Questin:

Janus kinase 2 (JAK2) contributes to intracellular signaling of leptin and erythropoietin, hormones protecting cells during energy depletion. The present study explored, whether JAK2 is activated by energy depletion and whether it regulates the activity of Na⁺/K⁺ ATPase, the major energy consuming pump in the cell membrane.

Methods:

JAK2 activity was determined by radioactive JAK2 kinase assay, activating JAK2 phosphorylation by western blotting and ouabain-sensitive currents (I_ouabain) reflecting Na⁺/K⁺-ATPase activity by whole cell patch clamp in Jurkat T- cells. Moreover, JAK2, active ^V617FJAK2 or, as negative control, inactive ^K882EJAK2 were expressed in Xenopus oocytes and I_ouabain determined by dual electrode voltage clamp.

Results:

Energy depletion of Jurkat T- cells by treatment with sodium azide (NaN₃) or 2,4- dinitro phenol (DNP) significantly increased JAK2 activity. Energy depletion with DNP significantly decreased I_ouabain in Jurkat T cells, an effect fully reversed by JAK2 inhibitor AG490. In Xenopus oocytes I_ouabain was significantly decreased by expression of JAK2 and of ^V617FJAK2 but not of ^K882EJAK2, an effect again reversed upon JAK2 inhibition. In ^V617FJAK2 expressing Xenopus oocytes DNP did not result in further decline of I_ouabain.

Conclusions:

JAK2 is a novel energy sensing kinase that regulates the activity of Na⁺/K⁺ ATPase.