A Bis-Indole–Derived NR4A1 Antagonist Induces PD-L1 Degradation and Enhances Antitumor Immunity

The orphan nuclear receptors NR4A1, NR4A2, and NR4A3 are immediate early genes induced by multiple stressors, and the NR4A receptors play an important role in maintaining cellular homeostasis and in pathophysiology. There is increasing evidence that these receptors are involved in important pathways in metabolic, cardiovascular, and neurologic functions as well as inflammation and inflammatory diseases and in immune functions and cancer (1, 2). NR4A1 is overexpressed in colon, ovarian, pancreatic, breast (estrogen receptor positive and negative), and lung tumors and high expression of NR4A1 in breast, colon, and lung tumors predicted decreased patient survival (3–9). The functional activity of NR4A1 in cancer has been extensively investigated in cancer cell lines by either knockdown or overexpression (9–20). Results of NR4A1 knockdown studies show that this receptor regulates pathways and genes associated with solid tumor-derived cancer cell growth, survival, migration, and invasion. We have also identified a series of bis-indole–derived ligands including 1,1-bis(3′-indolyl)-1-(p-hydroxyphenyl) methane (DIM-C-pPhOH, CDIM-8), which binds NR4A1 (K_d = 100 nmol/L; ref. 14) and acts as an antagonist inhibiting pro-oncogenic NR4A1-regulated pathways and genes in cancer cells (14–23).

Mechanistic studies show that for a number of genes including survivin, PAX3-FOXO1, and several integrins, NR4A1 acts as a nuclear cofactor that binds Sp1 or Sp4 and enhances expression of Sp1/Sp4-regulated genes (5, 17, 22, 23). Moreover, treatment with CDIM-8 and related NR4A1 antagonists inhibits expression of NR4A1/Sp-regulated genes. Although NR4A1/Sp-dependent gene regulation represents a novel pathway of gene regulation where NR4A1 does not directly bind promoter DNA, there are many examples of other nuclear receptors including the steroid hormone receptors that also activate genes through interactions with Sp transcription factors bound to their cognate GC-rich promoter elements (24). The PD-L1 gene also contains a proximal GC-rich promoter sequence and recent studies in human gastric cancer cells demonstrate that PD-L1 is a Sp1-regulated gene (25). We hypothesized that PD-L1 may also be an NR4A1/Sp–regulated gene that can be targeted by C-DIM/NR4A1 antagonists and thereby mimic the effects of checkpoint inhibitor antibodies. Checkpoint inhibitors represent a relatively new and effective breakthrough in cancer therapy where the blockade of cancer cell derived PD-L1 from interacting with PD-1 on T cells reactivates tumor-infiltrating lymphocytes (TIL). The effectiveness of antibody-derived checkpoint inhibitors is somewhat limited and associated with toxic side effects and development of resistance (26–32). Therefore, the major objective of this study is to determine whether PD-L1 is an NR4A1/Sp–regulated gene that can be targeted by NR4A1 antagonists and thereby activate immune surveillance. Our results confirm that PD-L1 is an NR4A1/Sp1–regulated gene and treatment with NR4A1 antagonists decreased expression of PD-L1 in triple-negative breast cancer cells and reactivated TILs in a syngeneic mouse model bearing 4T1 mouse mammary cancer cells. Thus, NR4A1 antagonists represent a new class of small molecules that act as immunotherapy mimics with clinical potential for treating patients expressing PD-L1 and NR4A1.

1,1-Bis(3′-indolyl)-1-(p-hydroxyphenyl) methane (DIM-C-pPhOH; CDIM-8) and 1,1-bis(3′-indolyl)-1-(3-chloro-4-hydroxy-5-methoxyphenyl) methane (Cl-OCH₃) were synthesized as described by the condensation of indole with p-hydroxybenzaldehyde or 3-chloro-4-hydroxy-5-methoxybenzaldehyde (21, 33). Indole and the substituted benzaldehydes were purchased from Sigma-Aldrich. Human mammary breast cancer MDA-MB-231, MCF7, MDA-MB-468, and SKBR3 cell lines and L3.6PL, Panc-1 (pancreatic cancer), SW480 (colon cancer), Rh30 (rhabdomyosarcoma), A549 (lung cancer), and 786-O (kidney cancer) cell lines were purchased from ATCC. SW480, L3.6pL, Panc1, MCF7, SKBR3, and MDA-MB-231 were authenticated by Biosynthesis. Human mammary tumor Sum159PT and HS578T cell lines were kindly provided by Dr. Weston Porter, Texas A&M University (College Station, TX) and were originally purchased from ATCC. Mouse mammary tumor 4T1 and luciferase-tagged 4T1-Luc cell lines were kindly provided by Dr. Mien-Chie Hung, MD Anderson Cancer Center (Houston, TX) and cells were initially purchased from ATCC. All tumor cells used in the experiments were Mycoplasma negative. Mycoplasma testing was routinely carried out to ensure cell lines were Mycoplasma free. MDA-MB-231, HS578, MCF7, MDA-MB-468, SKBR3, L3.6PL, SW480, Panc-1, and A549 were grown in DMEM supplemented with 10% FBS. 4T1, 4T1-Luc, and Sum159PT cells were maintained in (DMEM)/Ham's F-12 50/50 mix containing 2.5 mmol/L l-glutamine, 10% FBS. Rh30 and 786-O were maintained in RPMI1640 medium supplemented with l-glutamine, 10% FBS. All cells were maintained at 37°C in the presence of 5% CO₂, and the solvent (dimethyl sulfoxide, DMSO) used in the experiments was ≤0.2%. Antibodies, primers, and oligonucleotides are summarized in Supplementary Table S1. The wild-type and mutant PD-L1 promoter constructs (+66 to −263) containing the proximal (−5 to −15) wild-type GC-rich Sp1 binding (CCCGCCTCCGG), Mutant1 (CAAGCCTCCAA) and, Mutant2 (CCCGCCTCCAG) sequences were synthesized by DNA technologies (IDT). The constructs were cloned and verified by Eurofins Genomics LLC.

Whole-cell lysates from MDA-MB-231, Sum159PT, HS578T, MCF7, MDA-MB-468, SKBR3, 4T1, L3.6PL, SW480, Rh30, Panc-1, A549, and 786-O were analyzed by Western blot analysis as described previously (21, 22). Equal amounts of protein were separated by 10% and 15% SDS-PAGE and transferred to a polyvinylidene difluoride (PVDF) membrane. PVDF membranes were incubated overnight at 4°C with primary antibodies in 5% skimmed milk and incubated for 2–3 hours with secondary antibodies conjugated with horseradish peroxidase (HRP). Membranes were then exposed to HRP-substrate and immune reacted proteins were detected with chemiluminescence reagent using Kodak image developer. β-Actin was used as a reference loading control.

MDA-MB-231 and 4T1 cells (1.5 × 10⁵ cells/well) were plated in 6-well plates in the complete culture medium. After 24 hours, the cells were transfected with 100 nmol/L of each siRNA duplex for 6 hours using Lipofectamine 2000 reagent (Invitrogen) following the manufacturer's protocol. For siRNA-mediated transfection, culture media was changed to the fresh medium containing 10% FBS after 6 hours were incubated for 72 hours. Whole-cell lysates were obtained and analyzed by Western blots as described previously (21, 22).

MDA-MB-231 and 4T1 cells were seeded at density of 5 × 10⁶ and allowed to attach for 24 hours (using 150 × 20 mm TC dishes). Cells were treated with CDIM-8 and Cl-OCH₃ for 3 hours and subjected to chromatin immunoprecipitation (ChIP) analysis using the ChIP-IT Express Magnetic Chromatin Immunoprecipitation Kit (Active Motif) according to the manufacturer's protocol using 1% formaldehyde for crosslinking. The sonicated chromatin was immunoprecipitated with normal IgG (Santa Cruz Biotechnology), and antibodies for RNA polymerase II (pol II; Active Motif), NR4A1 (Abcam), Sp1 (Abcam), P300 (Santa Cruz Biotechnology) incubated with protein A–conjugated magnetic beads at 4°C for overnight. Magnetic beads were extensively washed and protein-DNA cross-links were reversed and eluted. DNA was extracted from the immunoprecipitants and PCR was performed using primers that encompassed the GC-rich regions of the human and mouse PD-L1 promoters. PCR products were resolved on a 2% agarose gel in the presence of ethidium bromide (EtBr; Denville Scientific Inc.).

GC-rich DNA binding of Sp1 was measured using an Episeeker DNA-Protein Binding Assay Kit (Abcam) according to the manufacturer's protocol. A biotinylated 25-bp double-stranded oligonucleotides of PD-L1 promoter region containing Sp1 core–binding consensus sequence was used as a capture probe, and 25 bp double-stranded unlabeled oligonucleotide containing the identical consensus sequences was used as a competitor. Nuclear extract from MDA-MB-231 was used in this experiment. A mutant probe with mutations in the GC-rich motif was also used.

Expression of PD-L1 after treatment with CDIM-8 and Cl-OCH₃ were measured using RT-PCR. MDA-MB-231 and 4T1 cells were plated in 6-well plates in the complete culture medium and were allowed to attach for 24 hours. Cells were treated with CDIM-8 and Cl-OCH₃ for 24 hours. Total RNA was extracted using Quick-RNA Mini-prep Kit (ZYMO Research) according to the manufacturer's instructions. Quantification of mRNA was performed using the iTaq Universal SYBR Green 1-Step Kit using the manufacturer's protocol with the CFX384 real-time PCR System (Bio-Rad). Values for each gene were normalized to expression levels of TATA-binding protein for human and GAPDH for mouse.

Growth inhibition and apoptosis assays for luciferase-tagged 4T1 cells after treatment with compounds were performed as described previously (15–23).

The BD-Matrigel Invasion Chamber (24-transwell with 8-μm pore size polycarbonate membrane) was used in a modified Boyden chamber assay. The medium in the bottom chamber contained the complete culture medium of 4T1-Luc, which acts as a chemoattractant. 4T1-Luc cells (5 × 10⁴ cells/insert) in serum-free medium were plated into the top chamber with or without various concentrations of compounds and incubated for 24 hours at 37°C, 5% CO₂; the noninvading cells were removed from the top surface of the membrane with a wet Q-tip/cotton swab. Ten percent formalin was used to fix the invading cells on the bottom surface for 10 minutes and stained in hematoxylin and eosin Y solution. After washing and drying, the numbers of cells in five adjacent fields of view were counted as described previously (15–23).

MDA-MB-231 and 4T1 cells (9 × 10⁴) per well were plated on 12-well plates in DMEM and DMEM/F-12 supplemented with 2.5% charcoal-stripped FBS and 0.22% sodium bicarbonate, respectively. After 24 hours, various amounts of DNA [i.e., pGL3-PD-L1 (500 ng), and β-gal (50 ng)] were transfected into each well by Lipofectamine 2000 reagent (Invitrogen) according to the manufacturer's protocol. After 5–6 hours after transfection, cells were treated with plating media (as above) containing either solvent (DMSO) or the indicated concentration of compounds for 24 hours. Cells were then lysed using a freeze–thaw protocol and 30 μL of cell extract was used for luciferase and β-gal assays. LumiCount (Packard) was used to quantify luciferase and β-gal activities. Luciferase activity values were normalized against corresponding β-gal activity values as well as protein concentrations determined by Lowry Method.

Female BALB/c mice (4–6 weeks old) were purchased from Charles River. Two separate studies were performed. In the first study, 4T1-Luc cells (2.5 × 10⁵) were harvested in 100 μL of DMEM and suspended in ice-cold Matrigel (1:1 ratio) and these cells were implanted orthotopically into the mammary fat pad region and after 2 weeks, mice were divided in to two groups of 7 animals each. One group received 100 μL of vehicle (corn oil), and the other group received an injection of 12.5 mg/kg/day of Cl-OCH₃ in 100 μL volume of corn oil intraperitoneally for three weeks. All mice were weighed once a week over the course of treatment to monitor changes in body weight. Tumor volumes were measured using Vernier caliper over the period of treatment and later calculated using Volume = L × W²/2. After three weeks of treatment, mice were sacrificed and tumor weights were determined. In the second phase of study, three different groups (7 mice/group) received 100 μL of corn oil (control) and 2.5 mg/kg/day or 7.5 mg/kg/day of Cl-OCH₃ in 100 μL volume of corn oil intraperitoneally for 3 weeks. Body weight and tumor volumes were measured and tumor weights were determined as described above. All animal studies were carried out according to the procedures approved by the Texas A&M University Institutional Animal Care and Use Committee.

Splenocytes were prepared from the spleen by filtering in 70-μm cell strainer and flow through was washed in 1% PBSA by centrifuging. Splenocytes (100 μL) were stained with 0.4% of Trypan blue and live cells were counted using hematocytometer; 5 × 10⁶ cells/mL in 1% PBSA were resuspended and dead cells were labeled and eliminated from the analysis using LIVE/DEAD Fixable Near-IR Dead Cell Stain Kit (Invitrogen). A total of 5 × 10⁶ cells/mL) in 200 μL PBSA were used for subsequent analysis. Function minus one (FMO) and compensation beads for all antibodies were prepared. All FMOs contain live and dead stain, whereas for compensation beads, just the antibodies with a drop of compensation beads were added following manufacturer's protocol (catalog no. 01-2222-42, Invitrogen). All samples were stained with Fc blocking solution (CD16/32) and stained with CD3e, marker of T-cell population. For cell surface staining, antibody markers such as CD4, CD25, and CD8 were also used in both control and Cl-OCH₃–treated samples. For intracellular staining (Foxp3), samples were fixed and permeabilized using manufacturer protocol (catalog no. 00-5521-00, Invitrogen). Stained samples were measured using a Beckman Coulter Moflo Astrios high speed cell sorter. The BV421 was excited using a 405 nm laser and emission detected using a 448/59 nm bandpass filter. The Alexa Fluor 488, PE- eFluor 610, and PE-Cy5.5 were excited using the 488 nm laser and emissions were detected using 513/26 nm, 620/29 nm, and 710/45 nm bandpass filters, respectively. The Alexa Fluor 647 was excited using a 640 nm laser and emission detected with a 671/30 nm bandpass filter. The Live/Dead fixable near-IR stain was excited using the 640 nm laser and emission detected using a 795/70 nm bandpass filter. The samples were run at a flow rate less than 3,000 events per second. The flow cytometry data was analyzed using FlowJo Software (Becton Dickinson and Company).

Mammary tumors were excised and disrupted mechanically with a surgical blade. Tumor digestion buffer was prepared in HBSS containing 400 U/mL collagenase IV (Worthington Biochemical Corporation) and 20 U/mL DNase I. Mechanically disrupted tumors were enzymatically digested using 500 μL of digestion buffer in 200 mg of tumor. Cell suspension was passed through 70-μm cell strainer and washed with 1× PBS containing 2.5 mmol/L EDTA. TILs were enriched on a Ficoll gradient (Sigma Aldrich) and 100 μL of cell suspension was stained with 0.4% Trypan blue and counted. Cells were than resuspended in 5 × 10⁶ cells/mL in 1% PBSA and analysis of immune cells was determined as described above.

After treatment with Cl-OCH₃ for 21 days, the mice were sacrificed, primary tumors were excised, and tumor weights were recorded. Common sites of mammary tumor metastasis were harvested (spleen, lung, liver, brain, and kidney) and homogenized in presence of protease and phosphatase inhibitor. Because initial inoculum in mice was made with 4T1-Luc–tagged cells, ex vivo detection of metastasis was determined using luciferase assay system (Promega) performed according to the manufacturer's instructions. Data are represented as percentage of luciferase-positive activity compared with those observed in the mammary tumors from mice receiving the corn oil (control; set at 1.0).

One-way ANOVA and Dunnett test were used to determine statistical significance between two groups. To confirm the reproducibility of the data, the experiments were performed at least three independent times and results were expressed as means ± SD. P values less than 0.05 were considered to be statistically significant.

Initial studies screened lysates from several breast (MDA-MB-231, SUM159PT, Hs578T, MCF7, MDA-MB-468, SKBR3, and 4T1) pancreatic (L36pL, Panc1), colon (SW480), lung (A549), kidney (786-0), and rhabdomyosarcoma (Rh30) cell lines and showed that NR4A1 was expressed in every cell line, whereas PD-L1 expression was variable (Fig. 1A and B). Treatment of the PD-L1 expressing A549, SW480, and 786-0 cells with CDIM-8 (Fig. 1C) or the second-generation Cl-OCH₃ analogue of CDIM-8 (Fig. 1D) decreased expression of PD-L1 in all three cell lines. We also observed similar results in four PD-L1–expressing breast cancer cell lines (including mouse 4T1 cancer cells) treated with CDIM-8 (Fig. 1E) and Cl-OCH₃ (Fig. 1F). Using MDA-MB-231 and 4T1 cells as models, we also show that NR4A1 ligand-dependent downregulation of NR4A1 protein was partially reversed in cells cotreated with the proteasome inhibitor MG132 (Fig. 1G). Thus, NR4A1 antagonists decrease expression of PD-L1 in multiple cancer cell lines and in some cell lines we also observed decreased expression of NR4A1 and Sp1 and quantitation of the individual band intensities (Fig. 1C–G) is summarized in Supplementary Figs. S1 and S2. Previous studies showed that Cl-OCH₃ was more potent than CDIM-8 in terms of gene expression and mammary tumor growth inhibition (15, 21); however, their potencies for decreasing PD-L1 protein expression in MDA-MB-231 and 4T1 cells were similar. EC₅₀ values for Cl-OCH₃ in MDA-MB-231 and 4T1 cells was 4.4 and 4.1 μmol/L, respectively, and the corresponding values for CDIM-8 were 4.8 and 6.0 μmol/L, respectively. We also determined PD-L1 mRNA levels in the panel of cancer cell lines (Supplementary Fig. S2C) and with few exceptions the relative expression of mRNA and protein levels were similar. We next focused on determining the possible role of NR4A1/Sp1 in regulating expression of PD-L1 in breast cancer cells.

Figure 2A and B show that knockdown of Sp1 by RNAi decreased expression of Sp1 and PD-L1 in human MDA-MB-231 mouse 4T1 cancer cell lines, respectively (quantitated in Supplementary Fig. S3A). Moreover, in a parallel experiment, knockdown of NR4A1 also decreased expression of PD-L1 in MDA-MB-231 and 4T1 cells (Fig. 2A and B). We also examined the effects of Sp4, Sp3, and p300 silencing on PD-L1 expression in MDA-MB-231 and 4T1 cells and observed minimal effects (Supplementary Fig. S3B), suggesting that in triple-negative breast cancer cells, NR4A1/Sp1 regulates expression of PD-L1. Interactions of NR4A1, Sp1, p300, and pol II with the GC-rich proximal; promoter region of the PD-L1 gene in MDA-MB-231 and 4T1 cells was determined in ChIP assays. In MDA-MB-231 cells, the ChIP assay detected interactions of NR4A1, Sp1, p300, and pol II with the proximal GC-rich region of the PD-L1 promoter and after treatment with CDIM-8 or Cl-OCH₃, interactions of these proteins with the PD-L1 promoter were decreased (Fig. 2C). Similar results were observed in 4T1 cells (Fig. 2D) and these data are consistent with the role for the NR4A1/Sp1 complex in regulating PD-L1 expression (Fig. 2E).

NR4A1/Sp1-dependent regulation of PD-L1 was further investigated at the gene and reporter gene level. Figure 3A and B show that both CDIM-8 and Cl-OCH₃ decrease PD-L1 mRNA levels in MDA-MB-231 and 4T1 cells. The NR4A1 antagonists also decreased luciferase activity in these cell lines transfected with a PD-L1-Luc construct containing the GC-rich region of PD-L1 promoter linked to a luciferase reporter gene (Fig. 3C). EC₅₀ values for downregulation of PD-L1 mRNA by CDIM-8 were 17.8 and 17.4 μmol/L in MDA-MB-231 and 4T1 cells, respectively, whereas the corresponding values for Cl-OCH₃ were 3.6 and 3.0 μmol/L, respectively. The higher potency of Cl-OCH₃ versus CDIM-8 in modulating gene expression was similar to results obtained for regulation of other NR4A1-regulated genes in breast cancer cells (21). We also observed decreased luciferase activity (transfected with PD-L1-Luc) in MDA-MB-231 and 4T1 cells after knockdown of NR4A1or Sp1 by RNA interference (Fig. 3D) thus confirming that NR4A1 and Sp1 play a role in regulating PD-L1 expression at the gene, reporter gene, and protein level.

A complementary approach was used to confirm the role of Sp1 in regulating PD-L1 gene expression. Treatment of MDA-MB-231 and 4T1 cells with mithramycin, a drug that complexes with GC-rich sites to prevent Sp1 binding (34, 35) decreased expression of PD-L1 and Sp1 proteins (Fig. 4A) and mithramycin also decreased PD-L1 mRNA levels (Fig. 4B) in MDA-MB-231 and 4T1 cells. Treatment with mithramycin also decreased interactions of NR4A1, Sp1, p300 (only in MDA-MB-231 cells), and pol II with the PD-L1 promoter as determined in ChIP assay (Fig. 4C). Two PD-L1 promoter constructs mutated in the GC-rich site were synthesized (Mt1 and Mt2) and transfections of wild-type and mutant constructs into MDA-MB-231 (Fig. 4D) and 4T1 (Fig. 4E) cells showed that the mutations significantly decreased constitutive luciferase activity and mithramycin inhibited activity only in cells transfected with the wild-type PD-L1 construct. A colorimetric Episeeker DNA-protein binding assay using nuclear extracts from MDA-MB-231 cells showed that the wild-type construct containing the GC-rich site in the human PD-L1 promoter exhibited protein binding that was decreased by competition with unlabeled wild-type oligonucleotide (Fig. 4F). Minimal binding of nuclear extracts were observed with a nucleotide containing mutations in the GC-rich sequence. These results (Figs. 3 and 4) demonstrate that NR4A1/Sp1 regulates PD-L1 gene expression in MDA-MB-231 and 4T1 cells through the proximal GC-rich sites and PD-L1 expression can be decreased by treatment with NR4A1 antagonists. In addition, we also confirmed that the NR4A1 antagonists (CDIM-8 and Cl-OCH₃) decreased the growth and invasion and induced apoptosis in 4T1 cells (Supplementary Fig. S4) as reported previously for MDA-MB-231 cells (15, 17).

The in vivo effects of Cl-OCH₃ and the activity of this compound as an immunotherapy mimic were investigated in two series of experiments using a syngeneic Balb/c mouse model and luciferase-expressing 4T1 cells injected into the mammary fat pad. Administration of Cl-OCH₃ at a dose of 12.5 mg/kg/day significantly inhibited tumor growth (volume; Fig. 5A), did not affect body weight (Fig. 5B), but decreased tumor weight (Fig. 5C). TIL profile analysis showed that tumor-bearing mice treated with 12.5 mg/kg/day of Cl-OCH₃ exhibited a significant decrease in the total number of intratumoral CD4⁺ cells with no change in the total number of intratumoral CD8⁺ cells compared with untreated mice (Fig. 5D). Moreover, the Cl-OCH₃ treatment significantly increased in the ratio of intratumoral CD8⁺ effector cells (T_eff) to CD4⁺ FoxP3⁺ Tregs compared with untreated mice. These results demonstrate that the Cl-OCH₃ treatment decreased the number of CD4⁺/FoxP3⁺ T cells in the tumor. Western blot analysis showed that Cl-OCH₃ treatment also decreased PD-L1, Sp1, and NR4A1 in tumors and this complemented the effects observed in cell culture (Fig. 5E).

Because of the potent tumor growth activity of 12.5 mg/kg/day Cl-OCH₃, we carried out a comparable second study using two lower doses (7.5 and 2.5 mg/kg/day) in the syngeneic mouse model with luciferase-expressing 4T1 cells. Both doses significantly inhibited tumor volumes (Fig. 6A), did not affect body weight (Fig. 6B), and decreased tumor weights (Fig. 6C). In this study, luciferase-tagged 4T1 cells were used and significant luciferase activity was observed in the mammary tumors and compared with control (corn oil) animals treated with Cl-OCH₃ exhibited decreased luciferase activity (Fig. 6D). Luciferase activity in the lungs of control animals was similar to that observed in the mammary tumors and treatment with Cl-OCH₃ decreased luciferase activity thus inhibiting tumor metastasis to the lung. Luciferase activity was also observed in the spleen from control animals and this was also lower in Cl-OCH₃–treated spleen; however, the percent decrease was less than observed in the mammary tumors and lungs. Luciferase activity was not observed in the brain, liver, and kidney. We further examined the response of the intratumoral and splenic CD3⁺ T-cell population to decreasing concentrations of Cl-OCH₃. In tumors from mice treated with corn oil (control), 2.5, and 7.5 mg/kg/day of Cl-OCH₃, the percentage of CD3⁺/CD8⁺ Teff cell population did not significantly change at any concentration of Cl-OCH₃ (Fig. 7A); however, the CD3⁺/CD4⁺/CD25⁺/FoxP3⁺ Treg cell population decreased in a dose-dependent manner (Fig. 7B). Moreover, the Teff/Treg ratio in the tumors and spleens increased in a dose-dependent manner (Fig. 7C); additional details on the FACS analysis are outlined in Supplementary Fig. S5. These results demonstrate that treatment with Cl-OCH₃ suppresses the percentage of regulatory T cells in the tumor and overall the results were similar to that observed in the 12.5 mg/kg/day study (Fig. 5). We also observed treatment-related downregulation of PD-L1, Sp1, and NR4A1 in the mammary tumors (Fig. 7D and E). Thus, the NR4A1 antagonist Cl-OCH₃ inhibited mammary tumor growth downregulated PD-L1 and increased Teff/Treg ratios in tumors and thus acted as a small-molecule immunotherapy mimic.

Immunotherapies for treating patients with cancer are an important recent breakthrough that has now been confirmed in multiple clinical trials for several tumor types using various checkpoint inhibitors primarily targeting PD-L1 and CTL-associated antigen (CTLA-4; refs. 36–39). The FDA only recently approved the checkpoint inhibitor immunotherapy using a PD-L1 antibody (atezolizumab) in combination with chemotherapy for treating triple-negative breast cancer patients that express PD-L1. Despite the remarkable success of immunotherapies, there are still concerns and issues with respect to the numbers of patients that do not respond, the duration of the response, the development of immunotherapy resistance, and toxicities associated with the immune checkpoint inhibitors (26–32). An alternative approach to immunotherapies that target checkpoints is the development of small molecules that specifically decrease expression of checkpoint genes and there has been some progress in this area. For example, drugs such as metformin, glycosylase inhibitors, the thalidomide-like drug pomalidomide and the JAK2 inhibitor SAR 302503 decrease PD-L1 expression in various tumors and related cell lines (40–44). Previous studies in this laboratory have demonstrated that NR4A1 antagonists are effective inhibitors of the growth, survival, and invasion of breast and other cancers in both in vitro and in vivo models (14–23). Moreover, mechanistic studies show that NR4A1 acted as an obligate cofactor for several Sp-regulated genes (5, 17, 22, 23) and based on a recent report in gastric cancer cells (25), we hypothesized that NR4A1 antagonists may also target PD-L1 in breast and other cancer cells.

RNA interference studies show that knockdown of Sp1 but not Sp3 or Sp4 and knockdown of NR4A1decreased expression of PD-L1 protein (Fig. 2A and B), and reporter gene activity (Fig. 3D) in MDA-MB-231 and 4T1 cells transfected with a construct containing the proximal GC region of the PD-L1 (mouse and human) promoters. ChIP assays show that Sp1 and NR4A1 are associated with the GC-rich region of the PD-L1 promoter (Fig. 2D) and treatment with mithramycin, an inhibitor of Sp-mediated gene expression inhibited PD-L1 protein and mRNA and reporter gene expression and blocked Sp1 and NR4A1 binding to the proximal regions of the PD-L1 promoter in MDA-MB-231 and 4T1 cells (Fig. 4). Similar results were observed after treatment with NR4A1 antagonists CDIM-8 and Cl-OCH₃ in MDA-MB-231 and 4T1 cells. Although CDIM-8 and Cl-OCH₃ decreased NR4A1 and PD-L1 proteins in cancer cell lines, there was not a parallel decrease in both genes and effects were ligand- and cell context-dependent (Fig. 1). These results suggest that the ligands were not only directly inhibiting PD-L1 gene expression (Fig. 3) but also inducing other pathways that affect expression of both genes. Results in Fig. 1G show that both NR4A1 ligands induce proteasome-dependent degradation of NR4A1 and the magnitude of this response is both ligand- and cell-context dependent. Previous studies in breast cancer cells did not determine CDIM/NR4A1 ligand effects on NR4A1 expression; however, the compounds decreased receptor expression in endometrial cancer (20), but not in colon cancer cell lines (14). Thus, CDIM-8 and Cl-OCH₃ decrease PD-L1 and NR4A1 expression through both transcriptional and posttranscriptional pathways and this explains, in part, why there is not a parallel ligand-dependent downregulation of PD-L1 and NR4A1 in the cancer cell lines.

A recent study has reported the anticancer activity of Cl-OCH₃ and related compounds as inhibitors of triple-negative mammary tumor growth in athymic nude mice bearing human MDA-MB-231 cells in an orthotopic model (21). In this study, we show that the NR4A1 antagonist Cl-OCH₃ also significantly inhibited mammary tumor growth in a syngeneic mouse model at doses of 12.5, 7.5, and 2.5 mg/kg/day (Figs. 5 and 6) with the lowest dose approximately representing an IC₅₀ for inhibition of tumor weight. Moreover, we also observed inhibition of lung tumor metastasis in all treatment groups and this was consistent with previous studies showing that NR4A1 antagonists inhibited growth, survival, and migration of breast and other cancer cell lines (11, 14–23). In addition, we also demonstrated that Cl-OCH₃ decreased expression of PD-L1, an NR4A1/Sp1–regulated gene, and the TIL profile analysis showed that in tumors from mice treated with Cl-OCH₃ exhibited a significant increase in the Teff/Treg ratio. Thus, like other small molecules such as glycosylase inhibitors, metformin, and pomalidomide (40–42, 44), Cl-OCH₃ is an immunotherapy mimic through targeting PD-L1. In breast cancer cells, C-DIM/NR4A1 ligands also downregulated NR4A1 expression (Figs. 1, 6, and 7) and a recent study showed that in NR4A1/NR4A2 double knockout mice or mice treated with agents that downregulated NR4A1 there was also inhibition of tumor growth and increased Teff/Treg ratios in the tumor (45). Because Cl-OCH₃ also decreased NR4A1 expression in mammary tumor cell and tumors by transcriptional and posttranscriptional (proteasome degradation) pathways, the activity of this compound as an immunotherapy mimic is associated with the dual targeting of both NR4A1 and PD-L1 gene expression.

In summary, this study demonstrates that PD-L1 is an NR4A1/Sp1 regulated gene in triple-negative breast cancer cells that can be targeted by bis-indole–derived NR4A1 antagonists such as Cl-OCH₃. This same compound inhibits mammary tumor growth and enhances Teff/Treg ratios in a TIL assay with the ratio increase due primarily to downregulation of CD3⁺/CD4⁺/CD25⁺/FoxP3⁺ Treg cells. Previous studies showed that administration of Cl-OCH₃ to mice at a dose of 25 mg/kg/day did not cause any toxicity (46), suggesting that there was a significant margin of safety for Cl-OCH₃ that exhibited anticancer activity at a dose as low as 2.5 mg/kg/day. Thus, NR4A1 antagonists are novel small-molecule immunotherapy mimics and current studies are optimizing the activities of these compounds and identify and test a lead compound for potential clinical applications.

No potential conflicts of interest were disclosed.

Conception and design: K. Karki, S. Safe

Development of methodology: K. Karki, G.A. Wright

Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): K. Karki, G.A. Wright, K. Mohankumar, U.-H. Jin, X.-H. Zhang

Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): K. Karki, G.A. Wright, K. Mohankumar, U.-H. Jin, S. Safe

Writing, review, and/or revision of the manuscript: K. Karki, G.A. Wright, S. Safe

Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): K. Karki, X.-H. Zhang, S. Safe

This work was supported by the NIH (P30-ES023512 to S. Safe; T32-ES026568 to K. Karki), Texas A&M AgriLife Research (to S. Safe), and the Sid Kyle Chair Endowment (to S. Safe).

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.