Clofazimine inhibits small-cell lung cancer progression by modulating the kynurenine/aryl hydrocarbon receptor axis

Small cell lung cancer (SCLC) is one of the highly metastatic malignancies that contributes to ∼15 % of all lung cancers. Most SCLC patients (50-60 %) develop osteolytic bone metastases, significantly affecting their quality of life. Among several factors, environmental pollutant 2,3,7,8-Tetrachlorodibenzodioxin (TCDD) and kynurenine (Kyn), an endogenous ligand derived from tryptophan (Trp) metabolism, activate the aryl hydrocarbon receptor (AhR) and are responsible for SCLC progression and metastasis. Further, elevated AhR expression in bone cells intensifies bone resorption, making the Kyn/AhR axis a potential target for the bone metastatic propensity of SCLC. We first assessed the expression profile of AhR in human SCLC cell lines and found a significantly increased expression compared to normal lung cells. Additionally, we also evaluated the clinical significance of AhR expression in the patient samples of SCLC along with the relevance of the same in the Rb1^fl/fl; Trp53^fl/fl; Myc^LSL/LSL (RPM) mouse model using immunohistochemistry and found the higher AhR expression in the patient samples and RPM mouse tumor tissues. Using computational simulations, we found that clofazimine (CLF) binds at the activator (Kyn) binding site by forming a stable complex with AhR. The CLF binding with AhR was favored by Van der Waals and hydrophobic forces, and the proteins retained their secondary structure. Furthermore, we found that Kyn treatment potentiates the migration and clonogenic ability of SCLC cell lines by activating Erk/Akt oncogenic signaling. Treatment with CLF reduces AhR expression, which inhibits Kyn-mediated proliferation of SCLC cells, induces apoptosis, and cell cycle arrest in the G2/M phase. Further, our examination indicates that Kyn treatment also promotes osteoblast-mediated osteoclast differentiation through RANKL. The treatment with CLF impedes RANKL expression and osteoclastogenesis, suggesting that CLF has the potential to be used as the therapeutic for SCLC patients having bone metastasis.