In light of the persuasive evidence that JAKs do indeed reside in the nucleus, one must wonder what are the roles and functions of nuclear JAK. The presence of GH receptor within the nucleus has been implicated in cellular proliferation within the context of hepatocyte regeneration, tumorigenesis, and metastasis (65
). In this model, the role of the nuclear JAKs (principally JAK2) would be ostensibly to initiate signaling off the receptor, including in particular the activation of STAT5 (66
). In contrast, while still involving nuclear JAK2, GH-induced STAT3 signaling may be less dependent on the presence of the GH receptor within the nucleus (40
). Such differences in the nuclear anchoring of JAK2 might play a role in the differential activation of STAT3 and STAT5 by GH as a function of dosage level and time (40
). As far as we know, noncanonical targets of the nuclear JAKs, however, have not been identified for GH signaling.
In response to prolactin stimulation, nuclear JAK2 was found to phosphorylate the transcription factor nuclear factor 1-C2 (NF1-C2) in mammary epithelial cells, thereby preventing its proteasomal degradation (68
). NF1-C2 plays an important role in milk gene expression and the temporal pattern of activated JAK2 in the nucleus and sustainability of NF1-C2 protein levels were consistent with a role for nuclear JAK2 in the initiation, as opposed to the maintenance, of milk gene expression. In these cells, another downstream consequence for increased NF1-C2 due to activated nuclear JAK2 may be the increased expression of tumor suppressor protein p53 (68
). These investigators subsequently provided evidence suggesting that nuclear JAK2/NF1-C2 suppresses breast cancer progression and metastasis by opposing the effects of FoxF1 (69
Members of the helicase-like transcription factor (HLTF) family would seem to be another target for nuclear JAK2 downstream of prolactin signaling in uterine epithelial cells and thereby forming the basis for prolactin-induced augmentation of progesterone-dependent transcriptional activity (70
). The rabbit HLTF ortholog RUSH-1α was shown to be tyrosine phosphorylated by nuclear JAK2 thereby enhancing DNA binding, while JAK2 inhibition blocked enhancement of progesterone-dependent gene induction by prolactin (70
). These findings support the existence and relevance of a JAK-RUSH/HLTF signaling pathway operating in parallel to the better known JAK-STAT pathway.
It is interesting to note that while nuclear JAK2 may play a role in preventing the epithelial-to-mesenchymal transition (EMT) associated with carcinoma metastasis (69
), accumulating evidence in the last few years has revealed JAK1 and JAK2 as epigenetic regulators of gene expression that may result in leukemogenesis. JAK overactivation was found to globally disrupt heterochromatic gene silencing in a Drosophila melanogaster
hematopoietic tumor model, while JAK loss of function enhanced heterochromatic gene silencing (72
). This action of JAK was circumvented by mutations in genes important in heterochromatic gene silencing through histone H3 methylation. Subsequently, others showed that JAK2 was present in the nucleus of hematopoietic cells and phosphorylated histone H3 on Y41. Phosphorylation of H3Y41 prevented binding of heterochromatin protein 1α (HP1α), which functions in the repression of heterochromatic genes (45
). Moreover, in the human erythroleukemia (HEL) cell line JAK2 activity directly correlated with expression of the lmo2
oncogene and H3Y41 phosphorylation at the lmo2
promoter, but inversely correlated with HP1α binding at the promoter.
Suggestive evidence was recently reported that the epigenetic actions of JAK2 that are mediated through H3Y41 phosphorylation may have physiological relevance, in this case in embryonic stem (ES) cell self-renewal (73
). The interleukin 6 type cytokine leukemia inhibitory factor (LIF) is a potent activator of JAK-STAT signaling and is used to maintain mouse embryonic stem cells in an undifferentiated state. Mouse ES cells that were engineered to contain the JAK2 V617F mutant allele were able to self-renew in the absence of LIF. In these cells, levels of HP1α bound to chromatin were lower, but were increased by JAK2 inhibition and this was associated with reduced histone H3Y41 phosphorylation. Furthermore, JAK2 inhibition reduced levels of Nanog, a transcription factor important for maintaining pluripotency, and this was associated with reduced H3Y41 phosphorylation and increased HP1α levels at the Nanog promoter.
Oncogenic JAK2 mutant kinases may exert epigenetic actions on chromatin structure in myeloproliferative neoplasms by phosphorylating the arginine methyltransferase PRMT5, which was originally identified as JAK-binding protein 1 (74
). Myeloproliferative neoplasms are stem cell disorders in which most patients express the JAK2V617F (or less commonly exon12 mutations) constitutively activated tyrosine kinase. These oncogenic JAK2 mutants show an enhanced association with PRMT5 compared to wild type JAK2 (74
). In addition, unlike wild type JAK2, the oncogenic JAKs were found to phosphorylate PRMT5 in HEL cells, resulting in reduced PRMT5 methyltransferase activity and decreased global histone H2A/H4 R3 methylation. The mutant JAKs were shown to impair PRMT5 activity by negatively impacting on its association with methylosome protein 50 (MEP50), which plays a role in enhancing the enzymatic activity of PRMT5. Enhanced phosphorylation of PRMT5 was detected in CD34+
cells and granulocytes of JAK2 V617F-positive patients. Moreover, knockdown of PRMT5 in human CD34+
cells increased colony formation and erythroid differentiation, providing evidence that PRMT5 phosphorylation contributes to myeloproliferative phenotype associated with the oncogenic JAK2 mutants. Intriguingly, PRMT5, MEP50, and the mutant JAK2 kinases were detected in both the nucleus and cytoplasm, raising the possibility of different consequences of JAK2-mediated PRMT5 phosphorylation depending upon the cellular compartment (74
Still not firmly established is whether wild type JAK2 (or JAK1 for that matter) is normally present in the nucleus of hematopoietic cells. A recent report established the presence of nuclear JAK2 in CD34+
bone marrow cells of patients with myeloproliferative neoplasia harboring the JAK2 V617F mutation, but not in those patients with wild type JAK2 (46
). In all patients, JAK2 was predominately cytoplasmic in differentiated granulocytic, megakaryocytic and erythroid cells. Again a correlation was noted between nuclear JAK2 activity and lmo2 expression. One possible explanation for these findings is that activation (phosphorylation) of JAK2, which is a fixture of undifferentiated CD34+
progenitor cells, is required for its nuclear accumulation (46
A growing appreciation of the broadening scope of functional roles of nuclear JAK offers new paradigms for phenomena incompletely explained by canonical JAK-STAT models (). The mitogenic role of JAK, especially JAK2, provides a good illustration. JAK2 is essential for the prolactin-stimulated proliferation of mammary epithelial cells, partially through a STAT-mediated activation of cyclin D1 gene transcription (75
), partially by promoting the nuclear accumulation of cyclin D1 protein (76
), and partially by direct phosphorylation of the cyclin-dependent kinase inhibitor p27Kip1
, which simultaneously impairs its ability to inhibit cyclin-dependent kinases and initiates a cascade of events culminating in proteasomal degradation of p27Kip1
). While it has not yet been unambiguously established that JAK2 catalyzes some of these critical phosphorylations in the nucleus, it is intriguing to note that conditions such as anoxia/hypoxia which are conducive to cyclin D1 gene expression (78
) have also been shown to trigger nuclear localization of JAK2 in other cell types (64
). Thus, understanding the functional role(s) of nuclear JAK2 may lead not only to a better understanding of epigenetic control, but also of cell cycle control ().
Broadening scope of a functional role for nuclear JAK
When trying to fully understand the biological significance of nuclear JAK, it is advisable to simultaneously consider JAK's cytoplasmic actions, as well as to consider the cell's developmental stage when JAK-catalyzed events occur, the magnitude and duration of JAK-catalyzed phosphorylation events and, of course, the specific substrates phosphorylated by JAK. We postulate that when these events are orchestrated during the correct stage of development and the activity levels are tightly controlled, nuclear and cytoplasmic JAK activities are essential; when these events are ill-timed, excessive, and/or prolonged, the results can be disastrous, leading to cancers and related diseases ().