c-Jun N-terminal Kinase (JNK) Pathway

1. What is c-Jun N-terminal Kinase?

The c-Jun N-terminal kinase (JNK) family, also known as stress-activated protein kinases (SAPKs), is a subgroup within the mitogen-activated protein kinase (MAPK) superfamily in mammalian systems. This serine/threonine (Ser/Thr) protein kinases are primarily localized to the cytoplasm but undergo rapid nuclear translocation upon activation by diverse stimuli such as environmental stress, pro-inflammatory cytokines, growth factors, microbial infections, or G-protein-coupled receptor ligands. Once activated, JNK phosphorylates key transcription factors, including TP53, c-Jun, and activating transcription factor 2 (ATF2), thereby modulating processes such as cell proliferation, differentiation, apoptosis, oxidative stress responses, and inflammatory signaling. These pathways play a central role in both physiological homeostasis and pathological conditions.

Fig.1 Overview of the regulatory system of the C-JUN N-terminal kinase (JNK) signaling cascade.^1,3

2. JNK Isoforms

Encoded by three distinct genes—JNK1 (MAPK8), JNK2 (MAPK9), and JNK3 (MAPK10)—the JNK family exhibits isoform-specific expression patterns. JNK1 and JNK2 are broadly expressed across diverse tissues, whereas JNK3 is predominantly found in specialized tissues, including the brain, heart, and testes. These genes undergo alternative splicing to yield ten isoforms, which display high sequence homology (>80%) but distinct functional profiles. The differential expression of JNK isoforms facilitates tissue-specific adaptations to JNK activation. This functional diversity is attributed to variations in substrate recognition, enabling distinct JNK isoforms to phosphorylate unique substrates or downstream nuclear transcription factors, thereby modulating context-dependent cellular responses.

Table 1. JNK isoforms.

3. JNK Pathway

3.1 Activation

The JNK pathway is organized into a three-tiered kinase cascade comprising MAP3Ks (e.g., ASK1), MAP2Ks (e.g., MKK4 and MKK7), and MAPKs (JNK1, JNK2, and JNK3). This pathway is activated by a wide range of stimuli, including inflammatory cytokines, environmental stressors (e.g., ischemia-reperfusion, oxidative stress, ultraviolet radiation, endoplasmic reticulum stress, hyperosmotic conditions, and DNA damage), G-protein-coupled receptors, growth factors, and physical stressors such as ultraviolet light or ionizing radiation. Upon cellular exposure to extracellular stress, MAP3Ks, such as MEKK1, are the first components activated within the JNK cascade. Activated MAP3Ks subsequently phosphorylate two members of the MAP2K family, MKK4 and MKK7, which activate the terminal kinases JNK1, JNK2, and JNK3.

Fig.2 The general activation mechanism of the JNK signaling pathway.^2,3

3.2 Components

The JNK signaling pathway is regulated by three components: upstream activators (e.g., MAP3Ks), downstream inhibitors (e.g., phosphatases), and scaffold proteins (e.g., JIPs).

• MAP3K Group: The MAP3K isoform transforming growth factor-β-activated kinase 1 (TAK1) is pivotal for JNK activation in response to inflammatory cytokines (e.g., interleukin-1, tumor necrosis factor, and transforming growth factor-β) and is essential for signaling through toll-like receptors (TLR-3, TLR-4, TLR-9), B-cell receptors, and T-cell receptors. Additionally, MAP3K subtypes TPL2 and MLK3 facilitate JNK activation in embryonic fibroblasts following tumor necrosis factor stimulation.
• MAPK Phosphatase (MKP) Family: MKPs function as negative regulators of JNK pathway. These phosphatases may directly dephosphorylate JNK or interact with it indirectly through scaffold-mediated mechanisms.
• Scaffold Proteins: The JNK pathway includes multiple scaffold proteins, such as APLIP1, JLP, UNC-16, and JIP4. These proteins contain modular domains that enable interactions with JNK and other pathway components. By forming multiprotein complexes, scaffold proteins coordinate the spatial organization and activity of JNK pathway members, thereby enhancing signaling efficiency, specificity, and cross-talk with other cellular pathways.

3.3 Substrates

Studies of c-Jun and associated transcription factors have revealed insights into the phosphorylation motifs and docking domains preferred by JNK. JNK has been shown to phosphorylate over 50 proteins, spanning processes such as protein degradation, signal transduction, and apoptosis.

Table 2. JNK substrates and their functions.

4. Cytokines Involved in the JNK Pathway

Cytokines such as TGF-β, TNF-α, and interleukins (ILs) are secreted proteins (6–70 kDa) that participate in diverse biological processes. Among these, TGF-β and TNF-α have been extensively studied for their roles in regulating cellular functions and signaling pathways.

• TGF-β

TGF-β, a pleiotropic cytokine, regulates cell growth, differentiation, and apoptosis by phosphorylating Smad proteins, which act as canonical mediators of TGF-β signaling. It also activates non-Smad pathways, including JNK. Additionally, activation of the TGF-β receptor initiates a TGF-β1-dependent reactive oxygen species (ROS) signaling cascade, which stimulates MMP-9 expression and enzymatic activity. ROS production further activates ERK1/2 and JNK, leading to NF-κB transcription factor activation.

• TNF-α

TNF-α, a pro-inflammatory cytokine, compromises endothelial function by binding to TNF receptor 1 (TNFR1) to form TNFR1 complex. This complex ubiquitinates TNF receptor-associated factors, thereby activating JNK and influencing hepatocyte survival and apoptosis during inflammatory responses.

• ILs

IL-17A promotes endothelial cell senescence by phosphorylating JNK and upregulating obesity-associated protein (FTO). IL-6 may enhance breast cancer cell migration and invasion by activating the JNK-c-JUN pathway.

5. The Biological Functions of JNKs Pathway

5.1 Apoptosis

The JNK signaling pathway induces apoptosis through both direct and indirect mechanisms. Activated JNK directly enhances the activity of transcription factors such as nuclear c-Jun, leading to the synthesis of the AP-1 complex and the subsequent upregulation of pro-apoptotic proteins. Additionally, activated JNK indirectly promotes apoptosis by activating the Bcl-2 family or facilitating the release of cytochrome C from mitochondria, thereby activating apoptosis.

Table 3. Genes Regulated by AP-1 and Involved in Apoptosis.

5.2 Autophagy

JNK influences autophagy through multiple mechanisms. Autophagy induction activates JNK, which then enhances c-Jun/c-Fos activity, boosting Beclin1 transcription.

5.3 Oxidative Stress

Oxidative stress, which arises from an imbalance between oxidants and antioxidants, can lead to cellular damage through processes such as senescence, apoptosis, inflammation, and necrosis. In this state, ASK1 activates JNK, which inhibits MEKK1 and NF-κB, promoting cell death over survival. ASK1, a MAP3K family kinase sensitive to redox changes, is activated by oxidative stress. It initiates signaling via p38 and JNK, leading to apoptosis, inflammatory responses, and fibrotic processes

5.4 Inflammatory Response

JNK activation drives endothelial cells to secrete pro-inflammatory factors, modulating inflammation. A decline in JNK phosphorylation is linked to reduced NF-κB phosphorylation and diminished levels of downstream inflammatory mediators, indicating JNK's regulatory role in inflammation via NF-κB.

5.5 Immune Response

The JNK pathway is crucial for immune functions, driving Th1/Th2 differentiation and T cell activation in the peripheral immune system. It also mediates immune responses in various cells, including microglia, macrophages, and splenic immune cells.

6. Related Products

Creative Biolabs is a trusted provider of high-quality JNK-related antibodies for research applications. We offer custom antibodies targeting key components of the MAP3Ks, phosphatase, and scaffold protein. Our products are optimized for various techniques, including Western blot, immunohistochemistry, and ELISA, supporting researchers in advancing their studies.