Abstract #3822: PDK1-SGK3 signaling in the absence of AKT activation in PIK3CA- mutant cancers

PIK3CA , which encodes the catalytic (p110\#945;, ) subunit of PI3 Kinase, is frequently mutated in a diverse range of human cancers including breast, colon and endometrial cancers. The most common tumor-associated PIK3CA mutations involve either the helical domain (exon 9; e.g., E542K and E545K) or the kinase domain (exon 20; e.g., H1047R) of p110\#945;, . These gain function PIK3CA mutations are thought to promote PI3K-mediated tumorigenesis primarily through downstream activation of the AKT/PKB kinase. However, we have found that many PIK3CA -mutant cancer cell lines and human tumors may transduce an AKT-independent tumor signaling mechanism. Systematic immunoblot studies showed that many PIK3CA -mutant cell lines lack activation of both AKT and its downstream effectors, do not require AKT signaling for anchorage independent growth, and exhibit reduced AKT membrane localization and PI(3,4,5)P3 levels. In contrast, PIK3CA -mutant cell lines and breast tumors exhibited robust activation of and reliance upon PDK1, another oncogenic kinase recruited to the membrane by activated PI3K. Possible mechanisms of PDK1-dependent, AKT -independent growth in PIK3CA-mutant cells were investigated by systematic RNAi screening using a lentiviral shRNA library targeting >1000 kinases, phosphatases and other known cancer genes, with a particular emphasis on 20 known PDK1 substrates. For this, PIK3CA- mutant cell lines were stratified into two groups based on p-AKT levels (high vs low) and analyzed the top 10% of 120 hairpins targeting PDK1 substrates whose effects on viability distinguished these two classes. This analysis found that shRNAs targeting SGK3 (serum/glucocorticoid regulated kinase 3) most strongly suppressed viability in \#8220;low p-AKT\#8221; PIK3CA -mutant cells. Further biochemical and functional studies confirmed that PDK1 activated SGK3, which localized to endosomes in these cells. Thus, PIK3CA -mutant cancer cells with low AKT signaling may exhibit a selective dependency on SGK3 for viability. Altogether, these results indicate that in the absence of AKT activation, PDK1 may transmit an alternative signal that engages the AGC kinase SGK3 in cancer cells that harbor oncogenic PIK3CA mutations. This study thereby nominates both PDK1 and SGK3 in addition to AKT as \#8220;druggable\#8221; oncogenic effectors downstream of activating PIK3CA mutations. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3822.