Based on the evolutionary relationships of their catalytic domains, the human kinome is divided into eight major groups, among which the AGC (PKA, PKG, PKC) kinases are the most evolutionarily conserved. These kinases are widely distributed across eukaryotes, including all vertebrates, invertebrates, fungi, plants, unicellular algae, and protozoa. In humans, the AGC kinase group accounts for approximately 12% of the kinome. This percentage varies across species, with 15% in yeast-like fungi, 20% in filamentous fungi, 4% in plants, 6-8% in trypanosomes, Leishmania, and amoebas, and 6% in Giardia.

The AGC kinase group is named after three representative families: cAMP-dependent protein kinase (PKA), cGMP-dependent protein kinase (PKG), and protein kinase C (PKC). In the human genome, 63 genes encode 61 AGC kinases and 2 pseudokinases, which are further divided into 14 families and 21 subfamilies. Additionally, 6 pseudogenes (e.g., MRCKps, GPRK6ps, PRKXps) are distributed across 5 different subfamilies. The number of AGC kinase families varies in more distant eukaryotes, with members of the PKA/PKG, PDK1, RSK, MAST, and NDR families being widely present. AGC kinase family members share several conserved structural features, including a catalytic domain, activation loop, hydrophobic motif, and turn motif. These structures work together to enable AGC kinases to precisely respond to upstream signals and relay these signals to downstream effectors.

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PDK1 Family

Within the AGC kinase family, PDK1 (3-phosphoinositide-dependent protein kinase-1) serves as the "master kinase." It phosphorylates and activates the activation loop sites of at least 23 other AGC kinases, playing a central role in the PI3K-AKT signaling pathway. PDK1 activity is primarily regulated by its cytoplasmic localization and its interaction with substrate kinases. In various cancers, aberrant activation of PDK1 is closely linked to tumorigenesis and progression, making PDK1 a potential anticancer target, with inhibitor development actively underway.

AKT/PKB Family

The AKT family, also known as protein kinase B (PKB), is another key member of the AGC kinase group, consisting of three isoforms: AKT1/PKBα, AKT2/PKBβ, and AKT3/PKBγ. The AKT family plays a central role in cellular signal transduction, with structural features that include an N-terminal PH domain, a central catalytic domain, and a C-terminal regulatory domain. Activation of AKT requires a series of precise steps: first, PI3K activation produces PIP3; then, AKT binds to PIP3 via its PH domain, translocating to the cell membrane; next, PDK1 phosphorylates AKT at the activation loop (Thr308); finally, mTORC2 phosphorylates AKT at the hydrophobic motif (Ser473). The three AKT isoforms have differentiated functions: AKT1 primarily regulates cell survival and growth, AKT2 is crucial for insulin signaling and glucose metabolism, and AKT3 plays an important role in nervous system development. AKT regulates various downstream effects through multiple mechanisms, including inhibition of apoptosis (e.g., by phosphorylating and inhibiting Bad, caspase-9), promotion of cell cycle progression (e.g., by inhibiting p21, p27), and regulation of metabolism (e.g., by promoting GLUT4 translocation, activating glycogen synthase). Due to its broad biological functions, dysregulation of the AKT signaling pathway is closely associated with various diseases. In cancer, AKT hyperactivation is common; in diabetes, AKT2 dysfunction is linked to insulin resistance; in neurological disorders, abnormal AKT signaling is associated with neurodegenerative diseases. Therefore, AKT has become an important therapeutic target, with researchers developing AKT inhibitors for cancer treatment and exploring AKT2 selective activators for diabetes therapy.

SGK Family

The SGK (serum- and glucocorticoid-inducible kinase) family is another significant branch of the AGC kinase group, consisting of three members: SGK1, SGK2, and SGK3. The SGK family plays a crucial role in cellular stress responses and ion channel regulation. Its structural features are similar to the AKT family but lack a PH domain. The activation mechanism of SGK is similar to that of AKT, requiring dual phosphorylation by PDK1 and mTORC2. SGK1 is the most extensively studied member of this family, involved in regulating various cellular processes, including cell proliferation, survival, ion transport, and transcriptional regulation. SGK1 expression is induced by various stimuli, such as serum, glucocorticoids, growth factors, and cellular stress. SGK1 plays essential roles under both physiological and pathological conditions. For example, in the kidneys, SGK1 regulates sodium ion reabsorption, influencing blood pressure regulation; in tumors, overexpression of SGK1 is associated with cell proliferation and anti-apoptosis. Recent studies suggest that SGK1 may contribute to the development of diabetes and its complications, as well as certain neurological disorders. Thus, the development of SGK inhibitors has become a new therapeutic strategy, with some progress made in treating diabetes-related hypertension.

Figure 1. Structure of SGK1. (Jang H, et al., 2022)

RSK Family

The RSK (ribosomal S6 kinase) family is another noteworthy member of the AGC kinase group. The RSK family includes four isoforms (RSK1-4), which are key downstream effectors of the Ras-ERK signaling pathway. RSKs are unique in that they contain two functional kinase domains: an N-terminal AGC kinase domain and a C-terminal CAMK family kinase domain. Activation of RSK is a complex, multi-step process involving ERK, PDK1, and autophosphorylation. Once activated, RSK phosphorylates a variety of substrates, regulating gene expression, cell cycle progression, and cell survival. RSKs play a role in several cellular processes, including differentiation, survival, and proliferation. Notably, loss-of-function mutations in RSK2 are associated with Coffin-Lowry syndrome, highlighting the importance of RSK in nervous system development. Recent research also indicates that RSK plays a crucial role in epithelial-mesenchymal transition in tumor cells, providing a theoretical basis for developing RSK inhibitors as anti-tumor metastasis drugs.

PKC Family

The PKC (protein kinase C) family is the most complex and diverse subgroup within the AGC kinase group. Based on structural features and activation requirements, the PKC family is divided into three subfamilies: classical PKC (cPKC, including α, βI, βII, and γ), novel PKC (nPKC, including δ, ε, η, and θ), and atypical PKC (aPKC, including ζ and ι/λ). PKC activation typically involves the binding of second messengers (e.g., calcium ions and diacylglycerol) and phosphorylation mediated by PDK1. Different PKC isoforms have distinct tissue distributions and functions, participating in the regulation of cell proliferation, differentiation, apoptosis, cytoskeletal reorganization, and gene expression. In terms of disease, PKC dysregulation is associated with various pathological conditions, such as cancer, diabetes, and neurodegenerative diseases. For example, atypical PKC iota (PKCι) is highly expressed in various cancers, and its inhibition can suppress tumor growth in lung cancer, among others. Meanwhile, novel PKC theta (PKCθ) plays a crucial role in T cell activation and is a potential therapeutic target for treating allergic and autoimmune diseases.

GRK Family

The GRK (G-protein-coupled receptor kinase) family is another significant branch of the AGC kinase group, comprising seven human homologs (GRK1-7). The primary function of GRKs is to phosphorylate activated G-protein-coupled receptors (GPCRs), leading to receptor desensitization. Beyond regulating GPCR signaling, GRKs interact with and phosphorylate a variety of other proteins, including non-GPCR receptors, membrane proteins, cytoplasmic and nuclear signaling proteins, and structural proteins. Abnormal GRK activity is associated with several human diseases, such as heart failure, Parkinson's disease, and mood disorders. Among them, GRK2 is considered a potential target for cardiovascular disease treatment, while GRK6 modulators may be used to treat Parkinson's disease.

In addition to the major members mentioned above, the AGC kinase family includes other important kinases such as PKG (protein kinase G), p70S6K (p70 ribosomal S6 kinase), and MSK (mitogen- and stress-activated protein kinase), which play specific roles in their respective signaling pathways, participating in the regulation of various physiological processes.

Structural Features of AGC Kinases

The AGC kinase family members share a highly conserved protein kinase fold, with the core catalytic domain comprising two functional regions: a small lobe and a large lobe. The ATP-binding site lies between these regions, with its top formed by a glycine-rich loop. The activation loop extends from the DFG motif (Asp-Phe-Gly) and lies between the α-C helix of the small lobe and the large lobe. The substrate-binding site also resides between the small and large lobes, and all identified structures indicate that substrates bind to this site in an extended

β-strand conformation: These structural characteristics have led to the discovery of multiple AGC kinase inhibitors. Although no inhibitors have been successfully developed for clinical use, they have greatly contributed to the understanding of kinase activation and regulation mechanisms.

Regulation of AGC Kinases

AGC kinases are regulated through a series of post-translational modifications, including phosphorylation and ubiquitination. The most common and crucial phosphorylation sites are the activation loop, hydrophobic motif, and turn motif. Additionally, AGC kinases are subject to allosteric regulation through interactions with phosphoinositides, scaffolding proteins, and protein-protein interactions, contributing to their precise regulation.

Therapeutic Potential and Future Directions

Given their central roles in various signaling pathways, AGC kinases have attracted significant attention as potential therapeutic targets for various diseases. In cancer, inhibitors targeting PDK1, AKT, SGK, and PKC isoforms are under active development. In cardiovascular diseases, modulators of GRK activity show promise, while in metabolic diseases, targeting the AKT and SGK pathways may offer new treatment options. Recent advances in the development of small-molecule inhibitors, peptides, and other therapeutic modalities have opened new avenues for targeting AGC kinases.

However, challenges remain in developing selective inhibitors for specific AGC kinase isoforms, given their high degree of sequence homology. The development of more selective and potent inhibitors requires a deeper understanding of the structural and regulatory mechanisms of AGC kinases. Furthermore, investigating the crosstalk between AGC kinases and other signaling pathways will provide new insights into their roles in disease and uncover additional therapeutic targets.

In conclusion, the AGC kinase family plays an integral role in cellular signaling, with broad implications for health and disease. Continued research in this field promises to yield novel therapeutic strategies for a wide range of diseases, including cancer, cardiovascular disorders, and metabolic diseases.