N-desmethyltamoxifen inhibits growth of MCF 7 human mammary carcinoma cells in vitro.
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Breast Neoplasms↗
Biomedical subjects
R R Reddel
Publications and source records attributed to R R Reddel.
Effects of oestrogens on cell proliferation and cell cycle kinetics. A hypothesis on the cell cycle effects of antioestrogens.
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Animals↗
Tamoxifen induces accumulation of MCF 7 human mammary carcinoma cells in the G0/G1 phase of the cell cycle.
When MCF 7 human mammary carcinoma cells in exponential growth phase were treated with tamoxifen a dose-dependent inhibition of cell growth was observed. This inhibition was accompanied by a dose-dependent decrease in the percentage of S phase cells and a concomitant increase in the percentage of cells in the G0/G1 phase of the cell cycle. Simultaneous treatment of cultures with a 10-fold lower concentration of oestradiol completely reversed the growth inhibitory and cell cycle effects of tamoxifen at doses below 5 microM but only partially reversed the effects of higher doses of this drug. It is concluded: (1) that tamoxifen-induced growth inhibition is associated with major changes in the cell cycle kinetic parameters of MCF 7 cells, indicating that this drug is a cell cycle phase-specific growth inhibitory agent and (2) that not all the anti-proliferative and cell cycle effects of tamoxifen in vitro are reversed by simultaneous treatment with oestradiol. This suggests that tamoxifen, in addition to having effects on cell proliferation that are reversed by oestrogens and are likely to be oestrogen receptor-mediated, has antitumor activity in vitro that involves biochemical mechanisms independent of the oestrogen receptor system.
Breast Neoplasms↗
Effects of biologically active metabolites of tamoxifen on the proliferation kinetics of MCF-7 human breast cancer cells in vitro.
The effects of two major metabolites of tamoxifen, N-demethyltamoxifen (DMT) and 4-hydroxytamoxifen (4OHT), on MCF-7 cell proliferation and cell cycle kinetic parameters were compared with those of the parent compound. All three compounds produced dose-dependent decreases in the rate of cell proliferation which were accompanied by decreases in the percentage of S- and G2-M-phase cells. 4OHT was 100- to 167-fold more potent than both tamoxifen and DMT in producing these effects, and this was correlated with their relative binding affinities (RBAs) for the cytoplasmic estrogen receptor (ER) (17 beta-estradiol = 100, 4OHT = 41, tamoxifen = DMT = 2). At doses less than or equal to 2.5 microM, these effects were completely reversed by 17 beta-estradiol, but the required 17 beta-estradiol:antiestrogen concentration ratios differed, i.e., 1:10 to 1:1 for 4OHT compared with 1:1000 to 1:100 for tamoxifen and DMT. Although the concentrations of 17 beta-estradiol required for reversal were related to affinity of the metabolite for ER, they were 5- to 20-fold lower than predicted from the measured RBAs. When the rate of cell proliferation was measured over a range of concentrations of antiestrogen, in the presence or absence of 17 beta-estradiol, it was highly correlated (r2 = 0.96) with the percentage of S-phase cells. In addition to these 17 beta-estradiol-reversible events, all three compounds caused 17 beta-estradiol-irreversible cytotoxicity at higher concentrations (greater than or equal to 7.5 microM DMT and 4OHT, 10 microM tamoxifen). The order of potency in producing this effect was DMT greater than 4OHT greater than tamoxifen, which correlated with neither the RBAs for ER nor the RBAs for the high-affinity microsomal antiestrogen binding site. These data support the concept that estrogens and antiestrogens compete for a common event which regulates the rate of cell proliferation probably by controlling the proportion of cells entering S phase. Although it appears likely that ER is intimately involved in this regulatory process, 17 beta-estradiol-irreversible mechanisms are also involved in antiestrogen action in vitro.
Biotransformation↗
Ototoxicity in patients receiving cisplatin: importance of dose and method of drug administration.
A total of 146 serial audiograms that were performed for 32 patients who had received cisplatin treatment were analyzed. Fifteen patients developed significant audiometric abnormality after a mean cumulative dose of 203 mg/m2. Ototoxicity was more severe with higher cumulative doses and higher individual doses, and was most marked in those patients receiving bolus injections. Loss of perception of higher frequencies was most common. Symptoms of ototoxicity occurred in 23 patients and included deafness, tinnitus, otalgia, and recruitment. In some patients the symptoms were transient, and not all patients with symptoms developed audiometric abnormalities. Audiometric abnormalities were invariably preceded by one or more symptoms of ototoxicity. Ototoxicity resulted in dose modification or cessation of treatment in only three patients, and in each case the decision to modify the dose was made on the basis of symptoms of ototoxicity rather than on the audiometric findings. We conclude that audiometry has a limited role in the routine management of patients receiving cisplatin treatment.
Cisplatin↗
Reassessment of immortalization complementation group D.
Previous somatic cell hybridization studies have assigned many human cell lines to one of four complementation groups (A-D) for immortalization. We report here that the A1698DM cell line, which contains selectable markers and has previously been defined as the immortalization group D representative, was derived from T24 cells rather than A1698. A1698DM did not undergo senescence when fused with cell lines assigned to groups A, B, or C. This raises the possibility that this cell line has undergone further evolution and lost multiple putative senescence genes so that it is now unable to complement any, or most, other cell lines for senescence. Cell lines previously assigned to group D may, therefore, be heterogeneous with respect to the genetic changes that resulted in their immortalization. This has important implications for strategies to clone senescence genes based on complementation groups.
Cell Fusion↗
The hTERTalpha splice variant is a dominant negative inhibitor of telomerase activity.
The telomerase catalytic subunit (hTERT) is an essential component of the holoenzyme complex that adds telomeric repeats to the ends of human chromosomes. Maintenance of telomeres by telomerase or another mechanism is required for cell immortalization, and loss of telomeric DNA has been proposed as a trigger for cellular senescence. Available evidence suggests that regulation of telomerase activity primarily depends on transcriptional control of hTERT. However, several human tissues as well as some normal cell strains have been shown to express low levels of hTERT mRNA even though they lack telomerase activity. We have previously identified six splice variants of hTERT, including a "deletion" variant (hTERTalpha) that is missing conserved residues from the catalytic core of the protein. Several of the deletion variants have been detected in normal and developing human tissues. We now show that hTERTalpha inhibits endogenous telomerase activity, which results in telomere shortening and chromosome end-to-end fusions. Telomerase inhibition induced a senescence-like state in HT1080 cells and apoptosis in a jejunal fibroblast cell line. These results suggest a possible role for hTERT splice variants in the regulation of telomerase activity.
Alternative Splicing↗
An N-terminal region of mot-2 binds to p53 in vitro.
The mouse mot-2 protein was earlier shown to bind to the tumor suppressor protein, p53. The mot-2 binding site of p53 was mapped to C-terminal amino acid residues 312-352, which includes the cytoplasmic sequestration domain. In the present study, we have found that both mot-1 and mot-2 bind to p53 in vitro. By using His-tagged deletion mutant proteins, the p53-binding domain of mot-2 was mapped to its N-terminal amino acid residues 253-282, which are identical in mot-1 and mot-2 proteins. Some peptides containing the p53-binding region of mot-2 were able to compete with the full-length protein for p53 binding. The data provided rationale for in vitro binding of mot-1 and mot-2 proteins to p53 and supported the conclusion that inability of mot-1 protein to bind p53 in vivo depends on secondary structure or its binding to other cellular factors. Most interestingly, the p53-binding region of mot-2 was common to its MKT-077, a cationic dye that exhibits antitumor activity, binding region. Therefore it is most likely that MKT-077-induced nuclear translocation and restoration of wild-type p53 function in transformed cells takes place by a competitional mechanism.
Amino Acids↗