Ethyl acetate extract of Squilla oratoria suppresses growth of HepG2 cells by inducing S phase arrest

Objective: The oceans and seas are a rich source of organisms from which anti-cancer drugs can be isolated and developed. Marine organisms have been screened in our laboratory, and organic solvent extracts of Squilla oratoria (ESO) have been shown to possess cytostatic effects on cancer cell lines of diverse origins. To explore the underlying mechanisms, the growth inhibition by ESO was investigated in the present study. Methods: Human hepatocellular carcinoma (HCC) derived cells (HepG2) were used. The cells were challenged with ESO, cell cycle profile was assayed, and level of proliferating cell nuclear antigen (PCNA) expression and that of cyclin D1 and cyclin A were evaluated with flow cytomtery. The in vivo antitumor effect of ESO was tested in nude mouse xenografts. PCNA expression was evaluated immunohistochemically in nude mouse xenograft tissues. Results: With the increase in dose of injected ESO, expression of PCNA by human HCC xenografts increased. ESO inhibited the growth of human HCC HepG2 cells both in vitro and in vivo. The effect was correlated with arrest of the cell cycle in S phase. Expression of PCNA, which is a cell-cycle regulator that promotes S phase entry, was elevated in both cell lines and xenografts whereas that of cyclins that promote M phase entry was down-regulated by exposure to ESO. Conclusion: Growth inhibition was explained by arrest of the cell cycle in S phase and down-regulation of molecules that promote cells to enter S phase.