Expression of G alpha 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 alpha 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 G alpha i2 in priming of macrophages by LPS, we expressed a mutant, activated form of alpha i2 (alpha i2Q205L) in P388D1 cells, and compared its effects on PAF-dependent C alpha 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 alpha i2 protein 2-fold. Both LPS treatment and expression of alpha i2Q205L increased the rate of PAF-induced C alpha influx across the cell membrane and arachidonic acid release, although neither altered release of C alpha from intracellular stores by PAF. Expression of alpha i2Q205L is sufficient to mimic the effects of LPS on the PAF-induced C alpha i signal and enhanced arachidonic acid release. Consequently, although increasing the expression of alpha i2 may not be the sole mechanism by which LPS enhances signalling by PAF, increased alpha i2 expression can account for the alterations in PAF-induced C alpha i regulation, and arachidonic acid release in LPS-primed P388D1 cells.