Although 3-phosphoinositide-dependent protein kinase-1 (PDK1) has been predominately linked to the phosphoinositide 3-kinase (PI3K)-AKT pathway, it may also evoke additional signaling outputs to promote tumorigenesis. Here, we report that PDK1 directly induces phosphorylation of Polo-like kinase 1 (PLK1), which in turn induces MYC phosphorylation and protein accumulation. We show that PDK1-PLK1-MYC signaling is critical for cancer cell growth and survival, and small-molecule inhibition of

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... LK1 provides an effective approach for therapeutic targeting of MYC dependency. Intriguingly, PDK1-PLK1-MYC signaling induces an embryonic stem cell-like gene signature associated with aggressive tumor behaviors and is a robust signaling axis driving cancer stem cell (CSC) self-renewal. Finally, we show that a PLK1 inhibitor synergizes with an mTOR inhibitor to induce synergistic antitumor effects in colorectal cancer by antagonizing compensatory MYC induction. These fi ndings identify a novel pathway in human cancer and CSC activation and provide a therapeutic strategy for targeting MYC-associated tumorigenesis and therapeutic resistance. SIGNIFICANCE: This work identifi es PDK1-PLK1-MYC signaling as a new oncogenic pathway driving oncogenic transformation and CSC self-renewal. Targeted inhibition of PDK1/PLK1 is robust in targeting MYC dependency in cancer cells. Thus, our fi ndings provide important insights into cancer and CSC biology and have signifi cant therapeutic implications. Cancer Discov; 3(10); 1156-71.