations, sumoylation of TCF21 could inhibit the growth of ERα-positive breast cancer cells and decreased the proportion of S-phase cells in the cell cycle. These findings suggested that TCF21 might act as a negative regulator of ERα, and its sumoylation inhibited the transcriptional activity of ERα through promoting the recruitment of HDAC1/2.

Biological reprogramming in acquired resistance to endocrine therapy of breast cancer

Endocrine therapies targeting the proliferative effect of 17β-estradiol through estrogen receptor α (ERα) are the most effective systemic treatment of ERα-positive breast cancer. However, most breast tumors initially responsive to these therapies develop resistance through molecular mechanisms that are not yet fully understood. The long-term estrogen-deprived (LTED) MCF7 cell model has been proposed to recapitulate acquired resistance to aromatase inhibitors in postmenopausal women. To elucidate this resistance, genomic, transcriptomic and molecular data were integrated into the time course of MCF7-LTED adaptation. Dynamic and widespread genomic changes were observed, including amplification of the ESR1 locus consequently linked to an increase in ERα. Dynamic transcriptomic profiles were also observed that correlated significantly with genomic changes and were predicted to be influenced by transcription factors known to be involved in acquired resistance or cell proliferation (for example, interferon regulatory transcription factor 1 and E2F1, respectively) but, notably, not by canonical ERα transcriptional function. Consistently, at the molecular level, activation of growth factor signaling pathways by EGFR/ERBB/AKT and a switch from phospho-Ser118 (pS118)- to pS167-ERα were observed during MCF7-LTED adaptation. Evaluation of relevant clinical settings identified significant associations between MCF7-LTED and breast tumor transcriptome profiles that characterize ERα-negative status, early response to letrozole and tamoxifen, and recurrence after tamoxifen treatment. In accordance with these profiles, MCF7-LTED cells showed increased sensitivity to inhibition of FGFR-mediated signaling with PD173074. This study provides mechanistic insight into acquired resistance to endocrine therapies of breast cancer and highlights a potential therapeutic strategy.

Use of global gene expression patterns in mechanistic studies of oestrogen action in MCF7 human breast cancer cells

Over the years, the MCF7 human breast cancer cell line has provided a model system for the study of cellular and molecular mechanisms in oestrogen regulation of cell proliferation and in progression to oestrogen and antioestrogen independent growth. Global gene expression profiling has shown that oestrogen action in MCF7 cells involves the coordinated regulation of hundreds of genes across a wide range of functional groupings and that more genes are downregulated than upregulated. Adaptation to long-term oestrogen deprivation, which results in loss of oestrogen-responsive growth, involves alterations to gene patterns not only at early time points (0-4 weeks) but continuing through to later times (20-55 weeks), and even involves alterations to patterns of oestrogen-regulated gene expression. Only 48% of the genes which were regulated > or =2-fold by oestradiol in oestrogen-responsive cells retained this responsiveness after long-term oestrogen deprivation but other genes developed de novo oestrogen regulation. Long-term exposure to fulvestrant, which resulted in loss of growth inhibition by the antioestrogen, resulted in some very large fold changes in gene expression up to 10,000-fold. Comparison of gene profiles produced by environmental chemicals with oestrogenic properties showed that each ligand gave its own unique expression profile which suggests that environmental oestrogens entering the human breast may give rise to a more complex web of interference in cell function than simply mimicking oestrogen action at inappropriate times.

Changes in oestrogen receptor-alpha and -beta during progression to acquired resistance to tamoxifen and fulvestrant (Faslodex, ICI 182,780) in MCF7 human breast cancer cells

Cell culture models of antioestrogen resistance often involve applying selective pressures of oestrogen deprivation simultaneously with addition of tamoxifen or fulvestrant (Faslodex, ICI 182,780) which makes it difficult to distinguish events in development of antioestrogen resistance from those in loss of response to oestrogen or other components. We describe here time courses of loss of antioestrogen response using either oestrogen-maintained or oestrogen-deprived MCF7 cells in which the only alteration to the culture medium was addition of 10(-6) M tamoxifen or 10(-7) M fulvestrant. In both oestrogen-maintained and oestrogen-deprived models, loss of growth response to tamoxifen was not associated with loss of response to fulvestrant. However, loss of growth response to fulvestrant was associated in both models with concomitant loss of growth response to tamoxifen. Measurement of oestrogen receptor alpha (ERalpha) and oestrogen receptor beta (ERbeta) mRNA by real-time RT-PCR together with ERalpha and ERbeta protein by Western immunoblotting revealed substantial changes to ERalpha levels but very little alteration to ERbeta levels following development of antioestrogen resistance. In oestrogen-maintained cells, tamoxifen resistance was associated with raised levels of ERalpha mRNA/protein. However by contrast, in oestrogen-deprived MCF7 cells, where oestrogen deprivation alone had already resulted in increased levels of ERalpha mRNA/protein, long-term tamoxifen exposure now reduced ERalpha levels. Whilst long-term exposure to fulvestrant reduced ERalpha mRNA/protein levels in the oestrogen-maintained cells to a level barely detectable by Western immunoblotting and non-functional in inducing gene expression (ERE-LUC reporter or pS2), in oestrogen-deprived cells the reduction was much less substantial and these cells retained an oestrogen-induction of both the ERE-LUC reporter gene and the endogenous pS2 gene which could still be inhibited by antioestrogen. This demonstrates that whilst ERalpha can be abrogated by fulvestrant and increased by tamoxifen in some circumstances, this does not always hold true and mechanisms other than alteration to ER must be involved in the development of antioestrogen resistant growth.

Comparison of the global gene expression profiles produced by methylparaben,n-butylparaben and 17β-oestradiol in MCF7 human breast cancer cells

Since the alkyl esters of p-hydroxybenzoic acid (parabens) can be measured intact in the human breast and possess oestrogenic properties, it has been suggested that they could contribute to an aberrant burden of oestrogen signalling in the human breast and so play a role in the rising incidence of breast cancer. However, although parabens have been shown to regulate a few single genes (reporter genes, pS2, progesterone receptor) in a manner similar to that of 17beta-oestradiol, the question remains as to the full extent of the similarity in the overall gene profile induced in response to parabens compared with 17beta-oestradiol. The GE-Amersham CodeLink 20 K human expression microarray system was used to profile the expression of 19881 genes in MCF7 human breast cancer cells following a 7-day exposure to 5 x 10(-4) M methylparaben, 10(-5) M n-butylparaben and 10(-8) M 17beta-oestradiol. At these concentrations, the parabens gave growth responses in MCF7 cells of similar magnitude to 17beta-oestradiol. The study identified genes which are upregulated or downregulated to a similar extent by methylparaben, n-butylparaben and 17beta-oestradiol. However, the majority of genes were not regulated in the same way by all three treatments. Some genes responded differently to parabens from 17beta-oestradiol, and furthermore, differences in expression of some genes could be detected even between the two individual parabens. Therefore, although parabens possess oestrogenic properties, their mimicry in terms of global gene expression patterns is not perfect and differences in gene expression profiles could result in consequences to the cells that are not identical to those following exposure to 17beta-oestradiol.

Insulin-like growth factor binding protein-3 prevents retinoid receptor heterodimerization: implications for retinoic acid-sensitivity in human breast cancer cells

Insulin-like growth factor binding protein-3 (IGFBP-3) has both IGF-dependent and -independent effects on cell growth, which are frequently growth-inhibitory. Interestingly, the development of a more aggressive phenotype in breast cancer cells (BCCs) correlates positively with elevated expression of IGFBP-3 and is often associated with all-trans-retinoic acid (atRA)-resistance. IGFBP-3 was previously demonstrated to interact directly with retinoid X receptor (RXR). In this study we have shown that IGFBP-5 also interacts with RXR and that both IGFBPs interact with retinoic acid receptor (RAR). To investigate whether the presence of IGFBP-3 regulates breast cancer cell responsiveness to atRA, we immuno-neutralized the IGFBP-3 expressed by the atRA-resistant Hs578T and MDA-MB-231 BCCs (which express IGFBP-3 constitutively) and showed that they become more sensitive to the growth-inhibitory effects of atRA. Similarly, in Hs578T cells expressing a reporter gene under the control of an RAR response element (RARE), depletion of IGFBP-3 resulted in the induction of reporter gene expression in response to atRA. In investigating possible mechanisms for IGFBP-3 regulation of atRA-sensitivity, we found that IGFBP-3 blocked the formation of RAR:RXR heterodimers and disrupted the ligand-inducible receptor complex. Thus, IGFBP-3 has the potential to reduce the RARE-mediated transactivation of target genes and modulate the atRA-response in BCCs.

Linkage between retinoid and fatty acid receptors: implications for breast cancer prevention

Certain dietary retinoids and polyunsaturated fatty acids (PUFAs) consistently inhibit progression of mammary carcinogenesis both in animal studies and cell culture, but clinically, their effect is inconsistent. New evidence of synergistic interaction between the nuclear receptors for the two groups of nutritional agents suggests that appropriate selective ligands from each group might be combined in breast cancer chemoprevention studies. Peroxisome proliferator-activated receptor (PPAR) gamma is a nuclear receptor that is activated by PUFAs, eicosanoids and antidiabetic agents such as troglitazone. Such activation can cause growth inhibition in human mammary cancer cells in culture and the effect is enhanced by ligands of retinoic acid receptor (RAR) and retinoid X receptor (RXR). In mouse mammary tissue in organ culture, an RXR-selective ligand has been shown to enhance the effect of troglitazone in suppressing carcinogen-induced pre-neoplastic changes. A PPAR/RXR heterodimer is involved in tumour growth inhibition and has been shown to bind directly to nuclear oestrogen response elements (ERE) independently of oestrogen receptor (ER) activity. A combination of an RXR-selective retinoid with either troglitazone or else a long-chain n-3 PUFA, is proposed for a short-term study in postmenopausal women after primary surgery for intraductal breast cancer. The resulting activation of PPAR/RXR expression may increase response to retinoid administration, especially in the presence of obesity and insulin resistance, because of the ability of PPAR gamma ligands to reduce insulin-like growth factor I (IGF-I) concentrations. Serial core biopsies of breast tissue over a short term are proposed to identify changes in phenotype, which may influence progression to invasiveness. In addition to cytomorphological criteria, expression of ER alpha and beta, RAR alpha and beta, and IGF-I receptor in the nucleus should be examined.

Insulin-like growth factor receptor levels are regulated by cell density and by long term estrogen deprivation in MCF7 human breast cancer cells

This work describes a reciprocal relationship between cell density and levels of insulin-like growth factor receptors (IGFR) in MCF7 human breast cancer cells, which adds a new dimension to the mechanism of cross-talk between estrogen and insulin-like growth factors in the regulation of breast cancer cell growth. The reduced binding of both (125)I-IGF1 and alphaIR3 anti-IGFR antibody to whole cells showed that IGFR are lost from the surface of MCF7 cells as cell density increases, and this occurred irrespective of the presence or absence of estradiol. Western immunoblotting further confirmed loss of type I IGFR from MCF7 cells with increasing cell density. Long term estrogen deprivation was found to increase the levels of IGFR at all cell densities, such that after 96 weeks of estrogen deprivation, IGFR levels had become similar at the highest cell density in the absence of estradiol to the IGFR levels at the lowest cell density in the estrogen-maintained cells, and the levels of IGFR could be increased still further by estradiol. This overexpression of IGFR in the estrogen-deprived cells correlated with a reversal of response to exogenously added ligand, in that concentrations of insulin, IGFI, and IGFII that had stimulated growth of the estrogen-maintained cells became growth inhibitory to the estrogen-deprived cells. Blockade of the IGFIR with the alphaIR3 anti-IGFR antibody could partially inhibit the growth of the estrogen-deprived cells, suggesting that up-regulation of IGFR in these cells may contribute to the mechanism of adaptation to growth in steroid-deprived conditions which results in progression to estrogen independence of cell growth.