Lysophosphatidylcholine and reactive oxygen species mediate the synergistic effect of mildly oxidized LDL with serotonin on vascular smooth muscle cell proliferation

Background-Mild oxidation of LDL enhances its atherogenic potential and ind uces a synergistic interaction with serotonin (5HT) on vascular smooth musc le cell (VSMC) proliferation. Because of its complex chemical nature, the m itogenic components of mildly oxidized LDL (moxLDL) remain unclear. Methods and Results-We examined both the effects of lysophosphatidylcholine (LPC) and hydrogen peroxide (H2O2), a donor of reactive oxygen species, as major components of moxLDL and their interactions with 5HT on VSMC prolife ration. Growth-arrested VSMCs were incubated with different concentrations of moxLDL, LPC, H2O2, or LPC with H2O2 in the absence or presence of 5HT. D NA synthesis in VSMCs was examined by [H-3]thymidine incorporation. MoxLDL, LPC, H2O2, and 5HT stimulated DNA synthesis in a dose-dependent manner. Mo xLDL had a maximal stimulatory effect at a concentration of 5 mug/mL (211%) , LPC at 15 mu mol/L (156%), H2O2 at 5 mu mol/L (179%), and 5HT at 50 mu mo l/L (205%). Added together, moxLDL (50 ng/mL) and 5HT (50 mu mol/L) synergi stically increased DNA synthesis (443%). Coincubation of LPC (1 mu mol/L) w ith H2O2 (0.5 mu mol/L) and 5HT (5 mu mol/L) resulted in a synergistic incr ease in DNA synthesis (439%), which was nearly equal to that of moxLDL with 5HT (443%), The combined effects of LPC, H2O2, and 5HT on DNA synthesis we re completely reversed by the combined use of an antioxidant, N-acetylcyste ine (400 mu mol/L) or butylated hydroxytoluene (20 mu mol/L), with a 5HT(2) receptor antagonist, LY281067 (10 mug/mL). Conclusions-Our results suggest that both LPC and reactive oxygen species m ay contribute to the mitogenic effect of moxLDL on VSMCs and its synergisti c effect with 5HT.