The Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and member of the basic helix-loop-helix Per/ARNT/Sim (bHLH/PAS) family of chemosensors and developmental regulators. As a member of the PAS domain family of transcription factors responsive to exogenous signals, the AhR exerts influence on many processes relating to cellular fate.
The activation of AhR is widely associated with toxic endpoints related to dioxin exposure. However, the AhR also activates endogenous gene programs related to development, cellular growth, and differentiation. The AhR is able to bind a variety of ligands, leading to a wide range of biological outcomes. Recent reports have shown that the AhR can mediate tumor suppressive effects. As a ligand-activated transcription factor, the AhR has the potential to actuate a variety of transcriptional programs that are dependent on the AhR ligand.
Our central hypothesis is that AhR ligands can be identified that are capable of initiating tumor suppressive functions of the AhR.
We utilized complementary cell-based and in silico virtual screening approaches to identify potential AhR ligands. We developed homology models of the AhR ligand-binding domain (LBD) for virtual ligand screening (VLS) of small molecule libraries. This led to the identification of new AhR ligands 5,7- dihydroxyflavanone!and 5-hydroxy-7-methoxyflavone. Additional small molecule libraries were screened in parallel that led to identification of flutamide as a putative AhR ligand. Flutamide is clinically approved for the treatment of prostate cancer due to its ability to antagonize androgen receptor mediated transcription. We investigated the biological effects of flutamide in AhR positive cancer cells that do not express the androgen receptor and found that flutamide inhibited the growth of HepG2 cells. Suppression of AhR expression reversed the anti-proliferative effects of flutamide.
We tested 15 structural analogs of flutamide, including the flutamide metabolite 2-hydroxyflutamide for activation of AhR transcriptional activity. Flutamide is unique in its ability to activate the AhR, and suppresses hepatoma cell growth. These data suggests that flutamide-induced AhR transcriptional activity is required to initiate the tumor suppressive effects. We examined changes in cell cycle checkpoint proteins after flutamide treatment and discovered increased expression of cell cycle inhibitory proteins p27[superscript Kip1] and p15[superscript INK]. We also found that transforming Growth Factor β1 (TGFβ1), which
regulates both p27[superscript Kip1] and p15[superscript INK], is upregulated by flutamide. We demonstrate
that TGFβ1 is upregulated by flutamide in an AhR-dependent manner and is
required for suppression of proliferation by flutamide. We identify specific and
unique transcriptional signatures of the AhR upon activation by flutamide, that
are distinct from the potent AhR agonist 2,3,7,8-Tetrachlorodibenzo-p-dioxin
In summary, we characterize flutamide as an AhR ligand and demonstrate
its AhR-dependent tumor suppressive effects in hepatoma cells. We provide the
first direct evidence that AhR regulates TGFβ signaling in a ligand dependent
manner. We demonstrate that the AhR-induced downstream transcriptional
signature and subsequent biological effects are specific to the AhR ligand. Our
studies have broad impact for characterizing the AhR as a new therapeutic target
in hepatocellular carcinoma.