Transcriptional regulation of LDL receptor-related protein by IFN-γ and the antagonistic activity of TGF-β1 in the RAW 264.7 macrophage-like cell line

Low density lipoprotein receptor-related protein (LRP) is a major receptor for multiple ligands, including chylomicron and VLDL remnants, bacterial toxins, viruses, proteinases, lipoprotein lipase, and activated alpha2-macroglobulin (alpha2M). In this study, we used Northern blot analyses and nuclear run-on experiments to demonstrate that interferon-gamma (IFN-gamma) causes a concentration-dependent decrease in steady-state LRP mRNA expression and gene transcription rate in RAW 264.7 cells. IFN-gamma also markedly increased expression of inducible nitric oxide synthase (NOS), as expected; however, the increase in nitric oxide was not responsible for the down-regulation of LRP expression since the NOS inhibitor, N(G)-monomethyl-L-arginine, did not preserve LRP expression in IFN-gamma-treated cells. Transforming growth factor-beta1 (TGF-beta1; 2.5 ng/mL) had no independent effect on LRP expression and did not modify the response to IFN-gamma when the two cytokines were added simultaneously to cultures. When TGF-beta1 was added 24 h prior to IFN-gamma, the extent of LRP down-regulation was significantly reduced. Specific binding of the LRP ligand, activated (125)I-alpha2M, was decreased by 76 +/- 5% in cells treated with 100 U/mL IFN-gamma, but only by 45 +/- 7% in cells treated with 100 U/mL IFN-gamma after TGF-beta1-pretreatment. The antagonistic activity of TGF-beta1 on the IFN-gamma response in RAW 264.7 cells did not result from a change in LRP mRNA stability or IFN-gamma receptor expression, as determined by Northern blot analyses and (125)I-IFN-gamma binding experiments. The studies presented here suggest that the balance between IFN-gamma and TGF-beta1 may be critical in determining LRP expression at sites of infection and inflammation.