LIPOPEPTIDES ACTIVATE G(I)-PROTEINS IN DIBUTYRYL CYCLIC AMP-DIFFERENTIATED HL-60 CELLS

Synthetic lipopeptides activate superoxide-anion (O2-) formation in hu man neutrophils in a pertussis-toxin (PTX)-sensitive manner, suggestin g the involvement of G-proteins of the G(i) family in the signal-trans duction pathway. We compared G-protein activation by lipopeptides and the chemotactic peptide N-formylmethionyl-leucyl-phenylalanine (fMLP) in dibutyryl-cyclic-AMP-differentiated HL-60 cells. The lipopeptide to ylamino-6-palmitoyloxymethyl-7-palmitoyloxyhepta noyl-SK4 (Pam3AhhSK4) and fMLP activated high-affinity GTPase, i.e. the enzymic activity of G-protein alpha-subunits, in HL-60 membranes in a time- and protein-d ependent manner, but they had no effect on Mg2+-ATPase and Na+/K+-ATPa se. Pam3AhhSK4 and fMLP increased V(max) of GTP hydrolysis. Pam3AhhSK4 activated GTP hydrolysis with half-maximal and maximal effects at abo ut 2 muM and 10 muM respectively. Other lipopeptides activated GTP hyd rolysis as well. Lipopeptides were less effective than fMLP to activat e GTPase. In membranes from PTX-treated cells, the stimulatory effects of lipopeptides and fMLP on GTPase were abolished. In N-ethylmaleimid e-treated membranes, the relative stimulatory effect of Pam3AhhSK4 on GTP hydrolysis was enhanced, whereas that of fMLP was diminished. fMLP and Pam3AhhSK4 activated GTPase in an over-additive manner in N-ethyl maleimide-treated membranes. Unlike fMLP, Pam3AhhSK4 did not enhance i ncorporation of GTP azidoanilide into, and cholera-toxin-catalysed ADP -ribosylation of G(i)-protein alpha-subunits in, HL-60 membranes and d id not induce rises in cytosolic Ca2+ concentration. Pam3AhhSK4 and fM LP stimulated phosphatidic acid formation in a PTX-sensitive manner. P am3AhhSK4 itself did not activate O2-formation, but potentiated the st imulatory effects of fMLP. Our data suggest that (i) lipopeptides acti vate the GTPase of G(i)-proteins, (ii) lipopeptides and fMLP activate G(i)-proteins differently, (iii) lipopeptides stimulate phospholipase D via G(i)-proteins, and (iv) phosphatidic acid formation is not suffi cient for activation of O2- formation.