Regulation of the surface expression of the platelet-activating factor receptor in IC-21 peritoneal macrophages. Effects of lipopolysaccharide.

The effect of bacterial lipopolysaccharide (LPS) on the expression of the receptor for platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine; AGEPC) was examined in cultured IC-21 peritoneal macrophages. AGEPC binding to its receptors reached saturation within 20 min at 25 degrees C and was reversible. Scatchard analysis revealed a single class of AGEPC receptors with a Bmax of approximately 170 fmol/mg cellular protein and a Kd of 0.25 nM. Preincubation of IC-21 cells with LPS (0.01-1,000 ng/ml) induced an increase in the surface expression of AGEPC receptors in a time- and concentration-dependent fashion. The maximal effect of LPS on the AGEPC receptor was observed between 5 and 8 h, with a typical increase between 150 and 200%. Scatchard analysis indicated that LPS treatment of IC-21 cells increased the number of AGEPC receptors on the cell surface without any apparent change in the affinity of the receptor for the ligand. The effect of LPS on the surface expression of the AGEPC receptor was nearly abolished by cycloheximide (0.1 mM) and by actinomycin D (3 microM), suggesting the involvement of enhanced receptor protein synthesis and mRNA production in this event. Moreover, LPS treatment increased the capability of the IC-21 cell to respond to AGEPC addition by elevating intracellular free Ca2+ without causing an increase in the basal level of intracellular Ca2+. The present study demonstrates that IC-21 peritoneal macrophages possess high affinity AGEPC receptors and provides the evidence that the number of functional AGEPC receptors on a cell can be increased significantly upon exposure to LPS.