JAK-STAT Signalling and Cytokines: The What, How, and Why

JAKs can phosphorylate tyrosines on the proteins they bind to, including cytokine receptors.

The family of JAK proteins plays a significant role in interpreting signals, regulating gene expression, and contributing to cellular equilibrium. All JAKs are tyrosine kinases. There are 4 different JAKs: JAK1, JAK2, JAK3, and TYK2 (JAK4). All JAKs have four domains: a N‐terminal FERM domain, an SH2 domain, and two kinase domains.​ The first of these kinase domains is a pseudokinase domain because it is catalysis inactive. The second kinase domain of JAK is responsible for phosphorylation of the receptor and subsequently STAT transcription factors.

STAT proteins have 2 functions - transducing signals from cytokines and promoting transcription of specific genes. The STATs are present in the cytoplasm as inactive dimers but are rapidly activated by cytokine signaling and transported into the nucleus. A STAT protein consists of an N‐terminal portion, a coiled-coil domain, a DNA binding domain, a linker region, an SH2 domain, and a C terminus transactivation domain. There is a single conserved tyrosine residue between the SH2 domain and the transactivation domain, which is where the STAT proteins are phosphorylated by JAKs and is required for their activation.

There are 7 STAT proteins in mammals; STAT1, STAT2, STAT3, STAT4, STAT5a, STAT5b, and STAT6. STAT1 is expressed at high levels in the heart, thymus and spleen. STAT2,3 and 6 are expressed in the majority of tissues. STAT4 is mainly located in the testis, thymus, and spleen. STAT 5a and 5b are deferentially expressed in muscle tissue, the brain, and the mammary gland and secretory organs (seminal vesicle and salivary gland).

JAK-STAT signalling is initiated when a ligand, e.g., cytokine, binds to the cognate transmembrane receptor. This result in two or more JAKs being brought into close proximity, triggering JAKs to phosphorylate both the receptor and JAKs themselves. This creates two binding sites for STAT proteins: JH domains on JAKs and phosphorylated tyrosines of receptors that have become bound by JAKs. The STAT protein then binds these sites through their Src homology 2 domain (SH2). When a tyrosine residue found between SH2 domain and the C-terminal transactivationon STATs are phosphorylated a homodimer (two identical copies) forms. These newly formed homodimers are stabilised by reciprocal phosphotyrosine and SH2 domain interactions.

This dimer has nuclear localization signals, meaning it can be transported into the cell nucleus from the cytoplasm. Once in the nucleus, STAT proteins bind DNA at specific enhancer sequences and activate or repress transcription. STAT proteins are important in JAK-STAT signalling as they control which genes are turned on or off.

Cytokines, compact yet potent proteins, hold a crucial role in cellular communication, orchestrating diverse physiological responses. These molecules act as intercellular messengers, transmitting vital information between cells and governing immune reactions, growth, and tissue specialization.

Produced by various cell types, including immune and stromal cells, cytokines bind to specific receptors on cell surfaces. This binding initiates a chain reaction of internal signals, culminating in cellular responses. Each cytokine type pairs with a particular receptor, often linked to JAK-STAT pathways, creating a tailored response mechanism.

Cytokine receptors serve as the architectural foundation for the intricate signaling pathways orchestrated by JAK-STAT systems. These receptors are poised on the cell surface, ready to translate extracellular cues into intracellular responses. Their structural and functional diversity allows cells to interpret a wide array of incoming signals, each triggering unique cellular outcomes.

Cytokine receptors are typically classified into several categories, including type I and type II receptors, based on their structural motifs and associated signaling mechanisms. Type I receptors, often linked to the JAK-STAT pathway, possess intrinsic JAK kinase activity. Upon cytokine binding, these receptors undergo conformational changes that bring their associated JAK kinases into close proximity, promoting autophosphorylation and subsequent activation. This activated JAK complex then phosphorylates specific tyrosine residues on the receptor, creating docking sites for downstream signaling molecules, such as STAT proteins.

Type II receptors, on the other hand, lack intrinsic kinase activity. They depend on a non-covalently associated JAK kinase for their signaling capacity. Upon cytokine binding, the receptor-JAK complex transduces the signal by a similar mechanism, initiating the phosphorylation cascade and STAT protein recruitment.

It's important to note that cytokine receptors also encompass cytokine-binding subunits and shared signaling components. These attributes contribute to the fine-tuning of signaling responses and the crosstalk between different signaling pathways.

Cytokine signalling begins as cytokines, the molecular messengers of the cell, interact with their corresponding receptors on the cell surface. Once engaged, this interaction prompts a sequence of events that trigger the JAK-STAT cascade.

Activated JAK proteins, now primed by cytokine-receptor binding, phosphorylate specific tyrosine residues on STAT proteins, inducing their activation and dimerization. These STAT dimers migrate to the nucleus, where they engage with DNA, initiating the transcription of target genes. This meticulous process culminates in the synthesis of proteins that mediate the cell's response to the initial cytokine signal.

The specificity of this signaling pathway is remarkable, as different cytokines engage distinct receptor-JAK-STAT combinations, resulting in unique cellular outcomes. Furthermore, the duration and magnitude of JAK-STAT signaling are tightly regulated, ensuring precise and controlled responses to changing environmental cues.

JAK-STAT signalling is responsible for many aspects of immunity and inflammation, including:

- the activation of T cells, B cells, and macrophages

- the production of cytokines

- the differentiation and activation of immune cells

JAK-STAT signalling plays an important role in the innate immune system. One example of this is macrophages, white blood cells that detect infections by binding pathogen-associated molecules and initiate inflammation by secreting cytokines. JAK-STAT proteins regulate many aspects of macrophage function, including differentiation from monocytes; maturation; activation; phagocytosis (engulfing and destroying pathogens); and production of cytokines.

Aberrant activation of JAK-STAT pathways has been implicated in various cancer types, acting as a driving force for cellular transformation, survival, proliferation, and immune evasion. Mutations in components of the JAK-STAT signaling cascade, such as JAK proteins and STAT factors, can unleash uncontrolled growth signals, contributing to tumor initiation and progression.

Moreover, the JAK-STAT pathways possess the ability to shape the tumor microenvironment, influencing immune responses and communication between cancer cells and their surroundings. Dysregulated cytokine signaling within the tumor milieu can create an immunosuppressive environment, hampering the body's natural defense mechanisms.

Despite the challenges posed by these hijacked pathways, the intricate nature of JAK-STAT signaling also presents opportunities for therapeutic interventions. Targeting specific components of these pathways has emerged as a promising strategy for cancer treatment.

Dysregulated JAK-STAT signaling has been implicated in a spectrum of immune disorders, ranging from autoimmune diseases to immunodeficiencies. In autoimmune conditions, abnormal activation of these pathways can lead to misguided immune responses against the body's own tissues, as observed in rheumatoid arthritis and multiple sclerosis. Conversely, deficiencies in JAK-STAT signaling can compromise immune cell development, impairing the body's ability to fend off infections, as observed in Severe Combined Immunodeficiency (SCID) and STAT1-defiecient hyper IgE syndrome. .

The complex interplay of JAK-STAT pathways within immune disorders as well as cancers reveals potential avenues for therapeutic intervention.

JAK-STAT inhibitors are drugs that interfere with the JAK kinases, key players in the signaling cascade. By halting JAK activity, these inhibitors effectively disrupt the downstream signaling events, curbing the overactive immune responses that contribute to immune disorders like rheumatoid arthritis, inflammatory bowel disease, and psoriasis. JAK inhibitors block JAKs from phosphorylating their substrates, such as receptors and domains on JAKs themselves. This prevents STAT proteins from dimerizing and binding DNA to activate or repress the transcription of genes involved in immune responses. Inhibition of JAK-STAT signalling has been shown to reduce inflammation by blocking the production of proinflammatory cytokines (e.g., TNF alpha). It also decreases other signalling pathways that are activated when Jak-STAT is not working properly, including NF kappa B (nuclear factor kappa beta).

The potential of JAK-STAT inhibitors extends beyond immune disorders. Cancers that exploit JAK-STAT pathways for growth and survival, such as certain types of leukemia and lymphomas, also stand to be impacted. By stifling these pathways, JAK-STAT inhibitors aim to stifle cancer cells' ability to proliferate and evade the body's defenses.

While the therapeutic prospects are promising, JAK-STAT inhibitors also bring challenges. Balancing effective suppression of aberrant signaling with the preservation of essential immune functions is a delicate tightrope walk. Ensuring the safety and long-term efficacy of these inhibitors remains a critical focus of ongoing research.

In conclusion, our exploration of JAK-STAT signaling has provided valuable insights into the intricacies of cellular communication. The interactions within cytokine dialogues and the roles played by JAK and STAT proteins have shed light on the mechanics of this crucial system.

The dynamic relationship between JAK-STAT pathways and conditions such as cancer and immune disorders underscores their dual role – both as instigators and prospective targets for therapeutic intervention. JAK-STAT inhibitors, with their precision-focused approach, hold significant promise for redefining treatment strategies.