Chromatin remodeling in hormone-dependent and -independent breast cancer cell lines.

Chromatin restricts the accessibility of DNA to regulatory factors; its remodelling over the regulatory regions contributes to the control of gene expression. An increasing number of evidence links defects in chromatin remodelling machinery and cancer. Our aim is to elucidate the role of chromatin structure in the control of the expression of hormone-induced genes in breast cell lines oestrogen-dependent or -independent for growth. Mammary tumour growth is controlled by steroid hormones via their nuclear receptor and by growth factors via tyrosine kinase receptors. 50% of these tumours elude to hormonal control. This limits the anti-oestrogen therapy. As a model, we have analysed in several cell lines the chromatin organisation of the regulatory regions of two genes, pS2 that is associated with a good prognostic, and cathepsin D (catD) that is a bad prognostic marker. The expression of the two genes is oestrogen-regulated in oestrogen-dependent cell line MCF7. In contrast in the hormone-independent cell line MDA MB 231, pS2 is not expressed and catD is constitutively expressed. Within the regulatory regions of pS2 gene, we have localised two regions that undergo a hormone-dependent change in chromatin structure in MCF7 cells but not in MDA MB 231 and that can be correlated with gene expression. In contrast catD regulatory regions did not display hormone-dependent changes in chromatin structure, suggesting that hormone regulation takes place within regions with a constitutively open chromatin structure.

[Chromatin remodeling in estrogen-dependent and independent human breast cancer cell lines]. / Remodelage de la structure chromatinienne des régions régulatrices de deux gènes oestrogéno-régulés dans des lignées de cellules cancéreuses mammaires humaines.

Chromatin restricts the accessibility of DNA to regulatory factors; its remodeling over the regulatory regions contributes to the control of gene expression. An increasing number of evidence links defects in chromatin remodeling machinery and cancer. Our aim is to elucidate the role of chromatin structure in the control of the expression of hormone-induced genes in breast cell lines estrogen-dependent or -independent for growth. Mammary tumor growth is controlled by steroid hormones via their nuclear receptor and by growth factors via tyrosine kinase receptors. 50 % of these tumors elude to hormonal control. This limits the anti-estrogen therapy. As a model, we have analyzed in several cell lines the chromatin organization of the regulatory regions of two genes, pS2 that is associated with a good prognostic, and cathepsin D (catD) that is a bad prognostic marker. The expression of the two genes is estrogen-regulated in estrogen-dependent cell line MCF7. In contrast in the hormone-independent cell line MDA MB 231, pS2 is not expressed and catD is constitutively expressed. Within the regulatory regions of pS2 gene, we have localized two regions that undergo a hormone-dependent change in chromatin structure in MCF7 cells but not in MDA MB 231. The lack of chromatin remodeling in MDA MB 231 cells is not due to the absence of expression of the estrogen receptor in the cell line. The expression of pS2 gene can be correlated with chromatin remodeling over the regulatory regions of pS2 gene. In contrast catD regulatory regions did not display hormone-dependent changes in chromatin structure, suggesting that hormone regulation takes place within regions with a constitutively open chromatin structure.

Chromatin structure of the regulatory regions of pS2 and cathepsin D genes in hormone-dependent and -independent breast cancer cell lines.

We have compared the DNase I hypersensitivity of the regulatory region of two estrogen-regulated genes, pS2 and cathepsin D in hormone-dependent and -independent breast carcinoma cell lines. This strategy allowed the identification of two important control regions, one in pS2 and the other in cathepsin D genes. In the hormone-dependent MCF7 cell line, within the pS2 gene 5'-flanking region, we detected two major DNase I hypersensitive sites, induced by estrogens and/or IGFI: pS2-HS1, located in the proximal promoter and pS2-HS4, located -10.5 Kb from the CAP site, within a region that has not been cloned. The presence of these two DNase I hypersensitive sites correlates with pS2 expression. Interestingly in MCF7 cells, estrogens and IGFI induced indistinguishable chromatin structural changes over the pS2 regulatory region, suggesting that the two transduction-pathways converge to a unique chromatin target. In two cell lines that do not express pS2, MDA MB 231, a hormone-independent cell line that lacks the estrogen receptor alpha, and HE5, a cell line derived from MDA MB 231 by transfection that expresses estrogen receptor alpha, there was only one hormone-independent DNase I hypersensitive site. This site, pS2-HS2, was located immediately upstream of pS2-HS1. In MCF7 cells, two major DNase I hypersensitive sites were present in the 5'-flanking sequences of the cathepsin D gene, which is regulated by estrogens in these cells. These sites, catD-HS2 and catD-HS3, located at positions -2.3 Kb and -3.45 Kb, respectively, were both hormone-independent. A much weaker site, catD-HS1, covered the proximal promoter. In MDA MB 231 cells, that express cathepsin D constitutively, we detected an additional strong hormone-independent DNase I hypersensitive site, catD-HS4, located at position -4.3 Kb. This region might control the constitutive over-expression of cathepsin D in hormone-independent breast cancer cells. All together, these data demonstrate that a local reorganization of the chromatin structure over pS2 and cathepsin D promoters accompanies the establishment of the hormone-independent phenotype of the cells.

Characterization of the membranous antiestrogen binding protein: I. Partial purification of the protein in its active state.

We previously demonstrated that, in addition to the estrogen receptor, the Antiestrogen Binding Site (ABS) is also a potent mediator of the antitumorous activity of the clinical drug tamoxifen. Because of report discrepancies in the binding parameters of rat liver ABS we first attempted to improve binding study conditions. In this way buffer, protein concentration, methodology for bound/free ligand separation and phospholipidic ratio were determined. This work was used to evaluate the Stoke radius (4.4 S) and isoelectric point (pH = 6.6) of the protein in its native state. These studies constituted the obligatory transition from rat liver to pure ABS protein.

Characterization of the membranous antiestrogen binding protein: II. Purification to homogeneity.

Our knowledge of the biological role of the antiestrogen binding site ABS in the antitumoral activity of tamoxifen, will be increased with the determination of its coding gene sequence. To this end our team has for some time attempted to purify this membranous protein. In this work we report the purification to homogeneity of ABS from rat liver in a six step succession. Specific photolabeling with a tritiated photoprobe, solubilization of rat liver microsomes, chromatofocusing of the labeled proteins, preparative electrophoresis on polyacrylamide gel, and two consecutive high performance liquid chromatography separations on C4 hydrophobic resin produced 2.5 micrograms of pure ABS by silver stain analysis of SDS-PAGE. The NH2-terminal residue of the protein appears to be blocked, which hinders the Edman degradation method for obtention of the whole protein sequence.

Antiestrogens inhibit the replication of the retroviral Moloney murine leukemia virus in vitro.

Widely used in breast cancer therapy, tamoxifen exhibits in vitro and in vivo pleiotropic activities that are generally attributed to its binding to the estrogen receptor. However, several reports have shown that the antiestrogen binding site (ABS) is also an intracellular target of the drug. This dual affinity determines at least two modes of action for the triphenylethylenic antiestrogens; one would be estrogen reversible and the other irreversible. Here, tamoxifen is shown to inhibit the production of Moloney murine leukemia virus virions by fibroblastic A9 cells, in which estrogen receptor is not detectable either by binding or by radioimmunoassay. Moreover, a specific ligand of the ABS induces effects equivalent to those of tamoxifen, suggesting that tamoxifen inhibits Moloney murine leukemia virus replication through an estrogen-independent pathway involving the ABS.

Cytosolic type II estrogen binding site in rat uterus: specific photolabeling with estrone.

A low affinity (Kd = 30 nM), large capacity (Bmax = 2.6 pmol/g tissue) estrogen binding site was photolabeled from estradiol-stimulated rat uterus cytosol. To maximize levels of this binding site and reduce those of the type I binding site, ovariectomized rats were injected with high doses of estradiol (10 micrograms per day) for four days with the last injection two hours before sacrifice. This treatment depleted type I estrogen receptors from the cytosol (by 90%) and raised levels of type II sites in the nucleus without affecting cytosolic type II levels. The type II estradiol binding sites were distinguished from the type I sites on the basis of their dissociation kinetics, pH-sensitivity and their behavior towards potassium chloride, somatostatin, sodium thiocyanate, sulfhydryl reagents and ammonium sulfate precipitation. These type II binding sites could be covalently photolabeled with tritiated estrone. A molecular weight of 43 kDa was found on SDS PAGE.

A potent and selective photoaffinity probe for the anti-estrogen binding site of rat liver.

The anti-estrogen binding site (ABS) is an apparently ubiquitous component of cells that has been shown to be intimately linked with the antiproliferative effects of certain antiestrogenic compounds, like tamoxifen, which is currently used for the treatment of breast cancer. However, the identification and in vitro study of this novel protein has been hampered to date by a lack of convenient probes that will efficiently label the molecule in nonpurified preparations. Thus, using a selective ABS ligand (4-benzylphenoxy-N-ethylmorpholine, MBPE) as starting material, we synthesized a photosensitive azido derivative, [(2-azido-4-benzyl)phenoxy]N-ethylmorpholine (azido-MBPE) that can be prepared in a tritiated form. Azido-MBPE has a high affinity for ABS (Kd = 3 nM), identical to that of tamoxifen, and covalently labels 5 and 12% of membrane-bound and detergent-solubilized ABS, respectively. Its incorporation is selectively and competitively inhibited by other ABS ligands (tamoxifen greater than nitromifen greater than hydroxytamoxifen). [3H]Azido-MBPE potently photolabels either membrane-bound or detergent-solubilized ABS as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under denaturing conditions revealing specific photoincorporation in a protein band of Mr = 40,000. This molecular weight is approximately two times lower than what we observed previously for ABS preparations studied under nondenaturing conditions and postlabeled with [3H]tamoxifen (Mr = 80,000-110,000). In chromatofocusing experiments with photolabeled ABS, a single specifically labeled protein fraction migrating with a pI of 6.4 was found to exhibit a Mr of 40,000 when subsequently electrophoresed on sodium dodecyl sulfate-polyacrylamide gels. These results indicate that [3H]azido-MBPE is a specific high affinity probe of ABS that will prove useful in the ultimate identification of this protein.