Expression of Gα(i2) mimics several aspects of LPS priming in a murine macrophage-like cell line

Priming of macrophages with low concentrations of lipopolysaccharide (LPS) enhances the ability of substances that act through heterotrimeric G proteins to stimulate immune cell functions. Although LPS-induced alterations in the expression and functions of G proteins of the α(i) family have been reported in hematopoietic cells, their effects on subsequent steps in LPS priming of macrophages have not been defined. To study the role of Gα(i2) in priming of macrophages by LPS, we expressed a mutant, activated form of α(i2) (α(i2Q2051)) in P388D₁ cells, and compared its effects on PAF-dependent Ca signalling and arachidonic acid release to those in cells treated with LPS. In control P388D₁ cells, treatment with LPS (100 ng/ml) for 1 hr increased the amount of α(i2) protein 2-fold. Both LPS treatment and expression of α(i2Q2051) increased the rate of PAF-induced Ca influx across the cell membrane and arachidonic acid release, although neither altered release of Ca from intracellular stores by PAF. Expression of α(i2Q2051) is sufficient to mimic the effects of LPS on the PAF-induced Ca(i) signal and enhanced arachidonic acid release. Consequently, although increasing the expression of α(i2), may not be the sole mechanism by which LPS enhances signalling by PAF, increased α(i2) expression can account for the alterations in PAF- induced Ca(i) regulation, and arachidonic acid release in LPS-primed P388D₁ cells.