Pim-2 phosphorylation of p21(Cip1/WAF1) enhances its stability and inhibits cell proliferation in HCT116 cells

CIP/KIP and INK4 families as hostages of oncogenic signaling

CIP/KIP and INK4 families of Cyclin-dependent kinase inhibitors (CKIs) are well-established cell cycle regulatory proteins whose canonical function is binding to Cyclin-CDK complexes and altering their function. Initial experiments showed that these proteins negatively regulate cell cycle progression and thus are tumor suppressors in the context of molecular oncology. However, expanded research into the functions of these proteins showed that most of them have non-canonical functions, both cell cycle-dependent and independent, and can even act as tumor enhancers depending on their posttranslational modifications, subcellular localization, and cell state context. This review aims to provide an overview of canonical as well as non-canonical functions of CIP/KIP and INK4 families of CKIs, discuss the potential avenues to promote their tumor suppressor functions instead of tumor enhancing ones, and how they could be utilized to design improved treatment regimens for cancer patients.

CDKN1A/p21 in Breast Cancer: Part of the Problem, or Part of the Solution?

Cyclin-dependent kinase inhibitor 1A (Cip1/Waf1/CDKN1A/p21) is a well-established protein, primarily recognised for its pivotal role in the cell cycle, where it induces cell cycle arrest by inhibiting the activity of cyclin-dependent kinases (CDKs). Over the years, extensive research has shed light on various additional mechanisms involving CDKN1A/p21, implicating it in processes such as apoptosis, DNA damage response (DDR), and the regulation of stem cell fate. Interestingly, p21 can function either as an oncogene or as a tumour suppressor in these contexts. Complicating matters further, the expression of CDKN1A/p21 is elevated in certain tumour types while downregulated in others. In this comprehensive review, we provide an overview of the multifaceted functions of CDKN1A/p21, present clinical data pertaining to cancer patients, and delve into potential strategies for targeting CDKN1A/p21 as a therapeutic approach to cancer. Manipulating CDKN1A/p21 shows great promise for therapy given its involvement in multiple cancer hallmarks, such as sustained cell proliferation, the renewal of cancer stem cells (CSCs), epithelial-mesenchymal transition (EMT), cell migration, and resistance to chemotherapy. Given the dual role of CDKN1A/p21 in these processes, a more in-depth understanding of its specific mechanisms of action and its regulatory network is imperative to establishing successful therapeutic interventions.

DAPL1 prevents epithelial-mesenchymal transition in the retinal pigment epithelium and experimental proliferative vitreoretinopathy

Epithelial-mesenchymal transition (EMT) of the retinal pigment epithelium (RPE) is a hallmark of the pathogenesis of proliferative vitreoretinopathy (PVR) that can lead to severe vision loss. Nevertheless, the precise regulatory mechanisms underlying the pathogenesis of PVR remain largely unknown. Here, we show that the expression of death-associated protein-like 1 (DAPL1) is downregulated in PVR membranes and that DAPL1 deficiency promotes EMT in RPE cells in mice. In fact, adeno-associated virus (AAV)-mediated DAPL1 overexpression in RPE cells of Dapl1-deficient mice inhibited EMT in physiological and retinal-detachment states. In a rabbit model of PVR, ARPE-19 cells overexpressing DAPL1 showed reduced ability to induce experimental PVR, and AAV-mediated DAPL1 delivery attenuated the severity of experimental PVR. Furthermore, a mechanistic study revealed that DAPL1 promotes P21 phosphorylation and its stabilization partially through NFκB (RelA) in RPE cells, whereas the knockdown of P21 led to neutralizing effects on DAPL1-dependent EMT inhibition and enhanced the severity of experimental PVR. These results suggest that DAPL1 acts as a novel suppressor of RPE-EMT and has an important role in antagonizing the pathogenesis of experimental PVR. Hence, this finding has implications for understanding the mechanism of and potential therapeutic applications for PVR.

Targeting Pim kinases in hematological cancers: molecular and clinical review

Decades of research has recognized a solid role for Pim kinases in lymphoproliferative disorders. Often up-regulated following JAK/STAT and tyrosine kinase receptor signaling, Pim kinases regulate cell proliferation, survival, metabolism, cellular trafficking and signaling. Targeting Pim kinases represents an interesting approach since knock-down of Pim kinases leads to non-fatal phenotypes in vivo suggesting clinical inhibition of Pim may have less side effects. In addition, the ATP binding site offers unique characteristics that can be used for the development of small inhibitors targeting one or all Pim isoforms. This review takes a closer look at Pim kinase expression and involvement in hematopoietic cancers. Current and past clinical trials and in vitro characterization of Pim kinase inhibitors are examined and future directions are discussed. Current studies suggest that Pim kinase inhibition may be most valuable when accompanied by multi-drug targeting therapy.

The Role of PIM Kinases in Pediatric Solid Tumors

PIM kinases have been identified as potential therapeutic targets in several malignancies. Here, we provide an in-depth review of PIM kinases, including their structure, expression, activity, regulation, and role in pediatric carcinogenesis. Also included is a brief summary of the currently available pharmaceutical agents targeting PIM kinases and existing clinical trials.

Protein kinase PIM2: A simple PIM family kinase with complex functions in cancer metabolism and therapeutics

PIM2 (proviral integration site for Moloney murine leukemia virus 2) kinase plays an important role as an oncogene in multiple cancers, such as leukemia, liver, lung, myeloma, prostate and breast cancers. PIM2 is largely expressed in both leukemia and solid tumors, and it promotes the transcriptional activation of genes involved in cell survival, cell proliferation, and cell-cycle progression. Many tumorigenic signaling molecules have been identified as substrates for PIM2 kinase, and a variety of inhibitors have been developed for its kinase activity, including SMI-4a, SMI-16a, SGI-1776, JP11646 and DHPCC-9. Here, we summarize the signaling pathways involved in PIM2 kinase regulation and PIM2 mechanisms in various neoplastic diseases. We also discuss the current status and future perspectives for the development of PIM2 kinase inhibitors to combat human cancer, and PIM2 will become a therapeutic target in cancers in the future.

RNA-Seq analysis reveals that spring viraemia of carp virus induces a broad spectrum of PIM kinases in zebrafish kidney that promote viral entry

PIM kinases are a family of serine/threonine protein kinases that potentiate the progression of the cell cycle and inhibit apoptosis. Because of this, they are considered to be proto-oncogenes, and they represent an interesting target for the development of anticancer drugs. In mammals, three PIM kinases exist (PIM-1, PIM-2 and PIM-3), and different inhibitors have been developed to block their activity. In addition to their involvement in cancer, some publications have reported that the PIM kinases have pro-viral activity, and different mechanisms where PIM kinases favour viral infections have been proposed. Zebrafish possess more than 300 Pim kinase members in their genome, and by using RNA-Seq analysis, we found a high number of Pim kinase genes that were significantly induced after infection with spring viraemia of carp virus (SVCV). Moreover, analysis of the miRNAs modulated by this infection revealed that some of them could be involved in the post-transcriptional regulation of Pim kinase abundance. To elucidate the potential role of the 16 overexpressed Pim kinases in the infectivity of SVCV, we used three different pan-PIM kinase inhibitors (SGI-1776, INCB053914 and AZD1208), and different experiments were conducted both in vitro and in vivo. We observed that the PIM kinase inhibitors had a protective effect against SVCV, indicating that, similar to what is observed in mammals, PIM kinases are beneficial for the virus in zebrafish. Moreover, zebrafish Pim kinases seem to facilitate viral entry into the host cells because when ZF4 cells were pre-incubated with the virus and then were treated with the inhibitors, the protective effect of the inhibitors was abrogated. Although more investigation is necessary, these results show that pan-PIM kinase inhibitors could serve as a useful treatment for preventing the spread of viral diseases.

Inhibition of PIM2 in liver cancer decreases tumor cell proliferation in vitro and in vivo primarily through the modulation of cell cycle progression

Liver cancer is the fourth leading cause of cancer-related mortality worldwide with limited therapeutic options. Thus, novel treatment strategies are urgently required. While the oncogenic kinase, proviral integration site for Moloney murine leukemia virus 2 (PIM2), has been shown to be overexpressed in liver cancer, little is known about the role of PIM2 in this tumor entity. In this study, we explored the functional relevance and therapeutic potential of PIM2 in liver cancer. Using PIM2-specific siRNAs, we examined the effects of PIM2 knockdown on proliferation (WST-1 assays and spheroid assays), 3D-colony formation and colony spread, apoptosis (flow cytometry and caspase 3/caspase 7 activity), as well as cell cycle progression (flow cytometry, RT-qPCR and western blot analysis) in the two liver cancer cell lines, HepG2 and Huh-7. In subcutaneous liver cancer xenografts, we assessed the effects of PIM2 knockdown on tumor growth via the systemic delivery of polyethylenimine (PEI)-complexed siRNA. The knockdown of PIM2 resulted in potent anti-proliferative effects in cells grown on plastic dishes, as well as in spheroids. This was due to G0/G1 cell cycle blockade and the subsequent downregulation of genes related to the S phase as well as the G2/M phase of the cell cycle, whereas the apoptotic rates remained unaltered. Furthermore, colony formation and colony spread were markedly inhibited by PIM2 knockdown. Notably, we found that HepG2 cells were more sensitive to PIM2 knockdown than the Huh-7 cells. In vivo, the therapeutic nanoparticle-mediated delivery of PIM2 siRNA led to profound anti-tumor effects in a liver cancer xenograft mouse model. On the whole, the findings of this study underscore the oncogenic role of PIM2 and emphasize the potential of targeted therapies based on the specific inhibition of PIM2 in liver cancer.

Effects of cisatracurium on epithelial-to-mesenchymal transition in esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (ESCC) is one of the most aggressive types of cancer worldwide, with a poor prognosis. The aim of the present study was to investigate the effect of cisatracurium (Cis) on epithelial-to-mesenchymal transition (EMT) in ESCC and its potential mechanism of action. In the present study, Cis was used to treat ECA-109 cells, with cell proliferation measured by a Cell Counting Kit-8 assay and the expression of TGF-β and phospho-Smad2/3 detected by western blotting. TGF-β was then applied to induce EMT. Flow cytometry, wound healing and Transwell assays were used to evaluate cell proliferation, apoptosis, invasion and migration. In addition, cell cycle-related proteins, including cyclin D1, p53 and p21, and EMT-associated proteins, including E-cadherin (E-cad), N-cadherin (N-cad), Vimentin and Slug, were examined by western blot analysis. The results revealed that Cis inhibited the proliferation and promoted apoptosis of ESCC cells. Following treatment with Cis, the expression of TGF-β and phosphorylation of Smad2/3 were downregulated. Cis also suppressed cancer cell invasion and migration induced by TGF-β. In addition, the expression levels of cyclin D1 were decreased, accompanied by increased p53 and p21 expression. In addition, the expression level of E-cad was increased, whereas N-cad, Vimentin and Slug were significantly reduced. Taken together, the results of the present study revealed that exposure of ESCC cells to Cis inhibited EMT and reduced cell invasion and metastasis through the TGF-β/Smad signaling pathway.

Negative regulation of AMPKα1 by PIM2 promotes aerobic glycolysis and tumorigenesis in endometrial cancer

Endometrial cancer (EC) is one of the most common gynecologic malignancies. However, the molecular mechanisms underlying the development and progression of EC remain unclear. Here, we demonstrated that the protein proviral insertion in murine lymphomas 2 (PIM2) was necessary for maintaining EC tumorigenesis in vivo and in vitro, and could inhibit AMPKα1 kinase activity in EC cells. Specifically, we found that PIM2 bound to AMPKα1, and directly phosphorylated it on Thr467. Phosphorylation of AMPKα1 by PIM2 led to decreasing AMPKα1 kinase activity, which in turn promoted aerobic glycolysis and tumor growth. In addition, PIM2 expression positively correlated with AMPKα1 Thr467 phosphorylation in EC tissues. Further, treatment with a combination of the PIM2 inhibitor SMI-4a and the AMPKα1 activator AICAR could effectively inhibit tumor growth. Thus, our findings provide insight into the role of PIM2 and AMPKα1 in EC and suggest that combination targeting of these proteins may represent a new strategy for EC treatment.

Pan-Pim Kinase Inhibitor AZD1208 Suppresses Tumor Growth and Synergistically Interacts with Akt Inhibition in Gastric Cancer Cells

Purpose

Pim kinases are highly conserved serine/threonine kinases, and different expression patterns of each isoform (Pim-1, Pim-2, and Pim-3) have been observed in various types of human cancers, including gastric cancer. AZD1208 is a potent and selective inhibitor that affects all three isoforms of Pim. We investigated the effects of AZD1208 as a single agent and in combination with an Akt inhibitor in gastric cancer cells.

Materials and Methods

The antitumor activity of AZD1208 with/without an Akt inhibitor was evaluated in a large panel of gastric cancer cell lines through growth inhibition assays. The underlying mechanism was also examined by western blotting, immunofluorescence assay, and cell cycle analysis.

Results

AZD1208 treatment decreased gastric cancer cell proliferation rates and induced autophagy only in long-term culture systems. Light chain 3B (LC3B), a marker of autophagy, was increased in sensitive cells in a dose-dependent manner with AZD1208 treatment, which suggested that the growth inhibition effect of AZD1208 was achieved through autophagy, not apoptosis. Moreover, we found that cells damaged by Pim inhibition were repaired by activation of the DNA damage repair pathway, which promoted cell survival and led the cells to become resistant to AZD1208. We also confirmed that the combination of an Akt inhibitor with AZD1208 produced a highly synergistic effect in gastric cancer cell lines.

Conclusion

Treatment with AZD1208 alone induced considerable cell death through autophagy in gastric cancer cells. Moreover, the combination of AZD1208 with an Akt inhibitor showed synergistic antitumor effects through regulation of the DNA damage repair pathway.

PIM-2 protein kinase negatively regulates T cell responses in transplantation and tumor immunity

PIM kinase family members play a crucial role in promoting cell survival and proliferation via phosphorylation of their target substrates. In this study, we investigated the role of the PIM kinases with respect to T cell responses in transplantation and tumor immunity. We found that the PIM-2 isoform negatively regulated T cell responses to alloantigen, in contrast to the PIM-1 and PIM-3 isoforms, which acted as positive regulators. T cells deficient in PIM-2 demonstrated increased T cell differentiation toward Th1 subset, proliferation, and migration to target organs after allogeneic bone marrow transplantation, resulting in dramatically accelerated graft-versus-host disease (GVHD) severity. Restoration of PIM-2 expression markedly attenuated the pathogenicity of PIM-2-deficient T cells to induce GVHD. On the other hand, mice deficient in PIM-2 readily rejected syngeneic tumor, which was primarily dependent on CD8+ T cells. Furthermore, silencing PIM-2 in polyclonal or antigen-specific CD8+ T cells substantially enhanced their antitumor response in adoptive T cell immunotherapy. We conclude that PIM-2 kinase plays a prominent role in suppressing T cell responses, and provide a strong rationale to target PIM-2 for cancer immunotherapy.

Specific up-regulation of p21 by a small active RNA sequence suppresses human colorectal cancer growth

The double stranded small active RNA (saRNA)- p21-saRNA-322 inhibits tumor growth by stimulating the p21 gene expression. We focused our research of p21-saRNA-322 on colorectal cancer because 1) p21 down-regulation is a signature abnormality of the cancer, and 2) colorectal cancer might be a suitable target for in situ p21-saRNA-322 delivery. The goal of the present study is to learn the activity of p21-saRNA-322 in colorectal cancer. Three human colorectal cancer cell lines, HCT-116, HCT-116 (p53–/−) and HT-29 were transfected with the p21-saRNA-322. The expression of P21 protein and p21 mRNA were measured using the Western blot and reverse transcriptase polymerase chain reaction (RT-PCR). The effect of p21-saRNA-322 on cancer cells was evaluated in vitro; and furthermore, a xenograft colorectal tumor mode in mice was established to estimate the tumor suppressing ability of p21-saRNA-322 in vivo. The results showed that in all three colorectal cancer cell lines, the expression of p21 mRNA and P21 protein were dramatically elevated after p21-saRNA-322 transfection. Transfection of p21-saRNA-322 caused apoptosis and cell cycle arrest at the G₀/G₁. Furthermore, anti-proliferation effect, reduction of colonies formation and cell senescence were observed in p21-saRNA-322 treated cells. Animal studies showed that p21-saRNA-322 treatment significantly inhibited the HT-29 tumor growth and facilitated p21 activation in vivo. These results indicated that, p21-saRNA-322-induceded up-regulation of p21 might be a promising therapeutic option for the treatment of colorectal cancer.

Downregulation of Pim-2 induces cell cycle arrest in the G0/G1 phase via the p53-non-dependent p21 signaling pathway

Pim-2 is a serine/threonine protein kinase that is highly expressed in various types of cancer, with essential roles in the regulation of signal transduction cascades, which promote cell survival and proliferation. The present study demonstrated that Pim-2 was expressed in cells lines derived from hematopoietic tumors and lung cancer. In vitro, downregulation of Pim-2 by short interfering RNA inhibited proliferation and delayed G₀/G₁ cell cycle progression in K562 leukemia, RPMI-8226 multiple myeloma, and H1299 and A549 non-small cell lung carcinoma cell lines. Furthermore, downregulation of Pim-2 resulted in upregulation of cyclin-dependent kinase (CDK) inhibitor p21, irrespective of the p53 status. In addition, the present study revealed that CDK2 and phosphorylated retinoblastoma (pRb) were significantly downregulated. This finding suggested that inhibition of CDK2 and pRb expression via upregulated p21 was involved in the downregulation of Pim-2-induced G₀/G₁ cell cycle arrest in lung cancer and hematopoietic malignancy cells. These results suggested that Pim-2 may serve a role in hematopoietic tumors, lung cancer proliferation and cell cycle progression by regulating the p21 signaling pathway. Downregulation of Pim-2 decreased cancer cell proliferation. Therefore, Pim-2 may be a potential therapy target in clinical cancer therapy.

Self-assembling HA/PEI/dsRNA-p21 ternary complexes for CD44 mediated small active RNA delivery to colorectal cancer

Our previous work proved that sequence specific double strand RNA (dsRNA-p21) effectively activated p21 gene expression of colorectal cancer (CRC) cells and consequently suppressed CRC growth. However, efficient delivery system is a significant challenge to achieve sufficient therapy. In this study, a self-assembled HA/PEI/dsRNA-p21 ternary complex (TC-dsRNA-p21) was developed for the tumor-target delivery of dsRNA-p21 into CRC cells. Hyaluronic acid (HA) was introduced to shield the PEI/dsRNA-p21 binary complexes (BC-dsRNA-p21) for reducing the cytotoxicity of PEI and for increasing the tumor-targeted intracellular uptake by cancer cells through HA-CD44 mediated endocytosis. Comparing to the BC-dsRNA-p21, the TC-dsRNA-p21 showed increase in size, decrease in zeta potential, low cytotoxicity as well as high stability in physiological conditions due to the anionic shielding. Confocal microscopy analysis and flow cytometry confirmed that TC-dsRNA-p21 had high transfection efficiency in the CD44-abundant Lovo cells, as compared with binary complex. In vitro physiological experiment showed that, comparing to the control group, the TC-dsRNA-p21 effectively activated the expression of p21 mRNA and P21 protein, causing blockage of cell cycle at G0/G1 phase and suppression of cancer cell proliferation as well as colony formation. Furthermore, in vivo distribution experiment demonstrated that the TC-dsRNA-p21 could effectively accumulate at rectal wall for up to 10 h, following in situ application. These findings indicated that TC-dsRNA-p21 might hold great potential for delivering dsRNA-p21 to treat CRC.

Pim-2 protects H9c2 cardiomyocytes from hypoxia/reoxygenation-induced apoptosis via downregulation of Bim expression

We know that silencing Bim, a pro-apoptosis protein, significantly attenuates glucose and oxygen-deprived induced apoptosis in cardiomyocytes. However, the mechanisms underlying the regulation of the Bim activation in the heart have remained unknown. Pim-2 is one of three Pim serine/threonine kinase family members thought to be involved in cell survival and proliferation. H9c2 cardiomyocytes were subjected to a hypoxia/reoxygenation (H/R) condition in vitro, mimicking ischemic/reperfusion injury in vivo. H/R augmented the expression of Bim, Cyt C, and Pim-2 and induced H9c2 cell apoptosis. Overexpression of Pim-2 attenuated apoptosis which induced by H/R in H9c2 cells, via downregulation of Bim and Cyt C expression. Silencing of Pim-2 promoted H/R-induced apoptosis via upregulation of Bim and Cyt C expression. Co-IP revealed the interaction between Pim-2 and Bim protein, with Bim Ser⁶⁵ phosphorylated by Pim-2. Furthermore, blocking proteasome activity by MG132 prevented Bim degradation, and Bim S65A mutation could reverse the anti-apoptotic role of Pim-2 which induced by H/R. These data demonstrated that Pim-2 is a novel Bim-interacting protein, which negatively regulates Bim degradation and protects H9c2 cardiomyocytes from H/R-induced apoptosis.

Patterns and Significance of PIM Kinases in Urothelial Carcinoma

BACKGROUND

The Provirus integrating site Moloney murine leukemia virus (Pim) family are proteins with serine/threonine kinase activity. Studies have demonstrated overexpression of Pims in cancer. To our knowledge, only a single study has examined Pim-1 in urothelial carcinoma. The aim of this investigation was to evaluate Pim-1, Pim-2, and Pim-3 in urothelial carcinoma and assess for expression that may contribute to disease progression and serve as a site for targeted therapy.

METHODS

This retrospective study included 137 cases taken from specimens from the University of Utah, Department of Pathology (2008 to 2011). Tissue was stained with antibodies against Pim-1, Pim-2, and Pim-3. Cases were classified into 3 groups, based upon current World Health Organization criteria (invasive high-grade urothelial carcinoma [IHG] [n=84], noninvasive high-grade urothelial carcinoma/carcinoma in situ [n=32], and noninvasive low-grade urothelial carcinoma [NILG] [n=21]). Cases were scored and recorded as positive or negative on the basis of the percentage of cells with cytoplasmic and/or nuclear staining.

RESULTS

NILG showed higher expression of Pim-1 (relative expression rate [RER]=2.28; 95% confidence interval [CI], 0.183-0.764) and Pim-3 (RER=3.06; 95% CI, 0.423-0.816) compared with other lesions. IHG had lower expression of Pim-1 (RER=0.31; 95% CI, 0.401-0.844) and Pim-3 (RER=0.354; 95% CI, 0.322-0.816) and noninvasive high-grade urothelial carcinoma (NIHG) demonstrated increased expression of Pim-1 and (RER=2.09; 95% CI, 0.124-0.739) and Pim-2 (RER=1.70; 95% CI, 0.151-0.591). At least 1 Pim kinase protein was expressed at the following rates: 49% in IHG, 66% in NIHG, and 76% in NILG.

CONCLUSION

A high percentage of urothelial carcinomas express Pim kinases. Pim expression differs in NILG, NIHG, and IHG lesions.

Targeting the Pim kinases in multiple myeloma

Multiple myeloma (MM) is a plasma cell malignancy that remains incurable. Novel treatment strategies to improve survival are urgently required. The Pims are a small family of serine/threonine kinases with increased expression across the hematological malignancies. Pim-2 shows highest expression in MM and constitutes a promising therapeutic target. It is upregulated by the bone marrow microenvironment to mediate proliferation and promote MM survival. Pim-2 also has a key role in the bone destruction typically seen in MM. Additional putative roles of the Pim kinases in MM include trafficking of malignant cells, promoting oncogenic signaling in the hypoxic bone marrow microenvironment and mediating resistance to therapy. A number of Pim inhibitors are now under development with lead compounds entering the clinic. The ATP-competitive Pim inhibitor LGH447 has recently been reported to have single agent activity in MM. It is anticipated that Pim inhibition will be of clinical benefit in combination with standard treatments and/or with novel drugs targeting other survival pathways in MM.

PIM kinases: an overview in tumors and recent advances in pancreatic cancer

The PIM kinases represent a family of serine/threonine kinases, which is composed of three different members (PIM1, PIM2 and PIM3). Aberrant expression of PIM kinases is observed in variety of tumors, including pancreatic cancer. The PIM kinases play pivotal roles in the regulation of cell cycle, apoptosis, properties of stem cells, metabolism, autophagy, drug resistance and targeted therapy. The roles of PIM kinases in pancreatic cancer include the regulation of proliferation, apoptosis, cell cycle, formation, angiogenesis and prediction prognosis. Blocking the activities of PIM kinases could prevent pancreatic cancer development. PIM kinases may be a novel target for cancer therapy.

Pim kinases in hematological malignancies: where are we now and where are we going?

The proviral insertion in murine (PIM) lymphoma proteins are a serine/threonine kinase family composed of three isoformes: Pim-1, Pim-2 and Pim-3. They play a critical role in the control of cell proliferation, survival, homing and migration. Recently, overexpression of Pim kinases has been reported in human tumors, mainly in hematologic malignancies. In vitro and in vivo studies have confirmed their oncogenic potential. Indeed, PIM kinases have shown to be involved in tumorgenesis, to enhance tumor growth and to induce chemo-resistance, which is why they have become an attractive therapeutic target for cancer therapy. Novel molecules inhibiting Pim kinases have been evaluated in preclinical studies, demonstrating to be effective and with a favorable toxicity profile. Given the promising results, some of these compounds are currently under investigation in clinical trials. Herein, we provide an overview of the biological activity of PIM-kinases, their role in hematologic malignancies and future therapeutic opportunities.

Identification of herpesvirus proteins that contribute to G1/S arrest

Lytic infection by herpesviruses induces cell cycle arrest at the G1/S transition. This appears to be a function of multiple herpesvirus proteins, but only a minority of herpesvirus proteins have been examined for cell cycle effects. To gain a more comprehensive understanding of the viral proteins that contribute to G1/S arrest, we screened a library of over 200 proteins from herpes simplex virus type 1, human cytomegalovirus, and Epstein-Barr virus (EBV) for effects on the G1/S interface, using HeLa fluorescent, ubiquitination-based cell cycle indicator (Fucci) cells in which G1/S can be detected colorimetrically. Proteins from each virus were identified that induce accumulation of G1/S cells, predominantly tegument, early, and capsid proteins. The identification of several capsid proteins in this screen suggests that incoming viral capsids may function to modulate cellular processes. The cell cycle effects of selected EBV proteins were further verified and examined for effects on p53 and p21 as regulators of the G1/S transition. Two EBV replication proteins (BORF2 and BMRF1) were found to induce p53 but not p21, while a previously uncharacterized tegument protein (BGLF2) was found to induce p21 protein levels in a p53-independent manner. Proteomic analyses of BGLF2-interacting proteins identified interactions with the NIMA-related protein kinase (NEK9) and GEM-interacting protein (GMIP). Silencing of either NEK9 or GMIP induced p21 without affecting p53 and abrogated the ability of BGLF2 to further induce p21. Collectively, these results suggest multiple viral proteins contribute to G1/S arrest, including BGLF2, which induces p21 levels likely by interfering with the functions of NEK9 and GMIP.

IMPORTANCE

Most people are infected with multiple herpesviruses, whose proteins alter the infected cells in several ways. During lytic infection, the viral proteins block cell proliferation just before the cellular DNA replicates. We used a novel screening method to identify proteins from three different herpesviruses that contribute to this block. Several of the proteins we identified had previously unknown functions or were structural components of the virion. Subsets of these proteins from Epstein-Barr virus were studied for their effects on the cell cycle regulatory proteins p53 and p21, thereby identifying two proteins that induce p53 and one that induces p21 (BGLF2). We identified interactions of BGLF2 with two human proteins, both of which regulate p21, suggesting that BGLF2 induces p21 by interfering with the functions of these two host proteins. Our study indicates that multiple herpesvirus proteins contribute to the cell proliferation block, including components of the incoming virions.

A regulatory feedback loop between HIF-1α and PIM2 in HepG2 cells

To survive under hypoxic conditions, cancer cells remodel glucose metabolism to support tumor progression. HIF transcription factor is essential for cellular response to hypoxia. The underlying mechanism how HIF is constitutively activated in cancer cells remains elusive. In the present study, we characterized a regulatory feedback loop between HIF-1α and PIM2 in HepG2 cells. Serine/threonine kinase proto-oncogene PIM2 level was induced upon hypoxia in a HIF-1α-mediated manner in cancer cells. HIF-1α induced PIM2 expression via binding to the hypoxia-responsive elements (HREs) of the PIM2 promoter. In turn, PIM2 interacted with HIF-1α, especially a transactivation domain of HIF-1α. PIM2 as a co-factor but not an upstream kinase of HIF-1α, enhanced HIF-1α effect in response to hypoxia. The positive feedback loop between PIM2 and HIF-1α was correlated with glucose metabolism as well as cell survival in HepG2 cells. Such a regulatory mode may be important for the adaptive responses of cancer cells in antagonizing hypoxia during cancer progression.

A novel regulatory mechanism of Pim-3 kinase stability and its involvement in pancreatic cancer progression

Translationally controlled tumor protein (TCTP/TPT1) was identified from a yeast 2-hybrid screen and shown to interact with Pim-3, a member of the proto-oncogene Pim family with serine/threonine kinase activity. TCTP was aberrantly expressed in human pancreatic cancer cells and malignant ductal epithelial cells, but not in normal pancreatic duct epithelial cells adjacent to tumor foci of human pancreatic cancer tissue. Moreover, TCTP colocalized with Pim-3 both in human pancreatic cancer cells and in clinical tissues. Mapping studies revealed that the interaction between Pim-3 and TCTP occurred through the C-terminal region of Pim-3 and N-terminal region of TCTP. Although Pim-3 had no effect on TCTP expression or phosphorylation, overexpression of TCTP increased the amount of Pim-3 in a dose-dependent manner. Interestingly, RNAi-mediated ablation of TCTP expression reduced Pim-3 protein but not mRNA, through a mechanism involving the ubiquitin-proteasome degradation system. As a consequence of Pim-3 instability and subsequent degradation, tumor growth in vitro and in vivo was inhibited by arresting cell-cycle progression and enhancing apoptosis. Furthermore, TCTP and Pim-3 expression were significantly correlated in pancreatic adenocarcinoma specimens, and patients with highly expressed TCTP and Pim-3 presented with a more advanced tumor stage. These observations indicate that TCTP enhances Pim-3 stability to simultaneously promote and prevent cell-cycle progression and apoptosis, respectively. Hence, TCTP and Pim-3 serve a pivotal role in human pancreatic cancer with important ramifications for clinical diagnostic and therapeutic implications.

IMPLICATIONS

The present study provides a new idea and experimental evidence for recognizing TCTP/Pim-3 pathway as a target for therapy in human pancreatic cancer.

Proviral insertion in murine lymphomas 2 (PIM2) oncogene phosphorylates pyruvate kinase M2 (PKM2) and promotes glycolysis in cancer cells

Pyruvate kinase M2 (PKM2) is a key player in the Warburg effect of cancer cells. However, the mechanisms of regulating PKM2 are not fully elucidated. Here, we identified the protein-serine/threonine kinase PIM2, a known oncogene, as a novel binding partner of PKM2. The interaction between PIM2 and PKM2 was confirmed by multiple biochemical approaches in vitro and in cultured cells. Importantly, we found that PIM2 could directly phosphorylate PKM2 on the Thr-454 residue, resulting in an increase of PKM2 protein levels. Compared with wild type, PKM2 with the phosphorylation-defective mutation displayed a reduced effect on glycolysis, co-activating HIF-1α and β-catenin, and cell proliferation, while enhancing mitochondrial respiration of cancer cells. These findings demonstrate that PIM2-dependent phosphorylation of PKM2 is critical for regulating the Warburg effect in cancer, highlighting PIM2 as a potential therapeutic target.

The PIM-2 kinase is an essential component of the ultraviolet damage response that acts upstream to E2F-1 and ATM

The oncogenic nature ascribed to the PIM-2 kinase relies mostly on phosphorylation of substrates that act as pro-survival/anti-apoptotic factors. Nevertheless, pro-survival effects can also result from activating DNA repair mechanisms following damage. In this study, we addressed the possibility that PIM-2 plays a role in the cellular response to UV damage, an issue that has never been addressed before. We found that in U2OS cells, PIM-2 expression and activity increased upon exposure to UVC radiation (2-50 mJ/cm(2)), and Pim-2-silenced cells were significantly more sensitive to UV radiation. Overexpression of PIM-2 accelerated removal of UV-induced DNA lesions over time, reduced γH2AX accumulation in damaged cells, and rendered these cells significantly more viable following UV radiation. The protective effect of PIM-2 was mediated by increased E2F-1 and activated ATM levels. Silencing E2F-1 reduced the protective effect of PIM-2, whereas inhibiting ATM activity abrogated this protective effect, irrespective of E2F-1 levels. The results obtained in this study place PIM-2 upstream to E2F-1 and ATM in the UV-induced DNA damage response.

Assembly, activation, and substrate specificity of cyclin D1/Cdk2 complexes

Previous studies have shown conflicting data regarding cyclin D1/cyclin-dependent kinase 2 (Cdk2) complexes, and considering the widespread overexpression of cyclin D1 in cancer, it is important to fully understand their relevance. While many have shown that cyclin D1 and Cdk2 form active complexes, others have failed to show activity or association. Here, using a novel p21-PCNA fusion protein as well as p21 mutant proteins, we show that p21 is a required scaffolding protein, with cyclin D1 and Cdk2 failing to complex in its absence. These p21/cyclin D1/Cdk2 complexes are active and also bind the trimeric PCNA complex, with each trimer capable of independently binding distinct cyclin/Cdk complexes. We also show that increased p21 levels due to treatment with chemotherapeutic agents result in increased formation and kinase activity of cyclin D1/Cdk2 complexes, and that cyclin D1/Cdk2 complexes are able to phosphorylate a number of substrates in addition to Rb. Nucleophosmin and Cdh1, two proteins important for centrosome replication and implicated in the chromosomal instability of cancer, are shown to be phosphorylated by cyclin D1/Cdk2 complexes. Additionally, polypyrimidine tract binding protein-associated splicing factor (PSF) is identified as a novel Cdk2 substrate, being phosphorylated by Cdk2 complexed with either cyclin E or cyclin D1, and given the many functions of PSF, it could have important implications on cellular activity.

Serine 123 phosphorylation modulates p21 protein stability and activity by suppressing ubiquitin-independent proteasomal degradation

The cyclin-dependent kinase inhibitor p21(Waf1/Cip1) is a major regulator of the cell cycle and plays an important role in many cellular processes, including differentiation, stress response, apoptosis, and tumorigenesis. We previously cloned the gene encoding dog p21 and found that unlike its human ortholog, dog p21 is expressed as two isoforms, one high molecular mass band of 23 kDa and one low molecular mass band of 19 kDa. In the current study, we found that the high molecular mass band is phosphorylated, whereas the low molecular mass band is hypophosphorylated. Moreover, by generating multiple mutants of dog p21, we found that serine 123 and proline 124, which form a consensus site for proline-directed phosphorylation, are required for expression of the high molecular mass p21 isoform through phosphorylation at serine 123. Most importantly, we showed that serine 123 phosphorylation inhibits ubiquitin-independent proteasomal degradation of p21 protein and subsequently, prolongs p21 protein half-life and enhances the ability of p21 to suppress cell proliferation. Taken together, these data reveal that serine 123 phosphorylation modulates p21 protein stability and activity by suppressing ubiquitin-independent proteasomal degradation.

MYC and PIM2 co-expression in mouse bone marrow cells readily establishes permanent myeloid cell lines that can induce lethal myeloid sarcoma in vivo

The hematopoietic cell malignancy is one of the most prevalent type of cancer and the disease has multiple pathologic molecular signatures. Research on the origin of hematopoietic cancer stem cells and the mode of subsequent maintenance and differentiation needs robust animal models that can reproduce the transformation and differentiation event in vivo. Here, we show that co-transduction of MYC and PIM2 proto-oncogenes into mouse bone marrow cells readily establishes permanent cell lines that can induce lethal myeloid sarcoma in vivo. Unlike the previous doubly transgenic mouse model in which coexpression of MYC and PIM2 transgenes exclusively induced B cell lymphoma, we were able to show that the same combination of genes can also transform primary bone marrow myeloid cells in vitro resulting in permanent cell lines which induce myeloid sarcoma upon in vivo transplantation. By inducing cancerous transformation of fresh bone marrow cells in a controlled environment, the model we established will be useful for detailed study of the molecular events involved in initial transformation process of primary myeloid bone marrow cells and provides a model that can give insight to the molecular pathologic characteristics of human myeloid sarcoma, a rare presentation of solid tumors of undifferentiated myeloid blast cells associated with various types of myeloid leukemia.

Why target PIM1 for cancer diagnosis and treatment?

The highly conserved proto-oncogenic protein PIM1 is an unusual serine or threonine kinase, in part because it is constitutively active. Overexpression of PIM1 experimentally leads to tumor formation in mice, while complete knockout of the protein has no observable phenotype. It appears to contribute to cancer development in three major ways when it is overexpressed; by inhibiting apoptosis, by promoting cell proliferation and by promoting genomic instability. Expression in normal tissues is nearly undetectable. However, in hematopoietic malignancies and in a variety of solid tumors, increased PIM1 expression has been shown to correlate with the stage of disease. This characteristic suggests it can serve as a useful biomarker for cancer diagnosis and prognosis. Several specific and potent inhibitors of PIM1’s kinase activity have also been shown to induce apoptotic death of cancer cells, to sensitize cancer cells to chemotherapy and to synergize with other anti-tumor agents, thus making it an attractive therapeutic target.