Retinoid-related Molecules Research Articles

Novel adamantyl retinoid-related molecules with POLA1 inhibitory activity

Atypical retinoids (AR) or retinoid-related molecules (RRMs) represent a promising class of antitumor compounds. Among AR, E-3-(3′-adamantan-1-yl-4′-hydroxybiphenyl-4-yl)acrylic acid (adarotene), has been extensively investigated. In the present work we report the results of our efforts to develop new adarotene-related atypical retinoids endowed also with POLA1 inhibitory activity. The effects of the synthesized compounds on cell growth were determined on a panel of human and hematological cancer cell lines. The most promising compounds showed antitumor activity against several tumor histotypes and increased cytotoxic activity against an adarotene-resistant cell line, compared to the parent molecule.The antitumor activity of a selected compound was evaluated on HT-29 human colon carcinoma and human mesothelioma (MM487) xenografts. Particularly significant was the in vivo activity of the compound as a single agent compared to adarotene and cisplatin, against pleural mesothelioma MM487. No reduction of mice body weight was observed, thus suggesting a higher tolerability with respect to the parent compound adarotene.

Abstract 4848: Preclinical antitumor activity of novel DNA polymerase 1 (POLA1) inhibitors

Abstract The anti-proliferative and pro-apoptotic effects of Retinoid-Related Molecules (RRMs) are described as independent from Retinoids' receptors-mediated transcriptional activity. Prototypes of this class are CD437 and its more potent analogue ST1926, which have a strong antitumor activity by targeting DNA polymerase 1 alpha (POLA1) (Han et al. Nat Chem Biol. 2016; Abdel-Samad et al. AJCR, in press). With aim to identify new RRMs with an improved pharmacological profile, we synthetized and screened a library of RRMs for their antitumor properties and inhibitory activity on POLA1. From this screening, four molecules, MIR002, MIR020, MIR072 and MIR075, were selected. All exert a potent anti-proliferative effect, with G1/S arrest and apoptosis, in more than 50 cancer cell lines derived from human hematological and solid tumors. From a mechanistic point of view, these RRMs modulate, at different extent, POLA1 activity and/or expression. Notably, NSCLC cells harboring POLA1-L764F mutation (H460-R9A), are resistant to both CD437 and ST1926 (IC50>50 higher than the one of wild type H460 cells), while they are sensitive to MIR002, showing for this RRM-derivative an improved/different pharmacological profile with respect to CD437 and ST1926. Hints on the possibility of these new RRMs to be orally absorbed were obtained using cancer cells overexpressing or not P-glycoprotein (Pgp), known as the major player limiting the oral absorption of several chemotherapeutic agents. Results from these experiments revealed that the Tested compounds are not Pgp substrate thus suggesting the possibility for their absorption via oral route MIR002 in vivo activity was assessed in tumors from Malignant Mesothelioma derived cells (MM487), and lung cancer cells (H460 and H460-R9A). In all the evaluated models, MIR002 induced a strong Tumor Growth Inhibition either alone or in combination with cisplatin (TGI>61% and TGI>80-100%, respectively). The treatment of orthotopic models of malignant mesothelioma (MM487) with MIR072 in combination with Cisplatin, resulted in a impressive synergic antitumor activity if compared to Cisplatin monotherapy (TGI 95% vs 55%). Tests on orthotopic transplants of hepatocellular carcinoma cells (HepG2), showed that MIR020 has significant antitumor effects (TGI 72%). Finally, the activity of MIR075 was evaluated on glioblastoma luciferase-expressing cells (U-87MG) intracranically injected. Also this compound displayed a significant tumor growth inhibition (TGI 72%), as measured by IVIS imaging system. Taken together, the results from in vitro and in vivo experiments indicate that this new class of RRMs, including MIR002, MIR072, MIR020, and MIR075, modulate POLA1 functions and activate pro-apoptotic pathways. The large spectrum of antitumor activity, together with the high tolerability observed, opens the possibility for their clinical investigation in different population of cancer patients. Citation Format: Claudio Pisano, Lucio Merlini, Sergio Penco, Raffaella Cincinelli, Nadine Darwiche, Mario B. Guglielmi, Ilaria La Porta, Giacomo Signorino, Gabriele De Rubis, Fabiana Colelli, Francesco Cardile, Alessandra Fucci, Egildo L. D'Andrea, Sabrina Dallavalle. Preclinical antitumor activity of novel DNA polymerase 1 (POLA1) inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4848.

Abstract 4195: MIR002: a new POLA1 inhibitor endowed with a large spectrum of antitumor activity

Abstract A recent work from Han et al. disclosed DNA polymerase 1 alpha (POLA1) as the key target for the anti-cancer effects of CD437, a molecule belonging to the class of Retinoid Related Molecules (RRMs). Before this evidence, this family of compounds, of which CD437 and ST1926 represent the prototypes, has been characterized for its antiproliferative, antitumor and pro-apoptotic activity. With aim to identify a new RRM with an improved pharmacological profile, we recently selected MIR002 as a novel compound endowed with a potent antitumor activity and a peculiar pharmacological profile. In vitro treatment with MIR002 leads to a G1/S arrest and exerts a potent anti-tumor/proapoptotic activity in a wide range of cancer cell lines, including H460-R9A cells, which are cross resistant to ST1926 and CD437 (IC50>70 fold of H460-R9A vs H460). Moreover, in H460-R9A cells, we identified the L764F mutation in POLA1, already described as a mutation conferring resistance to CD437. We also demonstrated that POLA1 is the molecular target of both MIR002 and ST1926 and that, interestingly, the expression of the L764F mutant, although associated with the resistance to RRM’s, only marginally reduces the cytotoxic effects of MIR002. In order to investigate the activity of MIR002 against tumor growth, we examined a panel of in vivo xenograft tumor models, including those originating from human Non-Small Cell Lung Cancer (NSCLC) and Malignant Pleural Mesothelioma (MPM). In these models, using a q2dx5x3w schedule in doses ranging from 50 to 70 mg/Kg administered orally, we observed a strong tumor growth inhibition (TGI>70%). Interestingly, the combination of MIR002 with Cisplatin (4mg/Kg; qd7x3w; i.v.) in MPM models showed a synergistic antitumor efficacy, reaching a TGI>90%, and cured animals. Interestingly, we previously identified POLA1 as a key pro-proliferative player in 9 primary MPM samples. The ex-vivo analyses indicated that POLA1 expression was strongly modulated upon MIR002 treatment. Furthermore, in the same models, we also observed that the expression/secretion of multiple circulating factors is affected by MIR002 and we identified some of them as potential biomarkers of tumor responsiveness to MIR002. Taken together, our results identify MIR002 as a novel anti-cancer compound and suggest POLA1 as a target for new antitumor strategies to be investigated in Clinical Trials. Citation Format: Claudio Pisano, Lucio Merlini, Sergio Penco, Raffaella Cincinelli, Nadine Darwiche, Mario B. Guglielmi, Antonietta Esposito, Ilaria La Porta, Giacomo Signorino, Gabriele De Rubis, Fabiana Colelli, Francesco Cardile, Alessandra Fucci, Egildo L. D'Andrea, Sabrina Dallavalle. MIR002: a new POLA1 inhibitor endowed with a large spectrum of antitumor activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4195. doi:10.1158/1538-7445.AM2017-4195

Inhibition of IκB kinase-β and IκB kinase-α by heterocyclic adamantyl arotinoids

We recently reported on a series of retinoid-related molecules containing an adamantyl group, a.k.a. adamantyl arotinoids (AdArs), that showed significant cancer cell growth inhibitory activity and activated RXRα (NR2B1) in transient transfection assays while devoid of RAR transactivation capacity. We have now explored whether these AdArs could also bind and inhibit IKKβ, a known target that mediates the induction of apoptosis and cancer cell growth inhibition by related AdArs containing a chalcone functional group. In addition, we have prepared and evaluated novel AdArs that incorporate a central heterocyclic ring connecting the adamantyl-phenol and the carboxylic acid at the polar termini. Our results indicate that the majority of the RXRα activating compounds lacked IKKβ inhibitory activity. In contrast, the novel heterocyclic AdArs containing a thiazole or pyrazine ring linked to a benzoic acid motif were potent inhibitors of both IKKα and IKKβ, which in most cases paralleled significant growth inhibitory and apoptosis inducing activities.

Upregulation of miR-150* and miR-630 Induces Apoptosis in Pancreatic Cancer Cells by Targeting IGF-1R

MicroRNAs have been implicated in many critical cellular processes including apoptosis. We have previously found that apoptosis in pancreatic cancer cells was induced by adamantyl retinoid-related (ARR) molecule 3-Cl-AHPC. Here we report that 3-Cl-AHPC-dependent apoptosis involves regulating a number of microRNAs including miR-150* and miR-630. 3-Cl-AHPC stimulated miR-150* expression and caused decreased expression of c-Myb and IGF-1R in the pancreatic cancer cells. 3-Cl-AHPC-mediated reduction of c-Myb resulted in diminished binding of c-Myb with IGF-1R and Bcl-2 promoters, thereby causing repression of their transcription and protein expression. Over-expression of miR-150* also resulted in diminished levels of c-Myb and Bcl-2 proteins. Furthermore, the addition of the miRNA inhibitor 2′-O-methylated miR-150 blocked 3-Cl-AHPC-mediated increase in miR-150* levels and abrogated loss of c-Myb protein. Knockdown of c-Myb in PANC-1 cells resulted in enhanced apoptosis both in the presence or absence of 3-Cl-AHPC confirming the anti-apoptotic property of c-Myb. Overexpression of miR-630 also induced apoptosis in the pancreatic cancer cells and inhibited target protein IGF-1R mRNA and protein expression. Together these results implicate key roles for miR-150* and miR-630 and their targeting of IGF-1R to promote apoptosis in pancreatic cancer cells.

Synthetic retinoid CD437 induces apoptosis and acts synergistically with TRAIL receptor-2 agonist in malignant melanoma

The novel synthetic retinoid, CD437, shows potent anti-tumor activity in a range of different cancer cell lines and now serves as a prototype for development of new retinoid related molecules (RRMs). The purpose of this study was to examine the effect and cellular targets of CD437 in the human metastatic melanoma cell lines FEMX-1 and WM239. We showed that treatment with CD437 led to cell cycle arrest and induced apoptosis through both the extrinsic- and intrinsic pathways (caspase 8, -9 and PARP cleavage) in both cell lines. Interestingly, apoptosis was induced independently of DNA-fragmentation in FEMX-1 cells, and appeared partially caspase-independent in the WM239 cells. Additionally, up-regulation of CHOP mRNA and cathepsin D protein expression, following retinoid treatment, suggests involvement of the endoplasmatic reticulum (ER) and lysosomes, respectively. Combination of suboptimal concentrations of CD437 and lexatumumab, a TRAIL death receptor-2 agonist, resulted in synergistic reduction of viable cells, along with increased PARP cleavage. These results indicate that CD437 has a strong anti-neoplastic effect alone and in combination with lexatumumab in melanoma cell lines.

Adamantyl Retinoid-Related Molecules Induce Apoptosis in Pancreatic Cancer Cells by Inhibiting IGF-1R and Wnt/β-Catenin Pathways

Pancreatic carcinoma has a dismal prognosis as it often presents as locally advanced or metastatic. We have found that exposure to adamantyl-substituted retinoid-related (ARR) compounds 3-Cl-AHPC and AHP3 resulted in growth inhibition and apoptosis induction in PANC-1, Capan-2, and MiaPaCa-2 pancreatic cancer cell lines. In addition, AHP3 and 3-Cl-AHPC inhibited growth and induced apoptosis in spheres derived from the CD44+/CD24+ (CD133+/EpCAM+) stem-like cell population isolated from the pancreatic cancer cell lines. 3-Cl-AHPC-induced apoptosis was preceded by decreasing expression of IGF-1R, cyclin D1, β-catenin, and activated Notch-1 in the pancreatic cancer cell lines. Decreased IGF-1R expression inhibited PANC-1 proliferation, enhanced 3-Cl-AHPC-mediated apoptosis, and significantly decreased sphere formation. 3-Cl-AHPC inhibited the Wnt/β-catenin pathway as indicated by decreased β-catenin nuclear localization and inhibited Wnt/β-catenin activation of transcription factor TCF/LEF. Knockdown of β-catenin using sh-RNA also induced apoptosis and inhibited growth in pancreatic cancer cells. Thus, 3-Cl-AHPC and AHP3 induce apoptosis in pancreatic cancer cells and cancer stem-like cells and may serve as an important potential therapeutic agent in the treatment of pancreatic cancer.

Ligand-Dependent Regulation of the Activity of the Orphan Nuclear Receptor, Small Heterodimer Partner (SHP), in the Repression of Bile Acid BiosyntheticCYP7A1andCYP8B1Genes

Small heterodimer partner (SHP) plays important roles in diverse biological processes by directly interacting with transcription factors and inhibiting their activities. SHP has been designated an orphan nuclear receptor, but whether its activity can be modulated by ligands has been a long-standing question. Recently, retinoid-related molecules, including 4-[3-(1-adamantyl)-4-hydroxyphenyl]-3-chlorocinnamic acid (3Cl-AHPC), were shown to bind to SHP and enhance apoptosis. We have examined whether 3Cl-AHPC acts as an agonist and increases SHP activity in the repression of bile acid biosynthetic CYP7A1 and CYP8B1 genes and delineated the underlying mechanisms. Contrary to this expectation, micromolar concentrations of 3Cl-AHPC increased CYP7A1 expression but indirectly via p38 kinase signaling. Nanomolar concentrations, however, repressed CYP7A1 expression and decreased bile acid levels in HepG2 cells, and little repression was observed when SHP was down-regulated by small hairpin RNA. Mechanistic studies revealed that 3Cl-AHPC bound to SHP, increased the interaction of SHP with liver receptor homologue (LRH)-1, a hepatic activator for CYP7A1 and CYP8B1 genes, and with repressive cofactors, Brahma, mammalian Sin3a, and histone deacetylase-1, and, subsequently, increased the occupancy of SHP and these cofactors at the promoters. Mutation of Leu-100, predicted to contact 3Cl-AHPC within the SHP ligand binding pocket by molecular modeling, severely impaired the increased interaction with LRH-1, and repression of LRH-1 activity mediated by 3Cl-AHPC. 3Cl-AHPC repressed SHP metabolic target genes in a gene-specific manner in human primary hepatocytes and HepG2 cells. These data suggest that SHP may act as a ligand-regulated receptor in metabolic pathways. Modulation of SHP activity by synthetic ligands may be a useful therapeutic strategy.

Adamantyl-Substituted Retinoid-Related Molecules Induce Apoptosis in Human Acute Myelogenous Leukemia Cells

The adamantyl-substituted retinoid-related (ARR) compounds 3-Cl-AHPC and AHP3 induce apoptosis in vitro and in vivo in a newly established human acute myelogenous leukemia (AML) cell line, FFMA-AML, and in the established TF(v-SRC) AML cell line. FFMA-AML and TF(v-SRC) cells displayed resistance to apoptosis mediated by the standard retinoids (including trans-retinoic acid, 9-cis-retinoic acid, and the synthetic retinoid TTNPB) but showed sensitivity to apoptosis mediated by 3-Cl-AHPC- and AHP3 in vitro and in vivo as documented by poly(ADP-ribose) polymerase (PARP) cleavage and apoptosis terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling assay. 3-Cl-AHPC or AHP3 exposure in vitro resulted in decreased expression of the antiapoptotic proteins (cellular inhibitor of apoptosis 1, X-linked inhibitor of apoptosis protein) and phospho-Bad and activated the NF-κB canonical pathway. A significant prolongation of survival was observed both in nonobese diabetic severe combined immunodeficient mice carrying FFMA-AML cells and treated with either 3-Cl-AHPC or AHP3 and in severe combined immunodeficient mice carrying TF(v-SRC) cells and treated with AHP3. We have previously shown that ARRs bind to the orphan nuclear receptor small heterodimer partner (SHP) and that the expression of SHP is required for ARR-mediated apoptosis. Induced loss of SHP in these AML cells blocked 3-Cl-AHPC- and AHP3-mediated induction of apoptosis. These results support the further development of 3-Cl-AHPC and AHP3 as potential therapeutic agents in the treatment of AML patients.

The Peptidomimetic, 1-Adamantyl-Substituted, and Flex-Het Classes of Retinoid-Derived Molecules: Their Structure-Activity Relationships and Retinoid Receptor-Independent Anticancer Activities

Increasing evidence demonstrates that three classes of molecules originally derived from all-trans-retinoic acid and its synthetic analogues, which function by interacting with the retinoid nuclear receptors, exert their anticancer activities through alternative signaling pathways. Thus, the methylene-linked analogues (4-HBR, 4-HPRCG, and 4-HBRCG) of N-(4-hydroxyphenyl) retinamide (4-HPR) and its O-glucuronide metabolite (4-HPROG), the cinnamic acid analogues (3-Cl-AHPC and AHPC/ST1926) of 6-[3'-(1-adamantyl)-4'-hydroxyphenyl)]-2-naphthalenecarboxylic acid, and N-(2,3-dihydro-2,2,4,4-tetramethyl-6-benzothiopyranyl),N'-(4-nitrophenyl)thiourea (SHetA2) induce cancer cell-cycle arrest and apoptosis mediated most likely through mitochondrial and/or endoplasmic reticulum stress responses. Structure-activity relationships and potential for clinical translation as anticancer therapeutics are presented.

SHP and Sin3A expression are essential for adamantyl-substituted retinoid-related molecule–mediated nuclear factor-κB activation, c-Fos/c-Jun expression, and cellular apoptosis

We previously found that the adamantyl-substituted retinoid-related molecules bind to the small heterodimer partner (SHP) as well as the Sin3A complex. In this report, we delineated the role of SHP and the Sin3A complex in 4-[3'-(1-adamantyl)-4'-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC)-mediated inhibition of cell growth and apoptosis. We examined the effect of loss of SHP and Sin3A expression in a number of cell types on 3-Cl-AHPC-mediated growth inhibition and apoptosis induction, 3-Cl-AHPC-mediated nuclear factor-kappaB (NF-kappaB) activation, and 3-Cl-AHPC-mediated increase in c-Fos and c-Jun expression. We found that loss of SHP or Sin3A expression, while blocking 3-Cl-AHPC-mediated apoptosis, had little effect on 3-Cl-AHPC inhibition of cellular proliferation. We have previously shown that 3-Cl-AHPC-mediated NF-kappaB activation is necessary for apoptosis induction. We have now shown that 3-Cl-AHPC-enhanced c-Fos and c-Jun expression is also essential for maximal 3-Cl-AHPC-mediated apoptosis. 3-Cl-AHPC induction of c-Fos and c-Jun expression as well as NF-kappaB activation was dependent on SHP protein levels. In turn, SHP levels are regulated by Sin3A because ablation of Sin3A resulted in a decrease in SHP expression. Thus, SHP and Sin3A play an important role in adamantyl-substituted retinoid-related induction of cellular apoptosis.

Highly twisted adamantyl arotinoids: Synthesis, antiproliferative effects and RXR transactivation profiles

Retinoid-related molecules with an adamantyl group (adamantyl arotinoids) have been described with selective activities towards the retinoid receptors as agonists for NR1B2 and NR1B3 (RARβ,γ) (CD437, MX3350-1) or RAR antagonists (MX781) that induce growth arrest and apoptosis in cancer cells. Since these molecules induce apoptosis independently of RAR transactivation, we set up to synthesize novel analogs with impaired RAR binding. Here we describe adamantyl arotinoids with 2,2′-disubstituted biaryl rings prepared using the Suzuki coupling of the corresponding fragments. Those with cinnamic and naphthoic acid end groups showed significant antiproliferative activity in several cancer cell lines, and this effect correlated with the induction of apoptosis as measured by caspase activity. Strikingly, some of these compounds, whereas devoid of RAR binding capacity, were able to activate RXR.

Epigenetic Inhibition of Nuclear Receptor Small Heterodimer Partner Is Associated With and Regulates Hepatocellular Carcinoma Growth

Aberrant hypermethylation of promoter regions in cytosine-guanine dinucleotides (CpG) islands has been shown to be associated with transcriptional silencing of tumor-suppressor genes in many cancers. This study evaluated the methylation profile and the tumor-suppressive function of the small heterodimer partner (SHP, NR0B2) in the development of human hepatocellular carcinoma (HCC). Human HCC pathologic specimens and cell lines were used as model systems in this study. The expression of SHP is diminished in HCC pathologic specimens and cell lines by epigenetic silencing owing to SHP promoter hypermethylation. In vitro methylation decreased SHP promoter transactivation and nuclear receptor LRH-1 binding, an event that was reversed by demethylation. Overexpression of SHP inhibited HCC foci formation, arrested HCC tumor growth in xenografted nude mice, and increased the sensitivity of HCC cells to apoptotic stimuli. Further analysis of a total of 19 normal liver and 57 HCC specimens showed that down-regulation of SHP gene expression may be a common denominator of HCC. We propose that SHP functions as a novel tumor suppressor in the development of HCC. These findings provide new insight into the molecular mechanisms leading to this common cancer and may have both diagnostic and therapeutic applications.

Synthesis and structure–activity relationships of new antiproliferative and proapoptotic retinoid-related biphenyl-4-yl-acrylic acids

Atypical retinoids, or retinoid-related molecules (RRMs), represent a class of proapoptotic agents with a promising potential in the treatment of neoplastic diseases. In the present work, the synthesis and structure–activity relationship of a series of 3′-adamantan-1-yl-biphenyl-4-yl-acrylic acids substituted in ring A were studied. The synthesized compounds were evaluated for their antiproliferative activity in a human promyelocitic leukemia cell line (NB4), and in an ovarian carcinoma cell system including IGROV-1, carrying a functional wild-type p53, and a cisplatin-resistant subline, IGROV-1/Pt-1. The presence of at least one oxygenated substituent in positions 4′ or 5′ appears determinant for the antiproliferative activity. With two substituents of this kind the activity increases, particularly in the case of alkylenedioxy compounds. The activation of DNA damage response as indicated by phosphorylation of H2AX histone, RPA-2 protein, and p53 at serine 15 by the most apoptotic compounds provides additional support to the hypothesis that the genotoxic stress is a critical event mediating apoptosis induction by compounds of this group.

Adamantyl-Substituted Retinoid-Related Molecules Bind Small Heterodimer Partner and Modulate the Sin3A Repressor

6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (CD437/AHPN) and 4-[3-(1-adamantyl)-4-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC/MM002) are inducers of apoptosis of malignant cells both in vitro and in vivo. Numerous mechanisms have been proposed for how these compounds exert this effect. This report shows that AHPN/3-Cl-AHPC binds specifically to the orphan nuclear receptor small heterodimer partner (SHP; NR0B2), and this binding promotes interaction of the receptor with a corepressor complex that minimally contains Sin3A, N-CoR, histone deacetylase 4, and HSP90. Formation of the SHP-Sin3A complex is essential for the ability of AHPN and 3-Cl-AHPC to induce apoptosis, as both knockout SHP and knockdown of Sin3A compromise the proapoptotic activity of these compounds but not other apoptosis inducers. These results suggest that AHPN/3-Cl-AHPC and their analogues are SHP ligands and their induction of apoptosis is mediated by their binding to the SHP receptor.

Antitumor Activity of the Retinoid-Related Molecules (E)-3-(4′-Hydroxy-3′-adamantylbiphenyl-4-yl)acrylic Acid (ST1926) and 6-[3-(1-Adamantyl)-4-hydroxyphenyl]-2-naphthalene Carboxylic Acid (CD437) in F9 Teratocarcinoma: Role of Retinoic Acid Receptor γ and Retinoid-Independent Pathways

The retinoid-related molecules (RRMs) ST1926 [(E)-3-(4'-hydroxy-3'-adamantylbiphenyl-4-yl)acrylic acid] and CD437 (6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid) are promising anticancer agents. We compared the retinoic acid receptor (RAR) trans-activating properties of the two RRMs and all-trans-retinoic acid (ATRA). ST1926 and CD437 are better RARgamma agonists than ATRA. We used three teratocarcinoma cell lines to evaluate the significance of RARgamma in the activity of RRMs: F9-wild type (WT); F9gamma-/-, lacking the RARgamma gene; F9gamma51, aF9gamma-/-derivative, complemented for the RARgamma deficit. Similar to ATRA, ST1926 and CD437 activate cytodifferentiation only in F9-WT cells. Unlike ATRA, ST1926 and CD437 arrest cells in the G2/M phase of the cell cycle and induce apoptosis in all F9 cell lines. Our data indicate that RARgamma and the classic retinoid pathway are not relevant for the antiproliferative and apoptotic activities of RRMs in vitro. Increases in cytosolic calcium are fundamental for apoptosis, in that intracellular calcium chelators abrogate the process. Comparison of the gene expression profiles associated with ST1926 and ATRA in F9-WT and F9gamma-/-indicates that the RRM activates a conspicuous nonretinoid response in addition to the classic and RAR-dependent pathway. The pattern of genes regulated by ST1926 selectively, in a RARgamma-independent manner, provides novel insights into the possible molecular determinants underlying the activity of RRMs in vitro. Furthermore, it suggests that RARgamma-dependent responses are relevant to the activity of RRMs in vivo. Indeed, the receptor hinders the antitumor activity in vivo, in that both syngeneic and immunosuppressed SCID mice bearing F9gamma-/- tumors have increased life spans after treatment with ST1926 and CD437 relative to their F9-WT counterparts.

The extrinsic and intrinsic apoptotic pathways are differentially affected by temperature upstream of mitochondrial damage

It is well known that mild hypothermia prevents neuronal cell death following cerebral ischemia, although it can also cause apoptosis in other cell types. Thus, incubation at room temperature (RT) has been shown to induce apoptosis in hematopoietic cells, including Jurkat T leukemia cells. To further understand the apoptotic events that can be activated at RT, we compared the induction of apoptosis by several apoptotic insults in Jurkat cells stimulated at 37 degrees C or RT. Retinoid-related molecules, which induce apoptosis via the intrinsic pathway, failed to induce apoptosis when cells were treated at RT, as determined by various apoptotic parameters including cytochrome c release and activation of caspase 3. In contrast, most apoptotic events were enhanced by lower temperatures when cells were stimulated with anti-Fas antibody via the extrinsic pathway. Ultraviolet radiation produced partial effects at RT, correlating with its capacity to activate both pathways. Our results indicate that the core caspase machinery is operational under mild hypothermia conditions. Experiments using purified recombinant caspases and cell-free assays confirmed that caspases are fully functional at RT. Other hallmark events of apoptosis, such as phosphatidylserine externalization and formation of apoptotic bodies were variably affected by RT in a stimulus-dependent manner, suggesting the existence of critical steps that are sensitive to temperature. Thus, analysis of apoptosis at RT might be useful to (i) discriminate between the extrinsic and intrinsic pathways in Jurkat cells treated with prospective stimuli, and (ii) to unravel temperature-sensitive steps of apoptotic signaling cascades.

Induction of GDF-15/NAG-1/MIC-1 in human lung carcinoma cells by retinoid-related molecules and assessment of Its role in apoptosis

Growth and Differentiation Factor-15 (GDF-15, also known as NAG-1 and MIC-1) is induced by several apoptosis-inducing agents including the retinoid-related molecule (RRM) 6-[3-(1-adamantyl-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437). It has been suggested that GDF-15 may be involved in the induction of apoptosis by CD437 in H460 lung cancer cells. The present study was designed to probe this hypothesis more directly. Several RRMs (CD437, ST1926, and MX3350) but not the retinoids all-trans-retinoic acid and 4HPR were able to induce GDF-15 in H460 cells. A similar differential effect of these retinoids was observed for the induction of p53, which has been reported to regulate GDF-15 expression. In H460 cells transfected with a neo vector control (H460-Neo), treatment with RRMs but not ATRA or 4HPR resulted in increases in p53, GDF-15 and apoptosis evidenced by polyADP ribose polymerase (PARP) cleavage. In contrast, RRMs failed to increase p53 or induce apoptosis in H460 cells in which p53 was inactivated by transfection of the human papillomavirus E6-6 (H460-E6-6). The increase in GDF-15 by RRMs was also compromised in the H460-E6-6 cells. Because PARP cleavage was only evident when GDF-15 levels where elevated it appeared that GDF-15 was mediating the pro-apoptotic effects of RRMs. However, silencing of GDF-15 induction by RNA interference failed to decrease the ability of CD437 and ST1926 to induce apoptosis. These results demonstrate that GDF-15 is dispensable for the proapoptotic activity of CD437 and ST1926.

Mechanism of Action and Therapeutic Potential of Novel Adamantyl Retinoid-Related Molecules

Mechanism of Action and Therapeutic Potential of Novel Adamantyl Retinoid-Related Molecules

Synthetic retinoids as potential antitumour agents

The retinoids, natural or synthetic derivatives of vitamin A, exhibit a variety of biological effects that may have implications in cancer chemoprevention and therapy. The chemopreventative action of classical retinoids has been ascribed to receptor-mediated modulation of a range of biological processes, including cell differentiation and proliferation. The therapeutic interest of several synthetic retinoids (also known as retinoid-related molecules) is related to their proapoptotic activity, which appears to be independent of the receptor-transactivating activity. This review focuses on the relevant features of recently reported synthetic retinoids, with particular reference to their therapeutic potential and current understanding of the mechanism of action. These agents are structurally heterogeneous compounds, including molecules closely related to natural retinoids and molecules with a retinoid-like structure (e.g., atypical retinoids) that include retinoic acid receptor (RAR) selective compounds or RAR antagonists. Promising agents of this class are adamantyl retinoids because of their apoptotic potency, in vivo antitumour activity and low toxicity. Activity of selected adamantyl retinoids (e.g., CD437, MX3350-1, ST1926) has been reported in models of various tumour types, including ovarian, breast, lung, head/neck squamous carcinoma and melanoma. The best in vivo antitumour efficacy was achieved by a protracted treatment. Orally available molecules (e.g., ST-1926) are the most suitable for this treatment schedule. Synthetic retinoids have shown synergistic interaction in combination with a variety of antitumour agents used in clinical therapy. This evidence is expected to expand the therapeutic options for clinical use of retinoids.