GGTase

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Introduction

The mutation frequency of Ras in human cancer is higher than any other oncogene. Therefore, this gene has always been the focus of cancer biologists' research on anti-cancer drugs. In 1989, several research groups showed that Ras proteins are post-translationally modified by 15-carbon farnesyl lipid, and later proved that this lipid plays a very important role in membrane association and transformation. Within a year, farnesyl transferase (FTase) was purified and characterized in the laboratory. Later, the same group discovered a second prenyltransferase, geranylgeranyl transferase type I (GGTase-I). When FTase was blocked, GGTase-I was later shown to modify Ras. Hamilton and Sebti pioneered the development of GGTase-I inhibitor (GGTI).

Prenyltransferases as drug targets

Ras is a member of a class of proteins that terminate in the CAAX sequence. These proteins undergo a series of post-translational modifications such as polyprenylation, proteolysis, and carboxyl methylation. The first rate-limiting step in CAAX processing is catalyzed by one of the two prenyltransferases. FTase and GGTase-I are cytoplasmic heterodimers, they have a common α subunit, but the β subunit is different. Since Ras protein is a substrate of FTase, and the inhibition of FTase can inhibit all subsequent modifications, this enzyme has become an obvious initial target for drug development. In the past 15 years, many pharmaceutical companies have conducted research on FTase and developed many FTase inhibitors (FTI), some of which have reached phase III clinical trials. Although a variety of regulatory proteins, including elements of the visual signal transduction pathway, are substrates of FTase, FTI is very well tolerated. Unfortunately, the efficacy of these drugs on tumors is disappointing.

Alternative prenylation of Ras is just one of the driving forces of GGTI development. GGTI targeting these substrates has now been developed. When used alone, GGTase-I-selective inhibitors have been shown to impair transformation in vitro and tumor growth in vivo. Unlike FTI, GGTI can cause cell cycle arrest in G1 and apoptosis. Interestingly, the severe toxicity of GGTI once predicted is far less significant than imagined. However, the anti-tumor targets of GGTIs are still uncertain, and it is clear that better preclinical models are needed to evaluate the consequences of GGTase-I inhibition in vivo.

Figure 1. Prenylation of CAAX proteins (Philips. M.R.; Cox, A.D. 2007)

GGTase-I deficiency ameliorates K-Ras–driven tumors

The establishment of a genetic model of GGTase-I deficiency will help to further develop the potential of GGTase-I as an anti-cancer drug target. Sjogren and his colleagues generated a conditional knockout allele (Pggt1b) for the β subunit of GGTase-I, in which exon 7 essential for enzyme activity is flanked by loxP sites. How does the GGTase-I deficiency limit the oncogenicity of K-Ras in the model reported by Sjogren et al.? The simplest explanation is the loss of function of one or more CAAX proteins. These proteins require geranylgeranyl to be active and are required for the complete expression of K-Ras-mediated oncogenesis.

The results reported by Sjogren et al. further validated the idea of targeting GGTase-I for anticancer drug discovery. But this research also left us with many unanswered questions. Is the anti-tumor effect of GGTase-I deficiency specific to Ras-dependent tumors? Does the ability to rescue GGTase-I-deficient cells proliferation with few farnesylated substrates that, contrary to previous expectations, many GGTase-I substrates can be alternatively prenylated by FTase in the setting of GGTase-I deficiency? What are the geranyl geranylated proteins required to maximize K-Ras tumorigenesis in this model?

The most important issue is whether the results reported in this new K-Ras-driven lung tumor model will translate into human cancer. However, even if the model proposed in the first report does not provide insight into human cancer, Pggt1b fl/fl mouse can be used by simply breeding with other cancer mouse models in strains that express Cre in a tissue-specific manner. In this way, Pggt1b fl/fl mice may be a valuable tool for evaluating the role of GGTase-I in oncogenesis.