The culture of HPBM in serum-containing medium induced a large accumulation of the protein cross-linking enzyme, tissue TGase. Immune IFN enhanced the expression of tissue TGase in cultured monocytes. Enzyme-inducing activity, both in normal and IFN-treated cells, was completely blocked by depleting the serum of the lipid fraction. The readdition of retinol at a physiologic concentration (1 micron) to delipidized serum completely restored the enzyme-inducing activity in cultured monocytes. Thus, serum retinoids seem to play an important regulatory role in the expression of tissue TGase gene in differentiating human monocytes.

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