UCN-01 suppresses E2F-1 mediated by ubiquitin-proteasome-dependent degradation.

E2F-1 regulates the transcription of genes required for DNA synthesis. Previously, we have reported that UCN-01 suppresses E2F-1 protein expression without any noticeable effect on its mRNA level in gastric cancer cell line SK-GT5 (Clin. Cancer Res., 4: 2201-2206, 1998). In this study, we investigated the mechanism responsible for the suppression of E2F-1 expression by UCN-01 in SK-GT5 cells. After 24-h exposure to 1 microM UCN-01, E2F-1 protein expression was decreased by >99%. The suppressive effect of UCN-01 could be reversed by ubiquitin-dependent proteasome inhibitors such as calpain inhibitor I and lactacystin. Transfection experiments using expression plasmids encoding full-length E2F-1 or truncated E2F-1 with deletion of the COOH-terminal region (which is required for eliciting ubiquitination and protein degradation) revealed that the expression of truncated E2F-1 was not affected by UCN-01. Other cell-cycle-related and ubiquitin-proteasome-regulated proteins such as p21, p27, and cyclin B1 were not repressed by UCN-01 in E2F-1-overexpressing cells. In vitro-translated, full-length E2F-1 degraded more rapidly upon incubation with extracts from UCN-01-treated cells when compared with truncated E2F-1. Taken together, these data indicate that UCN-01 suppresses E2F-1 protein expression mediated by the ubiquitin-proteasome pathway in a specific manner.