Identification of the autophosphorylation sites of the Xenopus laevis Pim-1 proto-oncogene-encoded protein kinase

Pim Kinases: Important Regulators of Cardiovascular Disease

Pim Kinases; Pim-1, Pim-2, and Pim-3, are a family of constitutively active serine/threonine kinases, widely associated with cell survival, proliferation, and migration. Historically considered to be functionally redundant, independent roles for the individual isoforms have been described. Whilst most established for their role in cancer progression, there is increasing evidence for wider pathological roles of Pim kinases within the context of cardiovascular disease, including inflammation, thrombosis, and cardiac injury. The Pim kinase isoforms have widespread expression in cardiovascular tissues, including the heart, coronary artery, aorta, and blood, and have been demonstrated to be upregulated in several co-morbidities/risk factors for cardiovascular disease. Pim kinase inhibition may thus be a desirable therapeutic for a multi-targeted approach to treat cardiovascular disease and some of the associated risk factors. In this review, we discuss what is known about Pim kinase expression and activity in cells of the cardiovascular system, identify areas where the role of Pim kinase has yet to be fully explored and characterised and review the suitability of targeting Pim kinase for the prevention and treatment of cardiovascular events in high-risk individuals.

Therapeutic targeting of PIM KINASE signaling in cancer therapy: Structural and clinical prospects

BACKGROUND

PIM kinases are well-studied drug targets for cancer, belonging to Serine/Threonine kinases family. They are the downstream target of various signaling pathways, and their up/down-regulation affects various physiological processes. PIM family comprises three isoforms, namely, PIM-1, PIM-2, and PIM-3, on alternative initiation of translation and they have different levels of expression in different types of cancers. Its structure shows a unique ATP-binding site in the hinge region which makes it unique among other kinases.

SCOPE OF REVIEW

PIM kinases are widely reported in hematological malignancies along with prostate and breast cancers. Currently, many drugs are used as inhibitors of PIM kinases. In this review, we highlighted the physiological significance of PIM kinases in the context of disease progression and therapeutic targeting. We comprehensively reviewed the PIM kinases in terms of their expression and regulation of different physiological roles. We further predicted functional partners of PIM kinases to elucidate their role in the cellular physiology of different cancer and mapped their interaction network.

MAJOR CONCLUSIONS

A deeper mechanistic insight into the PIM signaling involved in regulating different cellular processes, including transcription, apoptosis, cell cycle regulation, cell proliferation, cell migration and senescence, is provided. Furthermore, structural features of PIM have been dissected to understand the mechanism of inhibition and subsequent implication of designed inhibitors towards therapeutic management of prostate, breast and other cancers.

GENERAL SIGNIFICANCE

Being a potential drug target for cancer therapy, available drugs and PIM inhibitors at different stages of clinical trials are discussed in detail.

A functional SUMO-motif in the active site of PIM1 promotes its degradation via RNF4, and stimulates protein kinase activity

The PIM1 serine/threonine protein kinase mediates growth factor and survival signalling, and cooperates potently with c-MYC during tumorigenesis. PIM1 is overexpressed in many human cancers and is a promising target for drug development. PIM1 levels are regulated mainly through cytokine-induced transcription and protein degradation, but mechanisms regulating its activity and levels remain largely unexplored. Here, we show that PIM1 is modified in vitro and in cultured cells by the Small ubiquitin-like modifier (SUMO) on two independent sites: K169, within a consensus SUMOylation motif (IK¹⁶⁹DE¹⁷¹) in the active site of PIM1, and also at a second promiscuous site. Alanine substitution of E171 (within the consensus motif) abolished SUMOylation, significantly increased the half-life of PIM1, and markedly reduced its ubiquitylation. Mechanistically, SUMOylation promoted ubiquitin-mediated degradation of PIM1 via recruitment of the SUMO-targeted ubiquitin ligase, RNF4. Additionally, SUMOylated PIM1 showed enhanced protein kinase activity in vitro. Interestingly, the E171A mutant was active in vitro but displayed altered substrate specificity in cultured cells, consistent with the idea that SUMOylation may govern PIM1 substrate specificity under certain contexts. Taken together, these data demonstrate that the protein kinase activity and levels of PIM1 can be regulated by a covalent post-translational modification.

EBNA3C augments Pim-1 mediated phosphorylation and degradation of p21 to promote B-cell proliferation

Epstein–Barr virus (EBV), a ubiquitous human herpesvirus, can latently infect the human population. EBV is associated with several types of malignancies originating from lymphoid and epithelial cell types. EBV latent antigen 3C (EBNA3C) is essential for EBV-induced immortalization of B-cells. The Moloney murine leukemia provirus integration site (PIM-1), which encodes an oncogenic serine/threonine kinase, is linked to several cellular functions involving cell survival, proliferation, differentiation, and apoptosis. Notably, enhanced expression of Pim-1 kinase is associated with numerous hematological and non-hematological malignancies. A higher expression level of Pim-1 kinase is associated with EBV infection, suggesting a crucial role for Pim-1 in EBV-induced tumorigenesis. We now demonstrate a molecular mechanism which reveals a direct role for EBNA3C in enhancing Pim-1 expression in EBV-infected primary B-cells. We also showed that EBNA3C is physically associated with Pim-1 through its amino-terminal domain, and also forms a molecular complex in B-cells. EBNA3C can stabilize Pim-1 through abrogation of the proteasome/Ubiquitin pathway. Our results demonstrate that EBNA3C enhances Pim-1 mediated phosphorylation of p21 at the Thr¹⁴⁵ residue. EBNA3C also facilitated the nuclear localization of Pim-1, and promoted EBV transformed cell proliferation by altering Pim-1 mediated regulation of the activity of the cell-cycle inhibitor p21/WAF1. Our study demonstrated that EBNA3C significantly induces Pim-1 mediated proteosomal degradation of p21. A significant reduction in cell proliferation of EBV-transformed LCLs was observed upon stable knockdown of Pim-1. This study describes a critical role for the oncoprotein Pim-1 in EBV-mediated oncogenesis, as well as provides novel insights into oncogenic kinase-targeted therapeutic intervention of EBV-associated cancers.

The oncogenic serine/threonine kinase Pim-1 is upregulated in a number of human cancers including lymphomas, gastric, colorectal and prostate carcinomas. EBV nuclear antigen 3C (EBNA3C) is essential for EBV-induced transformation of human primary B-lymphocytes. Our current study revealed that EBNA3C significantly enhances Pim-1 kinase expression at both the transcript and protein levels. EBNA3C also interacts with Pim-1 and can form a complex in EBV-transformed cells. Moreover, EBNA3C increases nuclear localization of Pim-1 and stabilizes Pim-1 protein levels by inhibiting its poly-ubiquitination. Additionally, EBNA3C augments Pim-1 mediated phosphorylation of p21 and its proteosomal degradation. Stable knockdown of Pim-1 using si-RNA showed a significant decrease in proliferation of EBV transformed lymphoblastoid cell lines and subsequent induction of apoptosis by triggering the intrinsic apoptotic pathway. Therefore, our study demonstrated a new mechanism by which the oncogenic Pim-1 kinase targeted by EBV latent antigen 3C can inhibit p21 function, and is therefore a potential therapeutic target for the treatment of EBV-associated malignancies.

The PIM kinases in hematological cancers

The PIM genes represent a family of proto-oncogenes that encode three different serine/threonine protein kinases (PIM1, PIM2 and PIM3) with essential roles in the regulation of signal transduction cascades, which promote cell survival, proliferation and drug resistance. PIM kinases are overexpressed in several hematopoietic tumors and support in vitro and in vivo malignant cell growth and survival, through cell cycle regulation and inhibition of apoptosis. PIM kinases do not have an identified regulatory domain, which means that these proteins are constitutively active once transcribed. They appear to be critical downstream effectors of important oncoproteins and, when overexpressed, can mediate drug resistance to available agents, such as rapamycin. Recent crystallography studies reveal that, unlike other kinases, they possess a hinge region, which creates a unique binding pocket for ATP, offering a target for an increasing number of potent small-molecule PIM kinase inhibitors. Preclinical studies in models of various hematologic cancers indicate that these novel agents show promising activity and some of them are currently being evaluated in a clinical setting. In this review, we profile the PIM kinases as targets for therapeutics in hematologic malignancies.

Pim-1 kinase inhibits pathological injury by promoting cardioprotective signaling

Stem cells mediate tissue repair throughout the lifespan of an organism. However, the ability of stem cells to mitigate catastrophic damage, such as that sustained after major myocardial infarction is inadequate to rebuild the heart and restore functional capacity. However, capitalizing on the ability of these cells to attenuate damage in the myocardium, various maneuvers that enhance repair mechanisms to improve cardiac structure and function after injury are being investigated. These studies have led to discovery of various factors that mediate cardioprotection and enhance endogenous repair by 1) salvaging surviving myocardium, 2) promoting homing of stem cells and 3) increasing survival and proliferation of stem cell populations at the site of injury. Herein we report upon a downstream target of Akt kinase, named Pim-1, which promotes cardioprotective signaling and enhances cardiac structure and function after pathological injury. The compilation of studies presented here supports use of Pim-1 to enhance long-term myocardial repair after pathological damage. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Epstein-Barr virus latent membrane protein 1 increases chemo-resistance of cancer cells via cytoplasmic sequestration of Pim-1

Improved treatment of EBV positive lymphoma depends on the identification of molecular mechanism underlying chemo-resistance. LMP1 is an essential transmembrane protein for EBV-induced immortalization of hematopoietic cells. Herein, we show that an oncogenic Pim-1 is translocated to the cytoplasm by LMP1. Three lines of evidence indicate that cytoplasmic sequestration of Pim-1 may be required for LMP1-induced cancer cell survival. First, Pim-1 enhanced the survival of LMP1-overexpressing cells treated with doxorubicin. Second, nuclear export of Pim-1 was sufficient to increase the survival. Third, knockdown of Pim-1 effectively suppressed LMP-1-induced survival of cancer cells. Collectively, these data suggest that Pim-1 is a downstream target of LMP1, and that it contributes to the chemo-resistance of cancer cells.

PIM serine/threonine kinases in the pathogenesis and therapy of hematologic malignancies and solid cancers

The identification as cooperating targets of Proviral Integrations of Moloney virus in murine lymphomas suggested early on that PIM serine/threonine kinases play an important role in cancer biology. Whereas elevated levels of PIM1 and PIM2 were mostly found in hematologic malignancies and prostate cancer, increased PIM3 expression was observed in different solid tumors. PIM kinases are constitutively active and their activity supports in vitro and in vivo tumor cell growth and survival through modification of an increasing number of common as well as isoform-specific substrates including several cell cycle regulators and apoptosis mediators. PIM1 but not PIM2 seems also to mediate homing and migration of normal and malignant hematopoietic cells by regulating chemokine receptor surface expression. Knockdown experiments by RNA interference or dominant-negative acting mutants suggested that PIM kinases are important for maintenance of a transformed phenotype and therefore potential therapeutic targets. Determination of the protein structure facilitated identification of an increasing number of potent small molecule PIM kinase inhibitors with in vitro and in vivo anticancer activity. Ongoing efforts aim to identify isoform-specific PIM inhibitors that would not only help to dissect the kinase function but hopefully also provide targeted therapeutics. Here, we summarize the current knowledge about the role of PIM serine/threonine kinases for the pathogenesis and therapy of hematologic malignancies and solid cancers, and we highlight structural principles and recent progress on small molecule PIM kinase inhibitors that are on their way into first clinical trials.

Transcriptional regulation of Pim-1 kinase in vascular smooth muscle cells and its role for proliferation

The Ser/Thr-protein kinase Pim-1 has been discovered as a novel transducer of survival- and cell cycle promoting signals in the hematopoietic cell system. Although its significance for proliferation of vascular smooth muscle cells (VSMC) in vitro and neointima formation in vivo has been suggested recently, the mechanism has barely been characterized. This study aimed to foster the understanding of Pim-1 expression and regulation in murine VSMC in response to factors typically present within the atherosclerotic plaque. While oxidative stress, VEGF-A165 and angiotensin II did not have any effect on Pim-1 expression, VSMC strongly increased (3-fold) Pim-1 mRNA upon stimulation with PDGF(bb), followed by its protein upregulation. Half life of Pim-1 RNA and protein were determined to be 25 min and 6 h, respectively. PDGF(bb) induced a strong, 10-fold increase in BrdU-uptake, a marker of proliferation. This was effectively blocked by either Pim-1-specific inhibitor quercetagetin or adenovirally introduced Pim-1 shRNA. We further identified the signaling pathways linking PDGF(bb) to Pim-1 in VSMC: as expected, we determined transcriptional stimulation of Pim-1 via Janus-activated kinase (Jak), but also an additional pathway involving protein kinase C (PKC) and the mitogen-activated protein kinase Mek1/2. Blockade of Akt signaling did, however, not interfere with Pim-1 upregulation, suggesting an independence of either survival system. PDGF(bb)-induced proliferation of VSMC is partly attributed to transcriptionally upregulated Pim-1 and was assigned to distinct cell signaling. Our findings help to understand the fundamental processes of vasculoproliferative diseases thus opening avenues for its prevention and treatment.

PIM-1-specific mAb suppresses human and mouse tumor growth by decreasing PIM-1 levels, reducing Akt phosphorylation, and activating apoptosis

Provirus integration site for Moloney murine leukemia virus (PIM1) is a proto-oncogene that encodes a serine/threonine kinase with multiple cellular functions. Overexpression of PIM-1 plays a critical role in progression of prostatic and hematopoietic malignancies. Here we describe the generation of a mAb specific for GST-PIM-1, which reacted strongly with most human and mouse cancer tissues and cell lines of prostate, breast, and colon origin but only weakly (if at all) with normal tissues. The mAb binds to PIM-1 in the cytosol and nucleus as well as to PIM-1 on the surface of human and murine cancer cells. Treatment of human and mouse prostate cancer cell lines with the PIM-1-specific mAb resulted in disruption of PIM-1/Hsp90 complexes, decreased PIM-1 and Hsp90 levels, reduced Akt phosphorylation at Ser473, reduced phosphorylation of Bad at Ser112 and Ser136, and increased cleavage of caspase-9, an indicator of activation of the mitochondrial cell death pathway. The mAb induced cancer cell apoptosis and synergistically enhanced antitumor activity when used in combination with cisplatin and epirubicin. In tumor models, the PIM-1-specific mAb substantially inhibited growth of the human prostate cancer cell line DU145 in SCID mice and the mouse prostate cancer cell TRAMP-C1 in C57BL/6 mice. These findings are important because they provide what we believe to be the first in vivo evidence that treatment of prostate cancer may be possible by targeting PIM-1 using an Ab-based therapy.

Pim-1 regulates cardiomyocyte survival downstream of Akt

The serine-threonine kinases Pim-1 and Akt regulate cellular proliferation and survival. Although Akt is known to be a crucial signaling protein in the myocardium, the role of Pim-1 has been overlooked. Pim-1 expression in the myocardium of mice decreased during postnatal development, re-emerged after acute pathological injury in mice and was increased in failing hearts of both mice and humans. Cardioprotective stimuli associated with Akt activation induced Pim-1 expression, but compensatory increases in Akt abundance and phosphorylation after pathological injury by infarction or pressure overload did not protect the myocardium in Pim-1-deficient mice. Transgenic expression of Pim-1 in the myocardium protected mice from infarction injury, and Pim-1 expression inhibited cardiomyocyte apoptosis with concomitant increases in Bcl-2 and Bcl-X(L) protein levels, as well as in Bad phosphorylation levels. Relative to nontransgenic controls, calcium dynamics were significantly enhanced in Pim-1-overexpressing transgenic hearts, associated with increased expression of SERCA2a, and were depressed in Pim-1-deficient hearts. Collectively, these data suggest that Pim-1 is a crucial facet of cardioprotection downstream of Akt.

Pim-1 kinase phosphorylates RUNX family transcription factors and enhances their activity

Background

The pim family genes encode oncogenic serine/threonine kinases which in hematopoietic cells have been implicated in cytokine-dependent signaling as well as in lymphomagenesis, especially in cooperation with other oncogenes such as myc, bcl-2 or Runx family genes. The Runx genes encode α-subunits of heterodimeric transcription factors which regulate cell proliferation and differentiation in various tissues during development and which can become leukemogenic upon aberrant expression.

Results

Here we have identified novel protein-protein interactions between the Pim-1 kinase and the RUNX family transcription factors. Using the yeast two-hybrid system, we were able to show that the C-terminal part of human RUNX3 associates with Pim-1. This result was confirmed in cell culture, where full-length murine Runx1 and Runx3 both coprecipitated and colocalized with Pim-1. Furthermore, catalytically active Pim-1 kinase was able to phosphorylate Runx1 and Runx3 proteins and enhance the transactivation activity of Runx1 in a dose-dependent fashion.

Conclusion

Altogether, our results suggest that mammalian RUNX family transcription factors are novel binding partners and substrates for the Pim-1 kinase, which may be able to regulate their activities during normal hematopoiesis as well as in leukemogenesis.

The oncogenic serine/threonine kinase Pim-1 directly phosphorylates and activates the G2/M specific phosphatase Cdc25C

The proto-oncogene Pim-1 encodes a serine-threonine kinase which is a downstream effector of cytokine signaling and can enhance cell cycle progression by altering the activity of several cell cycle regulators among them the G1 specific inhibitor p21(Waf), the phosphatase Cdc25A and the kinase C-TAK1. Here, we demonstrate by using biochemical assays that Pim-1 can interact with the phosphatase Cdc25C and is able to directly phosphorylate the N-terminal region of the protein. Cdc25C is functionally related to Cdc25A but acts specifically at the G2/M cell cycle transition point and can be inactivated by C-TAK1-mediated phosphorylation. Immuno-fluorescence experiments showed that Pim-1 and Cdc25C co-localize in the cytoplasm of both epithelial and myeloid cells. We find that phosphorylation by Pim-1 enhances the phosphatase activity of Cdc25C and in transfected cells that are arrested in G2/M by bleomycin, Pim-1 can enhance progression into G1. Therefore, we propose that Pim-1 activates Cdc25C by a direct phosphorylation and can thereby assume the function of a positive cell cycle regulator at the G2/M transition.

Structure and substrate specificity of the Pim-1 kinase

The Pim kinases are a family of three vertebrate protein serine/threonine kinases (Pim-1, -2, and -3) belonging to the CAMK (calmodulin-dependent protein kinase-related) group. Pim kinases are emerging as important mediators of cytokine signaling pathways in hematopoietic cells, and they contribute to the progression of certain leukemias and solid tumors. A number of cytoplasmic and nuclear proteins are phosphorylated by Pim kinases and may act as their effectors in normal physiology and in disease. Recent crystallographic studies of Pim-1 have identified unique structural features but have not provided insight into how the kinase recognizes its target substrates. Here, we have conducted peptide library screens to exhaustively determine the sequence specificity of active site-mediated phosphorylation by Pim-1 and Pim-3. We have identified the major site of Pim-1 autophosphorylation and find surprisingly that it maps to a novel site that diverges from its consensus phosphorylation motif. We have solved the crystal structure of Pim-1 bound to a high affinity peptide substrate in complexes with either the ATP analog AMP-PNP or the bisindolylmaleimide kinase inhibitor 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)maleimide HCl. These structures reveal an unanticipated mode of recognition for basic residues upstream of the phosphorylation site, distinct from that of other kinases with similar substrate specificity. The structures provide a rationale for the unusually high affinity of Pim kinases for peptide substrates and suggest a general mode for substrate binding to members of the CAMK group.

Aberrant expression of serine/threonine kinase Pim-3 in hepatocellular carcinoma development and its role in the proliferation of human hepatoma cell lines

Most cases of human hepatocellular carcinoma develop after persistent chronic infection with human hepatitis B virus or hepatitis C virus, and host responses are presumed to have major roles in this process. To recapitulate this process, we have developed the mouse model of hepatocellular carcinoma using hepatitis B virus surface antigen transgenic mice. To identify the genes associated with hepatocarcinogenesis in this model, we compared the gene expression patterns between pre-malignant lesions surrounded by hepatocellular carcinoma tissues and control liver tissues by using a fluorescent differential display analysis. Among the genes that were expressed differentially in the pre-malignant lesions, we focused on Pim-3, a member of a proto-oncogene Pim family, because its contribution to hepatocarcinogenesis remains unknown. Moreover, the unavailability of the nucleotide sequence of full-length human Pim-3 cDNA prompted us to clone it from the cDNA library constructed from a human hepatoma cell line, HepG2. The obtained 2,392 bp human Pim-3 cDNA encodes a predicted open reading frame consisting of 326 amino acids. Pim-3 mRNA was selectively expressed in human hepatoma cell lines, but not in normal liver tissues. Moreover, Pim-3 protein was detected in human hepatocellular carcinoma tissues and cell lines but not in normal hepatocytes. Furthermore, cell proliferation was attenuated and apoptosis was enhanced in human hepatoma cell lines by the ablation of Pim-3 gene with RNA interference. These observations suggest that aberrantly expressed Pim-3 can cause autonomous cell proliferation or prevent apoptosis in hepatoma cell lines.

Pim-1 Ligand-bound Structures Reveal the Mechanism of Serine/Threonine Kinase Inhibition by LY294002

Pim-1 is an oncogene-encoded serine/threonine kinase primarily expressed in hematopoietic and germ cell lines. Pim-1 kinase was originally identified in Maloney murine leukemia virus-induced T-cell lymphomas and is associated with multiple cellular functions such as proliferation, survival, differentiation, apoptosis, and tumorigenesis (Wang, Z., Bhattacharya, N., Weaver, M., Petersen, K., Meyer, M., Gapter, L., and Magnuson, N. S. (2001) J. Vet. Sci. 2, 167-179). The crystal structures of Pim-1 complexed with staurosporine and adenosine were determined. Although a typical two-domain serine/threonine protein kinase fold is observed, the inter-domain hinge region is unusual in both sequence and conformation; a two-residue insertion causes the hinge to bulge away from the ATP-binding pocket, and a proline residue in the hinge removes a conserved main chain hydrogen bond donor. Without this hydrogen bond, van der Waals interactions with the hinge serve to position the ligand. The hinge region of Pim-1 resembles that of phosphatidylinositol 3-kinase more closely than it does other protein kinases. Although the phosphatidylinositol 3-kinase inhibitor LY294002 also inhibits Pim-1, the structure of the LY294002.Pim-1 complex reveals a new binding mode that may be general for Ser/Thr kinases.

Lymphocyte transformation by Pim-2 is dependent on nuclear factor-kappaB activation

Pim-2 is a transcriptionally regulated oncogenic kinase that promotes cell survival in response to a wide variety of proliferative signals. Deregulation of Pim-2 expression has been documented in several human malignancies, including leukemia, lymphoma, and multiple myeloma. Here, we show that the ability of Pim-2 to promote survival of cells is dependent on nuclear factor (NF)-kappaB activation. Pim-2 activates NF-kappaB-dependent gene expression by inducing phosphorylation of the oncogenic serine/threonine kinase Cot, leading to both augmentation of IkappaB kinase activity and a shift in nuclear NF-kappaB from predominantly p50 homodimers to p50/p65 heterodimers. Blockade of NF-kappaB function eliminates Pim-2-mediated survival in both cell lines and primary cells, and both Cot phosphorylation and expression are required for the prosurvival effects of Pim-2. Although Pim-2 cooperates with Myc to promote growth factor-independent cell proliferation, this feature is abrogated by NF-kappaB blockade. The ability of Pim-2 to serve as an oncogene in vivo depends on sustained NF-kappaB activity. Thus, the transcriptional induction of Pim-2 initiates a novel NF-kappaB activation pathway that regulates cell survival.

Structural basis of constitutive activity and a unique nucleotide binding mode of human Pim-1 kinase

Pim-1 kinase is a member of a distinct class of serine/threonine kinases consisting of Pim-1, Pim-2, and Pim-3. Pim kinases are highly homologous to one another and share a unique consensus hinge region sequence, ER-PXPX, with its two proline residues separated by a non-conserved residue, but they (Pim kinases) have <30% sequence identity with other kinases. Pim-1 has been implicated in both cytokine-induced signal transduction and the development of lymphoid malignancies. We have determined the crystal structures of apo Pim-1 kinase and its AMP-PNP (5'-adenylyl-beta,gamma-imidodiphosphate) complex to 2.1-angstroms resolutions. The structures reveal the following. 1) The kinase adopts a constitutively active conformation, and extensive hydrophobic and hydrogen bond interactions between the activation loop and the catalytic loop might be the structural basis for maintaining such a conformation. 2) The hinge region has a novel architecture and hydrogen-bonding pattern, which not only expand the ATP pocket but also serve to establish unambiguously the alignment of the Pim-1 hinge region with that of other kinases. 3) The binding mode of AMP-PNP to Pim-1 kinase is unique and does not involve a critical hinge region hydrogen bond interaction. Analysis of the reported Pim-1 kinase-domain structures leads to a hypothesis as to how Pim kinase activity might be regulated in vivo.

Pim-1 kinase promotes inactivation of the pro-apoptotic Bad protein by phosphorylating it on the Ser112 gatekeeper site

Constitutive expression of the Pim-1 kinase prolongs survival of cytokine-deprived FDCP1 cells, partly via maintenance of Bcl-2 expression. Here, we show that Pim-1 colocalizes and physically interacts with the pro-apoptotic Bad protein and phosphorylates it in vitro on serine 112, which is a gatekeeper site for its inactivation. Furthermore, wild-type Pim-1, but not a kinase-deficient mutant, enhances phosphorylation of this site in FDCP1 cells and protects cells from the pro-apoptotic effects of Bad. Our results suggest that phosphorylation of Bad by Pim-1 is one of several mechanisms via which the Pim-1 kinase can enhance Bcl-2 activity and promote cell survival.

The oncogenic serine/threonine kinase Pim-1 phosphorylates and inhibits the activity of Cdc25C-associated kinase 1 (C-TAK1): a novel role for Pim-1 at the G2/M cell cycle checkpoint

The Pim-1 oncogene encodes a serine-threonine kinase that relays signals from cytokine receptors and contributes to the formation of lymphoid tumors when expressed at high levels. Here we show that the protein kinase Cdc25 C-associated kinase 1 (C-TAK1) is a binding partner and a substrate of Pim-1. A physical interaction of Pim-1 and C-TAK1 could be shown biochemically and in yeast two-hybrid assays. Immunofluorescence experiments suggested that Pim-1.C-TAK1 complexes are predominantly cytoplasmic. When transiently transfected, Pim-1 was also found in the nucleus and could recruit C-TAK1 to this compartment. Both Pim-1 and C-TAK1 underwent autophosphorylation, but only Pim-1 was able to phosphorylate C-TAK1 but not vice versa. Mass spectrometry analysis of C-TAK1 suggested that the sites of autophosphorylation and Pim-1-mediated phosphorylation are distinct and not overlapping. Phosphorylation by Pim-1 decreased C-TAK1 kinase activity significantly, in particular its ability to phosphorylate and inactivate Cdc25C, a protein that actively promotes cell cycle progression at the G(2)/M phase. Hence our findings directly suggest a novel role for Pim-1 as a positive regulator at the G(2)/M transition of the cell cycle.

Mice deficient for all PIM kinases display reduced body size and impaired responses to hematopoietic growth factors

The Pim family of proto-oncogenes encodes a distinct class of serine/threonine kinases consisting of PIM1, PIM2, and PIM3. Although the Pim genes are evolutionarily highly conserved, the contribution of PIM proteins to mammalian development is unclear. PIM1-deficient mice were previously described but showed only minor phenotypic aberrations. To assess the role of PIM proteins in mammalian physiology, compound Pim knockout mice were generated. Mice lacking expression of Pim1, Pim2, and Pim3 are viable and fertile. However, PIM-deficient mice show a profound reduction in body size at birth and throughout postnatal life. In addition, the in vitro response of distinct hematopoietic cell populations to growth factors is severely impaired. In particular, PIM proteins are required for the efficient proliferation of peripheral T lymphocytes mediated by synergistic T-cell receptor and interleukin-2 signaling. These results indicate that members of the PIM family of proteins are important but dispensable factors for growth factor signaling.

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.

Intermolecular and intramolecular interactions regulate catalytic activity of myotonic dystrophy kinase-related Cdc42-binding kinase alpha

Myotonic dystrophy kinase-related Cdc42-binding kinase (MRCK) is a Cdc42-binding serine/threonine kinase with multiple functional domains. We had previously shown MRCKalpha to be implicated in Cdc42-mediated peripheral actin formation and neurite outgrowth in HeLa and PC12 cells, respectively. Here we demonstrate that native MRCK exists in high-molecular-weight complexes. We further show that the three independent coiled-coil (CC) domains and the N-terminal region preceding the kinase domain are responsible for intermolecular interactions leading to MRCKalpha multimerization. N terminus-mediated dimerization and consequent transautophosphorylation are critical processes regulating MRCKalpha catalytic activities. A region containing the two distal CC domains (CC2 and CC3; residues 658 to 930) was found to interact intramolecularly with the kinase domain and negatively regulates its activity. Its deletion also resulted in an active kinase, confirming a negative autoregulatory role. We provide evidence that the N terminus-mediated dimerization and activation of MRCK and the negative autoregulatory kinase-distal CC interaction are two mutually exclusive events that tightly regulate the catalytic state of the kinase. Disruption of this interaction by a mutant kinase domain resulted in increased kinase activity. MRCK kinase activity was also elevated when cells were treated with phorbol ester, which can interact directly with a cysteine-rich domain next to the distal CC domain. We therefore suggest that binding of phorbol ester to MRCK releases its autoinhibition, allowing N-terminal dimerization and subsequent kinase activation.

cDNA cloning, sequencing and characterization of bovine pim-1

The cDNA clone of bovine pim-1 has been isolated from phorbol-12-myristate-13-acetate (PMA) and concanavalin A (ConA)-activated peripheral blood lymphocytes (PBLs). The full-length cDNA contains a 411bp 5' untranslated region (5'-UTR), followed by a 939bp coding region and a 3' untranslated region (3'-UTR) that contains 1403bp. Comparison of the bovine pim-1 coding sequence with the human, rat, mouse, frog and zebrafish counterparts reveals 94, 90, 89, 67 and 40% homology at the nucleotide level, respectively. The predicted amino acid sequence of bovine Pim-1 shares 98.7, 97.1, 93.3, 68.8, and 52.4% similarity with the sequences of human, rat, mouse, frog, and zebrafish, respectively. The 5'-UTR of bovine pim-1 shares high sequence similarity to the human and mouse counterparts and is G/C-rich (75%) which may promote a high degree of secondary structure. The 3'-UTR of bovine pim-1 contains two potential polyadenylation sites and an A/T-rich motif which has been shown to decrease the stability of polyA mRNA molecules. Southern blot results indicate that a single copy of the gene exists in the bovine genome. Northern blot results show that PMA stimulation of PBLs increases the expression of the pim-1 mRNA. In addition, examination of Pim-1 protein expression in PBLs stimulated with a variety of mitogens including ConA, PMA, anti-CD3 and purified protein derivative (PPD) from Mycobacterium tuberculosis, reveals two different types of expression patterns during the course of a 24h period of stimulation. ConA and PPD gave a biphasic pattern of expression while PMA and anti-CD3 gave single transient pattern of expression suggesting that expression is controlled by more than one signaling pathway.

PAP-1, a novel target protein of phosphorylation by pim-1 kinase

Protooncogene, pim-1, has been reported to be a predisposition for lymphomagenesis along with myc, and its protein product, Pim-1, has been shown to be a serine/threonine protein kinase, whose activity is involved in proliferation and differentiation of blood cells. The signal transduction pathways neither to nor from Pim-1, however, have been clarified. We have cloned a cDNA encoding a novel Pim-1 binding protein, PAP-1, comprising 213 amino acids with a basic amino-acid cluster near the C-terminus. PAP-1 was colocalized with Pim-1 in human HeLa cell nuclei. The in vitro binding assays using GST fusion proteins of the wild-type and various deletion mutants revealed that the whole molecule of Pim-1 is required for the binding activity to PAP-1 and that Pim-1 binds to the region from amino-acid numbers 1-147 of PAP-1, or to two segments in the region. The association of PAP-1 with Pim-1 was also shown in vivo in transfected cells. Furthermore, PAP-1 was phosphorylated in vitro by Pim-1, but not a kinase-negative Pim-1 mutant. The two serine residues of PAP-1 at amino acids 204 and 206 near the C-terminus were phosphorylated by Pim-1. PAP-1 is thus thought to be a target protein for Pim-1 kinase.

The salt-inducible kinase, SIK, is induced by depolarization in brain

Membrane depolarization of neurons is thought to lead to changes in gene expression that modulate neuronal plasticity. We used representational difference analysis to identify a group of cDNAs that are induced by membrane depolarization or by forskolin, but not by neurotrophins or growth factors, in PC12 pheochromocytoma cells. One of these genes, SIK (salt-inducible kinase), is a member of the sucrose-nonfermenting 1 protein kinase/AMP-activated protein kinase protein kinase family that was also recently identified from the adrenal gland of rats treated with high-salt diets. SIK mRNA is induced up to eightfold in specific regions of the hippocampus and cortex in rats, following systemic kainic acid administration and seizure induction.

KID-1, a protein kinase induced by depolarization in brain

Membrane depolarization leads to changes in gene expression that modulate neuronal plasticity. Using representational difference analysis, we have identified a previously undiscovered cDNA, KID-1 (kinase induced by depolarization), that is induced by membrane depolarization or forskolin, but not by neurotrophins or growth factors, in PC12 pheochromocytoma cells. KID-1 is an immediate early gene that shares a high degree of sequence similarity with the family of PIM-1 serine/threonine protein kinases. Recombinant KID-1 fusion protein is able to catalyze both histone phosphorylation and autophosphorylation. KID-1 mRNA is present in a number of unstimulated tissues, including brain. In response to kainic acid and electroconvulsive shock-induced seizures, KID-1 is induced in specific regions of the hippocampus and cortex.

Effect of mutating the regulatory phosphoserine and conserved threonine on the activity of the expressed catalytic domain of Acanthamoeba myosin I heavy chain kinase

Phosphorylation of Ser-627 is both necessary and sufficient for full activity of the expressed 35-kDa catalytic domain of myosin I heavy chain kinase (MIHCK). Ser-627 lies in the variable loop between highly conserved residues DFG and APE at a position at which a phosphorylated Ser/Thr also occurs in many other Ser/Thr protein kinases. The variable loop of MIHCK contains two other hydroxyamino acids: Thr-631, which is conserved in almost all Ser/Thr kinases, and Thr-632, which is not conserved. We determined the effects on the kinase activity of the expressed catalytic domain of mutating Ser-627, Thr-631, and Thr-632 individually to Ala, Asp, and Glu. The S627A mutant was substantially less active than wild type (wt), with a lower kcat and higher Km for both peptide substrate and ATP, but was more active than unphosphorylated wt. The S627D and S627E mutants were also less active than phosphorylated wt, i.e., acidic amino acids cannot substitute for phospho-Ser-627. The activity of the T631A mutant was as low as that of the S627A mutant, whereas the T632A mutant was as active as phosphorylated wt, indicating that highly conserved Thr-631, although not phosphorylated, is essential for catalytic activity. Asp and Glu substitutions for Thr-631 and Thr-632 were inhibitory to various degrees. Molecular modeling indicated that Thr-631 can hydrogen bond with conserved residue Asp-591 in the catalytic loop and that similar interactions are possible for other kinases whose activities also are regulated by phosphorylation in the variable loop. Thus, this conserved Thr residue may be essential for the activities of other Ser/Thr protein kinases as well as for the activity of MIHCK.