Retinoid X Receptor Regulates Nur77/Thyroid Hormone Receptor 3-Dependent Apoptosis by Modulating Its Nuclear Export and Mitochondrial Targeting

Abstract

Retinoid X receptor (RXR) plays a central role in the regulation of intracellular receptor signaling pathways by acting as a ubiquitous heterodimerization partner of many nuclear receptors, including the orphan receptor Nur77 (also known as thyroid hormone receptor 3 or NGFI-B), which translocates from the nucleus to mitochondria, where it interacts with Bcl-2 to induce apoptosis. Here, we report that RXR␣ is required for nuclear export and mitochondrial targeting of Nur77 through their unique heterodimerization that is mediated by dimerization interfaces located in their DNA-binding domain. The effects of RXR␣ are attributed to a putative nuclear export sequence (NES) present in its carboxyl-terminal region. RXR␣ ligands suppress NES activity by inducing RXR␣ homodimerization or altering RXR␣/Nur77 heterodimerization. The RXR␣ NES is also silenced by RXR␣ heterodimerization with retinoic acid receptor or vitamin D receptor. Consistently, we were able to show that the mitochondrial targeting of the RXR␣/Nur77 heterodimer and its induction of apoptosis are potently inhibited by RXR ligands. Together, our results reveal a novel nongenotropic function of RXR␣ and its involvement in the regulation of the Nur77-dependent apoptotic pathway.Retinoid X receptors (RXRs) belong to the nuclear receptor superfamily, consisting of a large number of ligand-regulated transcription factors that mediate the diverse physiological functions of their ligands, such as steroid hormones, retinoids, thyroid hormone, and vitamin D 3 , in embryonic development, growth, differentiation, apoptosis, and homeostasis (29, 46). The superfamily also includes many orphan receptors whose ligands remain to be identified. All nuclear receptors consist of three major domains: the variable length N-terminal domain, the well-conserved DNA-binding domain (DBD), and the ligand-binding domain (LBD) (29,46). The C-terminal LBD is multifunctional and, in addition to harboring a ligand-binding site, contains regions for receptor dimerization and the ligand-dependent transactivation function (AF-2). The DBD also contains a dimerization interface that determines target gene specificity (38,55,60,85). RXRs mediate retinoid signaling through the RXR/retinoic acid receptor (RAR) heterodimer and the RXR/RXR homodimer (29,46,89). In addition, RXRs form heterodimers with many members of the subfamily 1 nuclear receptors, including vitamin D receptor (VDR), peroxisome proliferator-activated receptor (PPAR), and thyroid hormone receptor (TR), as well as several orphan receptors, such as liver X receptor, pregnane X receptor, constitutively activated receptor, and Nur77 (TR3 or NGFI-B) (29, 46). RXRs, therefore, play an essential role in the regulation of multiple nuclear hormone-signaling pathways through their unique and potent dimerization capacity. The vitamin A metabolite, 9-cis-retinoic acid (9-cis-RA), is a high-affinity ligand for RXRs (22,39). It induces transactivation of the RXR homodimer (26,29,46,88,90) and certain RXR heterodimers, such as the RXR/Nur77 ...

Regulation of Nur77 nuclear export by c-Jun N-terminal kinase and Akt

“…We previously reported that AHPN and its analog (E)-4-[3-(1-adamantyl)-4-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC) induced Nur77 mitochondrial targeting in prostate cancer and lung cancer cells (Dawson et al, 2001;Kolluri et al, 2003;Cao et al, 2004). Similar results were shown in ovarian cancer cells (Holmes et al, 2002(Holmes et al, , 2003.…”

“…To study whether JNK phosphorylation of Nur77 plays a role in mediating the effect of 3-Cl-AHPC on inducing Nur77 nuclear export and Nur77-dependent apoptosis (Dawson et al, 2001;Kolluri et al, 2003;Cao et al, 2004), we examined the effect of 3-Cl-AHPC on JNK activation. A panel of cancer cell lines, including H460, H292 and A549 lung cancer, LNCaP prostate cancer, and MDA-MB-231 breast cancer cells, were treated with 3-Cl-AHPC, and the activation of JNK was evaluated by an in vitro JNK kinase assay.…”

“…The observation that JNK inhibitor effectively suppressed 3-Cl-AHPC-induced apoptosis in these cells suggested that JNK might modulate Nur77 apoptotic activity. As Nur77 nuclear export is essential for the apoptotic effect of 3-Cl-AHPC in H460 cells (Kolluri et al, 2003;Cao et al, 2004;Lin et al, 2004), we examined whether JNK activation by 3-Cl-AHPC was involved in Nur77 nuclear export. H460 cells were treated with 3-Cl-AHPC in the absence or presence of a JNK inhibitor, then immunostained with an anti-Nur77 antibody ( Figure 1c).…”

“…We have previously demonstrated a new paradigm in cancer cell apoptosis, in which Nur77 is induced, then translocates from the nucleus to the mitochondria, leading to release of cytochrome c from the mitochondria (Li et al, 2000). The apoptosis-associated nuclear egress of Nur77 has been observed in cancer cells from the lung, ovary, colon, stomach and breast (Dawson et al, 2001;Holmes et al, 2002Holmes et al, , 2003Liu et al, 2002;Wu et al, 2002;Jeong et al, 2003;Kolluri et al, 2003;Wilson et al, 2003;Cao et al, 2004;Jacobs et al, 2004;Lin et al, 2004;Lee et al, 2005). Nur77 mitochondrial localization is also involved in Sindbis virus-induced apoptosis (Lee et al, 2002).…”

“…The Nur77-dependent apoptotic process is triggered by a number of apoptosis inducers, including retinoidrelated molecule 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecar-boxylic acid (AHPN) (also called CD437) (Li et al, 2000) and its analogs (Li et al, 2000;Dawson et al, 2001;Holmes et al, 2002Holmes et al, , 2003Kolluri et al, 2003;Cao et al, 2004;Lin et al, 2004), etoposide (Li et al, 2000;Liu et al, 2002;Wilson et al, 2003), and insulin-like growth factor binding protein-3 (Lee et al, 2005). The importance of Nur77-dependent apoptotic pathway in mediating the anticancer activity of chemotherapeutic agents is supported by the positive correlation between Nur77 family member Nor-1 expression and survival in diffuse large B-cell lymphoma patients treated with therapeutic drugs (Shipp et al, 2002).…”