Activation of phosphatidylinositol 3-kinase (PI3K) signaling is involved in carcinogenesis and cancer progression. Therefore, inhibitors targeting PI3K are considered to be candidate drugs for cancer therapy. Indeed, some PI3K inhibitors have entered clinical trials, but efforts are still underway to develop new PI3K inhibitors. To discover novel PI3K inhibitors, we took advantage of a drug screening system by using the yeast Saccharomyces cerevisiae with deleterious mutations in ATP-binding cassette transporter genes. This is because wild-type yeast employs drug efflux pumps for reducing intracellular drug concentrations. The screening system is based on growth inhibition induced by overexpression of membrane localized p110α in yeast, in which no endogenous p110 homolog is expressed. However, compounds with the ability to inhibit PI3K can rescue cells from p110α induced growth inhibition. Using this system, we screened the deposited chemical library of the Screening Committee of Anticancer Drugs (SCADS). As a result, the histone deacetylase (HDAC) inhibitor romidepsin (FK228) and its novel analogs were isolated. In vitro PI3K activity assays confirmed that these compounds directly inhibit PI3K activity at μM-range concentrations. FK-A5 analog was the most potent inhibitor. Western blotting revealed that these compounds inhibit phosphorylation of v-akt murine thymoma viral oncogene homolog (AKT) and downstream signaling components. Molecular modeling of the PI3K-FK228 complex indicated that FK228 binds to the ATP binding pocket of p110α. At µM range concentrations, FK228 and FK-A5 exhibit potent cytotoxicity even in HDAC inhibitor-resistant cells. Fluorescence activated cell sorting (FACS) and western blotting indicated apoptosis as the cause of cell death. Furthermore, it was suggested that HDAC/PI3K dual inhibitory activity of FK228 and FK-A5 at µM range concentrations contributed to the induction of potent apoptosis because of the following reasons. 1) FK228 or FK-A5 within µM range inhibited AKT phophorylation and acetylated histones simultaneously. 2) The apoptosis induction by FK228 or FK-A5 at µM range concentrations was mimicked by combination of HDAC inhibitor and PI3K inhibitor. In conclusion, we demonstrated that FK228 and its analogs directly inhibited PI3K activity and induce potent apoptosis exhibiting HDAC/PI3K dual inhibition at µM range concentrations. In future, optimizing potency of FK228 and its analogs against PI3K may contribute to the development of novel HDAC/PI3K dual inhibitors for cancer treatment.

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 872. doi:1538-7445.AM2012-872