Comparative analysis of the JAK/STAT signaling through erythropoietin receptor and thrombopoietin receptor using a systems approach
© Won et al; licensee BioMed Central Ltd. 2009
Published: 30 January 2009
The Janus kinase-signal transducer and activator of transcription (JAK/STAT) pathway is one of the most important targets for myeloproliferative disorder (MPD). Although several efforts toward modeling the pathway using systems biology have been successful, the pathway was not fully investigated in regard to understanding pathological context and to model receptor kinetics and mutation effects.
We have performed modeling and simulation studies of the JAK/STAT pathway, including the kinetics of two associated receptors (the erythropoietin receptor and thrombopoietin receptor) with the wild type and a recently reported mutation (JAK2V617F) of the JAK2 protein.
We found that the different kinetics of those two receptors might be important factors that affect the sensitivity of JAK/STAT signaling to the mutation effect. In addition, our simulation results support clinically observed pathological differences between the two subtypes of MPD with respect to the JAK2V617F mutation.
The Janus kinase-signal transducer and activator of transcription (JAK/STAT) pathway has been frequently reported to be responsible for oncogenic processes such as uncontrollable proliferation, apoptosis resistance, sustained angiogenesis, and immune evasion . Recently, several research groups have demonstrated the existence of a point mutation in JAK2 in most patients with polycythaemia vera (PV), and approximately half of patients with essential thrombocythaemia (ET) and primary myelofibrosis (PMF) which are subtypes of myeloproliferative disorder (MPD) [2–4]. The mutation results in a substitution of valine for phenylalanine at codon 617 of JAK2 (JAK2V617F) and leads to cytokine hypersensitivity and cytokine independent activation of the JAK/STAT pathway. As a consequence, the expression of JAK2V617F causes the production of hematopoietic colonies that are characteristic of MPD patients.
Frequency and mutational state of the JAK2V617F allele in MPD patients.
Furthermore, a dosage effect of JAK2V617F on JAK/STAT signaling has been reported. Constitutive activation of JAK/STAT signaling observed in cells transfected with only the JAK2V617F gene were diminished when cells were transfected with both the wild type and JAK2V617F gene .
Although several efforts have been made for modeling the JAK/STAT pathway [6–10], to the best of our knowledge, there has been no comparative analysis performed for JAK/STAT signaling through EpoR and TpoR that is related to specific disease phenotypes. In this work, we aimed to understand the role of the JAK2V617F mutation in the formation of the disease phenotypes, PV and ET, through systems analysis. The main question of this work is how the two cytokine ligands, Epo and Tpo, and the corresponding receptors are related to PV and ET in regard to the activation of the JAK/STAT pathway. To study the different dynamic characteristics, we considered not only the kinetics of the receptors but also the relationship of the receptors with JAK2. In addition, the behavior of the suppressor of cytokine signaling (SOCS) protein, a negative regulator of JAK/STAT signaling, was investigated.
Through this systems approach, we found that the JAK2V617F mutation led to the over-activation of JAK/STAT signaling and the effect of the mutation was different between PV and ET based on the different receptor kinetics, supporting the different mutation patterns clinically observed in the patients.
Results and Discussion
Comparison of wild type JAK/STAT signaling through EpoR and TpoR
Comparison of JAK/STAT signaling with wild type JAK2 and JAK2V617F mutant proteins
Comparison of JAK/STAT signaling with JAK2V617F through EpoR and TpoR
These findings suggesting that JAK/STAT signaling through TpoR is more sensitive to the mutation effect than through EpoR support that a lower frequency of the JAK2V617F mutation is observed in ET patients than in PV patients. JAK/STAT signaling through EpoR is considered as mainly related with the production of red blood cells, whose excessive increase is an important clinical characteristic of PV patients, and signaling through TpoR is related with the production of platelets whose excessive production is a manifestation of ET. In addition, the findings also support the rare homozygosity of JAK2V617F in ET patients while a significant population of PV patients carry homozygous JAK2V617F alleles.
This study has shown that a systems approach can be useful for a comparative analysis of receptor kinetics of a signal transduction pathway in the context of the pathological domain for understanding MPD, especially JAK/STAT signaling through EpoR and TpoR. The simulated results indicate that the difference of receptor kinetics between Epo-induced and Tpo-induced JAK/STAT pathway might affect the sensitivity of the pathway to JAK2V617F, a gain-of-function mutation. Particularly, in the absence of ligands (this condition might be critical in chronic diseases as well as for MPD), JAK/STAT signaling through TpoR with a higher cell surface expression and a lower disappearance rate was more sensitive to the mutation effect than through EpoR. Accordingly, the peak and steady state concentration level of STAT*Dn, regarded as an indicator of the amount of JAK/STAT signaling, was set to an abnormally high level by the mutant model of JAK/STAT signaling through TpoR. These observations can be in accord with the clinical observation of the rarer homozygosity of JAK2V617F in ET patients than in PV patients. The effect of the mutation in PV is relatively weak and the high rate of homozygosity or dosage of mutant JAK2 proteins might be required to induce the PV phenotype [12, 13].
In future work, to investigate the pathway with regard to the pathogenesis of PV and ET, the model needs to be extended further to include some proteins related with disease phenotypes. Since PV patients can be characterized by an abnormal increase in the number of three types of blood cells – erythrocytes, granulocytes, and platelets  – the model for PV should involve hematopoietic transcription factors such as GATA-1, GATA-2 [15, 16], C/EBPε and Fli-1 [17, 18] that correspond to each lineage. In addition, some previous studies have shown that the cytokine receptors are also needed for JAK2V617F-mediated transformation [19, 20]. Therefore, it might be informative to include JAK as a chaperone for the receptor kinetics into the model.
JAK/STAT pathway model
Receptor kinetics constants.
Relative surface expression 
Disappearance rate of receptor
JAK2V617F mutant model
JAK2V617F mutant kinases bind to cytokine receptors and are phosphorylated in the absence of ligands, leading to the ligand-independent activation of downstream signaling pathways . We assumed that the phosphorylation rate of cytoplasmic STAT depended on the concentration of the receptor-JAK complex (RJ) as well as active ligand binding receptors, as JAK2V617F is a gain-of-function mutation independent of ligand binding or negative regulation. The chemical reaction by which JAK2V617F was added to the wild type model is as follows:
d/dt(STAT*c) = WT reactions + kJAK2V617F × RJ
The rate constant kJAK2V617F was estimated based on reported experimental results .
This work was supported by the Samsung Biomedical Research Institute grant, #SBRI C-A7-101-2. DL was supported by the Korean National Research Laboratory Grant (2005-01450) from the Ministry of Education, Science and Technology (MEST). This study was supported by a grant of the Korea Healthcare technology R&D Project, Ministry for Health, Welfare and Family Affairs, Republic of Korea (A080588).
This article has been published as part of BMC Bioinformatics Volume 10 Supplement 1, 2009: Proceedings of The Seventh Asia Pacific Bioinformatics Conference (APBC) 2009. The full contents of the supplement are available online at http://www.biomedcentral.com/1471-2105/10?issue=S1
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