CD226 is expressed on the megakaryocytic lineage from hematopoietic stem cells/progenitor cells and involved in its polyploidization

Inhibition of CD226 Co-Stimulation Suppresses Diabetes Development in the NOD Mouse by Augmenting Tregs and Diminishing Effector T Cell Function

Aims/hypothesis

Immunotherapeutics targeting T cells are crucial for inhibiting autoimmune disease progression proximal to disease onset in type 1 diabetes. A growing number of T cell-directed therapeutics have demonstrated partial therapeutic efficacy, with anti-CD3 (α-CD3) representing the only regulatory agency-approved drug capable of slowing disease progression through a mechanism involving the induction of partial T cell exhaustion. There is an outstanding need to augment the durability and effectiveness of T cell targeting by directly restraining proinflammatory T helper type 1 (Th1) and type 1 cytotoxic CD8 + T cell (Tc1) subsets, while simultaneously augmenting regulatory T cell (Treg) activity. Here, we present a novel strategy for reducing diabetes incidence in the NOD mouse model using a blocking monoclonal antibody targeting the type 1 diabetes-risk associated T cell co-stimulatory receptor, CD226.

Methods

Female NOD mice were treated with anti-CD226 between 7-8 weeks of age and then monitored for diabetes incidence and therapeutic mechanism of action.

Results

Compared to isotype-treated controls, anti-CD226 treated NOD mice showed reduced insulitis severity at 12 weeks and decreased disease incidence at 30 weeks. Flow cytometric analysis performed five weeks post-treatment demonstrated reduced proliferation of CD4 + and CD8 + effector memory T cells in spleens of anti-CD226 treated mice. Phenotyping of pancreatic Tregs revealed increased CD25 expression and STAT5 phosphorylation following anti-CD226, with splenic Tregs displaying augmented suppression of CD4 + T cell responders in vitro. Anti-CD226 treated mice exhibited reduced frequencies of islet-specific glucose-6-phosphatase catalytic subunit related protein (IGRP)-reactive CD8 + T cells in the pancreas, using both ex vivo tetramer staining and single-cell T cell receptor sequencing (scTCR-seq) approaches. 51 Cr-release assays demonstrated reduced cell-mediated lysis of beta-cells by anti-CD226-treated autoreactive cytotoxic T lymphocytes.

Conclusions/interpretation

CD226 blockade reduces T cell cytotoxicity and improves Treg function, representing a targeted and rational approach for restoring immune regulation in type 1 diabetes.

Research in Context

What is already known about this subject?: The co-stimulatory receptor CD226 is upregulated upon activation and is highly expressed on NK cell subsets, myeloid cells, and effector T cells. A single nucleotide polymorphism in CD226 ( rs763361 ; C>T) results in a Gly307Ser missense mutation linked to genetic susceptibility for type 1 diabetes. Global knockout of Cd226 and conditional Cd226 knockout in FoxP3 + Tregs reduced insulitis severity and diabetes incidence in NOD mice, indicating a crucial role for CD226 in disease pathogenesis. What is the key question?: Can CD226 blockade reduce T cell cytotoxicity and improve Treg function to diminish diabetes incidence in NOD mice?What are the new findings?: Anti-CD226 treatment reduced insulitis, decreased disease incidence, and inhibited splenic CD4 + and CD8 + effector memory T cell proliferation. Pancreatic Tregs from anti-CD226 treated mice exhibited increased CD25 expression; splenic Tregs displayed augmented STAT5 phosphorylation and suppressive capacity in vitro . Anti-CD226 treatment reduced IGRP-specific pancreatic CD8 + T cell frequencies, and reduced autoreactive CD8 + T cell-mediated lysis of beta-cells in vitro . How might this impact on clinical practice in the foreseeable future?: CD226 blockade could reduce autoreactive T cell cytotoxicity, enhance Treg function, and slow disease progression in high-risk or recent-onset type 1 diabetes cases.

CD226 promotes renal fibrosis by regulating macrophage activation and migration

It has been found that CD226 plays an important role in regulating macrophage function, but its expression and function in macrophages during renal fibrogenesis have not been studied. Our data demonstrated that CD226 expression in macrophages was obviously upregulated in the unilateral ureteral obstruction model, while CD226 deficiency attenuated collagen deposition in renal interstitium along with fewer M1 within renal cortex and renal medulla and a lower level of proinflammatory factors compared to that of control littermates. Further studies demonstrated that Cd226-/- bone marrow-derived macrophages transferring could significantly reduce the tubular injury, collagen deposition, and proinflammatory cytokine secretion compared with that of Cd226+/+ bone marrow-derived macrophages transferring in the unilateral ureteral obstruction model. Mechanistic investigations revealed that CD226 promoted proinflammatory M1 macrophage accumulation in the kidney via suppressing KLF4 expression in macrophages. Therefore, our results uncovered a pathogenic role of CD226 during the development of chronic kidney disease by promoting monocyte infiltration from peripheral blood into the kidney and enhancing macrophage activation toward the inflammatory phenotype by suppressing KLF4 expression.

A subset of megakaryocytes regulates development of hematopoietic stem cell precursors

Understanding the regulatory mechanisms facilitating hematopoietic stem cell (HSC) specification during embryogenesis is important for the generation of HSCs in vitro. Megakaryocyte emerged from the yolk sac and produce platelets, which are involved in multiple biological processes, such as preventing hemorrhage. However, whether megakaryocytes regulate HSC development in the embryonic aorta-gonad-mesonephros (AGM) region is unclear. Here, we use platelet factor 4 (PF4)-Cre;Rosa-tdTomato+ cells to report presence of megakaryocytes in the HSC developmental niche. Further, we use the PF4-Cre;Rosa-DTA (DTA) depletion model to reveal that megakaryocytes control HSC specification in the mouse embryos. Megakaryocyte deficiency blocks the generation and maturation of pre-HSCs and alters HSC activity at the AGM. Furthermore, megakaryocytes promote endothelial-to-hematopoietic transition in a OP9-DL1 coculture system. Single-cell RNA-sequencing identifies megakaryocytes positive for the cell surface marker CD226 as the subpopulation with highest potential in promoting the hemogenic fate of endothelial cells by secreting TNFSF14. In line, TNFSF14 treatment rescues hematopoietic cell function in megakaryocyte-depleted cocultures. Taken together, megakaryocytes promote production and maturation of pre-HSCs, acting as a critical microenvironmental control factor during embryonic hematopoiesis.

Targeting TIGIT for cancer immunotherapy: recent advances and future directions

As a newly identified checkpoint, T cell immunoreceptor with immunoglobulin and tyrosine-based inhibitory motif (ITIM) domain (TIGIT) is highly expressed on CD4+ T cells, CD8+ T cells, natural killer (NK) cells, regulatory T cells (Tregs), and tumor-infiltrating lymphocytes (TILs). TIGIT has been associated with NK cell exhaustion in vivo and in individuals with various cancers. It not only modulates NK cell survival but also mediates T cell exhaustion. As the primary ligand of TIGIT in humans, CD155 may be the main target for immunotherapy due to its interaction with TIGIT. It has been found that the anti-programmed cell death protein 1 (PD-1) treatment response in cancer immunotherapy is correlated with CD155 but not TIGIT. Anti-TIGIT alone and in combination with anti-PD-1 agents have been tested for cancer immunotherapy. Although two clinical studies on advanced lung cancer had positive results, the TIGIT-targeted antibody, tiragolumab, recently failed in two new trials. In this review, we highlight the current developments on TIGIT for cancer immunotherapy and discuss the characteristics and functions of TIGIT.

Increased proportion of CD226 + CD14 + monocytes correlates with clinical features and laboratory parameters in patients with primary Sjögren's syndrome

OBJECTIVES

CD226 is widely expressed on the surface of immune cells as a co-stimulatory receptor, which is involved in the development of many autoimmune diseases. The purpose of this study was to investigate the proportion of CD226 on CD14 + monocytes in the peripheral circulation of patients with primary Sjögren's syndrome (pSS) and the clinical significance of pSS.

METHODS

The proportion of CD226 on the surface of CD14 + monocytes was measured by flow cytometry in 45 pSS patients and 25 healthy controls (HC). The correlations between the proportion of CD226 + CD14 + monocytes and the clinical features and laboratory parameters of pSS were analyzed. Meanwhile, we analyzed the change in proportion of CD226 + CD14 + monocytes before and after treatment, and the clinical significance of pSS was evaluated.

RESULTS

The proportion of CD226 on CD14 + monocytes markedly increased in pSS patients compared to HC (p < .01). We found the proportion of CD226 + CD14 + monocytes was positively correlated with the disease activity and severity of pSS patients. The proportion of CD226 + CD14 + monocytes in pSS patients with decayed tooth, fatigue, interstitial lung disease (ILD), low WBC, high IgG, anti-Ro60, and anti-SSB positive increased compared to that in negative patients (p < .05). Furthermore, the proportion of CD226 + CD14 + monocytes was significantly higher in active patients than in nonactive patients (p < .01). Additionally, the proportion of CD226 + CD14 + monocytes decreased in seven pSS patients after treatment (p < .01).

CONCLUSION

Our study suggested that an increased CD226 proportion on CD14 + monocytes was associated with the clinical manifestations, disease activity, and prognosis of pSS patients. CD226+ CD14 + monocytes may present a potential target and a biomarker for the prognosis and therapy of pSS patients.

CD96 as a Potential Immune Regulator in Cancers

The discovery of CTLA-4 and PD-1 checkpoints has prompted scientific researchers and the pharmaceutical industry to develop and conduct extensive research on tumor-specific inhibitors. As a result, the list of potential immune checkpoint molecules is growing over time. Receptors for nectin and nectin-like proteins have recently emerged as promising targets for cancer immunotherapy. Potential immune checkpoints, including CD226, TIGIT, and CD96, belong to this receptor class. Among them, CD96 has received little attention. In this mini-review, we aim to discuss the basic biology of CD96 as well as the most recent relevant research on this as a promising candidate for cancer immunotherapy.

TIGIT-CD226-PVR axis: advancing immune checkpoint blockade for cancer immunotherapy

Recent advances in understanding the roles of immune checkpoints in allowing tumors to circumvent the immune system have led to successful therapeutic strategies that have fundamentally changed oncology practice. Thus far, immunotherapies against only two checkpoint targets have been approved, CTLA-4 and PD-L1/PD-1. Antibody blockade of these targets enhances the function of antitumor T cells at least in part by relieving inhibition of the T cell costimulatory receptor CD28. These successes have stimulated considerable interest in identifying other pathways that may bte targeted alone or together with existing immunotherapies. One such immune checkpoint axis is comprised of members of the PVR/nectin family that includes the inhibitory receptor T cell immunoreceptor with Ig and immunoreceptor tyrosine-based inhibitory domains (TIGIT). Interestingly, TIGIT acts to regulate the activity of a second costimulatory receptor CD226 that works in parallel to CD28. There are currently over two dozen TIGIT-directed blocking antibodies in various phases of clinical development, testament to the promise of modulating this pathway to enhance antitumor immune responses. In this review, we discuss the role of TIGIT as a checkpoint inhibitor, its interplay with the activating counter-receptor CD226, and its status as the next advance in cancer immunotherapy.

CD226 Is Required to Maintain Megakaryocytes/Platelets Homeostasis in the Treatment of Knee Osteoarthritis With Platelet-Rich Plasma in Mice

Platelet-rich plasma (PRP) is a platelet-based application used to treat osteoarthritis (OA) clinically. The co-stimulatory molecule CD226 is expressed in T cells, NK cells, and also platelets. However, exact effects of CD226 on platelets and whether its expression level influences PRP efficacy are largely unknown. Here, CD226^fl/flPF4-Cre mice were obtained from mating CD226 ^fl/fl mice with PF4-Cre mice. Blood samples and washed platelets were collected from the mice eyeballs to undergo routine blood tests and transmission electron microscopy. Differentially expressed proteins were detected by iTRAQ-based proteomics analysis. Animal OA models were established through surgical destabilization of the medial meniscus (DMM) for C57BL/6 wildtype mice, followed by PRP injection to evaluate the effects of platelet CD226 on PRP efficacy. The results showed that deletion of platelet CD226 increased the number of megakaryocytes (MKs) in bone marrow (BM) but reduced MKs in spleen, combined with significantly decreased platelet amounts, α-granule secretion, and reduced immature platelets; indicating that absence of platelet CD226 may disrupt MK/platelet homeostasis and arrested platelet release from MKs. Sequencing analysis showed abnormal ribosomal functions and much downregulated proteins in the absence of platelet CD226. Autophagy-related proteins were also reduced in the CD226-absent MKs/platelets. Moreover, deletion of platelet CD226 diminished the protective effects of PRP on DMM-induced cartilage lesions in mice, and PDGF restored it. Therefore, deficiency of platelet CD226 inhibited platelet maturation, secretion, and normal ribosomal functions, which may lead to depressed PRP efficacy on OA, suggesting that CD226 is required to regulate platelet growth, functions, and its application.

CD226: An Emerging Role in Immunologic Diseases

CD226, a member of the immunoglobulin superfamily, is a functional protein initially expressed on natural killer and T cells. In recent years, the function of CD226 has been increasingly realized and researched. Accumulating evidence shows that CD226 is closely related to the occurrence of autoimmune diseases, infectious diseases, and tumors. Because of the CD226’s increasing importance, the author herein discusses the structure, mechanism of action, and role of CD226 in various pathophysiological environments, allowing for further understanding of the function of CD226 and providing the basis for further research in related diseases.

Deficiency of platelet adhesion molecule CD226 causes megakaryocyte development and platelet hyperactivity

This study used constitutive CD226 gene knockout (KO) mice as a model to investigate the functions and mechanisms of CD226 in megakaryocyte (MK) maturation and platelet activation. Although CD226 deficiency did not cause MK polyploidization or platelet granule abnormalities, increased MK counts were detected in the femora bone marrow (BM) and spleen of CD226 KO mice. Particularly, CD226 KO mice have a more extensive membrane system in MKs and platelets than wild-type (WT) mice. We also demonstrated that CD226 KO mice displayed increased platelet counts, shortened bleeding time, and enhanced platelet aggregation. CD226 KO platelets had an increased mature platelet ratio compared to the control platelets. In addition, the observed reduction in bleeding time may be due to decreased nitric oxide (NO) production in the platelets. Platelet-specific CD226-deficient mice showed similar increased MK counts, shortened bleeding time, enhanced platelet aggregation, and decreased NO production in platelets. Furthermore, we performed middle cerebral artery occlusion-reperfusion surgery on WT and CD226 KO mice to explore the potential effect of CD226 on acute ischemia-reperfusion injury; the results revealed that CD226 deficiency led to significantly increased infarct area. Thus, CD226 is a promising candidate for the treatment of thrombotic disorders.

Genetics of platelet traits in ischaemic stroke: focus on mean platelet volume and platelet count

Purpose/Aim of the study: The aim of this review is to summarize the role of genetic variants affecting mean platelet volume (MPV) and platelet count (PLT) leading to higher platelet reactivity and in turn to thrombotic events like stroke and cardiovascular diseases.

MATERIALS AND METHODS

A search was conducted in PUBMED, MEDLINE, EMBASE, PROQUEST, Science Direct, Cochrane Library, and Google Scholar related to the studies focussing on genome-wide association studies (GWAS), whole exome sequencing (WES), whole genome sequencing (WGS), phenome-wide association studies (PheWAS) and multi-omic analysis that have been employed to identify the genetic variants influencing MPV and PLT.

RESULTS

Antiplatelet agents underscore the crucial role of platelets in the pathogenesis of stroke. Higher platelet reactivity in terms of mean platelet volume (MPV) and platelet count (PLT) contributes significantly to the interindividual variation in platelet reaction at the site of vessel wall injury. Some individuals encounter thrombotic events as platelets get occluded at the site of vessel wall injury whereas others heal the injury without occluding the circulation. Evidence suggests that MPV and PLT have a strong genetic component. High throughput techniques including genome-wide association studies (GWAS), whole exome sequencing (WES), whole genome sequencing (WGS), phenome-wide association studies (PheWAS) and multi-omic analysis have identified different genetic variants influencing MPV and PLT.

CONCLUSIONS

Identification of complex genetic cross talks affecting PLT and MPV might help to develop novel treatment strategies in treating neurovascular diseases like stroke.

CD226 protein is involved in immune synapse formation and triggers Natural Killer (NK) cell activation via its first extracellular domain

CD226, an activating receptor that interacts with the ligands CD155 and CD112, activates natural killer (NK) cells via its immunoreceptor tyrosine-based activatory motif (ITAM). There are two extracellular domains of CD226; however, the comparative functional relevance of these domains remains unknown. In this study, two different deletion mutants, rCD226-ECD1 (the first extracellular domain) and rCD226-ECD (full extracellular domains), were recombinantly expressed. We observed that rCD226-ECD1, similar to rCD226-ECD, specifically bound to ligand-positive cell lines and that this interaction could be competitively blocked by an anti-CD226 mAb. In addition, rCD226-ECD1 was able to block the binding of CD112 mAb to tumor cells in a competitive binding assay. Importantly, based on surface plasmon resonance (SPR), we determined that rCD226-ECD1, similar to rCD226-ECD, directly bound to its ligand CD155 on a protein chip. Functionally, NK cell cytotoxicity against K562 or HeLa cells was blocked by rCD226-ECD1 by reducing the expression of CD69 and granzyme B, indicating the critical role of ECD1 in NK cell activation. We also examined the role of rCD226-ECD1 in effector/target interactions by using rCD226-ECD to block these interactions. Using flow cytometry, we found that the number of conjugates between IL-2-dependent NKL cells and HeLa cells was reduced and observed that the formation of immune synapses was also decreased under confocal microscopy. In addition, we prepared two anti-rCD226-ECD1 agonistic antibodies, 2E6 and 3B9. Both 2E6 and 3B9 antibodies could induce the phosphorylation of ERK in NK-92 cells. Taken together, our results show that CD226 functions via its first extracellular domain.

The genetics of normal platelet reactivity

Genetic and environmental factors contribute to a substantial variation in platelet function seen among normal persons. Candidate gene association studies represent a valiant effort to define the genetic component in an era where genetic tools were limited, but the single nucleotide polymorphisms identified in those studies need to be validated by more objective, comprehensive approaches, such as genome-wide association studies (GWASs) of quantitative functional traits in much larger cohorts of more carefully selected normal subjects. During the past year, platelet count and mean platelet volume, which indirectly affect platelet function, were the subjects of GWAS. The majority of the GWAS signals were located to noncoding regions, a consistent outcome of all GWAS to date, suggesting a major role for mechanisms that alter phenotype at the level of transcription or posttranscriptional modifications. Of 15 quantitative trait loci associated with mean platelet volume and platelet count, one located at 12q24 is also a risk locus for coronary artery disease. In most cases, the effect sizes of individual quantitative trait loci are admittedly small, but the results of these studies have led to new insight into regulators of hematopoiesis and megakaryopoiesis that would otherwise be unapparent and difficult to define.

A novel interface consisting of homologous immunoglobulin superfamily members with multiple functions

Immunoglobulin superfamily (IgSF) members account for a large proportion of cell adhesion molecules that perform important immunological functions, including recognizing a variety of counterpart molecules on the cell surface or extracellular matrix. The findings that CD155/poliovirus receptor (PVR) and CD112/nectin-2 are the ligands for CD226/platelet and T-cell activation antigen 1 (PTA1)/DNAX accessory molecular-1 (DNAM-1), CD96/tactile and Washington University cell adhesion molecule (WUCAM) and that CD226 is physically and functionally associated with lymphocyte function-associated antigen-1 (LFA-1) on natural killer (NK) and activated T cells have largely expanded our knowledge about the functions of CD226, CD96, WUCAM and LFA-1 and their respective ligands, CD155, CD112, intercellular adhesion molecule (ICAM)-1 and junctional adhesion molecule (JAM)-1. The interactions of these receptors and their ligands are involved in many key functions of immune cells including naive T cells, cytotoxic T cells, NK cells, NK T cells, monocytes, dendritic cells, mast cells and platelets/megakaryocytes.

Study of gene expression profile during cord blood-associated megakaryopoiesis

AIMS

To study the gene profile in cord blood (CB)-associated megakaryopoiesis.

METHODS

In vitro differentiation of megakaryocytes (Mks) was carried out using human CB CD34(+) cells under the stimulation of recombinant human interleukin-3, stem cell factor and thrombopoietin for 7 d, followed by thrombopoietin only for further 3 d. Lineage-specific differentiation of Mk was examined by the expression of CD41 using flow cytometry and confocal microscopy. Total cellular RNA was extracted from day-0 CD34(+), day-10 CD41(+) and CD41(-) populations were isolated by immunomagnetic sorting respectively. Microarray was performed, and the data were analyzed using the GeneChip Operating System, Spotfire software and Genomatix BiblioSphere.

RESULTS

Flow cytometric analysis showed 19.44 +/- 3.05% CD41(+) cells at day 10 of culture. The purity of CD41(+) population was enriched to 95.70 +/- 4.19% after sorting. Gene expression profiling revealed an upregulation of 285 and downregulation of 53 unique genes in the CD41(+) cells compared with CD41(-) and CD34(+) cells. Platelet-associated genes, such as thrombospondin 1, platelet glycoprotein IIIa, etc., were highly expressed in CD41(+) cells but not in CD41(-) cells and CD34(+) cells. Moreover, some genes that have not been reported to be associated with CB-derived megakaryopoiesis, such as Cbl-interacting proteins Sts-1, protocadherin 21, etc., are found to be highly expressed in the CD41(+) cells from this study.

CONCLUSIONS

This study reveals a global gene expression profile of in vitro human CB-derived megakaryopoiesis at day 10. Some of these genes may play regulatory roles during the development of CB-derived megakaryopoiesis.

Nicotinamide (vitamin B3) increases the polyploidisation and proplatelet formation of cultured primary human megakaryocytes

Megakaryocytic (Mk) cell maturation involves polyploidisation, and the number of platelets produced increases with Mk DNA content. Ploidy levels in cultured human MK cells are much lower than those observed in vivo. This study demonstrated that adding the water-soluble vitamin nicotinamide (NIC) to mobilised peripheral blood CD34+ cells cultured with thrombopoietin (Tpo) more than doubled the percentage of high-ploidy (> or = 8N) MK cells. This was observed regardless of donor-dependent differences in Mk differentiation. Furthermore, MK cells in cultures with NIC were larger, had more highly lobated nuclei, reached a maximum DNA content of 64N (vs. 16N with Tpo alone), and exhibited more frequent and more elaborate cytoplasmic extensions. NIC also increased the ploidy of cultured primary murine MK cells and a cell line model (CHRF-288) of Mk differentiation. However, NIC did not alter Mk commitment, apoptosis, or the time at which endomitosis was initiated. Despite the dramatic phenotypic differences observed with NIC addition, gene expression microarray analysis revealed similar overall transcriptional patterns in primary human Mk cultures with or without NIC, indicating that NIC did not disrupt the normal Mk transcriptional program. Elucidating the mechanisms by which NIC increases Mk maturation could lead to advances in the treatment of Mk and platelet disorders.

Generation of Rat Monoclonal Antibodies Against Murine PTA1/CD226

PTA1 (platelet and T cell antigen 1), designated as CD226, is one of the major activating receptors on NK cells. PTA1/CD226 is also broadly expressed on a variety of hematopoietic cells, including Th cells, CTLs, NK T cells, monocytes, DCs, and mast cells, and is involved in multiple immunoregulation. Murine PTA1 (mPTA1) is the homolog molecule of human PTA1. Using mPTA1-Fc as immunogen and the technique of rat B lymphocyte hybridoma, we raised five hybridoma cell lines secreting monoclonal antibodies (MAbs) to mPTA1, designated FMU-mCD226.1 approximately FMU-mCD226.5. Rat immunoglobulin class and subclass of the MAbs FMU-mCD226.1 approximately 5 were determined to be IgG2a, IgM, IgM, IgG1, and IgG1, respectively. FMU-mCD226.2, 3, 4, and 5 worked well in Western blot assay and could also recognize natural PTA1 on the surface of EL-4 cells and mPTA1 cDNA transfected 293T cells detected by FCM. Thus, successful production of rat anti-murine PTA1/CD226 monoclonal antibodies provides a new powerful tool for investigation of murine PTA1 function in the mouse model, both in vitro and in vivo.

Identification and characterization of the CD226 gene promoter

CD226 is one of the main activating receptors on natural killer cells, and it can induce cytotoxicity to target cells through interaction with its ligands CD155 or CD112. CD226 is also involved in T cell differentiation, activation, and cytotoxicity. The expression of CD226 on natural killer cells and T cells can be regulated by cytokines and chemical stimuli; however, the mechanism of the regulation of the CD226 gene is still unknown. In this study, we have identified two promoters in the human CD226 gene named P1 and P2, which are located at -810 to -287 bp and +33 to +213 bp, respectively, and a negative regulation element between P1 and P2. Both P1 and P2 can be regulated by phorbol ester (12-O-tetradecanoylphorbol-13-acetate) and calcium ionophore (A23187). Bioinformatics analysis shows that, within this CD226 gene region, there are putative binding sites for transcription factors AP-1, Sp1, PEA3, and Ets-1. We have found that transcription factor activating protein-1 (AP-1) can up-regulate CD226 promoters P1 and P2 in human hepatocarcinoma cells, a hepatocarcinoma cell line with low expression of endogenous AP-1 and Ets-1. Interestingly, the transcription factor Ets-1 promotes AP-1-induced P2 activity but inhibits AP-1-induced P1 activity for which a 10-bp AP-1/Ets-1 composite site (CCTTCCTTCC) in P1 may be responsible.