Search

Cathepsin D is a lysosomal protease produced and secreted in excess by most human breast cancer cells. In MCF7 cells, estrogens stimulate cathepsin D expression at the mRNA level via a mechanism independent of de novo protein synthesis. We have isolated the human cathepsin D gene and its 5' flanking sequences from a MCF7 genomic library. To demonstrate its transcriptional estrogen regulation, we constructed chimeric recombinants bearing different fragments of the cathepsin D gene 5' proximal region inserted in front of the bacterial chloramphenicol acetyl transferase reporter gene. By transient cotransfection with the estrogen receptor expression vector into MCF7 cells, we showed that a 240 bp fragment located in the 5' proximal region of the gene was able to mediate transcriptional estrogen activation. This induction was concentration-dependent and suppressed by the antiestrogen ICI 164, 384.

Cathepsin D, a lysosomal protease, is induced by estrogens in hormone responsive breast cancer cells, by progesterone in normal endometrium and expressed at high constitutive levels in estrogen receptor (ER)-negative cells. To investigate whether ER is the only transacting factor missing in ER negative cells to obtain estrogen regulation, we transfected an ER cDNA expression vector (HEO) into ER-negative Hela cells and showed that it could recover estrogen sensitivity for cathepsin D gene expression but not for cell growth regulation. These results show i. that the expression of an endogenous gene in humans can be stimulated by estradiol after ER supplementation, indicating that Hela cells contain sufficient amounts of the other transcription factors required for cathepsin D estrogen inducibility; ii. that cathepsin D expression is stimulated by estrogens in this cervix cancer cell line, as it is in the ER-positive breast cancer cells, which contrasts with its regulation by progesterone in normal endometrial cells.

In MCF7 cells, transcription of the lysosomal protease cathepsin D is stimulated by estrogens via a non-consensus estrogen responsive element (ERE). By contrast, in estrogen responsive Ishikawa endometrial cancer cells, the cathepsin D gene is unresponsive to estrogens. We now show that the transfected cathepsin D promoter, which can be induced by estrogens in several cell types, is insensitive in Ishikawa cells. The block is not due to a mutation in the cathepsin D promoter or estrogen receptor, but involves the cathepsin D ERE, and implies a C at position 3 of the ERE sequence. Our results suggest that in Ishikawa cells, cathepsin D insensitivity to estrogen most likely occurs through a specific interaction with the ER, or with an endometrial factor which may compete with the ER for binding to the cathepsin D ERE.

BRCA1, the susceptibility gene for hereditary breast-ovarian cancer, is located on chromosome 17q12-21 but has not yet been identified. Two tandem oestradiol 17 beta hydroxysteroid dehydrogenase genes (17HSD) are assigned to this region. The active 17HSDII gene encodes the normal enzyme which regulates local synthesis of oestrogens, whereas 17HSDI is considered to be a pseudogene. We used reverse transcription coupled to polymerase chain reaction (RT-PCR) and found that the 17HSDI gene was also transcribed in half of the human cell lines and most of the biopsies studied, suggesting that 17HSDI could modulate normal 17HSDII activity in oestrogen target cells. We hypothesize that altered 17HSDI gene expression could lead to both hereditary and/or sporadic breast cancer by increasing local oestrogen concentration and that it is still a potential candidate for BRCA1.

We previously demonstrated that antiestrogen 4-hydroxytamoxifen (OH-Tam) blocks the mitogenic activity of growth factors in breast cancer. We now investigate this mechanism by evaluating how OH-Tam affects growth factor binding and receptor tyrosine kinase activity. We show here that OH-Tam has an opposite effect on epidermal growth factor (EGF) and insulin-like growth factor-1 (IGF-1) binding in estrogen receptor (ER) positive cells. A decrease in IGF-1 binding sites may explain the reduced IGF-I mitogenic effect, whereas an increase in high affinity EGF binding associated with a decrease in in vitro receptor autophosphorylation rather favors the possibility of an alteration in EGF receptor tyrosine kinase activity. We conclude that OH-Tam may prevent growth factor action in ER+ cells both by modulating the concentration of growth factor binding sites and by altering growth factor receptor functionality.

The synthesis and secretion of pro-cathepsin D is increased by estrogens in MCF7 cells. We quantified the effect of estradiol on other lysosomal enzymes in order to investigate the mechanism of this hypersecretion. Precursors of β-hexosaminidase, cathepsin B and β-galactosidase, which are routed to lysosomes via the mannose-6-phosphate (Man-6-P) receptor, were secreted in much lower amounts than pro-cathepsin D, but their secretion was also increased by estradiol. The activity of acid phosphatase, which is routed to lysosomes via a different transmembrane mechanism, was not altered by estradiol. While estradiol stimulated gene expression of pro-cathepsin D, it had no effect on that of procathepsin B. We conclude that estradiol stimulates the secretion of several lysosomal pro-enzymes in MCF7 cells, suggesting that a general mechanism is responsible for this derouting rather than a specific alteration of cathepsin D structure.

An estrogen-induced 52-kDa glycoprotein secreted by human breast cancer cells and able to autostimulate the growth of MCF7 cells has been purified, using monoclonal antibodies, and characterized. The protein contains mannose 6-phosphate signals on its N-linked high-mannose chains, suggesting that it is a lysosomal enzyme. Both the secreted 52-kDa protein and its processed cellular forms (52-, 48- and 34-kDa) were identified as carboxyl proteinases having an optimal activity at pH 3.5 and being specifically inhibited by pepstatin. This protease is characterized by its inducibility by estrogens and its high concentration in proliferative benign and malignant mammary tissue, when detected by immunohistochemistry. The estrogen-induced secretion of this protease may help to understand how estrogens stimulate mammary tumor growth and/or invasion.

The antiproliferative effect of antiestrogens in breast cancer is believed to be entirely due to the inhibition of estrogen induced growth. We show here that non-steroidal antiestrogens inhibit the growth of the human breast cancer MCF7 cells in the complete absence of estrogens (phenol-red-free medium) when cell proliferation is stimulated by insulin or epidermal growth factor. This non-antiestrogenic effect of antiestrogens is, however, mediated by accessible estrogen receptor sites, as it is not observed in receptor negative hormone-independent breast cancers, and is rescued by estradiol but not by insulin. We conclude that antiestrogens inhibit cell proliferation by inhibiting growth factor action as well as estrogen action and that in both cases, accessible estrogen receptors are required.

Polyclonal antibodies against an estrogen induced 52 K protein released by human breast cancer cells have been developed by injecting rabbits with a crude cellular pellet of MCF7 human breast cancer cells. The rabbit antisera have been tested against [35S]Methionine labelled proteins released by the MCF7 cells followed by separation of the immune complexes with Protein A Sepharose. In spite of their low specificity and titer, these antisera allowed us to investigate the release of the 52 K protein in vitro by other mammary cancer or normal cells.

Different steroids and nafoxidine were tested in vitro on rat uteri for their ability to transfer the estrogen cytosol receptor (Rc) to the nucleus. This transfer was evaluated by testing the simultaneous decrease of the number of specific binding sites for estradiol in cytosol and increase of the same binding sites in nuclear extracts. Three classes of compounds could be distinguished according to their affinity for Rc and activity to transfer Rc to the nucleus ; those which bind and transfer Rc (estradiol, estrone and nafoxidine), those which neither bind nor transfer Rc (progesterone, cortisol) and finally those which do not bind Rc, but transfer it to the nucleus (androgens). It is suggested that androgens modify membrane permeability for the estrogen cytosol receptor.

We have studied the effect of R5020, a synthetic progestin, on the biosynthesis of cellular proteins extracted from the MCF7 and T47D human breast cancer cells, using gel electrophoresis. R5020 stimulates the synthesis, as measured after [35S]-methionine labelling, and the accumulation, as shown by silver staining, of a protein of molecular weight approximately equal to 250,000. The increase of the labelled 250-kilodalton protein was rapid (3 hours) and after 3 days this protein represented approximately equal to 6% of the total cellular proteins (approximately equal to 1 microgram/150,000 cells). The induction of the 250-kilodalton protein was obtained by physiologically active concentrations of several progestins and high concentrations of 5 alpha-dihydrotestosterone but not by estradiol or dexamethasone. It was inhibited by R486 , a progestin antagonist, but not by flutamide, an androgen antagonist. These results indicate a mediation by the progesterone receptor. The 250-kilodalton protein appears to be an excellent probe to study in cell culture the mechanism of action of progestin on human cells.

The interaction of the uterine estrogen receptor (R) with three anti-estrogens, Dimethylstilbestrol 1 , Tamoxifen and Nafoxidine, have been studied either indirectly by competitive experiments or directly using radioactive compounds. The affinity of these anti-estrogens for R was found much higher when determined directly without estradiol (E2) than when evaluated by competitive experiments. In the presence of E2, but not in its absence, the inhibitory activity of the anti-estrogens decreased slowly with time. The present report strongly suggests that R is transformed by E2 into a form less sensitive to anti-estrogen. This “desensitization” of R to the estrogen antagonists is proposed as another in vitro test for the E2 induced “activation” of its receptor.

We have compared the binding to the estrogen receptor (R) of different androgens and antiestrogens with their antiestrogenic activities on uterine growth. We found that estradiol (E2)1 and hydroxytamoxifen, a potent antiestrogen, displayed the same affinity for R. Conversely, androgens which have a much lower affinity for R and a much higher dissociation rate than E2, behave at high doses as full estrogens, with no significant antiestrogenic activity. We conclude that there is no correlation between the dissociation rate from R and the antiestrogenic activity of R ligands and that one cannot discriminate between estrogen and antiestrogen ligands by simply evaluating their in vitro binding to the cytosol R.