Enzymatically degraded low density lipoproteins are more potent inducers of egr-1 mRNA than oxidized or native low density lipoproteins

Hypochlorite-modified high-density lipoprotein promotes induction of HO-1 in endothelial cells via activation of p42/44 MAPK and zinc finger transcription factor Egr-1

Modification/chlorination of high-density lipoprotein (HDL) by hypochlorous acid (HOCl), formed by the myeloperoxidase-H₂O₂-chloride system of activated phagocytes, converts an anti-atherogenic lipoprotein into a pro-inflammatory lipoprotein particle. Chlorinated HDL is present in human lesion material, binds to and is internalized by endothelial cells and impairs expression and activity of endothelial nitric oxide synthase. The present study aimed at clarifying whether exposure of endothelial cells to pro-inflammatory HOCl-HDL impacts on expression of heme oxygenase-1, a potential rescue pathway against endothelial dysfunction. Our findings revealed that HDL modified by HOCl, added as reagent or generated enzymatically, induced phosphorylation of p42/44 mitogen-activated protein kinase, expression of transcription factor early growth response-1 (Egr-1) and enhanced expression of heme oxygenase-1 in human endothelial cells. Upregulation of heme oxygenase-1 could be blocked by an inhibitor upstream of p42/44 mitogen-activated protein kinase and/or knockdown of Egr-1 by RNA-interference. Electrophoretic mobility shift assays demonstrated HOCl-HDL-mediated induction of the Egr-1 DNA binding activity. Immunocytochemical and immunoblotting experiments demonstrated HOCl-HDL-induced translocation of Egr-1 to the nucleus. The present study demonstrates a novel compensatory pathway against adverse effects of HOCl-HDL, providing cytoprotection in a number of pathological conditions including cardiovascular disease.

E-LDL upregulates TOSO expression and enhances the survival of human macrophages

Uptake of modified lipoproteins by macrophages causes foam cell formation and promotes atherosclerosis. Atherogenic lipoproteins are cytotoxic and induce cell death under certain conditions but may also enhance macrophage survival. Macrophages treated with enzymatically modified LDL (E-LDL) were subjected to GeneChip analysis and the antiapoptotic gene TOSO was found induced. TOSO mRNA is upregulated and apoptosis is reduced in E-LDL but not in oxidized LDL (Ox-LDL) loaded macrophages. FLIP(L) abundance was suggested to mediate the antiapoptotic properties of TOSO; however, FLIP(L) was not changed. Ox-LDL is internalized predominantly by scavenger receptors such as CD36 while E-LDL particles are preferentially internalized by Fc- and complement-receptor dependent phagocytosis and internalization of phagobeads by macrophages upregulates TOSO. In COS-7 cells however, phagocytotic activity was not affected by TOSO. These data indicate that E-LDL-generated foam cells are protected from cell death most likely through the expression of TOSO by a FLIP(L) independent mechanism.

Chylomicron remnants regulate early growth response factor-1 in vascular smooth muscle cells

Early growth response factor-1 (Egr-1) is a zinc-finger transcription factor that induces genes that promote atherosclerosis. The goal of the present study was to determine whether Egr-1 expression is modulated by atherogenic, triglyceride rich lipoproteins known as chylomicron remnants. Chylomicron remnants induced Egr-1 mRNA and protein expression in rat cultured vascular smooth muscle cells (VSMCs) and activated extracellular signal-regulated kinase (ERK) 1/2 in VSMCs. Further, chylomicron remnant-induced Egr-1 expression was inhibited by PD98059, a selective inhibitor of MAPK kinase (MEK), suggesting that the action of chylomicron remnants on Egr-1 was dependent on the ERK/MEK pathway. Chylomicron remnants also induced mRNA expression of the pro-inflammatory cytokines, IL-2 and IFN-gamma in VSMCs. We conclude that chylomicron remnants act as atherogenic lipoproteins via induction of Egr-1 expression and via cytokine-mediated inflammation.

Early Growth Response-1 Promotes Atherogenesis

Early growth response-1 (Egr-1) regulates expression of proinflammatory and procoagulant genes in acute cell stress. Experimental evidence suggested that Egr-1 transcripts were upregulated in human atherosclerotic plaques versus adjacent unaffected tissue. To test the impact of Egr-1 in chronic vascular stress, we examined its role in a murine model of atherosclerosis. Real-time PCR analysis of aortae retrieved from apoE −/− mice demonstrated increased Egr-1 transcripts in an age-dependent manner, compared with aortae retrieved from C57BL/6 control animals. Therefore, homozygous Egr-1 −/− mice were bred into the apoE −/− background. Homozygous double-knockout mice (Egr-1 −/− /apoE −/− ) in the C57BL/6 background were maintained on normal chow diet. At age 14 and 24 weeks, atherosclerotic lesion area and complexity at the aortic root were strikingly decreased in mice deficient in both Egr-1 and apoE compared with mice deficient in apoE alone. In parallel, transcripts for genes regulating the inflammatory/prothrombotic response were diminished in Egr-1 −/− /apoE −/− aortae versus apoE −/− . In vitro, oxidized low-density lipoprotein (OxLDL), a key factor inciting atherogenic mechanisms in the vasculature, upregulated Egr-1 expression in monocytes via the MEK-ERK1/2 pathway. We conclude that Egr-1 broadly regulates expression of molecules critically linked to atherogenesis and lesion progression.