Mdm2-Mediated Downmodulation of GRK2 Restricts Centrosome Separation for Proper Chromosome Congression

GRK2-mediated AKT activation controls cell cycle progression and G2 checkpoint in a p53-dependent manner

Cell cycle checkpoints, activated by stressful events, halt the cell cycle progression, and prevent the transmission of damaged DNA. These checkpoints prompt cell repair but also trigger cell death if damage persists. Decision-making between these responses is multifactorial and context-dependent, with the tumor suppressor p53 playing a central role. In many tumor cells, p53 alterations lead to G1/S checkpoint loss and the weakening of the G2 checkpoint, rendering cell viability dependent on the strength of the latter through mechanisms not fully characterized. Cells with a strong pro-survival drive can evade cell death despite substantial DNA lesions. Deciphering the integration of survival pathways with p53-dependent and -independent mechanisms governing the G2/M transition is crucial for understanding G2 arrest functionality and predicting tumor cell response to chemotherapy. The serine/threonine kinase GRK2 emerges as a signaling node in cell cycle modulation. In cycling cells, but not in G2 checkpoint-arrested cells, GRK2 protein levels decline during G2/M transition through a process triggered by CDK2-dependent phosphorylation of GRK2 at the S670 residue and Mdm2 ubiquitination. We report now that this downmodulation in G2 prevents the unscheduled activation of the PI3K/AKT pathway, allowing cells to progress into mitosis. Conversely, higher GRK2 levels lead to tyrosine phosphorylation by the kinase c-Abl, promoting the direct association of GRK2 with the p85 regulatory subunit of PI3K and AKT activation in a GRK2 catalytic-independent manner. Hyperactivation of AKT is conditioned by p53's scaffolding function, triggering FOXO3a phosphorylation, impaired Cyclin B1 accumulation, and CDK1 activation, causing a G2/M transition delay. Upon G2 checkpoint activation, GRK2 potentiates early arrest independently of p53 through AKT activation. However, its ability to overcome the G2 checkpoint in viable conditions depends on p53. Our results suggest that integrating the GRK2/PI3K/AKT axis with non-canonical functions of p53 might confer a survival advantage to tumor cells.

GRK2 mediated degradation of SAV1 initiates hyperplasia of fibroblast-like synoviocytes in rheumatoid arthritis

Hyperplasia and migration of fibroblast-like synoviocytes (FLSs) are the key drivers in the pathogenesis of rheumatoid arthritis (RA) and joint destruction. Abundant Yes-associated protein (YAP), which is a powerful transcription co-activator for proliferative genes, was observed in the nucleus of inflammatory FLSs with unknown upstream mechanisms. Using Gene Expression Omnibus database analysis, it was found that Salvador homolog-1 (SAV1), the pivotal negative regulator of the Hippo-YAP pathway, was slightly downregulated in RA synovium. However, SAV1 protein expression is extremely reduced. Subsequently, it was revealed that SAV1 is phosphorylated, ubiquitinated, and degraded by interacting with an important serine-threonine kinase, G protein-coupled receptor (GPCR) kinase 2 (GRK2), which was predominately upregulated by GPCR activation induced by ligands such as prostaglandin E2 (PGE2) in RA. This process further contributes to the decreased phosphorylation, nuclear translocation, and transcriptional potency of YAP, and leads to aberrant FLSs proliferation. Genetic depletion of GRK2 or inhibition of GRK2 by paroxetine rescued SAV1 expression and restored YAP phosphorylation and finally inhibited RA FLSs proliferation and migration. Similarly, paroxetine treatment effectively reduced the abnormal proliferation of FLSs in a rat model of collagen-induced arthritis which was accompanied by a significant improvement in clinical manifestations. Collectively, these results elucidate the significance of GRK2 regulation of Hippo-YAP signaling in FLSs proliferation and migration and the potential application of GRK2 inhibition in the treatment of FLSs-driven joint destruction in RA.

G Protein-Coupled Receptor Kinases Take Central Stage

The relevance of the family of G protein-coupled receptor kinases (GRKs) is based on its key participation in the regulation and intracellular dynamics of the largest family of membrane receptors, namely G protein-coupled receptors (GPCRs) [...].