Fatty acid modulation of antiestrogen action and antiestrogen-binding protein in cultured lymphoid cells

Microsomal epoxide hydrolase of rat liver is a subunit of theanti-oestrogen-binding site

A tritiated photoaffinity labelling analogue of tamoxifen, [(2-azido-4-benzyl)-phenoxy]-N-ethylmorpholine (azido-MBPE), was used to identify the anti-oestrogen-binding site (AEBS) in rat liver tissue [Poirot, Chailleux, Fargin, Bayard and Faye (1990) J. Biol. Chem. 265, 17039-17043]. UV irradiation of rat liver microsomal proteins incubated with tritiated azido-MBPE led to the characterization of two photolabelled proteins of molecular masses 40 and 50 kDa. The amino acid sequences of proteolytic products from the 50 kDa protein were identical with those from rat microsomal epoxide hydrolase (mEH). Treatment of hepatocytes with anti-sense mRNA directed against mEH abolished AEBS in these cells. In addition we found that tamoxifen and N-morpholino-2-[4-(phenylmethyl)phenoxy]ethanamine, a selective ligand of AEBS, were potent inhibitors of the catalytic hydration of styrene oxide by mEH. However, functional overexpression of the human mEH did not significantly modify the binding capacity of [3H]tamoxifen. Taken together, these results suggest that the 50 kDa protein, mEH, is necessary but not sufficient to reconstitute AEBS.

Lipid composition changes induced by tamoxifen in a bacterial model system

A putative relationship between growth impairment of Bacillus stearothermophilus by tamoxifen (TAM) and TAM-induced perturbation of the physical properties of bacterial membrane lipids has been observed. The supplementation of the growth medium with Ca2+ (a membrane stabilizer) partially relieves growth inhibition by TAM, allowing growth at TAM concentrations that fully impair growth in the basal medium. B. stearothermophilus modifies the membrane lipid composition in response to the addition of TAM to the growth medium and the response is sensitive to Ca2+. Changes in lipid composition are observed in the acyl chains and in the polar head groups of phospholipids. The physical effects of alteration in these lipids was studied by fluorescence polarization of DPH and DPH-PA. Polar lipid dispersions from TAM-adapted cells grown in a Ca2+ medium show a shift of Tm to higher temperatures and a significant increase of the structural order as compared to lipids from control cells, suggesting that TAM-induced lipid composition changes compensate for the destabilizing effects of the cytostatic on membrane organization. The polar lipids from cells grown in the basal medium containing tamoxifen are also altered, but these alterations do not promote order increase of the bilayer in spite of a deviation of Tm to higher temperatures as detected by DPH. Data indicate that B. stearothermophilus controls the membrane lipid composition in response to tamoxifen, to compensate for TAM-promoted disordering in membranes and to provide an appropriate packing of phospholipid molecules in a stable bilayer, putatively disturbed by TAM incorporation.

Lymphoma cells selected for resistance against the cytotoxic effect of oxygenated sterols are also resistant to nonsteroidal antiestrogens

Oxygenated derivatives of cholesterol and related compounds (oxysterols) have long been known to be cytotoxic to many different cell types. The mechanism of this cytotoxic effect is not fully understood. Our laboratory has earlier reported that oxysterol cytotoxicity resembles that of nonsteroidal antiestrogens in some aspects: (i) the cytotoxic action of both types of compounds is blocked by inhibitors of protein or RNA synthesis, and (ii) both classes of compounds bind with high affinity to the microsomal antiestrogen binding site, a protein which may mediate the cytotoxicity of its ligands. We have now extended these studies by developing cell lines which are resistant to the cytotoxic action of oxysterols. Oxysterol-resistant cells were isolated by exposing two murine lymphoma cell lines, K36 and EL4, to incremental concentrations of 7-ketocholestanol. Intriguingly, the resistant cells thus obtained also exhibited considerable resistance to the cytotoxic effects of nonsteroidal antiestrogens such as tamoxifen and clomiphene, having LD50 values which were 10-100-times higher than that of the parental cells. The resistance appeared to be selective for oxysterols and antiestrogens and did not extend to non-specific toxic agents such as azide, ethanol, Triton-X100, or heat. The biochemical basis of the resistance is not clear but is not due to diminished cellular uptake or increased metabolism of the cytotoxic agents or to changes in the antiestrogen binding protein. The availability of the resistant cell lines should facilitate further studies on the mechanism of oxysterol- and antiestrogen-induced cell death.

Inhibitors of protein and RNA synthesis block the cytotoxic effects of non-steroidal antiestrogens

Non-steroidal antiestrogens such as tamoxifen are known to exert cytotoxic effects against various cell lines in culture. When the antiestrogens are present at sufficiently high concentrations, their cytotoxicity cannot be reversed by estrogens and is demonstrable even with cell lines which lack the estrogen receptor. The mechanism of this cytotoxicity, which is clearly independent of estrogen antagonism, remains unknown. Using two murine cancer cell lines (the K36 leukemia and the EL4 lymphoma cell line), the human breast cancer cell line MCF7, and two non-steroidal antiestrogens (tamoxifen and clomiphene), our laboratory attempted to determine whether the cytotoxic action of non-steroidal antiestrogens was mediated by a mechanism requiring protein or RNA synthesis. In the case of K36 and EL4 cells, inclusion of tamoxifen or clomiphene in the culture medium regularly caused the viable call count to fall below 20-30% of control in 36-48 h. Under these conditions, the addition of inhibitors of protein or RNA synthesis consistently increased viable cell count in a dose-dependent manner. With cultures of K36 cells grown in the presence of 10 microM tamoxifen, for example, the addition of appropriate concentrations of emetine, cycloheximide, puromycin, or actinomycin D increased the percentage of viable cells to 5.0, 2.4, 4.0, and 4.0 times that of control, respectively. Additional experiments revealed that the macromolecular synthesis inhibitors, while effective in inhibiting protein or RNA synthesis to varying degrees, did not affect the cellular uptake of [3H]tamoxifen, suggesting that their ability to protect cells against antiestrogen-induced cell death was not due to an inhibition of cellular uptake of antiestrogens. In the case of MCF7 cells, however, inhibition of protein synthesis did not protect the cells against the cytotoxic effect of tamoxifen. These observations suggest that non-steroidal antiestrogens may exert their cytotoxic effect by at least two different mechanisms; only one of these require de novo protein synthesis. The effect of antiestrogens on K36 and EL4 cells may provide a useful system for the identification of proteins involved in cell death.

Tamoxifen inhibits growth of oestrogen receptor-negative A549 cells

The non-steroidal anti-oestrogen tamoxifen inhibits proliferation of the A549 human lung adenocarcinoma cell line (EC50 congruent to 10 nM) yet there was no evidence of oestrogen receptor expression as determined by ligand binding assay and northern blotting. 17-beta-Oestradiol had no effect on A549 cell proliferation (1 pM-1 microM) and moreover a 100-fold excess failed to reverse the effect of 10 nM tamoxifen as did a 100-fold excess of the steroidal anti-oestrogens ICI 164384 and ICI 182780. However, 4-hydroxytamoxifen which had no significant effect on A549 cell growth (1 pM-1 microM) completely antagonized the effect of 10 nM tamoxifen when used at a 100-fold excess. In the presence of oleic acid and stearic acid (10 microM) the growth inhibitory effect of tamoxifen in A549 cells was greatly enhanced, unlike effects mediated by the anti-oestrogen binding protein described in other cells where these fatty acids had no effect. These results indicate the presence of a unique and highly sensitive mechanism in A549 cells whereby concentrations of tamoxifen relevant to classical receptor binding can inhibit cell growth in the absence of the oestrogen receptor.