Regulation of the Phosphoinositide Cascade by Polyamines in Brain

Abstract: The endogenous polyamines spermidine and spermine enhanced guanosine 5′-O-(3-thiotriphosphate) (GTP-γ-S)-stimulated phosphoinositide turnover with EC₅₀ values of 100 ± 30 and 50 ± 15 µM, respectively, whereas the synthetic polyamines N,N′-bis(3-aminopropyl)-1,3-propanediamine and -ethylenediamine inhibited GTP-γ-S-stimulated phosphoinositide turnover, with maximal inhibition at 1 mM. Kinetic analysis of GTP-γ-S-stimulated phosphoinositide turnover in the absence and presence of spermidine showed that the K_m for GTP-γ-S was not changed (1,303 ± 270 and 1,069 ± 214 nM, respectively), whereas the V_max was increased by 206% (1,566 ± 141 and 4,792 ± 84 cpm, respectively), indicating that spermidine and GTP-γ-S acted at different sites. Spermidine also enhanced Ca²⁺-stimulated phosphoinositide turnover in the absence of GTP-γ-S by decreasing the Ca²⁺ requirement of the phosphoinositide-specific phospholipase C. Arcaine and agmatine, polyamine antagonists at the NMDA receptor complex, did not block the effects of spermidine on GTP-γ-S- and Ca²⁺-induced phosphoinositide turnover, suggesting that the spermidine effects are not mediated through these specific polyamine sites. Furthermore, spermidine increased the level of [³H]phosphatidylinositol 4-phosphate (EC₅₀ = 120 ± 10 µM), without affecting significantly the levels of [³H]phosphatidylinositol and [³H]phosphatidylinositol 4,5-bisphosphate. Collectively these data indicate that the enhanced phosphoinositide turnover induced by spermidine in the presence of GTP-γ-S or Ca²⁺ is mediated through multiple levels of the phosphoinositide turnover cascade.