An experiment studies early atheroma development. Lipid streaks on arterial walls are examined microscopically and biochemically to determine their cellular and chemical constituents and the factors promoting their formation. Early lesions show increased attachment of monocytes to the endothelium. The monocytes migrate subendothelially and become macrophages; these macrophages transform themselves into foam cells. Which of the following substances is most likely to be responsible for the transformation of macrophages?