Unveiling the Role of CD73: A Key Player in Immune Regulation and Beyond

Exploring the Structure and Function of CD73

The CD73 gene finds its home in the 6q14.3 region of human chromosome 6, encoding a protein comprised of 549 amino acid residues. CD73 functions as a glycosylphospholipase, residing on the cell membrane surface and engaging with the extracellular matrix through its extracellular C-terminal tail domain. The protein's structure can be broken down into three key elements: a signal peptide sequence, an extracellular catalytic domain, and an intracellular phospholipid peptide binding domain.

CD73, also known as membrane-bound ecto-5′-nucleotidase NT5E, performs the crucial task of breaking down extracellular adenosine monophosphate (AMP) into adenosine and inorganic phosphate (P). Before reaching NT5E, adenosine triphosphate (ATP) undergoes a two-step hydrolysis process, transforming into AMP thanks to the enzyme ectonucleoside triphosphate diphosphohydrolase-1 (ENTPD1), also known as CD39.

The adenosine produced through these processes exerts potent anti-inflammatory effects. It achieves this by binding to the adenosine A2A receptor (ADORA2A), which is expressed by various immune cells like T cells, natural killer (NK) cells, and dendritic cells (DCs). This binding triggers a cascade involving cAMP, effectively blocking the effector functions of these immune cells. Notably, adenosine's influence also extends to the A2B receptor (ADORA2B), found on DCs and macrophages. These cells are similarly suppressed by adenosine.

Intriguingly, cancer cells exploit this mechanism to evade immune surveillance by ramping up NT5E protein levels. Additionally, adenosine binds to the A2B receptor expressed by cancer cells themselves, facilitating tumor cell survival and proliferation. Further complexity arises as cancer cells also express the adenosine A1 receptor (ADORA1) and A3 receptor (ADORA3). The binding of adenosine to these receptors triggers tumor cell migration and proliferation through signaling pathways involving Gαi proteins.

Beyond its immune-modulating functions, adenosine plays a role in adapting to hypoxic conditions and exhibits pro-angiogenic potential, further influencing the tumor microenvironment.

CD73 and Adenosine Receptor Activity Promotes Immunosuppression

Ecto-5′-nucleotidase (NT5E), commonly referred to as CD73, emerges as a central player in generating extracellular adenosine. When tissues face damage, inflammation, or hypoxic stress, adenosine triphosphate (ATP) is released from stressed, necrotic, or apoptotic cells. This ATP undergoes a stepwise transformation, first by ectonucleoside triphosphate diphosphohydrolase-1 (CD39), converting ATP into adenosine monophosphate (AMP), and then by CD73, converting AMP into extracellular adenosine.

ATP initially triggers inflammation through ATP receptors. However, the subsequent conversion of ATP into extracellular adenosine and the activation of adenosine receptors serve to temper this inflammation[1-3]. Extracellular adenosine communicates through four adenosine receptors: A1R, A2AR, A2BR, and A3R[4]. The earliest links between extracellular adenosine and immunosuppression were established through studies on methotrexate's anti-inflammatory activity[5]. Seminal research further revealed that A2AR signaling plays an essential role in quelling tissue-damaging inflammation[6].

Extracellular adenosine provides protective effects by mitigating inflammation in conditions such as myocardial injury[7], acute lung injury[8], intestinal ischemia-reperfusion injury[9], and inflammatory bowel disease[10]. In the context of cancer, tumors harness extracellular adenosine to shield cancer cells. Adenosine accumulates within tumors and hinders the activity of cytotoxic T cells and natural killer cells. Numerous studies employing syngeneic and spontaneous tumor models have demonstrated that genetic deletion or pharmacological inhibition of CD73 or A2AR significantly reduces tumor growth and metastasis. This effect is primarily attributed to the restoration of antitumor immunity. Moreover, these interventions enhance the sensitivity of these mice to chemotherapy[11] and curtail angiogenesis. Correspondingly, elevated CD73 expression is frequently observed in human tumors and is associated with a poor prognosis. CD73 is also implicated in drug resistance, epithelial-to-mesenchymal transition (EMT), and the proliferation and stemness of cancer cells[12].

Tumors also exhibit slower growth in mice lacking A2BR or when treated with A2BR antagonists. For the most part, the activation of A2AR, and to a lesser extent A2BR, on various types of immune cells fosters immunosuppression.

CD73 Expression and Its Impact on Tumor Cells

CD73 is found at higher expression levels in the majority of human solid tumors, and its presence is closely linked to tumor invasiveness and metastasis[15]. Research[16] has confirmed that the extracellular adenosine produced by CD73 on tumor cells plays a significant role in immune evasion, facilitating tumor growth and metastasis. To better understand the role of CD73 in tumorigenesis, multiple CD73-deficient tumor models have been studied, shedding light on its importance in both tumor cells and host cells[17].

Beyond its immune-regulating functions, CD73 influences various aspects of tumorigenesis, including proliferation, adhesion, migration, angiogenesis, and metastasis. It promotes tumor cell proliferation by regulating the cell cycle, apoptosis, and signaling pathways like EGFR, β-catenin/cyclin D1, VEGF, and AKT/ERK[18]. Interestingly, CD73, aside from its enzymatic function, also facilitates cell-to-cell adhesion, migration, invasion of cancer cells, and promotes stemness[19]. Notably, CD73, whether on tumor cells or host cells, plays a crucial role in tumor angiogenesis. Furthermore, studies highlight the significance of CD73-A2AR signaling in tumor-associated lymphangiogenesis[20], suggesting the potential use of adenosine-blocking agents to inhibit pathological lymphangiogenesis in cancers and prevent tumor dissemination.

Recent research has unveiled that cancer cell-intrinsic CD73 expedites metastasis through the PI3K/AKT signaling pathway and RICS/Rho GTPase signaling pathway. These findings correlate CD73 expression with worse prognosis[21] and poorer responses to therapeutic agents. However, it's important to note that CD73 isn't universally upregulated in all cancers; its expression can also be correlated with a positive prognosis. Aberrantly glycosylated CD73 and a human-specific CD73 isoform (CD73s)[22] have been identified in human hepatocellular carcinoma (HCC), leading to the functional suppression of tumor CD73. Additionally, CD73 expression was found to be downregulated in advanced-stage prostate, laryngeal, and high-grade colon carcinomas. Lower CD73 expression levels were observed in poorly differentiated and advanced-stage endometrial carcinomas compared to normal and well-differentiated, early-stage tumors, with higher CD73 expression associated with improved overall survival[23]. In early-stage endometrial tumors, CD73-generated adenosine has been shown to protect epithelial integrity via actin polymerization. Consequently, the role of CD73 in cancers appears to be complex, possibly due to its involvement in non-tumor-promoting effects mediated by CD73.

Navigating the Adenosinergic Signaling Pathway

The adenosinergic signaling pathway orchestrates intricate interactions within and outside cells. The extracellular milieu is governed by a web of molecules. ATP and ADP undergo hydrolysis, facilitated by CD39, which transforms them into AMP. The subsequent conversion of AMP to adenosine is managed by CD73. Extracellular adenosine assumes multiple roles: it triggers cellular signaling via adenosine receptors (ARs) through G proteins and associated effectors; it can degrade into inosine via ADA; or it can traverse the plasma membrane through either Na+-independent passive nucleoside transporters (ENTs) or Na+-dependent active nucleoside transporters (CNTs).

Within the cellular confines, the story continues. ATP undergoes hydrolysis, with NDPK and AK transforming it into ADP and AMP respectively. AMP within cells is then further refined into adenosine by cN. This internal adenosine encounters various fates: degradation into inosine through ADA, transportation to the extracellular realm, or resynthesis into ATP through metabolic enzymes.

Notably, the elevation of the AMP-to-ATP ratio prompts the activation of AMP-activated protein kinase (AMPK). This AMPK activation subsequently influences mitochondrial homeostasis, sculpting mitochondrial fission/fusion or mitophagy by modulating enzymes such as MFF or ULK1.

Even DNA methylation finds its place in this intricate dance. Intracellular adenosine comes into play, regulating DNA methylation by negatively impacting transmethylation pathways. Here, DNA methyltransferases (DNMTs) demethylate S-adenosyl-methionine (SAM) to S-adenosyl-homocysteine (SAH), which in turn boosts DNA methylation. The next step involves SAH's breakdown into adenosine and homocysteine, orchestrated by S-adenosyl-homocysteine hydrolase (SAHH). Interestingly, adenosine impairs SAHH activity, altering SAH levels and curbing SAM-dependent DNMT activity.

In this intricate narrative, a host of key players emerge: ADA (adenosine deaminase), ADK (adenosine kinase), AK (adenylate kinase), AMPK (AMP-activated protein kinase), ARs (adenosine receptors), CNTs (concentrative nucleoside transporters), ENTs (equilibrative nucleoside transporters), NDPK (nucleotide diphosphokinase), NT5C/cN (endonucleotidase), SAH (S-adenosyl-homocysteine), DNMTs (DNA methyltransferases), SAHH (S-adenosyl-homocysteine hydrolase), and SAM (S-adenosyl-methionine).

Fig.3 Regulatory components of the adenosinergic signaling pathway[24]

CD73 Protein

Recombinant Human CD73 Protein

Click here for more CD73

Synonym : 5' NT 5' nucleotidase (CD73) 5' nucleotidase precursor 5' nucleotidase, ecto 5' nucleotidase, ecto (CD73) 5'-NT 5'-nucleotidase 5NTD_HUMAN CD73 CD73 antigen E5NT Ecto 5' nucleotidase Ecto-5'-nucleotidase eN eNT NT NT5 NT5E NTE Purine 5 Prime Nucleotidase

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