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The mammalian target of rapamycin (mTOR) signaling network is central to the regulation of cell growth in response to both growth factors and nutrients. We developed a high-throughput, cell-based assay to identify small-molecule modulators of the mTOR signaling network. One such compound, which we name quinostatin, potently inhibits this network by directly targeting the lipid-kinase activity of the catalytic subunits of class Ia PI3Ks. This study illustrates the power of unbiased, phenotypic screening as a means for illuminating cell circuitry, and resulted in the identification of a chemotype for selective inhibition of the class Ia PI3Ks.