Enhancement of HL-60 Differentiation by a New Class of Retinoids with Selective Activity on Retinoid X Receptor

Abstract

Cellular responsiveness to retinoic acid and its metabolites is conferred through two distinct families of receptors: the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). Herein, we report on the identification and characterization of several conformationally restricted retinoids, which selectively bind and activate RX receptors. Under the influence of retinoids, HL-60 myelocytic leukemia cells differentiate into granulocytes. This effect is mediated by RAR␣, as has been demonstrated through the use of a selective RAR␣ antagonist (Apfel, C., Bauer, F., Crettaz, M., Forni, L., Kamber, M., Kaufmann, F., LeMotte, P., Pirson, W., and Klaus, M. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 7129 -7133). Here, we show that conformationally restricted RXR-specific retinoids, at doses that are per se inactive, are able to potentiate by up to one order of magnitude the pro-differentiating effects of all-trans retinoic acid and an RAR␣-selective synthetic retinoid. We also present evidence that these RXR-selective ligands are able to bind to a DNA RXR⅐RAR heterodimer complex. This finding demonstrates that agonists for RARs and RXRs can synergistically promote HL-60 differentiation, which could be mediated through a heterodimer of these receptors.A subclass of nuclear hormone receptors have been described, termed retinoid X receptors (RXR) 1 (2-4), which are ligand-inducible transcription factors responsive to the 9-cis isomer of retinoic acid (5, 6). A related subclass of receptors, the retinoic acid receptors (RAR) (7-12), also responds to 9-cis retinoic acid (9-cis RA), as well as to the all-trans isomer of retinoic acid (t-RA). A striking feature of RXRs is their ability to heterodimerize with several members of the steroid receptor superfamily, including the RA receptors, thyroid receptors, peroxisome proliferator-activated receptors, vitamin D receptor, and chicken ovalbumin upstream promoter-transcription factor (13-21). Growing evidence suggests that heterodimers are the active species for binding to promoter response elements.In view of the involvement of RXRs in multiple signaling pathways, RXR-selective ligands can possibly be expected to show medical usefulness through their modulation of the above mentioned and perhaps also other hormone-regulated processes. The pharmacological effects of RXR ligands, which are attributable solely to activity on RXRs, are difficult to assess using 9-cis RA as an activating ligand since 9-cis RA also functions as an RAR ligand and consequently elicits the full range of classical retinoid effects. Therefore, to gain deeper insight into RXR function as well as to evaluate their pharmacological usefulness, we have designed and characterized a number of RXR-selective compounds and report on some intriguing pharmacological properties. These compounds are sterically restricted analogues of 9-cis RA, which cannot isomerize. They exhibit high potency and high selectivity for RXRs versus RARs and belong to different structural classes than other RXR-selective compounds r...

The role of RAR and RXR activation in retinoid-induced tissue factor suppression

“…13 However, several studies have shown that in normal RAR␣ protein expressing myeloid cells the synergistic activation of both partners of the RAR-RXR complex is required for activation of RARE transcriptional activity and for many cellular responses, eg induction of cellular differentiation, growth arrest and retinoblastoma protein phosphorylation. 14,15,38 Consistent with these results we show that activation of RAR␣ or RXR alone by synthetic nuclear receptor agonists leads only to a moderate suppression of TF mRNA, TF antigen and TF procoagulant activity in the monoblastic U-937 cells. Full down-regulation was observed when both RAR␣ and RXR were activated.…”

Retinoids cause apoptosis in pancreatic cancer cells via activation of RAR-γ and altered expression of Bcl-2/Bax

“…Since the RXR agonist LG101305 alone had no effect on cell viability, RXR homodimers do not mediate the response. Instead, the fact that the effects of LG101093 and LGD1550 are potentiated by LG101305 supports a model where RAR/RXR heterodimers are optimally activated when both RAR and RXR bind their ligand (Apfel et al, 1995;Minucci et al, 1997). However, RXR activation is clearly not required, since this potentiation by LG101305 was abolished at higher concentrations of LG101093 and LGD1550.…”

Distinct retinoid X receptor activation function-2 residues mediate transactivation in homodimeric and vitamin D receptor heterodimeric contexts

“…This is consistent with previous in vitro data, which have indicated that the ligand status of the primary receptor (VDR) can allosterically regulate the response of its RXR partner to 9-cis RA . Such intra-heterodimeric subordination of the RXR AF-2 as a result of allosteric action by the 'primary' partner again mirrors the RXR-RAR heterodimer, where, as discussed above, RXR signaling is inactive unless the RAR partner is liganded, and can only operate through synergy with the RAR ligand (Apfel et al 1995, La Vista-Picard et al 1996, Chiba et al 1997, Minucci et al 1997, Vivat et al 1997. Although synergism by both cognate ligands of the RXR-VDR heterodimer has been reported , Kephart et al 1996, Li et al 1997, we observe no such synergistic activity by 9-cis RA at any concentration in the test systems described in this report.…”