Targeting oncogenic Ras signaling in hematologic malignancies

Abstract

Ras proteins are critical nodes in cellular signaling that integrate inputs from activated cell surface receptors and other stimuli to modulate cell fate through a complex network of effector pathways. Oncogenic RAS mutations are found in ∼25% of human cancers and are highly prevalent in hematopoietic malignancies. Because of their structural and biochemical properties, oncogenic Ras proteins are exceedingly difficult targets for rational drug discovery, and no mechanism-based therapies exist for cancers with RAS mutations. This article reviews the properties of normal and oncogenic Ras proteins, the prevalence and likely pathogenic role of NRAS, KRAS, and NF1 mutations in hematopoietic malignancies, relevant animal models of these cancers, and implications for drug discovery. Because hematologic malignancies are experimentally tractable, they are especially valuable platforms for addressing the fundamental question of how to reverse the adverse biochemical output of oncogenic Ras in cancer.

Figure 1

The Ras switch. Ras proteins are switches that relay signals initiated when transmembrane receptors bind ligand. Activated receptors recruit GEFs by assembly of multiprotein complexes (eg, including SOS) or more indirectly, by evoking lipid modifications that recruit GEFs, such as RasGRPs, to cytosolic membrane surfaces. GEFs promote exchange of GDP for GTP on Ras. When bound to GTP, Ras adopts a conformation in which the “switch” regions are stabilized and can interact productively with various downstream effectors. Thus, Ras connects extracellular stimuli to intracellular networks that compute and execute cell fate decisions. The Ras signal is terminated by GTP hydrolysis, which is largley dependent on GAPs, such as neurofibromin (NF1) or p120 RasGAP.

Figure 2

Ras processing and trafficking to subcellular compartments. H-Ras, N-Ras, K-Ras4a, and K-Ras4b proteins are identical in the first 85 amino acids, a region that includes the P loop (phosphate binding loop, amino acids 10-16), which binds the γ-phosphate of GTP, and the switch I (amino acids 30-38) and switch II (amino acids 60-76) regions, which regulate binding to Ras regulators and effectors. The next 78 amino acids show ∼ 85%-90% sequence homology. Amino acids that are shared by all isoforms are depicted in light gray, those that differ are depicted in dark gray. The final 24 (23 for K-Ras4b) amino acids, called the hypervariable region (HVR), specify posttranslational modifications and trafficking for each Ras isoform. All 4 isoforms have a C-terminal —CAAX motif, which is farnesylated (F) by FTase. The —AAX is removed by Ras-converting enzyme 1 (RCE1) and the cysteine methylated (M) by ICMT. K-Ras4b is then shuttled directly to the PM where it is stabilized by its polylysine domain (KKKKKK). The other 3 isoforms are shuttled to the Golgi apparatus, where they are palmitoylated (P) at one or more cysteines near the C-terminus before reaching the plasma membrane. On the membrane, H-Ras, N-Ras, and K-Ras4a can be depalmitoylated by acyl protein thioesterases 1 and 2 (APT1/APT2), directing them back to the Golgi. This palmitoylation-depalmitoylation, Golgi-plasma membrane cycle continues in a delicate balance until the proteins are degraded.