Protein myristoylation in human mononuclear phagocytes: Modulation by interferon-γ and tumor necrosis factor-α

Labelling of cells with [H]myristic acid and analysis of labelled proteins by SDS-PAGE and fluorography, enabled the identification of a limited number of myristoylated proteins in human monocytes and monocyte-derived macrophages. In human monocytes, cultivated for one to three days, major myristoylated proteins observed were of 18 kDa, 44 kDa, 60-62 kDa, 90 kDa, and a doublet of 38-40 kDa. Differentiation of monocytes to macrophages by in vitro cultivation was accompanied by a selective decrease in the 60-62 kDa protein. Cultivation of the cells in the presence of the macrophage-activating cytokines interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α), prevented the decrease in the expression of the 60-62 kDa myristoylated protein. The effect of cytokines was observed when monocytes were treated with IFN-γ or TNF-α for 24 or 48 h and protein myristoylation analyzed at day four of culture. Maintenance of monocytes in culture for up to nine days in the presence of cytokines prevented the decrease in the expression of the 60-62 kDa myristoylated protein. IFN-γ had additional effects on myristoylation of macrophage proteins. Treatment of monocytes with IFN-γ for a few hours caused the induction of a 66 kDa protein. Induction of this myristoylated protein by IFN-γ was time-dependent and peaked at six hours. Analysis of the subcellular distribution of the 66 kDa protein induced by IFN-γ showed that, analogously to other myristoylated proteins, most of it was associated with cell membranes. Initial attempts to identify the myristoylated proteins in human mononuclear phagocytyes showed that p60 is one of the myristoylated proteins which is down-modulated during differentiation of monocytes to macrophages, and that cultivation of monocytes in the presence of IFN-γ and TNF-α prevents this phenomenon.