Apolipoprotein E acts to increase nitric oxide production in macrophages by stimulating arginine transport

Abstract

Previous studies have shown that apolipoprotein E (apoE) plays a role in immune function by modulating tissue redox balance. Using a mouse macrophage cell line (RAW 264.7), we have examined the mechanism by which apoE regulates nitric oxide (NO) production in macrophages. ApoE potentiates NO production in immune activated RAW cells in combination with lipopolysaccharide or polyinosinic:polycytidylic acid (PIC), agents known to induce expression of inducible nitric oxide synthase mRNA and protein. The effect is not observed with apolipoprotein B or heat-inactivated apoE. The combination of PIC plus apoE produced more NO than the level expected from an additive effect of PIC and apoE alone. Furthermore, this increase was observed at submaximal extracellular arginine concentrations, suggesting that apoE altered arginine (substrate) availability. Examination of [(3)H]arginine uptake across the cell membrane demonstrated that arginine uptake was increased by PIC but further increased by PIC plus apoE. Treatment of RAW cells with apoE was associated with an increased apparent V(max) and decreased affinity for arginine as well as a switch in the induction of mRNA for subtypes of cationic amino acid transporters (CAT). Treatment of RAW cells with PIC plus apoE resulted in the loss of detectable CAT1 mRNA and expression of CAT2 mRNA. Regulation of arginine availability is a novel action of apoE on the regulation of macrophage function and the immune response.

Enteric infections, diarrhea, and their impact on function and development

“…An additional key amino acid for renewal of the intestinal epithelium is arginine, which is often deficient in malnourished patients. Indeed, it is an arginine-selective cationic amino acid transporter that is upregulated by the ApoE4 allele associated with protection from the cognitive effects of diarrhea and malnutrition (16,17), and this observation could explain, at least in part, the protective effect of the allele (57). Arginine might also provide a novel epithelial repairing therapy, and it was well tolerated in premature neonatal human infants, in whom it reduced the incidence of necrotizing enterocolitis (165).…”

“…For example, the APOE allele APOE4, which is associated with increased risk for cardiovascular and Alzheimer disease, was discovered to protect against the cognitive ravages of diarrhea (16). Because ApoE4 has been shown to drive an arginineselective transporter (17), these observations uncover a potential novel approach to repairing the damaged intestinal epithelium in individuals infected with enteric pathogens using arginine or its precursors, such as glutamine. It is therefore imperative that we understand the epidemiology, etiologies, and pathophysiology of enteric infections, as well as host-pathogen interactions, if we are to elucidate innovative interventions to control the still-devastating consequences of repeated malnourishing and disabling enteric infections in the most formative early years of childhood.…”

Apolipoprotein E3- and Nitric Oxide–Dependent Modulation of Endothelial Cell Inflammatory Responses

“…19 It has been demonstrated that apoE can directly increase intracellular NO via activation of NO synthase in both platelets and endothelial cells. 13,20,21 Conversely, epidemiological and experimental data have demonstrated an opposing role of apoE4 on various inflammatory processes. For example, studies have demonstrated that the 4 isoform is proinflammatory, 22 possibly through direct oxidant properties 19,23 or through deficits in NO signaling.…”

Disrupted Spermine Homeostasis: A Novel Mechanism in Polyglutamine-Mediated Aggregation and Cell Death

“…Arginine uptake was measured essentially as described by Colton et al (2001). CAD cells were plated into 24-well dishes and preequilibrated in serum-free media containing low arginine (10 M).…”

“…Intracellular arginine pools are primarily supplied by arginine transporters that move arginine across the plasma membrane to the interior of the cell. Although arginine transport can regulate NO production in macrophages (Kakuda et al, 1999;Colton et al, 2001), some neuronal populations maintain intracellular arginine pools by recycling citrulline through the citrulline-arginine-NO cycle, thus bypassing arginine uptake (Braissant et al, 1999;Heneka et al, 2001). To determine whether CAD cells synthesize arginine via this mechanism, we examined the expression of argininosuccinate lyase (AL) and argininosuccinate synthase (AS), the two enzymes necessary for the citrulline-arginine-NO cycle.…”