The ecto-5'-nucleotidase subunits in dimers are not linked by disulfide bridges but by non-covalent bonds

Targeting Metabolism of Extracellular Nucleotides via Inhibition of Ectonucleotidases CD73 and CD39

In the tumor microenvironment, unusually high concentrations of extracellular adenosine promote tumor proliferation through various immunosuppressive mechanisms. Blocking adenosine production by inhibiting nucleotide-metabolizing enzymes, such as ectonucleotidases CD73 and CD39, represents a promising therapeutic strategy that may synergize with other immuno-oncology mechanisms and chemotherapies. Emerging small-molecule ectonucleotidase inhibitors have recently entered clinical trials. This Perspective will outline challenges, strategies, and recent advancements in targeting this class with small-molecule inhibitors, including AB680, the first small-molecule CD73 inhibitor to enter clinical development. Specific case studies, including structure-based drug design and lead optimization, will be outlined. Preclinical data on these molecules and their ability to enhance antitumor immunity will be discussed.

Generation and Function of Non-cell-bound CD73 in Inflammation

Extracellular adenine nucleotides participate in cell-to-cell communication and modulate the immune response. The concerted action of ectonucleotidases CD39 and CD73 plays a major role in the local production of anti-inflammatory adenosine, but both ectonucleotidases are rarely co-expressed by human T cells. The expression of CD39 on T cells increases upon T cell activation and is high at sites of inflammation. CD73, in contrast, disappears from the cellular membrane after activation. The possibility that CD73 could act in trans would resolve the conundrum of both enzymes being co-expressed for the degradation of ATP and the generation of adenosine. An enzymatically active soluble form of CD73 has been reported, and AMPase activity has been detected in body fluids of patients with inflammation and cancer. It is not yet clear how CD73, a glycosylphosphatidylinositol (GPI)-anchored protein, is released from the cell membrane, but plausible mechanisms include cleavage by metalloproteinases and shedding mediated by cell-associated phospholipases. Importantly, like many other GPI-anchored proteins, CD73 at the cell membrane is preferentially localized in detergent-resistant domains or lipid rafts, which often contribute to extracellular vesicles (EVs). Indeed, CD73-containing vesicles of different size and origin and with immunomodulatory function have been found in the tumor microenvironment. The occurrence of CD73 as non-cell-bound molecule widens the range of action of this enzyme at sites of inflammation. In this review, we will discuss the generation of non-cell-bound CD73 and its physiological role in inflammation.

Targeting CD73 and downstream adenosine receptor signaling in triple-negative breast cancer

INTRODUCTION

Despite significant improvements in diagnosis and therapy over the past 20 years, breast cancer remains a worldwide public health issue. In particular, triple negative breast cancer (TNBC), a subset of very aggressive breast tumors, is associated with a poor prognosis and has very few efficient therapeutic options. The ectonucleotidase CD73 has recently emerged as a promising new target for TNBC in preclinical models. Pharmacological targeting of CD73 and downstream adenosine A2A/A2B receptor signaling is currently an active field of research that could lead to the development of new cancer therapeutics, including options against TNBC.

AREAS COVERED

This article reviews the basic structural and molecular features of CD73 and its role in the development of cancer, with a particular focus on CD73's role in the biology of TNBC.

EXPERT OPINION

It was recently demonstrated that CD73 expression in TNBC is associated with worse clinical outcomes and increased resistance to anthracycline chemotherapy. Targeted blockade of the CD73/A2A axis has been shown to impair various aspects of tumorigenesis and displays synergism with other anti-cancer treatments in preclinical studies. Hence, we strongly argue for the development of CD73 inhibitors and for the repositioning of A2A antagonists in cancer.

The lipid raft-bound alkaline phosphatase activity increases and the level of transcripts remains unaffected in liver of merosin-deficient LAMA2dy mouse

Alkaline phosphatase (AP) and other proteins add glycosylphosphatidylinositol (GPI) before addressing to raft domains of the cell membrane. Our previous report showing an increased density of lipid rafts in muscle of dystrophic Lama2dy mice prompted us to compare livers of normal (NL) and dystrophic mice (DL) for their levels of rafts. With this aim, hepatic rafts were isolated as Triton X-100 resistant membranes, and identified by their abundance of flotillin-2, alkaline phosphatase (AP) and other raft markers. The comparable abundance of cholesterol and flotillin-2 in rafts of NL and DL contrasted with the double AP activity both in rafts of DL and whole DL. The AP mRNA level was the same in NL and DL. Sedimentation analysis profiles revealed AP activity of NL distributed between dimeric (dAP) and monomeric AP (mAP), whose proportions and lectin-binding extent changed in DL. The increased AP activity and changed AP glycosylation in DL, the prevalence of mAP in NL and the enhanced stability of dAP in DL demonstrated the critical role that glycosylation and oligomerization play for AP catalysis. The higher AP activity of DL probably arises from dystrophy-associated changes in glycosyl transferases, which alter AP glycosylation and subunit folding with profitable effects for AP stability and catalysis.

Crystal structure of the human ecto-5'-nucleotidase (CD73): insights into the regulation of purinergic signaling

In vertebrates ecto-5'-nucleotidase (e5NT) catalyzes the hydrolysis of extracellular AMP to adenosine and represents the major control point for extracellular adenosine levels. Due to its pivotal role for activation of P1 adenosine receptors, e5NT has emerged as an appealing drug target for treatment of inflammation, chronic pain, hypoxia, and cancer. Crystal structures of the dimeric human e5NT reveal an extensive 114° conformational switch between the open and closed forms of the enzyme. The dimerization interface is formed by the C-terminal domains and exhibits interchain motions of up to 13°. Complex structures with adenosine and AMPCP indicate that structural control of the domain movement determines the selectivity for monophosphate nucleotides. Binding modes of nucleotide-derived and flavonoid-based compounds complexed to the C-terminal domain in the open form reveal an additional binding pocket of ∼210 Å(3) that might be explored to design more potent inhibitors.

Cellular function and molecular structure of ecto-nucleotidases

Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

Targeting of acetylcholinesterase to lipid rafts of muscle

Despite the great progress made in setting the basis for the molecular diversity of acetylcholinesterase (AChE), an explanation for the existence of two types of amphiphilic subunits, with and without glicosylphosphatidylinositol (GPI) (Types I and II), has not been provided yet. In searching whether, as for the deficiency of dystrophin, that of merosin (laminin-alpha2 chain) alters the number of caveolae in muscle, a high increase in caveolin-3 (Cav3) was observed in the Triton X-100-resistant membranes (TRM) isolated from muscle of merosin-deficient dystrophic mice (Lama2dy). The rise in Cav3 was accompanied by that of non-caveolar lipid rafts, as showed by the greater ecto-5'-nucleotidase (eNT) activity, a marker of non-caveolar rafts, in TRM of dystrophic muscle. The observation of AChE activity in TRM, the increased levels of rafts and raft-bound AChE activity in merosin-deficient muscle and the presence of phospholipase C-sensitive AChE dimers in TRM supported targeting of glypiated AChE to rafts. This issue and the involvement of TRM in conveying nicotinic receptors to the neuromuscular junction and particular muscarinic receptors to cardiac sarcolemma strongly support a role for lipid rafts in targeting ACh receptors and glypiated AChE. Their nearby location in the surface membrane may provide cells with a fine tuning for regulating cholinergic responses.

Nucleotide- and nucleoside-converting ectoenzymes: Important modulators of purinergic signalling cascade

The involvement of extracellular nucleotides and adenosine in an array of cell-specific responses has long been known and appreciated, but the integrative view of purinergic signalling as a multistep coordinated cascade has emerged recently. Current models of nucleotide turnover include: (i) transient release of nanomolar concentrations of ATP and ADP; (ii) triggering of signalling events via a series of ligand-gated (P2X) and metabotropic (P2Y) receptors; (iii) nucleotide breakdown by membrane-bound and soluble nucleotidases, including the enzymes of ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) family, ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) family, ecto-5'-nucleotidase/CD73, and alkaline phosphatases; (iv) interaction of the resulting adenosine with own nucleoside-selective receptors; and finally, (v) extracellular adenosine inactivation via adenosine deaminase and purine nucleoside phosphorylase reactions and/or nucleoside uptake by the cells. In contrast to traditional paradigms that focus on purine-inactivating mechanisms, it has now become clear that "classical" intracellular ATP-regenerating enzymes, adenylate kinase, nucleoside diphosphate (NDP) kinase and ATP synthase can also be co-expressed on the cell surface. Furthermore, data on the ability of various cells to retain micromolar ATP levels in their pericellular space, as well as to release other related compounds (adenosine, UTP, dinucleotide polyphosphates and nucleotide sugars) gain another important insight into our understanding of mechanisms regulating a signalling cascade. This review summarizes recent advances in this rapidly evolving field, with particular emphasis on the nucleotide-releasing and purine-converting pathways in the vasculature.

Ecto-5'-nucleotidase: Structure function relationships

Ecto-5’-nucleotidase (ecto-5’-NT) is attached via a GPI anchor to the extracellular membrane, where it hydrolyses AMP to adenosine and phosphate. Related 5’-nucleotidases exist in bacteria, where they are exported into the periplasmic space. X-ray structures of the 5’-nucleotidase from E. coli showed that the enzyme consists of two domains. The N-terminal domain coordinates two catalytic divalent metal ions, whereas the C-terminal domain provides the substrate specificity pocket for the nucleotides. Thus, the substrate binds at the interface of the two domains. Here, the currently available structural information on ecto-5’NT is reviewed in relation to the catalytic properties and enzyme function.

The increased ecto-5′-nucleotidase activity in muscle, heart and liver of laminin α2-deficient mice is not caused by an elevation in the mRNA content

We have previously shown that mouse muscle and liver contain catalytically active and inactive ecto-5'-nucleotidase (eNT) variants and that eNT activity in these tissues increases in laminin alpha2 (merosin)-deficient Lama2dy mice. These results prompted us to study whether: (1) the increase of eNT activity depends on the change in the content of merosin between healthy and dystrophic organs; (2) the active and inactive eNT variants arise from the same or distinct mRNAs; (3) the enhancement of the activity is caused by an increase in the eNT mRNA content. Compared to healthy organs, the activity in dystrophic organs increased four-fold in muscle, 1.7-fold in liver, 1.4-fold in heart and not at all in kidney and lung. The level of immunolabelled eNT protein per unit of activity suggested a similar ratio of inactive to active eNT in healthy liver, kidney, heart and muscle, which increased greatly in lung. The size of the eNT subunit in liver, kidney, heart and muscle (72 kDa) decreased to 66 kDa in lung. The identification of a single RT-PCR product suggested that active and inactive eNT arise from the same mRNA and are generated by a differential post-translational processing. Compared to the content in muscle, the amount of eNT mRNA was 12-fold higher in liver and kidney, eight-fold in heart and five-fold in lung. The relative content of eNT mRNA was unaffected by the deficiency of merosin.

Active and inactive ecto-5′-nucleotidase variants in liver of control and dystrophic Lama2dy mice

The ecto-5'-nucleotidase (eNT) activity and the eNT protein content in liver of normal and merosin-deficient dystrophic Lama2dy mice were studied. After the solubilization procedure, the eNT activity in the final extract was 9.2+/-2.5U/mg (nmol of phosphate released from AMP per min and per mg protein) in normal liver, and it rose to 16.1+/-3.9U/mg (P=0.005) in dystrophic liver. The increase of activity was less pronounced in Lama2dy liver (1.7-fold) than the one reported in muscle (four-fold), which probably reflects the lower content of merosin in liver. Similarly to muscle, liver contained active and inactive eNT, as demonstrated by the higher level of immunoreactive protein in normal than in dystrophic liver in Western blots performed with samples containing the same units of eNT activity. PNGase F digestion decreased the size of liver and muscle eNT from 72 and 69kDa, to 63 and 60kDa. Oligoglycan cleavage did not alter eNT activity or the sedimentation coefficient, revealing that oligosaccharides are not required for catalysis or for maintaining the dimeric structure. The eNT protein content in samples of normal liver decreased by 55 or 80% after the trypsinolysis of native or deglycosylated enzyme, but the activity did not change. Such a high proportion of inactive eNT is unlikely to come from aged enzyme, which suggests the involvement of inactive enzyme in non-catalytic actions.

Identification of inactive ecto-5'-nucleotidase in normal mouse muscle and its increased activity in dystrophic Lama2(dy) mice

Ecto-5'-nucleotidase (eNT) activity and protein in normal (NM) and merosin-deficient dystrophic (DM) Lama2(dy) mice muscle were studied. eNT activity in DM was three- to four-fold that in NM. eNT in NM and DM displayed the same kinetic properties. Slot and Western blotting revealed that the immunoreactive protein was two to three times more abundant in control muscle, when NM and DM samples with the same eNT activity were compared, indicating that mouse muscle contains catalytically inactive eNT components. eNT activity and protein peaks coincided in sedimentation analyses, revealing that inactive eNT occurs as dimers. Most eNT activity and protein of NM bound to Lens culinaris (LCA) or Ricinus communis (RCA) agglutinins, but half of the activity and one-third of the protein bound to wheat germ agglutinin (WGA). Although WGA interaction did not permit full separation of inactive eNT, the results suggest that similar proportions of active species with and without WGA reactivity occur in mouse muscle, whereas a great fraction of the inactive eNT variants lack WGA reactivity. Because the level of eNT protein was little modified in DM, the higher eNT activity in dystrophic than in control muscle may result from misregulation in the synthesis of active and inactive eNT species or from conversion of inactive into active components.