Inhibition of geranylgeranyl diphosphate synthase is a novel therapeutic strategy for pancreatic ductal adenocarcinoma

Rab proteins play an essential role in regulating intracellular membrane trafficking processes. Rab activity is dependent upon geranylgeranylation, a post-translational modification that involves the addition of 20-carbon isoprenoid chains via the enzyme geranylgeranyl transferase (GGTase) II. We have focused on the development of inhibitors against geranylgeranyl diphosphate synthase (GGDPS), which generates the isoprenoid donor (GGPP), as anti-Rab agents. Pancreatic ductal adenocarcinoma (PDAC) is characterized by abnormal mucin production and these mucins play important roles in tumor development, metastasis and chemo-resistance. We hypothesized that GGDPS inhibitor (GGDPSi) treatment would induce PDAC cell death by disrupting mucin trafficking, thereby inducing the unfolded protein response pathway (UPR) and apoptosis. To this end, we evaluated the effects of RAM2061, a potent GGDPSi, against PDAC. Our studies revealed that GGDPSi treatment activates the UPR and triggers apoptosis in a variety of human and mouse PDAC cell lines. Furthermore, GGDPSi treatment was found to disrupt the intracellular trafficking of key mucins such as MUC1. These effects could be recapitulated by incubation with a specific GGTase II inhibitor, but not a GGTase I inhibitor, consistent with the effect being dependent on disruption of Rab-mediated activities. In addition, siRNA-mediated knockdown of GGDPS induces upregulation of UPR markers and disrupts MUC1 trafficking in PDAC cells. Experiments in two mouse models of PDAC demonstrated that GGDPSi treatment significantly slows tumor growth. Collectively, these data support further development of GGDPSi therapy as a novel strategy for the treatment of PDAC.