Regulation of G-protein αi2 subunit expression by oxidized low-density lipoprotein

Abstract

Oxidized low-density lipoprotein (LDL) inhibits signalling pathways mediated by pertussis toxin-sensitive guanine nucleotide-binding proteins (Gi proteins). To determine whether this inhibition is due to altered G protein αi subunit expression, mRNA and protein levels of αi isoforms were assessed in bovine aortic endothelial cells treated with oxidized LDL (0-100 μg/ml, 0-72 h). Oxidized LDL did not affect the expression of αi3, but did cause time- and concentration-dependent decrease in αi2 mRNA and protein resulting in a 3.2- and 3.5-fold reduction, respectively, after 72 h. This decrease in αi2 coincided with a 86% decrease in αi2 GTPase activity. Nuclear run-off studies did not show any significant effect of oxidized LDL on αi2 or αi3 transcription. In the presence of actinomycin D, oxidized LDL shortened the t1/2 of αi2 mRNA from 16 h to 8 h which was attenuated by cycloheximide. In addition, pulse-chase labelling with [35S]methionine revealed that oxidized LDL reduced the t1/2 of αi2 protein from 27 to 14 h. Our results indicate that oxidized LDL can modulate receptor-Gi coupling by downregulating the expression of αi2, but not αi3. The mechanism involves both mRNA destabilization and protein degradation.