Tolerance to LPS decreases macrophage G protein content

The effects of tolerizing doses of LPS on mRNA and protein levels of three different G protein subunits were investigated to understand the mechanism( s) responsible for the reduction in Gi alpha protein content in LPS toleran ce. Tolerance was induced in rats using Salmonella enteritidis LPS (intrape ritoneal route) with a single dose of 100 mu g/kg. Peritoneal macrophages w ere harvested 6 and 24 h later. In some studies, a second dose of LPS 500 m u g/kg was given on the following day, and peritoneal macrophages were harv ested 5 days after the first injection. Macrophage RNA or a crude membrane fraction was prepared from macrophages, and the mRNA level or the protein c ontent for Gi(alpha 3), Gi(alpha 2), and Gs(alpha) was analyzed using North ern or Western blots, respectively. Compared with the control levels, the m essage for Gi(alpha 3) was reduced (p < .025) at 6 and 24 h and 5 day time periods after LPS treatment. The Gi(alpha 2) mRNA was increased relative to the control levels (p < .05) at 6 h and 5 days after LPS treatment, respec tively, and Gs alpha message was not significantly changed. The half-life o f Gi(alpha 3) mRNA was not significantly different in control versus tolera nt macrophages. The Gi(alpha 3) mRNA and membrane protein were not signific antly changed by incubation with LPS for intervals up to 6 h in vitro. Macr ophage membrane Gi(alpha 3) and Gi(alpha 1) and 2 protein content from tole rant rats were reduced compared with the controls at 6 and 24 h, respective ly (p < .05). These studies are consistent with our previous observations o f selective changes in macrophage Gi, protein content in LPS tolerance and raise the possibility that this may affect signal transduction events in th ese cells.