Inhibition of the mevalonate pathway by simvastatin interferes with mast cell degranulation by disrupting the interaction between Rab27a and double C2 alpha proteins.

Statins are well-known inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, which block the mevalonate pathway. The activity of statins not only decreases cholesterol levels but also ameliorates inflammation and modulates the immune system. In this study, we investigated the effects of simvastatin on histamine release using rat basophilic leukaemia (RBL-2H3) cells, and examined its interaction with proteins involved in the exocytosis process. Treatment with simvastatin for 24h inhibited histamine release in RBL-2H3 cells in a concentration-dependent manner after stimulation with dinitrophenylated bovine serum albumin (DNP-BSA, as an antigen), ionomycin (a calcium ion [Ca] ionophore), and thapsigargin (an inhibitor of Ca-ATPase in the endoplasmic reticulum). Simvastatin-induced inhibition was counteracted by co-administration of mevalonolactone or geranylgeraniol, but not farnesol. Indeed, several exocytotic proteins were post-translationally modified by isoprenylation, which is required for proper localization in the lipid membrane. RBL-2H3 cells express proteins involved in the fusion of granules and the plasma membrane, such as Ras-like protein in the brain 27a (Rab27a), synaptosome-associated protein 23 (SNAP23), and vesicle-associated membrane protein 7 (VAMP7), as well as Ca binding proteins, such as double C2 alpha (Doc2a), synaptotagmin2, and mammalian uncoordinated13-4 (munc13-4). The interaction of Rab27a and Doc2a proteins was detected using proximity ligation assays. Antigen stimulation caused these proteins to interact, and this interaction could be disrupted by co-administration of simvastatin. In conclusion, simvastatin inhibited the mevalonate pathway, which suppressed the geranylgeranylation of Rab27a by depleting geranylgeranyl pyrophosphate and interfering with the Rab27a-Doc2a interaction. This activity resulted in the inhibition of exocytosis in RBL-2H3 cells.