Functional genomics reveals that tumors with activating phosphoinositide 3-kinase mutations are dependent on accelerated protein turnover.

Activating mutations in the phosphoinositide 3-kinase (PI3K) signaling pathway are frequently identified in cancer. To identify pathways that support PI3K oncogenesis, we performed a genome-wide RNAi screen in isogenic cell lines harboring wild-type or mutant PIK3CA to search for PI3K synthetic-lethal (SL) genes. A combined analysis of these results with a meta-analysis of two other large-scale RNAi screening data sets in PI3K mutant cancer cell lines converged on ribosomal protein translation and proteasomal protein degradation as critical nononcogene dependencies for PI3K-driven tumors. Genetic or pharmacologic inhibition of either pathway alone, but not together, selectively killed PI3K mutant tumor cells in an mTOR-dependent manner. The expression of ribosomal and proteasomal components was significantly up-regulated in primary human colorectal tumors harboring PI3K pathway activation. Importantly, a PI3K SL gene signature containing the top hits of the SL genes identified in our meta-analysis robustly predicted overall patient survival in colorectal cancer, especially among patients with tumors with an activated PI3K pathway. These results suggest that disruption of protein turnover homeostasis via ribosome or proteasome inhibition may be a novel treatment strategy for PI3K mutant human tumors.

Protein kinase function and glutathionylation.

Intracellular reactive oxygen species are generated as a by-product of normal metabolic processes and can both damage cellular constituents and function as important signalling species. This signalling often involves changes in the thiol redox balance. As an antioxidant, glutathione serves in maintaining the reduced state of cellular protein thiol groups. The paper by Cross and Templeton appearing in this issue of the Biochemical Journal describes a mechanism by which glutathionylation plays a key role in the regulation of the kinase activity of MEKK1 [MAP (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase kinase; MAP3K] in response to oxidative stresses. This type of post-translational-modification glutathionylation may represent a general mechanism by which protein kinase function can be regulated.

The microtubule-associated Rho activating factor GEF-H1 interacts with exocyst complex to regulate vesicle traffic.

The exocyst complex plays a critical role in targeting and tethering vesicles to specific sites of the plasma membrane. These events are crucial for polarized delivery of membrane components to the cell surface, which is critical for cell motility and division. Though Rho GTPases are involved in regulating actin dynamics and membrane trafficking, their role in exocyst-mediated vesicle targeting is not very clear. Herein, we present evidence that depletion of GEF-H1, a guanine nucleotide exchange factor for Rho proteins, affects vesicle trafficking. Interestingly, we found that GEF-H1 directly binds to exocyst component Sec5 in a Ral GTPase-dependent manner. This interaction promotes RhoA activation, which then regulates exocyst assembly/localization and exocytosis. Taken together, our work defines a mechanism for RhoA activation in response to RalA-Sec5 signaling and involvement of GEF-H1/RhoA pathway in the regulation of vesicle trafficking.

Recurrent hemizygous deletions in cancers may optimize proliferative potential.

Tumors exhibit numerous recurrent hemizygous focal deletions that contain no known tumor suppressors and are poorly understood. To investigate whether these regions contribute to tumorigenesis, we searched genetically for genes with cancer-relevant properties within these hemizygous deletions. We identified STOP and GO genes, which negatively and positively regulate proliferation, respectively. STOP genes include many known tumor suppressors, whereas GO genes are enriched for essential genes. Analysis of their chromosomal distribution revealed that recurring deletions preferentially overrepresent STOP genes and underrepresent GO genes. We propose a hypothesis called the cancer gene island model, whereby gene islands encompassing high densities of STOP genes and low densities of GO genes are hemizygously deleted to maximize proliferative fitness through cumulative haploinsufficiencies. Because hundreds to thousands of genes are hemizygously deleted per tumor, this mechanism may help to drive tumorigenesis across many cancer types.

WNK1 and OSR1 regulate the Na+, K+, 2Cl- cotransporter in HeLa cells.

Oxidative stress-responsive kinase (OSR) 1 and sterile20-related, proline-, alanine-rich kinase (SPAK) are Ste20p-related protein kinases that bind to the sodium, potassium, two chloride cotransporter, NKCC. Here we present evidence that the protein kinase with no lysine [K] (WNK) 1 regulates OSR1, SPAK, and NKCC activities. OSR1 exists in a complex with WNK1 in cells, is activated by recombinant WNK1 in vitro, and is phosphorylated in a WNK1-dependent manner in cells. Depletion of WNK1 from HeLa cells by using small interfering RNA reduces OSR1 kinase activity. In addition, depletion of either WNK1 or OSR1 reduces NKCC activity, indicating that WNK1 and OSR1 are both required for NKCC function. OSR1 and SPAK are likely links between WNK1 and NKCC in a pathway that contributes to volume regulation and blood pressure homeostasis in mammals.

Structure of rabbit-muscle glyceraldehyde-3-phosphate dehydrogenase.

The crystal structure of the tetrameric form of D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH) isolated from rabbit muscle was solved at 2.4 A resolution after careful dynamic light-scattering experiments to find a suitable buffer for crystallization trials. The refined model has a crystallographic R factor of 20.3%. Here, the first detailed model of a mammalian GAPDH is presented. The cofactor NAD(+) (nicotinamide adenine dinucleotide) is bound to two subunits of the tetrameric enzyme, which is consistent with the negative cooperativity of NAD(+) binding to this enzyme. The structure of rabbit-muscle GAPDH is of interest because it shares 91% sequence identity with the human enzyme; human GAPDH is a potential target for the development of anti-apoptotic drugs. In addition, differences in the cofactor-binding pocket compared with the homology-model structure of GAPDH from the malaria parasite Plasmodium falciparum could be exploited in order to develop novel selective and potential antimalaria drugs.

Critical contribution of linker proteins to Raf kinase activation.

Genetic analysis of Ras signaling has unveiled the participation of non-enzymatic accessory proteins in signal transmission. These proteins, KSR, CNK, and Sur-8, can interact with multiple core components of the Ras/MAP kinase cascade and may contribute to the structural organization of this cascade. However, the precise biochemical nature of the contribution of these proteins to Ras signaling is currently unknown. Here we show directly that CNK and KSR are required for stimulus dependent Raf kinase activation. CNK is required for membrane recruitment of Raf, while KSR is likely required to couple Raf to upstream kinases. These results demonstrate that CNK and KSR are integral components of the cellular machinery mediating Raf activation.