Role of cystathionine β-synthase in human breast Cancer

Cystathionine β-synthase (CBS) is an enzyme in the transulfuration pathway that can catalyze the condensation of homocysteine (Hcy) and cysteine (Cys) to hydrogen sulfide (H₂S) and cystathionine (CTH). CBS-derived H₂S is important in angiogenesis and drug resistance in colon and ovarian cancers, respectively. However, the mechanisms by which cancer cell-derived H₂S is utilized by cancer cells as a protective agent against host-derived activated macrophages are not yet investigated. This study investigated the mechanistic role of CBS-derived H₂S in the protection of human breast cancer (HBC) cells against activated macrophages. HBC patient-derived tissue arrays and immunoblot analysis of HBC cells exhibited significantly increased levels of CBS when compared with their normal counterparts. This was associated with increased levels of H₂S and CTH. Silencing of CBS in HBC cells caused a significant decrease in the levels of H₂S and CTH but did not affect the growth of these cells per se, in in vitro cultures. However CBS-silenced cells exhibited significantly reduced growth in the presence of activated macrophages and in xenograft models. This was associated with an increase in the steady state levels of reactive aldehyde-derived protein adducts. Exogenous addition of H₂S countered the effects of CBS silencing in the presence of macrophages. Conversely overexpression of CBS in human breast epithelial (HBE) cells (which do not naturally express CBS) protected them from activated macrophages, which were otherwise susceptible to the latter.