REGULATION OF G-PROTEIN ALPHA(I2) SUBUNIT EXPRESSION BY OXIDIZED LOW-DENSITY-LIPOPROTEIN

Oxidized low-density lipoprotein (LDL) inhibits signalling pathways mediated by pertussis toxin-sensitive guanine nucleotide-binding proteins (G(i) proteins). To determine whether this inhibition is due to altered G protein alpha(i) subunit expression, mRNA and protein levels of alpha(i) isoforms were assessed in bovine aortic endothelial cells treated with oxidized LDL (0-100 mu g/ml, 0-72 h). Oxidized LDL did not affect the expression of alpha(i3), but did cause time- and concentration-dependent decrease in alpha(i2) mRNA and protein resulting in a 3.2- and 3.5-fold reduction, respectively, after 72 h. This decrease in alpha(i2) coincided with a 86% decrease in alpha(i2) GTPase activity, Nuclear run-off studies did not show any significant effect of oxidized LDL on alpha(i2) or alpha(i3) transcription. In the presence of actinomycin D, oxidized LDL shortened the t(1/2) of alpha(i2) mRNA from 16 h to 8 h which was attenuated by cycloheximide. In addition, pulse-chase labelling with [S-35]methionine revealed that oxidized LDL reduced the t(1/2) of alpha(i2) protein from 27 to 14 h. Our results indicate that oxidized LDL can modulate receptor-G(i) coupling by downregulating the expression of alpha(i2) but not alpha(i3). The mechanism involves both mRNA destabilization and protein degradation.