Stimulus-response coupling in FMLP-stimulated U937 monocytes: effect of differentiation on Gi2 expression

The effect of differentiation on FMLP-stimulated InsP production and G-protein expression was investigated in U937 monocytes. FMLP (0.01-10 microM) stimulated [3H]InsP production in dimethyl sulphoxide-differentiated, but not in immature, U937 cells. Ionomycin (1 and 10 microM) stimulated [3H]InsP production equally well in both cell types. The FMLP response was blocked by pertussis toxin (100 ng/ml for 4 h) which catalysed [32P]ADP ribosylation of a 40 kDa 'Gi-like' G-protein alpha subunit in these cells. This protein was also identified immunologically using anti-peptide antibodies that detect 'Gi-like' alpha subunits (SG2) or Gi2 alpha specifically (LE2). With LE2 a 5-fold increase in Gi2 alpha levels was seen following differentiation of the cells, suggesting that FMLP receptor expression is accompanied by an increase in the G-protein with which these receptors interact.