Tumor suppressor function of SHMT in a Drosophila RasDlg model: DNA damage and synergistic gene-nutrient interaction with PLP.

Serine hydroxymethyltransferase (SHMT) is a key enzyme in one-carbon (1 C) metabolism, essential for nucleotide synthesis and epigenetic maintenance. In mammals, there are two distinct SHMT isozymes: the cytosolic SHMT1 and the mitochondrial SHMT2. Several studies report that high SHMT levels in cancer contribute to metabolic reprogramming. Conversely, a limited number of studies have linked decreased SHMT1 expression to the progression and poor prognosis of hepatocellular carcinoma and renal cell carcinoma, suggesting that SHMT may play dual roles as an oncogene or tumor suppressor, depending on the cellular context. However, the molecular mechanisms underlying SHMT tumor suppressor role remain unknown. In this work, we used the Drosophila RasDlg cancer model to investigate the effects of SHMT depletion on cancer progression and the associated mechanisms. We found that RNAi-mediated SHMT silencing promotes the progression of RasDlg cancers by impairing thymidylate biosynthesis in the folate pathway. SHMT depletion in RasDlg cells causes DNA and chromosome damage and renders these cells sensitive to genotoxic stressors such as X-rays or hydroxyurea. Genome instability is correlated with cancer progression, and it is largely due to the generation of reactive oxygen species (ROS) and, to a lesser degree, to replicative stress and compromised DNA repair mechanisms, all arising from SHMT depletion. Antioxidant treatment with N-acetyl cysteine (NAC) significantly reduces both DNA damage and tumor progression. Intriguingly, the combined depletion of SHMT and its cofactor pyridoxal 5'-phosphate (PLP) further increases oxidative stress, leading to extensive DNA damage that induces apoptosis in RasDlg cells, thereby limiting the tumor growth. Taken together, our data suggest that a diminished SHMT activity may drive the progression of RasDlg cancers through ROS-induced genome instability. Additionally, our study points to a novel gene-nutrient interaction, SHMT-PLP, that impacts cancer growth with potential therapeutic implications.