Extracellular signal-regulated kinases – a potential pathway for GPCR-targeted drug discovery

“The desire to take medicine is perhaps the greatest feature which distinguishes man from animals.” Sir William Osler

Scientists have long sought to discover a “golden bullet” that would cure every disease without adverse effects. However, the journey to fine-tune human physiology is far from over. The more we know about the pathology of diseases, the more complicated it is to modulate them by a specific pathway without touching others. Notably, G-protein-coupled receptors (GPCRs), representing the biggest drug target, have been revealed to show functions through a plethora of signaling pathways. 

Historically, drug discovery efforts targeting GPCRs focused on G-protein-dependent signaling pathways, such as those involving cAMP and calcium mobilisation, to identify lead compounds. This emphasis may have caused other signaling pathways to be overlooked due to a need for adequate assay tools. While these signaling pathways are highly interconnected, they can also be regulated independently (Kenakin, 2019). Recent research has unveiled the emergence of G-protein-independent pathways, particularly those involving β-arrestins, which are now proving significant in drug discovery (Wei et al., 2003). β-arrestins, traditionally seen as terminators of G-protein signaling, are now recognised as critical players in activating extracellular signal-regulated kinases (ERK) pathways alongside the G-protein dependent pathways. This dual role opens new possibilities for more complex and nuanced cellular responses.

Extracellular signal-regulated kinases (ERK), a subset of the mitogen-activated protein kinase (MAPK) family, have long been known to play a critical role in the signaling pathways of GPCRs. This signaling cascade involves several steps. Initially, the activation of small GTPases like RAS leads to the activation of the MAP kinase kinase kinases (MAPKKKs), such as RAF. RAF then phosphorylates and activates the MAP kinase kinases (MAPKKs), MEK1 and MEK2, which in turn phosphorylate and activate ERK1 and ERK2. Once activated, ERKs translocate to the nucleus, phosphorylating various transcription factors, including ETS, c-Jun, and Fos, modulating gene expression and influencing cellular functions (Lu & Malemud, 2019). 

ERK signaling, tightly regulated through feedback mechanisms and spatial localisation within the cell, is a crucial determinant of cellular responses such as proliferation, differentiation, migration, survival, growth, growth arrest and apoptosis. The gravity of this regulation becomes even more apparent when we consider the potential consequences of dysregulation. In various pathological conditions, including cancer, aberrant ERK activity can lead to uncontrolled cell proliferation and survival​, highlighting the pressing need to comprehend and control these pathways (Sugiura et al., 2021).

ERK activation pathways can be categorised into two main sub-pathways based on their subcellular localisation: nuclear and cytosolic (Volmat & Pouysségur, 2001). The nuclear pathway typically involves ERK translocating into the nucleus to regulate gene expression, while the cytosolic pathway involves ERK acting in the cytoplasm to control other cellular functions. The choice of pathway can result in different cellular responses, underscoring the criticality of precise targeting in drug discovery. This further underscores the need for advanced assay technologies and a deep understanding of cellular signaling.

Recently, several high-throughput screening (HTS) assays for ERK activation have been developed, each utilising different detection technologies. These include infrared fluorescence, electrochemiluminescence, fluorescence emission, immunofluorescence staining, HTRF (homogeneous time-resolved fluorescence), and TR-FRET (time-resolved fluorescence resonance energy transfer). Each assay offers unique advantages for detecting ERK activation, contributing to the broader toolkit available for GPCR-related drug discovery (Eishingdrelo & Kongsamut, 2013).

In summary, targeting GPCR-activated ERK pathways represents a promising strategy for developing new therapeutics. By understanding the intricacies of GPCR signaling and utilising advanced assay technologies, researchers can identify novel drugs that selectively modulate this specific pathway, improving efficacy and safety profiles in clinical applications. 

References

Eishingdrelo, H., & Kongsamut, S. (2013). Minireview: targeting GPCR activated ERK pathways for drug discovery. Current Chemical Genomics and Translational Medicine, 7, 9. 

Kenakin, T. (2019). Biased receptor signaling in drug discovery. Pharmacological Reviews, 71(2), 267-315. 

Lu, N., & Malemud, C. J. (2019). Extracellular Signal-Regulated Kinase: A Regulator of Cell Growth, Inflammation, Chondrocyte and Bone Cell Receptor-Mediated Gene Expression. International Journal of Molecular Sciences, 20(15). 

Sugiura, R., Satoh, R., & Takasaki, T. (2021). ERK: a double-edged sword in cancer. ERK-dependent apoptosis as a potential therapeutic strategy for cancer. Cells, 10(10), 2509. 

Volmat, V., & Pouysségur, J. (2001). Spatiotemporal regulation of the p42/p44 MAPK pathway. Biology of the Cell, 93(1‐2), 71-79. 

Wei, H., Ahn, S., Shenoy, S. K., Karnik, S. S., Hunyady, L., Luttrell, L. M., & Lefkowitz, R. J. (2003). Independent β-arrestin 2 and G protein-mediated pathways for angiotensin II activation of extracellular signal-regulated kinases 1 and 2. Proceedings of the National Academy of Sciences, 100(19), 10782-10787.