LakeMountD
Doctor Science
Key (for those who aren't too familiar with biochemistry)
Antagonist: a substance that inhibits the normal physiological function of a receptor.
Agonist: a substance that binds to a receptor and triggers a response in the cell. An agonist is the opposite of an antagonist in the sense that while an antagonist also binds to the receptor, the antagonist fails to activate the receptor and actually blocks it from activation by agonists.
cAMP: a molecule that is important in many biological processes; it is derived from adenosine triphosphate (ATP). cAMP is a second messenger, used for intracellular signal transduction, such as transferring the effects of hormones like glucagon and adrenaline, which cannot get through the cell membrane. Its main purpose is the activation of protein kinases; it is also used to regulate the passage of Ca2+ through ion channels.
With a growing amount of people using nadrolones such as tren, NPP, and deca I figured I would look into some anti-progestin compounds such as RU-486 and ZK98299. At first I wanted to find out more of which one was more efficacious at preventing progesterone binding the the hPR but some of the things I came across were very very interesting.
First and foremost it appears that type II progestin antagonists such as RU-486 can actually be reversed and become agonists , which is the complete opposite of what we want to achieve. The mechanisms of action are becoming more and more known. One way that reversal of antagonistic action is seen is through elevated cAMP levels. While hPR occupied by the antagonist RU486 is transcriptionally inactive, the antagonist-occupied receptors become strong activators of transcription in the presence of 8-Br-cAMP. This suggests that the 8-Br-cAMP-induced transcriptional reversal requires that the antagonist-occupied receptors be bound to DNA. Even with agonist- occupied hPR, addition of 8-Br-cAMP results in a synergistic increase in transcriptional activity. This is interesting considering the antagonis ZK98299 does not allow hPR binding to DNA. It would seem that ZK98299 would be superior as an anti-progestin, however, it is hard to say considering the fact that I do not, yet, know what specific tissues this reversal can occur in. Not only that but I haven't found when this reversal occurs or whether it is caused by large dosages or occurs no matter what. Since RU-486 is also a glucocorticoid antagonist, it could spell disaster if it exhibits even a partial agonist activity.
L7/SPA is another way that agonist activity is greatly upregulated in these antagonists. It was shown that in the presence of L7/SPA, agonist activity was increased 3-10 fold and again ZK98299 cannot be up-regulated by L7/SPA.
The Partial Agonist Activity of Antagonist-Occupied Steroid Receptors Is Controlled by a Novel Hinge Domain-Binding Coactivator L7/SPA and the Corepressors N-CoR or SMRT
Twila A. Jackson, Jennifer K. Richer, David L. Bain, Glenn S. Takimoto, Lin Tung and Kathryn B. Horwitz
Departments of Medicine and Pathology and the Molecular, Biology Program, University of Colorado Health Sciences Center, Denver, Colorado 80262
Steroid receptor antagonists, such as the antiestrogen tamoxifen or the antiprogestin RU486, can have inappropriate agonist-like effects in tissues and tumors. To explain this paradox we postulated that coactivators are inadvertently brought to the promoters of DNA-bound, antagonist-occupied receptors. The human (h) progesterone receptor (PR) hinge-hormone binding domain (H-HBD) was used as bait in a two-hybrid screen of a HeLa cDNA library, in which the yeast cells were treated with RU486. We have isolated and characterized two interesting steroid receptor-interacting proteins that regulate transcription in opposite directions. The first is L7/SPA, a previously described 27-kDa protein containing a basic region leucine zipper domain, having no known nuclear function. When coexpressed with tamoxifen-occupied estrogen receptors (hER) or RU486-occupied hPR or glucocorticoid receptors (hGR), L7/SPA increases the partial agonist activity of the antagonists by 3- to 10-fold, but it has no effect on agonist-mediated transcription. The interaction of L7/SPA with hPR maps to the hinge region, and indeed, the hPR hinge region squelches L7/SPA-dependent induction of antagonist-mediated transcription. Interestingly, pure antagonists that lack partial agonist effects, such as the antiestrogen ICI164,384 or the antiprogestin ZK98299, cannot be up-regulated by L7/SPA. We also isolated, cloned, and sequenced the human homolog (hN-CoR) of the 270-kDa mouse (m) thyroid/retinoic acid receptor corepressor. Binding of hN-CoR maps to the hPR-HBD. mN-CoR, and a related human corepressor, SMRT, suppress RU486 or tamoxifen-mediated partial agonist activity by more than 90%. This suppression is completely squelched by overexpression of the hPR H-HBD. Additionally, both corepressors reverse the antagonist-dependent transcriptional up-regulation produced by L7/SPA. Our data suggest that the direction of transcription by antagonist-occupied steroid receptors can be controlled by the ratio of coactivators to corepressors recruited to the transcription complex by promoter-bound receptors. In normal tissues and in hormone-resistant breast cancers in which the agonist activity of mixed antagonists predominates, steroid receptors may be preferentially bound by coactivators. This suggests a strategy by which such partial agonist activity can be eliminated and by which candidate receptor ligands can be screened for this activity.
Antagonist-occupied human progesterone receptors bound to DNA are functionally switched to transcriptional agonists by cAMP
CA Sartorius, L Tung, GS Takimoto and KB Horwitz
Department of Medicine, University of Colorado Health Sciences Center, Denver 80262.
When steroid hormone antagonists have inappropriate agonist effects, the clinical consequences are grave. Progesterone antagonists bind to two naturally occurring isoforms of human progesterone receptors (hPR), hPRB and the NH2-terminally truncated hPRA, and usually inhibit agonist- stimulated transcription. It is shown here that elevation of cAMP levels in a human breast cancer cell line leads to the functional reversal of progesterone antagonist action. While hPR occupied by the antagonists RU486 and ZK112993 are transcriptionally inactive, the antagonist-occupied receptors become strong activators of transcription in the presence of 8-Br-cAMP.However, this functional switch does not occur with the progesterone antagonist ZK98299, which, unlike RU486 and ZK112993, is unable to induce hPR binding to DNA. This suggests that the 8-Br-cAMP-induced transcriptional reversal requires that the antagonist-occupied receptors be bound to DNA. Even with agonist- occupied hPR, addition of 8-Br-cAMP results in a synergistic increase in transcriptional activity. When hPRA alone are transiently expressed in COS-1 cells, transcription of a reporter gene is stimulated by the agonist R5020 and by 8-Br-cAMP and is synergistic when both are present; but the 8-Br-cAMP-dependent component of transcription proceeds in the absence of hPRA, in the absence of the progesterone response element, and in the presence of a DNA-binding domain mutant of hPRA that cannot bind to the progesterone response element. Additionally, under the intracellular conditions in which 8-Br-cAMP activates antagonist-hPR complexes, there is no protein kinase A- mediated phosphorylation of the receptors. We discuss a model in which a gene that is independently transcribed by cAMP-responsive factors and by hPR can be selected for positive or negative regulation on the transcription complex due to additive or cooperative interactions between the two DNA-bound factors.
Hormone-Induced Progesterone Receptor Phosphorylation Consists of Sequential DNA-Independent and DNA-Dependent Stages: Analysis with Zinc Finger Mutants and the Progesterone Antagonist ZK98299
GS Takimoto, DM Tasset, AC Eppert and KB Horwitz
Human progesterone receptors (hPRs) are phosphorylated at multiple serine residues, first in a basal step and then in a hormone-induced step. To determine whether hormone-induced phosphorylation precedes or follows the interaction of hPRs with DNA two strategies were used. (i) DNA binding was prevented or altered with site-specific mutants of the A form of hPR; (ii) DNA binding of wild-type hPR forms A and B was prevented with the progesterone antagonist ZK98299. Two hPRA mutants were constructed: DBDCys, which lacks a critical cysteine residue in the first zinc finger, and DBDsp, which is mutated at three discriminatory amino acids to change its DNA binding specificity from a progesterone response element to an estrogen response element. Receptors were transiently expressed in PR-negative cells and were intranuclear. DBDCys did not bind DNA in vitro and DBDsp bound only the estrogen response element. Transiently expressed hPRA and DBDsp showed the upward shift in electrophoretic mobility characteristic of hormone-induced phosphorylation; it was absent with DBDCys. Hormone-induced[32P]orthophosphate incorporation into transiently expressed DBDCys was reduced 60% compared to hPRA and DBDsp but was not eliminated. ZK98299 binds hPRs but prevents their interaction with DNA. Compared to R5020, the antagonist reduced phosphorylation of hPRB and hPRA in T47D breast cancer cells by 60% and totally prevented the mobility shift. We conclude that the hormone-induced phosphorylation of hPR includes DNA-independent and DNA-dependent stages and that only DNA-dependent sites contribute to the mobility shift.
Study 2
RU486 (mifepristone) has proved to be a remarkably active antiprogesterone and antiglucocorticosteroid agent in human beings. The mechanism of action involves the intracellular receptors of the antagonized hormones (progesterone and glucocorticosteroids). At the molecular level, the most important features are high binding affinity to the receptor, interaction of the phenylaminodimethyl group in the 11β-position with a specific region of the receptor binding pocket, and RU486-induced transconformation differences in the ligand-binding domain. These particularities have consequences at different steps of the receptor function as compared with agonists. However, the reasoning cannot be limited to the RU486-receptor interaction, and, for instance, there is the possibility of a switch from antagonistic property to agonist activity, depending on the intervention of other signaling pathways. It would be desirable to have derivatives with only one of the two antagonistic properties (antiprogestin, antiglucocorticosteroid) in spite of similarities between steroid structures, receptors involved, and responsive machineries in target cells. Clinically, the RU486-plus-prostaglandin method is ready to be used on a large scale and is close to being as convenient and safe as any medical method of abortion may be. The early use of RU486 as a contragestive as soon as a woman fears a pregnancy she does not want will help to defuse the abortion issue. Research should now be conducted to define an efficient and convenient contraceptive method with RU486 or other antiprogestins. The usefulness of RU486 for obstetric indications, including facilitation of difficult delivery, has to be assessed rapidly. Gynecologic trials, particularly in leiomyomata, should also be systematically continued. The very preliminary results obtained with tumors, including breast cancers, indicate that further studies are necessary.
The Antagonists RU486 and ZK98299 Stimulate Progesterone Receptor Binding to Deoxyribonucleic Acid in Vitro and in Vivo, but Have Distinct Effects on Receptor Conformation1
Elizabeth K. Gass, Susan A. Leonhardt, Steven K. Nordeen and Dean P. Edwards
Department of Pathology (E.K.G., S.A.L., S.K.N., D.P.E.) and Molecular Biology Program S.K.N. D.P.E.), University of Colorado Health Sciences Center, Denver, Colorado 80262
Three types of transfection experiments were used to detect the abilities of different classes of antagonists to stimulate binding of progesterone receptor (PR) to progesterone response elements (PRE) in intact mammalian cells. These included a promoter interference assay, in which PR binding to PREs positioned between the TATA box and the start of transcription is detected as a reduction of expression of a constitutively active reporter gene, competition of PR antagonist and glucocorticoid receptor agonist for a common glucocorticoid response element/PRE-controlled reporter construct, and activation of a chimeric receptor (PR-VP16) containing the constitutive trans-activation domain derived from the VP16 protein of herpes simplex virus. By each approach, all antagonists tested were equally effective in stimulating PR binding to PREs in the cell. This included previously designated type I (ZK98299) and type II (RU486, ZK98734, and ZK112993) 11ß-aryl substituted steroid analogs.Stimulation of PR binding to PREs in the cell by ZK98299 was of interest because this antagonist has been reported to lack the ability to stimulate PR-DNA binding in vitro by electrophoretic gel mobility shift assay compared with RU486, which promotes efficient binding of PR to PREs. To clarify the apparent discrepancy between intact cell and in vitro results with ZK98299, we altered electrophoretic gel mobility shift assay conditions to allow detection of less stable DNA complexes. Under these conditions, ZK98299 induced the formation of specific PR-PRE complexes. Further analysis of the ZK98299-induced DNA complexes revealed that they exhibited an electrophoretic mobility different from that of the complexes induced by RU486, and the off-rate of PR from DNA was faster than that of the PR bound to agonist. This suggests that ZK98299 promotes a conformational change within PR distinct from that induced by RU486. The present results are consistent with the conclusions that ZK98299 stimulates PR binding to target DNA sequences and that ZK98299 and RU486 represent two mechanistic classes of antagonists based on inducing different conformational changes in PR.
Antagonist: a substance that inhibits the normal physiological function of a receptor.
Agonist: a substance that binds to a receptor and triggers a response in the cell. An agonist is the opposite of an antagonist in the sense that while an antagonist also binds to the receptor, the antagonist fails to activate the receptor and actually blocks it from activation by agonists.
cAMP: a molecule that is important in many biological processes; it is derived from adenosine triphosphate (ATP). cAMP is a second messenger, used for intracellular signal transduction, such as transferring the effects of hormones like glucagon and adrenaline, which cannot get through the cell membrane. Its main purpose is the activation of protein kinases; it is also used to regulate the passage of Ca2+ through ion channels.
With a growing amount of people using nadrolones such as tren, NPP, and deca I figured I would look into some anti-progestin compounds such as RU-486 and ZK98299. At first I wanted to find out more of which one was more efficacious at preventing progesterone binding the the hPR but some of the things I came across were very very interesting.
First and foremost it appears that type II progestin antagonists such as RU-486 can actually be reversed and become agonists , which is the complete opposite of what we want to achieve. The mechanisms of action are becoming more and more known. One way that reversal of antagonistic action is seen is through elevated cAMP levels. While hPR occupied by the antagonist RU486 is transcriptionally inactive, the antagonist-occupied receptors become strong activators of transcription in the presence of 8-Br-cAMP. This suggests that the 8-Br-cAMP-induced transcriptional reversal requires that the antagonist-occupied receptors be bound to DNA. Even with agonist- occupied hPR, addition of 8-Br-cAMP results in a synergistic increase in transcriptional activity. This is interesting considering the antagonis ZK98299 does not allow hPR binding to DNA. It would seem that ZK98299 would be superior as an anti-progestin, however, it is hard to say considering the fact that I do not, yet, know what specific tissues this reversal can occur in. Not only that but I haven't found when this reversal occurs or whether it is caused by large dosages or occurs no matter what. Since RU-486 is also a glucocorticoid antagonist, it could spell disaster if it exhibits even a partial agonist activity.
L7/SPA is another way that agonist activity is greatly upregulated in these antagonists. It was shown that in the presence of L7/SPA, agonist activity was increased 3-10 fold and again ZK98299 cannot be up-regulated by L7/SPA.
The Partial Agonist Activity of Antagonist-Occupied Steroid Receptors Is Controlled by a Novel Hinge Domain-Binding Coactivator L7/SPA and the Corepressors N-CoR or SMRT
Twila A. Jackson, Jennifer K. Richer, David L. Bain, Glenn S. Takimoto, Lin Tung and Kathryn B. Horwitz
Departments of Medicine and Pathology and the Molecular, Biology Program, University of Colorado Health Sciences Center, Denver, Colorado 80262
Steroid receptor antagonists, such as the antiestrogen tamoxifen or the antiprogestin RU486, can have inappropriate agonist-like effects in tissues and tumors. To explain this paradox we postulated that coactivators are inadvertently brought to the promoters of DNA-bound, antagonist-occupied receptors. The human (h) progesterone receptor (PR) hinge-hormone binding domain (H-HBD) was used as bait in a two-hybrid screen of a HeLa cDNA library, in which the yeast cells were treated with RU486. We have isolated and characterized two interesting steroid receptor-interacting proteins that regulate transcription in opposite directions. The first is L7/SPA, a previously described 27-kDa protein containing a basic region leucine zipper domain, having no known nuclear function. When coexpressed with tamoxifen-occupied estrogen receptors (hER) or RU486-occupied hPR or glucocorticoid receptors (hGR), L7/SPA increases the partial agonist activity of the antagonists by 3- to 10-fold, but it has no effect on agonist-mediated transcription. The interaction of L7/SPA with hPR maps to the hinge region, and indeed, the hPR hinge region squelches L7/SPA-dependent induction of antagonist-mediated transcription. Interestingly, pure antagonists that lack partial agonist effects, such as the antiestrogen ICI164,384 or the antiprogestin ZK98299, cannot be up-regulated by L7/SPA. We also isolated, cloned, and sequenced the human homolog (hN-CoR) of the 270-kDa mouse (m) thyroid/retinoic acid receptor corepressor. Binding of hN-CoR maps to the hPR-HBD. mN-CoR, and a related human corepressor, SMRT, suppress RU486 or tamoxifen-mediated partial agonist activity by more than 90%. This suppression is completely squelched by overexpression of the hPR H-HBD. Additionally, both corepressors reverse the antagonist-dependent transcriptional up-regulation produced by L7/SPA. Our data suggest that the direction of transcription by antagonist-occupied steroid receptors can be controlled by the ratio of coactivators to corepressors recruited to the transcription complex by promoter-bound receptors. In normal tissues and in hormone-resistant breast cancers in which the agonist activity of mixed antagonists predominates, steroid receptors may be preferentially bound by coactivators. This suggests a strategy by which such partial agonist activity can be eliminated and by which candidate receptor ligands can be screened for this activity.
Antagonist-occupied human progesterone receptors bound to DNA are functionally switched to transcriptional agonists by cAMP
CA Sartorius, L Tung, GS Takimoto and KB Horwitz
Department of Medicine, University of Colorado Health Sciences Center, Denver 80262.
When steroid hormone antagonists have inappropriate agonist effects, the clinical consequences are grave. Progesterone antagonists bind to two naturally occurring isoforms of human progesterone receptors (hPR), hPRB and the NH2-terminally truncated hPRA, and usually inhibit agonist- stimulated transcription. It is shown here that elevation of cAMP levels in a human breast cancer cell line leads to the functional reversal of progesterone antagonist action. While hPR occupied by the antagonists RU486 and ZK112993 are transcriptionally inactive, the antagonist-occupied receptors become strong activators of transcription in the presence of 8-Br-cAMP.However, this functional switch does not occur with the progesterone antagonist ZK98299, which, unlike RU486 and ZK112993, is unable to induce hPR binding to DNA. This suggests that the 8-Br-cAMP-induced transcriptional reversal requires that the antagonist-occupied receptors be bound to DNA. Even with agonist- occupied hPR, addition of 8-Br-cAMP results in a synergistic increase in transcriptional activity. When hPRA alone are transiently expressed in COS-1 cells, transcription of a reporter gene is stimulated by the agonist R5020 and by 8-Br-cAMP and is synergistic when both are present; but the 8-Br-cAMP-dependent component of transcription proceeds in the absence of hPRA, in the absence of the progesterone response element, and in the presence of a DNA-binding domain mutant of hPRA that cannot bind to the progesterone response element. Additionally, under the intracellular conditions in which 8-Br-cAMP activates antagonist-hPR complexes, there is no protein kinase A- mediated phosphorylation of the receptors. We discuss a model in which a gene that is independently transcribed by cAMP-responsive factors and by hPR can be selected for positive or negative regulation on the transcription complex due to additive or cooperative interactions between the two DNA-bound factors.
Hormone-Induced Progesterone Receptor Phosphorylation Consists of Sequential DNA-Independent and DNA-Dependent Stages: Analysis with Zinc Finger Mutants and the Progesterone Antagonist ZK98299
GS Takimoto, DM Tasset, AC Eppert and KB Horwitz
Human progesterone receptors (hPRs) are phosphorylated at multiple serine residues, first in a basal step and then in a hormone-induced step. To determine whether hormone-induced phosphorylation precedes or follows the interaction of hPRs with DNA two strategies were used. (i) DNA binding was prevented or altered with site-specific mutants of the A form of hPR; (ii) DNA binding of wild-type hPR forms A and B was prevented with the progesterone antagonist ZK98299. Two hPRA mutants were constructed: DBDCys, which lacks a critical cysteine residue in the first zinc finger, and DBDsp, which is mutated at three discriminatory amino acids to change its DNA binding specificity from a progesterone response element to an estrogen response element. Receptors were transiently expressed in PR-negative cells and were intranuclear. DBDCys did not bind DNA in vitro and DBDsp bound only the estrogen response element. Transiently expressed hPRA and DBDsp showed the upward shift in electrophoretic mobility characteristic of hormone-induced phosphorylation; it was absent with DBDCys. Hormone-induced[32P]orthophosphate incorporation into transiently expressed DBDCys was reduced 60% compared to hPRA and DBDsp but was not eliminated. ZK98299 binds hPRs but prevents their interaction with DNA. Compared to R5020, the antagonist reduced phosphorylation of hPRB and hPRA in T47D breast cancer cells by 60% and totally prevented the mobility shift. We conclude that the hormone-induced phosphorylation of hPR includes DNA-independent and DNA-dependent stages and that only DNA-dependent sites contribute to the mobility shift.
Study 2
RU486 (mifepristone) has proved to be a remarkably active antiprogesterone and antiglucocorticosteroid agent in human beings. The mechanism of action involves the intracellular receptors of the antagonized hormones (progesterone and glucocorticosteroids). At the molecular level, the most important features are high binding affinity to the receptor, interaction of the phenylaminodimethyl group in the 11β-position with a specific region of the receptor binding pocket, and RU486-induced transconformation differences in the ligand-binding domain. These particularities have consequences at different steps of the receptor function as compared with agonists. However, the reasoning cannot be limited to the RU486-receptor interaction, and, for instance, there is the possibility of a switch from antagonistic property to agonist activity, depending on the intervention of other signaling pathways. It would be desirable to have derivatives with only one of the two antagonistic properties (antiprogestin, antiglucocorticosteroid) in spite of similarities between steroid structures, receptors involved, and responsive machineries in target cells. Clinically, the RU486-plus-prostaglandin method is ready to be used on a large scale and is close to being as convenient and safe as any medical method of abortion may be. The early use of RU486 as a contragestive as soon as a woman fears a pregnancy she does not want will help to defuse the abortion issue. Research should now be conducted to define an efficient and convenient contraceptive method with RU486 or other antiprogestins. The usefulness of RU486 for obstetric indications, including facilitation of difficult delivery, has to be assessed rapidly. Gynecologic trials, particularly in leiomyomata, should also be systematically continued. The very preliminary results obtained with tumors, including breast cancers, indicate that further studies are necessary.
The Antagonists RU486 and ZK98299 Stimulate Progesterone Receptor Binding to Deoxyribonucleic Acid in Vitro and in Vivo, but Have Distinct Effects on Receptor Conformation1
Elizabeth K. Gass, Susan A. Leonhardt, Steven K. Nordeen and Dean P. Edwards
Department of Pathology (E.K.G., S.A.L., S.K.N., D.P.E.) and Molecular Biology Program S.K.N. D.P.E.), University of Colorado Health Sciences Center, Denver, Colorado 80262
Three types of transfection experiments were used to detect the abilities of different classes of antagonists to stimulate binding of progesterone receptor (PR) to progesterone response elements (PRE) in intact mammalian cells. These included a promoter interference assay, in which PR binding to PREs positioned between the TATA box and the start of transcription is detected as a reduction of expression of a constitutively active reporter gene, competition of PR antagonist and glucocorticoid receptor agonist for a common glucocorticoid response element/PRE-controlled reporter construct, and activation of a chimeric receptor (PR-VP16) containing the constitutive trans-activation domain derived from the VP16 protein of herpes simplex virus. By each approach, all antagonists tested were equally effective in stimulating PR binding to PREs in the cell. This included previously designated type I (ZK98299) and type II (RU486, ZK98734, and ZK112993) 11ß-aryl substituted steroid analogs.Stimulation of PR binding to PREs in the cell by ZK98299 was of interest because this antagonist has been reported to lack the ability to stimulate PR-DNA binding in vitro by electrophoretic gel mobility shift assay compared with RU486, which promotes efficient binding of PR to PREs. To clarify the apparent discrepancy between intact cell and in vitro results with ZK98299, we altered electrophoretic gel mobility shift assay conditions to allow detection of less stable DNA complexes. Under these conditions, ZK98299 induced the formation of specific PR-PRE complexes. Further analysis of the ZK98299-induced DNA complexes revealed that they exhibited an electrophoretic mobility different from that of the complexes induced by RU486, and the off-rate of PR from DNA was faster than that of the PR bound to agonist. This suggests that ZK98299 promotes a conformational change within PR distinct from that induced by RU486. The present results are consistent with the conclusions that ZK98299 stimulates PR binding to target DNA sequences and that ZK98299 and RU486 represent two mechanistic classes of antagonists based on inducing different conformational changes in PR.
