Cancer Cell. 2009 Jul 7;16(1):21-32. doi: 10.1016/j.ccr.2009.04.012.

AKT-independent signaling downstream of oncogenic PIK3CA mutations in human cancer.

Vasudevan KM, Barbie DA, Davies MA, Rabinovsky R, McNear CJ, Kim JJ, Hennessy BT, Tseng H, Pochanard P, Kim SY, Dunn IF, Schinzel AC, Sandy P, Hoersch S, Sheng Q, Gupta PB, Boehm JS, Reiling JH, Silver S, Lu Y, Stemke-Hale K, Dutta B, Joy C, Sahin AA, Gonzalez-Angulo AM, Lluch A, Rameh LE, Jacks T, Root DE, Lander ES, Mills GB, Hahn WC, Sellers WR, Garraway LA.

Source

Department of Medical Oncology, Dana-Farber Cancer Institute, Harvard Medical School, Boston, MA 02115, USA.

Abstract

Dysregulation of the phosphatidylinositol 3-kinase (PI3K) signaling pathway occurs frequently in human cancer. PTEN tumor suppressor or PIK3CA oncogene mutations both direct PI3K-dependent tumorigenesis largely through activation of the AKT/PKB kinase. However, here we show through phosphoprotein profiling and functional genomic studies that many PIK3CA mutant cancer cell lines and human breast tumors exhibit only minimal AKT activation and a diminished reliance on AKT for anchorage-independent growth. Instead, these cells retain robust PDK1 activation and membrane localization and exhibit dependency on the PDK1 substrate SGK3. SGK3 undergoes PI3K- and PDK1-dependent activation in PIK3CA mutant cancer cells. Thus, PI3K may promote cancer through both AKT-dependent and AKT-independent mechanisms. Knowledge of differential PI3K/PDK1 signaling could inform rational therapeutics in cancers harboring PIK3CA mutations.

PMID:: 19573809; [PubMed - indexed for MEDLINE]
PMCID:: PMC2752826

Free PMC Article

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Figure 1

PTEN-null and PIK3CA-mutant cancer cells show different steady-state AKT pathway activation patterns. (A-C) Quantitative RPPA protein expression levels were determined in NCI60 cells showing loss of PTEN (PTEN-null) (green), helical PIK3CA mutation (Helical) (red), kinase PIK3CA mutation (Kinase) (blue) or a wild-type pattern for PTEN and PIK3CA (WT/WT) (gray). Relative protein levels of PTEN (A), p-AKT (Ser473) (B) and p-AKT (Thr308) (C) are shown. The diminished PTEN protein levels in PTEN-null cells are statistically significant (p < 0.001 when compared to both PIK3CA-mutant and “wild type” cells). PTEN-null cell lines have higher p-AKT (Ser473) and p-AKT (Thr308) levels than PIK3CA-mutant lines (p = 0.0006 and p = 0.002, respectively) or WT/WT lines (p = 0.0002 and p = 0.0003, respectively). The PIK3CA-mutant cell lines do not differ significantly from the WT/WT cell lines (p = 0.62 and p = 0.25, respectively, for each p-AKT residue). (D) Immunoblot analysis of AKT phosphorylation (p-AKT) at Ser473 and Thr308 across a panel of PTEN-null or PIK3CA-mutant cancer cell lines. (E, F) Quantitative RPPA levels were also determined for p-GSK3-α/β (E) p-Tuberin and (F). PTEN-null cells show higher phosphorylation of both AKT substrates than PIK3CA-mutant cells (p = 0.003 and p = 0.002, respectively) and WT/WT lines (p = 0.003 and 0.001, respectively). (G) Fluorescence microscopy of transient GFP-FOXO1 expression in PTEN-null or PIK3CA-mutant cancer cell lines. Lines with high p-AKT or low p-AKT are indicated. Scale bars = 30 μM.

AKT-independent signaling downstream of oncogenic PIK3CA mutations in human cancer