The Pim family of protein kinases: structure, functions and roles in hematopoietic malignancies

Phosphorylation is the universal regulatory mechanism in key physiological processes such as development, cell differentiation, proliferation, survival and malignant transformation. In this review we analyze serine/threonine protein kinases of the Pim (proviral integration of Moloney virus) family that have been initially discovered in experimental lymphomas. We provide data on gene structure, evolution, functions and substrates of Pim protein kinases. Focusing on Pim-1 as the major isoform, we analyze its role in the biology of hematopoietic malignancies. Pim-1 is a pro-proliferative and pro-survival protein kinase. It is constitutively active due to autophosphorylation, and its downstream partners positively regulate the cell cycle. Pim-1 cooperates with c-Myc oncoprotein in leukemogenesis; furthermore, Pim-1, like the Akt protein kinase, prevents cell death. Thus, Pim kinases are regarded as new therapeutic targets. Finally, we present an original test system f or screening of Pim inhibitors. In this test system the growth of a genetically engineered Escherichia coli strain in the presence of kanamycin is dependent on the phosphorylation of aminoglycoside-3' phosphotransferase VIII by Pim-1: pharmacological inhibition of this phosphorylation increases the bacterial cell lysis.     

Pim-1 kinase stability is regulated by heat shock proteins and the ubiquitin-proteasome pathway

Elevated expression of the serine/threonine kinase Pim-1 increases the incidence of lymphomas in Pim-1 transgenic mice and has also been found to occur in some human cancers. Pim-1 acts as a cell survival factor and may prevent apoptosis in malignant cells. It was therefore of interest to understand to what extent maintenance and degradation of Pim-1 protein is affected by heat shock proteins (Hsp) and the ubiquitin-proteasome pathway in K562 and BV173 human leukemic cells. The half-life of Pim-1 protein in these cells was found to increase from 1.7 to 3.1 hours when induced by heat shock or by treating the cells with the proteasome inhibitor PS-341 (bortezomib). The Hsp90 inhibitor geldanamycin prevented the stabilization of Pim-1 by heat shock. Using immunoprecipitation, it was determined that Pim-1 is targeted for degradation by ubiquitin and that Hsp70 is associated with Pim-1 under these circumstances. Conversely, Hsp90 was found to protect Pim-1 from proteasomal degradation. A luminescence-based kinase assay showed that Pim-1 kinase bound to Hsp70 or Hsp90 remains active, emphasizing the importance of its overall cellular levels. This study shows how Pim-1 levels can be modulated in cells through degradation and stabilization.     

Elevated levels of oncogenic protein kinase Pim-1 induce the p53 pathway in cultured cells and correlate with increased Mdm2 in mantle cell lymphoma

Mutation of the p53 gene is a common event during tumor pathogenesis. Other mechanisms, such as mdm2 amplification, provide alternative routes through which dysfunction of the p53 pathway is promoted. Here, we address the hypothesis that elevated expression of pim oncogenes might suppress p53 by regulating Mdm2. At a physiological level, we show that endogenous Pim-1 and Pim-2 interact with endogenous Mdm2. Additionally, the Pim kinases phosphorylate Mdm2 in vitro and in cultured cells at Ser(166) and Ser(186), two previously identified targets of other signaling pathways, including Akt. Surprisingly, at high levels of Pim expression, as would occur in tumors, active, but not inactive, Pim-1 or Pim-2 blocks the degradation of both p53 and Mdm2 in a manner that is independent of Mdm2 phosphorylation, leading to increased p53 levels and, proportionately, p53-dependent transactivation. Additionally, Pim-1 induces endogenous ARF, p53, Mdm2, and p21 in primary murine embryo fibroblasts and stimulates senescence-associated beta-galactosidase levels, consistent with the induction of senescence. Immunohistochemical analysis of a cohort of 33 human mantle cell lymphomas shows that elevated expression of Pim-1 occurs in 42% of cases, with elevated Pim-2 occurring in 9% of cases, all of which also express Pim-1. Notably, elevated Pim-1 correlates with elevated Mdm2 in MCL with a p value of 0.003. Taken together, our data are consistent with the idea that Pim normally interacts with the p53 pathway but, when expressed at pathological levels, behaves as a classic dominant oncogene that stimulates a protective response through induction of the p53 pathway.     

Elevated levels of the polyadenylation factor CstF 64 enhance formation of the 1kB Testis brain RNA-binding protein (TB-RBP) mRNA in male germ cells

The single copy mouse Testis Brain RNA-Binding Protein (TB-RBP) gene encodes three mRNAs of 3.0, 1.7, and 1.0 kb which only differ in their 3' UTRs. The 1 kb TB-RBP mRNA predominates in testis, while somatic cells preferentially express the 3.0 kb TB-RBP mRNA. Here we show that the 1 kb mRNA is translated several-fold more efficiently than the 3 kb TB-RBP in rabbit reticulocyte lysates and cells with elevated levels of the 1 kB TB-RBP mRNA express high levels of TB-RBP. To determine if the cleavage stimulatory factor CstF 64 can modulate the alternative splicing of the TB-RBP pre-mRNA and therefore TB-RBP expression, CstF 64 levels and binding to alternative polyadenylation sites were examined. CstF 64 is abundant in the testis and preferentially binds to a distal site in the TB-RBP pre-mRNA that produces the 3 kb TB-RBP. Moreover, upregulation or overexpression of CstF 64 increases the poly(A) site selection for the 1 kb TB-RBP mRNA. We propose that the level of the polyadenylation factor CstF 64 modulates the level of TB-RBP synthesis in male germ cells by an alternative processing of the TB-RBP pre-mRNA.     

CD40 signaling in B cells regulates the expression of the Pim-1 kinase via the NF-kappa B pathway.

The ability of CD40 signaling to regulate B cell growth, survival, differentiation, and Ig class switching involves many changes in gene expression. Using cDNA expression arrays and Northern blotting, we found that CD40 signaling increased the mRNA levels for pim-1, a protooncogene that encodes a serine/threonine protein kinase. Subsequent experiments showed that CD40 engagement also increased both Pim-1 protein levels and Pim-1 kinase activity in B cells. We then investigated the signaling pathways by which CD40 regulates Pim-1 expression and found that CD40 up-regulates Pim-1 primarily via the activation of NF-kappaB. Inhibiting the activation of NF-kappaB, either by treating cells with a chemical inhibitor, BAY11-7082, or by inducibly expressing a superrepressor form of IkappaBalpha, significantly impaired the ability of CD40 to increase Pim-1 protein levels. Because Pim-1 expression is associated with cell proliferation and survival, we asked whether this correlated with the ability of CD40 signaling to prevent anti-IgM-induced growth arrest in the WEHI-231 murine B cell line, a model for Ag-induced clonal deletion. We found that the anti-IgM-induced growth arrest in WEHI-231 cells correlated with a substantial decrease in Pim-1 levels. In contrast, culturing WEHI-231 cells with either anti-CD40 Abs or with the B cell mitogen LPS, both of which prevent the anti-IgM-induced growth arrest, also prevented the rapid decline in Pim-1 levels. This suggests that Pim-1 could regulate the survival and proliferation of B cells.     

Characterization of the herpes simplex virus type 1 glycoprotein D mRNA and expression of this protein in Xenopus oocytes

We have identified and characterized a 3.0 kilobase (kb) mRNA containing coding sequences of the herpes simplex virus type 1 (HSV-1) glycoprotein D (gD) gene. The synthesis of this 3.0 kb mRNA was unaffected by the presence of cytosine arabinoside, but was made in greatly reduced amounts in cells infected with HSV-1 in the presence of cycloheximide: it was, therefore, classified as an early mRNA. By nuclease protection experiments, it was found that the 3.0 kb mRNA is unspliced and, further, that it is 3' co-terminal with a smaller 1.6 kb early mRNA which is transcribed from a DNA sequence 3' to the gD coding sequence. We describe the use of the Xenopus laevis oocyte system to produce HSV-1 gD in vitro. Oocytes injected with mRNA isolated from HSV-1-infected Vero cells synthesized gD, which was identified by immunoprecipitation. Injection of a plasmid clone containing the HSV-1 BamHI J fragment (0.89 to 0.93 map units) into the nuclei of Xenopus oocytes also resulted in synthesis of gD.     

Molecular cloning of porcine alpha 1-microglobulin/HI-30 reveals developmental and tissue-specific expression of two variant messenger ribonucleic acids.

A 1008 basepair (bp) cDNA clone encoding 335 amino acids followed by an inframe TGA translation termination codon and a 295-nucleotide 3' untranslated (UT) region has been isolated from a pig liver cDNA library. Based on the deduced amino acid and nucleotide sequence homology to a human cDNA (Kaumeyer, J.F., Polazzi, J.O. and Kotick, M.P. (1986) Nucleic Acids Res. 14, 7839-7850), the 5' amino terminus was found to code for alpha 1-microglobulin (alpha 1-M), a 183 amino acid protein belonging to the lipocalin protein superfamily (Pervaiz, S. and Brew, K. (1985) Science 228, 335-337). The 3' half encoded HI-30 which constitutes the Kunitz-type proteinase inhibitory (L-chain) domain of porcine inter-alpha-trypsin inhibitor (I alpha TI). In Northern blot hybridization, this cDNA identified two equally abundant mRNA species of approx. 1.3 kb and 1.6 kb in length. However, a 125 bp cDNA probe derived from the 3' UT region of the cDNA hybridized only to the 1.6 kb mRNA. The differences observed in the 3' UT region of these mRNAs suggest the utilization of alternative polyadenylation signals or presence of unprocessed nuclear RNA. Densitometric scanning of Northern blots indicated that alpha 1-M/HI-30 mRNA levels were higher (5-8-fold) in fetal and neonatal liver compared to that of primiparous pigs. In contrast, the RNA levels did not change significantly during pregnancy. Dot blot analysis of RNA indicated liver to be the major site of alpha 1-M/HI-30 mRNA expression with lower levels observed in the stomach. The results suggest that modulation of alpha 1-M/HI-30 gene expression could play a role during porcine growth. Increased I alpha TI L-chain mRNA levels may be particularly important in fetal and neonatal development when regulation of the inflammatory response and protection of macromolecules from proteolytic degradation is vital to survival and sustained growth.     

Sequential events of apoptosis involving docetaxel, a microtubule-interfering agent: A cytometric study

Background  

Pim-1, 2 and 3 are a group of enzymes related to the calcium calmodulin family of protein kinases. Over-expression of Pim-1 and Pim-2 in mice promotes the development of lymphomas, and up-regulation of Pim expression has been observed in several human cancers.     

Autocrine growth and progression of murine X-ray-induced T cell lymphomas.

Primary tumors of X-ray-induced murine T cell lymphomas comprise autocrine, growth factor-dependent cells. We have grown cell lines from primary X-ray-induced thymic lymphomas (PXTLs) under conditions which minimize the progression of the cells from factor dependence to factor independence. All (22) PXTL lines grown secrete a growth factor which supports their own growth and which we will call lymphoma growth factor LGF. LGF-dependent cells are non-tumorigenic or poorly tumorigenic, do not clone in soft agar, have no detectable rearrangements in the c-myc or Pim-1 region and possess near diploid or pseudodiploid karyotypes without evidence for trisomy of chromosomes nos. 15 or 17. PXTL-secreted LGF has no interleukin 1, 2, or 3 activity nor do LGF-secreting cells synthesize detectable IL-1, -2, or -3 mRNA. LGF contains no detectable interferon or GM-CSF activity in specific bioassays. Purified EGF, TGF beta, and interleukin preparations are inactive on LGF-dependent PXTL cells. Thus LGF appears to be a new growth factor that is required for the proliferation of non-progressed T lymphoma cells. Upon progression PXTL cells become growth factor independent, are highly tumorigenic in vivo, clone in soft agar, and assume a near triploid karyotype containing numerous chromosomal aberrations. Thus in X-ray-induced lymphomagenesis an autocrine, LGF-dependent phase precedes the progressed phase characterized by rearrangements in the myc and/or Pim-1 regions as well as by many chromosomal aberrations visible in the karyotype.     

Comparative molecular genetic analysis of lymphomas from six inbred mouse strains.

Previous studies of 21 highly lymphomatous AKXD recombinant inbred mouse strains demonstrated correlations between lymphoma type, the somatic proviral DNA content of the lymphoma, and the frequency of virally induced rearrangements in eight common sites of viral integration (Myc, Pim-i, Pvt-1, Mlvi-1, Mlvi-2, Fis-1, Myb, and Evi-1). In this study we analyzed lymphomas from six inbred mouse strains, AKR/J, C58/J, HRS/J (hr/hr and hr/+), SJL/J, SEA/GnJ, and CWD/LeAgl, to determine whether these correlations are also evident in these strains. Mice of the AKR/J, C58/J, and HRS/J strains died exclusively of T-cell lymphomas. In contrast to earlier studies which showed a great disparity in the rate and incidence of lymphomas in HRS/J hr/hr and HRS/J hr/+ mice, we found a high incidence of T-cell lymphomas and the same mean age of onset of disease in both strains. SJL/J mice died primarily of pre-B-cell lymphomas, whereas CWD/LeAgl and SEA/GnJ mice died primarily of B-cell lymphomas. Somatically acquired mink cell focus-forming proviruses were detected only in T-cell lymphomas, whereas ecotropic proviruses were found in lymphomas from all hematopoietic cell lineages. No rearrangements were detected in the Fis-1, Mlvi-2, and Myb loci, whereas rearrangements were detected in the Mlvi-1, Myc, Pim-1, Pvt-1, and Evi-1 loci. Most rearrangements were found in T-cell lymphomas, and many were virally induced. These results are similar to those we obtained previously for lymphomas of 21 highly lymphomatous AKXD recombinant inbred mouse strains.