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[Cancer Research 61, 4723-4730, June 15, 2001]
© 2001 American Association for Cancer Research

Experimental Therapeutics

Apoptosis Induction in Cancer Cells by a Novel Analogue of 6-[3-(1-Adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic Acid Lacking Retinoid Receptor Transcriptional Activation Activity1

Marcia I. Dawson2, Peter D. Hobbs, Valerie J. Peterson, Mark Leid, Christopher W. Lange, Kai-Chia Feng, Guo-quan Chen, Jian Gu, Hui Li, Siva Kumar Kolluri, Xiao-kun Zhang, Yuxiang Zhang and Joseph A. Fontana

Department of Medicinal Chemistry, Molecular Medicine Research Institute, Mountain View, California 94043 [M. I. D., C. W. L., K-C. F., G-q. C.]; Retinoid Program, SRI, Menlo Park, California 94025 [P. D. H.]; Laboratory of Molecular Pharmacology, Oregon State University School of Pharmacy, Corvallis, Oregon 97331 [V. J. P., M. L.]; The Burnham Institute, La Jolla, California 92037 [J. G., H. L., S. K. K., X-k. Z.]; and John D. Dingell Veterans Affairs Medical Center and Karmanos Cancer Institute, Wayne State University, Detroit, Michigan 48201 [Y. Z., J. A. F.]


   ABSTRACT
Top
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN) is reported to have anticancer activity in vivo. Induction of cell cycle arrest and apoptosis in cancer cell lines refractory to standard retinoids suggests a retinoid-independent mechanism of action for AHPN. Conformational studies suggested that binding of AHPN does not induce an unusual conformation in retinoic acid receptor (RAR) {gamma}. The 3-chloro AHPN analogue MM11453 inhibited the growth of both retinoid-resistant (HL-60R leukemia, MDA-MB-231 breast, and H292 lung) and retinoid-sensitive (MCF-7 breast, LNCaP prostate, and H460 lung) cancer cell lines by inducing apoptosis at similar concentrations. Before apoptosis, MM11453 induced transcription factor TR3 expression and loss of mitochondrial membrane potential characteristic of apoptosis. MM11453 lacked the ability to significantly activate RARs and retinoid X receptor {alpha} to initiate (TREpal)2-tk-CAT reporter transcription. These results, differential proteolysis-sensitivity assays, and glutathione S-transferase-pulldown experiments demonstrate that, unlike AHPN or the natural or standard synthetic retinoids, MM11453 does not behave as a RAR or retinoid X receptor {alpha} transcriptional agonist. These studies strongly suggest that AHPN exerts its cell cycle arrest and apoptotic activity by a signaling pathway independent of retinoid receptor activation.


   INTRODUCTION
Top
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The natural RAs3 and their synthetic analogues are being investigated as chemotherapeutic agents because they inhibit proliferation, induce apoptosis in cancer cells, and retard tumor xenograft growth (1) . These standard retinoids exert their antiproliferative effects by influencing the transcriptional activity of RAR and RXR subtypes {alpha}, ß, and {gamma} (reviewed in Ref. 2 ). Retinoids complexed to a RXR/RAR can activate or repress gene transcription from RA response elements in the promoter of retinoid-sensitive genes. A retinoid bound to an RXR can modulate activation by other transcription factors with which it dimerizes (2) . Retinoid receptor-ligand complexes also compete with other transcription factors for coactivator proteins (3 , 4) , whereas nonliganded dimers compete for corepressors (5) .

The diversity from the six subtypes and variations in their expression patterns (2 , 6, 7, 8, 9) , response element sequences, intermediary proteins, and other transcription factors (2) led to the identification of receptor-selective retinoids to enhance efficacy by reducing the systemic toxicity associated with retinoids activating all receptors (10) . Receptor class and subtype-selective compounds (reviewed in Refs. 1 and 11 ) also provide a means for studying individual receptor-signaling pathways.

On evaluating RAR{gamma}-selective retinoids, we observed that AHPN (CD437 [1] in Fig. 1Citation ; Ref. 12 ) rapidly caused detachment of retinoid-sensitive MCF-7 breast and NIH:OVCAR-3 ovarian cancer cells (13 , 14) . This atypical retinoid activity extended to retinoid-resistant lines, including MDA-MB-231 breast cancer and HL-60R leukemia (13) . AHPN induced cyclin-dependent kinase inhibitor p21WAF1/CIP1 expression (13) , G0-G1 cell cycle arrest (13) , and apoptotic events, such as caspase activation, gadd45 expression (15) , poly(adenosyl diphosphate-ribose) polymerase cleavage, and DNA fragmentation (13) . Interestingly, apoptosis occurred in the absence of functional tumor suppressor p53 (13) , the gene for which is mutated in many cancers (16) . Apoptosis by AHPN and its derivatives and analogues was subsequently observed in other lines derived from tumors and their metastases (17, 18, 19, 20, 21, 22, 23, 24, 25) .



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  		Fig. 1. AHPN [1], MM11453 [2], trans-RA [3], 9-cis-RA [4], TTAB [5], MM11254 [6], MM11253 [7], and BMS270394 [8].

 
The efficacy against retinoid-resistant cancer cells prompted studies on how AHPN induces apoptosis. To reduce complications, we conducted mechanistic studies in cells lacking functional retinoid receptors (26) and used apoptotic AHPN analogues lacking retinoid agonist transactivation activity, such as MM11453 [2]. MM11453 induced apoptosis by a cascade that included mitochondrial translocation of transcription factor TR3/nur77/NGFIB-II (TR3), cytochrome c release, caspase activation, and DNA fragmentation (27) . Binding of MM11453 to RARs and RXR{alpha} did not cause the conformational changes of AHPN that led to corepressor loss and coactivator recruitment. We report here the characterization and anticancer activity of MM11453, the prototype for new nonretinoidal apoptotic agents with potential for cancer treatment.


   MATERIALS AND METHODS
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Retinoids.
AHPN [1] was prepared by modifying a reported procedure (28) . AHPN (MM11453) [2] was synthesized as follows. The biaryl bond was introduced by palladium(0)-catalyzed coupling between 3-(1-adamantyl)-4-benzyloxybenzeneboronic acid and ethyl 6-bromo-3-chloro-2-naphthalenecarboxylate [palladium(triphenylphosphine)4 (Aldrich, St. Louis, MO), aqNa2CO3, dimethoxyethane, reflux, 6 h], followed by chromatography (6% EtOAc/hexane on silica gel) to give the benzyl-protected ethyl ester of MM11453 (66%). Benzyl group cleavage [BBr3, CH2Cl2, -78°C, 2 h] to the phenol (91%) and ester group hydrolysis (aq NaOH, ethanol, 90°C, 2 h; aq HCl) gave MM11453 (95%) as a white powder, melting point 294°C–296°C (decomp.). IR (KBr): 3200, 1706, 1277, 1244, 991, 815, and 680 cm-1. 1H nuclear magnetic resonance (300 MHz, Me2SO-d6, {delta}): 1.75, 2.06, 2.17 (s, 6, adamantyl CH2; s, 3, adamantyl CH; s, 6, adamantyl CH2), 6.97 (d, J = 9.0 Hz, 1, ArH-5), 7.51 (s, 1, ArH-2), 7.52 (d, J = 9.0 Hz, 1, ArH-6), 7.98 (d, J = 9.0 Hz, 1, NapH-8), 8.06 (s, 1, NapH-5), 8.25 (d, J = 8.6 Hz, 1, NapH-7), 8.27 (s, 1, NapH-4), 8.60 (s, 1, NapH-1), 9.68 (s, 1, ArOH). High-resolution mass spectrometry for C27H25ClO3 (M+): calculated, 432.1492; found, 432.1492. trans-RA [3] was purchased (Sigma Chemical Co.), as was [11,12-3H)2]9-cis-RA (specific activity, 43 Ci/mmol; DuPont NEN, Boston, MA). 9-cis-RA [4] was prepared as reported (29) .

Computational Analysis.
CAChe Software (Fujitsu, Beaverton, OR) was used to identify low-energy conformers within 2 kcal of the global energy minimum (MM3 force field, conjugate-gradient minimization, 30° search label variation, exclusion of >=9 Å van der Waals interactions, and energy change <0.001 kcal/mol). Conformers were superimposed by using least-squares rigid fit of atoms corresponding to the 1, 5–9, and 15 carbon molecules of trans-RA.

Receptor Transcriptional Activation.4
CV-1 cells (1,000 per well) were grown in DMEM (Irving Scientific, Santa Ana, CA) with 10% charcoal-treated FCS (Tissue Culture Biologicals, Tulare, CA) for 16–24 h before transfection, as described (30 , 31) . Briefly, 100 ng of (TREpal)2-tk-CAT reporter, ß-galactosidase expression vector pCH 110 (Pharmacia, Piscataway, NJ), and a RAR expression vector (or 20 ng of RXR{alpha}) were mixed with carrier DNA (pBluescript; Stratagene, La Jolla, CA) to give 1 µg of total DNA/well. CAT activity was normalized using ß-galactosidase as the control. Activation after subtraction of constitutive activity is expressed relative to that of 1.0 µM trans-RA for RARs (100%) or 1.0 µM 9-cis-RA for RXR{alpha} (100%) and represents the average of three determinations.

Receptor Binding.
Competitive radioligand binding on crude bacterial lysates at 0°C for 2 h used ~25 µmol of recombinant human RAR subtype or mouse RXR{alpha}-GST fusion proteins in 200 µl of binding buffer [10 mM HEPES (Sigma Chemical Co.; pH 7.8), 100 mM NaCl, 0.1 mM EDTA, 0.5 mM DTT, and 10% glycerol] with 1–2 nM [3H2]9-cis-RA (43 Ci/mmol). Bound [3H]9-cis-RA was isolated (Sephadex G-50; Pharmacia) and counted. Nonspecific [3H]9-cis-RA binding at 1 µM nonlabeled 9-cis-RA generally was <10% of total label bound.

DPSA.
[35S]Met-labeled RAR{alpha}, RARß, RAR{gamma}, and RXR{alpha}, prepared by in vitro transcription/translation (32) , were used in DPSA as described (33) . [35S]Met-labeled receptors were incubated with 0.1% ethanol alone, 1.0 µM 9-cis-RA, or MM11453 for 30 min at 0°C. Limited proteolysis (trypsin-tosyl phenylalanyl chloromethyl ketone; Sigma Chemical Co.) for 15 min at 22°C, followed by termination by Laemmli sample buffer and boiling and separation (10% acrylamide gel under denaturing conditions), afforded PFs for visualization by autoradiography (33 , 34) .

GST-Pulldown.
Experiments were performed as described using GST-p300 1–450 (35) and GST-NCoR 2110–2453 (36) fusion proteins and [35S]Met-labeled human RAR{gamma}.

Cell Lines and Culture.
RA-resistant HL-60R cells, having a mutant RAR{alpha} that does not significantly bind trans-RA and lacking RARß and RAR{gamma} (26) , and MDA-MB-231 cells were grown as described (20) . MCF-7, LNCaP prostate, H460 and retinoid-resistant H292 lung cancer cells and Jurkat lymphoma cells (American Type Culture Collection, Rockville, MD) were grown in RPMI 1640 (Irving Scientific) with 10% charcoal-treated FCS.

Cell Growth Inhibition.
HL-60R and MDA-MB-231 cells (50,000 and 100,000 per well, respectively) and 0.1–1.0 µM MM11453, AHPN, or Me2SO alone were incubated for 24 or 120 h (72-h medium change), respectively. Results are expressed relative to Me2SO control as mean ± SE of triplicate experiments. SEs were <10%. MCF-7, LNCaP, H292, and H460 cells (3,000 per well in 96-well plates) were treated with 1.0 µM MM11453, AHPN, trans-RA, or ethanol alone for 48 h before viable cell numbers were determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (7 , 9 , 27) . Data shown are representative of three experiments.

Apoptosis Detection.
DNA fragmentation and apoptotic bodies were assessed in at least 500 HL-60R or MDA-MB-231 cells after incubation with MM11453 for 24 or 120 h, respectively, as described above, and acridine orange staining (15) . The percentage of apoptotic cells was expressed relative to the Me2SO control as the mean ± SE of triplicate experiments. MCF-7, LNCaP, H292, H460, and Jurkat cells (3,000 per well) were treated with 1.0 µM MM11453, trans-RA, or ethanol alone for 48 h, trypsinized, washed (PBS), fixed (3.7% paraformaldehyde), and stained with 4',6-diamidino-2-phenylindole (1 µg/ml) to visualize nuclei by fluorescent microscopy (21) . Cells with apoptotic nuclear morphology were scored in each 400-cell sample using a fluorescence microscope. The data are representative of three experiments.

Northern Analysis.
Total RNAs were prepared (RNeasy Mini kit; Qiagen, Germany), and TR3 expression was determined on 30 µg of total RNA from each line treated with 1.0 µM MM11453, trans-RA, or ethanol alone. Blotting conditions were as described (27) with ß-actin expression as the control.

TR3 Mitochondrial Targeting.
The expression vector for TR3/{Delta}DBD-GFP, a TR3 mutant lacking the DNA-binding domain fused to the green fluorescent protein expression vector, was transiently transfected into H460 cells, as described for LNCaP cells (27) . Cells were treated with 1.0 µM MM11453 or ethanol alone for 6 h and then immunostained with anti-Hsp60 antibody (Santa Cruz Biotechnology, Santa Cruz, CA) and Cy3-conjugated secondary antibody (Sigma Chemical Co.) to indicate mitochondria to which Hsp60 is restricted. Confocal microscopy was used to detect TR3/{Delta}DBD-GFP (green fluorescence) and Hsp60 (red). Images were overlaid to show colocalization.

Mitochondrial Membrane Potential Assay.
LNCaP, MCF-7, and MDA-MB-231 cells (10,000,000) were treated with 1.0 µM MM11453 for 18 h before incubation with 5 µg/ml Rh123 for 30 min at 37°C. Rh123-fluorescing cells were scored depolarized by flow cytometry (FACScalibur system; BD Biosciences, San Jose, CA; Ref. 37 ). The data shown are representative of three experiments. Wild-type Jurkat cells or Jurkat cells stably expressing either Bcl-2 or control vector (38) were treated similarly.


   RESULTS
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Close-Fitting of Energy-minimized AHPN and Retinoid Conformers.
Energy-minimized conformers of AHPN [1], RAR-selective trans-RA [3], and RAR-selective TTAB [5] (39) were overlapped. The trans-RA conformer was that reported in the RAR{gamma} LBD (40 , 41) . Three orthogonal views of these overlaps are shown in Fig. 2ACitation . The major structural difference was the 1-adamantyl group of AHPN, which extended 2.2 Å more than the trans-RA 18-methyl group. In Fig. 2BCitation , the energy-minimized conformers of RAR{gamma}-selective agonists AHPN and MM11254 [6], a (Z)-oxime (14) of 6-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenylcarbonyl)-2-naphthalenecarboxylic acid (42) , are shown overlapped with RAR{gamma}-selective agonist BMS270394 [8], as found in the ligand-binding pocket of crystallized holo-RAR{gamma} (40) . The AHPN 1-adamantyl group overlaps the saturated portion of the 5,6,7,8-tetrahydro-5,5,8,8-tetramethylnaphthalene rings of both RAR{gamma}-selective retinoids, and the AHPN phenolic oxygen is near the oxygen molecules in the oxime group of MM11254 (2.5 Å) and the bridge hydroxyl of BSM270394 (3.9 Å). Such hydroxyl groups are reported to confer RAR{gamma} selectivity by hydrogen bonding to the Met-272 sulfur molecule of RAR{gamma} (41) . Placement of these overlapped conformers (Fig. 2B)Citation in the RAR{gamma} ligand-binding site gives ligand O–Met-272–S distances of 4.09, 2.55, and 3.32 Å, respectively. These studies suggest that binding of AHPN to RAR{gamma} occurs in the same manner as that of standard retinoid agonists.



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  		Fig. 2. Comparison of energy-minimized AHPN and retinoid conformers. Conformational analysis was performed as described in "Materials and Methods." A, orthogonal views of superimposed conformers of AHPN (blue), trans-RA (red), and TTAB (yellow). B, superimposed conformers of AHPN (blue), MM11254 (green), and BMS270394 (magenta).

 
MM11453 Lacked RAR Transcriptional Activation of AHPN.
Although originally reported as RAR{gamma} selective on the (TREpal)2-tk-CAT reporter in cotransfected HeLa cells (12) , we observed on the (TREpal)2-tk-CAT in CV-1 cells (4) that high RAR{gamma} selectivity occurred at 0.1 µM and below (14) . At 0.5–1.0 µM, at which the natural retinoid trans-RA [3] inhibits retinoid-sensitive cancer cells, AHPN significantly activated RARß. At 1.0 µM AHPN, reporter activation by RAR{alpha}, RARß, and RAR{gamma} was 9, 37, and 54%, respectively, of that caused by 1.0 µM trans-RA (Fig. 3)Citation . Unlike 1.0 µM trans-RA or 9-cis-RA, 1.0 µM MM11453 did not adequately activate any RAR subtype or RXR{alpha} to induce even modest (TREpal)2-tk-CAT transcription. MM11453 did not activate RAR{alpha} or RXR{alpha} and only activated RARß and RAR{gamma} to 5 and 10%, respectively, of that of trans-RA or 13 and 19%, respectively, of that of AHPN. Thus, MM11453 is an analogue with substantially reduced capacity for RAR activation.



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  		Fig. 3. Transcriptional activation of retinoid receptors by MM11453 on the (TREpal)2-tk-CAT reporter. CV-1 cells were transiently transfected as described in "Materials and Methods," treated with 1.0 µM MM11453, AHPN, trans-RA, or 9-cis-RA, and assayed for CAT activity after 24 h. Reporter gene activation is expressed relative to 1.0 µM trans-RA on the RARs or 1.0 µM 9-cis-RA on RXR{alpha}.

 
Retinoid Receptors Bound MM11453.
Competitive ligand binding was used to determine whether MM11453 bound directly to RARs and RXR{alpha}. MM11453 at 1.0 µM displaced 61 ± 6% of [3H2]9-cis-RA bound to RAR{gamma}, whereas displacement from other receptors was lower [RAR{alpha} (11 ± 2%), RARß (25 ± 5%), and RXR{alpha} (18 ± 5%); Fig. 4Citation ].



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  		Fig. 4. Binding affinity of MM11453 to recombinant RAR and RXR{alpha}. Competition radioligand binding was conducted as described in "Materials and Methods." The data represent the means (n = 3) of the percentages of [11,12-3H2]9-cis-RA bound that were inhibited by 1.0 µM MM11453; bars, SE.

 
MM11453 Did Not Induce an Agonist-bound RAR Conformation.
DPSA on 9-cis-RA-bound RAR{alpha}, RARß, and RAR{gamma} produced 27-kDa PF27{alpha}, 35-kDa PF35ß, and 32-kDa PF32{gamma}, respectively (Lane 3 in Fig. 5, A–CCitation ). DPSA on AHPN-bound RARs produced the same fragments (data not shown). These PFs were not observed on incubation with ethanol or MM11453 (Lanes 2 and 4, respectively, in Fig. 5, A–CCitation ). Unlike 9-cis-RA, neither MM11453 nor AHPN altered the proteolytic sensitivity of RXR{alpha} (data not shown). The lack of PFs from RAR-MM11453 complexes suggests that MM11453 does not promote an agonist-bound conformation. Similar to RAR{gamma}-selective antagonist MM11253,5 MM11453 did not prevent RAR/RXR agonist 9-cis-RA from inducing this conformation in RAR{alpha}, RARß, or RAR{gamma} (data not shown).



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  		Fig. 5. MM11453 is not a RAR agonist. A, DPSA on [35S]Met-labeled RAR{alpha} in ethanol or 1 µM 9-cis-RA or MM11453. The migration of the 9-cis-RA-induced PF27{alpha} of RAR{alpha} is indicated. In B and C, DPSA on RARß and RAR{gamma}, respectively, were conducted as in A and "Materials and Methods." Positions of RARß PF35ß and RAR{gamma} PF35ß are indicated. Left, marker migration (molecular mass).

 
MM11453 Failed to Dissociate Corepressor NCoR-RAR{gamma} in Vitro.
GST-pulldown was used to test whether MM11453 dissociated NCoR (5) from RAR{gamma}, as 9-cis-RA does. As indicated (Fig. 6A)Citation , 9-cis-RA (Lane 3), but not MM11453 (Lane 4) or vehicle (Lane 2), disrupted the NCoR-RAR{gamma} complex.



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  		Fig. 6. MM11453 does not induce NCoR corepressor dissociation from RAR{gamma} or coactivator p300 recruitment to RAR{gamma}. A, NCoR-RAR{gamma} dissociation using [35S]Met-RAR{gamma} and GST-NCoR 2110–2453 is as described in "Materials and Methods." Only 9-cis-RA induced NCoR-RAR{gamma} dissociation (Lane 3). In B, RAR{gamma} coactivator recruitment using GST-p300 (1–450) and [35S]Met-RAR{gamma} is as in "Materials and Methods." Only 9-cis-RA enhanced binding of p300 to RAR{gamma}. Lane 1 in A and B represents ~15% of [35S]Met-RAR{gamma}. Left, marker migration (molecular mass).

 
MM11453 Failed to Recruit Coactivator p300 to RAR{gamma}.
We compared the abilities of MM11453 and 9-cis-RA (36) to recruit p300 (43) to RAR{gamma}. Vehicle (Lane 2 in Fig. 6BCitation ) or MM11453 (Lane 4) did not enhance p300 recruitment, whereas 9-cis-RA did (Lane 3). These findings, which agree with results on MM11453 in RAR{gamma} DPSAs (Fig. 5)Citation and corepressor-dissociation experiments (Fig. 6A)Citation , confirm that MM11453 does not induce a RAR{gamma}-agonist conformation.

MM11453 Inhibited Cancer Cell Growth.
Increasing evidence, including retinoid-resistant cancer cell growth inhibition (13 , 27) , suggests that AHPN action is independent of retinoid receptors (21 , 44) . Cell counting and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays were conducted to show that MM11453 inhibited growth similarly. MM11453 inhibited HL-60R and MDA-MB-231 growth with IC50s of 0.17 and 0.32 µM (Fig. 7, A and B)Citation , respectively, compared with AHPN values of 0.15 and 0.30 µM, respectively. Inhibition by 1.0 µM trans-RA was <=5% (13) . The effects of MM11453 on H460, H292, LNCaP, and Jurkat cells were then examined. As shown (Fig. 7C)Citation , 1.0 µM MM11453 significantly reduced growth by 70, 46, 64, and 70%, respectively, whereas 1.0 µM trans-RA reduced H460 growth by 15% and had no evident effect on the other lines (0–3%). Some of us reported previously that 1.0 µM AHPN for 48 h inhibited the growth of H460, H292, and LNCaP cells by 62 ± 6% (21) , 53 ± 5% (21) , and 100 ± 5% (25) , respectively, whereas Jurkat growth was inhibited by 84% and 80 ± 3% after 24 and 96 h, respectively (22) . Thus, both AHPN and MM11453 similarly retard the growth of these cell lines.



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  		Fig. 7. MM11453 inhibits cell growth and induces apoptosis. HL-60R cells (A and D) and MDA-MB-231 cells (B and E) were treated with 10 nM to 1.0 µM MM11453, AHPN, or Me2SO alone for 24 h and 120 h, respectively, as described in "Materials and Methods," and then harvested and counted (A and B) or assayed for apoptosis (D and E) as in "Materials and Methods." The results shown represent the means of three replicates; bars, SE. In C and F, H460, H292, LNCaP, and Jurkat cells were treated with ethanol alone, 1.0 µM trans-RA, or MM11453 for 48 h before viability was determined (C) or treated with 1.0 µM MM11453 or ethanol alone for 48 h before nuclear morphology was analyzed (F) as in "Materials and Methods." The experiments shown are representative of three triplicates.

 
MM11453 Induced Cancer Cell Apoptosis.
The MM11453 EC50s for inducing nuclear fragmentation in HL-60R and MDA-MB-231 cells were 0.12 and 0.13 µM, respectively (Fig. 7, D and E)Citation , which are similar to AHPN EC50s of 0.07 and 0.35 µM, respectively (13) . HL-60R apoptosis inhibition by 1.0 µM MM11453 and AHPN was 82 ± 3% and 91 ± 4%, respectively, and MDA-MB-231 apoptosis was 75 ± 1% and 76 ± 7%, respectively. Thus, both MM11453 and AHPN are similarly apoptotic in retinoid-resistant cells. MM11453 at 1.0 µM induced apoptosis in LNCaP (38%), H460 (47%), and MCF-7 (57%) cells, as demonstrated by nuclear morphological changes (Fig. 7F)Citation . In other experiments using these cells, 1.0 µM AHPN was found to induce 21 (21) , 37, and 42% apoptosis, respectively (data not shown). Thus, both MM11453 and AHPN also induce apoptosis in retinoid-sensitive cells.

MM11453 Induced TR3 Expression.
TR3 expression must be induced for AHPN to cause lung cancer cell apoptosis (21) . To determine whether 1.0 µM MM11453 had this capability, H460 and LNCaP cells were treated for 6 h. MM11453 strongly induced TR3 expression, whereas trans-RA did not (Fig. 8)Citation .



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  		Fig. 8. MM11453 induces TR3 expression in H460 and LNCaP cells. Cells were treated with ethanol alone, 1.0 µM trans-RA, or MM11453 for 6 h. Total RNAs were prepared and analyzed for TR3 expression by Northern blotting. Expression of ß-actin was the RNA-loading control.

 
MM11453 Induced TR3 Mitochondrial Targeting.
MM11453 at 1.0 µM induced the migration of transiently expressed TR3/{Delta}DBD-GFP to mitochondria in H460 cells, as indicated in Fig. 9Citation by colocalization of GFP fluorescence with that of immunostained Hsp60. Colocalization did not occur in vehicle-alone-treated cells (data not shown). Thus, both AHPN and MM11453 induce TR3 targeting to mitochondria.



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  		Fig. 9. MM11453 induces TR3 translocation to H460 mitochondria. Cells were transiently transfected with TR3/{Delta}DBD-GFP expression vector and then treated with 1.0 µM MM11453 for 6 h as in "Materials and Methods." Immunostained mitochondrial Hsp60 (red) and TR3/{Delta}DBD-GFP protein (green) were visualized by confocal microscopy, and images were overlaid (Overlay) to indicate colocalization (yellow).

 
MM11453 Altered Mitochondrial Membrane Potential.
We found that MM11453 induced TR3 targeting to the mitochondrial outer membrane of breast and prostate cancer cells to initiate cytochrome c release and apoptosis (27) . A loss of inner mitochondrial membrane potential or depolarization, which may signify outer membrane or permeability transition pore opening (45) and has been suggested as causing cytochrome c release (45) , is associated with apoptosis. The effect of MM11453 on this process was explored using Rh123, which cells incorporate on depolarization. MM11453 increased MCF-7, MDA-MB-231, and LNCaP cell Rh123 fluorescence 2.2-, 1.9-, and 5.4-fold, respectively (Fig. 10)Citation . Again, MM11453 behaves similarly to AHPN (46) .



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  		Fig. 10. Effect of MM11453 on LNCaP, MCF-7, and MDA-MB-231 mitochondrial membrane potential. Cells were treated with or without 1.0 µM MM11453 for 18 h and then with Rh123 as in "Materials and Methods." Rh123-fluorescencing cells are expressed as a percentage of the total.

 
Bcl-2 Attenuated Mitochondrial Membrane Depolarization by MM11453.
Because overexpression of antiapoptotic, mitochondrial membrane-surface protein Bcl-2 is reported to block cancer cell apoptosis (47) , its effect on apoptosis by MM11453 was explored in Jurkat cells transfected with an expression vector containing bcl-2 or the vector alone. In MM11453-treated nontransfected cells and MM11453-treated vector alone-transfected cells, depolarized cell numbers increased 4.8- and 5.5-fold, respectively, over that of the nontreated control, whereas cell numbers increased only 2-fold in treated cells overexpressing bcl-2 (Fig. 11)Citation . Thus, bcl-2 modified the effect of MM11453 on mitochondrial membranes.



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  		Fig. 11. Bcl-2 inhibits Jurkat mitochondrial membrane potential decrease by MM11453. Nontransfected cells stably expressing vector alone (Jurkat/Neo) and transfected cells stably expressing Bcl-2 (Jurkat/Bcl-2) were treated with 1.0 µM MM11453 or ethanol alone for 18 h and analyzed for change in mitochondrial membrane potential as in "Materials and Methods."

 

   DISCUSSION
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
AHPN induces apoptosis in cancer cell lines (13 , 14 , 18, 19, 20, 21, 22 , 24) . How AHPN initiates this process remains to be completely defined. A report of the RAR{gamma} selectivity of AHPN (12) led to the hypothesis of an apoptotic role for RAR{gamma} in breast cancer, melanoma, and neuroblastoma cells (17 , 23 , 24) . To support this, RAR{gamma} transcriptionally active AHPN derivatives and analogues were also reported to inhibit growth and induce apoptosis (28 , 48 , 49) . Other reports present data strongly suggesting an RAR-independent pathway, such as growth inhibition and apoptosis of retinoid-resistant cancer cells (13 , 14 , 21 , 27 , 42 , 44 , 50) .5 Our results support the latter by showing that MM11453, although unable to activate retinoid receptors on a reporter with the efficacy of standard retinoids or AHPN, strongly inhibited growth and induced apoptosis in retinoid-resistant cancer cell lines.

The near absence of RAR subtype and RXR{alpha} transcriptional activation by MM11453 was confirmed by limited proteolysis. DPSAs suggest that MM11453 is not a RAR or RXR{alpha} agonist. MM11453 did not induce a protease-resistant RAR{gamma} conformation, characteristic of binding a retinoid agonist, such as MM11254 [6], but behaved as the RAR{gamma}-selective antagonist MM11253 [7], a dithiane (14 , 39) of 6-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenylcarbonyl)-2-naphthalenecarboxylic acid].5 MM11453 did not detectably dissociate NCoR from RAR{gamma} or recruit p300 to RAR{gamma}, as agonists did. Thus, the behavior of MM11453 contrasts with that of RAR-agonist AHPN (12 , 14) . A retinoid receptor-independent pathway for anticancer activity has precedent in the mechanism of action of N-(4-hydroxy)phenyl retinamide, which inhibits the growth of cancer cells that resist standard retinoids (51 , 52) .

DPSA (data not shown) and molecular modeling (Fig. 2)Citation suggest that AHPN does not induce a unique conformation in the RAR{gamma} LBD that could account for apoptosis-inducing activity. RAR{gamma} on binding AHPN, trans-RA, or MM11254 produced the same PFs,5 whereas 1.0 µM MM11253 [7] did not induce this conformation5 or transcriptionally activate RAR{gamma} (14) . Both transactivation and DPSA show that the bulky 1-adamantyl group of AHPN (Fig. 2A)Citation did not prevent an agonist-induced RAR{gamma} conformation, and modeling shows the 1-adamantyl group occupying the same region as the tetrahydronaphthalene rings of agonists MM11254 and BMS270394 [8] (Ref. 41 ; Fig. 2BCitation ). The three hydroxyl and carboxyl oxygen molecules are also close. Thus, on the basis of the strategy used by Klaholz et al. (41) that the low-energy conformation of a ligand approximates its bound form, our findings suggest that pharmacophoric AHPN groups are not responsible for inducing any unique conformation in RAR{gamma}. Only the 3-chloro group ortho to the COOH group distinguishes MM11453 from AHPN. How the chloro group inhibits transcriptional activation remains to be determined. Both its steric and electronic properties may perturb hydrogen bonding by the COOH group or shift van der Waals contacts of RAR{gamma} LBD pendant groups, thereby preventing the conformational changes in the receptor necessary for coactivator recruitment and transcriptional activation.

The inhibition of [3H]9-cis-RA binding to RARs by MM11453 suggests direct binding, whereas transfection indicates minimal RAR or RXR{alpha} agonism. Thus, MM11453 may function as a moderately selective RAR{gamma} antagonist. Although how RAR{gamma} antagonism or that of another RAR or RXR subtype contributes to MM11453 activity remains to be defined, the lack of growth inhibition by antagonist MM11253 (data not shown) suggests that the contribution, if any, is small. Unlike trans-RA, both MM11453 and AHPN strongly inhibited HL-60R, MDA-MB-231, LNCaP, and H292 cell growth and induced apoptosis. EC50s for inhibiting growth in HL-60R and MDA-MB-231 cells were comparable, and their apoptotic EC50s were similar (Fig. 7)Citation . These results indicate that MM11453 functions independently of RARs and RXR{alpha} and strongly suggest a similar mode of action for AHPN. Both AHPN (21) and MM11453 (Fig. 8)Citation induced TR3 expression in H460 and LNCaP cells and TR3 mitochondrial translocation (Ref. 27 and Fig. 9Citation , respectively) and caused inner mitochondrial membrane depolarization in MCF-7, MDA-MB-231, LNCaP, and Jurkat cells (Figs. 10Citation and 11Citation ). These results demonstrate that MM11453 retains the apoptotic properties of AHPN without behaving as a competent RAR{gamma} agonist and, thus, indicate that RAR{gamma} activation is not required for apoptotic activity. The recent report that AHPN induces apoptosis in RAR{gamma}-negative myeloma cells through a mitochondrial pathway (46) supports this conclusion. Reporter and limited proteolysis assays on MM11453 and AHPN suggest that their apoptotic activity does not involve RAR{alpha}, RARß, or RXR{alpha} activation.

Transactivation by liganded RAR{gamma} is reported to correlate with retinoid toxicity (53 , 54) . The lack of retinoid receptor activation activity by MM11453 suggests that toxic side effects characteristic of retinoid receptor activation (reviewed in Ref. 11 ) should be reduced in this class of apoptosis inducers, thereby affording more effective candidates for development as cancer chemotherapeutic agents.


   ACKNOWLEDGMENTS
 
We thank Pierre Chambon (Institute de Genetique et de Biologie Moleculaire et Cellulaire, Illkirch, France), David M. Livingston (Dana-Farber Cancer Institute, Boston, MA), and Thorsten Heinzel (German Cancer Research Center, Heidelberg, Germany) for constructs and Drs. Anne Hamburger (University of Maryland Cancer Center, Baltimore, MD) and Steve Collins (University of Washington, Seattle, WA) for MDA-MB-231 and HL-60R cells, respectively.


   FOOTNOTES
 
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1 Supported in part by NIH Grant P01 CA51993 (to M. I. D., J. A. F., M. L., and X-k. Z.) and State of California Grant 6RT-2012 (to M. I. D. and X-k. Z.). Back

2 To whom requests for reprints should be addressed, at The Burnham Institute, 10901 North Torrey Pines Road, La Jolla, CA 92037. Phone: (858) 646-3165; Fax: (858) 646-3195; E-mail: mdawson{at}burnham.org Back

3 The abbreviations used are: RA, retinoic acid; RAR, retinoic acid receptor; RXR, retinoid X receptor; AHPN, 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid; aq, aqueous; GST, glutathione S-transferase; tk, thymidine kinase; CAT, chloramphenicol acetyltransferase; DPSA, differential protease sensitivity assay; TRE, thyroid hormone receptor response element; TREpal, palindromic TRE; PF, protease-resistant fragment; Hsp, heat shock protein; LBD, ligand-binding domain; Met, methionine; NCoR, nuclear receptor corepressor; Rh123, rhodamine green; TTAB, 4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-anthracenyl)benzoic acid. Back

4 Assay conducted at The Burnham Institute, La Jolla, CA, under a license agreement with Ligand Pharmaceuticals, San Diego, CA. Back

5 V. J. Peterson, M. I. Deinzer, M. I. Dawson, K-C. Feng, A. Fields, and M. Leid. Mass spectrometric analysis of agonist-induced retinoic acid receptor {gamma} conformational change, unpublished results. Back

Received 12/20/00. Accepted 4/17/01.


   REFERENCES
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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