SOCS-mediated downregulation of mutant Jak2 (V617F, T875N and K539L) counteracts cytokine-independent signaling.
Recently, mutations in the gene of Janus kinase 2 (Jak2) were discovered in patients suffering from chronic myeloproliferative disorders (MPD) and leukemia. As suppressors of cytokine signaling (SOCS) proteins are potent feedback inhibitors of Jak-mediated signaling, we investigated their role in signal transduction through constitutively active Jak2 mutants. We selected two mutants, Jak2-V617F and Jak2-K539L, found in patients with MPDs and Jak2-T875N identified in acute megakaryoblastic leukemia. We found SOCS family members to be induced through Jak2-V617F in human leukemia cell lines expressing the mutant allele and in stable HEK transfectants inducibly expressing constitutively active Jak2 mutants. SOCS proteins were recruited to the membrane and bound to the constitutively active Jaks. In contrast to wild-type Jak2, the mutant proteins were constitutively ubiquitinated and degraded through the proteasome. Taken together, we show a SOCS-mediated downregulation of the constitutively active, disease-associated mutant Jak2 proteins. Furthermore, a threshold level of mutant Jak expression has to be overcome to allow full cytokine-independent constitutive activation of signaling proteins, which may explain progression to homozygocity in MPDs as well as gene amplification in severe phenotypes and leukemia.
Haan
S
S
Life Sciences Research Unit, University of Luxembourg, Luxembourg, Luxembourg. serge.haan@uni.lu
Wüller
S
S
Kaczor
J
J
Rolvering
C
C
Nöcker
T
T
Behrmann
I
I
Haan
C
C
D045744:Cell Line, Tumor; D016207:Cytokines / Q000502:physiology*; D015536:Down-Regulation; D006801:Humans; D053614:Janus Kinase 2 / Q000502:physiology*; D009154:Mutation; D046988:Proteasome Endopeptidase Complex / Q000502:physiology; D021381:Protein Transport; D012333:RNA, Messenger / Q000032:analysis; D050799:STAT5 Transcription Factor / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D000071222:Suppressor of Cytokine Signaling 1 Protein; D000071223:Suppressor of Cytokine Signaling 3 Protein; D050826:Suppressor of Cytokine Signaling Proteins / Q000235:genetics / Q000502:physiology*
D016207:Cytokines; D012333:RNA, Messenger; C108944:SOCS1 protein, human; C106644:SOCS3 protein, human; D050799:STAT5 Transcription Factor; D000071222:Suppressor of Cytokine Signaling 1 Protein; D000071223:Suppressor of Cytokine Signaling 3 Protein; D050826:Suppressor of Cytokine Signaling Proteins; C507924:JAK2 protein, human; D053614:Janus Kinase 2; D046988:Proteasome Endopeptidase Complex