PET imaging of integrins
All cells (excluding blood cells) adhere to the surrounding extracellular matrix (ECM) by membrane receptors, mainly integrins. Integrins are heterodimeric proteins, consisting of α and β subunits, which have a large extracellular domain, a single transmembrane domain, and short intracellular tail. The specificity of the integrin is determined by the combination of the α and β subunits. The cytoplasmic tail of integrins are connected to the cells cytoskeleton. Binding to and separation from their cytoplasmic signalling molecules, cytoskeleton, and ECM ligands are associated via conformational changes. Integrin receptors can signal bidirectionally: intracellular signalling pathways may activate or deactivate the extracellular part via large conformational changes.
ECM proteins expose short peptide loops, such as Arg-Gly-Asp (RGD) sequence, which are recognized by the integrins. RGD motif is found for example in fibronectin, vitronectin and fibrinogen. At least 24 integrins are known, and eight of them recognize RGD sequence (Steider et al., 2021). Endothelial cells express endoglin, which also contains the RGD motif.
Integrins are important in tissue remodelling, cell migration, proliferation, and differentiation. Haemostasis and regulation of blood clotting is largely based on integrins. Integrins mediate the entry of lymphocytes into inflamed tissue, and can therefore be targeted in therapy for autoimmune diseases. Integrins expressed by the cells of CNS are important in synaptic plasticity, synapse formation, and regulation of myelination and status of microglia.
Shank-associated RH domain-interacting protein (SHARPIN, alias SIPL1) is a cytosolic protein that controls integrin-dependent cell adhesion and migration in several normal and malignant cell types, and loss of SHARPIN correlates with increased integrin activity in mice (Siitonen et al., 2019).
Integrin αvβ3 is a cell membrane glycoprotein receptor ("vitronectin receptor") that interacts with specific ligands in extracellular matrix (ECM), playing a key role in angiogenesis, inflammation, fibrosis, osteoporosis, and tumour metastasis formation. Expression of αvβ3 integrin is increased in activated endothelial cells and tumour cells, but is low in most normal tissues.
Extracellular (interstitial) matrix proteins, such as vitronectin, fibronectin, and thrombospondin) contain a Arg-Gly-Asp (RGD) tripeptide sequence, which binds to many integrins, including αvβ3. Radiolabelled peptides containing the RGD peptide can be used to detect the increased expression of αvβ3 integrin, and multimerization of the RGD-domains increases the affinity and specific uptake of the radiotracer (Dijkgraaf et al., 2011). [68Ga]Ga-DOTA-E-[c(RGDfK)]2 is one of the proposed PET radiotracers for αvβ3 imaging.
Increased integrin αvβ3 expression is also a potential biomarker of successful tissue repair processes. After myocardial infarction, its expression is upregulated during angiogenesis. In rat model of myocardial infarction, increased uptake of [18F]galacto-RGD in the defect area predicted better healing (Sherif et al., 2012). Similar results have been obtained in patients with myocardial infarction using [18F]fluciclatide ([18F]AH111585) (Jenkins et al., 2017) and [18F]galacto-RGD (Makowski et al., 2021). Uptake of [68Ga]Ga-NODAGA-RGD is increased in active inflammatory lesions in rat model or autoimmune myocarditis (Jahandideh et al., 2021).
Integrin αvβ5 has a role in regulation of the integrity of endothelial barrier; VEGF-induced vascular leakage is diminished when αvβ5 inactivated. Integrin αvβ5 is expressed on endothelial and epithelial cells, and fibroblasts. Blocking αvβ5 reduces ischemia-reperfusion damage in rat model of acute kidney injury (McCurley et al., 2017).
[68Ga]DOTA-RGD binds to both αvβ3 and αvβ5 integrins (Haukkala et al., 2009).
Epithelial integrin αvβ6 is expressed by many carcinoma cell types, providing a target for radionuclide therapy and imaging (Färber et al., 2018). Integrin αvβ6 activates transforming growth factor β (TGFβ, with isoforms 1-3), a growth-inhibiting cytokine. Carcinoma cells become insensitive to TGFβ-induced growth inhibition, and thus overexpression of αvβ6 helps tissue invasion as normal tissue growth is inhibited (Brown & Marshall, 2019).
PET radioligands targeting αvβ6 have been developed (Hausner et al., 2009 and 2015; Singh et al., 2014; Notni et al., 2017), and used in humans for imaging carcinomas (Altmann et al., 2017; Roesch et al., 2018; Müller et al., 2019; Flechsig et al., 2019; Hausner et al., 2019) and idiopathic pulmonary fibrosis (Kimura et al., 2019; Lukey et al., 2020; Maher et al., 2020).
Integrin αvβ8 is an activator of TGFβ but by a different mechanism as αvβ6 (Mu et al., 2002). It is expressed in many human carcinomas (Takasaka et al., 2018). [68Ga]Triveoctin can be used for in vivo imaging of αvβ8 integrin (Quigley et al., 2020).
Lymphocyte functions associated antigen 1 (LFA-1) is the heterodimer of integrins alpha L (CD11a) and beta 2 (CD18) chains. LFA-1 is found on lymphocytes, macrophages, and neutrophils, and it is involved in leukocyte trafficking from blood into the tissue, and binding to antigen presenting cells.
Macrophage-1 antigen receptor (Mac-1) is the heterodimer of integrins alpha L (CD11b) and beta 2 (CD18) chains. Mac-1 is one of complement receptors, found on neutrophils, macrophages and NK cells, and it is involved in the innate immune system. Other complement receptors are formed with combination of CD18 and CD11c or CD11c.
- Extracellular matrix
- Transglutaminase type 2
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Updated at: 2022-02-09
Created at: 2017-03-05
Written by: Vesa Oikonen