Molecular mechanisms linking geranylgeranyl diphosphate synthase to cell survival and proliferation

Abstract

Geranylgeranyl diphosphate is a 20-carbon isoprenoid phospholipid whose lipid moiety can be post-translationally incorporated into proteins to promote membrane association. The process of geranylgeranylation has been implicated in anti-proliferative effects of clinical agents that inhibit enzymes of the mevalonate pathway (i.e. statins and nitrogenous bisphosphonates) as well as experimental agents that deplete geranylgeranyl diphosphate. Inhibitors of geranylgeranyl diphosphate synthase are an attractive way to block geranylgeranylation because they possess a calcium-chelating substructure to allow localization to bone and take advantage of a unique position of the enzyme within the biosynthetic pathway. Here, we describe recent advances in geranylgeranyl diphosphate synthase expression and inhibitor development with a particular focus on the molecular mechanisms that link geranylgeranyl diphosphate to cell proliferation via geranylgeranylated small GTPases.

Keywords: Bisphosphonates; GGDPS; Rac; Ras; Rho; small GTPase.

Figure 1. Location of geranylgeranyl diphosphate synthase (GGDPS) within the human isoprenoid biosynthetic pathway

The first and rate-limiting step of human isoprenoid synthesis is the conversion of HMG-CoA to mevalonate by HMG-CoA reductase (HMGCR). This enzyme is the molecular target of the statin drugs. Several steps downstream, farnesyl diphosphate synthase (FDPS) catalyzes the production of farnesyl diphosphate (FPP) from dimethylallyl diphosphate DMAPP and two equivalents of isopentenyl diphosphate (IPP). This enzyme is the molecular target of the nitrogenous bisphosphonates. Farnesyl diphosphate is a key branch point of isoprenoid metabolism, giving rise to squalene in a reaction catalyzed by squalene synthase (SQS) and geranylgeranyl diphosphate (GGPP) in the GGDPS reaction.

Figure 2. Key feedback mechanisms that control isoprenoid biosynthetic gene expression at the transcriptional level

A) Sterol regulatory element-binding proteins (SREBPs) regulate transcription of HMG-CoA reductase and other enzymes of isoprenoid biosynthesis in response to cellular sterol levels, which indirectly affects synthesis of geranylgeranyl diphosphate. B) Egr-1 regulates transcription of isoprenoid biosynthesis in response to activation of the ERK pathway, which directly regulates GGPDS expression at the transcriptional level.

Figure 3. Structural evolution of GGDPS inhibitors

Small nitrogen-containing bisphosphonates are inhibitors of farnesyl diphosphate synthase (FDPS). Lipophilic bisphosphonates can inhibit the enzymes of farnesyl diphosphate metabolism, including GGDPS and squalene synthase. Branched bisalkyl bisphosphonates retain specificity for GGDPS. Non bisphosphonate inhibitors of GGDPS have also been identified.

Figure 4. Geranylgeranylation of small GTPases including Rac, Rho, Rab, and Cdc42 promotes localization to the membrane domain, affecting proliferation

Geranylgeranyl transferase I catalyzes the addition of geranylgeranyl diphosphate to the C-terminus of Rho, Rac, or Cdc42 while geranylgeranyl transferase II catalyzes the addition of one or two geranylgeranyl moieties to Rab GTPases. Rab GTPases are primarily involved in vesicle trafficking, though disruption of Rab geranylgeranylation can affect cell viability and proliferation. Rho, Rac, and Cdc42 are involved in proliferation in addition to adhesion and migration regulation.