Structure, Energetics, and Dynamics of Binding Coactivator Peptide to the Human Retinoid X Receptor alpha Ligand Binding Domain Complex with 9-cis-Retinoic Acid

Retinoid X receptors (RXRs) are ligand-dependent nuclear receptors, which are activated by the potent agonist 9-cis-retinoic acid (9cRA). 9cRA binds to the ligand binding domain (LBD) of RXRs and recruits coactivator proteins for gene transcription. Using isothermal titration calorimetry, the binding of a 13-mer coactivator peptide, GRIP-1, to the hRXR alpha-LBD homodimer complex containing 9cRA (hRXR alpha-LBD:9cRA:GRIP-1) is reported between 20 and 37 degrees C. Delta G is temperature independent (-8.5 kcal/mol), and GRIP-1 binding is driven by Delta H (-9.2 kcal/mol) at 25 degrees C. Delta C-p is large and negative (-401 cal mol(-1) K-1). The crystal structure of hRXR alpha-LBD:9cRA:GRIP-1 is reported at 2.05 angstrom. When the structures of hRXR alpha-LBD:9cRA:GRIP-1 and hRXR alpha-LBD:9cRA (1FBY) homodimers are compared, E453 and E456 on helix 12 bury and form ionic interactions with GRIP-1. R302 on helix 4 realigns to form new salt bridges to both E453 and E456. F277 (helix 3), F437 (helix 11), and F450 (helix 12) move toward the hydrophobic interior. The changes in the near-UV spectrum at 260 nm of the hRXR alpha-LBD:9cRA:GRIP-1 support this structural change. Helix 11 tilts toward helix 12 by approximate to 1 angstrom, modifying the ring conformation of 9cRA. Hydrogen-deuterium exchange mass spectroscopy indicates GRIP-1 binding to hRXR alpha-LBD:9cRA significantly decreases the exchange rates for peptides containing helices 3 (F277), 4 (R302), 11 (F437), and 12 (E453, E456). The structural changes and loss of dynamics of the GRIP-1-bound structure are used to interpret the energetics of coactivator peptide binding to the agonist-bound hRXR alpha-LBD.close

The Novel Retinoid, 9cUAB30, Inhibits Telomerase and Induces Apoptosis in HL60 Cells1

Telomerase, a ribonucleoprotein important to neoplastic immortality, is up-regulated in approximately 85% of cancers, including leukemias. In this study, 9cUAB30, a novel retinoic acid, resulted in differentiation of HL60 leukemia cells as indicated by morphologic changes characteristic of granulocytes. It also caused a down-regulation of hTERT gene expression and a decrease in telomerase activity. Telomerase inhibition was followed by loss of proliferative capacity, induction of apoptosis, and partial differentiation. These findings demonstrate the effectiveness of 9cUAB30 at inhibiting telomerase activity by down-regulating hTERT gene expression in human leukemic cells

Retinoid X Receptor Agonists Upregulate Genes Responsible for the Biosynthesis of All-Trans-Retinoic Acid in Human Epidermis.

UAB30 is an RXR selective agonist that has been shown to have potential cancer chemopreventive properties. Due to high efficacy and low toxicity, it is currently being evaluated in human Phase I clinical trials by the National Cancer Institute. While UAB30 shows promise as a low toxicity chemopreventive drug, the mechanism of its action is not well understood. In this study, we investigated the effects of UAB30 on gene expression in human organotypic skin raft cultures and mouse epidermis. The results of this study indicate that treatment with UAB30 results in upregulation of genes responsible for the uptake and metabolism of all-trans-retinol to all-trans-retinoic acid (ATRA), the natural agonist of RAR nuclear receptors. Consistent with the increased expression of these genes, the steady-state levels of ATRA are elevated in human skin rafts. In ultraviolet B (UVB) irradiated mouse skin, the expression of ATRA target genes is found to be reduced. A reduced expression of ATRA sensitive genes is also observed in epidermis of mouse models of UVB-induced squamous cell carcinoma and basal cell carcinomas. However, treatment of mouse skin with UAB30 prior to UVB irradiation prevents the UVB-induced decrease in expression of some of the ATRA-responsive genes. Considering its positive effects on ATRA signaling in the epidermis and its low toxicity, UAB30 could be used as a chemoprophylactic agent in the treatment of non-melanoma skin cancer, particularly in organ transplant recipients and other high risk populations

The retinoid X receptor agonist, 9-cis UAB30, inhibits cutaneous T-cell lymphoma proliferation through the SKP2-p27kip1 axis

BACKGROUND Bexarotene (Targretin) is currently the only FDA approved retinoid X receptor (RXR) -selective agonist for the treatment of cutaneous T-cell lymphomas (CTCLs). The main side effects of bexarotene are hypothyroidism and elevation of serum triglycerides (TGs). The novel RXR ligand, 9-cis UAB30 (UAB30) does not elevate serum TGs or induce hypothyroidism in normal subjects. OBJECTIVES To assess preclinical efficacy and mechanism of action of UAB30 in the treatment of CTCLs and compare its action with bexarotene. METHODS With patient-derived CTCL cell lines, we evaluated UAB30 function in regulating growth, apoptosis, cell cycle check points, and cell cycle-related markers. RESULTS Compared to bexarotene, UAB30 had lower half maximal inhibitory concentration (IC) values and was more effective in inhibiting the G1 cell cycle checkpoint. Both rexinoids increased the stability of the cell cycle inhibitor, p27kip1 protein, in part, through targeting components involved in the ubiquitination-proteasome system: 1) decreasing SKP2, a F-box protein that binds and targets p27kip1 for degradation by 26S proteasome and 2) suppressing 20S proteasome activity (cell line-dependent) through downregulation of PSMA7, a component of the 20S proteolytic complex in 26S proteasome. CONCLUSIONS UAB30 and bexarotene induce both early cell apoptosis and suppress cell proliferation. Inhibition of the G1 to S cell cycle transition by rexinoids is mediated, in part, through downregulation of SKP2 and/or 20S proteasome activity, leading to increased p27kip1 protein stability. Because UAB30 has minimal effect in elevating serum TGs and inducing hypothyroidism, it is potentially a better alternative to bexarotene for the treatment of CTCLs