Sepia ink oligopeptide (SIO) is a tripeptide extracted from Sepia ink. To test the hypothesis that SIO inhibits prostate cancer by inducing apoptosis, the effects of SIO on the proliferation of three human prostate cancer cell lines were examined using a CCK-8 assay. SIO significantly inhibited the proliferation of DU-145, PC-3 and LNCaP cells in a time- and dose-dependent manner. Flow cytometry studies showed that exposing DU-145, PC-3 and LNCaP cells to 5, 10, or 15 mg/mL SIO for 24 h increased the percentage of the early-stage apoptotic cells from 11.84% to 38.26% (DU-145), 22.76% to 39.96% (PC-3) and 5.05% to 16.11% (LNCaP), respectively. In addition, typical morphologic changes were observed in the cells with acridine orange/ethidium bromide staining. SIO treatment induced strong S and G 2/M phase cell cycle arrest in a dose-dependent manner in DU-145 and LNCaP. In contrast, SIO treatment induced strong Sub G 1 and G 0/G 1 phase cell cycle arrest in a dose-dependent manner in PC-3. SIO exposure for 24 h decreased the expression of the anti-apoptotic protein Bcl-2 and increased the expression of the apoptogenic protein Bax. Moreover, the Bax/Bcl-2expression ratio was increased. Concurrently, the expression of caspase-3 was upregulated. These data support our hypothesis that SIO has anticarcinogenic properties.
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