Stellettin B was isolated from marine sponge Jaspis stellifera. In vitro antitumor activities were investigated on 39 human cancer cell lines. Stellettin B exhibited highly potent inhibition against the growth of a human glioblastoma cell line SF295, with a GI50 of 0.01 μM. In contrast, stellettin B showed very weak inhibitory activity on normal cell lines including HMEC, RPTEC, NHBE and PrEC, with GI50s higher than 10 μM, suggesting its relatively selective cytotoxicity against human cancer cells compared to normal human cell lines. We then focused on the antitumor activity of this compound on SF295 cells. Flow cytometric analysis indicated that stellettin B induced apoptosis in SF295 cells in a concentration-dependent manner. Further study indicated that stellettin B increased the production of ROS, the activity of caspase 3/7, as well as the cleavage of PARP, each of which is known to be involved in apoptosis. To investigate the molecular mechanism for cell proliferation inhibition and apoptosis induction, effect on the phosphorylation of several signal proteins of PI3K/Akt and RAS/MAPK pathways was examined. Stellettin B inhibited the phosphorylation of Akt potently, with no activity on p-ERK and p-p38, suggesting that inhibition of PI3K/Akt pathway might be involved in the antiproliferative and apoptosis-inducing effect. However, homogenous time-resolved fluorescence (HTRF) assay indicated that stellettin B did not inhibit PI3K activity, suggesting that the direct target might be signal protein upstream of Akt pathway other than PI3K.

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Novel agents are still urgently expected for therapy of chronic myeloid leukemia (CML). The in vitro anti-leukemia activity of Stellettin B (Stel B), a triterpenoid we isolated from marine sponge Jaspis stellifera, on human CML K562 and KU812 cells was recently investigated. Stel B inhibited K562 and KU812 cell proliferation with IC50 as 0.035 μM and 0.95 μM respectively. While no obvious cell cycle arrest was observed, apoptosis was induced in K562 cells after Stel B treatment. The Stel B-induced apoptosis might be in mitochondrial pathway, with increase of Bad and Bax, decrease of Bcl-2 and activation of caspase-9. In addition, dose-dependent increase of reactive oxygen species (ROS) and loss of mitochondrial membrane potential (MMP) occurred. Meanwhile, Stel B inhibited phosphorylation of Stat5, expression of 4 PI3K catalytic isoforms, and phosphorylation of the downstream effectors including PDK1 and Akt, suggesting that inhibition against Stat5 and PI3K might be involved in the apoptosis-inducing effect. Combination of Stel B with Imatinib with ratio as IC50 Stel B: 5×IC50 Imatinib led to synergistic effect. Stel B might become a promising candidate for CML therapy alone or together with Imatinib.

Tumor metastasis is the main cause of lethality of prostate cancer, because conventional therapies like surgery and hormone treatment rarely work at this stage. Tumor cell migration, invasion and adhesion are necessary processes for metastasis. By providing nutrition and an escape route from the primary site, angiogenesis is also required for tumor metastasis. Phosphatidylinositol 3-kinases (PI3Ks) are well known to play important roles in tumorigenesis as well as metastasis. ZSTK474 is a specific PI3K inhibitor developed for solid tumor therapy. In the present report, antimetastatic activities of ZSTK474 were investigated in vitro by determining the effects on the main metastatic processes. ZSTK474 exhibited inhibitory effects on migration, invasion and adhesive ability of prostate cancer PC3 cells. Furthermore, ZSTK474 inhibited phosphorylation of Akt substrate-Girdin, and the secretion of matrix metalloproteinase (MMP), both of which were reported to be closely involved in migration and invasion. On the other hand, ZSTK474 inhibited the expression of HIF-1α and the secretion of vascular endothelial growth factor (VEGF), suggesting its potential antiangiogenic activity on PC3 cells. Moreover, we demonstrated the antiangiogenesis by determining the effect of ZSTK474-reduced VEGF on tube formation of human umbilical vein endothelial cells (HUVECs). In conclusion, ZSTK474 was demonstrated to have potential in vitro antimetastatic effects on PC3 cells via dual mechanisms: inhibition of metastatic processes including cell migration, invasion and adhesion, and antiangiogenesis via blockade of VEGF secretion.

Chronic myelogenous leukemia (CML) is a malignant hematological disorder mainly caused by the Bcr-Abl tyrosine kinase. While Bcr-Abl inhibitors including Imatinib showed antitumor efficacy on many CML patients, resistance was frequently reported in recent years. Therefore, novel drugs for CML are still expected. ZSTK474 is a specific phosphatidylinositol 3-kinase (PI3K) inhibitor that we identified. In the present study, the efficacy of ZSTK474, alone or in combination with Imatinib, on K562 CML cells as well as on its multidrug resistance counterpart K562/A02 cells, was investigated. ZSTK474 inhibited the cell proliferation with an IC50 of 4.69 μM for K562 and 7.57 μM for K562/A02 cells, respectively. Treatment by ZSTK474 resulted in cell cycle arrest in G1 phase, which might be associated with upregulation of p27, and downregulation of cyclin D1. ZSTK474 also inhibited phosphorylation of Akt and GSK-3β, which might be involved in the effect on the above cell cycle-related proteins. Moreover, combination of ZSTK474 and Imatinib indicated synergistic effect on both cell lines. In conclusion, ZSTK474 exhibited antileukemia activity alone, and showed synergistic effect when combined with Imatinib, on CML K562 cells as well as the multidrug resistant ones, providing a potential therapeutic approach for CML patients.