Length increase of the side chain of idoxifene does not improve its antagonistic potency in breast-cancer cell lines

The terminal substituents of 7α, 6-hexanyl derivatives of estradiol determine their selective estrogen receptor modulator versus agonist activities

Pure antiestrogens were clinically developed as alternative therapies for estrogen receptor (ER) positive breast cancers. Unlike the selective estrogen receptor modulators (SERMs), these antiestrogens are devoid of tissue-specific ER agonist activity. Many of these compounds are steroidal in nature, containing an estradiol (E2) structural core with long alkyl side chains at the C-7α position. Two novel 7α-substituted E2 derivatives were evaluated that retain high binding affinity for ER. Compared to known pure antiestrogens, these compounds, referred to as compound 13 (C13) and C14, contain shorter 7α alkyl side chains and differ only in their terminal substituent: a hydroxyl moiety versus a benzyloxy group, respectively. Herein we assessed the effects of these compounds on ER transcriptional activity and report that despite their similar overall structure, C13 and C14 produce distinct cell type-specific responses. Of note, C13 functions as a mixed agonist/antagonist in Hela cells, inducing only weak ER transcriptional activity while preventing coactivator recruitment and stabilizing ER expression. However, this compound effectively stimulates ER activity in MCF-7 cells, does not increase ER levels and promotes cell proliferation on par with E2. Conversely, C14 stimulates transcriptional activity in both cell types and enhances ER-coactivator interactions. The activities of both compounds were inhibited by the pure antiestrogen ICI 182,780. Taken together, these results reveal that C13 is a SERM while C14 is an ER agonist, and indicate that the terminal modification of the C-7α hexanyl side chain of these estradiol derivatives is an important determinant of the biocharacter of these ER ligands.

Mechanisms governing the accumulation of estrogen receptor alpha in MCF-7 breast cancer cells treated with hydroxytamoxifen and related antiestrogens

This study aimed at a better understanding of estrogen receptor alpha (ER) up regulation induced by partial estrogen antagonists. Effect of treatment with hydroxytamoxifen (OH-Tam) on ER level in MCF-7 cells was investigated by an approach combining ER measurement (enzyme immunoassay) and morphological demonstration (immunofluorescence). Furthermore, the influence of drug exposure on the rates of ER synthesis and degradation was assessed by determining [35S]methionine incorporated into the receptor in different experimental conditions (measurement of synthesis or pulse-chase experiments). ER up regulation was already induced by a 1-h pulse treatment with OH-Tam, thus a continuous exposure was not required. This process appeared reversible (i.e. ER accumulation due to OH-Tam rapidly vanished upon subsequent exposure to 17beta-estradiol (E2) or the pure antiestrogen RU 58668). While OH-Tam did not affect the rate of [35S]methionine incorporation into ER, it clearly caused an impairment of ER degradation (pulse-chase experiments) indicating that up regulation results from a stabilization of the receptor associated with the maintenance of its synthesis. Various tamoxifen derivatives, as well as a few related partial antiestrogens, were compared on the basis of binding ability and propensity to induce ER up regulation. A close relationship was found between both properties. Structure-activity analysis revealed that the capacity of these compounds to induce ER up regulation is associated with characteristics of their aminoalkyle side-chain, similar to those required for antiestrogenicity.

N-Alkoxypyrazoles as biomimetics for the alkoxyphenyl group in tamoxifen

The preparation of a series of novel analogues of the selective antiestrogen tamoxifen is reported. 1Z-alkoxyphenyl group in tamoxifen has been replaced by a N-alkoxypyrazole, while functionalised phenyl groups or heteroaromatics were introduced at the 2Z-position using sequential Suzuki cross coupling of 1,2-(bis)borylpinacol 1-phenylbutene with 4- or 5-iodo-1-N,N-dimethylaminoethyl or propyl-pyrazoles. Approximately 50 tamoxifen analogues were obtained and tested in an estrogen receptor (ERalpha) affinity assay. Several compounds exhibited binding affinities 2-5-fold lower than tamoxifen. Dose-response experiments with six selected compounds were carried out using two different human breast cancer cell lines, MCF-7 and the tamoxifen resistant cell line MCF-/TAM(R)-1. Both cell lines exhibited growth inhibition upon treatment with the tamoxifen analogues. Co-treatment of the cells, with estradiol and the individual compounds, were also performed. The results indicated that the observed growth inhibitory effect was mediated by the ERalpha. Analogues of the potent antiestrogen 4-hydroxytamoxifen (4-OHT) were synthesised where the 1E-4-hydroxyphenyl was replaced by a 1-hydroxypyrazol-4-yl group. However, modest growth inhibition of MCF-7 cells was observed upon treatment with these analogues. In contrast, 1Z-, 2Z-ringclosed tamoxifen analogue (59) was shown to possess antiproliferative effects on MCF-7 and MCF-/TAM(R)-1 cells in lower doses than tamoxifen.

Evidence of an estrogen receptor form devoid of estrogen binding ability in MCF-7 cells

In MCF-7 breast cancer cells, hydroxytamoxifen (OH-Tam) up-regulates the estrogen receptor (ER) in a form unable to bind [(3)H]estradiol (E(2)). We show here that this property is not restricted to this antiestrogen. [(3)H]E(2) binding assays (whole cell assays, DCC assays on cell extracts) and enzyme immunoassays (Abbott) performed in parallel, establish the permanent presence of such unusual ERs in the absence of any exposure of the cells to a ligand. E(2) and the pure antiestrogen RU 58 668, which down-regulate ER, also decrease [(3)H]E(2) binding. In control cells, these ERs represent about the half of the whole receptor population; they also display a tendency to stabilize within the cell nucleus. Loss of E(2) binding ability appears irreversible, since we failed to label receptor accumulated under OH-Tam with [(3)H]E(2) or [(3)H]tamoxifen aziridine (TAZ). Cycloheximide (CHX), which blocks E(2)-induced down regulation of ER, failed to stabilize [(3)H]E(2) binding (whole cell assay) after an [(3)H]E(2) pulse (1 h), confirming that regulation of E(2) binding and peptide level are related to different regulatory mechanisms. Loss of binding ability could not be ascribed to any ER cleavage as demonstrated by Western blotting with a panel of ER antibodies raised against its various domains (67 kDa ER solely detected). We propose that loss of E(2) binding ability is related to the aging process of the receptor, i.e. it is progressively converted to a form devoted to degradation after it has accomplished its physiological role. Ligands may favor (E(2), RU 58 668) or impede (OH-Tam) this elimination process.