RAS proteins (KRAS4A, KRAS4B, NRAS and HRAS) function as GDP-GTP-regulated binary on-off switches, which regulate cytoplasmic signaling networks that control diverse normal cellular processes. Gain-of-function missense mutations in RAS genes are found in ∼25% of human cancers, prompting interest in identifying anti-RAS therapeutic strategies for cancer treatment. However, despite more than three decades of intense effort, no anti-RAS therapies have reached clinical application. Contributing to this failure has been an underestimation of the complexities of RAS. First, there is now appreciation that the four human RAS proteins are not functionally identical. Second, with >130 different missense mutations found in cancer, there is an emerging view that there are mutation-specific consequences on RAS structure, biochemistry and biology, and mutation-selective therapeutic strategies are needed. In this Cell Science at a Glance article and accompanying poster, we provide a snapshot of the differences between RAS isoforms and mutations, as well as the current status of anti-RAS drug-discovery efforts.

The BRAF kinase is mutated, typically V600E, to induce an active oncogenic state in a large fraction of melanoma, thyroid, hairy cell leukemia, and to a lesser extent, a wide spectrum of other cancers1,2. BRAFV600E phosphorylates and activates the kinases MEK1 and MEK2, which in turn phosphorylate and activate the kinases ERK1 and ERK2, stimulating the MAPK pathway to promote cancer3. Targeting MEK1/2 is proving to be an important therapeutic strategy, as a MEK1/2 inhibitor provides a survival advantage in metastatic melanoma4, which is increased when co-administered with a BRAFV600E inhibitor5. In this regard, we previously found that copper (Cu) influx enhances MEK1 phosphorylation of ERK1/2 through a Cu-MEK1 interaction6. We now show that genetic loss of the high affinity Cu transporter Ctr1 or mutations in MEK1 that disrupt Cu binding reduced BRAFV600E-driven signaling and tumorigenesis. Conversely, a MEK1-MEK5 chimera that phosphorylates ERK1/2 independent of Cu or an active ERK2 restored tumor growth to cells lacking Ctr1. Importantly, Cu chelators used in the treatment of Wilson disease7 reduced tumor growth of both BRAFV600E-transformed cells and cells resistant to BRAF inhibition. Taken together, these results suggest that Cu-chelation therapy could be repurposed to treat BRAFV600E mutation-positive cancers.

Allele-specific signaling by different KRAS alleles remains poorly understood. The KRAS G12R mutation displays uneven prevalence among cancers that harbor the highest occurrence of KRAS mutations: it is rare in lung and colorectal cancers (~1%), yet relatively common (~20%) in pancreatic ductal adenocarcinoma (PDAC), suggesting context-specific properties. We evaluated whether KRAS G12R is functionally distinct from the more common KRAS G12D or KRAS G12V mutant proteins (KRAS G12D/V ). We found that KRAS G12D/V but not KRAS G12R drives macropinocytosis and that MYC is essential for macropinocytosis in KRAS G12D/V but not KRAS G12R -mutant PDAC. Surprisingly, we found that KRAS G12R is defective for interaction with a key effector, p110α phosphoinositide 3-kinase (PI3Kα), due to structural perturbations in switch II. Instead, upregulated KRAS-independent PI3Kγ activity was able to support macropinocytosis in KRAS G12R -mutant PDAC. Finally, we determined that KRAS G12R -mutant PDAC displayed a distinct drug sensitivity profile compared with KRAS G12D -mutant PDAC but is still responsive to the combined inhibition of ERK and autophagy.Hobbs et al.

Significance: Oxidation and reduction events are critical to physiological and pathological processes and are highly regulated. Herein, we present evidence for the role of Ras and Rho GTPases in controlling these events and the unique underlying mechanisms. Evidence for redox regulation of Ras GTPases that contain a redox-sensitive cysteine (X) in the conserved NKXD motif is presented, and a growing consensus supports regulation by a thiyl radical-mediated oxidation mechanism. We also discuss the debate within the literature regarding whether 2e -oxidation mechanisms also regulate Ras GTPase activity. Recent Advances: We examine the increasing in vitro and cell-based data supporting oxidant-mediated activation of Rho GTPases that contain a redox-sensitive cysteine at the end of the conserved phosphoryl- binding loop (p-loop) motif (GXXXXG[S/T]C). While this motif is distinct from Ras, these data suggest a similar 1e-oxidation-mediated activation mechanism. Critical Issues: We also review the data showing that the unique p-loop placement of the redox-sensitive cysteine in Rho GTPases supports activation by 2e -cysteine oxidation. Finally, we examine the role that Ras and Rho GTPases play in controlling key oxidant-regulating enzymes in the cell, and we speculate on a feedback mechanism. Future Directions: Given that these GTPases and redox-regulating enzymes are involved in multiple physiological and pathological processes, we discuss future experiments that may clarify the interplay between them. Antioxid. Redox Signal. 18, 250-258.

Aim-Tau protein is a prominent component of paired helical filaments in Alzheimer's disease (AD) and other tauopathies. While the abnormal phosphorylation of tau on serine and threonine has been well established in the disease process, its phosphorylation on tyrosine has only recently been described. We previously showed that the Src family non-receptor tyrosine kinases (SFKs) Fyn and Src phosphorylate tau on Tyr18 and that phospho-Tyr18-tau was present in AD brain. In this study, we have investigated the appearance of phospho-Tyr18-tau, activated-SFK, and Proliferating Cell Nuclear Antigen (PCNA) during disease progression in a mouse model of human tauopathy.Methods-We have used JNPL3, which expresses human tau with P301L mutation, and antibodies specific for phospho-Tyr18-tau (9G3), ser/thr phosphorylated tau (AT8), activated-SFK, and PCNA. Antibody staining was viewed by either epifluorescence or confocal microscopy.Results-Phospho-Tyr18-tau appeared concurrently with AT8-reactive tau as early as 4 months in JNPL3. Some 9G3-positive cells also contained activated-SFKs and PCNA. We also investigated the triple transgenic mouse model of AD and found that unlike the JNPL3 model, the appearance of 9G3 reactivity did not coincide with AT8 in the hippocampus, suggesting that the presence of APP/presenilin influences tau phosphorylation. Also, thioflavin-S positive plaques were 9G3 negative, suggesting that phospho-Tyr18 tau is absent from the dystrophic neurites of the mouse triple transgenic brain.Conclusions-Our results provide evidence for the association of tyrosine-phosphorylated tau with mechanisms of neuropathogenesis and indicate that SFK activation and cell cycle activation are also involved in JNPL3.