Blockade of the STAT3/BCL-xL Axis Leads to the Cytotoxic and Cisplatin-Sensitizing Effects of Fucoxanthin, a Marine-Derived Carotenoid, on Human Bladder Urothelial Carcinoma Cells

Bladder cancer is a globally prevalent urological malignancy, with transitional carcinoma (TCC) representing the majority of cases. Cisplatin is the primary drug for metastatic bladder cancer chemotherapy: however, its application is limited by nephrotoxicity and resistance. Signal Transducer and Activator of Transcription 3 (STAT3) is an oncogenic transcription factor often overactivated in various cancers, making it an appealing drug target. Fucoxanthin, a marine carotenoid, has significant anticancer properties. This study explored Fucoxanthin&rsquo:s cytotoxic effects and its potential to potentiate the efficacy of Cisplatin, along with the mechanisms underlying these effects, on human bladder TCC cells. We demonstrated that Fucoxanthin is cytotoxic to bladder TCC cells by inducing apoptosis, evidenced by z-VAD-fmk-mediated annulment of Fucoxanthin&rsquo:s cytotoxicity. Furthermore, Fucoxanthin reduced the levels of inherent or interleukin-6-induced tyrosine 705-phosphorylated STAT3 accompanied by downregulating BCL-xL, a well-established STAT3 target. Notably, ectopic expression of STAT3-C, a dominant-active STAT3 mutant, or BCL-xL thwarted Fucoxanthin&rsquo:s proapoptotic and cytotoxic actions. Moreover, Fucoxanthin at subtoxic dosages enhanced the susceptibility to Cisplatin-induced apoptosis of bladder TCC cells initially resistant to Cisplatin. Remarkably, this Cisplatin-sensitizing effect of Fucoxanthin was abrogated when cells ectopically expressed STAT3-C or BCL-xL. Overall, for the first time, we proved that the proapoptotic, cytotoxic, and Cisplatin-sensitizing effects of Fucoxanthin on human bladder TCC cells are attributed to the blockade of the STAT3/BCL-xL axis. Our findings highlight that targeting the STAT3/BCL-xL axis is a promising strategy to eliminate bladder TCC cells and facilitate Cisplatin sensitization, and further support the potential of incorporating Fucoxanthin into Cisplatin-based chemotherapy for treating bladder cancer.