PECAM-1 (also known as CD31) is a cellular adhesion and signaling receptor comprised of six extracellular immunoglobulin (Ig) - like homology domains, a short transmembrane domain, and a 118 amino acid cytoplasmic domain that becomes serine and tyrosine phosphorylated upon cellular activation. PECAM-1 expression is restricted to blood and vascular cells. In circulating platelets and leukocytes, PECAM-1 functions largely as an inhibitory receptor that, via regulated sequential phosphorylation of its cytoplasmic domain, limits cellular activation responses. PECAM-1 is also highly expressed at endothelial cell intercellular junctions, where it functions as a mechanosensor, as a regulator of leukocyte trafficking, and in the maintenance of endothelial cell junctional integrity. In this review we will describe (1) the functional domains of PECAM-1 and how they contribute to its barrier-enhancing properties, (2) how the physical properties of PECAM-1 influence its subcellular localization and its ability to influence endothelial cell barrier function, (3) various stimuli that initiate PECAM-1 signaling and/or function at the endothelial junction, and (4) cross-talk of PECAM-1 with other junctional molecules, which can influence endothelial cell function.

Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a cell adhesion and signaling receptor that is expressed on hematopoietic and endothelial cells. PECAM-1 is vital to the regulation of inflammatory responses, as it has been shown to serve a variety of pro-inflammatory and anti-inflammatory functions. Pro-inflammatory functions of PECAM-1 include the facilitation of leukocyte transendothelial migration and the transduction of mechanical signals in endothelial cells emanating from fluid shear stress. Anti-inflammatory functions include the dampening of leukocyte activation, suppression of pro-inflammatory cytokine production, and the maintenance of vascular barrier integrity. Although PECAM-1 has been well-characterized and studied, the mechanisms through which PECAM-1 regulates these seemingly opposing functions, and how they influence each other, are still not completely understood. The purpose of this review, therefore, is to provide an overview of the pro-and anti-inflammatory functions of PECAM-1 with special attention paid to mechanistic insights that have thus far been revealed in the literature in hopes of gaining a clearer picture of how these opposing functions might be integrated in a temporal and spatial manner on the whole organism level. A better understanding of how inflammatory responses are regulated should enable the development of new therapeutics that can be used in the treatment of acute and chronic inflammatory disorders.

Abstract-Enhanced tissue angiotensin (Ang) II levels have been reported in diabetes and might lead to cardiac dysfunction through oxidative stress. This study examined the effect of blocking the Ang II type 1 (AT 1 ) receptor on high glucose-induced cardiac contractile dysfunction. Rat ventricular myocytes were maintained in normal-(NG, 5.5 mmol/L) or high-(HG, 25.5 mmol/L) glucose medium for 24 hours. Mechanical and intracellular Ca 2ϩ properties were assessed as peak shortening (PS), time to PS (TPS), time to 90% relengthening (TR 90 ), maximal velocity of shortening/relengthening (ϮdL/dt), and intracellular Ca 2ϩ decay (). HG myocytes exhibited normal PS; decreased ϮdL/dt; and prolonged TPS, TR 90 , and . Interestingly, the HG-induced abnormalities were prevented with the AT 1 blocker L-158,809 (10 to 1000 nmol/L) but not the Janus kinase-2 (JAK2) inhibitor AG-490 (10 to 100 mol/L). The only effect of AT 1 blockade on NG myocytes was enhanced PS at 1000 nmol/L. AT 1 antagonist-elicited cardiac protection against HG was nullified by the NADPH oxidase activator sodium dodecyl sulfate (80 mol/L) and mimicked by the NADPH oxidase inhibitors diphenyleneiodonium (10 mol/L) or apocynin (100 mol/L). Western blot analysis confirmed that the protein abundance of NADPH oxidase subunit p47 phox and the AT 1 but not the AT 2 receptor was enhanced in HG myocytes. In addition, the HG-induced increase of p47 phox was prevented by L-158,809. Enhanced reactive oxygen species production observed in HG myocytes was prevented by AT 1 blockade or NADPH oxidase inhibition. Collectively, our data suggest that local Ang II, acting via AT 1 receptor-mediated NADPH oxidase activation, is involved in hyperglycemia-induced cardiomyocyte dysfunction, which might play a role in diabetic cardiomyopathy.

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SummaryPECAM-1 (CD31) is a cellular adhesion and signaling receptor that is highly expressed at endothelial cell-cell junctions in confluent vascular beds. Previous studies have implicated PECAM-1 in the maintenance of vascular barrier integrity; however, the mechanisms behind PECAM-1-mediated barrier protection are still poorly understood. The goal of the present study, therefore, was to examine the pertinent biological properties of PECAM-1 (i.e. adhesion and/or signaling) that allow it to support barrier integrity. We found that, compared with PECAM-1-deficient endothelial cells, PECAM-1-expressing endothelial cell monolayers exhibit increased steady-state barrier function, as well as more rapid restoration of barrier integrity following thrombin-induced perturbation of the endothelial cell monolayer. The majority of PECAM-1-mediated barrier protection was found to be due to the ability of PECAM-1 to interact homophilically and become localized to cell-cell junctions, because a homophilic binding-crippled mutant form of PECAM-1 was unable to support efficient barrier function when re-expressed in cells. By contrast, cells expressing PECAM-1 variants lacking residues known to be involved in PECAM-1-mediated signal transduction exhibited normal to near-normal barrier integrity. Taken together, these studies suggest that PECAM-1-PECAM-1 homophilic interactions are more important than its signaling function for maintaining the integrity of endothelial cell junctions.

BackgroundPolarized macrophage populations can orchestrate both inflammation of the kidney and tissue repair during CKD. Proinflammatory M1 macrophages initiate kidney injury, but mechanisms through which persistent M1-dependent kidney damage culminates in fibrosis require elucidation. Krüppel-like factor 4 (KLF4), a zinc-finger transcription factor that suppresses inflammatory signals, is an essential regulator of macrophage polarization in adipose tissues, but the effect of myeloid KLF4 on CKD progression is unknown.MethodsWe used conditional mutant mice lacking KLF4 or TNFα (KLF4’s downstream effector) selectively in myeloid cells to investigate macrophage KLF4’s role in modulating CKD progression in two models of CKD that feature robust macrophage accumulation, nephrotoxic serum nephritis, and unilateral ureteral obstruction.ResultsIn these murine CKD models, KLF4 deficiency in macrophages infiltrating the kidney augmented their M1 polarization and exacerbated glomerular matrix deposition and tubular epithelial damage. During the induced injury in these models, macrophage-specific KLF4 deletion also exacerbated kidney fibrosis, with increased levels of collagen 1 and α-smooth muscle actin in the injured kidney. CD11b+Ly6Chi myeloid cells isolated from injured kidneys expressed higher levels of TNFα mRNA versus wild-type controls. In turn, mice bearing macrophage-specific deletion of TNFα exhibited decreased glomerular and tubular damage and attenuated kidney fibrosis in the models. Moreover, treatment with the TNF receptor-1 inhibitor R-7050 during nephrotoxic serum nephritis reduced damage, fibrosis, and necroptosis in wild-type mice and mice with KLF4-deficient macrophages, and abrogated the differences between the two groups in these parameters.ConclusionsThese data indicate that macrophage KLF4 ameliorates CKD by mitigating TNF-dependent injury and fibrosis.

Our findings suggest a pathologic role of glutathione depletion in cardiac dysfunction and the therapeutic potential of antioxidants.

Abstract-One key mechanism for endothelial dysfunction is endothelial NO synthase (eNOS) uncoupling, whereby eNOS generates O 2 •Ϫ rather than NO because of deficient eNOS cofactor tetrahydrobiopterin (BH4). This study was designed to examine the effect of BH4 deficiency on cardiac morphology and function, as well as the impact of metallothionein (MT) on BH4 deficiency-induced abnormalities, if any. Friend virus B (FVB) and cardiac-specific MT transgenic mice were exposed to 2,4-diamino-6-hydroxy-pyrimidine (DAHP; 10 mmol/L, 3 weeks), an inhibitor of the BH4 synthetic enzyme GTP cyclohydrolase I. DAHP reduced plasma BH4 levels by 85% and elevated blood pressure in both FVB and MT mice. Echocardiography found decreased fractional shortening and increased end-systolic diameter in DAHPtreated FVB mice. Cardiomyocytes from DAHP-treated FVB mice displayed enhanced O 2•Ϫ production, contractile and intracellular Ca 2ϩ defects including depressed peak shortening and maximal velocity of shortening/relengthening, prolonged duration of relengthening, reduced intracellular Ca 2ϩ rise, and clearance. DAHP triggered mitochondrial swelling/myocardial filament aberrations and mitochondrial O 2•Ϫ accumulation, assessed by transmission electron microscopy and MitoSOX Red fluorescence, respectively. DAHP also promoted the N G -nitro-L-arginine methyl ester-inhibitable O 2•Ϫ production and eNOS phosphorylation at Thr497. Although MT had little effect on cardiac mechanics and ultrastructure, it attenuated DAHP-induced defects in cardiac function, morphology, O 2•Ϫ production, and eNOS phosphorylation (Thr497). The DAHP-induced cardiomyocyte mechanical responses were alleviated by in vitro BH4 treatment. DAHP inhibited mitochondrial biogenesis, mitochondrial uncoupling protein 2, and chaperone heat shock protein 90, and all but uncoupling protein 2 were rescued by MT. Our data suggest a role for BH4 deficiency in cardiac dysfunction and the therapeutic potential of antioxidants against eNOS uncoupling in the heart. (Hypertension. 2009;53:1023-1031.)