Key points are not available for this paper at this time.
We have shown that four metabolites of all-trans-retinoic acid (ATRA) (4-oxo-, 4-OH-, 18-OH-, and 5,6-epoxy-RA) can induce maturation of NB4 promyelocytic leukemia cells (Idres, N., Benoit, G., Flexor, M. A., Lanotte, M., and Chabot, G. G. (2001) Cancer Res. 61, 700–705). To better understand the mechanism of action of ATRA metabolites and isomers, we assessed their binding to retinoic acid receptors (RARs) and activation of RAR-mediated transcription via a retinoic acid response element (RARE). Competition binding experiments with tritiated ATRA showed that all metabolites could bind to RARs with variable affinity. For transactivation studies, COS-7 cells were cotransfected with RARα, β, or γ expression vectors and the reporter plasmid RARE-tk-Luc, and the retinoid concentrations for half-maximal luciferase activity (EC50) were determined. All retinoids tested could activate the three RAR isotypes. The lowest EC50 value for RARα was with 9-cis-RA (13 nm), followed by 4-oxo-RA (33 nm), 5,6-epoxy-RA (77 nm), 13-cis-RA (124 nm), 18-OH-RA (162 nm), ATRA (169 nm), and 4-OH-RA (791 nm). For RARβ, the EC50 values increased as follows: 4-oxo-RA (8 nm), ATRA (9 nm), 18-OH-RA (14 nm), 5,6-epoxy-RA (35 nm), 13-cis-RA (47 nm), 4-OH-RA (64 nm), and 9-cis-RA (173 nm). For RARγ the EC50 values were: ATRA (2 nm), 5,6-epoxy-RA (4 nm), 18-OH-RA (14 nm), 13-cis-RA (36 nm), 9-cis-RA (58 nm), 4-oxo-RA (89 nm), and 4-OH-RA (94 nm). By comparing the -fold induction of luciferase activity, all retinoids tested were equipotent at transactivating RARE-tk-Luc whatever the RAR considered. However, the best induction of the transcription was obtained for RARα, which was 5-fold higher than for RARβ and 10-fold higher than for RARγ. In conclusion, these data show that ATRA metabolites can bind to and activate the three RARs with variable relative affinity but with similar efficacy. These results suggest that ATRA metabolites may activate several signaling pathways and probably play an important role in cellular physiology and cancer therapy. We have shown that four metabolites of all-trans-retinoic acid (ATRA) (4-oxo-, 4-OH-, 18-OH-, and 5,6-epoxy-RA) can induce maturation of NB4 promyelocytic leukemia cells (Idres, N., Benoit, G., Flexor, M. A., Lanotte, M., and Chabot, G. G. (2001) Cancer Res. 61, 700–705). To better understand the mechanism of action of ATRA metabolites and isomers, we assessed their binding to retinoic acid receptors (RARs) and activation of RAR-mediated transcription via a retinoic acid response element (RARE). Competition binding experiments with tritiated ATRA showed that all metabolites could bind to RARs with variable affinity. For transactivation studies, COS-7 cells were cotransfected with RARα, β, or γ expression vectors and the reporter plasmid RARE-tk-Luc, and the retinoid concentrations for half-maximal luciferase activity (EC50) were determined. All retinoids tested could activate the three RAR isotypes. The lowest EC50 value for RARα was with 9-cis-RA (13 nm), followed by 4-oxo-RA (33 nm), 5,6-epoxy-RA (77 nm), 13-cis-RA (124 nm), 18-OH-RA (162 nm), ATRA (169 nm), and 4-OH-RA (791 nm). For RARβ, the EC50 values increased as follows: 4-oxo-RA (8 nm), ATRA (9 nm), 18-OH-RA (14 nm), 5,6-epoxy-RA (35 nm), 13-cis-RA (47 nm), 4-OH-RA (64 nm), and 9-cis-RA (173 nm). For RARγ the EC50 values were: ATRA (2 nm), 5,6-epoxy-RA (4 nm), 18-OH-RA (14 nm), 13-cis-RA (36 nm), 9-cis-RA (58 nm), 4-oxo-RA (89 nm), and 4-OH-RA (94 nm). By comparing the -fold induction of luciferase activity, all retinoids tested were equipotent at transactivating RARE-tk-Luc whatever the RAR considered. However, the best induction of the transcription was obtained for RARα, which was 5-fold higher than for RARβ and 10-fold higher than for RARγ. In conclusion, these data show that ATRA metabolites can bind to and activate the three RARs with variable relative affinity but with similar efficacy. These results suggest that ATRA metabolites may activate several signaling pathways and probably play an important role in cellular physiology and cancer therapy. all-trans-retinoic acid 4-hydroxy-all-trans-retinoic acid 4-oxo-all-trans-retinoic acid 18-hydroxy-all-trans-retinoic acid 6-epoxy-RA, 5,6-epoxy-all-trans-retinoic acid 9-cis-retinoic acid 13-cis-retinoic acid retinoic acid receptor retinoic acid response element retinoid X receptor direct repeat cytochrome P450 effective concentration for half-maximal response concentration of unlabeled compound required to prevent 50% of the radiolabeled ligand from binding to the receptor glucocorticoid receptor thyroid hormone receptor estrogen receptor DNA-binding domain ligand-binding domain Dulbecco's modified Eagle's medium phosphate-buffered saline Vitamin A and its derivatives (retinoids) are natural compounds that play central roles in several physiological processes such as embryonic development, proliferation, differentiation, and apoptosis (reviewed in Ref. 1Gudas L.J. Sporn M.B. Roberts A.B. Sporn M.B. Roberts A.B. Goodman D.S. The Retinoids: Biology, Chemistry, and Medicine. Raven Press, New York1994: 443-520Google Scholar). Beside their role in the physiology of normal cells, retinoids possess pharmacological properties used in dermatology and in cancer therapy, including epithelial cancers, precancerous lesions (2Hong W.K. Itri L.M. Sporn M.B. Roberts A.B. Goodman D.S. The Retinoids: Biology, Chemistry, and Medicine. Raven Press, New York1994: 597-630Google Scholar), and acute promyelocytic leukemia (3Huang M.E., Ye, Y.C. Chen S.R. Chai J.R., Lu, J.X. Zhoa L., Gu, L.J. Wang Z.Y. Blood. 1988; 72: 567-572Crossref PubMed Google Scholar). All-trans-retinoic acid (ATRA1) (see Fig. 1), which is considered as one of the most active retinoid, is metabolized by several cytochrome P450s (CYPs) (4Marill J. Cresteil T. Lanotte M. Chabot G.G. Mol. Pharmacol. 2000; 58: 1341-1348Crossref PubMed Scopus (174) Google Scholar). CYPs are heme proteins catalyzing the oxidation of several endobiotics and xenobiotics such as environmental pollutants and drugs. The CYP-mediated metabolism may transform some substrates into inactive compounds but can also lead to the formation of biologically active metabolites. ATRA is metabolized into several oxidized metabolites, including 4-oxo-RA, 4-OH-RA, 18-OH-RA, and 5,6-epoxy-RA (Fig. 1) (reviewed in Ref. 5Blaner W.S. Olson J.A. Sporn M.B. Roberts A.B. Goodman D.S. The Retinoids: Biology, Chemistry, and Medicine. Raven Press, New York1994: 229-256Google Scholar). All of these metabolites have shown biological activity, e.g. 4-oxo-RA is a highly active modulator in embryogenesis (6Pijnappel W.W. Hendriks H.F. Folkers G.E. van den Brink C.E. Dekker E.J. Edelenbosch C. van der Saag P.T. Durston A.J. Nature. 1993; 366: 340-344Crossref PubMed Scopus (246) Google Scholar). It has also been shown that 4-oxo-RA, 4-OH-RA, and 5,6-epoxy-RA can inhibit the growth of several breast cancer cell lines (7van der Leede B.M. van den Brink C.E. Pijnappel W.W. Sonneveld E. van der Saag P.T. van der Burg B. J. Biol. Chem. 1997; 272: 17921-17928Abstract Full Text Full Text PDF PubMed Scopus (57) Google Scholar, 8van Heusden J. Wouters W. Ramaekers F.C. Krekels M.D. Dillen L. Borgers M. Smets G. Br. J. Cancer. 1998; 77: 26-32Crossref PubMed Scopus (37) Google Scholar). Some of these metabolites can inhibit growth and induce differentiation of rhabdomyosarcoma cells (9Ramp U. Gerharz C.D. Eifler E. Biesalski H.K. Gabbert H.E. Biol. Cell. 1994; 81: 31-37Crossref PubMed Scopus (16) Google Scholar) and regulate the expression of several genes involved in differentiation and embryogenesis (6Pijnappel W.W. Hendriks H.F. Folkers G.E. van den Brink C.E. Dekker E.J. Edelenbosch C. van der Saag P.T. Durston A.J. Nature. 1993; 366: 340-344Crossref PubMed Scopus (246) Google Scholar, 10Reynolds N.J. Fisher G.J. Griffiths C.E. Tavakkol A. Talwar H.S. Rowse P.E. Hamilton T.A. Voorhees J.J. J. Pharmacol. Exp. Ther. 1993; 266: 1636-1642PubMed Google Scholar). We have recently shown that ATRA metabolites, including 4-oxo-, 4-OH-, 18-OH-, and 5,6-epoxy-RA, can induce granulocytic differentiation of NB4 acute promyelocytic leukemia cells, elicit nuclear bodies reorganization, and induce the degradation of the chimeric protein PML-RARα (11Idres N. Benoit G. Flexor M.A. Lanotte M. Chabot G.G. Cancer Res. 2001; 61: 700-705PubMed Google Scholar). The retinoid signal is transduced by two families of nuclear receptors, the retinoic acid receptor (RAR) family comprising three isotypes, RARα, RARβ, and RARγ, and the retinoid X receptor (RXR) family comprising also three isotypes, RXRα, RXRβ, and RXRγ (12Mangelsdorf D.J. Umesono K. Evans R.M. Sporn M.B. Roberts A.B. Goodman D.S. The Retinoids: Biology, Chemistry, and Medicine. Raven Press, New York1994: 319-350Google Scholar). Each RAR and RXR isotype includes several isoforms. These receptors belong to the superfamily of nuclear hormone receptors and act as ligand-activated transcription factors (reviewed in Refs. 12Mangelsdorf D.J. Umesono K. Evans R.M. Sporn M.B. Roberts A.B. Goodman D.S. The Retinoids: Biology, Chemistry, and Medicine. Raven Press, New York1994: 319-350Google Scholar and 13Chambon P. FASEB J. 1996; 10: PubMed Scopus Google Scholar). RARs as a with The act as transcription of several genes by binding to retinoic acid response (RARE). of were retinoic the which is and the which is the most The RXR can also act as a transcription activation via the retinoid X response element and as a with several nuclear receptors, e.g. the thyroid hormone receptor the and the their response The natural for the RARs are ATRA and its 9-cis-RA and are by 9-cis-RA ATRA and metabolites are biologically active (6Pijnappel W.W. Hendriks H.F. Folkers G.E. van den Brink C.E. Dekker E.J. Edelenbosch C. van der Saag P.T. Durston A.J. Nature. 1993; 366: 340-344Crossref PubMed Scopus (246) Google Scholar, der Leede B.M. van den Brink C.E. Pijnappel W.W. Sonneveld E. van der Saag P.T. van der Burg B. J. Biol. Chem. 1997; 272: 17921-17928Abstract Full Text Full Text PDF PubMed Scopus (57) Google Scholar, 8van Heusden J. Wouters W. Ramaekers F.C. Krekels M.D. Dillen L. Borgers M. Smets G. Br. J. Cancer. 1998; 77: 26-32Crossref PubMed Scopus (37) Google Scholar, U. Gerharz C.D. Eifler E. Biesalski H.K. Gabbert H.E. Biol. Cell. 1994; 81: 31-37Crossref PubMed Scopus (16) Google Scholar, 10Reynolds N.J. Fisher G.J. Griffiths C.E. Tavakkol A. Talwar H.S. Rowse P.E. Hamilton T.A. Voorhees J.J. J. Pharmacol. Exp. Ther. 1993; 266: 1636-1642PubMed Google Scholar, N. Benoit G. Flexor M.A. Lanotte M. Chabot G.G. Cancer Res. 2001; 61: 700-705PubMed Google Scholar), the of was to their relative binding and transactivating RARα, RARβ, and RARγ. binding experiments showed that ATRA metabolites and can bind the three RARs with affinity. all ATRA metabolites tested could the three RAR isotypes. experiments showed that the effective concentrations of the metabolites to half-maximal luciferase activity (EC50) were highly variable for the RAR isotypes. However, the of the of induction of luciferase activity showed that all the retinoids tested were equipotent at transactivating whatever the RAR isotype considered. and 13-cis-RA were from 4-OH-RA, 4-oxo-RA, 18-OH-RA, and 5,6-epoxy-RA were by and and was from and were by The RAR receptors, RARα RARβ and RARγ were by and of retinoids were in at and from at COS-7 cells were in Dulbecco's modified Eagle's medium with and The cells were at in a was assessed with an and of retinoids in were in medium to the concentration in The concentration of in medium of retinoids were for and show by All were The cells were at a of cells the of were the to the with of or The in of was with of and for at (4 with of was and the were for at were the cells were with to and of were The were to the cells and for at The was increased to with with to cells were with of and at for at The cell were with and with of protein and The cells were to three of in and in a for cells The were at for at protein were and protein concentrations were with the The protein were and at protein of or COS-7 cells were with and concentrations to of unlabeled 4-oxo-RA, 4-OH-RA, 18-OH-RA, 5,6-epoxy-RA, and in a of of binding at in the of a A was and the was and for at to the The were at for at The were and in the to of of the was for the in of in the of a of unlabeled ATRA was from binding to The cells were at a of cells that were the of were the to the with of reporter that the expression of the reporter of or expression vectors and of the that the luciferase as an to for in The with of was with of and for at with of was and for at were the cells were with to and of was The was to the cells and at of the was increased to with with all were with the cells were and with the retinoid at the concentration and The cells were with phosphate-buffered saline and with of and for at The were at for at concentrations were with the cell from COS-7 cells (2 were and were with in with a RARα RARβ or RARγ as C. M. P. J. Biol. PubMed Scopus Google Scholar). were with at at for Each was followed by three in was as in the luciferase and luciferase were of the luciferase The was with a of luciferase and of and luciferase All the experiments were in or and the luciferase activity was with the of and with the protein response or from RARs by metabolites, were to a data are as the concentration of unlabeled compound required to prevent 50% of the radiolabeled ligand from binding to the for the transactivation are as EC50 which the effective concentration for half-maximal luciferase To the binding of ATRA metabolites (4-oxo-, 4-OH-, 18-OH-, 5,6-epoxy-RA) and and the three RAR isotypes, we binding from COS-7 cells with RARα, RARβ, or RARγ were with (2 and concentrations of the unlabeled nm). All the retinoids tested could with ATRA binding to the three RAR isotypes. However, the of the concentration of unlabeled ligand required to inhibit 50% of binding the receptors show that the values were highly variable and could values for metabolites. of the binding with and 4-oxo-RA are in Fig. The best for RARα and RARβ were 9-cis-RA with values of and followed by 4-oxo-RA with values of and and 13-cis-RA with values of and 18-OH-RA and 5,6-epoxy-RA were also for RARα nm), and RARβ nm), In the of RARγ, the best was 4-oxo-RA with an of The binding to the three RAR isotypes, to of retinoid concentrations required to prevent from binding to RAR in a COS-7 cells were cotransfected with the reporter the of a retinoic acid response with the expression of the retinoic acid receptor RARα, RARβ, or RARγ. were to that RARα, RARβ, and RARγ were in COS-7 cells Fig. that these nuclear receptors were with of its and and its four oxidized metabolites (4-oxo-, 4-OH-, 18-OH-, and 5,6-epoxy-RA) the transcription of the reporter were the cells were with concentrations of the retinoids to for and the concentrations required to half-maximal luciferase activity (EC50) were by of the experiments for RAR Fig. obtained with ATRA and the 18-OH-RA for A in luciferase activity was for the three and the EC50 values with RARα and RARβ were similar for ATRA and the EC50 values of ATRA and the metabolites for the three These data show that 9-cis-RA was the most effective retinoid for RARα activation with an EC50 value of followed by 4-oxo-RA with an EC50 value of The retinoids showed EC50 with for 5,6-epoxy-RA, for for 18-OH-RA, and for The 4-OH-RA the EC50 value of used to the effective concentrations for half-maximal luciferase activity EC50 for RAR RAR β, or RAR cells were cotransfected with the reporter with RARα, RARβ, or RARγ expression as The were in at concentrations from to of the retinoid for The the best to the activity is as of retinoids required to half-maximal transactivation (EC50) of RARE-tk-Luc transcription via retinoic acid receptors in a For RARβ, the most effective were 4-oxo-RA, and 18-OH-RA with EC50 values of and and 4-OH-RA were also of RARβ and EC50 values of and that were to higher than the EC50 value of The effective retinoid for RARβ activation was 9-cis-RA with an EC50 value of The lowest EC50 values were with RARγ. 5,6-epoxy-RA, and 18-OH-RA were the most effective retinoids with EC50 values of and and 9-cis-RA showed EC50 values for RARγ of and 4-oxo-RA and 4-OH-RA were the effective retinoids tested for RARγ, with EC50 values of and all the retinoids tested could bind to and activate the three we their to the transcription of the reporter plasmid RARE-tk-Luc in the of RAR isotype by the -fold induction of luciferase COS-7 cells were cotransfected with RARE-tk-Luc plasmid and expression vectors for RARα, RARβ, or RARγ, the cells were with a concentration of of the retinoids for The of these was experiments that that was a retinoid concentration at The of luciferase showed that for RAR isotype its isomers, and metabolites were equipotent for the transactivation of the transcription of the RARE-tk-Luc reporter as shown in However, the best induction of the transcription was obtained for RARα with all can in Fig. the -fold induction of luciferase activity with RARα was 5-fold higher than with RARβ and 10-fold higher than with RARγ, whatever the retinoid ATRA can metabolized into several oxidized metabolites, we the transactivation of the reporter RARE-tk-Luc with ATRA and its metabolites could have been as a of To the induction of the transcription of the reporter by RARα with ATRA and two of its metabolites, 4-oxo-RA and 5,6-epoxy-RA, was formation is shown in Fig. ATRA or the metabolites the of of luciferase activity and of a of luciferase was at and with the three the of transactivation of the luciferase were similar for ATRA and its metabolites. These results that the luciferase with of or used in the experiments a direct the of ATRA and its metabolites at transactivating the reporter was ATRA and the oxidized metabolites of ATRA were considered as degradation biological In ATRA oxidation to compounds was of as a to and to regulate its biological However, several have shown that ATRA metabolites can induce the growth and the differentiation of several cancer cell lines (7van der Leede B.M. van den Brink C.E. Pijnappel W.W. Sonneveld E. van der Saag P.T. van der Burg B. J. Biol. Chem. 1997; 272: 17921-17928Abstract Full Text Full Text PDF PubMed Scopus (57) Google Scholar, 8van Heusden J. Wouters W. Ramaekers F.C. Krekels M.D. Dillen L. Borgers M. Smets G. Br. J. Cancer. 1998; 77: 26-32Crossref PubMed Scopus (37) Google Scholar, U. Gerharz C.D. Eifler E. Biesalski H.K. Gabbert H.E. Biol. Cell. 1994; 81: 31-37Crossref PubMed Scopus (16) Google Scholar, N. Benoit G. Flexor M.A. Lanotte M. Chabot G.G. Cancer Res. 2001; 61: 700-705PubMed Google Scholar). It has also been shown that some may active than e.g. 4-oxo-RA is effective in the of in and in the transcription induction of the genes and (6Pijnappel W.W. Hendriks H.F. Folkers G.E. van den Brink C.E. Dekker E.J. Edelenbosch C. van der Saag P.T. Durston A.J. Nature. 1993; 366: 340-344Crossref PubMed Scopus (246) Google Scholar). In to their activity of acute promyelocytic leukemia cells, ATRA metabolites can also induce the degradation of the chimeric protein PML-RARα and the of nuclear bodies (11Idres N. Benoit G. Flexor M.A. Lanotte M. Chabot G.G. Cancer Res. 2001; 61: 700-705PubMed Google Scholar). We have also that the differentiation activity of ATRA metabolites to via the RARα (11Idres N. Benoit G. Flexor M.A. Lanotte M. Chabot G.G. Cancer Res. 2001; 61: 700-705PubMed Google Scholar). the biological of ATRA metabolites, we were to understand their mechanism of by their binding to the three retinoic acid receptors RARα, RARβ, and RARγ, and to their at transactivating a reporter via a retinoic acid response element (RARE). data show that all ATRA metabolites and its were to bind the three RAR isotypes. were also to induce transcription of the reporter luciferase the of a in the of of the three The of the values the of the metabolites to inhibit binding to RARs and the of the EC50 values as a of the of these metabolites to induce transcription of the RARE-tk-Luc reporter have shown that and EC50 were in a of the binding all the metabolites could bind to the three was that some of these retinoids 4-oxo-RA, and 13-cis-RA for RARα and RARβ for RARα and metabolites 4-OH-RA for RARα and RARγ and For transactivating of the RARs by ATRA metabolites, the EC50 values obtained with the experiments also a of these values were in than obtained in binding The of the EC50 values for RAR show that the lowest values were with RARγ for all the retinoids tested nm), for 9-cis-RA for which the lowest EC50 was obtained with RARα (13 nm), and 4-oxo-RA for which the lowest EC50 value was obtained with RARβ (8 nm). It is to that ATRA and 5,6-epoxy-RA and similar EC50 values (2 and with RARγ. The EC50 values obtained with RARβ from to were also than obtained with RARα, for which showed the EC50 value (173 nm). The experiments of the transactivation of the transcription of RARE-tk-Luc with RARα the of EC50 values with RARβ and RARγ. In the EC50 values from for 9-cis-RA to for ATRA a EC50 (169 that was similar to the EC50 value for the of the 4-oxo-RA and the 5,6-epoxy-RA EC50 values than ATRA (33 and for The EC50 values in showed that the EC50 values for RARα from to with ATRA P. Evans R.M. Nature. PubMed Scopus Google Scholar, N. M. A. P. A. P. A. Nature. 1988; PubMed Scopus Google Scholar, A. U. A. P. Voorhees J.J. Res. PubMed Scopus Google Scholar, D.J. J.A. Evans R.M. Nature. PubMed Scopus Google Scholar, D.J. J.A. G. Evans R.M. C. Cell. Full Text PDF PubMed Scopus Google Scholar), from to with 9-cis-RA D.J. J.A. G. Evans R.M. C. Cell. Full Text PDF PubMed Scopus Google Scholar, L.J. T. C. G. J. C. M. A. Nature. PubMed Scopus Google Scholar), and from to with 13-cis-RA A. U. A. P. Voorhees J.J. Res. PubMed Scopus Google Scholar, D.J. J.A. G. Evans R.M. C. Cell. Full Text PDF PubMed Scopus Google Scholar). A similar is with ATRA metabolites, and the in the the 4-oxo-RA, the 4-OH-RA, and the 5,6-epoxy-RA, whatever the RAR considered A. Griffiths C.E. P. Voorhees J.J. J. PubMed Scopus Google Scholar). is probably to the used in all these studies, such as the of retinoic response the thyroid response element N. M. A. P. A. P. A. Nature. 1988; PubMed Scopus Google Scholar) and the glucocorticoid response element P. Evans R.M. Nature. PubMed Scopus Google Scholar), at the these were the were In some of these used chimeric receptor such as glucocorticoid or estrogen response of may the EC50 of for ATRA with RARα, glucocorticoid a glucocorticoid response that may the these is the cell lines All the and In the the results obtained with the of a retinoic acid response element probably better the of the and the of its isomers, and its metabolites to induce transcription of retinoid In to is the the 18-OH-RA activity the which to a ligand for and as by the EC50 values of for The results obtained with 9-cis-RA is a for RARs and We that the with in of induction of the transcription of the luciferase via the could have been in to the binding of 9-cis-RA to the RXR of the for the activation of the However, the RXR to the probably have been a a EC50 value (13 was with RARα, EC50 values were obtained for RARβ and RARγ (173 and with the retinoids that the RXR protein was the in all the as was for the RXR of the COS-7 cells, the variable in these experiments was the of one of the the that 9-cis-RA could also bind to RXR the that the EC50 values were the RAR and the action of the ligand these In has been shown in the of the the of RXR are to the binding of the ligand RXR is to the binding of the RAR ligand B.M. Umesono K. Chen J. Evans R.M. Cell. 81: Full Text PDF PubMed Scopus Google Scholar, J. M. J. B. Nature. 1994; PubMed Scopus Google Scholar). the of the RXR in the response obtained with ATRA metabolites, some that the RXR probably the response of luciferase activity at for 4-oxo-RA, a binding to has been to RARβ with a affinity (6Pijnappel W.W. Hendriks H.F. Folkers G.E. van den Brink C.E. Dekker E.J. Edelenbosch C. van der Saag P.T. Durston A.J. Nature. 1993; 366: 340-344Crossref PubMed Scopus (246) Google Scholar). experiments in have shown that ATRA metabolites are at differentiation of the cells, in with J. M. A. Flexor, and G. G. Chabot, these cells have a RAR signaling E. Benoit G. Flexor M. Lanotte M. 2000; PubMed Scopus Google Scholar). It that these cells can in the of RXR and 9-cis-RA in with probably by G. L. Flexor M. J. W. Lanotte M. J. PubMed Scopus Google Scholar). these data suggest that ATRA metabolites bind to the In of results and of the data in the that ATRA metabolites and act the the activation of the transcription of the RARE-tk-Luc reporter is by the binding of these retinoids to the RAR of the the binding values of all the metabolites for the three RARs were with the EC50 values obtained with of transcription via a with the could by the that the binding is in with cell and that to the transactivation in cellular the binding to the the of the to bind to the response and the with proteins such as and that transcription All of these which are for in the in binding may the of the binding of the their receptors, and the values in these in binding may the experiments have shown that the EC50 values for its isomers, and its metabolites for the RARs were we have also that all these retinoids could the RARE-tk-Luc reporter plasmid with the whatever the RAR isotype considered. the EC50 values and the of the retinoids tested could probably as follows: the EC50 values to the affinity of the receptors for the the -fold induction of the luciferase transcription most the of the with the response element and its to induce as was for retinoids A. U. A. P. Voorhees J.J. Res. PubMed Scopus Google Scholar). In we also showed that were in the of the retinoids tested to the transcription via to a RAR All the retinoids tested were 5-fold with RARα than with RARβ, and 10-fold than with RARγ, the lowest EC50 values with most of the retinoids tested were for RARγ. have been which the RARγ, and to a the RARβ, are at transcription than RARα, or the cells required for activation of RARβ and RARγ A. U. A. P. Voorhees J.J. Res. PubMed Scopus Google Scholar). In conclusion, we have shown that ATRA metabolites can bind to the three RARs and can induce the transactivation of transcription via the retinoic acid response with the three RAR isotypes. These data show that ATRA metabolites may activate several signaling pathways the RAR isotype considered and suggest that these metabolites probably play an important role in cellular In these compounds could considered as for the of to retinoid therapy. We are to and of the for the RARs used in and to for of and We also and as as for the ATRA metabolites used in
Idres et al. (Thu,) studied this question.