The serine-threonine kinase Pim-1 belongs to the Calmodulin-dependent protein kinase family together with two other highly conserved family members (Pim-2 and Pim-3). Pim-1 is the preferential site of integration for the Moloney murine leukemia virus (Proviral Integration for Moloney Virus) discovered over 25 years ago (Selten et al. 1985). Pim-1 plays pivotal roles in cellular proliferation, differentiation, metabolism, and survival by phosphorylating and interacting with many targets. A literature search reveals the dynamic expression and activity of Pim-1 depends upon cell type and response to stimuli, either pathologic or homeostatic. Specifically, Pim-1 is expressed in various hematopoietic sites including thymus, spleen, bone marrow, and fetal liver, but can also be found in the heart, oral epithelia, prostate, hippocampus, vascular smooth muscle, and many tumorigenic cell types (reviewed in Nawijn et al. (2011)). Expression of Pim-1 is preferentially elevated in the hematopoietic system and during fetal development coinciding with periods of increased cell cycling. Upon maturation, Pim-1 is downregulated in most organs until induced by pathologic stimuli to promote survival (Muraski et al. 2007). Presence of Pim-1 in neoplastic cell types can result in a poor prognosis depending on the type of malignancy. For example, the presence of Pim-1 together with a synergistic partner named c-Myc support a good prognosis in prostate adenocarcinoma, yet the opposite is true in the case of mantle cell lymphomas (reviewed in Nawijn et al. (2011)). Ironically, Pim-1 promotes survival and regeneration in heart tissue considered to be resistant to neoplastic transformation (Muraski et al. 2007). Therefore, the same molecular mechanisms that produce relatively bleak outcomes in cancer patients have the potential to produce therapeutic interventions in patients with cardiomyopathy.
Structure and Function
Genomic mapping identified Pim-1 on chromosome 17 in mice and chromosome 6 in humans. Six exons make up the 313 amino acid protein which has two isoforms stemming from alternative start sites; the larger 44 kD and smaller 34 kD isoforms contain a kinase domain, a proton acceptor site, a glycine loop motif, and a phosphate-binding site (Bachmann and Moroy 2005). Both isoforms of Pim-1 are short-lived (∼5 min–6 h depending on cell type) and constitutively active. Paradoxically, Pim-1 is known to phosphorylate itself despite the fact that it does not contain the consensus sequence needed for phosphorylation. Selective peptide mapping identified the Pim-1 consensus sequence as either (K/R)3-X-S/T-X or R/K-R/K-R-R/K-X-S/T-X where X is an amino acid with a small side chain but neither basic nor acidic (Bachmann et al. 2006). Pim-1 has numerous targets in various cellular compartments, giving rise to speculation that subcellular localization defines effect, with nuclear Pim-1 stimulating proliferation and mitochondrial/cytoplasmic localization promoting survival.
Regulation of Pim-1 occurs primarily at transcriptional and translational levels. Pim-1 gene expression is induced by a large array of cytokines: most interleukins, granulocyte macrophage colony-stimulating factor (GM-CSF), epidermal growth factor (EGF), leukemia inhibitory factor (LIF), and interferon-alpha. Cytokine stimulation results in the activation of multiple pathways such as the Janus kinase-signal transducer and activation of transcription (Jak-STAT) and nuclear factor kB (NFKB)–growth factor signaling pathways. Interleukins 5 and 7 (IL-5 and IL-7) activate STAT5, whereas GM-CSF and EGF activate STAT 3. STAT3/STAT5 are both able to bind to the Pim-1 promoter and induce transcription (Bachmann and Moroy 2005). Platelet-derived growth factor (PDGF) also increases Pim-1 transcription in vascular smooth muscle cells resulting in enhanced proliferation that can be blunted with Jak-STAT pathway inhibitors (Willert et al. 2010). The hormone prolactin stimulates Pim-1 transcription independent of STAT transcription factors. Alternatively, transcription is induced through the PI3K/AKT signaling pathway establishing Akt as an upstream regulator of Pim-1 activity (Krishnan et al. 2003). In addition, pathological stimuli such as DNA damaging agents including peroxide or 5-fluorouracil stimulate Pim-1 through kruppel-like factor 5 binding to the Pim-1 promoter activating transcription and subsequently preventing apoptosis (Zhao et al. 2008). Hypoxia elevates levels of Pim-1 mRNA and protein by inhibiting ubiquitin-mediated proteasomal degradation and causes Pim-1 translocation from the cytoplasm to the nucleus (Chen et al. 2009).
Pim1 mRNA has a short half-life due to five copies of the AUUU(A) destabilization motif in the 3’ untranslated region (UTR) (Selten et al. 1985). The 5’ UTR of Pim-1 contains multiple secondary structures due to repetitive GC rich regions, necessitating a 7-methylguanosine cap. By capping the 5’ end of the message, ribosomal assembly is supported resulting in an abundant increase in Pim-1 protein (reviewed in Nawijn et al. (2011).
In addition to transcriptional control, Pim-1 is regulated posttranslationally through protein stabilization. Interactions with several molecules and complexes inhibit ubiquitination and dephosphorylation resulting in accumulation and persistence. Several facets of Pim-1 activity rely on synergistic partners that enhance and sustain kinase activity by preventing degradation. One such partner is heat shock binding partner 90 (Hsp90), association of Pim-1 and Hsp90 prevent ubiquitin-mediated destruction, whereas inhibition or silencing of Hsp90 promote rapid Pim-1 proteolysis (Shay et al. 2005). Conversely, Hsp70 binds ubiquitinated Pim-1 and directs Pim-1 to the proteosome for degradation (Shay et al. 2005). Similar to Akt, Pim-1 is inactivated by protein phosphatases including protein phosphatase 2A (PP2A). Once dephosphorylated, Pim-1 is quickly ubiquitinated and shuttled to the proteosome for degradation (reviewed in Bachmann and Moroy (2005)).
Pim-1 and Cell Cycle
During mitosis Pim-1 is enriched in the nucleus facilitating cell division (Bhattacharya et al. 2002). Pim-1 stabilizes spindle poles by interacting with Nuclear Mitotic Apparatus protein (NuMA) during mitosis, most likely phosphorylating NuMA as cells with kinase dead forms of Pim-1 do not co-localize with NuMA and have a higher frequency of apoptosis (Bhattacharya et al. 2002). Pim-1 phosphorylates NuMA to promote complex formation at the microtubule (−) end and provide a docking site for dynein and dynactin to promote proper chromosome segregation (Bhattacharya et al. 2002). In addition to NuMA, Pim-1 also phosphorylates heterochromatin protein-1 (HP-1) to further contribute to spindle fiber assembly during mitosis.
The transcription factor c-Myc is a well-characterized partner for Pim-1 known to promote cellular proliferation and differentiation when the two are co-expressed. Pim-1 binds, phosphorylates, and stabilizes c-Myc, facilitating cell cycle progression by promoting c-Myc-dependent transcription of target genes (Zhang et al. 2008). Pim-1 contributes to the regulation of around 20% of c-Myc target genes by promoting heterodimerization between c-Myc and Max that bind to E-box promoter elements present in approximately 15% of all human genes, (Zippo et al. 2007). During mitosis, c-Myc activates CDC25A, CDK2, and CDK4 driving entry and progression through the cell cycle. Overexpression of Pim-1 in cardiac progenitor cells results in increased cell cycling and elevated c-Myc expression (Cottage et al. 2010). Pim-1 regulates c-Myc-dependent transcription by phosphorylating Histone 3 on serine 10, unraveling chromatin providing E box-binding sites for c-Myc to bind and promote transcription (Zippo et al. 2007).
Pim-1 and Cell Survival
In summary, Pim-1 is capable of manipulating protective signaling by promoting proliferative and survival signaling in a variety of ways and in a variety of cell types. Oncologists view Pim-1 as an enzyme capable of driving tumorigenesis with transformation promoting partners like c-Myc. In most hematopoietic malignancies, Pim-1 propels cell proliferation while simultaneously promoting survival creating a “recipe for disaster” and poor prognosis. To slow metastasis, pharmacologists are actively synthesizing new Pim-1 inhibitors in the hope of reversing mitogenic signaling. In contrast, cardiovascular biologists view increased proliferation as a breakthrough capable of resuscitating cardiac cells during times of myocardial damage. Pim-1-engineered cardiac stem cells (CSCs) have been adoptively transferred into damaged myocardium in order to promote repair. Several weeks after transplantation, hearts receiving Pim-1-engineered stem cells contain regenerated cardiomyocytes and possess increased functional output (Fischer et al. 2009). In addition to enhancing stem cell therapy, cardiotropic adenoviruses (AAV-9) overexpressing Pim-1 promote cellular proliferation and survival in cardiac cells. Tail vein injections of Pim-1 overexpressing AAV-9 reversed the effects of diabetic cardiomyopathy without surgery (Katare et al. 2011). In summary, inhibiting Pim-1 may serve as a powerful therapeutic to halt cancer progression whereas overexpressing Pim-1 empowers stem cells and reverses the deleterious effects of cardiovascular disease.