Independent actions on cyclin-dependent kinases and aryl hydrocarbon receptor mediate the antiproliferative effects of indirubins

Targeted protein degradation might present a novel therapeutic approach in the fight against African trypanosomiasis

African trypanosomiasis (AT) is a hemoparasitic disease caused by infection with African trypanosomes and it is prevalent in many sub-Saharan African countries, affecting both humans and domestic animals. The disease is transmitted mostly by haematophagous insects of the genus Glossina while taking blood meal, in the process spreading the parasites from an infected animal to an uninfected animal. The disease is fatal if untreated, and the available drugs are generally ineffective and resulting in toxicities. Therefore, it is still pertinent to explore novel methods and targets for drug discovery. Proteolysis-targeting chimeras (PROTACs) present a new strategy for development of therapeutic molecules that mimic cellular proteasomal-mediated protein degradation to target proteins involved in different disease types. PROTACs have been used to degrade proteins involved in various cancers, neurodegenerative diseases, and immune disorders with remarkable success. Here, we highlight the problems associated with the current treatments for AT, discuss the concept of PROTACs and associated targeted protein degradation (TPD) approaches, and provide some insights on the future potential for the use of these emerging technologies (PROTACs and TPD) for the development of new generation of anti-Trypanosoma drugs and the first "TrypPROTACs".

Synergistic effect of plasma-activated medium and novel indirubin derivatives on human skin cancer cells by activation of the AhR pathway

Due to the increasing number of human skin cancers and the limited effectiveness of therapies, research into innovative therapeutic approaches is of enormous clinical interest. In recent years, the use of cold atmospheric pressure plasma has become increasingly important as anti-cancer therapy. The combination of plasma with small molecules offers the potential of an effective, tumour-specific, targeted therapy. The synthesised glycosylated and non glycosylated thia-analogous indirubin derivatives KD87 and KD88, respectively, were first to be investigated for their pharmaceutical efficacy in comparison with Indirubin-3'-monoxime (I3M) on human melanoma (A375) and squamous cell carcinoma (A431) cells. In combinatorial studies with plasma-activated medium (PAM) and KD87 we determined significantly decreased cell viability and cell adhesion. Cell cycle analyses revealed a marked G2/M arrest by PAM and a clear apoptotic effect by the glycosylated indirubin derivative KD87 in both cell lines and thus a synergistic anti-cancer effect. I3M had a pro-apoptotic effect only in A431 cells, so we hypothesize a different mode of action of the indirubin derivatives in the two skin cancer cells, possibly due to a different level of the aryl hydrocarbon receptor and an activation of this pathway by nuclear translocation of this receptor and subsequent activation of gene expression.

Modulation of glioma-inflammation crosstalk profiles in human glioblastoma cells by indirubin-3'-(2,3 dihydroxypropyl)-oximether (E804) and 7-bromoindirubin-3'-oxime (7BIO)

Indirubins E804 (indirubin-3'-(2,3 dihydroxypropyl)-oximether) and 7BIO (7-Bromoindirubin-3'-oxime) are synthetic derivatives of natural indirubin, the active compound in Danggui Longhui Wan, a traditional Chinese remedy for cancer and inflammation. Herein, we explore E804 and 7BIO for their potential to modulate key pro-inflammatory genes and cytokines in LN-18 and T98G glioblastoma cells. High grade gliomas typically secrete large amounts of inflammatory cytokines and growth factors that promote tumor growth in an autocrine fashion. Inflammation is emerging as a key concern in the success of new treatment modalities for glioblastomas. Studies indicate that select indirubin derivatives bind and activate signaling of the AHR pathway, as well as inhibit cyclin-dependent kinases and STAT3 signaling. AHR signaling is involved in hematopoiesis, immune function, cell cycling, and inflammation, and thus may be a possible target for glioma treatment. To determine the significance of the AHR pathway in LN-18 and T98G glioma inflammatory profiles, and on the effects of E804 and 7BIO on these profiles, we used 6,2',4'-trimethoxyflavone (TMF), a putative selective AHR antagonist. It was confirmed that E804 and 7BIO activates the AHR leading to cyp1b1 expression, and that TMF antagonizes expression. We then employed a commercial cancer inflammation and immunity crosstalk qRT-PCR array to screen for anti-inflammatory related properties. TMF alone inhibited expression of ifng, ptsg2, il12b, tnfa, il10, il13, the balance between pd1 and pdl1, and even expression of mhc1a/b. E804 was very potent in suppressing many pro-inflammatory genes, including il1a, il1b, il12a, ptgs2, tlr4, and others. E804 also affected expression of il6, vegfa, and stat3. Conversely, 7BIO induced cox2, but suppressed a different selection of pro-inflammatory genes including nos2, tnfa, and igf1. Secretion of IL-6 protein, an iconic inflammatory cytokine, was decreased by E804. VEGF (vascular endothelial growth factor) protein secretion was upregulated by 7BIO, yet downregulated by E804 and E804 plus TMF. Thus, E804 is both an AHR ligand and regulator of important pro-inflammatory cytokines such as IL-6 and oncogene STAT3, among others. Our results point to the use of E804 and TMF in combination as a promising new treatment for glioblastoma.

Identification of GSK-3 as a Potential Therapeutic Entry Point for Epilepsy

In view of the clinical need for new antiseizure drugs (ASDs) with novel modes of action, we used a zebrafish seizure model to screen the anticonvulsant activity of medicinal plants used by traditional healers in the Congo for the treatment of epilepsy, and identified a crude plant extract that inhibited pentylenetetrazol (PTZ)-induced seizures in zebrafish larvae. Zebrafish bioassay-guided fractionation of this anticonvulsant Fabaceae species, Indigofera arrecta, identified indirubin, a compound with known inhibitory activity of glycogen synthase kinase (GSK)-3, as the bioactive component. Indirubin, as well as the more potent and selective GSK-3 inhibitor 6-bromoindirubin-3'-oxime (BIO-acetoxime) were tested in zebrafish and rodent seizure assays. Both compounds revealed anticonvulsant activity in PTZ-treated zebrafish larvae, with electroencephalographic recordings revealing reduction of epileptiform discharges. Both indirubin and BIO-acetoxime also showed anticonvulsant activity in the pilocarpine rat model for limbic seizures and in the 6-Hz refractory seizure mouse model. Most interestingly, BIO-acetoxime also exhibited anticonvulsant actions in 6-Hz fully kindled mice. Our findings thus provide the first evidence for anticonvulsant activity of GSK-3 inhibition, thereby implicating GSK-3 as a potential therapeutic entry point for epilepsy. Our results also support the use of zebrafish bioassay-guided fractionation of antiepileptic medicinal plant extracts as an effective strategy for the discovery of new ASDs with novel mechanisms of action.

Indirubin, a bis-indole alkaloid binds to tubulin and exhibits antimitotic activity against HeLa cells in synergism with vinblastine

Indirubin, a bis-indole alkaloid used in traditional Chinese medicine has shown remarkable anticancer activity against chronic myelocytic leukemia. The present work was aimed to decipher the underlying molecular mechanisms responsible for its anticancer attributes. Our findings suggest that indirubin inhibited the proliferation of HeLa cells with an IC₅₀ of 40 μM and induced a mitotic block. At concentrations higher than its IC₅₀, indirubin exerted a moderate depolymerizing effect on the interphase microtubular network and spindle microtubules in HeLa cells. Studies with goat brain tubulin indicated that indirubin bound to tubulin at a single site with a dissociation constant of 26 ± 3 μM and inhibited the in vitro polymerization of tubulin into microtubules in the presence of glutamate as well as microtubule-associated proteins. Molecular docking analysis and molecular dynamics simulation studies indicate that indirubin stably binds to tubulin at the interface of the α-β tubulin heterodimer. Further, indirubin stabilized the binding of colchicine on tubulin and promoted the cysteine residue modification by 5,5'-dithiobis-2-nitrobenzoic acid, indicating towards alteration of tubulin conformation upon binding. In addition, we found that indirubin synergistically enhanced the anti-mitotic and anti-proliferative activity of vinblastine, a known microtubule-targeted agent. Collectively our studies indicate that perturbation of microtubule polymerization dynamics could be one of the possible mechanisms behind the anti-cancer activities of indirubin.

Effects of indirubin and isatin on cell viability, mutagenicity, genotoxicity and BAX/ERCC1 gene expression

CONTEXT

Indigofera suffruticosa Miller (Fabaceae) and I. truxillensis Kunth produce compounds, such as isatin (ISA) and indirubin (IRN), which possess antitumour properties. Their effects in mammalian cells are still not very well understood.

OBJECTIVE

We evaluated the activities of ISA and/or IRN on cell viability and apoptosis in vitro, their genotoxic potentials in vitro and in vivo, and the IRN- and ISA-induced expression of ERCC1 or BAX genes.

MATERIALS AND METHODS

HeLa and/or CHO-K1 cell lines were tested (3 or 24 h) in the MTT, Trypan blue exclusion, acridine orange/ethidium bromide, cytokinesis-blocked micronucleus (CBMN) and comet (36, 24 and 72 h) tests after treatment with IRN (0.1 to 200 μM) or ISA (0.5 to 50 μM). Gene expression was measured by RT-qPCR in HeLa cells. Swiss albino mice received IRN (3, 4 or 24 h) by gavage (50, 100 and 150 mg/kg determined from the LD₅₀ - 1 g/kg b.w.) and submitted to comet assay in vivo.

RESULTS

IRN reduced the viability of CHO-K1 (24 h; 5 to 200 μM) and HeLa cells (10 to 200 μM), and was antiproliferative in the CBMN test (CHO-K1: 0.5 to 10 μM; HeLa: 5 and 10 μM). The drug did not induce apoptosis, micronucleus neither altered gene expression. IRN and ISA were genotoxic for HeLa cells (3 and 24 h) at all doses tested. IRN (100 and 150 mg/kg) also induced genotoxicity in vivo (4 h).

CONCLUSION

IRN and ISA have properties that make them candidates as chemotherapeutics for further pharmacological investigations.

Identification of a Raloxifene Analog That Promotes AhR-Mediated Apoptosis in Cancer Cells

We previously reported that raloxifene, an estrogen receptor modulator, is also a ligand for the aryl hydrocarbon receptor (AhR). Raloxifene induces apoptosis in estrogen receptor-negative human cancer cells through the AhR. We performed structure–activity studies with seven raloxifene analogs to better understand the structural requirements of raloxifene for induction of AhR-mediated transcriptional activity and apoptosis. We identified Y134 as a raloxifene analog that activates AhR-mediated transcriptional activity and induces apoptosis in MDA-MB-231 human triple negative breast cancer cells. Suppression of AhR expression strongly reduced apoptosis induced by Y134, indicating the requirement of AhR for Y134-induced apoptosis. Y134 also induced apoptosis in hepatoma cells without having an effect on cell cycle regulation. Toxicity testing on zebrafish embryos revealed that Y134 has a significantly better safety profile than raloxifene. Our studies also identified an analog of raloxifene that acts as a partial antagonist of the AhR, and is capable of inhibiting AhR agonist-induced transcriptional activity. We conclude that Y134 is a promising raloxifene analog for further optimization as an anti-cancer agent targeting the AhR.

Indirubin inhibits the migration, invasion, and activation of fibroblast-like synoviocytes from rheumatoid arthritis patients

OBJECTIVES

To evaluate the inhibition of indirubin in FLSs migration, invasion, activation, and proliferation in RA FLSs.

METHODS

The levels of IL-6 and IL-8 in cultural supernatants were measured by ELISA. RA FLS migration and invasion in vitro were measured by the Boyden chamber method and the scratch assay. Signal transduction protein expression was measured by western blot. FLS proliferation was detected by Edu incorporation. F-actin was measured by immunofluorescence staining.

RESULTS

We found that indirubin reduced migration, invasion, inflammation, and proliferation in RA FLSs. In addition, we demonstrated that indirubin inhibited lamellipodium formation during cell migration. To gain insight into molecular mechanisms, we evaluated the effect of indirubin on PAK1 and MAPK activation. Our results indicated that indirubin inhibited the activity of PAK1 and MAPK.

CONCLUSIONS

Our observations suggest that indirubin may be protective against joint destruction in RA by regulating synoviocyte migration, invasion, activation, and proliferation.

Dietary Indoles Suppress Delayed-Type Hypersensitivity by Inducing a Switch from Proinflammatory Th17 Cells to Anti-Inflammatory Regulatory T Cells through Regulation of MicroRNA

Aryl hydrocarbon receptor (AhR) has been shown to have profound influence on T cell differentiation, and use of distinct AhR ligands has shown that whereas some ligands induce regulatory T cells (Tregs), others induce Th17 cells. In the present study, we tested the ability of dietary AhR ligands (indole-3-carbinol [I3C] and 3,3'-diindolylmethane [DIM]) and an endogenous AhR ligand, 6-formylindolo(3,2-b)carbazole (FICZ), on the differentiation and functions of Tregs and Th17 cells. Treatment of C57BL/6 mice with indoles (I3C or DIM) attenuated delayed-type hypersensitivity (DTH) response to methylated BSA and generation of Th17 cells while promoting Tregs. In contrast, FICZ exacerbated the DTH response and promoted Th17 cells. Indoles decreased the induction of IL-17 but promoted IL-10 and Foxp3 expression. Also, indoles caused reciprocal induction of Tregs and Th17 cells only in wild-type (AhR(+/+)) but not in AhR knockout (AhR(-/-)) mice. Upon analysis of microRNA (miR) profile in draining lymph nodes of mice with DTH, treatment with I3C and DIM decreased the expression of several miRs (miR-31, miR-219, and miR-490) that targeted Foxp3, whereas it increased the expression of miR-495 and miR-1192 that were specific to IL-17. Interestingly, treatment with FICZ had precisely the opposite effects on these miRs. Transfection studies using mature miR mimics of miR-490 and miR-1192 that target Foxp3 and IL-17, respectively, or scrambled miR (mock) or inhibitors confirmed that these miRs specifically targeted Foxp3 and IL-17 genes. Our studies demonstrate, to our knowledge for the first time, that the ability of AhR ligands to regulate the differentiation of Tregs versus Th17 cells may depend on miR signature profile.

Indirubin derivatives: a patent review (2010 - present)

INTRODUCTION

Indirubins are bisindole alkaloids naturally occurring in indigo-bearing plants or in mollusks from the Muricidae family. They belong to the rather small family of indigoids, which has nevertheless found an extreme importance in the fields of dyes and medicinal chemistry. Indirubin has been found to be the active ingredient of a traditional Chinese Medicine used to treat the symptoms of leukemia. Further biological explorations revealed the ability of indirubin to bind cyclin-dependent kinases and 6-bromoindirubin, extracted from mollusks, to bind glycogen synthase kinase-3. The high affinity displayed by the two natural products has opened a vast field of research and triggered the development of hundred of derivatives with biological activities.

AREAS COVERED

The traditional use of indirubin for the treatment of leukemia has prompted different research groups to study the cytotoxic effect of indirubin derivatives on both solid tumors and leukemia. Moreover, the affinity of indirubins for kinases also allowed the exploration of their activity towards stem cells.

EXPERT OPINION

The derivatives presented are in accordance with first discoveries and establish the close relation between activity and kinase inhibition. New derivatives have been patented and new interferences in signaling pathways are described. However, few in vivo studies have been performed and more efficient solutions are needed to unravel the major issue of solubility.

Indirubin and Indirubin Derivatives for Counteracting Proliferative Diseases

Indirubin is the active component of Danggui Longhui Wan, a traditional Chinese medicine formulation. The encouraging clinical results from the 1980s obtained in chronic myelocytic leukemia patients treated with indirubin stimulated numerous studies on this compound. These investigations explored the use of indirubin in different types of cancer and reported the synthesis of novel derivatives with improved chemical and pharmacokinetic properties. In this paper, we review the impressive progress that has been made in elucidating the mechanistic understanding of how indirubin and its derivatives affect physiological and pathophysiological processes, mainly by inhibition of cell proliferation and induction of cell death. Furthermore, we survey the therapeutic use of these compounds in combating proliferative diseases such as cancer, restenosis, and psoriasis.

3,4-Dihydroxy-benzohydroxamic acid (Didox) suppresses pro-inflammatory profiles and oxidative stress in TLR4-activated RAW264.7 murine macrophages

Didox (3,4-dihydroxy-benzohydroxamic acid), is a synthetic ribonucleotide reductase (RR) inhibitor derived from polyhydroxy-substituted benzohydroxamic acid, and originally developed as an anti-cancer agent. Some studies indicate that didox may have anti-oxidative stress-like properties, while other studies hint that didox may have anti-inflammatory properties. Using nitric oxide production in response to LPS treatment as a sensitive screening assay for anti-inflammatory compounds, we show that didox is very potent at levels as low as 6.25 μM, with maximal inhibition at 100 μM. A qRT-PCR array was then employed to screen didox for other potential anti-inflammatory and anti-oxidative stress-related properties. Didox was very potent in suppressing the expression of these arrayed mRNA in response to LPS, and in some cases didox alone suppressed expression. Using qRT-PCR as a follow up to the array, we demonstrated that didox suppresses LPS-induced mRNA levels of iNOS, IL-6, IL-1, TNF-α, NF-κβ (p65), and p38-α, after 24h of treatment. Treatment with didox also suppresses the secretion of nitric oxide, IL-6, and IL-10. Furthermore, oxidative stress, as quantified by intracellular ROS levels in response to macrophage activators LPS and phorbol ester (PMA), and the glutathione depleting agent BSO, is reduced by treatment with didox. Moreover, we demonstrate that nuclear translocation of NF-κβ (p65) in response to LPS is inhibited by didox. These findings were supported by qRT-PCR for oxidative stress genes SOD1 and catalase. Overall, this study supports the conclusion that didox may have a future role in managing acute and chronic inflammatory diseases and oxidative stress due to high production of ROS.

Pityriazepin and other potent AhR ligands isolated from Malassezia furfur yeast

Malassezia furfur yeast strains isolated from diseased human skin preferentially biosynthesize indole alkaloids which can be detected in the human skin and are highly potent activators of the aryl hydrocarbon receptor (AhR) and AhR-dependent gene expression. Chemical analysis of an EtOAc extract of a M. furfur strain obtained from diseased human skin and grown on l-tryptophan agar revealed several known AhR active tryptophan metabolites along with a previously unidentified compound, pityriazepin. While its structure resembled that of the known alkaloid pityriacitrin, the comprised pyridine ring had been transformed into an azepinone. The indoloazepinone scaffold of pityriazepin is extremely rare in nature and has only been reported once previously. Pityriazepin, like the other isolated compounds, was found to be a potent activator of the AhR-dependent reporter gene assay in recombinant cell lines derived from four different species, although significant species differences in relative potency were observed. The ability of pityriazepin to competitively bind to the AhR and directly stimulate AhR DNA binding classified it as a new naturally-occurring potent AhR agonist. M. furfur produces an expanded collection of extremely potent naturally occurring AhR agonists, which produce their biological effects in a species-specific manner.

Identification of small molecules that support human leukemia stem cell activity ex vivo

Leukemic stem cells (LSCs) are considered a major cause of relapse in acute myeloid leukemia (AML). Defining pathways that control LSC self-renewal is crucial for a better understanding of underlying mechanisms and for the development of targeted therapies. However, currently available culture conditions do not prevent spontaneous differentiation of LSCs, which greatly limits the feasibility of cell-based assays. To overcome these constraints we conducted a high-throughput chemical screen and identified small molecules that inhibit differentiation and support LSC activity in vitro. Similar to reports with cord blood stem cells, several of these compounds suppressed the aryl-hydrocarbon receptor (AhR) pathway, which we show to be inactive in vivo and rapidly activated ex vivo in AML cells. We also identified a compound, UM729, that collaborates with AhR suppressors in preventing AML cell differentiation. Together, these findings provide newly defined culture conditions for improved ex vivo culture of primary human AML cells.

Indirubin-3'-(2,3 dihydroxypropyl)-oximether (E804) is a potent modulator of LPS-stimulated macrophage functions

Indirubin is a deep-red bis-indole isomer of indigo blue, both of which are biologically active ingredients in Danggui Longhui Wan, an ancient Chinese herbal tea mixture used to treat neoplasia and chronic inflammation and to enhance detoxification of xenobiotics. Multiple indirubin derivatives have been synthesized and shown to inhibit cyclin-dependent kinases (CDKs) and glycogen-synthase kinase (GSK-3β) with varying degrees of potency. Several indirubins are also aryl hydrocarbon receptor (AhR) agonists, with AhR-associated activities covering a wide range of potencies, depending on molecular structure. This study examined the effects of indirubin-3'-(2,3 dihydroxypropyl)-oximether (E804), a novel indirubin with potent STAT3 inhibitory properties, on basal and LPS-inducible activities in murine RAW264.7 macrophages. Using a focused commercial qRT-PCR array platform (SuperArray®), the effects of E804 on expression of a suite of genes associated with stress and toxicity were determined. Most genes up-regulated by LPS treatment were suppressed by E804; including LPS-induced expression of pro-inflammatory cytokines and receptors, apoptosis control genes, and oxidative stress response genes. Using qRT-PCR as a follow up to the commercial arrays, E804 treatment suppressed LPS-induced COX-2, iNOS, IL-6 and IL-10 gene expression, though the effects on iNOS and COX-2 protein expression were less dramatic. E804 also inhibited LPS-induced secretion of IL-6 and IL-10. Functional endpoints, including iNOS and lysozyme enzymatic activity, phagocytosis of fluorescent latex beads, and intracellular killing of bacteria, were also examined, and in each experimental condition E804 suppressed activities. Collectively, these results indicate that E804 is a potent modulator of pro-inflammatory profiles in LPS-treated macrophages.

Malassezia-derived indoles activate the aryl hydrocarbon receptor and inhibit Toll-like receptor-induced maturation in monocyte-derived dendritic cells

BACKGROUND

The aryl hydrocarbon receptor (AhR) is a nuclear receptor and transcriptional regulator with pleiotropic effects. The production of potent AhR ligands by Malassezia yeasts, such as indirubin, indolo[3,2-b]carbazole (ICZ), tryptanthrin and malassezin, has been associated with the pathogenesis of seborrhoeic dermatitis and pityriasis versicolor. Antigen-presenting cells in the skin can encounter microbes in the presence of these bioactive metabolites that could potentially modulate their function.

OBJECTIVES

To study the effects of the aforementioned naturally occurring ligands on AhR activation and Toll-like receptor (TLR)-induced maturation in human monocyte-derived dendritic cells (moDCs).

METHODS

These indoles were screened for AhR activation capacity in moDCs employing CYP1A1 and CYP1B1 induction as read out and for their effects on the function of moDCs after TLR-ligand stimulation.

RESULTS

Indirubin and ICZ were the most potent AhR ligands and were selected for subsequent experiments. Concurrent exposure of moDCs to indirubin or ICZ together with TLR agonists significantly augmented the AhR-mediated CYP1A1 and CYP1B1 gene expression. Additionally, mature DCs that were subsequently stimulated with AhR ligands showed increased AhR target gene expression. Moreover, these ligands limited TLR-induced phenotypic maturation (CD80, CD83, CD86, MHC II upregulation) of moDCs, reduced secretion of the inflammatory cytokines interleukin (IL)-6 and IL-12, and decreased their ability to induce alloreactive T-lymphocyte proliferation.

CONCLUSIONS

These results demonstrate that AhR agonists of yeast origin are able to inhibit moDC responses to TLR ligands and that moDCs can adapt through increased transcription of metabolizing enzymes such as CYP1A1 and CYP1B1.

The Malassezia genus in skin and systemic diseases

In the last 15 years, the genus Malassezia has been a topic of intense basic research on taxonomy, physiology, biochemistry, ecology, immunology, and metabolomics. Currently, the genus encompasses 14 species. The 1996 revision of the genus resulted in seven accepted taxa: M. furfur, M. pachydermatis, M. sympodialis, M. globosa, M. obtusa, M. restricta, and M. slooffiae. In the last decade, seven new taxa isolated from healthy and lesional human and animal skin have been accepted: M. dermatis, M. japonica, M. yamatoensis, M. nana, M. caprae, M. equina, and M. cuniculi. However, forthcoming multidisciplinary research is expected to show the etiopathological relationships between these new species and skin diseases. Hitherto, basic and clinical research has established etiological links between Malassezia yeasts, pityriasis versicolor, and sepsis of neonates and immunocompromised individuals. Their role in aggravating seborrheic dermatitis, dandruff, folliculitis, and onychomycosis, though often supported by histopathological evidence and favorable antifungal therapeutic outcomes, remains under investigation. A close association between skin and Malassezia IgE binding allergens in atopic eczema has been shown, while laboratory data support a role in psoriasis exacerbations. Finally, metabolomic research resulted in the proposal of a hypothesis on the contribution of Malassezia-synthesized aryl hydrocarbon receptor (AhR) ligands to basal cell carcinoma through UV radiation-induced carcinogenesis.

A SAR study of novel antiproliferative ruthenium and osmium complexes with quinoxalinone ligands in human cancer cell lines

A series of ruthenium(II) arene complexes with 3-(1H-benzimidazol-2-yl)-1H-quinoxalin-2-one, bearing pharmacophoric groups of known protein kinase inhibitors, and related benzoxazole and benzothiazole derivatives have been synthesized. In addition, the corresponding osmium complexes of the unsubstituted ligands have also been prepared. The compounds have been characterized by NMR, UV-vis, and IR spectroscopy, ESI mass spectrometry, elemental analysis, and by X-ray crystallography. Antiproliferative activity in three human cancer cell lines (A549, CH1, SW480) was determined by MTT assays, yielding IC(50) values of 6-60 μM for three unsubstituted metal-free ligands, whereas values for the metal complexes vary in a broad range from 0.3 to 140 μM. Complexation with osmium of quinoxalinone derivatives with benzimidazole or benzothiazole results in a more consistent increase in cytotoxicity than complexation with ruthenium. For selected compounds, the capacity to induce apoptosis was confirmed by fluorescence microscopy and flow-cytometric analysis, whereas cell cycle effects are only moderate.

Exploring the anticancer activity of functionalized isoindigos: synthesis, drug-like potential, mode of action and effect on tumor-induced xenografts

Meisoindigo has been used as an indirubin substitute for the treatment of chronic myeloid leukemia (CML) for several years. In view of its poor solubility and erratic absorption, several investigations have focused on developing analogues with more desirable physicochemical profiles. Here, we investigated the structure-activity relationship (SAR) of meisoindigo with respect to its antiproliferative activity on leukemic K562 cells and found that appending a phenalkyl side chain onto the lactam NH resulted in analogues that retained good activity. Furthermore, analogues in which the phenyl ring was substituted with a basic heterocycle were significantly more soluble than meisoindigo while retaining acceptable antiproliferative profiles. The most promising analogue (E)-1-(2-(4-methylpiperazin-1-yl)ethyl)-[3,3'-biindolinylidene]-2,2'-dione (5-4) is more potent than meisoindigo across a panel of malignant cells, with at least 40 times greater solubility than meisoindigo, little or no tendency to aggregate in solution and capable of significantly extending the lifespans of animals with K562 induced xenografts. Mechanistically, it induced apoptotic cell death and disrupted the progression of K562 cells from the G(1) to G(2) phase. Taken together, our findings highlighted the feasibility of addressing the physicochemical deficits of the isoindigo scaffold by systematic modifications which was achieved without overt loss of growth inhibitory activity.

Impact of eugenol and isoeugenol on AhR translocation, target gene expression, and proliferation in human HaCaT keratinocytes

The phenolic derivatives eugenol and isoeugenol, which are naturally found in essential oils of different spices, are commonly used as fragrances. Recently data demonstrated that growth suppression produced by these substances occurs in keratinocytes and that the effects may be mediated via aryl hydrocarbon receptor (AhR) interactions. In this study the effects of eugenol and isoeugenol were determined on intracellular localization of AhR, AhR target gene expression, AhR-dependent cell cycle regulation, and proliferation in HaCaT cells. Both compounds produced a rapid and marked translocation of AhR into the nucleus, induced the expression of the AhR target genes cytochrome P-450 1A1 (CYP1A1) and AhR repressor (AhRR), and inhibited proliferation of HaCaT cells. Among the G(1) phase cell cycle-related proteins, levels of the retinoblastoma protein (RB), which is known to interact with AhR, and levels of the cyclin dependent kinase (CDK) 6 were reduced by eugenol and isoeugenol, whereas steady-state levels of CDK2 and CDK4 remained unaffected. Protein levels of CDK inhibitor (CKI) p27(KIP1), known to be modulated in an AhR-dependent manner, were increased after treatment with both substances. In conclusion, data show that the antiproliferative properties of eugenol and isoeugenol in HaCaT cells are mediated through AhR, and thereby the molecular mechanisms of action in these cells were identified for the first time in this study.

Identification of benzothiazole derivatives and polycyclic aromatic hydrocarbons as aryl hydrocarbon receptor agonists present in tire extracts

Leachate from rubber tire material contains a complex mixture of chemicals previously shown to produce toxic and biological effects in aquatic organisms. The ability of these leachates to induce Ah receptor (AhR)-dependent cytochrome P4501A1 expression in fish indicated the presence of AhR active chemicals, but the responsible chemicals and their direct interaction with the AhR signaling pathway were not examined. Using a combination of AhR-based bioassays, we have demonstrated the ability of tire extract to stimulate both AhR DNA binding and AhR-dependent gene expression and confirmed that the responsible chemicals were metabolically labile. The application of CALUX (chemical-activated luciferase gene expression) cell bioassay-driven toxicant identification evaluation not only revealed that tire extract contained a variety of known AhR-active polycyclic aromatic hydrocarbons but also identified 2-methylthiobenzothiazole and 2-mercaptobenzothiazole as AhR agonists. Analysis of a structurally diverse series of benzothiazoles identified many that could directly stimulate AhR DNA binding and transiently activate the AhR signaling pathway and identified benzothiazoles as a new class of AhR agonists. In addition to these compounds, the relatively high AhR agonist activity of a large number of fractions strongly suggests that tire extract contains a large number of physiochemically diverse AhR agonists whose identities and toxicological/biological significances are unknown.

Third-generation Ah receptor-responsive luciferase reporter plasmids: amplification of dioxin-responsive elements dramatically increases CALUX bioassay sensitivity and responsiveness

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and related dioxin-like chemicals are widespread and persistent environmental contaminants that produce diverse toxic and biological effects through their ability to bind to and activate the Ah receptor (AhR) and AhR-dependent gene expression. The chemically activated luciferase expression (CALUX) system is an AhR-responsive recombinant luciferase reporter gene-based cell bioassay that has been used in combination with chemical extraction and cleanup methods for the relatively rapid and inexpensive detection and relative quantitation of dioxin and dioxin-like chemicals in a wide variety of sample matrices. Although the CALUX bioassay has been validated and used extensively for screening purposes, it has some limitations when screening samples with very low levels of dioxin-like chemicals or when there is only a small amount of sample matrix for analysis. Here, we describe the development of third-generation (G3) CALUX plasmids with increased numbers of dioxin-responsive elements, and stable transfection of these new plasmids into mouse hepatoma (Hepa1c1c7) cells has produced novel amplified G3 CALUX cell bioassays that respond to TCDD with a dramatically increased magnitude of luciferase induction and significantly lower minimal detection limit than existing CALUX-type cell lines. The new G3 CALUX cell lines provide a highly responsive and sensitive bioassay system for the detection and relative quantitation of very low levels of dioxin-like chemicals in sample extracts.

Could Malassezia yeasts be implicated in skin carcinogenesis through the production of aryl-hydrocarbon receptor ligands?

Malassezia yeasts are found on the skin of all humans and many warm-blooded animals. In vitro they have the ability to synthesize potent ligands (indolo[3,2-b]carbazole, malassezin and indirubin) of the aryl-hydrocarbon receptor (AhR; synonym: dioxin receptor) when the sweat contained L-tryptophan is used as the single nitrogen source. The production of these AhR-ligands has been associated with pathogenic strains of a certain Malassezia species (Malassezia furfur) but recent evidence shows that this property is widely distributed in almost all currently known Malassezia species. AhR is associated with carcinogenesis and the potential connection of these ubiquitous skin symbionts, and putative pathogens, with skin neoplasia should be evaluated mainly focusing on mechanisms related to the distinctive ability of the yeast to produce potent AhR ligands.

HYPOTHESIS

Synthesis of available pertinent data show a possible link between Malassezia produced AhR ligands and skin carcinogenesis, particularly of basal cell carcinoma (BCC). BCCs are almost exclusively observed in animal species colonized by Malassezia. In humans and animals there is overlapping in the skin regions colonized by this yeast and affected by BCC. The potent AhR ligands synthesized by pathogenic Malassezia strains could contribute to tumor promotion by: modification of the UV radiation carcinogenesis, alterations in the salvage/survival of initiated tumor cells, inhibition of cell senescence, interaction with vitamin D metabolism, promotion of immune tolerance and finally pro-carcinogenic modulation of cell cycle progression and apoptosis.

Design, synthesis, and testing of an 6-O-linked series of benzimidazole based inhibitors of CDK5/p25

Alzheimer's disease (AD) is a progressive neurodegenerative disease resulting in cognitive and behavioral impairment. The two classic pathological hallmarks of AD include extraneuronal deposition of amyloid β (Aβ) and intraneuronal formation of neurofibrillary tangles (NFTs). NFTs contain hyperphosphorylated tau. Tau is the major microtubule-associated protein in neurons and stabilizes microtubules (MTs). Cyclin dependent kinase 5 (CDK5), when activated by the regulatory binding protein p25, phosphorylates tau at a number of proline-directed serine/threonine residues, resulting in formation of phosphorylated tau as paired helical filaments (PHFs) then in subsequent deposition of PHFs as NFTs. Beginning with the structure of Roscovitine, a moderately selective CDK5 inhibitor, we sought to conduct structural modifications to increase inhibitory potency of CDK5 and increase selectivity over a similar enzyme, cyclin dependent kinase 2 (CDK2). The design, synthesis, and testing of a series of 1-isopropyl-4-aminobenzyl-6-ether-linked benzimidazoles is presented.

Proliferative and androgenic effects of indirubin derivatives in LNCaP human prostate cancer cells at sub-apoptotic concentrations

Certain indirubin derivatives are potent cyclin-dependent kinase (CDK) and glycogen synthase kinase (GSK-3β) inhibitors and may be effective against various cancers. We evaluated the effects of aloisine A, alsterpaullone, aminopurvalanol, indirubin-3'-oxime, 6-Br-indirubin-3'-oxime, kenpaullone, olomoucine and roscovitine on cell proliferation, prostate-specific antigen (PSA) expression, androgen receptor (AR) activation, and GSK-3β and β-catenin expression in androgen-dependent LNCaP human prostate cancer cells. Effects were also evaluated in MDA-kb2 human breast cancer cells containing an AR-responsive luciferase construct. Steroid-deprived LNCaP cells were exposed to indirubins±dihydrotestosterone (DHT, 0.1 nM) and cell proliferation was assessed by MTT assay after 120 h. PSA expression was determined by real-time quantitative RT-PCR after 24h. Cytoplasmic and nuclear GSK-3β/β-catenin expression and phosphorylation status was determined by Western blotting. Effects on MDA-kb2 luciferase expression were determined after 24h using Steady-Glo (Promega). Indirubin-3'-oxime, 6-Br-indirubin-3'-oxime, alsterpaullone and kenpaullone increased LNCaP cell proliferation and PSA expression (0.03-1 μM; apoptosis occurred >1 μM), whereas aminopurvalanol significantly (p<0.05) reduced DHT-stimulated PSA expression (31%) at 1 nM. The other indirubin derivatives had no effect. The same was observed for induction of AR-dependent MDA-kb2 luciferase expression. Kenpaullone (1, 3 μM) decreased the active- and increased the inactive form of cytoplasmic GSK-3β, and increased nuclear AR and β-catenin accumulation. Flutamide (10 μM), unexpectedly, also strongly increased nuclear β-catenin accumulation. Indirubin derivatives that were potent GSK-3β inhibitors (relative to CDK1) stimulated LNCaP cell proliferation and other androgenic responses, suggesting (in a cancer treatment context) these compounds may increase AR-dependent prostate cancer growth if not used within an appropriate therapeutic dose-range.

Differential effects of indirubin and 2,3,7,8-tetrachlorodibenzo-p-dioxin on the aryl hydrocarbon receptor (AhR) signalling in liver progenitor cells

In the present study, we investigated the effects of potential endogenous ligand indirubin on the aryl hydrocarbon receptor (AhR) signalling, with a focus on the AhR-dependent gene expression and cell cycle progression in rat liver progenitor cells, and compared them with the effects of a model toxic AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The low (picomolar and nanomolar) doses of indirubin, corresponding to expected endogenous levels, induced a transient translocation of AhR to the nucleus, while high (micromolar) doses induced a long-term AhR nuclear translocation, followed by its degradation, similar to the effects of TCDD. Whereas high doses of indirubin recruited AhR/ARNT1 dimer to rat Cyp1a1 promoter, the low doses did not induce its DNA binding, as revealed by the chromatin immunoprecipitation assay. This corresponded with the fact that the micromolar doses of indirubin significantly increased Cyp1a1/1b1 mRNA in a way similar to TCDD, while the low doses of indirubin were only poor inducers of Cyp1a1/1b1 expression. Comparable patterns of expression were observed also for other AhR gene targets, such as Nqo1 and Nrf2. Also, only micromolar doses of indirubin were able to mimic the effects of TCDD on cell cycle and proliferation of liver progenitor cells or hepatoma cells. Nevertheless, indirubin at low concentrations may have unique effects on gene expression in non-tumorigenic cells. Although both TCDD and the high doses of indirubin repressed plakoglobin (Jup) expression, the picomolar doses of indirubin, unlike the equimolar doses of TCDD, increased mRNA levels of this important desmosomal and adherens junctions constituent. These present data suggest that the outcome of AhR activation induced by indirubin at concentrations expected in vivo may differ from the AhR signalling triggered by exogenous toxic ligands, such as TCDD.

CH223191 is a ligand-selective antagonist of the Ah (Dioxin) receptor

The aryl hydrocarbon (dioxin) receptor (AhR) is a ligand-dependent transcription factor that produces a wide range of biological and toxic effects in many species and tissues. Whereas the best-characterized high-affinity ligands include structurally related halogenated aromatic hydrocarbons (HAHs) and polycyclic aromatic hydrocarbons (PAHs), the AhR is promiscuous and can also be activated by structurally diverse exogenous and endogenous chemicals. However, little is known about how these diverse ligands actually bind to and activate the AhR. Utilizing AhR ligand binding, DNA binding, and reporter gene expression assays, we have identified a novel ligand-selective antagonist (CH223191) that preferentially inhibits the ability of some classes of AhR agonists (2,3,7,8-tetrachlorodibenzo-p-dioxin and related HAHs), but not others (PAHs, flavonoids, or indirubin), to bind to and/or activate the AhR and AhR signal transduction. HAH-specific antagonism of AhR-dependent reporter gene expression by CH223191 was observed with mouse, rat, human, and guinea pig cell lines. Ligand- and species-selective antagonism was also observed with the AhR antagonists 3'-methoxy-4'-nitroflavone and 6,2',4',-trimethoxyflavone. Our results suggest that the differences in the binding by various ligands to the AhR contribute to the observed structural diversity of AhR ligands and could contribute in ligand-specific variation in AhR functionality and the toxic and biological effects of various classes of AhR agonists.

A novel 7-azaisoindigo derivative-induced cancer cell apoptosis and mitochondrial dysfunction mediated by oxidative stress

This research focused on a novel 7-azaisoindigo derivative [namely N(1)-(n-butyl)-7-azaisoindigo, 7-AI-b], and investigated its molecular antitumor mechanism by exploring the means of cell death and the effects on mitochondrial function. 7-AI-b inhibited cancer cell proliferation in a dose- and time-dependent way. The morphological and nuclei changes in H(2) B-GFP-labeled HeLa cells were observed using a live cell system. The results suggested that cell death induced by 7-AI-b is closely related to apoptosis. 7-AI-b induced release of cytochrome C from mitochondria to cytosol and activation of caspase-3, showing that the apoptosis is mediated by the mitochondrial pathway. Furthermore, our data indicated that 7-AI-b triggers apoptosis through reactive oxygen species (ROS): cellular ROS levels were increased after 3 h exposure of 7-AI-b, which was reversed by the ROS scavenger N-acetyl-L-cysteine. As a consequence, 7-AI-b-mediated cell death, mitochondrial transmembrane potential collapse and ATP level were partly blocked by N-acetyl-L-cysteine. Further study showed that 7-AI-b could induce mitochondrial dysfunction: collapse of the mitochondrial transmembrane potential and reduction of intracellular ATP level. In summary, the novel synthesized 7-AI-b was demonstrated to be effective in killing cancer cells via an ROS-promoted and mitochondria- and caspase-dependent apoptotic pathway.

Indirubin shows anti-angiogenic activity in an in vivo zebrafish model and an in vitro HUVEC model

AIM OF THE STUDY

Indirubin is an active ingredient of the traditional Chinese medicine, Dang Gui Long Hui Wan, commonly used for the treatment of chronic myelocytic leukemia (CML) and other inflammatory conditions. These anti-leukemic and anti-inflammatory activities may be mediated by anti-angiogenic action. To investigate the anti-angiogenic activity of indirubin, we tested its inhibitory effect on blood vessel formation in zebrafish embryos and on endothelial cell proliferation in culture.

MATERIALS AND METHODS

The anti-angiogenic activity of indirubin was tested using transgenic zebrafish embryos with fluorescent vasculature and human umbilical vein endothelial cells (HUVECs). Apoptosis was analyzed with a terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) assay.

RESULTS

Indirubin dose-dependently inhibited intersegmental vessel formation in zebrafish embryos. It also inhibited HUVEC proliferation by the induction of cellular apoptosis and cell-cycle arrest at the G0/G1 phase.

CONCLUSIONS

The anti-angiogenic activity of indirubin may partly contribute to its anti-leukemic and anti-psoriatic properties and may be valuable for the treatment of diseases with excessive angiogenesis. The zebrafish model of angiogenesis was further validated in this study.

A novel role of the aryl hydrocarbon receptor (AhR) in centrosome amplification - implications for chemoprevention

BACKGROUND

Centrosome aberrations can cause genomic instability and correlate with malignant progression in common human malignancies such as breast and prostate cancer. Deregulation of cyclin/cyclin-dependent kinase 2 (CDK2) activity has previously been shown to be critically involved in centrosome overduplication. We therefore test here whether small molecule CDK inhibitors derived from the bis-indole indirubin can be used to suppress centrosome aberrations as a novel approach to chemoprevention of malignant progression.

RESULTS

As expected, we found that the CDK inhibitor indirubin-3'-oxime (IO) suppresses centrosome amplification in breast cancer cells. However, we made the unexpected discovery that indirubin-derived compounds that have been chemically modified to be inactive as kinase inhibitors such as 1-methyl-indirubin-3'-oxime (MeIO) still significantly reduced centrosome amplification. All indirubins used in the present study are potent agonists of the aryl hydrocarbon receptor (AhR), which is known for its important role in the cellular metabolism of xenobiotics. To corroborate our results, we first show that the coincidence of nuclear AhR overexpression, reflecting a constitutive activation, and numerical centrosome aberrations correlates significantly with malignancy in mammary tissue specimens. Remarkably, a considerable proportion (72.7%) of benign mammary tissue samples scored also positive for nuclear AhR overexpression. We furthermore provide evidence that continued expression of endogenous AhR is critical to promote centriole overduplication induced by cyclin E and that AhR and cyclin E may function in the same pathway. Overexpression of the AhR in the absence of exogenous ligands was found to rapidly disrupt centriole duplication control. Nonetheless, the AhR agonists IO and MeIO were still found to significantly reduce centriole overduplication stimulated by ectopic AhR expression.

CONCLUSIONS

Our results indicate that continued expression of endogenous AhR promotes centrosome amplification in breast cancer cells in a pathway that involves cyclin E. AhR agonists such as indirubins inhibit centrosome amplification even when stimulated by ectopic expression of the AhR suggesting that these compounds are potentially useful for the chemoprevention of centrosome-mediated cell division errors and malignant progression in neoplasms in which the AhR is overexpressed. Future studies are warranted to determine whether individuals in which nuclear AhR overexpression is detected in benign mammary tissue are at a higher risk for developing pre-cancerous or cancerous breast lesions.

Indirubins deplete striatal monoamines in the Intact and MPTP-treated mouse brain and block kainate-induced striatal astrogliosis

The indirubins long have been used in Chinese medicine for treatment of myelocytic leukemia. Among the many more recently described biological activities of the indirubins, attention has been directed toward the ability of these compounds to inhibit GSK-3 and CDKs, kinases implicated in neurodegenerative conditions. Little information is available on effects of indirubins on chemically-induced neurodegeneration. Here we examined the influence of three indirubins on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)- and kainic acid (KA)-induced neurotoxicity in the mouse. The three indirubins examined were 6-bromoindirubin-3'-oxime (6BIO), 5-bromoindirubin-3'-oxime (5BIO) and 5-amino-6-bromoindirubin (5A6BI). The first two derivatives were previously described indirubins with low nanomolar inhibitory activity against GSK-3 and CDKs. The third compound was synthesized by the dimerization of 5-amino-6-bromoisatin with 3-acetoxyindol. The synthesis of the key compound 5-amino-6-bromoisatin was based on the bromination of the ketal of 5-amino-isatin. All indirubins examined decreased various measures associated with dopaminergic neurotransmission in striatum. These effects occurred alone or over and above the decrements seen following administration of the dopaminergic neurotoxicant, MPTP. Striatal serotonin and serotonin turnover were decreased by the indirubins in MPTP-treated mice. None of these striatal effects of the indirubins alone were associated with evidence of astrogliosis, an indicator of underlying neuropathology, nor did they potentiate the astrogliosis accompanying administration of MPTP. In general, the indirubins reduced KA-associated mortality and striatal but not hippocampal astrogliosis due to this toxicant. The data suggest that indirubins affect striatal biogenic amine levels and turnover in intact mice. The data do not indicate a neuroprotective action of indirubins in mice treated with MPTP but that they do suggest that they may be neuroprotective against KA-induced injury of the neostriatum.

Synthesis and antiproliferative activity of 7-azaindirubin-3'-oxime, a 7-aza isostere of the natural indirubin pharmacophore

The bis-indole alkaloid indirubin and its analogues bear a very interesting natural pharmacophore. They are recognized mainly as kinase inhibitors, but several other activities make them possible candidates for preclinical studies. Based on the previously reported activity of 7-bromoindirubin-3'-oxime and its derivatives, the synthesis of indirubins bearing a heterocyclic nitrogen atom at position 7 was carried out. Herein, we report the first synthesis of 7-azaindirubin-3'-oxime (12) as well as its antiproliferative activity against 57 cancer cell lines and its inhibitory activity against a series of kinases. 7-Azaindirubin (10) and its 3'-oxime derivative (12) showed reduced activity as kinase inhibitors in comparison with other known indirubin derivatives, but antiproliferative activity with a best GI(50) value of 0.77 microM.

Anti-psoriatic effects of indigo naturalis on the proliferation and differentiation of keratinocytes with indirubin as the active component

BACKGROUND

Indigo naturalis has shown efficacy in treating psoriasis in our previous clinical studies.

OBJECTIVES

To investigate the potential effect of indigo naturalis on regulating keratinocyte proliferation and differentiation.

METHODS

Skin samples from six patients were analyzed for proliferating cell nuclear antigen (PCNA) and involucrin expression by immunohistochemical staining. In addition, indigo naturalis extracts from 10 to 500 microg/ml were added to cultured keratinocytes and cell viability determined. Real-time RT-PCR, Western blotting analysis and indirect immunofluorescent labeling were used to investigate the messenger (m)RNA and protein expressions of PCNA and involucrin. Finally, high-performance liquid chromatography (HPLC) was used to identify major components of indigo naturalis and their in vitro effects compared.

RESULTS

Immunohistochemical results demonstrated decreased PCNA and increased involucrin in psoriatic lesions after indigo naturalis treatment. Cultured keratinocytes decreased after indigo naturalis treatment, while G(0)/G(1) arrest was observed to dose-dependently increase. Staining revealed decreased PCNA-stained nuclei and increased cytosolic involucrin in treated keratinocytes. Decreased PCNA and increased involucrin at both the mRNA and protein levels were confirmed. Both major components, indirubin and indigo, could cause G(0)/G(1) phase arrest; however, only indirubin modulated the expressions of PCNA and involucrin similar to indigo naturalis.

CONCLUSIONS

Together, these findings indicate that the anti-psoriatic effects of indigo naturalis are mediated, at least in part, by modulating the proliferation and differentiation of keratinocytes, with indirubin as the major active component.

Soluble 3',6-substituted indirubins with enhanced selectivity toward glycogen synthase kinase -3 alter circadian period

Glycogen synthase kinase -3 (GSK-3) is a key enzyme involved in numerous physiological events and in major diseases, such as Alzheimer's disease, diabetes, and cardiac hypertrophy. Indirubins are bis-indoles that can be generated from various natural sources or chemically synthesized. While rather potent and selective as GSK-3 inhibitors, most indirubins exhibit low water solubility. To address the issue of solubility, we have designed novel analogues of 6-bromo-indirubin-3'-oxime with increased hydrophilicity based on the GSK-3/indirubins cocrystal structures. The new derivatives with an extended amino side chain attached at position 3' showed potent GSK-3 inhibitory activity, enhanced selectivity, and dramatically increased water solubility. Furthermore, some of them displayed little or no cytotoxicity. The new indirubins inhibit GSK-3 in a cellular reporter model. They alter the circadian period measured in rhythmically expressing cell cultures, suggesting that they might constitute tools to investigate circadian rhythm regulation.

Activation of the aryl hydrocarbon receptor by the calcium/calmodulin-dependent protein kinase kinase inhibitor 7-oxo-7H-benzimidazo[2,1-a]benz[de]isoquinoline-3-carboxylic acid (STO-609)

This study was designed to analyze the effects of the Ca2+/calmodulin-dependent protein kinase kinase (CaMKK) inhibitor STO-609 (7-oxo-7H-benzimidazo[2,1-a]benz[de]isoquinoline-3-carboxylic acid) toward the aryl hydrocarbon receptor (AhR) pathway because Ca2+/calmodulin-dependent protein kinase (CaMK) Ialpha, known as a downstream CaMKK effector, has been recently shown to contribute to the AhR cascade. STO-609 failed to alter up-regulation of the AhR target CYP1A1 in response to the potent AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in MCF-7 cells. STO-609, used at a 25 muM concentration known to fully inhibit CaMKK activity, was surprisingly found to markedly induce CYP1A1 expression and activity by itself in MCF-7 cells; it similarly up-regulated various other AhR target genes in human macrophages. STO-609-related CYP1A1 induction was prevented by chemical inhibition or small interfering RNA-mediated knockdown expression of AhR. Moreover, STO-609 was demonstrated to physically interact with the ligand-binding domain of AhR, as assessed by TCDD binding competition assay, and to induce AhR translocation to the nucleus. As already reported for AhR agonists, STO-609 triggered the increase of [Ca2+](i) and activation of CaMKIalpha, whose inhibition through the use of the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester or the CaMK inhibitor KN-93 (2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine), respectively, prevented STO-609-mediated CYP1A1 activity induction. Taken together, these results demonstrate that the CaMKK inhibitor STO-609 can act as an AhR ligand and, in this way, fully activates the Ca2+/CaMKIalpha/AhR cascade. Such data, therefore, make unlikely any contribution of CaMKK activity to the AhR pathway and, moreover, suggest that caution may be required when using STO-609 as a specific inhibitor of CaMKKs.