JAK1

Detailed Information

Janus kinases (JAKs) are non-receptor tyrosine kinases involved in regulating immune responses, inflammatory reactions, and hematopoietic functions related to various cytokines and growth factors. The JAK family in mammals consists of four different subtypes: JAK1, JAK2, JAK3, and tyrosine kinase 2 (TYK2). JAKs are composed of seven homologous domains organized into four conserved structural regions: the C-terminal protein tyrosine kinase (PTK) domain, pseudokinase domain, SH2 domain, and N-terminal FERM domain.

• PTK: JH1 is the catalytically active kinase domain and the characteristic region that distinguishes JAK proteins from other protein tyrosine kinases.
• Pseudokinase: JH2 is a domain that inhibits tyrosine kinase activity and induces cytokine signal transduction. Mutations in this domain often lead to abnormal JAK activity, resulting in certain diseases.
• SH2: JH3 and JH4 form the SH2 domain, which stabilizes the JAK conformation.
• FERM domain: The FERM region formed by JH5-JH7 can interact with the intracellular region of cytokine receptors, mediating the specific binding of JAKs to appropriate receptors. Mutations in this region may inhibit or enhance kinase activity, though it lacks important residues with catalytic function.

JAKs and their downstream signal transducers and activators of transcription (STATs) form the JAK-STAT signaling pathway, which participates in processes including immunity, cell division, cell death, and tumor formation. Abnormalities in this pathway can lead to various diseases, including skin conditions, cancers, and diseases affecting the immune system.

JAK/STAT Signaling Pathway

STAT proteins are key signal molecules downstream of JAK kinases. The STAT family includes seven members: STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B, and STAT6. The STAT protein family has six domains: N-terminal domain, coiled-coil domain, DNA binding domain, linker domain, SH2 domain, and transactivation domain.

The N-terminal and coiled-coil domains drive STAT dimer formation, while the helical structure regulates the dynamic balance of protein nuclear import and export. The DNA binding domain enables STATs to bind DNA as transcription factors, and the SH2 domain specifically recognizes phosphorylated tyrosine sites on cytokine receptors.

When receptor tyrosine residues are phosphorylated, cytoplasmic STAT proteins are recruited to the activated receptor, where their own tyrosine sites are phosphorylated, triggering STAT dimer formation. Subsequently, STAT dimers enter the nucleus as core components of transcription factor complexes, activating the transcription of specific target genes. After completing transcriptional regulation, STAT proteins are dephosphorylated in the nucleus and return to the cytoplasm.

Notably, STAT3, as a central hub of this family, plays a key role in signal cascade transmission from the plasma membrane to the nucleus and has become an important target for drug development.

The JAK-STAT signaling pathway consists of three core components:

• Surface receptors: Usually cytokine receptors that recognize and bind specific cytokines, such as interleukins (ILs) and interferons (IFNs).
• Janus kinases (JAKs): Non-receptor tyrosine kinases including JAK1, JAK2, JAK3, and TYK2, which couple with cytokine receptors and play a key role in signal transduction.
• Signal transducers and activators of transcription (STATs): Including STAT1, STAT2, STAT3, STAT4, STAT5(A/B), and STAT6, which are transcription factors that regulate gene expression in the nucleus.

Typical Activation of JAK/STAT Signaling Pathway

The JAK-STAT signaling pathway mediates various physiological and pathological responses in the body and is an important regulatory mechanism. JAKs can bind to transmembrane receptors for cytokines and growth factors. When two JAK molecules bind to a cytokine receptor, the conformation of the receptor's cytoplasmic region changes, activating JAK proteins while phosphorylating the receptor's cytoplasmic region, providing docking sites for STATs.

STATs exist as dimers and, through phosphorylation by JAKs, cause the subunits within the STAT dimer to reposition. Depending on the receptor and the molecular docking sites produced by tyrosine phosphorylation in the cytoplasmic region, any one or more of the six members of the STAT family (STAT1, STAT2, STAT3, STAT4, STAT5, or STAT6) may enter the nucleus via the SH2 domain, affecting gene replication, transcription, and expression.

Phosphorylated STAT homodimers or heterodimers translocate to the nucleus, bind to corresponding promoter regions of target genes, activate gene transcription and expression, leading to changes in cellular function, and thereby affecting cell proliferation, differentiation, and death.

Figure 1. Canonical activation and negative regulation of JAK-STAT signaling pathways. (Xue C, et al., 2023)

Negative Regulation of JAK/STAT Signaling Pathway

The activation of the JAK-STAT signaling pathway is subject to multi-level negative regulation, mainly involving three types of regulatory factors:

• Suppressors of cytokine signaling (SOCS): The SOCS family is a core group of pathway inhibitors, including CIS and SOCS1-7. All SOCS proteins have an SH2 domain and a conserved SOCS functional domain. Their expression can be induced by cytokines such as IL-2, IL-3, and IFN-γ. Activated STATs enter the nucleus and establish a negative feedback loop by enhancing SOCS gene transcription. SOCS proteins exert inhibitory effects through the following mechanisms: blocking the binding of STATs to receptors; inactivating JAKs through the N-terminal kinase inhibitory domain; mediating ubiquitination of JAKs or STATs and promoting their degradation via the proteasome.
• Protein inhibitors of activated STATs (PIAS): The PIAS family (including PIAS1, PIAS3, PIASx, PIASy, etc.) inhibits STAT dimerization by binding to STAT monomers or prevents STAT dimers from binding to DNA.
• Protein tyrosine phosphatases: These act in two ways: directly interacting with receptors to dephosphorylate JAKs, or catalyzing the dephosphorylation of STAT dimers, thereby terminating signal transduction.

JAK inhibitors are small molecule inhibitors targeting the JAK/STAT signaling pathway. Currently, multiple JAK inhibitors have been approved for treating various diseases, including autoimmune diseases and tumors.

Development Process of JAK Inhibitors

Since 2010, multiple JAK inhibitors have been approved globally for autoimmune diseases, inflammation, and other indications. From the first JAK inhibitor, ruxolitinib, approved in 2011, followed by other first-generation non-selective JAK inhibitors like tofacitinib and baricitinib, to second-generation selective JAK inhibitors such as upadacitinib, filgotinib, and abrocitinib entering the market, until the first third-generation JAK inhibitor, deucravacitinib, was successfully launched in 2022, spanning a full twenty years.

First-Generation JAK Inhibitors

Novartis's ruxolitinib was the first JAK inhibitor (JAK1/JAK2), approved by the FDA in 2011 for treating myelofibrosis. It entered the market in 2017. Subsequent first-generation JAK inhibitors include Pfizer's tofacitinib (pan-JAK), Eli Lilly's baricitinib (JAK1/JAK2), Astellas's peficitinib (pan-JAK), and Japan Tobacco/Torii Pharmaceutical's delgocitinib (pan-JAK).

First-generation JAK inhibitors target pan-JAK targets with relatively low selectivity, inhibiting multiple signaling pathways simultaneously. While treating diseases, they also affect normal pathways, leading to various toxic side effects. For example, improper inhibition of JAK2 can lead to thrombocytopenia and anemia, while improper inhibition of JAK3 may cause T cell and B cell deficiency, leading to immunodeficiency and infections.

The FDA issued black box warnings for first-generation JAK inhibitors, including risks of cardiovascular issues, tumors, thrombosis, infections, and death. The FDA believes that subsequent JAK inhibitor products have similar potential safety risks, so black box warnings have been issued for all subsequently approved JAK inhibitors.

To reduce the occurrence of side effects, highly selective second-generation inhibitors emerged.

Second-Generation JAK Inhibitors

Currently approved second-generation JAK inhibitors in the market mainly target JAK1 and JAK3 combination inhibitors. Upadacitinib, developed by AbbVie, is the first second-generation JAK inhibitor (JAK1), currently approved for multiple indications globally. Subsequent second-generation JAKs include Gilead's filgotinib (JAK1), Pfizer's abrocitinib (JAK1), and CTI BioPharma's pacritinib (JAK2).

Compared to first-generation JAK inhibitors, the advantage of second-generation inhibitors lies in their high selectivity, inhibiting specific disease-related signaling pathways while maintaining other cytokine functions unaffected, reducing the occurrence of toxic side effects and improving the safety of clinical medication. However, despite having higher selectivity than first-generation inhibitors and significantly reducing risks in cardiovascular and infection aspects, the FDA still issued black box warnings.

Third-Generation JAK Inhibitors

Deucravacitinib, developed by Bristol Myers Squibb, is the first and currently only approved third-generation JAK inhibitor (TYK2), used to treat moderate to severe plaque psoriasis. It is the only JAK inhibitor without any black box warnings. In clinical trials, deucravacitinib did not report serious adverse reactions, with adverse events being primarily mild or moderate, the most common being upper respiratory tract infections and nasopharyngitis.