Signal transduction in human alveolar macrophages: diminished chemotactic response to FMLP correlates with a diminished density of Gi proteins and FMLP receptors

Alveolar macrophages (AM) migrate less well in response to chemotactic ligands than do monocytes and neutrophils. The response of monocytes and neutrophils to chemotactic ligands is mediated at least in part by pertussis toxin-sensitive guanine nucleotide binding proteins (Gi proteins). Whether this is also true in AM is uncertain. We hypothesized that decreased chemotaxis by AM was due in part to diminished Gi protein and/or chemotactic receptor density in AM. G proteins are heterotrimers made up of alpha, beta, and gamma subunits; the predominant pertussis toxin-sensitive Gi proteins are those containing alpha i2 or alpha i3 subunits. Pertussis toxin pretreatment (0.5 microgram/ml) significantly reduced AM, monocyte, and neutrophil chemotaxis to N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) and human zymosan-activated serum (P less than 0.05). However, as previously noted, AM chemotaxis was much less than that observed in monocytes and neutrophils. Immunoblots using antibodies that are specific for alpha i2 and alpha i3 showed that AM contained approximately 3-fold less alpha i2 and approximately 10-fold less alpha i3 per microgram of plasma membrane protein than did monocytes or neutrophils. Similar results were obtained in immunoblots made using antibodies to common alpha subunit determinants and to the beta 36 subunit. A comparable approximately 4-fold reduction in density of receptors for [3H]FMLP was found in AM compared to neutrophils. The diminished density of Gi proteins and FMLP receptors was not due to a generally decreased density of plasma membrane proteins in AM, since the density of the membrane-associated tyrosine kinase hck was similar in AM, monocytes, and neutrophils.(ABSTRACT TRUNCATED AT 250 WORDS)