Sterigmatocystin-induced checkpoint adaptation depends on Chk1 in immortalized human gastric epithelial cells in vitro

Sterigmatocystin (ST) is a common contaminant detected in food and animal feed that has been recognized as a possible human carcinogen. Our previous studies demonstrate that ST causes DNA damage and subsequently triggers cell cycle arrest in G(2) and apoptosis in immortalized human gastric epithelial cells (GES-1). Recently, studies have shown that in certain contexts, cells with DNA damage may escape checkpoint arrest and enter mitosis without repairing the damage. The term for this process is "checkpoint adaptation," and it increases the risk of unstable genome propagation, which may contribute to carcinogenesis. Thus, we aimed to investigate whether checkpoint adaptation occurs in GES-1 cells treated with ST and explored the underlying molecular mechanisms that contribute to this phenotype. In this study, we found that ST treatment for 24 h in GES-1 cells led to an initial G(2) arrest; however, a fraction of GES-1 cells became large and rounded, and the number of p-H3-positive cells increased sharply after ST treatment for 48 h. Moreover, collection of the large and rounded cells by mechanical shake-off revealed that the majority of these large cells were found in the mitotic phase of the cell cycle. Importantly, we found that these rounded cells entered mitosis despite damaged DNA and that a small subset of this cell population survived and continued to propagate. These results suggest that ST induces an initial G(2) arrest that is subsequently followed by G(2) phase checkpoint adaptation, which may potentially promote genomic instability and result in tumorigenesis. Furthermore, we showed that activation of Chk1 contributes to the G(2) arrest in GES-1 cells that are treated with ST for 24 h and that prolonged treatment of cells with ST for 48 h led to a decrease in the total protein and phosphorylation levels of Chk1 in mitotic cells, indicating that checkpoint adaptation may be driven by inactivation of Chk1. Knockdown studies confirmed that cells entered mitosis following inactivation of Chk1. Taken together, we show that ST treatment for 24 h activates Chk1 and induces a G(2) arrest in GES-1 cells. However, prolonged ST treatment for 48 h led to Chk1 inactivation in GES-1 cells, which promotes checkpoint adaptation and entry of cells into mitosis despite damaged DNA. Importantly, checkpoint adaptation in GES-1 cells treated with ST may potentially promote genomic instability and drive tumorigenesis.