Potentiation of Gi-mediated phospholipase C activation by retinoic acid in HL-60 cells: Possible role of gγ2

Abstract

Differentiated HL-60 cells acquire responsiveness to fMet-Leu-Phe (fMLP), which activates phospholipase C and O-2 generation in a pertussis toxin-sensitive manner. Addition of retinoic acid (RA) for the last 24 h during dimethyl sulfoxide (Me2SO)-induced differentiation enhanced fMLP-dependent signals and interaction between fMLP receptor and Gi. RA modifies both the function and subunit composition of Gi2, the predominant Gi of HL-60 membranes, as shown by comparing purified Gi2 from membranes of Me2SO-treated cells (D-Gi2) to Gi2 from membranes of cells treated with both Me2SO and RA (DR-Gi2). As compared to D-Gi2, DR-Gi2 induced more fMLP binding when added to membranes of pertussis toxin-treated HL-60 cells and, in the presence of GTPγS, simulated βγ-sensitive phospholipase C in extracts of HL-60 cells to a much greater extent and at lower concentrations. Immunoblots revealed that RA induced expression of the γ2 subunit, which was otherwise undetectable in Gi2 purified from HL-60 cells or in HL-60 membranes. Possibly by inducing expression of γ2, RA alters two functions of the Gi βγ subunit, modulation of fMLP receptor-Gi2 coupling and activation of the effector, phospholipase C.