CD73, also called ecto-5-nucleotidase, is a glycosylphosphatidylinositol (GPI) anchored cell-surface homodimer, without membrane-embedded protein segments. CD73 comprises two identical 70-kD subunits. CD73 is characterized by an N-terminal domain-containing binding sites for zinc and cobalt ions and a C-terminal domain containing the adenosine monophosphate (AMP)-binding site. Its enzymatic activity is closely coupled to that of ectoapyrase (CD39), which generates the AMP substrate for CD73. In addition, pyrophosphatases cleave nicotinamide adenine dinucleotide (NAD+) to yield AMP and b-nicotinamide mononucleotide, thereby providing the substrate for CD73. CD73 is an ectoenzyme that converts AMP to adenosine (Ado). Besides, CD73 also acts as an adhesion molecule, participating in the migration of normal and neoplastic cells.
Fig.1 Structure and role of CD73. (Antonioli, 2016)
Extracellular Ado can activate 4 types of G protein-coupled Ado receptors (ARs): A1R, A2AR, A2BR, and A3R. Adenylate cyclase (AC) is inhibited by A1R/A3R and activated by A2AR/A2BR. ARs can also activate phospholipase C (PLC), phosphoinositide 3-kinase (PI3K), mitogen-activated protein kinase (MAPK), and ion channels. Alternatively, Ado can be taken up intracellularly via equilibrative nucleoside transporters (ENT). Among the numerous physiological responses regulated by Ado receptors, epithelial ion and fluid transport, tissue barrier maintenance, hypoxia, ischemic preconditioning, and inflammation have been closely linked to CD73 activity. CD73-Ado pathway is a major immunosuppressive mechanism to escape anti-tumor immunity. Abundant Ado exerts immunosuppressive activity on a variety of immune cells.
Several studies have revealed important functions of CD73 that are independent of its activity as an AMPase, including
CD73 is expressed on a variety of cells found within the tumor microenvironment, and the hypoxia-inducible factor (HIF-1α) is a major regulator of its expression. Within the tumor environment, ATP is released and is converted into adenosine by upregulated CD39 and CD73. CD73 expressed on stromal cells or tumor cells contributes substantially to tumor-induced immune suppression. Adenosine promotes cancer growth by acting directly on neoplastic cells through A1, A2A, A2B, and/or A3 Ado receptor activation and by subsequent enhancement of invasiveness and metastatic ability. In addition, CD39/CD73 can be viewed as ‘immunological switches’ that shift ATP-driven proinflammatory immune cell activity toward an anti-inflammatory state mediated by Ado. The Ado generated by CD39/CD73 expressed either on primary neoplastic cells or cancer exosomes generates an immunosuppressive environment by acting on macrophages, neutrophils, dendritic cells (DCs), and T cells. For example, Ado is a strong stimulator of M2 macrophage polarization and suppresses DCs to limit their stimulation of effector T cells. Ado decreases the function of antitumor T cells and promotes T cell apoptosis, thereby contributing to tumor immune evasion. What's more, Ado is responsible for a substantial component of the immunosuppressive and anti-inflammatory functions of Tregs. All of these provide a shield against the antitumor immune response.
Fig.2 CD73 in the regulation of the immunosuppressive tumor microenvironment. (Allard, 2019)
The CD73 metabolic immune checkpoint orchestrates a crucial homeostatic balance as part of a negative feedback mechanism to control inflammatory responses within a stressed or damaged tissue microenvironment. Creative Biolabs provides a full set of CD73 assay portfolio services, which including but not limited to:
Creative Biolabs focuses on developing customizable tumor marker assay services (e.g., CD73 assay portfolio service). We provide multiple well-tailored assay portfolio services to satisfy your various demands. We are pleased to assist our customers with their projects. Please feel free to contact us.
References
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