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.

Tumorigenesis driven by the oncogene BRAFV600E is shown both to depend on the BRAF substrates MEK1/2 associating with copper, and to be sensitive to copper-chelating drugs, suggesting merit in testing such drugs for the treatment of BRAF mutation-positive cancers. A large proportion of melanomas and some other cancers harbour mutations in the BRAF gene, most of them at codon 600, causing constitutive activation of the MAPK (mitogen-activated protein kinase) pathway. Following the discovery that copper transport promotes MAPK signalling in Drosophila by binding to and activating the kinase MEK, Chris Counter and colleagues now show that oncogenic signalling by mutant BRAF requires copper binding to MEK, promoting activation of ERK1/2, the next kinases in the cascade. Interfering with copper availability by genetic means or with copper-chelating agents reduces BRAF-driven tumour growth in vivo in mouse models, and also of cancer cells that have become resistant to BRAF inhibitors. Thus copper chelators, already in the clinic for other indications, may prove useful for the treatment of BRAF-mutant tumours in combination with BRAF inhibitors, and potentially to prevent resistance. The BRAF kinase is mutated, typically Val 600→Glu (V600E), to induce an active oncogenic state in a large fraction of melanomas, thyroid cancers, hairy cell leukaemias and, to a smaller extent, a wide spectrum of other cancers1,2. BRAFV600Ephosphorylates and activates the MEK1 and MEK2 kinases, which in turn phosphorylate and activate the ERK1 and ERK2 kinases, stimulating the mitogen-activated protein kinase (MAPK) pathway to promote cancer3. Targeting MEK1/2 is proving to be an important therapeutic strategy, given that a MEK1/2 inhibitor provides a survival advantage in metastatic melanoma4, an effect that is increased when administered together with a BRAFV600Einhibitor5. We previously found that copper (Cu) influx enhances MEK1 phosphorylation of ERK1/2 through a Cu–MEK1 interaction6. Here we show decreasing the levels of CTR1 (Cu transporter 1), or mutations in MEK1 that disrupt Cu binding, decreased BRAFV600E-driven signalling and tumorigenesis in mice and human cell settings. Conversely, a MEK1–MEK5 chimaera that phosphorylated ERK1/2 independently of Cu or an active ERK2 restored the tumour growth of murine cells lacking Ctr1. Cu chelators used in the treatment of Wilson disease7decreased tumour growth of human or murine cells transformed by BRAFV600E or engineered to be resistant to BRAF inhibition. Taken together, these results suggest that Cu-chelation therapy could be repurposed to treat cancers containing the BRAFV600Emutation.

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 (∼1%) in lung and colorectal cancers, 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 PI3K (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. Significance: We determined that KRAS^G12R is impaired in activating a key effector, p110α PI3K. As such, KRAS^G12R is impaired in driving macropinocytosis. However, overexpression of PI3Kγ in PDAC compensates for this deficiency, providing one basis for the prevalence of this otherwise rare KRAS mutant in pancreatic cancer but not other cancers. See related commentary by Falcomatà et al., p. 23. This article is highlighted in the In This Issue feature, p. 1

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.

While numerous studies support regulation of Ras GTPases by reactive oxygen and nitrogen species, the Rho subfamily has received considerably less attention. Over the last few years, increasing evi...