Effects of deletion of SAT1, a polyamine-metabolic enzyme, on mouse HeLa xenograft tumor

An acidic tumor microenvironment plays a critical role in tumor progression. However, understanding of metabolic reprogramming of tumors in response to acidic extracellular pH has remained elusive. Using comprehensive metabolomic analyses, we demonstrated that acidic extracellular pH (pH 6.8) leads to the accumulation of N1-acetylspermidine, a pro-tumor metabolite, through upregulation of the expression of spermidine/spermine acetyl transferase 1 (SAT1). Inhibition of SAT1 expression suppressed the accumulation of intra- and extracellular N1-acetylspermidine at acidic pH. Conversely, overexpression 3 of SAT1 increased intra- and extracellular N1-acetylspermidine levels, supporting the proposal that SAT1 is responsible for accumulation of N1-acetylspermidine. While inhibition of SAT1 expression only had a minor effect on cancer cell growth in vitro, SAT1 knockdown significantly decreased tumor growth in vivo, supporting a contribution of the SAT1-N1-acetylspermidine axis to pro-tumor immunity. Immune cell profiling revealed that inhibition of SAT1 expression decreased neutrophil recruitment to the tumor, resulting in impaired angiogenesis and tumor growth. We showed that anti-neutrophil neutralizing antibodies suppressed growth in control tumors to a similar extent to that seen in SAT1 knockdown tumors in vivo. Further, a SAT1 signature was found to be correlated with poor patient prognosis. Our findings demonstrate that extracellular acidity stimulates recruitment of pro-tumor neutrophils via the SAT1-N1-acetylspermidine axis, which may represent a novel target for anti-tumor immune therapy. To investigate the function of SAT1 on immune cells within tumours, HeLa cells knocked down SAT1 with shRNA were transplanted into scid/scid mice and excised 21 days later.