Enantioselective syntheses of potent retinoid X receptor ligands: differential biological activities of individual antipodes

Novel (2E,4E,6Z)-7-(2-alkoxy-3,5-dialkylbenzene)-3-methylocta-2,4,6-trienoic acid retinoid X receptor modulators are active in models of type 2 diabetes

Previous data have shown that RXR-selective agonists (e.g., 3 and 4) are insulin sensitizers in rodent models of non-insulin-dependent diabetes mellitus (NIDDM). Unfortunately, they also produce dramatic increases in triglycerides and profound suppression of the thyroid hormone axis. Here we describe the design and synthesis of new RXR modulators that retain the insulin-sensitizing activity of RXR agonists but produce substantially reduced side effects. These molecules bind selectively and with high affinity to RXR and, unlike RXR agonists, do not activate RXR homodimers. To further evaluate the antidiabetic activity of these RXR modulators, we have designed a concise and systematic structure-activity relationship around the 2E,4E,6Z-7-aryl-3-methylocta-2,4,6-trienoic acid scaffold. Selected compounds have been evaluated using insulin-resistant rodents (db/db mice) to characterize effects on glucose homeostasis. Our studies demonstrate the effectiveness of RXR modulators in lowering plasma glucose in the db/db mouse model.

Treatment of non-insulin-dependent diabetes mellitus

Diabetes mellitus has been declared to be at an epidemic level by the World Health Organization. The syndrome is characterised as either Type I (insulin-dependent) or Type II (non-insulin-dependent) diabetes mellitus. Impaired glucose tolerance for extended periods of time results in serious complications such as kidney damage and impaired blood circulation and is the main cause for blindness and amputations in patients with diabetes. A combination of life-style change, dietary change and oral medications can treat Type II diabetes mellitus effectively and prevent long-term complications. Combination therapy appears to be the most effective approach in controlling blood glucose levels. This review updates the progress made in medicinal chemistry towards promising biological targets, with the development of a new generation of small molecules having improved efficacy and safety profiles.

The modified firefly luciferase reporter gene (luc+) but not Renilla luciferase is induced by all-trans retinoic acid in MCF-7 breast cancer cells

Luciferase genes are widely used as reporters to analyze promoter and regulatory elements. We found that a luciferase reporter gene vector with a modified firefly luciferase gene (luc+), but not Renilla luciferase (Rluc), was induced by all-trans retinoic acid (tRA) in the MCF-7 breast cancer cell line. tRA (5 x 10(-6) M) increased luciferase activity of the pGL3 promoter vector (containing luc+) up to approximately 3.8-fold in MCF-7 cells, but not in LNCaP prostate cancer cells or JEG-3 choriocarcinoma cells. Chimeric plasmids were constructed and showed that tRA-induction required the luc+ gene, but not any specific promoter or vector sequence. Time course and dose-response studies of tRA-induction indicated that longer treatment (> 24h) and higher tRA dose (> 10(-6) M) were required for luc+ induction compared with those for a positive retinoic acid response element (maximum induction at 6 h and 10(-8) M tRA). Studies with the translation inhibitor, cycloheximide, indicated the half-life of the luc+ protein was increased from 9.7 +/- 1.5 to 22.1 +/- 3.1 h with tRA treatment. Other retinoids, TTNPB, a retinoic acid receptor beta/gamma-specific ligand, and a retinoid X receptor ligand, did not significantly increase luc+ expression. Caution is needed in analysis of retinoid responsive gene regulation with the luciferase reporter system in MCF-7 cells, especially at high retinoid concentrations.

Transcriptional induction of mitogen-activated protein kinase phosphatase 1 by retinoids. Selective roles of nuclear receptors and contribution to the antiapoptotic effect

All-trans-retinoic acid (t-RA) inhibits hydrogen peroxide (H(2)O(2))-induced apoptosis by inhibiting the c-Jun N-terminal kinase (JNK)-activator protein 1 (AP-1) pathway. In this report, we examined the involvement of mitogen-activated protein kinase phosphatase 1 (MKP-1) in suppression of JNK and the antiapoptotic effect of t-RA and the roles of nuclear receptors in the regulation of MKP-1 by t-RA. We found that not only t-RA, but also a selective agonist of retinoic acid receptor (RAR), a selective agonist of retinoid X receptor (RXR), and a pan-agonist of RAR and RXR all induced MKP-1 at the transcriptional level. Activation of RAR was required for all of these triggering effects, but activation of RXR was required only for the RXR agonist-induced MKP-1 expression. Among the three RAR subtypes, RARalpha and RARgamma, but not RARbeta, mediated the t-RA-induced MKP-1 expression. The antiapoptotic effect of t-RA on H(2)O(2)-induced apoptosis in several cell types was correlated with the inducibility of MKP-1 by t-RA. Inhibition of MKP-1 by vanadate enhanced JNK phosphorylation and attenuated the antiapoptotic effect of t-RA. Furthermore, overexpression of MKP-1 inhibited H(2)O(2)-induced JNK phosphorylation and apoptosis. To our knowledge, this is the first to demonstrate that 1) MKP-1 is inducible by retinoids at the transcriptional level, 2) RXR and individual RAR subtypes have different roles in this process, and 3) the induced MKP-1 plays a significant role in mediating both JNK inhibition and the antiapoptotic effect of t-RA in oxidative stress.

RXR receptor agonist suppression of thyroid function: central effects in the absence of thyroid hormone receptor

High-affinity agonists for the retinoic acid X receptors (RXR) have pleotropic effects when administered to humans. These include induction of hypertriglyceridemia and hypothyroidism. We determined the effect of a novel high-affinity RXR agonist with potent antihyperglycemic effects on thyroid function of female Zucker diabetic rats and nondiabetic littermates and in db/db mice. In both nondiabetic and ZFF rats, AGN194204 causes a 70-80% decrease in thyrotropin (TSH), 3,3',5-triiodothyronine, and thyroxine (T(4)) concentrations. In the db/db mouse, AGN194204 causes a time-dependent decrease in thyroid hormone levels with the fall in TSH that was significant after 1 day of treatment preceding the fall in T(4) levels that was significant at 3 days of treatment. Treatment with AGN194204 caused an initial increase in hepatic 5'-deiodinase mRNA levels which then fell to undetectable levels by 3 days of treatment and continued to be low at 7 days of treatment. After treatment for 5 days with AGN194204, both wild-type and thyroid hormone receptor beta (TR beta(-/-))-deficient mice demonstrated a nearly 50% decrease in serum TSH and T(4) concentrations. The results suggest that a high-affinity RXR agonist with antihyperglycemic activity can cause central hypothyroidism independently of TR beta, the main mediator of hormone-induced TSH suppression.

Vitamin A enhances in vitro Th2 development via retinoid X receptor pathway

Vitamin A deficiency diminishes Th2-mediated Ab responses, and high-level dietary vitamin A or treatment with the vitamin A metabolite retinoic acid (RA) enhances such responses. To identify a potential mechanism(s) underlying this in vivo activity of vitamin A, we examined the effects of all-trans and 9-cis RA on development of Th1 and Th2 cell populations using in vitro stimulation of Ag-naive Th0 cells from the DO11.10 TCR-transgenic mouse. Treatment with 9-cis, but not with all-trans RA, at primary stimulation strongly enhanced Th2 development. IL-4-neutralizing Ab blocked this activity, but IL-12- and IFN-gamma-neutralizing Ab did not. Because 9-cis RA regulates gene transcription via either RA receptors or retinoid X receptors (RXRs), we tested the Th2-enhancing activities of the RXR- and RA receptor-selective agonists AGN194204 and 4-((E)-2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1-propenyl)benzoic acid (TTNPB). AGN194204 strongly enhanced Th2 development, whereas TTNPB did not. This RXR agonist also enhanced Th2 development when purified, naive Th0 cells (L-selectin(high)/CD4(+)) were stimulated with CD3 and CD28 Abs in the absence of APCs. During primary antigenic stimulation of naive Th0 cells from DO11.10 mice, AGN194204 increased IL-4 and IL-5 production, decreased IFN-gamma production, increased mRNA in responding T cells for genes involved in Th2 development (IL-4, GATA-3, and c-maf), and decreased mRNA for genes involved in Th1 development (IFN-gamma, T-bet, and IL-12R). These data show that stimulation of the RXR pathway enhances Th2 development, perhaps by affecting the relative expression of pertinent transcription factors, cytokines, and cytokine receptors.