P-glycoprotein transport of neurotoxic pesticides

P-glycoprotein (P-gp) has been associated with a number of neurodegenerative diseases, including Parkinson's disease, although the mechanisms remain unclear. Altered transport of neurotoxic pesticides has been proposed in Parkinson's disease, but it is unknown whether these pesticides are P-gp substrates. We used three in vitro transport models, stimulation of ATPase activity, xenobiotic-induced cytotoxicity, and inhibition of rhodamine-123 efflux, to evaluate P-gp transport of diazinon, dieldrin, endosulfan, ivermectin, maneb, 1-methyl-4-phenyl-4-phenylpyridinium ion (MPP⁺), and rotenone. Diazinon and rotenone stimulated ATPase activity in P-gp-expressing membranes, with V_max values of 22.4 ± 2.1 and 16.8 ± 1.0 nmol inorganic phosphate/min per mg protein, respectively, and K_m values of 9.72 ± 3.91 and 1.62 ± 0.51 mM, respectively, compared with the P-gp substrate verapamil, with a V_max of 20.8 ± 0.7 nmol inorganic phosphate/min per mg protein and K_m of 0.871 ± 0.172 mM. None of the other pesticides stimulated ATPase activity. We observed an increased resistance to MPP¹ and rotenone in LLC-MDR1-WT cells compared with LLC-vector cells, with 15.4- and 2.2-fold increases in EC₅₀ values, respectively. The resistance was reversed in the presence of the P-gp inhibitor verapamil. None of the other pesticides displayed differential cytotoxicity. Ivermectin was the only pesticide to inhibit P-gp transport of rhodamine-123, with an IC₅₀ of 0.249 ± 0.048 mM. Our data demonstrate that dieldrin, endosulfan, and maneb are not P-gp substrates or inhibitors. We identified diazinon, MPP¹, and rotenone as P-gp substrates, although further investigation is needed to understand the role of P-gp transport in their disposition in vivo and associations with Parkinson's disease.