JAK2 and SHP2 reciprocally regulate tyrosine phosphorylation and stability of proapoptotic protein ASK1

Area of Science:

• Cellular signaling pathways
• Protein regulation and degradation
• Cytokine-mediated cellular responses

Background:

• Apoptosis signal-regulating kinase 1 (ASK1) degradation is mediated by Suppressor of Cytokine Signaling 1 (SOCS1) binding to phosphorylated Tyrosine 718 (Tyr-718).
• The specific kinase and phosphatase regulating ASK1 phosphorylation and dephosphorylation at Tyr-718 were previously unidentified.

Purpose of the Study:

• To identify the kinase and phosphatase responsible for ASK1 phosphorylation and dephosphorylation at Tyr-718.
• To elucidate the roles of these enzymes in regulating ASK1 stability and downstream signaling in response to cytokines.

Main Methods:

• Utilized specific kinase inhibitors and phosphatase-inactive mutants.
• Investigated protein-protein interactions using co-immunoprecipitation assays.
• Employed knockout endothelial cells (EC) deficient in SOCS1 or SHP2.
• Assessed protein phosphorylation and degradation via Western blotting and proteasomal inhibition.

Main Results:

• Identified Janus kinase 2 (JAK2) as the Tyr-718-specific kinase and Src homology 2 domain-containing phosphatase-2 (SHP2) as the Tyr-718-specific phosphatase for ASK1.
• JAK2 directly phosphorylates ASK1 at Tyr-718, promoting SOCS1 binding and subsequent ASK1 degradation, particularly induced by Interferon-gamma (IFN-gamma).
• SHP2 dephosphorylates ASK1 at Tyr-718, dissociates SOCS1, enhances ASK1 stability, and regulates ASK1-JNK signaling and endothelial cell (EC) apoptosis, especially in response to Tumor Necrosis Factor (TNF).

Conclusions:

• JAK2 and SHP2 act as reciprocal regulators of ASK1 phosphorylation and stability.
• These findings reveal a novel regulatory mechanism for ASK1 in response to diverse cytokine signals, impacting cellular fate.
• The interplay between JAK2-SOCS1 and SHP2 provides critical insights into ASK1-mediated cellular processes.