RGS4 inhibits angiotensin II signaling and macrophage localization during renal reperfusion injury independent of vasospasm.

Vascular inflammation is a major contributor to the severity of acute kidney injury. In the context of vasospasm-independent reperfusion injury we studied the potential anti-inflammatory role of the Gα-related RGS protein, RGS4. Transgenic RGS4 mice were resistant to 25 min injury, although post-ischemic renal arteriolar diameter was equal to the wild type early after injury. A 10 min unilateral injury was performed to study reperfusion without vasospasm. Eighteen hours after injury, blood flow was decreased in the inner cortex of wild-type mice with preservation of tubular architecture. Angiotensin II levels in the kidneys of wild-type and transgenic mice were elevated in a sub-vasoconstrictive range 12 and 18 h after injury. Angiotensin II stimulated pre-glomerular vascular smooth muscle cells (VSMCs) to secrete the macrophage chemoattractant RANTES, a process decreased by angiotensin II R2 (AT2) inhibition. However, RANTES increased when RGS4 expression was suppressed implicating Gα protein activation in an AT2-RGS4-dependent pathway. RGS4 function, specific to VSMC, was tested in a conditional VSMC-specific RGS4 knockout showing high macrophage density by T2 MRI compared with transgenic and non-transgenic mice after the 10 min injury. Arteriolar diameter of this knockout was unchanged at successive time points after injury. Thus, RGS4 expression, specific to renal VSMC, inhibits angiotensin II-mediated cytokine signaling and macrophage recruitment during reperfusion, distinct from vasomotor regulation.

An unbiased chemical biology screen identifies agents that modulate uptake of oxidized LDL by macrophages.

Macrophage-derived foam cells are thought to play a major role in atherosclerotic lesion formation and progression. An automated assay was established to evaluate the uptake of fluorescently labeled oxidized low-density lipoprotein (oxLDL) by a monocyte/macrophage cell line. The assay was used to screen 480 known bioactive compounds. Twenty-two active compounds were identified. Efficacy studies in peritoneal macrophages demonstrated a high rate of concordance with the initial screening results. Inhibitory compounds confirmed important previous findings and identified new drugs of interest including: 3 blockers of nuclear factor kappab activation, 2 protein kinase C inhibitors, a phospholipase C inhibitor, and 2 antipsychotic drugs. In addition, an opioid receptor agonist was found to increase the oxLDL uptake of macrophages. The involvement of nuclear factor kappaB in oxLDL uptake was validated in peritoneal macrophages in vivo. The results support a model in which oxLDL uptake is dependent on the activation of multiple intracellular signaling pathways that culminate in actin-mediated lipoprotein internalization.

1,25(OH)2 vitamin d inhibits foam cell formation and suppresses macrophage cholesterol uptake in patients with type 2 diabetes mellitus.

BACKGROUND: Cardiovascular disease is the leading cause of death among those with diabetes mellitus. Vitamin D deficiency is associated with an increased risk of cardiovascular disease in this population. To determine the mechanism by which vitamin D deficiency mediates accelerated cardiovascular disease in patients with diabetes mellitus, we investigated the effects of active vitamin D on macrophage cholesterol deposition. METHODS AND RESULTS: We obtained macrophages from 76 obese, diabetic, hypertensive patients with vitamin D deficiency (25-hydroxyvitamin D <80 nmol/L; group A) and 4 control groups: obese, diabetic, hypertensive patients with normal vitamin D (group B; n=15); obese, nondiabetic, hypertensive patients with vitamin D deficiency (group C; n=25); and nonobese, nondiabetic, nonhypertensive patients with vitamin D deficiency (group D; n=10) or sufficiency (group E; n=10). Macrophages from the same patients in all groups were cultured in vitamin D-deficient or 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] -supplemented media and exposed to modified low-density lipoprotein cholesterol. 1,25(OH)(2)D(3) suppressed foam cell formation by reducing acetylated or oxidized low-density lipoprotein cholesterol uptake in diabetic subjects only. Conversely, deletion of the vitamin D receptor in macrophages from diabetic patients accelerated foam cell formation induced by modified LDL. 1,25(OH)(2)D(3) downregulation of c-Jun N-terminal kinase activation reduced peroxisome proliferated-activated receptor-gamma expression, suppressed CD36 expression, and prevented oxidized low-density lipoprotein-derived cholesterol uptake. In addition, 1,25(OH)(2)D(3) suppression of macrophage endoplasmic reticulum stress improved insulin signaling, downregulated SR-A1 expression, and prevented oxidized and acetylated low-density lipoprotein-derived cholesterol uptake. CONCLUSIONS: These results identify reduced vitamin D receptor signaling as a potential mechanism underlying increased foam cell formation and accelerated cardiovascular disease in diabetic subjects.

Requirement for p38 mitogen-activated protein kinase activity in neointima formation after vascular injury.

BACKGROUND: Angioplasty and stent delivery are performed to treat atherosclerotic vascular disease but often cause deleterious neointimal lesion formation. Previously, growth factor receptor-bound protein 2 (Grb2), an intracellular linker protein, was shown to be essential for neointima formation and for p38 mitogen-activated protein kinase (MAPK) activation in vascular smooth muscle cells (SMCs). In this study, the role of vascular SMC p38alpha MAPK in neointimal development was examined. METHODS AND RESULTS: Compound transgenic mice were generated with doxycycline-inducible SMC-specific expression of dominant-negative p38alpha MAPK (DN-p38alpha). Doxycycline treatment resulted in the expression of DN-p38alpha mRNA and protein in transgenic arteries. Doxycycline-treated compound transgenic mice were resistant to neointima formation 21 days after carotid injury and showed reduced arterial p38 MAPK activation. To explore the mechanism by which p38alpha MAPK promotes neointima formation, an in vitro SMC culture system was used. Inhibition of p38alpha MAPK in cultured SMCs by treatment with SB202190 or small interfering RNA blocked platelet-derived growth factor-induced SMC proliferation, DNA replication, phosphorylation of the retinoblastoma protein, and induction of minichromosome maintenance protein 6. CONCLUSIONS: SMC p38alpha MAPK activation is required for neointima formation, perhaps because of its ability to promote retinoblastoma protein phosphorylation and minichromosome maintenance protein 6 expression.

Grb2 is required for atherosclerotic lesion formation.

OBJECTIVE: Grb2 is a ubiquitously expressed linker protein that couples growth factor receptor activation to downstream mitogen-activated protein kinase (MAPK) cascades. Macrophage proliferation and uptake of modified lipoproteins are critical components of atherogenesis which require MAPK activation. However, the precise role of upstream signaling factors and the interrelationship of various MAPK cascades in the pathogenesis of atherosclerosis remains uncertain. Complete deletion of Grb2 in mice results in early embryonic lethality. However, Grb2 heterozygous mice appear normal at birth. To test the role of the Grb2 adapter protein in atherosclerotic lesion formation, we generated Grb2+/- mice in the apoE-/- genetic background. METHODS AND RESULTS: Grb2+/- apoE-/- and apoE-/- mice exhibited similar body weight and serum lipid profiles. However, Grb2+/- apoE-/- mice on a Western diet had reduced lesion formation compared with apoE-/- mice by aortic sinus and en face assays. Transplantation of apoE-/- mice with Grb2+/- apoE-/- or apoE-/- bone marrow indicated that Grb2 haploinsufficiency in blood-borne cells confers resistance to Western diet-induced atherosclerosis. Cell culture experiments with bone marrow-derived macrophages showed that Grb2 is required for oxidized low density lipoprotein (oxLDL)-induced MAPK activation and foam cell formation. CONCLUSIONS: Grb2 is required for atherosclerotic lesion formation and uptake of oxidized LDL by macrophages.

IL4/PGE2 induction of an enlarged early endosomal compartment in mouse macrophages is Rab5-dependent.

The endosomal compartment and the plasma membrane form a complex partnership that controls signal transduction and trafficking of different molecules. The specificity and functionality of the early endocytic pathway are regulated by a growing number of Rab GTPases, particularly Rab5. In this study, we demonstrate that IL4 (a Th-2 cytokine) and prostaglandin E2 (PGE2) synergistically induce Rab5 and several Rab effector proteins, including Rin1 and EEA1, and promote the formation of an enlarged early endocytic (EEE) compartment. Endosome enlargement is linked to a substantial induction of the mannose receptor (MR), a well-characterized macrophage endocytic receptor. Both MR levels and MR-mediated endocytosis are enhanced approximately 7-fold. Fluid-phase endocytosis is also elevated in treated cells. Light microscopy and fractionation studies reveal that MR colocalizes predominantly with Rab5a and partially with Rab11, an endosomal recycling pathway marker. Using retroviral expression of Rab5a:S34N, a dominant negative mutant, and siRNA Rab5a silencing, we demonstrate that Rab5a is essential for the large endosome phenotype and for localization of MR in these structures. We speculate that the EEE is maintained by activated Rab5, and that the EEE phenotype is part of some macrophage developmental program such as cell fusion, a characteristic of IL4-stimulated cells.