Molecular mechanisms underlying TXNIP's anti-tumor role in breast cancer, including interaction with a novel, pro-tumor partner: CAST.

Thioredoxin-interacting protein (TXNIP) plays a pivotal role in glucose metabolism and redox signaling. Its emerging function as a potent suppressor of cell proliferation in various cancer contexts underscores its importance in cancer development. In a previous study, we found TXNIP activation by UNC0642, an inhibitor of histone methyltransferase G9A, significantly inhibited MDA-MB-231 breast cancer cell proliferation in vitro and tumor growth in vivo. Here, we demonstrated that TXNIP knockdown increased MDA-MB-231 tumor growth and metastasis in a mouse model. Reintroducing TXNIP into TXNIP-deficient HCC-1954 breast cancer cells decreased cell proliferation and migration while boosting the generation of reactive oxygen species, alongside reductions in mitochondrial respiration, mitochondrial membrane potential, and glycolysis. To elucidate the mechanisms underlying TXNIP's antitumor effects in breast cancer cells, we conducted co-immunoprecipitation and proteomic analyses that revealed calpastatin (CAST) as a novel TXNIP-interacting protein in MDA-MB-231 and HCC-1954 cells. Overexpression of CAST, an endogenous inhibitor of calpains, significantly increased xenograft tumor growth for both MDA-MB-231 and HCC-1954 cells, underscoring its novel role as a tumor promoter. In addition, we identified a positive correlation between the expression of TXNIP and interleukin-24 (IL-24), a molecule that induces cancer-specific apoptosis in several breast cancer cell lines. Our findings also show TXNIP's ability to decrease activation of STAT3, a key driver of oncogenesis. Finally, cells with high levels of TXNIP expression displayed increased susceptibility to IL-24 and WP1066, a specific STAT3 inhibitor, suggesting possible predictive value for TXNIP. Collectively, these findings unveil novel TXNIP-dependent pathways that may contribute to breast cancer pathogenesis, enriching our understanding of this molecule's intricate role in cancer and potentially paving the way for clinical translation.