Defining the tumor suppressive mechanism of the kinase STK25 - Abstract The maintenance of cellular and tissue homeostasis relies on the tight regulation of cell growth and proliferation. This regulation is predominantly controlled by two evolutionarily conserved pathways: the mammalian Target of Rapamycin (mTOR) pathway and the Hippo pathway. Dysregulation of both the mTOR and Hippo pathways promotes aberrant cell proliferation, tissue overgrowth and tumor development. Elucidating how the mTOR and Hippo signaling pathways coordinate their activities is therefore a vitally important, yet poorly understood, area of cell cancer biology. Our lab previously demonstrated that STK25, a serine/threonine kinase, functions as an upstream activator of Hippo signaling to limit cell growth. Using unbiased approaches, we have now generated strong preliminary data suggesting that the STK25 kinase also functions to impair mTORC1 signaling. We have found that that genetic depletion of STK25 leads to both mTORC1 pathway activation and Hippo pathway inactivation. These data suggest that STK25 may play a critical role in regulating cell proliferation and tumor suppression by coordinating the activities of both the mTORC1 and Hippo signaling pathways. However, the mechanism by which STK25 regulates mTORC1 is unknown. In this proposal, we hypothesize that STK25 directly regulates and phosphorylates key regulatory components of the mTORC1 signaling pathway, ultimately attenuating mTOR signaling. Further, our preliminary studies of STK25 knockout in mice demonstrate that conditional depletion of STK25 is sufficient to induce tumorigenesis, which is congruent with bioinformatic evidence that STK25 is significantly focally deleted in a variety of human cancers. Combining this with our in vitro findings, we also hypothesize that STK25 deficient tumors are dependent on YAP/TAZ and/or mTORC1 activity for growth and survival. These hypotheses will be tested in two specific aims. In Aim 1, we will use cell biological and biochemical approaches to mechanistically define the role of STK25 kinase in regulating mTORC1 activity by characterizing its direct phosphorylated substrates. In Aim 2, we will use a conditional STK25 knockout mouse that we have already generated to determine if STK25 loss facilitates tumorigenesis through increased YAP/TAZ and/or mTORC1 activity. Successful completion of these aims will advance our understanding of mTORC1 and Hippo signaling, and reveal significant new insights into the underlying mechanisms driving tumor development. Moreover, the proposed training plan will greatly enhance my repertoire of skills in microscopy and imaging techniques, molecular cell biology, mouse cancer studies and liquid chromatography mass spectrometry (LC-MS/MS) analysis. Altogether, this will facilitate the development of my scientific expertise in the fields of mTOR and Hippo signaling in cancer biology required for success as an independent scientist.
