The RAS superfamily consists of five subfamilies – Ras, Rho, Rab, Arf, and Ran – that contain approximately 150 proteins. Abberant RAS genes are common in a wide number of human cancers. As one of the first human oncogenes identified, point mutations or aberrant transcription of RAS genes is estimated to result in 15-30% of all cancers. From these, it is known that there are three major downstream effector families: Raf, PI3K, and RalGEF.
The Ras-Raf signaling activity is critical to the Ras/MAPK signaling pathway. The Raf protein family is made up of serine/threonine kinases (A-Raf, B-Raf, and C-Raf/Raf-1) that are activated by the binding to GTP-bound Ras. The resultant effect is that the MAPK cascade signaling pathway becomes activated, leading to MEK and ERK phosphorylation that results in the activation of various transcription factors and cell cycle regulatory proteins. For tumors, constitutive activation of RAS leads to cell proliferation and survival.
RAS also shows selectivity for Class I PI3K isoforms. More specifically, while H-RAS, K-RAS, N-RAS, R-RAS, TC21, and M-RAS can activate p110α and p110γ; meanwhile, R-RAS and TC21 only activate the p110δ isoform. It has been suggested that because RAS shows specificity for p110 isoforms, there may be unique consequences of RAS activation in different cell types. As a consequence, strategies to target RAS in oncology will need to consider isoform-specific inhibitors to inhibit PI3K activity where oncogenic RAS mutations are implicated.
Tumors consisting of RAS mutations are capable of exploiting both MAPK and PI3K pathways to result in mitosis, apoptosis, motility, proliferation, and differentiation. Aside from MAPK and PI3K, RAS also interacts with a range of other proteins (p120-GAP, NF1-GAP, MEKK1, AF-6, Nore-1, Rin-1 and Canoe), however, their role in tumor activity is unclear. Interestingly, of all the subfamily proteins only three constituents have been found to be mutated in a number of human cancers (K-RAS, N-RAS, and H-RAS). K-RAS mutations are the most frequently mutated of the three RAS-members, and is implicated especially in pancreatic adenocarcinomas, colorectal tumorigenesis, and lung cancer. In contrast, H-RAS mutations have been shown to be associated in 10-24% of prostate cancer cases (rate varies by demographics), small intestinal tumors, esophageal tumorigensis, and uterine carcinogenesis.
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