PIK3CA is mutated in diverse human cancers, but the functional effects of these mutations have not been defined. To evaluate the consequences of PIK3CA alterations, the two most common mutations were inactivated by gene targeting in colorectal cancer (CRC) cells. Biochemical analyses of these cells showed that mutant PIK3CA selectively regulated the phosphorylation of AKT and the forkhead transcription factors FKHR and FKHRL1. PIK3CA mutations had little effect on growth under standard conditions, but reduced cellular dependence on growth factors. PIK3CA mutations resulted in attenuation of apoptosis and facilitated tumor invasion. Treatment with the PI3K inhibitor LY294002 abrogated PIK3CA signaling and preferentially inhibited growth of PIK3CA mutant cells. These data have important implications for therapy of cancers harboring PIK3CA alterations.

Tumor progression is driven by genetic mutations, but little is known about the environmental conditions that select for these mutations. Studying the transcriptomes of paired colorectal cancer cell lines that differed only in the mutational status of their KRAS or BRAF genes, we found that GLUT1, encoding glucose transporter-1, was one of three genes consistently upregulated in cells with KRAS or BRAF mutations. The mutant cells exhibited enhanced glucose uptake and glycolysis and survived in low glucose conditions, phenotypes that all required GLUT1 expression. In contrast, when cells with wild-type KRAS alleles were subjected to a low glucose environment, very few cells survived. Most surviving cells expressed high levels of GLUT1 and 4% of these survivors had acquired new KRAS mutations. The glycolysis inhibitor, 3-bromopyruvate preferentially suppressed the growth of cells with KRAS or BRAF mutations. Together, these data suggest that glucose deprivation can drive the acquisition of KRAS pathway mutations in human tumors.Mutations in oncogenes and tumor suppressor genes endow cancer cells with the ability to outgrow their neighboring cells in situ (1). Though numerous studies have identified the downstream effects of such mutations and their biochemical mediators, there is relatively little known about the microenvironmental conditions that provide the selective advantage that allows cells with such mutations to clonally expand. Mutations in KRAS commonly occur in colorectal, pancreatic, and some forms of lung cancer, while BRAF mutations occur commonly in melanomas as well as in colorectal tumors without KRAS mutations (2-4). BRAF and KRAS mutations are mutually exclusive, that is, do not occur in the same tumor, suggesting a common origin and effect. Indeed, KRAS binds to and activates BRAF, thereby activating MAPK signaling pathways (5,6). Despite advances in the molecular delineation of the RAS/ RAF pathway, the specific environmental pressures that drive KRAS and BRAF mutations and how KRAS and BRAF mutations alleviate these pressures are unknown.

Mutations in oncogenes often promote tumorigenesis by changing the conformation of the encoded proteins, thereby altering enzymatic activity. The PIK3CA oncogene, which encodes p110␣, the catalytic subunit of phosphatidylinositol 3-kinase alpha (PI3K␣), is one of the two most frequently mutated oncogenes in human cancers. We report the structure of the most common mutant of p110␣ in complex with two interacting domains of its regulatory partner (p85␣), both free and bound to an inhibitor (wortmannin). The N-terminal SH2 (nSH2) domain of p85␣ is shown to form a scaffold for the entire enzyme complex, strategically positioned to communicate extrinsic signals from phosphopeptides to three distinct regions of p110␣. Moreover, we found that Arg-1047 points toward the cell membrane, perpendicular to the orientation of His-1047 in the WT enzyme. Surprisingly, two loops of the kinase domain that contact the cell membrane shift conformation in the oncogenic mutant. Biochemical assays revealed that the enzymatic activity of the p110␣ His1047Arg mutant is differentially regulated by lipid membrane composition. These structural and biochemical data suggest a previously undescribed mechanism for mutational activation of a kinase that involves perturbation of its interaction with the cellular membrane.