846 Resistance to apoptosis and PD-L1 induction in melanoma is associated with suppressed aryl hydrocarbon receptor signaling

Abstract

Metastatic melanoma is an aggressive form of skin cancer with a 5-year survival rate of 15% to 20%. Unlike primary melanoma, metastatic disease is difficult to treat due to acquisition of chemoresistance and immune evasion through PD-L1 expression. Highlighting the intrinsic differences in signaling networks that are overactive or suppressed between primary and metastatic melanoma will provide essential cues to understanding mechanisms of therapeutic resistance and immune evasion. We detected reduced cytochrome p450 enzymes, Cyp1a1 and Cyp1b1, in vertical growth phase melanoma cell line, WM793, and its metastatic variant, 1205Lu, compared to primary melanoma cells WM1552c. Since their expression is directly controlled by aryl hydrocarbon receptor (AhR), whose expression was also decreased in 1205Lu and WM793 cells, we tested whether reduced AhR signaling in melanoma cells confers chemoresistance and immune evasion properties. We used paclitaxel (ptx)-mediated apoptosis and interferon gamma (IFNg)-mediated PD-L1 induction as end points to evaluate chemotherapeutic resistance and immune evasion, respectively. 1205Lu and WM793 cells were resistant to ptx-mediated apoptosis (<10%) and PD-L1 expression increased by 3 to 6-fold following IFNg treatment. WM1552c cells, however, were sensitive to ptx-mediated apoptosis (>25%) with minimal induction of PD-L1 (1.5-fold) following IFNg treatment. Treatment with AhR antagonist, CH223191, prior to ptx decreased the percentage of apoptotic cells to <10% in WM1552 cells. IFNg-induced PD-L1 expression also increased by 3-folds in WM1552c cells following AhR inhibition although the mean fluorescence intensity was still lower than 1205Lu cells. Our data indicates that a suppressed AhR signaling in vertical phase and metastatic melanoma cells may have been evolved to resist apoptosis to chemotherapy and promote immune evasion via enhanced PD-L1 induction. Elucidating components of AhR pathway that are affected can be of high therapeutic value for metastatic melanoma.