Lactate inhibits interferon-α response in ovarian cancer by inducing STAT1 ubiquitin degradation

The emergence of lactate, produced by lactate dehydrogenase A (LDHA), as an important regulator of the immune response in tumor development has garnered attention in recent research. But, many questions still need to be clarified regarding the relationship between lactate and anti-tumor immunity. Here, we reported that both exogenous and endogenous lactate reduced the protein level and activation of the signal transducer and activator of transcription 1(STAT1) in ovarian cancer cells. As a consequence, the expression of IFNα-STAT1 regulated genes was weakened. This, in turn, weakened the antitumor effect of IFNα by impeding NKT and CD8+T cells recruitment. Strikingly, we found that LDHA knockdown did not result in the downregulation of STAT1 mRNA level in ovarian cancer cells. Instead, we observed that lactate triggered the degradation of STAT1 through the proteasomal pathway. Notably, we identified that lactate reduced the stability of STAT1 by promoting the expression of F-box only protein 40 (Fbxo40). This protein interacts with STAT1 and potentially acts as an E3 ubiquitin ligase, leading to the induction of STAT1 polyubiquitination and degradation. Importantly, ectopic over-expression of the Fbxo40 gene significantly inhibited the expression of ISGs in LDHA knockdown cells. In the TCGA tumor data, we observed that high expression of Fbxo40 negatively correlates with overall survival in ovarian cancer patients. Collectively, our findings reveal lactate as a negative regulator of the IFNα-STAT1 signaling axis in ovarian cancer. This discovery suggests that strategies aimed at targeting lactate for ovarian cancer prevention and treatment should consider the impact on the IFNα-STAT1 response.