We have previously shown that minimally oxidized LDL (MM-LDL) activated endothelial cells to increase their interaction with monocytes but not neutrophils, inducing monocyte but not neutrophil binding and synthesis of monocyte chemotactic protein-i and monocyte colony-stimulating factor (M-CSF). In the present studies we have examined the signaling pathways by which this monocyte-specific response is induced. Both induction of monocyte binding and mRNA levels for M-CSF by MM-LDL were not inhibited in protein kinase C-depleted endothelial cells. A number of our studies indicate that cAMP is the second messenger for the effects of MM-LDL cited above. Incubation of endothelial cells with MM-LDL caused a 173% increase in intracellular cAMP levels. Agents which increased cAMP levels, including cholera toxin, pertussis toxin, dibutyryl cAMP, and isoproterenol mimicked the actions of MM-LDL. Agents which elevated cAMP were also shown to activate NFKB, suggesting a role for this transcription factor in activation of monocyte-endothelial interactions. Although endothelial leukocyte adhesion molecule (ELAM) mRNA synthesis can be regulated by NFKB, ELAM was not expressed and ELAM mRNA was only slightly elevated in response to MM-LDL. We present evidence that induction of neutrophil binding by LPS is actually suppressed by agents that elevated cAMP levels. (J. Clin. Invest. 1993. 92:471-478.)

There is considerable evidence that oxidized lipoproteins play a role in the development of atherosclerosis. Early studies showed oxidized lipoproteins to be present in atherosclerotic lesions (1-3). Studies on hyperlipidemic animals demonstrated that oxidation products were increased in lesions (4). In addition, antioxidants such as probucol and butylated hydroxytoluene have been found to reduce the development and severity of lesions in a number of animal models (5, 6).Mononuclear cell adhesion to vascular endothelium was observed in the initial steps of fatty streak formation (7,8). Recent studies (9, 10) using mice null for monocyte chemotactic protein-1 (MCP-1) and the MCP-1 receptor have demonstrated the important role of monocytes in lesion development. Our group has used minimally modified low-density lipoprotein (MM-LDL)-stimulated aortic endothelial cells to model the development of the fatty streak (11). We have demonstrated that monocyte, but not neutrophil, adhesion and transmigration across the aortic endothelium is increased in response to MM-LDL (11). Active oxidized phospholipids isolated from MM-LDL were found to be increased in rabbit atherosclerotic lesions (12).Previous studies from our group have identified MM-LDL-induced molecules involved in several phases of monocyte/endothelial interactions. Previous work (13,14) suggests that in vivo entry of leukocytes into the vessel wall involves at least three steps; rolling, activation, and firm adhesion to the endothelium. The rolling step has been shown to involve the interaction of selectins on the endothelium, with their ligands on leukocytes. Studies from our group and others (15-18) suggest that Pselectin is an important rolling molecule for monocytes in atherosclerosis. Using in vitro studies, we have shown that levels of P-selectin in human aortic endothelial cells (HAEC) are increased by MM-LDL (18), whereas levels of E-selectin are decreased (19). We and others have also shown that highly oxidized low-density lipoprotein (LDL) leads to P-selectin release to the upper cell surface (18,20). Specific cytokines and chemokines that activate monocyte adhesion ligands have been found in lesions We have shown previously that treatment of human aortic endothelial cells (HAECs) with minimally modified low-density lipoprotein (MM-LDL) induces monocyte but not neutrophil binding. This monocyte binding was not mediated by endothelial E-selectin, P-selectin, vascular cell adhesion molecule-I, or intercellular adhesion molecule-I, suggesting an alternative monocyte-specific adhesion molecule. We now show that moncytic α4β1 integrins mediate binding to MM-LDL-treated endothelial cells. We present data suggesting that the expression of the connecting segment-1 (CS-1) domain of fibronectin (FN) is induced on the apical surface of HAEC by MM-LDL and is the endothelial α4β1 ligand in MM-LDLtreated cells. Although the levels of CS-1 mRNA and protein were not increased, we show that MM-LDL treatment causes deposition of FN on the apical surface by activation ...

We have previously shown that treatment of endothelial cells with minimally modified LDL (MM-LDL) induces the binding of monocytes to unknown endothelial receptor molecules. We now report that a member of the GRO family of chemokines plays a role in MM-LDL-induced monocyte binding. A cDNA library made from rabbit aortic endothelial cells (RAEC) treated with MM-LDL was expression screened for molecules inducing binding of a human monocyte cell line (THP-1). A cDNA was isolated with 75% homology to GRO. GRO mRNA levels were significantly elevated after exposure of RAEC or human aortic endothelial cells (HAEC) to MM-LDL. HAEC treated with MM-LDL displayed an increase in a surface-associated protein that bound to antibody against GRO despite low levels of GRO in the medium. Antibody to GRO significantly inhibited the binding of monocytes to MM-LDL-treated RAEC and HAEC. The increase in GRO expression and monocyte binding were reduced by incubating MM-LDL-treated endothelial cells with heparin (in a method that releases heparan sulfate bound molecules from the cell surface). These results suggest that GRO related chemokines are bound to the surface of MM-LDL-treated endothelial cells and may contribute to the monocyte adhesion induced by MM-LDL. (J. Clin.

We have previously shown that treatment of aortic endothelial cells with minimally oxidized LDL (MM-LDL) induces their interaction with monocytes but not neutrophils and that these induced responses are associated with increased cAMP levels. Here we studied the mechanism of by which MM-LDL elevates cAMP levels. Treatment of human aortic endothelial cells with MM-LDL resulted in a saturable dose-dependent increase in cAMP levels. Studies using a combination of pertussis toxin and MM-LDL suggested that part of the cAMP increase was due to the stimulation of Gs complexes. Studies with pertussis toxin-treated membranes in which Gi was completely inhibited were used to directly address the effect of MM-LDL on the Gs pathway. MM-LDL and an oxidized lipid (palmitoyl arachidonyl phosphatidylcholine), the effects of which mimic those of MM-LDL, caused a 40% to 100% increase in cAMP levels in these isolated membranes that was augmented by GTP, thus showing Gs stimulation. These results also show that MM-LDL increases cAMP levels by inhibiting Gi. MM-LDL inhibited ADP ribosylation of Gi by about 30% and completely abolished the ability of serotonin to interact with Gi complexes, whereas direct activation of Gi by mastoparan was not inhibited. This observation suggests that MM-LDL interferes with the interaction of Gi molecules with inhibitory receptors. There was no direct effect of MM-LDL on adenylate cyclase. Overall, these studies show that MM-LDL increases cAMP levels both by stimulating Gi and inhibiting Gi complexes.

NCT02590744.