Regulation of Skp2 Levels by the Pim-1 Protein Kinase

Department of Medicine, Medical University of South Carolina, Charleston, South Carolina 29425, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 09/2010; 285(38):29128-37. DOI: 10.1074/jbc.M110.137240
Source: PubMed


The Pim-1 protein kinase plays an important role in regulating both cell growth and survival and enhancing transformation
by multiple oncogenes. The ability of Pim-1 to regulate cell growth is mediated, in part, by the capacity of this protein
kinase to control the levels of the p27, a protein that is a critical regulator of cyclin-dependent kinases that mediate cell
cycle progression. To understand how Pim-1 is capable of regulating p27 protein levels, we focused our attention on the SCFSkp2 ubiquitin ligase complex that controls the rate of degradation of this protein. We found that expression of Pim-1 increases
the level of Skp2 through direct binding and phosphorylation of multiple sites on this protein. Along with known Skp2 phosphorylation
sites including Ser64 and Ser72, we have identified Thr417 as a unique Pim-1 phosphorylation target. Phosphorylation of Thr417 controls the stability of Skp2 and its ability to degrade p27. Additionally, we found that Pim-1 regulates the anaphase-promoting
complex or cyclosome (APC/C complex) that mediates the ubiquitination of Skp2. Pim-1 phosphorylates Cdh1 and impairs binding
of this protein to another APC/C complex member, CDC27. These modifications inhibit Skp2 from degradation. Marked increases
in Skp2 caused by these mechanisms lower cellular p27 levels. Consistent with these observations, we show that Pim-1 is able
to cooperate with Skp2 to signal S phase entry. Our data reveal a novel Pim-1 kinase-dependent signaling pathway that plays
a crucial role in cell cycle regulation.

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    • "We found that T-cells deprived and then re-stimulated with IL-7, which mimics the in vivo conditions in which T-cells encounter IL-7, immediately increased Pim-1 levels in a JunD/AP-1 dependent manner. This is consistent with the proliferative function of Pim-1, since it phosphorylates and induces the activity of proteins involved in cell cycling, including Cdc25A [47] and Skp2 [48]. Our conclusions are supported by the fact that Pim-1 is an effector of the IL-7 signaling pathway, as its expression reconstituted thymic cellularity in IL-7 deficient mice [49], and that Pim-1, 2 and 3 are required for proliferation of peripheral T-cells [50]. "
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    ABSTRACT: Interleukin-7 (IL-7) is an essential cytokine for lymphocyte growth that has the potential for promoting immune reconstitution. This feature makes IL-7 an ideal candidate for therapeutic development. As with other cytokines, signaling through the IL-7 receptor induces the JAK/STAT pathway. However, the broad scope of IL-7 regulatory targets likely necessitates the use of other signaling components whose identities remain poorly defined. To this end, we used an IL-7 dependent T-cell line to examine how expression of the glycolytic enzyme, Hexokinase II (HXKII) was regulated by IL-7 in a STAT5-independent manner. Our studies revealed that IL-7 promoted the activity of JNK (Jun N-terminal Kinase), and that JNK, in turn, drove the expression of JunD, a component of the Activating Protein 1 (AP-1) transcription factors. Gel shifts showed that the AP-1 complex induced by IL-7 contained JunD but not c-Fos or c-Jun. Inhibition of JNK/JunD blocked glucose uptake and HXKII gene expression, indicating that this pathway was responsible for promoting HXKII expression. Because others had shown that JunD was a negative regulator of cell growth, we performed a bioinformatics analysis to uncover possible JunD-regulated gene targets. Our search revealed that JunD could control the expression of proteins involved in signal transduction, cell survival and metabolism. One of these growth promoters was the oncogene, Pim-1. Pim-1 is an IL-7-induced protein that was inhibited when the activities of JNK or JunD were blocked, showing that in IL-7 dependent T-cells JunD can promote positive signals transduced through Pim-1. This was confirmed when the IL-7-induced proliferation of CD8 T-cells was impaired upon JunD inhibition. These results show that engagement of the IL-7 receptor drives a signal that is more complex than the JAK/STAT pathway, activating JNK and JunD to induce rapid growth stimulation through the expression of metabolic and signaling factors like HXKII and Pim-1.
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    ABSTRACT: INTRODUCTION: Inhibition of protein kinases has become a standard of modern clinical oncology. PIM1 belongs to a novel class of serine/threonine kinases with distinct molecular and biochemical features regulating various oncogenic pathways, for example hypoxia response, cell cycle progression and apoptosis resistance. PIM1 is overexpressed in human cancer diseases and has been associated with metastasis and overall treatment response; in experimental models, inhibition of PIM1 suppressed cell proliferation and migration, induced apoptotic cell death and synergized with other chemotherapeutic agents. AREAS COVERED: A PubMed literature search was performed to review the currently available data on PIM1 expression, regulation and targets; its implication in different types of cancer and its impact on prognosis are described. We present ATP-competitive PIM1 inhibitors and the state of the art of PIM1 inhibitor design. Finally, we highlight the development of the unusual class of highly selective and potent organometallic PIM1 inhibitors. EXPERT OPINION: As PIM1 possesses oncogenic functions and is overexpressed in various kinds of cancer diseases, its inhibition provides a new option in cancer therapy. Based on the ability of highly selective organometallic PIM1 inhibitors, promising in vivo applicability is expected.
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