Biochemical and molecular changes at the cellular level in response to exposure to environmental estrogen-like chemicals

Estrogen-like chemicals are unique compared to nonestrogenic xenobiotics, because in addition to their chemical properties, the estrogenic property of these compounds allows them to act like sex hormones. Whether weak or strong, the estrogenic response of a chemical, if not overcome, will add extra estrogenic burden to the system. At elevated doses, natural estrogens and environmental estrogen-like chemicals are known to produce adverse effects. The source of extra or elevated concentration of estrogen could be either endogenous or exogenous. The potential of exposure for humans and animals to environmental estrogen-like chemicals is high. Only a limited number of estrogen-like compounds, such as diethylstilbestrol (DES), bisphenol A, nonylphenol, polychlorinated biphenyls (PCBs), and dichlorodiphenyltrichloroethane (DDT), have been used to assess the biochemical and molecular changes at the cellular level. Among them, DES is the most extensively studied estrogen-like chemical, and therefore this article is focused mainly on DES-related observations. In addition to estrogenic effects, environmental estrogen-like chemicals produce multiple and multitype genetic and/or nongenetic hits. Exposure of Syrian hamsters to stilbene estrogen (DES) produces several changes in the nuclei of target organ for carcinogenesis (kidney): (1) Products of nuclear redox reactions of DES modify transcription regulating proteins and DNA; (2) transcription is inhibited; (3) tyrosine phosphorylation of nuclear proteins, including RNA polymerase II, p53, and nuclear insulin-like growth factor-1 receptor, is altered; and (4) DNA repair gene DNA polymerase beta transcripts are decreased and mutated. Exposure of Noble rats to DES also produces several changes in the mammary gland: proliferative activity is drastically altered; the cell cycle of mammary epithelial cells is perturbed; telomeric length is attenuated; etc. It appears that some other estrogenic compounds, such as bisphenol A and nonylphenol, may also follow a similar pattern of effects to DES, because we have recently shown that these compounds alter cell cycle kinetics, produce telomeric associations, and produce chromosomal aberrations. Like DES, bisphenol A after metabolic activation is capable of binding to DNA. However, it should be noted that a particular or multitype hit(s) will depend upon the nature of the environmental estrogen-like chemical. The role of individual attack leading to a particular change is not clear at this stage. Consequences of these multitypes of attack on the nuclei of cells could be (1) nuclear toxicity/cell death; (2) repair of all the hits and then acting as normal cells; or (3) sustaining most of the hits and acting as unstable cells. Proliferation of the last type of cell is expected to result in transformed cells.

Transcriptional pausing at +62 of the HIV-1 nascent RNA modulates formation of the TAR RNA structure

A strong transcriptional pause delays human RNA polymerase II three nt after the last potentially paired base in HIV-1 TAR, the RNA structure that binds the transactivator protein Tat. We report here that the HIV-1 pause depends in part on an alternative RNA structure (the HIV-1 pause hairpin) that competes with formation of TAR. By probing the nascent RNA structure in halted transcription complexes, we found that the transcript folds as the pause hairpin before and at the pause, and rearranges to TAR concurrent with or just after escape from the pause. The pause signal triggers a 2 nt reverse translocation by RNA polymerase that may block the active site and be counteracted by formation of TAR. Thus, the HIV-1 pause site modulates nascent RNA rearrangement from a structure that favors pausing to one that both recruits Tat and promotes escape from the pause.

Roles of RNA:DNA hybrid stability, RNA structure, and active site conformation in pausing by human RNA polymerase II

Human RNA polymerase II recognizes a strong transcriptional pause signal in the initially transcribed region of HIV-1. We report the use of a limited-step transcription assay to dissect the mechanism underlying recognition of and escape from this HIV-1 pause. Our results suggest that the primary determinant of transcriptional pausing is a relatively weak RNA:DNA hybrid that triggers backtracking of RNA polymerase II along the RNA and DNA chains and displaces the RNA 3' OH from the active site. In contrast, two alternative RNA secondary structures, TAR and anti-TAR, are not required for pausing and affect it only indirectly, rather than through direct interaction with RNA polymerase II. TAR accelerates escape from the pause, but anti-TAR inhibits formation of TAR prior to pause escape. The behavior of RNA polymerase II at a mutant pause signal supports a two-step, non-equilibrium mechanism in which the rate-determining step is a conformational change in the enzyme, rather than the changes in nucleic-acid base-pairing that accompany backtracking.

In vivo binding of diethylstilbestrol to nuclear proteins of kidneys of Syrian hamsters

We demonstrate here that stilbene estrogen (diethylstilbestrol) is converted to nuclear protein binding metabolite(s) both in vitro and in vivo. In vitro reaction of DES with nuclei from hamster liver or kidney in the presence of cumene hydroperoxide or NADPH revealed binding of [3H]DES in nuclear proteins (histones; nonhistones precipitable by 2% TCA, NH2; nonhistones soluble in 2% TCA, NH30). The binding was significantly inhibited by cytochromes P450 inhibitors. In an in vitro system [3H]DES quinone, one of the metabolites of DES, was able to bind to pure nonhistone proteins RNA polymerase and DNA polymerase. The binding of [3H]DES quinone to nonhistones RNA polymerase and DNA polymerase was inhibited by low molecular weight thiols, i.e. glutathione and cysteine, or thiol modifiers, such as n-ethylmaleimide, dithionitrobenzoic acid and hydroxymercuric benzoate. DES and DES metabolites inhibited transcriptional activity. In vivo [3H]DES was able to bind to nuclear proteins of hamster liver, kidneys and testes. The level of in vivo [3H]DES binding to all three types of nuclear proteins (histones, NH2, NH30) in the kidney (target organ) was two or more fold higher than that observed in the liver or testis (nontarget organs). Four nuclear NH30 proteins (mol wts.: 56, 37, 33 and 28 kDa) were irreversibly bound to [3H]DES in vivo. The in vivo binding of [3H]DES to transcriptionally active chromatin NH30 proteins also was observed. The data reported here establish that DES was able to bind to liver or kidney nuclear proteins in vitro, which was catalyzed by nuclear enzymes when fortified with an appropriate cofactor. DES quinone may be one of the protein binding metabolites. DES and DES metabolites inhibited transcriptional activity. The level of in vivo binding of [3H] DES to nuclear proteins of kidney (target organ) was double in comparison with that observed in liver or testis (nontarget organs). In vivo modifications in the chromatin proteins may be a factor in the development of DES-induced renal carcinogenesis is not clear.

Phosphorylation of tyrosine residues of RNA polymerase II and other nuclear proteins by active chromatin tyrosine kinase(s)

We demonstrate here for the first time that protein tyrosine kinases are present in the active chromatin of nucleus. The presence of tyrosine kinase activity in the active chromatin was initially determined using poly (Glu,Na-Tyr;4:1) (PGT) as a tyrosine phosphorylatable substrate. Active chromatin in the presence of cofactors phosphorylated PGT at a rate of 40 pmol/min. The phosphorylation of PGT by active chromatin was inhibited by 41, 47 and approximately 95% with genistein, n-ethylmaleimide and quercetin (known inhibitors of tyrosine kinases), respectively. A Lineweaver-Burk plot revealed an apparent Km of 50 micrograms/ml and Vmax of 45 pmol/min for active chromatin tyrosine kinase(s). Analyses of phosphorylation of endogenous substrates by immunoprecipitation, western blotting and phosphoamino acids revealed that active chromatin protein tyrosine kinase(s) are able to phosphorylate tyrosine residues of the large subunit of RNA pol II and several other active chromatin proteins. The ability of AC-PTKs to phosphorylate many proteins of active chromatin components argues strongly for its role(s) in regulating transcription.

Organ-specific inhibition of types I, II and III transcriptional activity in hamsters exposed to stilbene estrogen

We have previously shown that stilbene estrogen (diethyl-stilbestrol, DES) covalently binds to nonhistone nuclear proteins both in vivo and in vitro. In this study, we demonstrate the differential effects of DES exposure on in organelle transcriptional activity in nuclei isolated from kidney (target organ of cancer) and liver (non target organ) of hamsters. Kidney RNA polymerase (RNA pol) I and III activities were significantly inhibited by 50% at days 8 and 15 of DES exposure compared to that of controls. Liver RNA pol I and III activities were only modestly inhibited (17 and 22%, respectively) by 2 and 8 days of DES exposure, respectively. However, longer exposure of DES to animals did not produce any significant effects on RNA pol I activity. The activity of RNA pol II was affected by DES exposure in both liver and kidney. DES treatment for two days resulted in an increase in RNA pol II activity in kidney. The enhanced enzyme activity was decreased to 50% of that of the control at 15 days of DES treatment. Unlike RNA pol I and III, RNA pol II activity in the liver was inhibited in a time-dependent fashion in response to DES exposure. To understand the mechanism of transcriptional inhibition by DES, we analyzed the effect of DES exposure on the expression of hepatic RNA pol II at both mRNA and protein levels and also phosphorylation of hepatic RNA pol II. The total amount of transcripts or protein contents of hepatic RNA pol II was not altered in response to DES exposure to hamster for 15 days. Total phosphorylation of hepatic RNA pol II was also not affected by 15 days DES exposure. However tyrosine phosphorylation of hepatic RNA pol II was lowered by 2.8-fold compared to that of control enzyme in response to DES exposure for 15 days. An inhibitory effect of DES on the total RNA polymerase activity in both kidney and liver nuclei in the presence of endogenous template was observed in vitro. No inhibitory effect of DES was observed in vitro on transcriptional activity in the presence of exogenously added DNA template. Based on these data it appears that the in vivo inhibition of transcription by DES may be due to alterations in chromatin template or the level of transcription regulating proteins and not due to decreased availability of the chromatin template and/or RNA polymerase. Whether DES related inhibition of transcriptional activity plays a role in the development of kidney cancer is not clear.

Mechanism of antiproliferative activity of luteolin against stilbene-estrogen stimulation of proliferation of hamster renal epithelial cells

In the present study, we have investigated the influence of a naturally occurring plant flavone luteolin on cell proliferation, nuclear tyrosine kinase activity, and expression of insulin-like growth factor-I (IGF-I) receptor in Syrian hamster renal epithelial cells. Diethylstilbestrol (DES)-induced cell proliferation was inhibited in a dose-dependent manner by cotreatment with luteolin. We also tested the influence of luteolin on nuclear protein tyrosine kinase activity and the expression of IGF-I receptor. The nuclear tyrosine kinase activity was inhibited in a dose-dependent manner by both genistein and luteolin. Luteolin was five times more effective in inhibiting nuclear tyrosine kinase activity than genistein. Data of kinetic constants suggest that luteolin is a noncompetitive inhibitor of nuclear protein tyrosine kinases. Co-treatment of luteolin significantly inhibited DES-induce IGF-I receptor expression. These findings indicate that blocking effects of DES-induced cell proliferation by luteolin may be through the inhibition of nuclear tyrosine kinase activity and of IGF-I receptor expression.

Stilbene estrogen-mediated enhanced tyrosine phosphorylation of the nuclear protein p53 by nuclear matrix associated tyrosine kinase(s)

We have previously shown that protein tyrosine kinases are present in the nuclei of hamster renal epithelial cells. In this study we have identified an endogenous, critically important nuclear matrix protein as a substrate for tyrosine phosphorylation by nuclear matrix tyrosine kinases (NM-PTKs) and its modulation by stilbene estrogen, diethylstilbestrol (DES). Analyses of tyrosine phosphorylation of nuclear matrix proteins by endogenous labeling revealed that NM-PTKs were able to phosphorylate the tyrosine residues of several proteins. Both amino acid analysis and immuno-precipitation of nuclear matrix proteins with the monoclonal antibody that recognizes wild-type p53 revealed that p53 was phosphorylated on its tyrosine residues by NM-PTK(s). Moreover, DES exposure resulted in the enhanced tyrosine phosphorylation of p53 to a tune of 3.6 fold. Several other proteins of the nuclear matrix were also differentially phosphorylated in response to DES exposure. The DES-mediated enhanced tyrosine phosphorylation of p53 coincided with increased cell proliferation of renal tubular epithelial cells of Syrian hamster. The results of this study suggest that p53 protein is phosphorylated at tyrosine residues and enhanced tyrosine phosphorylation of p53 by DES exposure may play a role in the proliferation of hamster renal tubular epithelial cells.