Monitoring of phosphorylated peptides by radioactive assay and matrix-assisted laser desorption-ionization time-of-flight mass spectrometry

Unveiling the Role of GRK2: From Immune Regulation to Cancer Therapeutics

G protein-coupled receptors (GPCRs) represent humans' most prominent family of membrane proteins. In contrast, G protein-coupled receptor kinases (GRKs) play a pivotal role in the rapid desensitization of GPCRs. GRK2 is a particularly significant member of the GRK family. Recent studies have demonstrated that GRK2 primarily regulates immune cell function and homeostasis through receptor desensitization. Over the past decade, substantial progress has been made in elucidating the role of GRK2 in various human diseases. Notably, GRK2 is implicated in a range of autoimmune disorders, including rheumatoid arthritis (RA), inflammatory bowel disease (IBD), multiple sclerosis (MS), Sjögren's syndrome (SS), autoimmune myocarditis, hepatitis, and Graves' disease. Furthermore, emerging research has expanded our understanding of GRK2's involvement in cancer biology. Comprehensive investigations into the biological and pathological functions of GRK2 have facilitated the development of therapeutic strategies aimed at targeting the GRK2 signaling pathway in cancer, inflammation, and autoimmune diseases. Promising results have been observed with targeted biologics in preclinical and clinical trials. This review aims to elucidate the multifaceted role of GRK2 in immune function, autoimmune diseases, and cancer to uncover the remaining complexities associated with this kinase. A thorough understanding of GRK2 may position it as a potent therapeutic target in treating inflammation and cancer.

Design of substrates and inhibitors of G protein-coupled receptor kinase 2 (GRK2) based on its phosphorylation reaction

The G protein-coupled receptor kinase (GRK) family consists of seven cytosolic serine/threonine (Ser/Thr) protein kinases, and among them, GRK2 is involved in the regulation of an enormous range of both G protein-coupled receptors (GPCRs) and non-GPCR substrates that participate in or regulate many critical cellular processes. GRK2 dysfunction is associated with multiple diseases, including cancers, brain diseases, cardiovascular and metabolic diseases, and therefore GRK2-specific substrates/inhibitors are needed not only for studies of GRK2-mediated cellular functions but also for GRK2-targeted drug development. Here, we first review the structure, regulation and functions of GRK2, and its synthetic substrates and inhibitors. We then highlight recent work on synthetic peptide substrates/inhibitors as promising tools for fundamental studies of the physiological functions of GRK2, and as candidates for applications in clinical diagnostics.

A high-affinity peptide substrate for G protein-coupled receptor kinase 2 (GRK2)

We synthesized a previously identified β-tubulin-derived G protein-coupled receptor kinase 2 (GKR2) peptide (GR-11-1; DEMEFTEAESNMN) and its amino-terminal extension (GR-11-1-N; GEGMDEMEFTEAESNMN) and carboxyl-terminal extension (GR-11-1-C; DEMEFTEAESNMNDLVSEYQ) peptides with the aim of finding a high-affinity peptide substrate for GRK2. GR-11-1-C showed high affinity for GRK2, but very low affinity for GKR5. Its specificity and sensitivity for GKR2 were greater than those of GR-11-1 and GR-11-1-N. These findings should be useful in designing tools for probing GKR2-mediated intracellular signaling pathways, as well as GRK2-specific drugs.

Role of amino acid residues surrounding the phosphorylation site in peptide substrates of G protein-coupled receptor kinase 2 (GRK2)

A series of amino acid substitutions was made in a previously identified β-tubulin-derived GRK2 substrate peptide (⁴⁰⁴DEMEFTEAESNMN⁴¹⁶) to examine the role of amino acid residues surrounding the phosphorylation site. Anionic amino acid residues surrounding the phosphorylation site played an important role in the affinity for GRK2. Compared to the original peptide, a modified peptide (Ac-EEMEFSEAEANMN-NH₂) exhibited markedly higher affinity for GRK2, but very low affinity for GRK5, suggesting that it can be a sensitive and selective peptide for GRK2.