Tumor suppressor IRF-1 mediates retinoid and interferon anticancer signaling to death ligand TRAIL

Identification of TRAIL as an interferon regulatory factor 3 transcriptional target

Interferon production and apoptosis in virus-infected cells are necessary to prevent progeny virus production and to eliminate infected cells. Paramyxovirus infection induces apoptosis through interferon regulatory factor 3 (IRF-3), but the exact mechanism of how IRF-3 functions is unknown. We show that IRF-3 is involved in the transcriptional induction of TRAIL, a key player in the apoptosis pathway. IRF-3 upregulates TRAIL transcription following viral infection and binds an interferon-stimulated response element in the TRAIL promoter. The mRNA for TRAIL and its receptor, DR5, are induced following viral infection. These studies identify TRAIL as a novel IRF-3 transcriptional target.

Valproic acid, in combination with all-trans retinoic acid and 5-aza-2'-deoxycytidine, restores expression of silenced RARbeta2 in breast cancer cells

Epigenetic silencing of tumor suppressor genes has been established as an important process of carcinogenesis. The retinoic acid (RA) receptor beta2 (RARbeta2) gene is one such tumor suppressor gene often silenced during carcinogenesis. The combined use of histone deacetylase and DNA methyltransferase inhibitors has been shown to reverse the epigenetic silencing of numerous growth regulatory genes. Valproic acid (VPA), which has long been used in the treatment of epilepsy, was shown recently to be an effective histone deacetylase inhibitor that can induce differentiation of neoplastically transformed cells. In this study, we show for the first time that VPA, in combination with RA and the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine (Aza-dC), can overcome the epigenetic barriers to transcription of a prototypical silenced tumor suppressor gene, RARbeta2, in human breast cancer cells. Chromatin immunoprecipitation assays show that the combination of VPA, RA, and Aza-dC increases histone acetylation at the silenced RARbeta2 promoter of MCF-7 breast cancer cells. Furthermore, reverse transcription-PCR analyses reveal cell type-specific effects in the actions of VPA on RARbeta2 expression in cultured human breast cancer cells. Finally, we show that VPA, in combination with RA and Aza-dC, inhibits the proliferation of both estrogen receptor alpha-positive (MCF-7) and estrogen receptor alpha-negative (MDA-MB-231) breast cancer cell lines. These data suggest that VPA may ultimately be useful in combination therapies in the treatment of human breast cancers.

Retinoic-acid-induced apoptosis in leukemia cells

Retinoic acid (RA) cures more than 75% of patients with acute promyelocytic leukemia (APL). Here, we review the various anti-cancer activities of retinoids and rexinoids, alone and in combination with other drugs, with emphasis on the RA-dependent induction of a cancer-cell-selective apoptosis signaling pathway to which multiple anti-cancer signals converge. These findings identify the TRAIL (tumor-necrosis-factor-related apoptosis-inducing ligand) pathway as a central cell-autonomous anti-cancer weapon that can act independently of the immune system.

Retinoic acid determines life span of leukemic cells by inducing antagonistic apoptosis-regulatory programs

As a single signal, retinoids induce terminal differentiation. This implies that they activate differentiation and apoptosis in a temporally defined order to allow expression of the differentiated phenotype well before death. We report that two apparently contradictory retinoid-induced programs have the capacity to define cellular life span. Anti-apoptotic factors are activated concomitantly with differentiation, while retinoids induce at the same time also pro-apoptotic signaling. We have assessed the roles of two key factors, Bcl2A1 and TRAIL, in the temporal programming of cell death and differentiation. We demonstrate that PLB985 are type II cells in which TRAIL induces apoptosis through the extrinsic and--via Bid activation--also the intrinsic death pathways. Bcl2A1, ectopically over-expressed, or endogenously induced by retinoic acid receptor agonists, protected cells from apoptosis triggered by TRAIL, whose induction required the activation of both the retinoic acid and retinoid X receptors. Bcl2A1 prevented loss of mitochondrial membrane potential and caspase-9, but not caspase-8, activation. The expression of anti-sense Bcl2A1 sensitized PLB985 cells to TRAIL. Co-culture experiments revealed protection from fraternicide if sister cells were pre-exposed to retinoic acid. Collectively, our data support a model in which retinoids orchestrate a life span-regulatory program comprising Bcl2A1 induction to temporally protect against concomitantly induced TRAIL death signaling. Termination of this life span in presence of Bcl2A1 is most likely a consequence of the Bid-independent TRAIL action. Thus, depending on the retinoic acid and retinoid X receptor activation potential of a ligand and the relative efficacies of the intrinsic and extrinsic death pathways in a given cell, a single retinoid triggers the life span of a differentiated phenotype.

Tumor-selective action of HDAC inhibitors involves TRAIL induction in acute myeloid leukemia cells

Chromatin is a dynamic macromolecular structure epigenetically modified to regulate specific gene expression. Altered chromatin function can lead to aberrant expression of growth regulators and may, ultimately, cause cancer. That many human diseases have epigenetic etiology has stimulated the development of 'epigenetic' therapies. Inhibitors of histone deacetylases (HDACIs) induce proliferation arrest, maturation and apoptosis of cancer cells, but not normal cells, in vitro and in vivo, and are currently being tested in clinical trials. We investigated the mechanism(s) underlying this tumor selectivity. We report that HDACIs induce, in addition to p21, expression of TRAIL (Apo2L, TNFSF10) by directly activating the TNFSF10 promoter, thereby triggering tumor-selective death signaling in acute myeloid leukemia (AML) cells and the blasts of individuals with AML. RNA interference revealed that the induction of p21, TRAIL and differentiation are separable activities of HDACIs. HDACIs induced proliferation arrest, TRAIL-mediated apoptosis and suppression of AML blast clonogenicity irrespective of French-American-British (FAB) classification status, karyotype and immunophenotype. No apoptosis was seen in normal CD34(+) progenitor cells. Our results identify TRAIL as a mediator of the anticancer action of HDACIs.

Antiestrogens, aromatase inhibitors, and apoptosis in breast cancer

Antiestrogens have been the therapeutic agents of choice for breast cancer patients whose tumors express estrogen receptors, regardless of menopausal status. Unfortunately, many patients will eventually develop resistance to these drugs. Antiestrogens primarily act by preventing endogenous estrogen from activating estrogen receptors and promoting cell growth, which can ultimately lead to tumor cell death. Understanding the mechanisms by which antiestrogens cause cell death or apoptosis is critical to our efforts to develop ways to circumvent resistance. This article focuses on antiestrogen-induced apoptosis both in vitro and in vivo. We review the clinical utility of both antiestrogens and aromatase inhibitors and their apoptogenic mechanisms in cell culture models. Among the key signaling components discussed are the roles of Bcl-2 family members, several cytokines, and their receptors, p53, nuclear factor kappa B (NFkappaB), IRF-1, phosphatidylinositol 3-kinase (PI3K)/Akt, and specific caspases. Finally, we discuss the evidence supporting a role for apoptotic defects in acquired and de novo antiestrogen resistance.

Principles for modulation of the nuclear receptor superfamily

Nuclear receptors are major targets for drug discovery and have key roles in development and homeostasis, as well as in many diseases such as obesity, diabetes and cancer. This review provides a general overview of the mechanism of action of nuclear receptors and explores the various factors that are instrumental in modulating their pharmacology. In most cases, the response of a given receptor to a particular ligand in a specific tissue will be dictated by the set of proteins with which the receptor is able to interact. One of the most promising aspects of nuclear receptor pharmacology is that it is now possible to develop ligands with a large spectrum of full, partial or inverse agonist or antagonist activities, but also compounds, called selective nuclear receptor modulators, that activate only a subset of the functions induced by the cognate ligand or that act in a cell-type-selective manner.

Monitoring ligand-mediated nuclear receptor-coregulator interactions by noncovalent mass spectrometry

Retinoid receptors are ligand-dependent transcription factors belonging to the nuclear receptor superfamily. Retinoic acid (RARalpha, beta, gamma) and retinoid X (RXRalpha, beta, gamma) receptors mediate the retinoid/rexinoid signal to the transcriptional machineries by interacting at the first level with coactivators or corepressors, which leads to the recruitment of enzymatically active noncovalent complexes at target gene promoters. It has been shown that the interaction of corepressors with nuclear receptors involves conserved LXXI/HIXXXI/L consensus sequences termed corepressor nuclear receptor (CoRNR) boxes. Here we describe the use of nondenaturing electrospray ionization mass spectrometry (ESI-MS) to determine the characteristics of CoRNR box peptide binding to the ligand binding domains of the RARalpha-RXRalpha heterodimer. The stability of the RARalpha-RXRalpha-CoRNR ternary complexes was monitored in the presence of different types of agonists or antagonists for the two receptors, including inverse agonists. These results show unambiguously the differential impact of distinct retinoids on corepressor binding. We show that ESI-MS is a powerful technique that complements classical methods and allows one to: (a) obtain direct evidence for the formation of noncovalent NR complexes; (b) determine ligand binding stoichiometries and (c) monitor ligand effects on these complexes.