Pim1 promotes human prostate cancer cell tumorigenicity and c-MYC transcriptional activity
© Kim et al; licensee BioMed Central Ltd. 2010
Received: 23 September 2009
Accepted: 1 June 2010
Published: 1 June 2010
The serine/threonine kinase PIM1 has been implicated as an oncogene in various human cancers including lymphomas, gastric, colorectal and prostate carcinomas. In mouse models, Pim1 is known to cooperate with c-Myc to promote tumorigenicity. However, there has been limited analysis of the tumorigenic potential of Pim1 overexpression in benign and malignant human prostate cancer cells in vivo.
We overexpressed Pim1 in three human prostate cell lines representing different disease stages including benign (RWPE1), androgen-dependent cancer (LNCaP) and androgen-independent cancer (DU145). We then analyzed in vitro and in vivo tumorigenicity as well as the effect of Pim1 overexpression on c-MYC transcriptional activity by reporter assays and gene expression profiling using an inducible MYC-ER system. To validate that Pim1 induces tumorigenicity and target gene expression by modulating c-MYC transcriptional activity, we inhibited c-MYC using a small molecule inhibitor (10058-F4) or RNA interference.
Overexpression of Pim1 alone was not sufficient to convert the benign RWPE1 cell to malignancy although it enhanced their proliferation rates when grown as xenografts in vivo. However, Pim1 expression enhanced the in vitro and in vivo tumorigenic potentials of the human prostate cancer cell lines LNCaP and DU145. Reporter assays revealed increased c-MYC transcriptional activity in Pim1-expressing cells and mRNA expression profiling demonstrated that a large fraction of c-MYC target genes were also regulated by Pim1 expression. The c-MYC inhibitor 10058-F4 suppressed the tumorigenicity of Pim1-expressing prostate cancer cells. Interestingly, 10058-F4 treatment also led to a reduction of Pim1 protein but not mRNA. Knocking-down c-MYC using short hairpin RNA reversed the effects of Pim1 on Pim1/MYC target genes.
Our results suggest an in vivo role of Pim1 in promoting prostate tumorigenesis although it displayed distinct oncogenic activities depending on the disease stage of the cell line. Pim1 promotes tumorigenicity at least in part by enhancing c-MYC transcriptional activity. We also made the novel discovery that treatment of cells with the c-MYC inhibitor 10058-F4 leads to a reduction in Pim1 protein levels.
Pim1 is a constitutively active serine/threonine kinase , whose activity is therefore primarily regulated at the level of expression and stability. Pim1 enhances cell cycle progression by phosphorylating Cdc25A, Cdc25C, p21cip1, p27kip1 and c-Tak1 [2–5] or by associating with protein complexes required for mitosis . Pim1 also inhibits apoptosis by phosphorylating apoptotic proteins including Bad , FOXO3a  and ASK1 . PIM1 has been implicated as an oncogene whose expression is dysregulated in several human cancers including lymphomas, gastric, colorectal and prostate cancers .
The oncogenic activity of Pim1 was first discovered in lymphomagenesis. PIM1 was identified as a non-immunoglobulin (IG)/BCL6 translocation partner gene and 6p21, its chromosomal locus, was amplified in B-cell lymphomas [10, 11]. PIM1 is also known to be a target locus for aberrant somatic hypermutation in some lymphomas [12–15]. Eμ-Pim1 transgenic mice engineered to overexpress Pim1 in lymphocytes develop T cell lymphomas and cooperate with another proto-oncogene Myc to accelerate the disease progression [16–18].
In human prostate cancer, PIM1 expression is known to be elevated in ~50% of human prostate cancer specimens and its cooperation with MYC was also proposed . Prostate cancer induced by mouse prostate-specific overexpression of c-MYC oncogene demonstrated Pim1 mRNA upregulation, suggesting possible synergistic effect between two oncogenes . However, the oncogenic activity of Pim1 itself in prostate cancer using in vivo models has not been fully characterized. One study used PC3 human prostate carcinoma cells to show that Pim1 overexpression accelerates tumorigenicity in these cells associated with elevated levels of c-MYC and the phosphorylation of proteins involved in protein synthesis . Here we sought to determine the effects of Pim1 overexpression on the tumorigenic potential of human prostate cells representing distinct stages of disease progression, including benign/non-tumorigenic, tumorigenic/androgen-sensitive and tumorigenic/androgen-independent stages. Using these cells, we analyzed the effects of Pim1 on in vitro and in vivo tumorigenicity as well as c-MYC transcriptional activity.
Cell lines and cell culture
Cell lines were obtained from American Type Culture Collection. Vector control, Pim1 or kinase dead mutant Pim1 (K67M)-overexpressing cells were generated as described . pBabe-Puro-MYC-ER plasmid (gift from Dr. Gerard Evan, University of California at San Francisco, CA, USA) was used to generate retroviruses and infect RWPE1-Neo and RWPE1-Pim1 cells to generate RWPE1-Neo/MYC-ER and RWPE1-Pim1/MYC-ER cells and the cells were maintained as described . To activate c-MYC in chimeric MYC-ER protein, 100 nM of 4-hydroxytamoxifen (4OHT) in ethanol was added to the cells. LNCaP and DU145 cells were maintained in RPMI with 10% fetal bovine serum.
Western blot analyses
Western blotting was performed as described  using following antibodies: anti-Pim1 (mouse, 1:500, Santa Cruz), anti-beta-Actin (goat, 1:1000, Santa Cruz), anti-phospho-p21 (rabbit, 1:1000, Santa Cruz), anti-p21 (mouse, 1:1000, Santa Cruz), anti-cyclin E (rabbit, 1:1000, Santa Cruz), anti-androgen receptor (rabbit, 1:500, Santa Cruz) and anti-c-Myc (mouse, 1:500, Santa Cruz).
Cell cycle analysis
Cell cycle was analyzed by fluorescence-activated cell sorting (FACS) as described .
MTT cell proliferation assay
MTT Cell Proliferation Assay Kit was purchased from ATCC and assay was performed in accordance with the manufacturer's protocol. Individual absorbance was measured with plate photometer (Bio-tek).
Soft agar assays
These were performed as described . 100 × 103 and 20 × 103 of LNCaP and DU145 cells were used, respectively and total number of colonies (≥ cutoff sizes) was counted. For c-Myc inhibitor (10058-F4) treatment, 50 × 103 of LNCaP and DU145 were used and 100 uM 10058-F4 in 0.25% DMSO or 0.25% DMSO alone was added. Three random low power view-fields were selected and each number of colonies (≥ cutoff sizes) was added.
Xenografts in nude mice
Male nude (nu/nu) mice were obtained from Charles River laboratories. 3 × 106, 3 × 106 and 5 × 106 of RWPE1, LNCaP and DU145 cells were mixed with 200 μl of Matrigel (BD Biosciences), respectively. Cells were injected subcutaneously in both flanks of nude mice. Tumor volumes and cross-sectional areas were calculated as described [26, 27]. Animal care and experiments were carried out according to the protocols approved by the Institutional Animal Care and Use Committees at Vanderbilt University.
Histology and immunohistochemical analyses
Nude mice were sacrificed after 30-38 (RWPE1) or 6-8 (LNCaP and DU145) weeks. Xenografted lesions were taken, photographed, fixed in formalin (10%) and embedded in paraffin for subsequent histology as described . Immunohistochemical analyses were performed as described  using following antibodies: activated Caspase 3 (rabbit, 1:500, Cell Signaling), Ki67 (rabbit, 1:50, abcam) and phospho-Histone H3 (rabbit, 1:500, Upstate).
Analysis of androgen-dependent proliferation
15,000 LNCaP cells were plated on 24 well plates. The next day, cells were washed with phosphate-buffered saline (PBS) and phenol red-free RPMI media with charcoal-striped serum was added for androgen starvation. After 2-day starvation, DHT (5α-Dihydrotestosterone) or carrier (ethanol) was added. MTT assays were performed from next day (Day 1). MTT values of DHT-treated cells were divided by those of carrier-untreated cells and triplicate data per group were analyzed.
DHT treatment and quantitative RT-PCR
For DHT treatment, LNCaP cells were washed with phosphate-buffered saline (PBS) and phenol red-free RPMI media with charcoal-striped serum was added for androgen starvation. After 2-day starvation, DHT (0.1, 1, 10 and 100 nM) or carrier (ethanol) was added and the cells were incubated for 2 days. Total RNA isolation, reverse transcription and subsequent quantitative PCR were performed as described . The following primers were used: Prostate specific antigen (PSA) forward (5'-CAA CCC TGG ACC TCA CAC CTA-3'), PSA reverse (5'-GGA AAT GAC CAG GCC AAG AC-3'); human GAPDH forward (5'-ATG GAA ATC CCA TCA CCA TCT T-3'), human GAPDH reverse (5'-CGC CCC ACT TGA TTT TGG-3'); LAMC2 forward (5'-GGA TGA GAA TCC TGA CAT TGA GTG T-3'), LAMC2 reverse (5'-GTC GTG CGG ATC GTT GTA GA-3'); MT1F forward (5'-ACC TGC CCC ACT GCT TCT T-3'), MT1F reverse (5'-TTG CAA GCC GAG GAG AGA CT-3'); UPP1 forward (5'-TCT GGA GGC AGC CTA TGC A-3'), UPP1 reverse (5'-GCA AAC ACC GAG GAC TCC AT-3'); CDKN1C forward (5'-GCC TCT GAT CTC CGA TTT CTT C-3'), CDKN1C reverse (5'-CAT CGC CCG ACG ACT TCT-3'); CUL3 forward (5'-AGA TTT TGA GGC TCC TTT TTT GG-3'), CUL3 reverse (5'-AAA ATT TCT GGC TTT CCA TCT GAA-3'); SOD2 forward (5'-GCT GCA CCA CAG CAA GCA-3'), SOD2 reverse (5'-TCG GTG ACG TTC AGG TTG TTC-3'); VAV3 forward (5'-CTG GTG AAC AAG GGA CAC TCA A-3'), VAV3 reverse (5'-TTT AGG AGT TCT TCG CAG TCC ATT-3'); mouse Pim1 (5'-ATT CCG TTT GAG CAC GAT GAA-3'), mouse Pim1 reverse (5'-TGA AGA GAC AGT TTG CCT GAA GAA-3'). Each mRNA expression was normalized with GAPDH expression.
300,000 RWPE1 cells were plated on 12 well plates and grown without supplements (bovine pituitary extract and epidermal growth factor) for 24 hours. Transient transfection was performed with the following plasmids: c-Myc-responsive 4× E-box reporter (gift from Dr. Stephen Hann, Vanderbilt University, TN, USA), pMSCV-MYC and pMSCV empty vector. After 30 hours, cell lysates were harvested and luciferase activity was measured using luciferase assay system (Promega). Luciferase activity was normalized to protein concentration and triplicate data per group were analyzed.
c-Myc inhibitor treatment
c-Myc inhibitor (10058-F4) was purchased from Sigma. Cells were plated on 6-well plates and 50, 100 and 200 uM 10058-F4 in 0.5% DMSO or 0.5% DMSO alone was added on next day when cells were sub-confluent. Cells were harvested after 20 hours and RT-PCR and western blotting were performed from the isolated mRNA and cell lysates, respectively.
c-Myc gene silencing with small hairpin RNA (shRNAmir)
Control and Pim1-expressing LNCaP cells were plated on 60 mm dishes. Lentiviral shRNAmirs (Open Biosystems) against c-MYC and pGIPZ control plasmid were transiently transfected. Transfection efficiency was monitored by GFP fluorescence. After 3 days, cells were harvested, and mRNA isolation followed by reverse transcription and qRT-PCR and western blotting were performed.
Total RNA was isolated from RWPE1-Neo/MYC-ER and RWPE1-Pim1/MYC-ER cells with and without 24 hr 4OHT treatment. Genechip analysis was performed in duplicates according to manufacturer's protocol (Affymetrix) using U133A chips. Genes whose expressions were altered at least 1.4-fold with significance (P < 0.005) in pairwise comparisons were identified.
Each group was compared using t-test. Values are considered statistically significant at P < 0.05. Quantitative variables are expressed as means + standard deviation while categorical variables are expressed as numbers (%).
Generation of human prostate cell lines at different disease stages, with stable Pim1 overexpression
Pim1 has been reported to induce polyploidy and chromosomal instability (CIN) in a passage-dependent manner [22, 24] and the resultant polyploidy/CIN was shown to increase in vitro and in vivo tumorigenicity . We therefore examined the cell cycle profiles of the cells we used in the tumorigenicity assays in the current study to ensure that we utilize cells that have not progressed to polyploidy. As shown in Figure 1B, there were no differences in ploidy between control and Pim1-expressing cells and this is probably due to relatively early passage number (RWPE1, passage 18; LNCaP, passage 27; DU145, passage 23).
Pim1 promotes proliferation and attenuates apoptosis of RWPE1 cells in vivo without malignant conversion
Pim1 promotes in vitro and in vivotumorigenicity of LNCaP and DU145 cells
Androgen receptor signaling does not affect in vitrocellular proliferation but its transcriptional activity is induced by Pim1
Pim1 enhances c-MYC transcriptional activity
Pim1 tumorigenic function has been intimately linked to c-Myc in several systems. Pim1 is thought to cooperate with c-Myc to promote tumorigenicity by increasing c-Myc protein expression [21, 37] or stimulating the binding of RNA polymerase II leading to increase the transcription of c-Myc target genes . To assess c-MYC transcriptional activity in Pim1-expressing prostate cells we used a c-Myc-responsive 4x-Ebox reporter in luciferase assays. RWPE1-Pim1 cells demonstrated higher c-MYC reporter activity compared to RWPE1-Neo cells (Figure 2A). Furthermore, activity of the c-Myc reporter was suppressed in RWPE1-K67M cells consistent with a dominant negative function of the kinase-dead mutant Pim1 (K67M). This dominant negative action of the K67M mutant is again evident as repression of expression of Cyclin E, a known c-MYC target gene, in RWPE1-K67M cells (Figure 2B).
List of common genes altered by Myc induction and by Pim1 expression in RWPE1-MYC-ER cells
Probe Set ID
connective tissue growth factor
X-linked retinopathy protein-like
immediate early response 3
laminin, gamma 2
protease, serine, 3
tissue factor pathway inhibitor 2
uridine phosphorylase 1
acyl-CoA synthetase long-chain family member 1
aldo-keto reductase family 1, member C1
aldo-keto reductase family 1, member C2
aldehyde dehydrogenase 3 family, memberA1
aldehyde dehydrogenase 3 family, member B2
amylase, alpha 1A/1B/1C/2A/2B
apolipoprotein L, 1
aquaporin 3 (Gill blood group)
butyrophilin, subfamily 3, member A2/A3
complement component 1, r subcomponent
complement component 1, r subcomponent-like
complement component 2/complement factor B
carbonic anhydrase XII
carbonic anhydrase II
cyclin-dependent kinase inhibitor 1C (p57, Kip2)
chloride intracellular channel 3
ceroid-lipofuscinosis, neuronal 5
DEAD (Asp-Glu-Ala-Asp) box polypeptide 60
erythrocyte membrane protein band 4.1
intestinal cell (MAK-like) kinase
killer cell lectin-like receptor subfamily C, member1/2
killer cell lectin-like receptor subfamily C, member 3
methyltransferase like 7A
matrix-remodelling associated 5
niacin receptor 2
phospholipase C, beta 4
S100 calcium binding protein A8
serum amyloid A1/A2
spermidine/spermine N1-acetyltransferase 1
serpin peptidase inhibitor, clade B, member 13
superoxide dismutase 2, mitochondrial
single-stranded DNA binding protein 2
ST6 beta-galactosamide alpha-2,6-sialyltranferase 1
tumor necrosis factor, alpha-induced protein 3
tumor necrosis factor (ligand) superfamily, member 10
tripartite motif-containing 22
UDP glucuronosyltransferase 1 family, A1/A3-A10
vav 3 guanine nucleotide exchange factor
c-MYC inhibition by 10058-F4 treatment or RNA interference abrogates in vitro cellular growth and alters target gene expression of Pim1-expressing prostate cancer cells
Remarkably, 10058-F4 also repressed both the endogenous human and transgenic murine Pim1 protein expression in both cell lines (Figure 8B). This effect is not due to a global, non-specific effect on protein expression since neither beta-Actin expression used as a loading control in both cells nor MYC in DU145 cells were changed by inhibitor treatment (Figure 8B). To determine if 10058-F4 treatment affects Pim1 expression at the protein or mRNA levels, we performed RT-PCR to detect the transfected murine Pim1  in LNCaP and DU145 cells. Pim1 mRNA levels were not dramatically changed after inhibitor treatment (Figure 8B), suggesting that 10058-F4 may inhibit Pim1 protein levels via post-translational regulation in addition to c-MYC transcriptional activity.
This novel function of c-MYC inhibitor controlling Pim1 protein expression can not let us examine if decreased tumorigenicity by c-MYC inhibitor is due to inhibited c-MYC activity, repressed Pim1 expression or both. Therefore, to prove that Pim1-induced tumorigenicity is through c-MYC activity, it is necessary to selectively inhibit c-MYC using small hairpin RNA (shRNA). As shown in Figure 8, depending on the levels of c-MYC repression, cells with ~50% c-MYC knock-down (sh1) displayed significant reversal in target gene expression such as LAMC2 and VAV3, but cells with ~25% c-MYC knock-down (sh2) only showed relatively minor change (Figure 8C and 8D). Overall, these results support the notion that MYC is an important mediator for some of the Pim1 effects observed in this model.
Recent studies have increasingly implicated overexpression of the PIM1 kinase in several human tumors, including lymphomas, leukemia's, gastrointestinal, pancreatic and prostate cancers . In human prostate cancer, whether PIM1 plays a role in tumor initiation and/or tumor progression has not been clearly defined. Some studies have found absent or weak expression of PIM1 in most high-grade prostatic intraepithelial neoplasia (HGPIN) lesions, the putative precursor lesion for prostate cancer , consistent with a role for PIM1 in tumor progression, rather than tumor initiation. By contrast, others have noted PIM1 overexpression in a significant fraction of HGPIN lesions [42, 43]. There has been limited in vivo experimental investigation of Pim1 oncogenic activity in the prostate. One study used, PC3, an aggressive androgen-independent human prostate cancer cell line, to examine in vivo tumorigenicity of Pim1. The results demonstrated that tumor growth in Pim1-expressing PC3 cells was accelerated compared to control cells, and this could be partly due to c-MYC protein induction . Our previous study examining tumorigenic potential of Pim1 in the benign human prostate epithelial cell line RWPE1 indicates that Pim1 overexpression alone is not sufficient for the malignant conversion of these cells , a finding that was confirmed in the current study. Rather, time-dependent additional genetic events, propelled by chromosomal instability appeared to be required for Pim1-expressing RWPE1 cells to form tumors in vivo . These conclusions are supported by our recent observations that Pim1 expression resulted in mild pathological alterations in normal adult mouse prostate epithelial cells using a tissue recombination system, but it greatly accelerated the progression of c-MYC-initiated tumors .
Our findings that Pim1 expression could significantly promote tumorigenicity in established human prostate cancer cell lines (LNCaP and DU145) indicate a clear role for Pim1 in tumor progression. The data also showed effects of Pim1 in promoting tumor proliferation and inhibiting apoptosis in vivo. Interestingly, these effects of Pim1 are not always obvious in cells grown on plastic tissue culture plates in vitro. Notably, both LNCaP and DU145 cells express appreciable levels of Pim1, and it is likely that at least part of the mechanism by which Pim1 expression promotes tumorigenicity in these cells is via enhancing c-MYC transcriptional activity. This hypothesis is supported by the results of the luciferase assays showing that the transcriptional activity of c-MYC was significantly enhanced by Pim1 in a kinase-dependent manner, by gene expression profiling using a MYC-ER inducible system, and by the finding that changes in target gene expression were reversed by c-MYC inhibition when small hairpin RNA (shRNA) against c-MYC was introduced.
Our finding that 10058-F4, a small molecule c-Myc inhibitor could target both Pim1 protein expression and c-Myc transcriptional activity in both androgen-dependent and -independent prostate cancer cells is intriguing and of potential clinical significance. Since Pim1 and c-Myc cooperate to promote the development of lymphoma [16–18] and prostate cancer [19–21, 44], targeting both molecules with one drug could dramatically enhance the efficacy of the treatment in cancer patients. However, a limitation of using this particular inhibitor as a drug relates to short half-life because it is rapidly metabolized in vivo . Efforts to improve the efficacy of this compound  could lead to a potentially important way to target MYC/Pim1-expressing cancers.
It is important to determine the roles that specifically relevant oncogenes play in the course of tumorigenesis in vivo in order to aid appropriate therapeutic targeting. We have shown that Pim1 promotes the tumorigenicity of human prostate cells (LNCaP and DU145) partially by enhancing activities of the MYC and AR signaling pathways. Our findings support the targeting of Pim1 in human cancer and suggest a potential for using the same inhibitor (such as 10058-F4) to target both MYC and PIM1 in human tumors.
mutation at Lysine 67 (to Methionine)
4OHT-inducible Myc-Estrogen Receptor fusion protein
Fluorescence-activated cell sorting
American Type Culture Collection
Prostate specific antigen
Hematoxylin and Eosin
Activated Caspase 3
small hairpin RNA
This work was supported by grant number RO1CA123484 from the National Cancer Institute. We thank Dr. Isam-Eldin Eltoum (University of Alabama at Birmingham, Birmingham, AL, USA) for assistance with pathological analysis.
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