Human MCP Chemokine Cluster
Chemokines are small chemotactic cytokines that promote directional cell movement and originally were shown to regulate inflammation. By now the superfamily of chemokines and their receptors consists of 50 chemokines and 19 receptors (Zlotnik et al. 2006). Chemokines are classified into four groups CC, CXC, C, and CX3C, based on the position of the conserved cysteines in the amino-terminal domain. They bind to G-coupled protein receptors, with one ligand usually binding to more than one receptor and with the receptors exhibiting high affinity for more than one ligand (Rollins 1997). The chemokine superfamily is characterized, besides the low molecular mass (8–10 kDa) and the four conserved cysteines, by the ability of its members to bind heparin. The major chemokine clusters are the CC and the CXC, with the first originally identified as chemoattractant for monocytes and the latter for neutrophils. The CC and CXC chemokine clusters in humans are located on chromosome 17q12 and 4q13, respectively. The mouse CC and CXC genes are located on the chromosomes 11 and 5 respectively (Nomiyama et al. 2001).
CCL2 (Monocyte Chemoattractive Protein-1, MCP-1)
CCL2 gene consists of two introns and three exons, encoding for a 99 amino acid-long protein with a signal sequence of 23 residues. The mature protein is composed of 76 amino acids containing two cysteine bridges (Cys11-Cys36, and Cys12-Cys52) that are conserved in the C-C chemokine family. CCL2 has the highest chemotactic activity on monocytes and T lymphocytes (CD4+ memory T lymphocytes) at concentrations as low as 0.1 nM. In addition it induces the migration of NK cells, basophils, and dendritic cells, but not the migration of neutrophils (Proost et al. 1996).
CCL2 is produced by fibroblasts, mononuclear cells, endothelial cells, epithelial cells, keratinocytes, melanocytes, smooth muscle cells, mesothelial cells, mesangial cells, chondrocytes, osteoblasts, liver fat-storing cells, astrocytes, and microglia. Interferon gamma (IFN-γ), Interleukin-1 (IL-1), and tumor necrosis factor alpha (TNF-α) are potent inducers of its expression.
The amino terminal region of CCL2 is crucial for its biological activity (Yadav et al. 2010).
CCL2 exerts its chemoattractive activity by binding mostly to the receptor CCR2 and to a lesser extend through CCR11. It also binds to the decoy receptors D6 and DARC.
Binding of CCL2 to CCR2 results in mobilization of intracellular calcium (Ca2+), inhibition of adenylyl cyclase, and arachidonic acid release, and is mediated by a Pertussis toxin-sensitive G-protein(s).
Another receptor of CCL2 is CCR11 however binding to CCR11 does not result in increased intracellular calcium mobilization, which is essential for chemotaxis. Furthermore, CCL2 has a lower affinity for CCR11 than other chemokines (Schweickart et al. 2000).
CCL2 is overexpressed in synovium and synovial fluids of arthritis patients (rheumatoid arthritis, osteoarthritis, gout, and traumatic arthritis) and in the bronchial epithelium of idiopathic pulmonary fibrosis and asthma patients, as well as in tuberculosis effusions (Proost et al. 1996).
High levels of CCL2 were also detected in skin diseases, in atherosclerotic lesions, in chronic acute hepatitis, in experimental autoimmune encephalomyelitis, and in serum of septic patients (Proost et al. 1996).
CL2 is one of the most studied chemokines among the MCP cluster with increasing evidence for its involvement in pathogenic conditions such as multiple sclerosis, atherosclerosis, insulin-resistant diabetes, rheumatoid arthritis, lupus nephritis, and cancer (Yadav et al. 2010). In murine inflammatory models, CCL2 knockout mice have been shown to have defects in monocyte recruitment.
Studies in ischemic brain injury with CCL2KO mice showed that CCL2 deficiency is neuroprotective and this finding has been further confirmed using CCR2KO mice (Dimitrijevic et al. 2007). It was also demonstrated that CCL2 and its receptor (CCR2) besides regulating the infiltration of inflammatory cells could have an important role in regulating blood-brain barrier (BBB) permeability.
Subsequent studies showed that depending on the context CCL2 can be either tumor promoting or tumor suppressive.
Overexpression of human or mouse CCL2 in cancer cells suppressed tumor formation in immunocompromised mice. The tumors in the particular mouse models were characterized by a lower growth rate and infiltration of mononuclear cells, suggesting that CCL2 can activate host antitumor responses in a T lymphocyte-independent manner (Rollins and Sunday 1991; Bottazzi et al. 1990).
On the other hand, there is accumulating evidence for the protumorigenic effects of CCL2, also supported by the fact that CCL2 is produced by a variety of cancer cells. This also explains the initial designation of CCL2 as tumor-derived chemotactic factor.
A study in human breast cancer samples revealed that CCL2 is responsible for the recruitment of tumor-associated macrophages and affects angiogenesis and survival of patients. It is conceivable that the opposing effects of CCL2 in tumorigenesis reflect the pro- and antioncogenic effects of inflammation with which CCL2 is implicated (Ueno et al. 2000).
These findings were further supported by a number of studies, indicating that CCL2 can be highly expressed by both the tumor and surrounding stromal cells. It has been now established that CCL2 can promote tumor progression either through autonomous or nonautonomous mechanisms. Tumor-derived CCL2 can activate the recruitment of monocytes, providing growth and antiapoptotic signals to the tumor, or tumor cells can activate the production of CCL2 from the stroma contributing to tumor progression.
Inhibition of this paracrine activity of CCL2 by using a neutralizing antibody treatment of tumor-bearing mice resulted in the reduction of infiltrated macrophages and tumor growth suggesting the potential therapeutic application of CCL2 antibodies (Fujimoto et al. 2009).
In addition, in the context of neoplastic disease, recent studies implicated epithelial cell-derived CCL2 and CCR2-expressing macrophages as an important axis in metastasis of breast cancer to the bone and lungs.
Administration of CCL2 in mice increased organ-specific metastasis of human breast cancer cells and inhibition of CCL2 using neutralizing antibodies suppressed metastasis. CCL2 neutralization has shown positive outcomes in several murine models of mammary, prostate, and lung cancers (Lu and Kang 2009). Noteworthy, a recent study showed the enhancement of disease severity after interruption of anti-CCL2 treatment, resulting in induction of breast cancer metastases (Bonapace et al. 2014).
CCL8 (Monocyte Chemoattractive Protein-2, MCP-2)
Human CCL8, similarly to CCL2, was identified as monocyte chemotactic factor and isolated from osteosarcoma cells. Its primary structure is related to that of CCL2, also consisting of the four conserved cysteine residues. Its function is similar to CCL2, attracting monocytes, but not neutrophils.
CCL8 gene consists of two introns and three exons, encoding for a 99 amino acid-long protein and a 76 amino acid-long mature protein.
CCL8 induces migration of monocytes, with the same potency as CCL2 at 0.1 nM, with a typical bell-shaped curve.
CCL8 besides monocytes also induces the migration of T lymphocytes (memory and naïve) and at higher concentrations the migration of natural killer cells (NK), basophils, mast cells, and eosinophils.
CCL8 is produced by monocytes and fibroblasts under stimulation with IFN-γ and tumor cells.
In contrast to CCL2, CCL8-induced chemotaxis is blocked by cholera toxin and relatively unaffected by pertussis toxin. In addition, CCL8 signaling has a minor effect on calcium mobilization or arachidonic acid release, suggesting that CCL8 utilizes different receptors and signal transduction pathways and might have different properties. However, CCL8 attenuates the CCL2-induced monocyte migration, through homologous deactivation, suggesting the utilization of the same CCL2 receptor domains. Further studies by using ectopically expressed CCL8 are showing that CCL8, unlike CCL2, uses both CCR1 as well as CCR2B as its functional receptors, which explains its unique activities (Sozzani et al. 1994; Proost et al. 1996).
Other CCL8 receptors include CCR3 and CCR5, with the latter being activated particularly in T cells in response to CCL8 (Ruffing et al. 1998). Furthermore, it binds to the atypical decoy receptor D6.
As regards the biological activity, CCL8 through binding to CCR5 on CD4+ T lymphocytes inhibits the entry and replication of human immunodeficiency virus type 1 (HIV-1) (Yang et al. 2002).
Studies in mouse plasma indicate that CCL8 expression is correlated with the progression of graft vs.host disease (GVHD) after hematopoietic stem-cell transplantation (HSCT) and could serve as a marker for its early diagnosis (Yamamoto et al. 2011).
CCL8 expression in human plasma could be a promising new biomarker for the diagnosis of Mycobacterium tuberculosis infection (Ruhwald et al. 2008).
Furthermore, studies regarding inflammation in mouse skin identified an additional receptor for mouse Ccl8, CCR8. Mouse CCL8 is found to recruit CCR8-expressing inflammatory T helper type 2 (Th2) cells, regulating chronic allergic inflammation (Islam et al. 2011).
The involvement of CCL8 in cancer appears to be through modulation of the tumor-promoting activity of tumor microenvironment. It has been shown that breast cancer cells can stimulate CCL8 production in adjacent stromal fibroblasts and similarly CCL8 expression is increased in cancer-associated fibroblasts in a colon cancer mouse model. In addition, CCL8 expression has been shown in the melanoma microenvironment.
CCL8 is involved in breast cancer metastasis, through the establishment of self-sustained gradients between the stroma, the periphery, and the neoplastic epithelium that promote the dissemination of breast cancer cells (Farmaki et al. 2016). The prometastatic activity of CCL8 has been associated with the recruitment of regulatory T cells (Tregs) in an orthotopic mammary tumor model (Halvorsen et al. 2016). Finally, Ccl8 expression was found to be increased in the plasma of mice during the growth of breast cancers. Studies in CCL8-deficient mice showed that CCL8 promotes the dissemination of breast cancer cells (Farmaki et al. 2016).
CCL7 (Monocyte Chemoattractive Protein-3, MCP-3)
CCL7 was initially isolated along with CCL8 from osteosarcoma cells and is biochemically and biologically related to CCL2 and CCL8. CCL7 gene similarly to CCL2 and CCL8 consists of two introns and three exons, encoding for a 99 amino acid-long protein and a 76 amino acid-long mature protein (Proost et al. 1996).
CCL7 has a broader spectrum of action as regards chemotaxis compared to CCL2 and CCL8. Apart from monocytes and lymphocytes, also NK cells, eosinophils, basophils, dendritic cells, and neutrophils respond to CCL7 at low nanomolar concentrations.
Similarly to CCL2, monocyte chemotaxis stimulated by CCL7 is sensitive to pertussis toxin but not cholera toxin. Furthermore, consistent with CCL2, signaling activated by CCL7 results in calcium influx and arachidonic acid release.
CCL7 is produced by certain tumor cell lines, fibroblasts, colonic epithelial cells, and macrophages.
It binds to CCR1, CCR2, CCR3, and CCR5 chemokine receptors and the atypical decoy receptors DARC and D6 (Rollins 1997).
CCL7 plays an important role in chronic inflammatory skin diseases such as psoriasis where it is found to be upregulated in both epidermal and dermal tissues, regulating the infiltration of a variety of inflammatory cells.
CCL7 stimulates human coronary smooth muscle cell proliferation (Maddaluno et al. 2011), suggesting a potential role for this chemokine in vascular pathology. CCL7 is induced in smooth muscle cells and in carotid artery after balloon angioplasty, implicated in the pathogenesis of restenosis and atherosclerosis.
In addition, CCL7 recruits mesenchymal stem cells (MSCs) to sites of injured tissue after myocardial infarction and improves cardiac remodeling (Schenk et al. 2007).
As regards cancer, it has been shown that CCL7 can enhance the invasion and dissemination of prostate cancer, oral squamous cell carcinoma, and gastric cancer. CCL7 and its receptors are overexpressed in liver metastasis of colorectal cancer (Cho et al. 2012).
In colon cancer, CCL7 enhances cancer progression and metastasis via epithelial-mesenchymal transition (EMT) through its receptor CCR3 and mitogen-activated protein kinases (MAPKs) signaling pathways (Song et al. 2016).
CCL11, initially named as eotaxin (composite of the words eosinophil chemotaxin), was identified in an in vivo model (guinea pig) of allergic airways inflammation as an eosinophil selective chemoattractant. The cloning of human CCL11 few years later confirmed that it is a strong and specific eosinophil chemoattractant in vitro. Furthermore, it is a strong chemoattractant for monocytes but not for T cells and NK cells (Jose et al. 1994).
CCL11 is produced by eosinophils, macrophages, lymphocytes, fibroblasts, smooth muscle endothelial cells, epithelial cells, and chondrocytes.
Consistent with other cytokines, the amino-terminal domain of CCL11 is important for its biological activity. CCL24 (eotaxin-2) and CCL25 (eotaxin-3) are functional homologs of CCL11 but they are not part of the MCP cluster. Unlike other chemokines, the human mRNA for eotaxin-1 is constitutively expressed in the small intestine and colon.
The main receptor for CCL11 is CCR3. Recent evidence indicates that CCL11 may also bind to other receptors such as CCR2, CCR5, and possibly CXCR3, suggesting a more widespread regulatory role beyond the recruitment of eosinophils.
In cancer, CCL11 and CCR3 are found to be overexpressed in glioblastoma cells promoting the proliferation, migration, and invasion. Furthermore, CCL11-CCR3 axis appears to be a useful prognostic and predictive biomarker in glioblastoma patients (Tian et al. 2016).
CCL11 levels are increased in the serum of patients with prostate cancer compared to control group. Even though a prognostic role for CCL11 could not be established, the chemokine may serve as a diagnostic marker to distinguish between disease-free prostates and cancer (Heidegger et al. 2015).
CCL13 was identified in a library constructed from human fetal RNA. It consists of 75 amino acids and has functional similarities to CCL7 and CCL11. CCL13 is a chemoattractant of high efficacy for monocytes and T lymphocytes and also eosinophils, basophils, and immature dendritic cells. Its chemoattracting activity for eosinophils is similar to CCL11, but less potent than CCL2 in attracting monocytes or T cells. Similar to other CC chemokines, CCL13 binds to several chemokine receptors and particularly CCR1, CCR2, and CCR3 (Uguccioni et al. 1996).
CCL13 expression levels are high in organs such as small intestine, colon, and lung, consistent with an immediate infiltration of eosinophils in response to invading pathogens.
Unlike other MCP chemokines, CCL13 expression is not affected by proinflammatory mediators such as lipopolysaccharide (LPS) and TNF-a.
CCL13 is involved in many chronic inflammatory diseases, regulating the selective recruitment of cell lineages to the inflamed tissues and their subsequent activation.
CCL13 is also highly expressed in cartilage from patients with rheumatoid arthritis and its concentration is found to be elevated in sera from these patients. CCL13 may be associated with disease progression through the induction of macrophage infiltration, and synovial tissue angiogenesis in rheumatoid arthritis patients (Yamaguchi et al. 2013).
Furthermore, CCL13 levels are increased in the sera of patients with systemic sclerosis (Yanaba et al. 2010).
In addition, the involvement of CCL13 in asthma has been shown, where it not only predicts susceptibility to asthma but is also directly associated with exacerbations (Kalayaci et al. 2004).
CCL13 is a human-only chemokine for which mouse homolog has not been identified.
Human CCL1 was initially named as I-309 and identified as a chemokine secreted by activated T lymphocytes, around the same time that most of chemokines of the MCP cluster were discovered.
CCL1 is expressed also by monocytes, dendritic cells and activated mast cells and endothelial cells, however its expression levels in tissue cells is low. In the skin where CCL1 is well studied, besides T lymphocytes it is also produced by Langerhans cells, melanocytes, and endothelial cells.
Inflammatory cytokines and microbial products dramatically induce CCL1 expression.
Its expression is associated with atopic dermatitis, allergy, and asthma; however the results are controversial (McCully and Moser 2011).
Unlike the other chemokines of the cluster, CCL1 has one unique receptor CCR8 that was cloned in the late 1990s. CCR8 is expressed on lymphocytes of the Th2 lineage, including Th1, Th2, Treg cells, and also on thymocytes with natural Treg function. Expression and function of CCR8 in monocytes, dendritic cells, and NK cells is still controversial.
CCR8 expression has been found on phagocytic macrophages and activated microglial cells in the human central nervous system and also on active demyelinating multiple sclerosis (MS) lesions, in progressive multifocal leukoencephalopathy (PML), and in cerebral ischemia. In addition, CCR8 is detected on endothelial-derived spindle cells of human Kaposi sarcoma biopsies. Signaling through CCR8 is sensitive to pertussis toxin, suggesting coupling to a Gi-type G protein (McCully and Moser 2011).
In cancer, the CCL1–CCR8 axis has been implicated mostly in leukemia and in lymphoma development. In vitro studies demonstrated that CCL1 protects lymphoma and T cell leukemia cells from apoptosis and supports transformation of T cells. Furthermore, CCL1 has been shown to play a role in cancer metastasis by regulating the entry of tumor cells into the lymph node (Das et al. 2013).
Finally, CCL1 is found to be upregulated by fibroblasts in the microenvironment of bladder cancer promoting tumor invasion (Yeh et al. 2015).
The role of chemoattractive cytokines (chemokines) receives increased appreciation for the regulation of several physiological processes and the development of pathological conditions. The MCP cluster of chemokines is located on 17q12 chromosome in humans and contains six chemokines, CCL2, CCL7, CCL8, CCL11, CCL13, and CCL1. In the mouse, orthologs of these genes are located on chromosome 11C with the exception of CCL13 that exists only in the human genome and Ccl12 that exists only in the mouse genome. The MCP cluster chemokines are produced by several different cell types and by acting through their G-protein coupled receptors establish autocrine/paracrine/endocrine networks of communication that promote directed cell migration.
- Bottazzi B, Colotta F, Sica A, Nobili N, Mantovani A. A chemoattractant expressed in human sarcoma cells (tumor-derived chemotactic factor, TDCF) is identical to monocyte chemoattractant protein-1/monocyte chemotactic and activating factor (MCP-1/MCAF). Int J Cancer. 1990;45:795–7.PubMedCrossRefGoogle Scholar