Hyo Sang Jang

The Aryl Hydrocarbon Receptor Mediates Leflunomide-Induced Growth Inhibition of Melanoma Cells

A novel role of the dihydroorotatedehydrogenase (DHODH) inhibitor leflunomide as a potential anti-melanoma therapy was recently reported (Nature 471∶518-22, 2011). We previously reported that leflunomide strongly activates the transcriptional activity of the Aryl Hydrocarbon Receptor (AhR). We therefore tested whether the AhR regulates the anti-proliferative effects of leflunomide in melanoma. We first evaluated the expression of AhR in melanoma cells and found that AhR is highly expressed in A375 melanoma as well as in several other cancer cell types. To evaluate whether AhR plays a role in regulating the growth inhibitory effects of leflunomide in A375 cells, we generated a stable cell line from parental A375 cells expressing a doxycycline (DOX) inducible AhR shRNA. Using these cells in the absence or presence of DOX (normal AhR levels or AhR-knockdown, respectively) we found that the anti-proliferative effects of leflunomide, but not its metabolite A771726, were strongly dependent upon AhR expression. It has been well established that supplementation of cells with exogenous uridine completely rescues the anti-proliferative effects due to DHODH inhibition. Thus, we performed uridine rescue experiments in A375 cells to determine whether the anti-proliferative effects of leflunomide are solely due to DHODH inhibition as previously reported. Interestingly, saturating levels of uridine only modestly rescued A375 cells from the anti-proliferative effects of both leflunomide and A771726, indicating additional mechanism(s), apart from DHODH inhibition are responsible for the anti-proliferative effects of leflunomide in melanoma cells. Uridine also did not rescue MDA-MB-435S melanoma cell proliferation after leflunomide treatment. Our results reveal that the AhR is a molecular target of leflunomide and support the feasibility of the clinical application of leflunomide for treating melanoma. Furthermore, analysis of expression data from 967 cancer cell lines revealed that AhR is expressed in multiple different cancer types supporting the intriguing possibility of targeting the AhR for therapy in a number of cancers.

Calpain 2 is Required for Glioblastoma Cell Invasion: Regulation of Matrix Metalloproteinase 2

Invasion of glioblastoma cells significantly reduces the effectiveness of current treatments, highlighting the importance of understanding dispersal mechanisms and characteristics of the invasive population. Induction of calcium fluxes into glioblastoma cells by autocrine glutamate is critical for invasion. However, the target(s) by which calcium acts to stimulate the dispersal of glioblastoma cells is not clear. In this study, we tested the hypothesis that the calcium-activated protease calpain 2 is required for glioblastoma cell invasion. Knockdown of calpain 2 expression using shRNA or chemical inhibition of calpain activity reduced glioblastoma cell invasion by 90%. Interestingly, decreased expression of calpain 2 did not influence morphology or migration, suggesting regulation of invasion specific mechanisms. Consistent with this idea, 39% less extracellular MMP2 was measured from knockdown cells identifying one mechanism by which calpain 2 mediates glioblastoma cell invasion. This is the first report demonstrating that calpain 2 is required for glioblastoma cell invasion.

Phosphoinositide binding to the substrate regulates susceptibility to proteolysis by calpain.

Calpain-mediated proteolysis regulates cytoskeletal dynamics and is altered during aging and the progression of numerous diseases or pathological conditions. Although several cytoskeletal proteins have been identified as substrates, how localized calpain activity is regulated and the mechanisms controlling substrate recognition are not clear. In this study, we report that phosphoinositide binding regulates the susceptibility of the cytoskeletal adhesion protein α-actinin to proteolysis by calpains 1 and 2. At first, α-actinin did not appear to be a substrate for calpain 2; however, phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) binding to α-actinin resulted in nearly complete proteolysis of the full-length protein, producing stable breakdown products. Calpain 1 was able to cleave α-actinin in the absence of phosphoinositide binding; however, PtdIns(3,4,5)P3 binding increased the rate of proteolysis, and phosphatidylinositol 4,5-diphosphate (PtdIns(4,5)P2) binding significantly inhibited cleavage. Phosphoinositide binding appeared to regulate calpain proteolysis of α-actinin by modulating the exposure of a highly sensitive cleavage site within the calponin homology 2 domain. In U87MG glioblastoma cells, which contain elevated levels of PtdIns(3,4,5)P3, α-actinin colocalized with calpain within dynamic actin cytoskeletal structures. Furthermore, proteolysis of α-actinin producing stable breakdown products was observed in U87MG cells treated with calcium ionophore to activate the calcium-dependent calpains. Additional evidence of PtdIns(3,4,5)P3-mediated calpain proteolysis of α-actinin was observed in rat embryonic fibroblasts. These results suggest that PtdIns(3,4,5)P3 binding is a critical determinant for α-actinin proteolysis by calpain. In conclusion, phosphoinositide binding to the substrate is a potential mechanism for regulating susceptibility to proteolysis by calpain.

The aryl hydrocarbon receptor mediates raloxifene-induced apoptosis in estrogen receptor-negative hepatoma and breast cancer cells

Identification of new molecular targets for the treatment of breast cancer is an important clinical goal, especially for triple-negative breast cancer, which is refractory to existing targeted treatments. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor known primarily as the mediator of dioxin toxicity. However, the AhR can also inhibit cellular proliferation in a ligand-dependent manner and act as a tumor suppressor in mice, and thus may be a potential anticancer target. To investigate the AhR as an anticancer target, we conducted a small molecule screen to discover novel AhR ligands with anticancer properties. We identified raloxifene, a selective estrogen receptor (ER) modulator currently used in the clinic for prevention of ER-positive breast cancer and osteoporosis in post-menopausal women, as an AhR activator. Raloxifene directly bound the AhR and induced apoptosis in ER-negative mouse and human hepatoma cells in an AhR-dependent manner, indicating that the AhR is a molecular target of raloxifene and mediates raloxifene-induced apoptosis in the absence of ER. Raloxifene selectively induced apoptosis of triple-negative MDA-MB-231 breast cancer cells compared with non-transformed mammary epithelial cells via the AhR. Combined with recent data showing that raloxifene inhibits triple-negative breast cancer xenografts in vivo (Int J Oncol. 43(3):785-92, 2013), our results support the possibility of repurposing of raloxifene as an AhR-targeted therapeutic for triple-negative breast cancer patients. To this end, we also evaluated the role of AhR expression on survival of patients diagnosed with breast cancer. We found that higher expression of the AhR is significantly associated with increased overall survival and distant metastasis-free survival in both hormone-dependent (ER-positive) and hormone-independent (ER and progesterone receptor (PR)-negative) breast cancers. Together, our data strongly support the possibility of using the AhR as a molecular target for the treatment of hormone-independent breast cancers.

Benzimidazoisoquinolines: A New Class of Rapidly Metabolized Aryl Hydrocarbon Receptor (AhR) Ligands that Induce AhR-Dependent Tregs and Prevent Murine Graft-Versus-Host Disease

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that plays multiple roles in regulation of immune and inflammatory responses. The ability of certain AhR ligands to induce regulatory T cells (Tregs) has generated interest in developing AhR ligands for therapeutic treatment of immune-mediated diseases. To this end, we designed a screen for novel Treg-inducing compounds based on our understanding of the mechanisms of Treg induction by the well-characterized immunosuppressive AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We screened a ChemBridge small molecule library and identified 10-chloro-7H-benzimidazo[2,1-a]benzo[de]Iso-quinolin-7-one (10-Cl-BBQ) as a potent AhR ligand that was rapidly metabolized and not cytotoxic to proliferating T cells. Like TCDD,10-Cl-BBQ altered donor CD4+ T cell differentiation during the early stages of a graft versus host (GVH) response resulting in expression of high levels of CD25, CTLA-4 and ICOS, as well as several genes associated with Treg function. The Treg phenotype required AhR expression in the donor CD4+ T cells. Foxp3 was not expressed in the AhR-induced Tregs implicating AhR as an independent transcription factor for Treg induction. Structure-activity studies showed that unsubstituted BBQ as well as 4, 11-dichloro-BBQ were capable of inducing AhR-Tregs. Other substitutions reduced activation of AhR. Daily treatment with 10-Cl-BBQ during the GVH response prevented development of GVH disease in an AhR-dependent manner with no overt toxicity. Together, our data provide strong support for development of select BBQs that activate the AhR to induce Tregs for treatment of immune-mediated diseases.

Glycine-rich region regulates cysteine-rich protein 1 binding to actin cytoskeleton

Cysteine-rich protein 1 (CRP1) has a unique structure with two well separated LIM domains, each followed by a glycine-rich region. Although CRP1 has been shown to interact with actin-binding proteins and actin filaments, the mechanism regulating localization to the actin cytoskeleton in cells is not clear. Experiments using truncated forms showed that the first LIM domain and glycine-rich region are necessary for CRP1 bundling of actin filaments and localization to the actin cytoskeleton. Furthermore, domain swapping experiments replacing the first glycine-rich region with the second resulted in the loss of CRP1 bundling activity and localization to the actin cytoskeleton, identifying seven critical amino acid residues. These results highlight the importance of the first glycine-rich region for CRP1 bundling activity and localization to the actin cytoskeleton. In addition, this work identifies the first LIM domain and glycine-rich region as a distinct actin filament bundling module.

Anti-androgen flutamide suppresses hepatocellular carcinoma cell proliferation via the aryl hydrocarbon receptor mediated induction of transforming growth factor-β1.

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and a member of the basic helix-loop-helix PER/ARNT/SIM family of chemosensors and developmental regulators. The AhR is widely known as a mediator of dioxin toxicity; however, it also suppresses cancer cell proliferation and recent findings have implicated its role as a tumor suppressor. We conducted a chemical library screen to identify nontoxic AhR ligands with anti-cancer effects and discovered flutamide (Eulexin) as a putative AhR ligand. Flutamide is an androgen receptor (AR) antagonist approved by the United States Food and Drug Administration for the treatment of prostate cancer. We found that flutamide inhibited the growth of several cancer cell lines independent of AR status, and that suppression of AhR expression reversed the anti-proliferative effects of flutamide. We investigated the AhR-dependent mechanism of action of flutamide in human hepatocellular carcinoma cells and identified that transforming growth factor-β1 (TGF-β1) is induced by flutamide in an AhR-dependent manner. In contrast, the potent AhR agonist 2,3,7,8-Tetrachlorodibenzo-p-dioxin had no effect on TGF-β1 expression, indicating the ligand specificity of AhR activation. We also determined that TGF-β1 induction is required for the AhR-dependent growth inhibitory effects of flutamide. Therefore, flutamide may be effective in AhR-positive cancers that are sensitive to TGF-β1 signaling, such as hepatocellular carcinoma.

Retinoid-X-Receptors (α/β) in Melanocytes Modulate Innate Immune Responses and Differentially Regulate Cell Survival following UV Irradiation

Understanding the molecular mechanisms of ultraviolet (UV) induced melanoma formation is becoming crucial with more reported cases each year. Expression of type II nuclear receptor Retinoid-X-Receptor α (RXRα) is lost during melanoma progression in humans. Here, we observed that in mice with melanocyte-specific ablation of RXRα and RXRβ, melanocytes attract fewer IFN-γ secreting immune cells than in wild-type mice following acute UVR exposure, via altered expression of several chemoattractive and chemorepulsive chemokines/cytokines. Reduced IFN-γ in the microenvironment alters UVR-induced apoptosis, and due to this, the survival of surrounding dermal fibroblasts is significantly decreased in mice lacking RXRα/β. Interestingly, post-UVR survival of the melanocytes themselves is enhanced in the absence of RXRα/β. Loss of RXRs α/β specifically in the melanocytes results in an endogenous shift in homeostasis of pro- and anti-apoptotic genes in these cells and enhances their survival compared to the wild type melanocytes. Therefore, RXRs modulate post-UVR survival of dermal fibroblasts in a “non-cell autonomous” manner, underscoring their role in immune surveillance, while independently mediating post-UVR melanocyte survival in a “cell autonomous” manner. Our results emphasize a novel immunomodulatory role of melanocytes in controlling survival of neighboring cell types besides controlling their own, and identifies RXRs as potential targets for therapy against UV induced melanoma.

The aryl hydrocarbon receptor is required for induction of p21 cip1/waf1 expression and growth inhibition by SU5416 in hepatoma cells

The aryl hydrocarbon receptor (AhR) is a potential clinical target for cancer and autoimmune dysfunction. Identifying selective AhR modulators that produce desirable clinical outcomes represents an opportunity for developing new anti-cancer agents. Repurposing clinically-used drugs with established safety profiles that activate the AhR represents a good starting place to pursue this goal. In this study, we characterized the AhR-dependent effects of SU5416 (Semaxanib) following its identification in a small-molecule library screen. SU5416 potently activated AhR-dependent reporter genes, induced AhR nuclear localization, facilitated AhR-DNA binding, and increased, expression of its endogenous target genes. SU5416 significantly inhibited proliferation of Hepa1 hepatoma cells in an AhR-dependent manner, but did not induce apoptosis. SU5416 also inhibited the growth of human HepG2 liver cancer cells. The effects of SU5416 correlated with an increased G1 population and increased expression of cell cycle inhibitor p21cip1/waf1 at both the mRNA and protein level. Increased expression of p21cip1/waf1 by SU5416 required expression of both AhR and Arnt. In addition, evidence for long-term activation of the AhR in vivo by a single dose of SU5416 was identified by analyzing published microarray data. Our results provide support for continued investigation of the AhR as therapeutic for cancers such as hepatocellular carcinoma. In addition, our findings raise the possibility that some of the previously observed anti-proliferative effects of SU5416 may be due to activation of the AhR.

Distinctive Nuclear Localization Signals in the Oomycete Phytophthora sojae

To date, nuclear localization signals (NLSs) that target proteins to nuclei in oomycetes have not been defined, but have been assumed to be the same as in higher eukaryotes. Here, we use the soybean pathogen Phytophthora sojae as a model to investigate these sequences in oomycetes. By establishing a reliable in vivo NLS assay based on confocal microscopy, we found that many canonical monopartite and bipartite classical NLSs (cNLSs) mediated nuclear import poorly in P. sojae. We found that efficient localization of P. sojae nuclear proteins by cNLSs requires additional basic amino acids at distal sites or collaboration with other NLSs. We found that several representatives of another well-characterized NLS, proline-tyrosine NLS (PY-NLS) also functioned poorly in P. sojae. To characterize PY-NLSs in P. sojae, we experimentally defined the residues required by functional PY-NLSs in three P. sojae nuclear-localized proteins. These results showed that functional P. sojae PY-NLSs include an additional cluster of basic residues for efficient nuclear import. Finally, analysis of several highly conserved P. sojae nuclear proteins including ribosomal proteins and core histones revealed that these proteins exhibit a similar but stronger set of sequence requirements for nuclear targeting compared with their orthologs in mammals or yeast.