Modulation of the transcriptional rate of Fc gamma receptor mRNA in human mononuclear phagocytes.

Human monocytes and macrophages bear three classes of cell surface receptors for the Fc portion of IgG (Fc gamma RI, Fc gamma RII, and Fc gamma RIII). These receptors mediate phagocytosis and other effector functions and are important in the pathophysiology of hematologic disease, inflammation, and host defense. We have previously shown that interferon-gamma (IFN-gamma) and dexamethasone modulate total Fc gamma RII mRNA levels as well as Fc gamma RI and Fc gamma RII protein expression on monocytes and the monocyte-like cell line U937. In this study, we investigated the modulation of Fc gamma RI mRNA. Additionally, we utilized mRNA stability and nuclear run-on assays to study the mechanism of the modulation of Fc gamma receptor transcripts in the monocyte/macrophage cell line U937. In U937 cells, IFN-gamma increased Fc gamma RI mRNA levels 7.5-fold. Treatment with dexamethasone reduced Fc gamma RI mRNA levels to 0.6-fold of baseline and inhibited by 20-60% the increase in mRNA observed after treatment of the U937 cells with IFN-gamma. On monocytes, treatment with IFN-gamma increased monocyte Fc gamma RI mRNA 6.7-fold. Cotreatment of the IFN-gamma-stimulated monocytes with dexamethasone resulted in a 160% further increase in Fc gamma RI expression. Fc gamma RI and Fc gamma RII mRNA half-lives were then determined in U937 cells by incubation with dexamethasone and/or IFN-gamma for 16 hr and then arresting mRNA transcription with actinomycin-D (10 micrograms/ml). The mRNA half-lives in untreated U937 cells were 3.3 +/- 0.3 hr (Fc gamma RI) and 3.1 +/- 0.3 hr (Fc gamma RII). For either Fc gamma RI and Fc gamma RII, the effect of dexamethasone and/or IFN-gamma on mRNA half-life was not significant (P > 0.5). We also performed nuclear run-on experiments with U937 cells which indicated that IFN-gamma increased the transcription of Fc gamma RI 4.2-fold and Fc gamma RII 1.7-fold. Our data suggest that these changes in Fc gamma RI and Fc gamma RII protein are likely due, at least in part, to increases in mRNA levels secondary to alteration in gene transcription.