CONSTITUTIVE NUCLEAR-LOCALIZATION OF JANUS KINASE-1 AND KINASE-2

Both GH and the GH receptor have been reported to undergo rapid nuclea r translocation. Janus kinases (JAK) 1 and 2 have been implicated in G H receptor signaling, and both of these kinases are phosphorylated by GH stimulation. In this report, we have investigated the subcellular d istribution of JAK1 and JAK2. Both JAK1 and JAK2 exhibit a nucleocytop lasmic distribution by immunocytochemistry in unstimulated serum depri ved CHO cells stably transfected with rat GH receptor complementary DN A (cDNA). The nucleocytoplasmic localization of JAK2 was verified by i mmunogold electron microscopy in both rat liver hepatocytes and CHO ce lls stably transfected with rat GH receptor cDNA. Nucleocytoplasmic lo calization of JAK2 was also verified by transient tranfection of CHO c ells with a Haemophilus influenzae haemagglutinin (HA) epitope tagged JAK2 expression plasmid and subsequent localization of HA immunoreacti vity. Western blot analysis of purified nuclear extracts revealed the presence of immunoreactive JAK1 at 130 kDa and immunoreactive JAK2 at 128 kDa. No change in the nuclear content of JAK1 or JAK2 was observed upon ligand stimulation of GH receptor cDNA transfected cells with 10 0 nM human GH for 5, 10, 15, 30, or 60 min. GH stimulation caused, how ever, the appearance of tyrosine phosphorylated 42- and 44-kDa protein s as well as tyrosine phosphorylated JAK2 in the nucleus. The constitu tive nuclear localization of the Janus Kinases is suggestive of a nove l nuclear role for JAK family members, in addition to their described cytosolic function and presents an interesting challenge to the subcel lular site of hormone action.