An endogenous aryl hydrocarbon receptor ligand inhibits proliferation and migration of human ovarian cancer cells

Highlights

• AhR is widely present in different histotypes of human ovarian cancer.

• ITE suppresses human ovarian cancer cell proliferation and migration via AhR.

• ITE inhibits human ovarian cancer cell growth in mice.

• ITE could potentially be used as a therapeutic drug for human ovarian cancer.

Abstract

The aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor mediates many biological processes. Herein, we investigated if 2-(1’H-indole-3’-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE, an endogenous AhR ligand) regulated proliferation and migration of human ovarian cancer cells via AhR. We found that AhR was widely present in many histotypes of ovarian cancer tissues. ITE suppressed OVCAR-3 cell proliferation and SKOV-3 cell migration in vitro, which were blocked by AhR knockdown. ITE also suppressed OVCAR-3 cell growth in mice. These data suggest that the ITE might potentially be used for therapeutic intervention for at least a subset of human ovarian cancer.

Introduction

To date, ovarian cancer is still the most lethal female genital cancer, largely because cancer cells acquire a chemoresistant phenotype after initial cytoreductive surgery and chemotherapy in the majority of cases [1], [2]. Another major challenge of current cancer therapies is severe side effects and toxicity of chemotherapy drugs used. Thus, since human ovarian cancer is characterized by its high degree of heterogeneity at the cellular and molecular levels, understanding individual type of ovarian cancer is critical to develop an efficacious, but low side-effect cancer therapy [1], [2].

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcriptional factor [3]. A classic AhR ligand is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which is a potent environmental toxicant and carcinogen [3]. The AhR mediated biological action is well known to involve a multi-step signal transduction process. Specifically, upon binding to its ligand and dissociation from its associated proteins, AhR translocates from the cytoplasm into the nucleus and dimerizes with AhR nuclear translocator (ARNT), activating a series of downstream genes (e.g., enzyme cytochrome P450 [CYP], family 1, member A1 and B1 [CYP1A1 and CYP1B1]), ultimately initiating the xenobiotic metabolizing process [3]. Once the metabolizing process is initiated, AhR in the nucleus transports back to the cytoplasm, in which AhR is degraded by the 26S proteasome system [4]. To date, it is well established that besides its participations in metabolizing xenobiotics, AhR also mediates a variety of other biological processes such as normal ovarian growth and function as evidenced by the fact that either AhR knockdown in mice or exposure to TCDD in rats could decrease the number of pre-antral and antral follicles and reduce or block ovulation [5], [6], [7].

The AhR ligand also can adversely impact estrogen receptor (ER) signaling directly via binding to ER target gene promoters [8] or indirectly via regulating the CYP family (e.g., CYP1A1 and CYP1B1) [9], which is also the key enzyme for estrogen metabolism. Thus, the AhR signaling could potentially affect behaviors of those estrogen-sensitive cells such as ovarian cancer cells [9], even though estrogen may differently regulate ovarian cancer cell growth, plausibly depending on individual subtypes of ovarian cancer, concentrations of estrogen, and patients’ ages (e.g., pre- vs. post-menopause) [1], [10].

The reports on potential roles of AhR in human cancer are controversial. Epidemiological studies have suggested that occupational exposures to high levels of TCDD did not increase risk of human ovarian cancer and endometriosis [11]; however, such exposures could be associated with a decreased risk in breast and endometrial cancers [12], [13] and with an increased mortality from other cancer sites (e.g., lung cancer in men) [11], [12], [13], [14], [15]. Paradoxically, levels of AhR expression may not be positively correlated with the incidences of these cancers as the increased AhR expression has been reported in human breast and lung cancers [15], [16]. Moreover, AhR gene polymorphisms are also closely associated with an increased risk of lung and breast cancers [16], [17]. Nonetheless, AhR activation has been reported to suppress growth of breast, pancreatic, and liver cancers [15], [18], [19], [20]. To date, little is known regarding the role of AhR in human ovarian cancer [20], [21], although TCDD has been shown to stimulate proliferation of CAOV-3 cells, a human ovarian cancer cell line [21], suggesting that AhR could be used as a therapeutic target for treating ovarian cancer.

Many endogenous AhR ligands have been discovered [22], including 2-(1’H-indole-3’-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE), which was first isolated from porcine lungs [23]. Based on its activity on the dioxin responsive element, the biological potency of ITE was ∼ 100-fold lower than that of TCDD [23]. Moreover, the estimated Kd value for ITE in mouse hepatoma cells was ∼5–6-fold greater than that of TCDD [23]. However, given its naturally producing feature and specific binding to AhR [23], ITE could potentially be used for interfering ovarian cancer growth, particularly because of its lack of toxicity (e.g., cleft palate and hydronephrosis, which typically associated with perinatal TCDD exposure in the mouse fetus) [24], [25].

To date, the roles of AhR in human ovarian cancer are poorly understood, and it is also unknown if activation of AhR by its endogenous ligand can affect ovarian cancer growth. Thus, given that the AhR signaling has been shown to suppress growth of breast, pancreatic, and liver cancers [12], [15], [16], [17], in this study, we tested the hypothesis that the AhR activation by its endogenous ligand inhibited human ovarian cancer progress via attenuating growth and/or migration of human ovarian cancer cells. We first determined AhR expression in human ovarian tissues and then examined if ITE regulated ovarian cancer cell proliferation and migration via AhR using in vitro and/or in vivo models.