The isoprenoid donor for protein geranylgeranylation reactions, geranylgeranyl diphosphate (GGDP), is the product of the enzyme GGDP synthase (GGDPS) that condenses farnesyl diphosphate (FDP) and isopentenyl pyrophosphate. GGDPS inhibition is of interest from a therapeutic perspective for multiple myeloma because we have shown that targeting Rab GTPase geranylgeranylation impairs monoclonal protein trafficking, leading to endoplasmic reticulum stress and apoptosis. We reported a series of triazole bisphosphonate GGDPS inhibitors, of which the most potent was a 3:1 mixture of homogeranyl (HG) and homoneryl (HN) isomers. Here we determined the activity of the individual olefin isomers. Enzymatic and cellular assays revealed that although HN is approximately threefold more potent than HG, HN is not more potent than the original mixture. Studies in which cells were treated with varying concentrations of each isomer alone and in different combinations revealed that the two isomers potentiate the induced-inhibition of protein geranylgeranylation when used in a 3:1 HG:HN combination. A synergistic interaction was observed between the two isomers in the GGDPS enzyme assay. These results suggested that the two isomers bind simultaneously to the enzyme but within different domains. Computational modeling studies revealed that HN is preferred at the FDP site, that HG is preferred at the GGDP site, and that both isomers may bind to the enzyme simultaneously. These studies are the first to report a set of olefin isomers that synergistically inhibit GGDPS, thus establishing a new paradigm for the future development of GGDPS inhibitors.
- Received November 3, 2016.
- Accepted December 28, 2016.
This project was supported by the National Institutes of Health National Cancer Institute [Grant R01 CA172070], the American Society of Hematology, and the Roy J. Carver Charitable Trust.
Portions of these data were presented as abstracts at the following meeting: Allen C, Metzger JI, Wills VS, Tong H, Kortagere S, Wiemer DF, and Holstein SA (2016) Synergistic inhibition of geranylgeranyl diphosphate synthase by a mixture of olefin stereoisomers. American Society of Hematology Annual Meeting; 2016 Dec 3-6; San Diego, CA. American Society of Hematology, Washington.
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