STUDY OF SULFUR METABOLISM ALTERATIONS IN HUMAN CELL AND MOUSE MODELS OF COLORECTAL CANCER

Globally, colorectal cancer (CRC) is the third leading cause of cancer-related death and the incidence is still increasing. Our gastrointestinal tract is home to hundreds of bacterial species, constituting the gut microbiome. Intestinal cells are exposed to hydrogen sulfide (H2S) produced by commensal gut bacteria contained in the microbiome. H2S is also produced in mammalian cells through side reactions via the following transsulfuration (TS) pathway enzymes: cystathionine-β-synthase (CBS), cystathionine-γ-lyase (CSE) and combined action of cysteine aminotransferase (CAT) and 3-mercaptopyruvate sulfurtransferase (MPST). H2S and persulfides have been shown to affect signaling pathways and metabolic functions involved in carcinogenesis. The sulfur oxidation pathway plays a key role in the catabolism of H2S in mammalian cells and notably in the intestinal epithelium, but the sequence of reactions involved in this pathway remains poorly understood.
We elucidated the roles of enzymes previously linked to H2S oxidation, namely sulfide quinone oxidoreductase (SQRDL), sulfur dioxygenase (ETHE1), thiosulfate sulfurtransferase (TST), and thiosulfate:glutathione sulfurtransferase (TSTD1), by using HCT116 cell lines in which these enzymes were knocked out via the Crispr/Cas9 technology. Our findings indicate that the SQRDL and ETHE1 enzymes are crucial for H2S detoxification and thiosulfate production, while TSTD1 is dispensable. We examined the role of the Sulfur oxidation pathway in the detoxification of H2S and in mitigating H2S effects on the metabolism of cancer cells: reduction of the TCA cycle flux, inhibition of succinate dehydrogenase function in the succinate-fumarate direction leading to disruption of the glycerol-phosphate shuttle and a decrease in the NAD+/NADH ratio. Cell lines with compromised sulfur oxidation functionality demonstrated metabolic perturbations at lower sulfide concentrations compared to wild type cells.
Our second objective was to explore changes in sulfur metabolism and disruptions in H2S homeostasis during CRC tumorigenesis and progression using the APCmin mouse model. Through analysis of tumor and epithelial tissue samples from APCmin mice, we discovered that the capacity to detoxify H2S via the sulfur oxidation pathway decreases in tumor tissue, aligned with reduced endogenous H2S production via the transsulfuration pathway. Despite decreased expression of cysteine biosynthesis enzymes, comparable cysteine levels were measured in tumor and healthy intestinal segments. Concurrently, taurine biosynthesis pathway enzymes showed decreased expression in tumor tissue, potentially contributing to maintaining Cys homeostasis. Our metagenomic analyses of fecal samples collected at tumor sites and in healthy intestinal sections revealed no significant differences in microbiota composition and genes linked to sulfur metabolism in bacteria. This suggests that the observed alterations in tumor sulfur metabolism are attributable to inherent tumor metabolic changes rather than alterations in microbiome composition.
Our research underscores the significance of the sulfur oxidation pathway in maintaining H2S homeostasis, its relationship with endogenous H2S production, and highlights a potential cysteine catabolic pathway that may be utilized by tumors to preserve cysteine levels. Further investigation into dietary impacts on microbiota composition, specifically relating to cysteine and H2S producers, and tumor metabolic response to diet could be pivotal in devising medical treatment strategies that synergize with dietary interventions.