Novel Mechanisms of Nuclear Phosphoinositide Signaling in the Regulation of the YAP/TAZ Pathway in Triple-Negative Breast Cancer

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Project Summary: “Novel Mechanisms of Nuclear Phosphoinositide Signaling in the Regulation of the YAP/TAZ Pathway in Triple-Negative Breast Cancer” Phosphoinositides (PIs) are lipid messengers that control many aspects of human physiology. A significant fraction of PIs (>40% of total PIs) is found in the nucleus, however the nature and functions of the nuclear PIs are largely unknown. We discovered that phosphatidylinositol 4,5-bisphosphate (PI4,5P2) is an abundant PI species in the nucleus and the PI4,5P2-generating kinase phosphatidylinositol-4-phosphate-5-kinase type 1 alpha (PIPKI) is a major enzyme modulating nuclear PI4,5P2 signaling. Nuclear PI4,5P2 can be further phosphorylated by a nuclear-localizing PI3-kinase (PI3K) inositol phosphate multikinase (IPMK) to produce a PI species, PI3,4,5P3, that has been implicated in oncogenesis. This suggests that PIPKI and IPMK are potential targets for cancer therapy. Consistently, we showed that depletion or pharmacological inhibition of PIPKI and IPMK leads to cancer cell death by apoptosis in triple negative breast cancer (TNBC) cells. Moreover, in TNBC cells, we discovered that depletion of PIPKI and IPMK significantly reduces the expression of YAP/TAZ target genes that have established contributions to oncogenesis. TNBC is the most aggressive subtype of breast cancer and associated with poor patient survival due to lack of alternatives to current chemotherapies. As a result, there is an urgent need for discovering novel targeted therapeutics in TNBC. The YAP/TAZ-PI kinases (PIPKI and IPMK) pathways are attractive drug targets because 1) aberrant activation of YAP/TAZ is frequently found in breast cancer particularly in TNBC, PIPKI gene is commonly amplified in TNBC, and 2) the nuclear PI3,4,5P3 pathway is frequently dysregulated in TNBC. Precise understanding of nuclear PIs-mediated YAP/TAZ pathway will provide knowledge which can be utilized for developing targeted therapeutics against TNBC. In this proposal, we will 1) elucidate molecular mechanisms by which the YAP/TAZ pathway is controlled by PIPIK and IPMK via extensive biochemical and cell biological approaches and 2) investigate contributions of PIPIK and IPMK in TNBC pathogenesis in cultured TNBC cells and mouse models. This project will provide pivotal information how the YAP/TAZ pathway is regulated by the PI signaling and illuminate new routes to target the YAP/TAZ pathway in cancer by the understudied kinases of PIPIK and IPMK.