Intercellular Adhesion Molecule 1
General Features of Intercellular Adhesion Molecule 1
The molecular weight of ICAM-1 varies from 80 to 114 kDa which mainly depends on glycosylation level but core protein having molecular weight of ~60 kDa. It contains five extracellular immunoglobulins like domains constituting ~453 amino acids with higher hydrophobicity. Every extracellular domain is stabilized by disulfide bonds. This extracellular portion is connected with short cytoplasmic tail by hydrophobic transmembrane region of around 24 amino acid residues. Single tyrosine residue at cytoplasmic tail may be significantly involved in signaling.
In 1989, Greve et al. described that ICAM-1 gene consists of 15 kb and is located on human chromosome 19 (cytogenetic location: 19p13.2) (Greve et al. 1989). Recently, extensive alternative splicing and products of spliced proteins or isoforms of ICAM-I differing in their expression and ligand binding are also observed (Gottrand et al. 2015).
The gene sequence of ICAM-1 consists of seven exons, among which exon 1 encodes the signal sequence, exons 2–6 each encodes one of the five extracellular domains, and exon 7 encodes transmembrane region and cytoplasmic tail. Various nucleotide polymorphisms have been observed in exons 4, 5, and 6. The substitution from glutamic acid to lysine in exon 6 was found to be associated with coronary heart disease, myocardial infarction, and peripheral artery disease (Lawson and Wolf 2009).
ICAM-1 plays an important role in both innate and adaptive immune responses. It is involved in the transendothelial migration of leukocytes to sites of inflammation, as well as interactions between antigen-presenting cells (APC) and T cells (immunological synapse formation). Normally, ICAM-1 protein is encoded by ICAM-1 gene, expressed constitutively in low concentrations in the membrane of endothelial cells and also on some lymphocytes and monocytes. However, ICAM-1 expression can be increased significantly in the presence of pro-inflammatory cytokines like TNFα, IL-1, IFNγ, bacterial lipopolysaccharide (LPS), and reactive oxygen species (ROS) in the vascular endothelium and other cells. Upon activation by cytokines, activated leukocytes bind to endothelial cells via ICAM-1 and LFA-1 interaction, integrin LFA-1 being the receptor for ICAM-1 on leukocytes. Thus, leukocytes transmigrate into endothelial tissues causing extravasation and inflammation. However, soluble LFA-1 is also found which seems to bind and block ICAM-1.
The regulation of ICAM-1 gene expression is cell type specific which involves different signaling pathways and several enhancer elements such as palindromic interferon-gamma-responsive element (plγRE), NF-κB, Ets, C/EBP, AP1, Sp1, and retinoic acid response elements. Signal transducers and activators of transcription (STAT) factors, more specifically STAT-1 and STAT-3, bind to ICAM-1 promoter pIγRE and are strongly involved in ICAM-1 gene expression in different cell types. Similarly, NF-κB is also implicated to play a pivotal role in ICAM-1 gene regulation where RelA (p65)/RelA, RelA/c-Rel, and RelA/NF-κB dimers can potently induce ICAM-1 expression in several cell types. In addition, Ets gene family of transcriptional factors is also involved in the regulation of ICAM-1 expression. The control of highly diverse sets of genes by Ets proteins involves their own regulation at different levels that include many phosphorylation events mediated by the Ras-MAPK pathway. Thus, in response to extracellular cytokine stimulation, transcription factors like JAK-STAT, NF-κB, and Ets, pathways are shown to be modulated by phosphorylation events leading to their translocation into the nucleus for initiation of ICAM-1 gene expression. ICAM-1 participates in a positive feedback loop, and ICAM-1 ligation was found to upregulate its own expression at both mRNA and protein level. ICAM-1 also competes with ICAM-2 to maintain a pro-inflammatory environment during the leukocyte endothelial transmigration (Myers et al. 1992).
Structure of Intercellular Adhesion Molecule 1
In 2007, Chen et al. reveal the X-ray crystallographic structure of ICAM-1 protein. ICAM-1 protein is a member of the immunoglobulin superfamily which also consists of antibodies and T-cell receptors. The molecular weight of ICAM-1 protein is determined to be 57.8 kDa with 532 amino acids. ICAM1 protein forms Protein forms a dimeric structure with two amino terminals (D1 and D2), three carboxy terminals (D3–D5), a transmembrane region, and a cytosolic tail. ICAM-1 protein’s extracellular domains contain multiple loops, and each loop contains disulfide bridges within loop. Electron micrograph results revealed bent rod shape of ICAM-1 with 18.7 nm length and five immunoglobulin-like domains arranged from extracellular head to cytoplasmic tail. Domain 1 of ICAM-1 contains the primary site of contact for both natural receptor LFA-1 and clinical ligand rhinovirus. Binding sites of both LFA-1 and rhinovirus are overlapping in nature but not same which is also demonstrated by the involvement of domains 3–5 in the binding of rhinovirus but not for LFA-1.
Further analysis of domain D1 of ICAM-I shows its relationship with “I” set of the immunoglobulin superfamily which is critical for binding to cognate receptor LFA-1. There are flexible FG and BC loops on the tip of domain D1, and domain D2 has conserved region in the BC loop. However, the interphase region between two domains is highly hydrophobic. Domain D2 of ICAM-1 belongs to the V, C1, C2, and I set of immunoglobulin superfamily and is similar to domain 2 of the VCAM-1 and ICAM-2. Domain D3 belongs to the I-1 subset of the immunoglobulin superfamily and is made up of two β sheets similar to ICAM-I D1. Domain D4 has only five β strands, and D4 has floppy irregular region present within it. A short 16-residue region of D4 is capable to unfold and form large biological D4 super-domain dimer. Domain 5 is in distorted region with 12 subsets. Four cysteine residues are found clustered in the central part of the Domain 5 D5 (Yang et al. 2004; Casasnova et al. 1998). Moreover, its structure is heavily glycosylated by posttranslational glycosylations at Thr 62, Asn 130, Asn 145, Asn 183, Asn 202, Asn 267, Asn 296, Asn 385, and Asn 406 positions.
Functions of Intercellular Adhesion Molecule 1
Basic Function of Intercellular Adhesion Molecule 1
Macrophage adhesion ligand-1 (Mac-1) and leukocyte function-associated antigen 1 (LFA-1) are expressed on endothelial cells and leukocytes. Therefore, when these proteins interact and bind to ICAM-1, transmigration of leukocytes across vascular endothelia is facilitated resulting in extravasation and endothelial inflammation. Due to these binding properties, ICAM-1 is largely implicated in intercellular adhesion. ICAM-1 has been shown to interact with CD11a, EZR, and CD18. Thus, the basic function of ICAM-1 is the generation of a specific and reversible cell-cell adhesion resulting in intercellular communication, T-cell-mediated defense mechanism, and inflammatory response (Rothlein et al. 1986; Marlin and Springer 1987; Diamond et al. 1990). ICAM-1 has also played a major role in spermatogenesis in testis (Cai et al. 2016). However, the role of ICAM-1 as a simple adhesion molecule became more broadened with new discovery of ICAM-1’s ability to serve as respiratory epithelial cell surface receptor of human rhinovirus, the causative agent of most common colds (Greve et al. 1989; Mukhopadhyay et al. 2014). ICAM-1 is also found to bind with plasmodium falciparum-infected erythrocytes. This demonstrates ICAM-1 having a unique role in viral and parasitic infectious diseases in addition to role in cell surface adhesion molecule (Berendt et al. 1992).
Sligh et al. (1993) observed that surface ICAM-1 expression is not a prerequisite for survival as confirmed from their gene knock down study. However, ICAM-1 deficiency caused aberrant immune and inflammatory responses such as moderate granulocytosis, impaired neutrophil migration during chemical peritonitis, and reduced contact hypersensitivity (Sligh et al, 1993). Gottrand et al. (2015) observed that disrupted ICAM-1 express truncated ICAM-1 isoforms lacking transmembrane domain. Their group further studied the effect of ICAM-I disruption, and results revealed impaired thymocyte development, peripheral T-cell distribution, T-cell activation, and T regulatory (Treg)-suppressive activity in mice containing disrupted ICAM-I which further supported vitality of ICAM-I in Treg development and suppressive function in immune response (Gottrand et al. 2015).
Role of Intercellular Adhesion Molecule 1 in Cell Signaling
In recent years, a potential role for ICAM-1 in signal transduction is explored. Besides its classical role as an adhesion and viral entry molecule, ICAM-1 is also involved in pro-inflammatory cell signaling pathways and recruits macrophages and granulocytes during inflammation. ICAM-1 has specific binding properties thus displaying important role in cell-to-cell communication. Moreover, ICAM-1 is lately implicated as cellular receptor of human rhinovirus. ICAM-1, having distinct role in immune responses, has distinct functions in signal transduction. Depending upon cell type, ICAM-1 contributes in the transport of inflammatory cells, antigen presentation, microbial pathogenesis, and signal transduction through outside in signaling events. ICAM-1 has also been observed to activate phosphorylation-dependent kinases resulting in activation of transcription factors, cytokine production, membrane-bound protein expression, reactive oxygen species (ROS) production, and cell proliferation. Interestingly, ICAM-I ligation resulted in upregulation of the production of chemokines such as regulated upon activation of normal T-cell expressed and secreted (RANTES) which mediated macrophage and granulocyte activation. However, the reticular nature of signaling cascades of ICAM-1 involving these downstream kinases are largely unknown (Lawson and Wolf 2009).
Pathological Role of Intercellular Adhesion Molecule 1 and Anti-intercellular Adhesion Molecule 1 in Therapy
ICAM-1 is constitutively present on endothelial cells, but its expression is increased by pro-inflammatory cytokines. The endothelial expression of ICAM-1 is increased in atherosclerotic and transplant-associated atherosclerotic tissue and in animal models of atherosclerosis. A soluble ICAM-1 (sICAM-1) is elevated in the serum of patients with cardiovascular disease, autoimmune disorders, as well as various cancers, and several studies have correlated serum levels of sICAM-1 with the severity of these diseases. While increased level of inflammatory condition enhances ICAM-1 expression, once expressed, ICAM-1 further enhances inflammation and oxidative stress and thereby complicates various diseases. A mouse-human ICAM-1 chimera has been developed in transgenic BALB/c mice which aid in the understanding of molecular mechanism in ICAM-1-mediated cellular inflammation (Bartlett et al. 2008). ICAM-1 has an important role in hypersensitivity type I reaction. ICAM-1 interacts with its receptors and thus recruits pro-inflammatory lymphocytes and mast cells in ocular allergies.
As the first murine anti-ICAM-1 mAb administered to humans as an anti-inflammatory agent, R6.5 (enlimomab) proved to be beneficial in suppressing disease activity in patients with refractory rheumatoid arthritis (Kavanaugh et al. 1994). Increased expression of endothelial ICAM-1 has been found in humans to be associated with neutrophil emigration into ischemic brain tissue (Lindsberg et al. 1996). It is presently not known whether R6.5 can prevent neutrophil infiltration in human cerebral infarction. Similarly, anti-ICAM therapy is not an effective treatment in ischemic stroke in mouse model and may significantly worsen stroke outcome.
In 1992, Isobe et al. used monoclonal antibody against ICAM-1 and LFA-1 and indicated as a useful therapeutic agent to prevent ICAM-1-LFA-1 adhesion for allograft rejection of experimental mice model. In another study, Ozer et al. (2001) investigated the effects of anti-ICAM-1 and anti-LFA-1 therapy in the rat hind-limb-cremaster transplantation model. The experimental results showed that the combination of anti-ICAM-1 and anti-LFA-1 monoclonal antibodies significantly prolonged allograft survival in this composite tissue transplantation model.
In subarachnoid hemorrhagic patients, ICAM-1 level is significantly elevated as compared to control. However, ICAM-1 is not directly linked to cerebral vasospasm in these patients, but an anti-ICAM-1 antibody therapy relieved the symptoms in 70% of patients implicating the role of ICAM-1 in this disease. In addition, ICAM-1 is also involved in many complications such as cancers of myeloid and lymphoid origin, acquired immunodeficiency syndrome, and allergic asthma. In Sjogren syndrome, expression of HLA-DR antigen and ICAM-1 in human conjunctival epithelium is upregulated in patients with dry eyes. In 1999, Tsubota et al. have shown upregulation of ICAM-1 in Sjogren syndrome patients may be controlled by treating with IFNγ through the activation of transcription factor NF-κB. Furthermore, Zhai et al. observed that anti-ICAM-1 therapy with enlimomab (murine anti-ICAM-1 antibody) may increase the prognosis in sepsis. Recently in a comprehensive review article, we have discussed the importance of anti-ICAM1 therapy against asthma and rhinitis (Mukhopadhyay et al. 2014).
ICAM-1 is a cell surface glycoprotein which belongs to immunoglobulin superfamily of proteins and is expressed by both hematopoietic and non-hematopoietic cells. The natural ligands of ICAM-1 are macrophage adhesion ligand 1 (Mac-1) expressed on the surface of macrophages, leukocyte function-associated antigen 1 (LFA-1) expressed on the surface of T cells, fibrinogen, an ECM protein and human rhino virus (HRV), the common cold virus. ICAM-1 helps in homing and trafficking of these cells. Engagement of ICAM-1 assists tight cell-to-cell interactions during antigen presentation and outside in signal transduction events. ICAM-1 expression is not a prerequisite for survival as ICAM-1 knockout (KO) mice have been found to grow and mature normally. However, neutrophil migration is impaired in ICAM-1 KO mice, under certain conditions. Recently, a mouse-human ICAM-1 chimera has been developed in transgenic BALB/c mice which aid in the understanding of molecular mechanism in ICAM-1-mediated cellular inflammation. High level expression of ICAM-1 is related to increased level of inflammation and subsequent complication of various diseases. While pro-inflammatory condition including treatment with various cytokines (e.g., TNFα/IFNγ) induces ICAM-1 expression, once expressed, ICAM-1 further enhances inflammatory condition and thereby complicates various inflammatory diseases. Thus, anti-ICAM-1therapy may be useful against a wide variety of diseases where increased level of inflammation and oxidative stress is a prerequisite to complicate the disease.
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