Acquisition of secretion of transforming growth factor-β1 leads to autonomous suppression of scavenger receptor activity in a monocyte- macrophage cell line, THP-1

Abstract

Macrophage cells derived from the human monocytic leukemia cell line, THP-1, accumulate esterified cholesterol when cultivated in the presence of acetylated low density lipoprotein (Ac-LDL) through scavenger receptors (ScR). In the present study, we isolated a subtype of THP-1 cells that failed to accumulate esterified cholesterol when cultivated in the presence of Ac- LDL. The cells had negligible amounts of cell association and degradation of Ac-LDL compared with the parent THP-1 cells. The subtype THP-1 cells did not express ScR mRNA as well as that of lipoprotein lipase. In contrast, the expression of apolipoprotein E mRNA was greater than that found in parent THP-1 cells. The culture medium of subtype THP-1 cells treated with 12-O- tetradecanoylphorbol-13-acetate inhibited the uptake of Ac-LDL and the expression of ScR in parent THP-1 cells. After a 48-h incubation in the culture medium containing 12-Otetradecanoylphorbol-13-acetate, the culture medium of differentiated subtype THP-1 cells contained 6.9 ng/ml transforming growth factor (TGF)-β1, while that of parent THP-1 cells secreted below detection level, which was less than 3 ng/ml. This inhibitory effect of the conditioned medium on the expression of ScR in parent THP-1 cells was abolished by pretreatment of the culture medium with anti-TGF-β1 antibodies. Parent THP-1 cells expressed as much TGF-β1 mRNA as sTHP-1 cells after stimulation of differentiation. Although the precursor forms of TGF-β1 that were synthesized in both parent and subtype THP-1 cells were of similar size and were expressed at similar levels, latent TGF-β1-binding protein, which is necessary for the secretion of TGF-β1, could only be co- immunoprecipitated with antiTGF-β1 antibody from subtype THP-1 cells. This suggests that subtype THP-1 cells secrete TGF-β1 into the medium by forming a functional complex with the latent TGF-β1-binding protein. We conclude that subtype THP-1 cells could not take up Ac-LDL because ScR was inhibited (leading to a loss of function) caused by the secreted TGF-β1.