Abstract
Platelet-derived growth factor (PDGF) is a potent mitogen of mesenchymal cells. A and B chains of PDGF are transcribed in atherosclerotic plaques but details of the mechanism of the expression of either chains in vivo are not understood. It has been suggested that differentiated macrophages, which are the major origin of foam cells, but not monocytes hardly secrete PDGF in vitro. In the present study, we investigated the effect of 1) tumor necrosis factor-α (TNF-α), a macrophage derived cytokine which promote the differentiation from monocytes to macrophages, and 2) interferon-y (IFN-γ), a T cell derived macrophage activating factor, on the the production and mRNA expression of PDGF A and B chains in a human monocytic leukemia cell line THP-1. THP-1 cells were cultured in the presence of one of the experimental cytokines with or without the addition of phorbol 12-myristate 13-acetate (PMA). PMA is known to induce the differentiation of monocytes (including THP-1 cells) to macrophages within a period of 24 hours. Thus the treatment of THP-1 cells with a cytokine and/or PMA in vitro was expected to mimic the effect of the cytokine during the differentiation of monocytes to macrophages in vivo.
Unstimulated THP-1 cells expressed no detectable amount of PDGF A or B mRNA. The treatment of THP-1 cells with PMA resulted in the strong expression of both PDGF A and B mRNAs. Stimulated with TNF-α (50 U/ml) on day 0, THP-1 cells expressed PDGF A mRNA significantly from 1 up to 3 days after the treatment, whereas the expression of PDGF B and mRNA was only weakly detected on day 2. The maximum signal of PDGF A mRNA was ten times strongerr than that of PDGF B mRNA. Within the same period, relatively small number of TNF-a treated THP-1 cells had differentiated to macrophages as detected by the reduction of nitro blue tetrazolium (NBT) (35% of total cells) and by the counting of adherent cells (35% of total cells). THP-1 cells simultaneously treated with TNF-α and PMA strongly expressed PDGF A and B gene. Most remarkably, the expression of PDGF B mRNA was greatly enhanced with compared to the treatment with PMA alone. In addition, TNF-α augmented the secretion of PDGF proteins in PMA stimulated THP-1 cells.
In contrast, INFγ per se had no effect on PDGF gene expression, NBT reducing activity and cell adherence in unstimulated THP-1 cells. In PMA-treated THP-1 cells, however, 100 U/ml of IFNy decreased the secretion of PDGF proteins and the expression of PDGF A and B mRNA in time dependent manner without changing the morphology of cells. Taken together, the data suggested that TNF-α stimulated PDGF A gene expression in monocytes and this PDGF A expression was correlated with their differentiation to macrophages, while in differentiated macrophages TNF-α stimulated PDGF B gene expression and PDGF secretion more significantly. It was suggested that IFNγ inhibited both PDGF A and B expression and PDGF secretion in macrophages without affecting their state of differentiation. Modulating the secretion of PDGF from monocytes and macrophages in atherosclerotic plaques, cytokines may thus control the proliferation of smooth muscle cells as well.
