Exogenous cysteine and cystine promote cell proliferation in CaCo‐2 cells

. Previous studies have shown that intracellular glutathione, a ubiquitous intracellular thiol, is related to cell proliferation and that cysteine or its disulphide form, cystine, also induces cell proliferation. Cysteine is a thiol containing amino acid and a rate‐limiting precursor of glutathione. Therefore, it is still unresolved as to whether the proliferative effect of cysteine or cystine is entirely mediated by a change in the intracellular glutathione status. The objective of this study was to delineate the relationship among cysteine/cystine (thereafter referred to as cyst(e)ine), intracellular glutathione and cell proliferation in the human colon cancer CaCo‐2 cell line. CaCo‐2 cells were cultured in cyst(e)ine‐free Dulbecco’s Modified Eagle Medium without serum, and treated with 200 µ m cysteine and/or 200–400 µ m cystine for 24 h. In the presence of DL‐buthionine‐[S, R]‐sulfoximine (BSO), a glutathione synthesis inhibitor, exogenously administered cyst(e)ine did not change the intracellular glutathione content, but increased the intracellular cysteine as well as cystine level. Addition of exogenous cyst(e)ine following 5 m m BSO treatment significantly increased cell proliferation as measured by 3 H‐thymidine incorporation and protein content. Cell cycle analyses revealed that cyst(e)ine promoted cell progression from the G 1 phase to the S phase. Correspondingly, cyst(e)ine treatment induced expression of cyclin D1 and phosphorylation of retinoblastoma protein (Rb). In conclusion, these data indicate that both cysteine and cystine have proliferative effects in CaCo‐2 cells independent of an increase in intracellular glutathione. Induction of cyclin D1, phosphorylation of Rb, and subsequent facilitation of G 1 ‐to‐S phase transition were involved in the proliferative effect of exogenous cyst(e)ine.