1. Introduction
CD147/Basigin, originally cloned as a carrier of the Lewis X carbohydrate antigen, is a transmembrane protein belonging to the immunoglobulin super family [
1,
2]. CD147 performs pleiotropic functions by binding to various molecules including monocarboxylate transporters (MCTs). MCTs, MCT-1 and MCT-4, play important roles in the regulation of glycolysis, the enzymatic conversion of glucose to pyruvate and adenosine triphosphate (ATP), to fuel the pathophysiological activities of cellular growth and proliferation [
3]. CD147 is highly associated with MCT-1 and MCT-4 and facilitates their expression on the plasma membrane [
4].
Malignant melanoma (MM) is highly malignant because of its rapid growth and metastatic properties. In MM, CD147 expression is enhanced on the cell membrane and CD147 plays an important role in tumor proliferation, production of matrix metalloproteinases (MMPs), angiogenesis, invasiveness, and metastatic activity
in vitro and
in vivo [
5,
6]. In human MM cells, CD147 co-localizes with MCT-1 and MCT-4 on the plasma membrane and promotes the proliferation, invasiveness, and metastasis of MM by regulating glycolysis. Silencing of CD147 by a small interfering RNA (siRNA) clearly abrogates the expression of MCT-1 and MCT-4 and their co-localization with CD147, dramatically decreases the glycolysis rate, extracellular pH, and the production of ATP. Thus, cell proliferation, invasiveness, and VEGF production are significantly decreased by siRNA. These previous observations strongly suggest that CD147 interacts with MCT-1 and MCT-4 to promote tumor cell glycolysis leading to the progression of MM [
5,
6,
7].
T-cells also depend on glycolysis to generate energy for their differentiation, proliferation, and activation [
8]. Psoriasis is a chronic inflammatory dermatosis characterized by recurrent episodes of sharply demarcated scaly erythematous plaques. Differentiation of naïve CD4
+ T-cells into Th17 cells is important for the development of psoriasis [
9,
10]. We speculated that CD147 is involved in the pathogenesis of psoriasis through the regulation of glycolysis and demonstrated that CD147 is essential for the development of psoriasis via the induction of Th17 cell differentiation. The serum level of CD147 is increased in psoriasis patients and the expression of CD147 and MCT-1 is elevated on their dermal CD4
+ RORγt
+ Th17 cells.
In vitro, the potential of naïve CD4
+ T cells to differentiate into Th17 cells is abrogated in T-cells from CD147-deficient mice. Imiquimod (IMQ)-induced psoriasis-like dermatitis is significantly milder in CD147-deficient mice and bone marrow chimeric mice lacking CD147 in bone marrow hematopoietic cells. These findings demonstrate that CD147 is essential for the development of psoriasis via the induction of Th17 cell differentiation [
11].
Here, we review the function of CD147 in glycolysis and its involvement in the development of malignant tumors and immune disorders.
3. Chaperone-like function of CD147/Basigin
Previous studies have revealed that CD147 has various binding partners such as cyclophilin A (CyPA) [
33], integrins [
34,
35], P-glycoprotein [
12,
30,
36], and MCTs [
4]. The pleiotropic functions of CD147 are attributable to its binding partners. CD147 acts as a chaperone for their proper plasma membrane expression and catalytic activity and participates in many pathophysiological processes through these molecules [
12,
30].
CyPA, the major target of the immunosuppressive drug cyclosporin A, is a ubiquitously distributed intracellular protein. CyPA is secreted by cells in response to inflammatory stimuli and is a potent neutrophil and eosinophil chemoattractant. In the process of inflammation, CD147 acts as a cell surface receptor for CyPA and initiates signaling cascades leading to ERK activation [
33]. CD147 binds to β1-integrin. β1-integrin induces aggregation of promonocyte line U937 cells via protein tyrosine phosphorylation. The antibody against CD147 inhibits the aggregation and the tyrosine phosphorylation by blocking the binding between β1-integrin and CD147 [
34]. Multidrug resistance (MDR) of cancer cells is often associated with over expression of P-glycoprotein (P-gp), a transmembrane ATP-dependent transporter. Expression of P-gp and CD147 is up-regulated in Adriamycin (ADR)-resistant human mammary carcinoma cell line MCF7 (MCF7/Adr) compared to its non-ADR-resistant counterpart MCF7. Silencing of CD147 in MCF7/Adr by shRNA targeting CD147 resulted in the down-regulation of P-gp expression and the reduction of drug resistance [
36].
The association between CD147 and MCTs yielded important clues for understanding the role of CD147. Cross-linking studies showed that CD147 forms a homo-oligomer [
37]. Homodimerized CD147 binds to two MCT monomers and facilitates its proper folding and expression on the cell membrane (
Figure 1). In MCT-transfected COS cells, expressed MCT proteins accumulate in the perinuclear compartment, whereas co-transfection with CD147 cDNA enables the expression of functional MCT-1 or MCT-4 on the plasma membrane [
4]. As mentioned previously, CD147 consists of two Ig-like extracellular domains: a single transmembrane domain and a short C-terminal cytoplasmic tail [
1]. MCT-1 directly binds to the transmembrane domain of CD147 [
30,
38]. The cryo-EM structure of the CD147/MCT complex has been determined [
39,
40] and Glu218 in the CD147 transmembrane domain is the binding site for MCT-1 [
39,
41]. Philp et al. demonstrated the role of CD147 in the retina, in association with MCTs [
42]. CD147-deficient mice exhibit visual impairment. In CD147-deficient mice, morphology of the retina at the light microscopic level and the fundus and the fluorescein fundus angiography appeared to be normal until 8 weeks of age, whereas the amplitude of all components of both the photopic and scotopic electroretinograms was decreased indicating that both rod and cone functions were severely affected in CD147-deficient mice [
43]. In these mice, cell membrane expression of MCT-1 and MCT-4 was greatly reduced in retinal photoreceptor cells and adjacent Müller cells. Müller cells are retinal glial cells, whose major role is to maintain the functional and structural stability of photoreceptor cells. Müller cells provide photoreceptor cells with lactate as fuel for normal functions [
42]. Because of the absence of MCTs on the membrane of facing cells, the flux of lactate from Müller cells to photoreceptor cells is disrupted, and photoreceptor cell activity is lost owing to energy depletion [
42]. The absence of CD147 results in the impaired expression of MCTs on the cell membrane leading to loss of vision.
Other binding partners of CD147 include glucose transporter-1 (GLUT1), CD44, the major hyaluronan receptor, CD43, CD98, γ-secretase, NOD2, γ-catenin, platelet glycoprotein VI (GPVI), and apolipoprotein D. Molecular interactions between CD147 and these binding partners have been previously documented [
30].
4. CD147 and glycolysis
Elucidation of the mechanism of lactate flux by CD147 and MCTs in the retina prompted us to investigate the role of CD147 in glycolysis. Glycolysis is the enzymatic conversion of glucose to pyruvate to generate energy, which is stored in the form of ATP. Pyruvate is further converted to lactic acid, which is exported through the plasma membrane and is required for metabolism and intracellular pH regulation [
38]. Lactic acid is transported by proton-linked/lactate co-transporters, MCTs, on the plasma membrane. The detailed metabolic pathway of glycolysis was initially studied in cancer cells. Under physiological conditions, cellular energy is provided by mitochondrial oxidative phosphorylation and glycolysis results from anaerobic enzymatic conversion. Warburg first reported that cancer cells depend on glycolysis for energy even in the presence of oxygen, i.e., under aerobic conditions [
3]. They take up excess glucose through GLUT-1 or GLUT-3, which is then enzymatically converted into ATP to fuel the pathophysiological activities of cellular growth and proliferation [
44]. During aerobic glycolysis in cancer cells, pyruvate is converted into lactic acid, which is excreted from the cytoplasm into the surrounding extracellular microenvironment through MCTs. Altered metabolism requires tumor cells to rapidly efflux lactate into the surrounding microenvironment to prevent self-poisoning. MCTs facilitate proton-linked monocarboxylate transport, leading to a decrease in the extracellular pH of tumors. The acidity of tumor microenvironment produces more aggressive phenotypes in cancer cells that exhibit increased proliferation, invasiveness, metastasis, and VEGF production [
45,
46,
47,
48].
Based on the observation in the retina, we investigated the involvement of CD147 in cancer cell glycolysis using MM cells. Human MM cells (A375) expressed higher levels of MCT-1, MCT-4, and CD147 and showed an increased glycolysis rate compared to normal human melanocytes. CD147 co-localized with MCT-1 and MCT-4 on the A375 cell membrane. CD147 silencing by siRNA abrogated MCT1 and MCT-4 expression and their co-localization with CD147. The glycolysis rate and ATP production were dramatically decreased and the extracellular pH increased. Subsequently, cell proliferation, invasiveness, and VEGF production were significantly inhibited [
5]. Gallagher et al. documented similar findings in highly metastatic breast cancer cell line MDA-MB-231. In accordance with the findings in MM, lactate efflux was mediated by MCTs and the accessory subunit, CD147. CD147 was highly expressed on MDA-MB-231 cells and its expression was linked to MCT expression. MCT-4 mRNA and protein expression were increased in MDA-MB-231 cells compared to cells derived from normal mammary tissue. MCT-4 co-localized with CD147 in the plasma membrane. CD147 silencing resulted in the loss of MCT4 in the plasma membrane and the accumulation of the transporter in endo-lysosomes. On the other hand, silencing of MCT-4 impaired the maturation and trafficking of CD147 to the cell surface, resulting in accumulation of CD147 in the endoplasmic reticulum [
49]. These findings strongly suggest that CD147 interacts with MCT-1 and MCT-4 to promote glycolysis in tumor cells, resulting in tumor progression (
Figure 2).
5. T cell differentiation/proliferation and glycolysis
Glycolysis is also important for differentiation, proliferation, and activation of lymphocytes including T cells, B cells, and natural killer cells. Activated lymphocytes engage in robust growth and rapid proliferation. For these processes, lymphocytes adopt glycolysis [
8]. Stimulated CD4
+ T cells differentiate into effector T cells or inducible regulatory T cells. Differentiation of CD4
+ T cells into distinct subsets, Th1, Th2, and Th17 cells, requires aerobic glycolysis. Th1, Th2, and Th17 cells express high surface levels of the GLUT-1 and are highly glycolytic [
50]. Halestrap and Wilson demonstrated the importance of MCT-1 in T cell activation and proliferation. In T cells, energy metabolism is largely glycolytic even under aerobic conditions and lactic acid efflux from T cells is mediated by MCT-1. MCT-1 is important during the activation and proliferation of resting T cells, which is accompanied by a switch from aerobic to glycolytic metabolism and remarkable increase in lactate production and export [
51]. In line with this, Murray et al. showed that a potent and specific MCT-1 inhibitor blocked T-cell proliferation and acted as an immunosuppressive agent [
52]. T cell glycolysis is regulated by CD147 because MCT-1 requires CD147 as an ancillary protein in T cells [
51]. Studies on the role of CD147 in T cell biology by Hahn et al. documented that CD147 is critical for energy metabolism and is involved in the development, activation, proliferation, migration, invasion, and adhesion of T-cells. The rapid proliferation and activation of T cells require glycolysis instead of oxidative phosphorylation to respond to their energy demands. Although glycolysis generates energy faster than oxidative phosphorylation, energy production is less efficient and leads to the intracellular accumulation of lactic acid. CD147 plays a role in alleviating lactate efflux for cell stability [
53]. In patients with RA, CD147 mRNA expression was elevated in peripheral blood mononuclear cells. CD147 promotes the differentiation of CD4
+ T cells into Th17 cells and induces production of Th17-secreting cytokine, interleukin (IL)-17, and Th17 differentiation-regulating cytokines, IL-6 and IL-1β [
54]. These findings strongly suggest that CD147 is involved in the pathogenesis of T cell-mediated immune disorders.
6. Psoriasis
Psoriasis is a chronic inflammatory keratotic dermatosis characterized clinically by recurrent episodes of sharply demarcated scaly erythematous plaques and histologically by hyperkeratosis with parakeratosis, Munro microabscess, absence of granular layer, regular elongation of the rete ridges, marked dilatation of blood vessels in the papillary dermis, and dense clusters of inflammatory cells consisting of T cells and dendritic cells in the dermis. Our molecular and cellular understanding of the immunopathogenesis of psoriasis has progressed, and recent studies have revealed that Th17 cells and related signaling pathways play pivotal roles in the development of psoriasis [
9,
10,
55]. First, tumor necrosis factor (TNF) and inducible nitric oxide synthase (iNOS)-producing dendritic cells (TIP-DC) which reside in the dermis are activated and produce TNF-α and IL-23. TNF-α is required for the maintenance of activatied state of TIP-DC and IL-23 promotes the differentiation of naïve CD4
+ T cells into Th17 cells in human. Th17 cells produce the effector cytokines IL-17 and IL-22 which activate signal transducer and activator of transcription (STAT) 3 signaling in keratinocytes, leading to the production of chemokines, cytokines, and proinflammatory mediators such as IL-6, IL-8, GM-CSF, CXCL10, and CCL20 which are responsible for the proliferation of epidermal keratinocytes to develop psoriasis lesions [
56]. Th17 cells are a distinct T cell subset derived from naïve CD4
+ helper T cells and the mechanism underlying Th17 cell differentiation is known [
57]. The combination of IL-6 and transforming growth factor-β (TGF-β) activates the retinoic acid receptor-related orphan nuclear receptor γt (RORγt) that is the key transcription factor directing the differentiation into Th17 cells [
58]. Because the differentiation of naïve CD4
+ helper T cells into Th17 cells is the fundamental process of psoriasis development [
59,
60] and requires glycolysis as an energy source, it is possible that CD147 participates in the pathogenesis of psoriasis.
CD147/Basigin and psoriasis
Previous studies showed that CD147 is highly expressed in peripheral blood neutrophils and neutrophils in the lesioned skin of patients with psoriasis [
61]. The expression level was significantly correlated with disease severity as evaluated by the psoriasis area and severity index (PASI) [
61]. In this study, inhibitory effect of CD147 on neutrophil chemotaxis was presented. An accepted model system for studying neutrophil chemotaxis using all-
trans retinoic acid (ATRA)-induced differentiated HL-60 promyelocytic leukemia cells was employed. By treatment with ATRA, HL-60 cells were differentiated into neutrophils, which was confirmed by the expression of CD11b, cytoplasm to nucleus ratio, and nuclear segmentation accompanied by chromatin condensation. IL-8 and N-formyl-methyonylleucyl-phenylalanine (fMLP), well-known neutrophil chemo-attractants, induced the chemotaxis of ATRA-treated HL-60 cells. Silencing of CD147 significantly decreased the migration of these cells [
61]. CD147 is cleaved and released into the peripheral blood as a 22-kDa fragment of the N-terminal extracellular domain [
62]. Soluble CD147 has been detected in patients with malignant tumors such as MM and immune disorders including psoriasis and systemic lupus erythematosus (SLE) [
63]. In patients with psoriasis, the serum level of soluble CD147 is significantly elevated compared to that in normal controls and correlates with PASI [
64]. The effect of soluble CD147 on keratinocyte proliferation was examined. When HaCaT cells, an immortalized non-tumorigenic keratinocyte cell line, were treated with plasma from patients with psoriasis, proliferation was induced compared with that from normal subjects [
64].
Regarding T cells in psoriasis, CD147 expression is increased in the CD3
+ T cells in the dermis of psoriasis lesions. Its expression was significantly higher on CD4
+ RORγt
+ Th17 cells than on CD4
+ RORγt
- non-Th17 T cells. The expression level of MCT-1 on RORγt
+ Th17 cells was upregulated and positively correlated with CD147 expression. These findings indicate that together with MCT-1, CD147 modulates CD4
+ T-cell differentiation into Th17 cells [
11].
The involvement of CD147 in psoriasis development was investigated in wild-type (WT) and CD147-deficient mice. MCT-1 was absent on CD4
+ T cells from CD147-deficient mice. Naïve CD4
+ T-cells from spleen of CD147-deficient mice showed a low potential for differentiating into Th17 cells in response to stimulation with IL-6 and TGF-β, which are the differentiation stimuli into Th17 cells (
Figure 3). Topical application of imiquimod (IMQ), a toll-like receptor-7 agonist, on the shaved skin induces psoriasis-like dermatitis. IMQ-induced dermatitis is an accepted psoriasis mouse model. After shaving the dorsal skin of CD147-deficient mice, 5% IMQ cream was applied for 7 consecutive days. The control WT mice were shaved but not treated with IMQ. The degree of skin inflammation was evaluated using the cumulative disease severity scoring system modified from the PASI score (modified PASI score). The degree of skin erythema, induration, and scaling was classified as 0 = none, 1 = mild, 2 = moderate, 3 = severe, and 4 = maximum. These scores were summed, resulting in a theoretical maximum score of 12. IMQ-induced skin inflammation was significantly milder in CD147-deficient mice than in WT mice (
Figure 4). The modified PASI score in WT mice and CD147-deficient mice were 8.75 and 5.2 at day 6, and 8.0 and 4,6 at day 7, respectively. These studies on mice deficient in the CD147 gene yielded direct evidence that CD147 plays a major role in the development of psoriasis [
11].
CD147 is ubiquitously expressed in various cells, including epidermal keratinocytes and lymphocytes. Epidermal CD147 plays a role in the pathogenesis of psoriasis via the production of IL-22 and subsequent STAT3 activation in the K5-promotor transgenic mice designed to overexpress CD147 in epidermal keratinocytes [
65]. To examine the importance of CD147 in immune cells in psoriasis, bone marrow chimeric mice lacking CD147 in the bone marrow hematopoietic cells were produced. To produce bone marrow chimeric mice, Ly5.1 recipient mice were exposed to 10 Gy of total body irradiation. Bone marrow lumps were obtained from the femur of CD147-deficient and their wild type (WT) counterparts, and the lumps were transplanted to the recipient mice trans-venously. Leukocytes from the Ly5.1 recipients and the C57BL/6J donors expressed CD45.1 and CD45.2, respectively, which enables to distinguish the origin of the immune cells in the lesional skin. IMQ-induced skin lesions appeared significantly milder in the CD147-deficient chimeric mice than that in the WT chimeric mice. The modified PASI score in WT chimeric mice and CD147-deficient chimeric mice were 5.33 and 1.8 at day 6, and 5.67 and 1.4 at day 7, respectively. These results show that CD147 in immune cells is responsible for the development of psoriasis.
7. CD147/Basigin and other immune disorders
The involvement of CD147 in other immune disorders has been previously documented. As mentioned above, CD147 promotes Th17 cell differentiation in patients with RA. Expression of CD147 mRNA was approximately 3-fold higher in the peripheral blood mononuclear cells (PBMCs) from RA patients than those from normal subjects. Differentiation of CD4
+ T cells into Th17 cells was promoted when CD4
+ T cells were co-cultured with CD14
+ monocytes in response to lipopolysaccharide (LPS) and anti-CD3 stimulation. The proportion of Th17 cells was significantly higher in RA patients than in normal subjects. Th17-secreting cytokine, IL-17, and Th17-regulating cytokines, IL-6 and IL-1β, were highly induced in the culture supernatants of the CD4
+ /CD14
+ co-culture. Anti-CD147 antibody reduced the proportion of Th17 cells and inhibited the production of IL-17, IL-6, and IL-1β [
54].
Lung interstitial fibrosis is a chronic pulmonary disease characterized by excessive accumulation of extracellular matrix and often develops as a complication of various autoimmune diseases [
66]. In a murine bleomycin-induced lung interstitial fibrosis model, CD147 neutralizing monoclonal antibodies (HAb18 mAbs) markedly reduced collagen accumulation and downregulated the proportion of M1 macrophages and Th17 cells. In vitro, M1 macrophages induced Th17 differentiation, which was significantly inhibited by treatment with HAb18 mAbs or by silencing CD147 by lentivirus interference in M1 macrophages. These observations indicate that CD147 promotes M1 macrophages and induces Th17 cell differentiation in lung interstitial fibrosis [
67].
Multiple sclerosis (MS) is a demyelinating and neurodegenerative autoimmune disorder associated with the migration of activated lymphocytes and macrophages into central nervous system (CNS). These leukocytes enter the white matter by penetrating the BBB in areas of inflammation called perivascular cuffs and through the CNS-meningeal barrier. Accumulation of activated leukocytes in the lesion causes the loss BBB integrity leading to the transmigration of leukocytes into the brain parenchyma. Metabolic switch is seen in T cells and macrophages in the lesion; they heavily depend on glycolysis as energy source. In the experimental autoimmune encephalomyelitis (EAE) model of MS, macrophages within the perivascular cuffs of postcapillary venules were highly glycolytic as shown by strong expression of lactate dehydrogenase (LDHA) which converts pyruvate to lactate. These macrophages expressed prominent level of MCT-4, which engages the secretion of lactate from the cytoplasm of glycolytic cells. Silencing of LDHA or MCT-4 by each siRNA resulted in decreased lactate secretion and macrophage transmigration, indicating the functional relevance of glycolysis in the pathogenesis of EAE. In EAE, CD147 binds to MCT-4 and regulates its expression on the cell membrane of macrophages, which was confirmed by co-immunoprecipitation. Glycolysis and glycolytic capacity were significantly abrogated by CD147-knockdown by siRNA as measured by extracellular acidification rate (ECAR). These results demonstrate the importance of CD147, in association with MCT-4, in governing macrophage glycolysis and its migration in inflammatory perivascular cuffs of EAE. Relevance of MS was confirmed by the strong expression of CD147, MCT-4, and LDHA in perivascular macrophages in MS brains of human [
68].
8. Conclusion and future directions
CD147 functions as a chaperone for various membrane proteins including CyPA, integrins, P-gp, MCTs, and so forth; and it supports their plasma membrane expression. In this review, we focused on MCTs. In association with MCT-1 and MCT-4, CD147 regulates glycolysis, an enzymatic metabolic system that generates energy that is stored as ATP. Glycolysis is required by cancer cells for their proliferation, invasiveness, and VEGF production and by T cells for their differentiation, proliferation, and activation. CD147 contributes to the development of malignant tumors and Th17 cell-mediated immune disorders including psoriasis. Therefore, CD147 is a promising therapeutic target for patients with these disorders.