The potential role of estrogen in aromatase regulation in the breast

Aromatase is expressed in both normal and malignant breast tissues. Aromatase activity in the breast varies over a wide range. Our previous studies have demonstrated that in situ aromatization contributes to the estrogen content of breast tumors to a major extent. Consequently, alterations of aromatase activity could serve as a major determinant of tissue estradiol content. However, the mechanisms and extent of aromatase regulation in breast tissues have not been fully established. We have observed an inverse correlation between tumor aromatase activity and estrogen content in nude mice bearing xenografts of MCF-7 cells transfected with the aromatase gene. To investigate the potential role of estrogen in aromatase regulation in the breast, studies were carried out in an in vitro model. In this model, MCF-7 cells were cultured long term in estrogen-deprived medium and called by the acronym, LTED cells. We found that long-term estrogen deprivation enhanced aromatase activity by 3–4-fold when compared to the wild-type MCF-7 cells. Re-exposure of LTED cells to estrogen reduced aromatase activity to the levels of the wild-type MCF-7 cells. We also measured aromatase activity in 101 frozen breast carcinoma specimens and compared tumor aromatase activities in pre-menopausal patients versus post-menopausal patients and in post-menopausal patients with or without hormone replacement therapy (HRT). Although statistically not significant, there was a trend paralleling that observed in the in vitro studies. Aromatase activity was higher in breast cancer tissues from the patients with lower circulating estrogen levels. Our data suggest that estrogen may be involved in the regulation of aromatase activity in breast tissues.     

A new role for the transcriptional corepressor SIN3; regulation of centromeres

Centromeres play a vital role in maintaining the genomic stability of eukaryotes by coordinating the equal distribution of chromosomes to daughter cells during mitosis and meiosis. Fission yeast (S. pombe) centromeres consist of a 4-9 kb central core region and 30-100 kb of flanking inner (imr/B) and outer (otr/K) repeats. These sequences direct a laminar kinetochore structure similar to that of human centromeres. Centromeric heterochromatin is generally underacetylated. We have previously shown that inhibition of histone deacetylases (HDACs) caused hyperacetylation of centromeres and defective chromosome segregation. SIN3 is a HDAC corepressor that has the ability to mediate HDAC targeting in the repression of promoters. In this study, we have characterized S. pombe sin three corepressors (Pst1p and Pst2p) to investigate whether SIN3-HDAC is required in the regulation of centromeres. We show that only pst1-1 and not pst2Delta cells displayed anaphase defects and thiabendazole sensitivity. pst1-1 cells showed reduced centromeric silencing, increased histone acetylation in centromeric chromatin, and defective centromeric sister chromatid cohesion. The HDAC Clr6p and Pst1p coimmunoprecipitated, and Pst1p colocalized with centromeres, particularly in binucleate cells. These data are consistent with a model in which Pst1p-Clr6p temporally associate with centromeres to carry out the initial deacetylation necessary for subsequent steps in heterochromatin formation.     

The role of proton and sodium ions in energy transduction by respiratory complex I

Respiratory complex I plays a central role in energy transduction. It catalyzes the oxidation of NADH and the reduction of quinone, coupled to cation translocation across the membrane, thereby establishing an electrochemical potential. For more than half a century, data on complex I has been gathered, including recently determined crystal structures, yet complex I is the least understood complex of the respiratory chain. The mechanisms of quinone reduction, charge translocation and their coupling are still unknown. The H(+) is accepted to be the coupling ion of the system; however, Na(+) has also been suggested to perform such a role. In this article, we address the relation of those two ions with complex I and refer ion pump and Na(+)/H(+) antiporter as possible transport mechanisms of the system. We put forward a hypothesis to explain some apparently contradictory data on the nature of the coupling ion, and we revisit the role of H(+) and Na(+) cycles in the overall bioenergetics of the cell.     

Conservation of the acetylation pattern of histones and the transcriptional activity in Ehrlich ascites tumor cells by sodium butyrate

Histone deacetylases of Ehrlich ascites tumor cells are active at low temperatures (0-4 degrees C). The so-called hyperacetylated state of histones is the physiological state of histones in intact Ehrlich ascites tumor cells which is conserved by the continuous presence of 10 mM sodium butyrate during the preparation of nuclei and histones. Isolation of histones in the absence of butyrate causes an artificial decrease in histone acetylation. This artificial loss of histone acetylation produces a decrease of the elongation reaction in the RNA synthesis. The initiation of RNA synthesis is not affected.