Guanosine triphosphatase stimulation of oncogenic Ras mutants

Substrate assisted catalysis -- application to G proteins

The idea that both the substrate and the enzyme contribute to catalysis (substrate assisted catalysis; SAC) is applicable to guanine nucleotide-binding proteins (G proteins). Naturally occurring SAC uses GTP as a general base in the GTPase reaction catalyzed by G proteins. Engineered SAC has identified a putative rate-limiting step for the GTPase reaction and shown that GTPase-deficient oncogenic Ras mutants are not irreversibly impaired. Thus, anti-cancer drugs could potentially be designed to restore the blocked GTPase reaction.

Small-molecule inhibitors of cell signaling

Small-molecule inhibitors of several intracellular signaling proteins, mostly protein kinases, show tremendous selectivity and potency. The complexity and redundancy of signaling pathways presents opportunities for therapeutic selectivity and some clinical results are remarkable. New strategies are being developed to interfere with previously intractable targets, such as protein-protein interactions.

RAS inhibitors: potential for cancer therapeutics

As RAS oncoproteins play a major role in human malignancy, inhibiting RAS function is a promising approach for developing anticancer therapies. Among these approaches are agents such as farnesyltransferase inhibitors (FTIs) and the nontoxic farnesylcysteine analogue farnesylthiosalicylic acid (FTS) that dislodges all RAS isoforms from the membrane, as well as methods to restore regulation of RAS-GTP levels and to alter the interaction of RAS-GTP with downstream targets.

Regulation of GTPases in the bacterial translation machinery

Several GTPases participate in bacterial protein biosynthesis. Initiation factor 2 controls the formation of the ribosomal initiation complex and places initiator fMet-tRNAfMet in the ribosomal P-site. Elongation factors Tu and G are responsible for codon-specific binding of the aminoacyl-tRNA to the A-site, and peptidyl-tRNA to the P-site, respectively, during the elongation phase of protein biosynthesis. Release factor 3, a GTPase which is not ubiquitous, is involved in termination and release of the nascent polypeptide. Other translation factors, including initiation factors 1 and 3, elongation factor Ts, release factors 1 and 2, and ribosomal release factor do not belong to the family of GTP/GDP binding proteins. The guanosine nucleotide binding domains of the GTPases involved in translation are structurally related to the Galpha subunit of heterotrimeric G proteins and to the proteins of the Ras family. We have identified and sequenced all genes coding for translation factors in the extreme thermophile Thermus thermophilus. The proteins were overproduced in Escherichia coli, purified, biochemically characterised and used for crystallisation and structural analysis. Further biochemical investigations were aimed at gaining insight into the molecular mechanism underlying the regulation of the GTPase activity of the translation factors, and to elucidate the role of their ribosomal binding sites in this process.

Concepts in Ras-directed therapy

Ras proteins are key transducers of growth signals regulated by cell surface receptors. They are anchored to the inner surface of the cell membrane where receptor-mediated signalling induces Ras activation (GDP/GTP exchange) and inactivation (stimulation of Ras GTPase activity). Ras-GTP in turn activates a multitude of signalling cascades controlling cell growth and differentiation. Aberrant Ras function (mostly constitutive activation) contributes to the development of many types of neoplastic human diseases. Activating mutations in ras genes, leading to the expression of Ras proteins insensitive to Ras-GTPase activating proteins, are found in as many as 30% of all human tumours. This suggests that Ras is an appropriate target for drug design. Remarkable improvements in the understanding of post-translational modifications in Ras that promote Ras-membrane anchorage, in the mechanisms of activation and inactivation of Ras, and in the interactions of Ras with a plethora of effector molecules have led to the development of new concepts for Ras-directed therapy. The most advanced approach has been that of farnesyltransferase inhibitors (FTIs) designed to inhibit the farnesylation of Ras required for membrane anchorage and transforming activity. FTIs now in clinical trials have been extensively reviewed. Here we review the progress in the development of FTIs and in the development of other promising concepts for Ras-directed therapy. These include compounds such as S-farnesylthiosalicylic acid (FTS), which disrupt the proper anchorage of Ras with the cell membrane and inhibit human tumour growth in animal models, and compounds that interfere with interactions of Ras with its downstream effectors. We conclude with a description of a recently described novel drug concept that could restore the defective GTPase activity of oncogenic Ras and with the interesting results of reovirus-induced tumour regression observed in animal models of human tumours containing an intact Ras signalling pathway.