Activation-induced apoptosis and cell surface expression of Fas (CD95) ligand are reciprocally regulated by retinoic acid receptor alpha and gamma and involve nur77 in T cells

It has been previously shown that CD4+ T cells enter the apoptotic suicide program via the Fas ligand (FasL)/Fas-mediated pathway upon T cell receptor (TCR) stimulation. In Jurkat cells TCR stimulation regulates the de novo synthesis of FasL, while in the influenza hemagglutinin-specific CD4+ murine T cell hybridoma (IP-12-7) the cell surface appearance of a preformed FasL is initiated. Both processes are dependent on new mRNA and protein synthesis, involve up-regulation of nur77, and can be inhibited by retinoic acids (RA). Two groups of nuclear receptors for RA have been identified: retinoic acid receptors (RAR) and retinoid X receptors (RXR). In this study various synthetic retinoids were used to define which receptors regulate TCR-mediated apoptosis. It is demonstrated that the inhibition is mediated via RARalpha, while RARgamma enhances TCR-mediated apoptosis, and when both receptors are stimulated, the costimulation by RXR will promote the effect of RARalpha. Evidence is presented that these receptors affect the transcriptional activity of nur77 and consequently the expression of FasL. Our data suggest a complex interaction between the various isoforms of retinoid receptors in regulating T cell death and demonstrate that the target through which retinoids regulate TCR-mediated apoptosis is nur77.

Activation-induced apoptosis and cell surface expression of Fas (CD95) ligand are reciprocally regulated by retinoic acid receptor alpha and gamma and involve nur77 in T cells

It has been previously shown that CD4+ T cells enter the apoptotic suicide program via the Fas ligand (FasL)/Fas-mediated pathway upon T cell receptor (TCR) stimulation. In Jurkat cells TCR stimulation regulates the de novo synthesis of FasL, while in the influenza hemagglutinin-specific CD4+ murine T cell hybridoma (IP-12-7) the cell surface appearance of a preformed FasL is initiated. Both processes are dependent on new mRNA and protein synthesis, involve up-regulation of nur77, and can be inhibited by retinoic acids (RA). Two groups of nuclear receptors for RA have been identified: retinoic acid receptors (RAR) and retinoid X receptors (RXR). In this study various synthetic retinoids were used to define which receptors regulate TCR-mediated apoptosis. It is demonstrated that the inhibition is mediated via RARalpha, while RARgamma enhances TCR-mediated apoptosis, and when both receptors are stimulated, the costimulation by RXR will promote the effect of RARalpha. Evidence is presented that these receptors affect the transcriptional activity of nur77 and consequently the expression of FasL. Our data suggest a complex interaction between the various isoforms of retinoid receptors in regulating T cell death and demonstrate that the target through which retinoids regulate TCR-mediated apoptosis is nur77.

Activation-induced apoptosis and cell surface expression of Fas (CD95) ligand are reciprocally regulated by retinoic acid receptor alpha and gamma and involve nur77 in T cells

It has been previously shown that CD4+ T cells enter the apoptotic suicide program via the Fas ligand (FasL)/Fas-mediated pathway upon T cell receptor (TCR) stimulation. In Jurkat cells TCR stimulation regulates the de novo synthesis of FasL, while in the influenza hemagglutinin-specific CD4+ murine T cell hybridoma (IP-12-7) the cell surface appearance of a preformed FasL is initiated. Both processes are dependent on new mRNA and protein synthesis, involve up-regulation of nur77, and can be inhibited by retinoic acids (RA). Two groups of nuclear receptors for RA have been identified: retinoic acid receptors (RAR) and retinoid X receptors (RXR). In this study various synthetic retinoids were used to define which receptors regulate TCR-mediated apoptosis. It is demonstrated that the inhibition is mediated via RARalpha, while RARgamma enhances TCR-mediated apoptosis, and when both receptors are stimulated, the costimulation by RXR will promote the effect of RARalpha. Evidence is presented that these receptors affect the transcriptional activity of nur77 and consequently the expression of FasL. Our data suggest a complex interaction between the various isoforms of retinoid receptors in regulating T cell death and demonstrate that the target through which retinoids regulate TCR-mediated apoptosis is nur77.

Limited proteolysis for assaying ligand binding affinities of nuclear receptors

The binding of natural or synthetic ligands to nuclear receptors is the triggering event leading to gene transcription activation or repression. Ligand binding to the ligand binding domain of these receptors induces conformational changes that are evidenced by an increased resistance of this domain to proteases. In vitro labeled receptors were incubated with various synthetic or natural agonists or antagonists and submitted to trypsin digestion. Proteolysis products were separated by SDS-PAGE and quantified. The amount of trypsin-resistant fragments was proportional to receptor occupancy by the ligand, and allowed the determination of dissociation constants (kDa). Using the wild-type or mutated human retinoic acid receptor alpha as a model, kDa values determined by classical competition binding assays using tritiated ligands are in agreement with those measured by the proteolytic assay. This method was successfully extended to human retinoic X receptor alpha, glucocorticoid receptor, and progesterone receptor, thus providing a basis for a new, faster assay to determine simultaneously the affinity and conformation of receptors when bound to a given ligand.

Inhibition of activation-induced apoptosis of thymocytes by all-trans- and 9-cis-retinoic acid is mediated via retinoic acid receptor alpha

Thymocytes can be induced to undergo apoptotic cell death by activation through the T-cell receptor (TCR). This process requires macromolecular synthesis and has been shown to be inhibited by retinoic acids (RAs). Two groups of nuclear receptors for RAs have been identified: retinoic acid receptors (RARs) and retinoid X receptors (RXRs). All-trans-RA is the high-affinity ligand for RARs, and 9-cis-RA additionally binds to RXRs with high affinity. Because 9-cis-RA is much more potent in inhibiting TCR-mediated death than all-trans-RA, it was suggested that RXRs participate in the process. In the present study various synthetic retinoid analogues were used to address this question further. The results presented suggest that the inhibitory effect of RAs on activation-induced death of thymocytes is mediated via RARalpha, because (1) it can be reproduced by various RARalpha analogues both in vitro and in vivo, (2) the effect of RAs can be inhibited by the addition of an RARalpha antagonist, (3) CD4+CD8+thymocytes, which die on TCR stimulation, express RARalpha. Stimulation of RARgamma, in contrast, enhances the activation-induced death of thymocytes and inhibits its prevention by RARalpha stimulation. RXR co-stimulation suspends this inhibitory effect of RARgamma and permits the preventive function of RARalpha on activation-induced death. Our results suggest a complex interaction between the various isoforms of retinoid receptors and demonstrate that low (physiological) concentrations of all-trans-RA do not affect the activation-induced death of thymocytes because the RARalpha-mediated inhibitory and the RARgamma-mediated enhancing pathways are in balance, whereas if 9-cis-RA is formed, additional stimulation of RXRs permits the inhibitory action of RARalpha.

Induction of apoptosis by retinoids and retinoic acid receptor gamma-selective compounds in mouse thymocytes through a novel apoptosis pathway

Retinoic acids are morphogenic signaling molecules that are derived from vitamin A and involved in a variety of tissue functions. Two groups of their nuclear receptors have been identified: retinoic acid receptors (RARs) and retinoic acid X receptors (RXRs). All-trans retinoic acid is the high affinity ligand for RARs, and 9-cis retinoic acid also binds to RXRs with high affinity. In cells at high concentrations, all-trans retinoic acid can be converted to 9-cis retinoic acid via unknown mechanisms. It was previously shown that retinoic acids prevents activation-induced death of thymocytes. Here, we report that both all-trans and 9-cis retinoic acid induce apoptosis of mouse thymocytes and purified CD4+CD8+ cells in ex vivo cultures, with 9-cis retinoic acid being 50 times more effective. The induction of apoptosis by retinoic acids is mediated by RARgamma because (a) the phenomenon can be reproduced only by RARgamma-selective retinoic acid analogs, (b) the cell death induced by either retinoic acids or RARgamma analogs can be inhibited by RARgamma-specific antagonists, and (c) CD4+CD8+ thymocytes express RARgamma. In vivo administration of an RARgamma analog resulted in thymus involution with the concomitant activation of the apoptosis-related endonuclease and induction of tissue transglutaminase. The RARgamma pathway of apoptosis is RNA and protein synthesis dependent, affects the CD4+CD8+ double positive thymocytes, and can be inhibited by the addition of either Ca2+ chelators or protease inhibitors. Using various RAR- and RXR-specific analogs and antagonists, it was demonstrated that stimulation of RAR alpha inhibits the RARgamma-specific death pathway (which explains the lack of apoptosis stimulatory effects of all-trans retinoic acid at physiological concentrations) and that costimulation of the RXR receptors (in the case of 9-cis retinoic acid) can neutralize this inhibitory effect. It is suggested that formation of 9-cis retinoic acid may be a critical element in regulating both the positive selection and the "default cell death pathway" of thymocytes.

Synthesis, structure-affinity relationships, and biological activities of ligands binding to retinoic acid receptor subtypes

The retinoic acid receptors (RARs) transduce retinoid dependant gene regulation, and many biological effects of retinoids are mediated through binding and activation of three closely related receptor subtypes (RAR alpha, RAR beta, and RAR gamma). In order to investigate the role of receptor subtypes, we have carried out a chemical synthesis program to seek selective retinoids for these receptors. We measured receptor binding affinity using recombinant RAR alpha, -beta, and -gamma proteins and assessed cellular differentiating activity in F9 murine teratocarcinoma cells (F9 cells). This research has identified the 4-substituted-3-(1-adamantyl)phenyl moiety as a new pharmacophore which can replace the beta-cyclogeranylidene ring of the naturally occurring all-trans-retinoic acid. Two chemical series derived from the general structures 6-(3-tertioalkylphenyl)-2-naphthoic acid (series I) and 4-[(E)-2-(3-tertioalkylphenyl)propenyl]benzoic acid (series II) were developed. In particular, we have obtained the RAR gamma selective derivatives 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthoic acid (7) [Ki(RAR alpha) = 6500 nM, Ki(RAR beta) = 2480 nM, Ki(RAR gamma) = 77 nM] and 4-[(E)-2-[3-(1-adamantyl)-4-hydroxyphenyl]propenyl]benzoic acid (19) [Ki(RAR alpha) = 1,144 nM, Ki(RAR beta) = 1245 nM, Ki(RAR gamma) = 53 nM]. In series I, the presence of a phenol group, irrespective of the nature of tertioalkyl group, imparted at least partial RAR gamma selectivity, whereas in series II, the presence of both adamantyl and phenol groups is needed to confer RAR gamma selectivity. The RAR gamma selective ligands induce differentiation in F9 cells (7, AC50 = 33 nM; 19, AC50 = 66 nM). From series I, a mixed RAR beta-gamma agonist with potent cellular differentiating activity was selected for development as a topical antiacne agent, 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid (5, CD 271) [Ki(RAR alpha) = 1100 nM, Ki-(RAR beta) = 34 nM, Ki(RAR gamma) = 130 nM, AC50(F9) = 37 nM]. Finally, from series II, we have obtained a weak antagonist in the F9 cellular differentiation assay, 4-[(E)-2-(3-tert-butyl-4-hydroxyphenyl)propenyl]benzoic acid (15, IC50 = 700 nM).

Identification of synthetic retinoids with selectivity for human nuclear retinoic acid receptor gamma

The action of retinoids on gene regulation is mediated by three distinct nuclear retinoic acid receptor (RAR) subtypes called RAR alpha, beta and gamma. Since RAR gamma is predominantly expressed in adult skin, specific ligands for this subtype could (i) represent valuable tools to evaluate the biological role of RAR gamma in skin and (ii) provide therapeutic entities with a higher therapeutic index at lower teratogenic risk. Using in vitro binding studies and a functional transactivation assay, we have identified three compounds with high RAR gamma selectivity.

Selective synthetic ligands for human nuclear retinoic acid receptors

From a series of naphthalene and benzoic acid derivatives we have identified synthetic retinoic acid analogues exhibiting high selectivity for the nuclear retinoic acid receptors RAR alpha (Am 580), RAR beta (CD 2019) and RAR gamma (CD 437) as well as ligands sharing high affinities for all RAR subtypes (CD 367). The compounds were evaluated in two complementary screening systems: (1) binding to nuclear proteins extracted from COS-7 cells after transfection with the appropriate expression vectors, and (2) induction of plasminogen activator in the embryonic mouse teratocarcinoma cell line F9. All compounds behaved as retinoic acid agonists in the F9 test.

Determination of retinoid activity by an enzyme-linked immunosorbent assay

In normal human keratinocytes, retinoic acid suppresses the expression of the plasma membrane associated enzyme transglutaminase (TGm) at the pretranslational level. This finding led us to develop an enzyme-linked immunosorbent assay (ELISA) for the evaluation of the biological activity of retinoids, i.e., natural and synthetic derivatives of vitamin A. In this assay, keratinocytes are cultured in a 96-well cluster in the presence of different retinoid concentrations. The expression of TGm is then quantified, without any extraction or purification step, using a TGm-specific monoclonal antibody and a peroxidase-conjugated secondary antibody. The dose-response curves obtained show this ELISA to be a sensitive and reproducible assay to determine the potency of retinoids.

Differentiation of F9 embryonal carcinoma cells by synthetic retinoids: amplitude of plasminogen activator production does not depend on retinoid potency or affinity for F9 nuclear retinoic acid receptors

Retinoic acid and analogues (retinoids) are able to induce the differentiation of F9 murine embryonal carcinoma stem cells into endoderm-like cells. The secretion of plasminogen activator (PA) which accompanies this differentiation is a good index of the biological response of F9 cells to retinoids. We have previously reported that the potency of a series of natural and synthetic retinoids, evaluated by the concentration which provokes half-maximal induction of PA, correlates well with the affinity of these compounds for the endogenous F9 nuclear retinoic acid receptors, but not for the cytosolic retinoic acid binding protein, CRABP. In this paper we show that various retinoids differ, not only in terms of potency, i.e. the dilution at which they are active, but also in terms of the amount of PA that they induce. This parameter, called amplitude, is used to quantify the extent of PA induction by a given retinoid relative to retinoic acid. The amplitude parameters of synthetic retinoids are found to vary over a wide range and are independent of both potency and binding affinity for F9 retinoic acid receptors. It is proposed that the amplitude of the biological response to a given retinoid is the resultant of three factors: (i) the total or partial agonist character of the retinoid; (ii) the binding spectrum of the retinoid for the various types of retinoic acid receptors; (iii) the chemical and metabolic stability of the retinoid in the test system.