Abstract The PI3K signaling cascade is frequently activated in human cancer, and AKT is commonly considered its major transforming conduit. Thus, PI3K and AKT represent relevant targets for therapeutic purposes. However, the many essential of roles played by this pathway in normal cells poses the problem of reaching therapeutic efficacy without harming healthy tissues. In principle, it would be ideal to target only molecules directly involved in the transformation process, thus reducing the toxicity associated with wide-range inhibitors. PDK1 plays a key role in the PI3K cascade, directly activating AKT. In addition, PDK1 possesses a substrate docking site named “PIF pocket” that is required to phosphorylate and activate members of the AGC kinase family, including S6K, PKC, SGK, and RSK. Mice in which the PI3K pathway is constitutively activated in the thyroid epithelium through loss of Pten are born with thyroid hyperplasia that progresses to invasive and metastatic follicular carcinoma, a process that is accelerated by the simultaneous loss of p27 or p53, and by Ras or Braf activation. Using in vivo as well as ex vivo and in vitro genetic and pharmacological approaches, we now show that although Akt activation is necessary, it is not sufficient to transform thyroid epithelial cells. Concomitant activation of Pdk1-dependent pathways is absolutely required to develop neoplastic lesions in vivo, and to induce cell proliferation ex vivo. Pten mutants develop adenomas and carcinomas by one year of age. Strikingly, complete rescue of this phenotype is observed in compound mutants in which, in addition to thyroid-specific Pten loss, the PIF-pocket of Pdk1 has been genetically disrupted, maintaining constitutive activation of Akt and mTor, but not of PIF-pocket dependent AGC kinases. We also show that the transcription factor STAT3 represents a key mediator of this essential Pdk1-dependent pathway: STAT3 phosphorylation is increased in Pten mutants, and abolished in compound mutants. Furthermore, genetic and pharmacological inhibition of STAT3 drastically impairs cell proliferation in vitro and rescues the hyperplastic phenotype of Pten mutant mice in vivo. Thus, we have identified an essential and druggable signaling cascade that critically cooperates with AKT activation to transform thyroid epithelial cells. Citation Format: Arturo Orlacchio, Valeria Antico Arciuch, Antonio Di Cristofano. PDK1-dependent activation of AGC kinases is an absolute requirement for AKT oncogenic activity. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4776. doi:10.1158/1538-7445.AM2014-4776