Redox regulation of Janus kinase: The elephant in the room

Abstract

The redox regulation of Janus kinases (JAKs) is a complex subject. Due to other redox-sensitive kinases in the kinome, redox-sensitive phosphatases, and cellular antioxidant systems and reactive oxygen species (ROS) production systems, the net biological outcomes of oxidative stress on JAK-dependent signal transduction vary according to the specific biological system examined. This review begins with a discussion of the biochemical evidence for a cysteine-based redox switch in the catalytic domain of JAKs, proceeds to consider direct and indirect regulatory mechanisms involved in biological experiments, and ends with a discussion of the role(s) of redox regulation of JAKs in various diseases.

Keywords: Janus kinases; cysteine modification; cytokine biology; oxidative stress; redox regulation; thiol chemistry.

Figure 1. A simple equilibrium model for the direct redox regulation of Janus kinases. This model, based on the 3D structural coordinates of the JAK2 catalytic domain, shows the reduced (thiol) state of the redox switch in yellow, with Cys866 on the left and Cys917 on the right, and with the essential Lys882 residue shown in purple. Upon oxidation, these two residues can become oxidized to form a disulfide bond, shown in orange, or either one can become independently oxidized to a sulfenic acid, where the S-OH moiety is shown as bonded orange-pink-gray spheres. The dynamic equilibrium between these states shifts in response to the redox state of the environment, which can be naturally or artificially manipulated by an excess of reductants (shifting left) or oxidants (shifting right). Note: the disulfide form does not need to transition through a sulfenic acid state to become the fully reduced state, and vice versa.

Figure 2. Conservation of four critical cysteine residues within 22 Janus kinases. Twenty-two JAK amino acid sequences from diverse metazoan species were aligned using the CLUSTAL OMEGA multiple sequence alignment algorithm.^, Sections of the alignment including rat JAK2 cysteine residues 866, 917, 1094, and 1105 are shown, with conserved cysteine residues highlighted in yellow.

Figure 3. The canonical cytokine/JAK/STAT/SOCS pathway. The canonical cytokine/JAK/STAT/SOCS pathway is illustrated with the example of erythropoieitin/JAK2/STAT5/SOCS3. Canonical pathways stimulate cytokine and cytokine-like hormone receptor-dependent activation of tyrosine kinase activity in JAKs, resulting in an elevation of JAK activity state, phosphorylation of docking sites on the receptor, and the recruitment and tyrosine phosphorylation of STATs. Tyrosine-phosphorylated STATs oligomerize, translocate into the nucleus, and stimulate gene transcription. One of the transcribed genes encodes SOCS, which primarily provides negative feedback via inhibition of JAK's action and initiating proteasomal degradation of the activated receptor complex; upon tyrosine phosphorylation it also promotes survival via the Ras pathway. Other tyrosine phosphorylation-dependent interactions initially catalyzed by JAK, such as coupling the EPO receptor (EPOR) to the RAS/RAF/MEK/ERK pathway via GRB2/SOS, or coupling EPOR to the PI₃K/AKT pathway, may not be considered by all authors to be part of the canonical pathway per se, yet they must be considered to understand the cytokine-proximal events of cell biology.

Figure 4. Permutations of JAK signal transduction outcomes due to the complexity of redox-associated pathways within the cell. To illustrate the importance of cell-specific context in determining the net biological outcomes of redox-regulation of JAK-dependent signaling, this cartoon depicts a few of the redox-related pathways, biomolecules, and processes capable of interacting with JAKs. Each of these components, such as protein tyrosine phosphatases (PTP, purple tetragons) and protein tyrosine kinases (yellow and orange rectangles) will be present in variable abundances from cell type to cell type, and their expression dynamically fluctuates in response to redox and non-redox regulation. Moreover, ROS is not generated uniformly throughout the cell, but is compartmentalized, such that their concentrations are gradients which can be transient or sustained according to the intensity and duration of their production. NADPH oxidases (NOX, gray pentagons) have specific subcellular localizations, as do thioredoxin reductases (TxR) and superoxide dismutases (SOD); cell-specific nitric oxide synthetases (NOS), glutathione reductase (GSR), and neighboring cells also affect the cellular redox state. Several of these redox regulators are in turn regulated by JAKs and other key signal transduction enzymes.