[WITHDRAWN] Novel FOXM1 inhibitor STL001 sensitizes human cancers to a broad-spectrum of cancer therapies

The full text of this preprint has been withdrawn by the authors while they make corrections to the work. Therefore, the authors do not wish this work to be cited as a reference. Questions should be directed to the corresponding author.

Abstract LB-356: Micelle-encapsulated thiostrepton: an effective nanomedicine for inhibiting tumor growth

The thiazole antiobiotic, thiostrepton, has been found to suppress the expression of FOXM1, the oncogenic forkhead transcription factor overexpressed in cancer cells. Thiostrepton has also been shown to induce cell death in cancer cells through a proteasome inhibitory mechanism. As thiostrepton is highly hydrophobic, encapsulation into nano-sized delivery vehicles not only aides its solubilization but also improves its targeting for tumors in vivo. This is achieved by encapsulating thiostrepton into the interior of an amphiphilic micelle, composed from an assembly of lipid-PEG (polyethylene glycol) conjugates. In this study, we examined the anti-tumor effects of micelle-thiostrepton in both subcutaneous xenograft models (HepG2 liver cancer, Capan-2 pancreatic cancer and MDA-MB-231 breast cancer) and endogenous liver tumor models (DEN/PB-induced adenocarcinomas). In the case of the xenograft tumor-bearing mice, biodistribution studies showed micelle-thiostrepton to accumulate into both the liver (30% of injected dose) and the xenograft tumors (60% of injected dose) from 4 hours post-administration. In the endogenous liver cancer models, micelle-thiostrepton accumulated into the liver only, allowing a high concentration of tumor-associated thiostrepton drug. Continuous administration of micelle-thiostrepton at doses of 40mg/kg, 3 times a week for 30 days retarded the growth rate of all subcutaneous tumors and reduced the occurrence of adenocarinomas in endogenous liver cancer models. Immunoblotting and immunohistochemical staining revealed that treatment with micelle-thiostrepton inhibited the expression of the oncogenic transcription factor FOXM1 in tumors, a reduction of which generally coincides with suppressed cell proliferation and increased cell death. Furthermore, it was found that the reduced tumor growth rate of thiostrepton-treated tumors was indeed related to increased apoptosis, as indicated by the elevated expression of cleaved caspase-3 in such tumors samples. Our data suggest that the thiazole antibiotic/proteasome inhibitor thiostrepton, when formulated into nanoparticles, may be highly suited as a nanomedicine for treating cancer, particularly that of liver cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-356. doi:1538-7445.AM2012-LB-356

Activation of Akt/protein kinase B overcomes a G(2)/m cell cycle checkpoint induced by DNA damage

Activation of Akt, or protein kinase B, is frequently observed in human cancers. Here we report that Akt activation via overexpression of a constitutively active form or via the loss of PTEN can overcome a G(2)/M cell cycle checkpoint that is induced by DNA damage. Activated Akt also alleviates the reduction in CDC2 activity and mitotic index upon exposure to DNA damage. In addition, we found that PTEN null embryonic stem (ES) cells transit faster from the G(2)/M to the G(1) phase of the cell cycle when compared to wild-type ES cells and that inhibition of phosphoinositol-3-kinase (PI3K) in HEK293 cells elicits G(2) arrest that is alleviated by activated Akt. Furthermore, the transition from the G(2)/M to the G(1) phase of the cell cycle in Akt1 null mouse embryo fibroblasts (MEFs) is attenuated when compared to that of wild-type MEFs. These results indicate that the PI3K/PTEN/Akt pathway plays a role in the regulation of G(2)/M transition. Thus, cells expressing activated Akt continue to divide, without being eliminated by apoptosis, in the presence of continuous exposure to mutagen and accumulate mutations, as measured by inactivation of an exogenously expressed herpes simplex virus thymidine kinase (HSV-tk) gene. This phenotype is independent of p53 status and cannot be reproduced by overexpression of Bcl-2 or Myc and Bcl-2 but seems to counteract a cell cycle checkpoint mediated by DNA mismatch repair (MMR). Accordingly, restoration of the G(2)/M cell cycle checkpoint and apoptosis in MMR-deficient cells, through reintroduction of the missing component of MMR, is alleviated by activated Akt. We suggest that this new activity of Akt in conjunction with its antiapoptotic activity may contribute to genetic instability and could explain its frequent activation in human cancers.