TGF-β1 downregulates CD36 and scavenger receptor A but upregulates LOX-1 in human macrophages

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Abstract
Transforming growth factor-β1 (TGF-β1), a key cytokine for control of cell growth, extracellular matrix formation, and inflammation control, is secreted by many cells present in the arteriosclerotic plaque. Lipid accumulation in the vessel wall is regarded as an early step in atherogenesis and depends on uptake of modified low-density lipoprotein (LDL) by macrophages through scavenger receptors and their transformation into foam cells. Prominent members of the scavenger receptor family are the class A type I and II receptors (ScR-A), the class B receptor CD36, and the recently detected lectin-like oxidized LDL receptor-1 (LOX-1), which, unlike the native LDL receptor (LDL-R), are not feedback controlled. CD36 is responsible for >50% of modified LDL uptake into human monocyte-derived macrophages. We therefore studied whether TGF-β1 influences expression and function of ScR-A, CD36, and LOX-1 in monocytes using RT-PCR and flow cytometry. Total uptake of oxidized LDL by monocytoid cells, reflecting the combined function of all scavenger receptors, was significantly reduced by TGF-β1. At initially low picomolar concentrations, TGF-β1 decreased CD36 mRNA and protein surface expression and ScR-A mRNA levels in the human monocytic cell line THP-1 and in freshly isolated and cultivated human monocytes, whereas LOX-1 mRNA was increased. Expression of LDL-R and β-actin was not affected by TGF-β1. In conclusion, depression of scavenger receptor function in monocytes by TGF-β1 in low concentrations reduces foam cell formation. Together with matrix control by TGF-β1, this may be important for atherogenesis and plaque stabilization.

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