Energy-sensitive regulation of Na⁺/K⁺-ATPase by Janus kinase 2

Janus kinase 2 (JAK2) contributes to intracellular signaling of leptin and erythropoietin, hormones protecting cells during energy depletion. The present study explores whether JAK2 is activated by energy depletion and regulates Na⁺/K⁺-ATPase, the major energy-consuming pump. In Jurkat cells, JAK2 activity was determined by radioactive kinase assay, phosphorylated JAK2 detected by Western blotting, ATP levels measured by luciferase assay, as well as Na⁺/K⁺-ATPase α1-subunit transcript and protein abundance determined by real-time PCR and Western blotting, respectively. Ouabain-sensitive K⁺-induced currents (I_pump) were measured by whole cell patch clamp. I_pump was further determined by dual-electrode voltage clamp inXenopus oocytes injected with cRNA-encoding JAK2, active ^V617FJAK2, or inactive ^K882EJAK2. As a result, in Jurkat T cells, JAK2 activity significantly increased following energy depletion by sodium azide (NaN₃) or 2,4- dinitro phenol (DNP). DNP- and NaN₃-induced decrease of cellular ATP was significantly augmented by JAK2 inhibitor AG490 and blunted by Na⁺/K⁺-ATPase inhibitor ouabain. DNP decreased and AG490 enhanced I_pump as well as Na⁺/K⁺-ATPase α1-subunit transcript and protein abundance. The α1-subunit transcript levels were also enhanced by signal transducer and activator of transcription-5 inhibitor CAS 285986-31-4. In Xenopus oocytes,I_pump was significantly decreased by expression of JAK2 and ^V617FJAK2 but not of ^K882EJAK2, effects again reversed by AG490. In ^V617FJAK2-expressingXenopus oocytes, neither DNP nor NaN₃ resulted in further decline of I_pump. In Xenopus oocytes, the effect of ^V617FJAK2 on I_pump was not prevented by inhibition of transcription with actinomycin. In conclusion, JAK2 is a novel energy-sensing kinase that curtails energy consumption by downregulating Na⁺/K⁺-ATPase expression and activity.