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Tumor Biology |
McKusick-Nathans Institute of Genetic Medicine, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21287-3914
| ABSTRACT |
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protein expression in response to PGE2 stimulation. Inhibition of C-SRC tyrosine kinase activity also blocks PGE2-induced HIF-1
protein and VEGF mRNA expression without blocking ERK phosphorylation. In contrast, phosphorylation of AKT is dependent on ERK and C-SRC activity. Thus, the activity of multiple signal transduction pathways is required for the HIF-1-mediated induction of VEGF expression in colon cancer cells exposed to PGE2. | INTRODUCTION |
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The mechanism by which COX2-generated PGE2 induces VEGF expression in colon cancer cells has not been determined. VEGF expression is also induced in colon cancer cells by other stimuli, including hypoxia, activation of the IGF-1R, or p53 loss-of-function, and, in these cases, transcriptional activation of the VEGF gene is mediated by HIF-1 (16
, 17)
. HIF-1 is a heterodimeric protein consisting of a constitutively expressed HIF-1ß subunit and a HIF-1
subunit, the expression of which is regulated by the cellular O2 concentration and diverse signal transduction pathways leading from cell surface receptors to kinase cascades (18)
. Under hypoxic conditions, the O2-dependent hydroxylation of proline residues 402 and 564, which is required for ubiquitination and proteasomal degradation of HIF-1
, is inhibited (19, 20, 21)
. In contrast, activation of the PI3K and ERK pathways after ligand binding to the IGF-1R stimulates increased synthesis of HIF-1
protein (16)
. HIF-1
overexpression has been demonstrated in human colon cancer biopsies (22
, 23)
, and forced overexpression of HIF-1
in human colon carcinoma HCT116 cells increases tumor growth and angiogenesis in nude mice (17)
.
In the present study, we investigated the mechanism by which PGE2 exposure induces VEGF gene expression in HCT116 human colon carcinoma cells. We demonstrate that PGE2 induces expression of HIF-1
protein and VEGF mRNA and that inhibition of HIF-1
expression by RNA interference blocks the induction of VEGF mRNA. We also provide evidence that the activity of multiple signal transduction pathways is required for the HIF-1-mediated induction of VEGF expression in colon cancer cells exposed to PGE2.
| MATERIALS AND METHODS |
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PGE2 and Inhibitor Treatments.
HCT116 cells were plated at a density of 2.5 x 106/10-cm or 8.6 x 105/6-cm dish. Subconfluent cells were serum starved (0.1% FBS) for 24 h before PGE2 or 17-pt-PGE2 was added. Kinase inhibitors PD98059, Wortmannin, rapamycin, and SB203580 were added 1 h before exposure to PGE2, 1% O2, or 100 µM CoCl2. JNK inhibitor and PP2 were added 20 min and 2 h, respectively, before the exposure to PGE2 or 1% O2. EP1 receptor antagonist SC-51322 was added 30 min before exposure to PGE2. Cycloheximide was added to the media of HCT116 cells that had been serum starved and treated with CoCl2 for 4 h or PGE2 for 24 h, and whole cell extracts were prepared at 0, 20, and 40 min.
Immunoblot Assays.
Whole cell extracts were prepared using radioimmunoprecipitation assay buffer, fractionated by SDS-PAGE, and transferred to a nitrocellulose filter. For HIF-1
and HIF-1ß, 150-µg aliquots of protein were analyzed using a monoclonal antibody against HIF-1
(H1
67; Ref. 23
) or HIF-1ß (H1ß234; Novus Biologicals, Littleton, CO; Ref. 24
) at 1:1000 dilution (16)
. Aliquots (50 µg) were analyzed using antibodies (1:1000 dilution) specific for phosphorylated (Thr202/Tyr204) or total p44/p42 MAP kinase and phosphorylated (Ser473) or total AKT (Cell Signaling Technology, Beverly, MA, and Santa Cruz Biotechnology, Santa Cruz, CA). Horseradish peroxidase-conjugated secondary antibodies against mouse and rabbit IgG (1:2500 dilution) and enhanced chemiluminescence reagents were from Amersham Biosciences (Piscataway, NJ).
RT-PCR Assays.
Total RNA was extracted from HCT116 cells using TRIzol reagent (Invitrogen Corp.). Aliquots (5 µg) of RNA were reverse transcribed to cDNA using Superscribe First-Strand Synthesis System (Invitrogen Corp.). Aliquots (1, 2, and 4 µl) of cDNA were used as template for PCR of HIF-1
, VEGF, and 18S rRNA sequences. The following oligonucleotides were used as primers: (a) 5'-GGGAGAAAATCAAGTCGTGC-3' and 5'-AGCAAGGAGGGCCTCTGATG-3' (HIF-1
); (b) 5'-TACCTCCACCATGCCAAGTG-3' and 5'-AAGATGTCCACCAGGGTCTC-3 (VEGF); and (c) 5'-ATCCTGCCAGTAGCATATGC-3' and 5'-ACCGGGTTGGTTTTGATCTG-3' (18S rRNA). Thermocycling conditions were 30 s at 95°C, 30 s at 55°C, and 30 s at 72°C for 25 (HIF-1
), 27 (VEGF), or 13 (18S rRNA) cycles.
RNA Interference.
To generate siRNAHIF-1
, two oligonucleotides consisting of ribonucleosides, except for the presence of 2'-deoxyribonucleosides at the 3' end (5'-AGAGGUGGAUAUGUGUGGGdTdT-3' and 5'-CCCACACAUAUCCACCUCUdTdT-3'), were synthesized and annealed (Dharmacon Research, Inc., Lafayette, CO). HCT116 cells were plated at 2 x 106 cells/10-cm dish and exposed to 100 nM siRNAHIF-1
in the presence of Oligofectamine (Invitrogen Corp.) for 4 h and then cultured for 24 h in complete media (25)
. Cells were serum starved for another 24 h and exposed to 100 µM PGE2 or vehicle for 24 h, and total RNA was isolated for RT-PCR analysis of HIF-1
and VEGF mRNA. As control, cells were exposed to Oligofectamine without siRNAHIF-1
. Neither mock transfection nor transfection with an siRNA targeted to an irrelevant mRNA inhibited HIF-1
mRNA or protein expression (26)
.
| RESULTS |
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protein expression was induced with peak levels observed 1832 h after the addition of PGE2 to the culture media (Fig. 1A)
protein expression is observed 8 h after IGF-1 addition to HCT116 cells (16)
. Thus, compared with IGF-1, PGE2 induces HIF-1
protein expression with delayed kinetics. HIF-1
mRNA expression was not induced by PGE2 stimulation (Fig. 1A)
protein levels resulted from either increased protein synthesis or decreased protein degradation. VEGF mRNA expression was induced by PGE2 with kinetics similar to those observed for HIF-1
protein expression (Fig. 1A)
protein and VEGF mRNA expression in a dose-dependent manner, although the responses were modest in comparison with the responses induced by hypoxia (Fig. 1B)
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protein expression in HCT116 cells (Fig. 2A)
expression in a dose-dependent manner (Fig. 2B)
expression in HCT116 cells.
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protein levels result from a decreased rate of degradation and an increased rate of synthesis, respectively (16)
. To determine which of these mechanisms is involved in PGE2-induced HIF-1
expression, cells were cultured for 24 h in the presence of PGE2 or CoCl2, which inhibits HIF-1
degradation, similar to the effect of hypoxia. The half-life of HIF-1
protein was >40 min in CoCl2-treated cells but <20 min in PGE2-treated cells (Fig. 3)
protein and therefore must stimulate HIF-1
protein synthesis, similar to the effect of IGF-1.
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protein and VEGF mRNA that was induced by PGE2 (Fig. 4A)
protein and VEGF mRNA expression. Neither the inducers or inhibitors had any consistent effect on the expression of HIF-1ß protein or HIF-1
mRNA (Fig. 4A)
protein expression in a dose-dependent manner (Fig. 4B)
protein expression (Fig. 4C)
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protein expression induced by PGE2 but not by hypoxia (Fig. 4D)
protein expression (Fig. 4AD)
The data presented above (Figs. 1, A and B
and 4A)
demonstrate a remarkably consistent correlation between HIF-1
protein and VEGF mRNA expression in cells treated with various kinase inhibitors and/or PGE2. To further demonstrate that HIF-1
is required for the induction of VEGF mRNA expression in response to PGE2, HCT116 cells were mock transfected or transfected with a small interfering RNA (siRNAHIF-1
) that targets HIF-1
mRNA for degradation. After transfection, the cells were serum starved and exposed to vehicle or PGE2. As expected, HIF-1
mRNA levels were reduced in cells transfected with siRNAHIF-1
(Fig. 5)
. VEGF mRNA levels were slightly reduced in siRNAHIF-1
-transfected cells treated with vehicle and dramatically reduced in PGE2-treated cells. Thus, inhibition of HIF-1
expression is sufficient to block PGE2-induced VEGF mRNA expression.
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| DISCUSSION |
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protein and VEGF mRNA expression (16)
. We demonstrate that, unlike hypoxia, PGE2 treatment is not associated with an increase in the half-life of HIF-1
protein. Our data suggest that, as in the case of IGF-1, PGE2 increases the rate of HIF-1
protein synthesis. The induction of HIF-1
protein and VEGF mRNA expression in PGE2-treated cells is completely or partially blocked by inhibitors of MEK and PI3K, respectively, as was also observed in IGF-1-treated HCT116 cells (16)
. Inhibition of C-SRC tyrosine kinase activity also blocks the induction of HIF-1
protein and VEGF mRNA expression in PGE2-treated cells without blocking ERK phosphorylation. Our data indicate that the signal transduction from PGE2 receptor binding to HIF-1
expression is complex, and additional studies are required to determine the mechanisms and consequences of ERK, PI3K, and C-SRC activation in PGE2-treated HCT116 cells.
Although HIF-1
protein and VEGF mRNA expression are induced in HCT116 cells exposed to either IGF-1 or PGE2, there are several notable differences in the respective signal-transduction pathways: (a) in contrast to IGF-1, PGE2 induces ERK phosphorylation and HIF-1
expression with delayed kinetics, which suggests a requirement for gene expression; and (b) AKT phosphorylation is dependent on ERK activation in PGE2-treated HCT116 cells but independent of ERK activation in IGF-1-treated cells. PGE2-induced AKT phosphorylation is also dependent on C-SRC tyrosine kinase activity. Induction of HIF-1
protein and VEGF mRNA expression in V-SRC-transfected rodent cells has been demonstrated previously (27)
.
While this study was in preparation, the induction of HIF-1
expression by PGE2 treatment of PC-3ML human prostate cancer cells was reported (28)
. HIF-1
expression was induced by EP2 and EP4 but not EP3 receptor-selective agonists, whereas the EP1 receptor was not expressed. PD98059 inhibited PGE2-induced HIF-1
expression. However, HIF-1
expression was induced within 4 h in PGE2-treated PC-3ML cells (28)
. These results suggest that different signal transduction pathways are activated by PGE2 in prostate and colon cancer cells, although MEK-ERK activation appears to play an essential role in both cases. The effect of PGE2 in PC-3ML cells was attributed to stabilization of HIF-1
protein, but no data were presented to support this conclusion. Additional studies are required to address this issue.
VEGF expression is induced in colon and other cancer cells as a result of hypoxia and multiple genetic alterations, including p53 and PTEN loss-of-function, RAS and SRC gain-of-function, and autocrine tyrosine kinase signaling pathways involving epidermal growth factor receptor, HER2neu, and IGF-1R (29, 30, 31, 32, 33, 34, 35)
. In each case, VEGF gene expression is activated by HIF-1 (16
, 17
, 27
, 36, 37, 38, 39)
. Based on the analysis of prostate cancer cells recently reported (28)
and colon cancer cells presented in this study, this list can now be extended to include the increased VEGF expression resulting from COX2-generated PGE2. COX2 inhibitors, either alone or in combination with traditional cancer therapies such as radiation, have antiangiogenic effects (5
, 40)
. As in the case of the tyrosine kinase-signaling pathways described above, the antiangiogenic effects of COX2 inhibitors appear to be attributable in part to their inhibition of HIF-1
expression (28
, 41)
. Efforts to identify small molecules that directly inhibit HIF-1 activity are under way (42)
, and such compounds may represent useful additions to the armamentarium of anticancer agents that target signal transduction pathways and angiogenesis.
| FOOTNOTES |
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1 Supported by NIH grants (to G. L. S.). ![]()
2 To whom requests for reprints should be addressed, at The Johns Hopkins University School of Medicine, CMSC-1004, 600 North Wolfe Street, Baltimore, MD 21287-3914. Fax: (410) 955-0484; E-mail: gsemenza{at}jhmi.edu ![]()
3 The abbreviations used are: VEGF, vascular endothelial growth factor; IGF, insulin-like growth factor; ERK, extracellular signal-regulated kinase; COX2, cyclooxygenase 2; HIF-1, hypoxia-inducible factor 1; PI3K, phosphatidylinositol-3-kinase; 17-pt-PGE2, 17-phenyl-trinor-prostaglandin E2; IGF-1R, insulin-like growth factor-1 receptor; MEK, mitogen-activated protein/extracellular signal-regulated kinase kinase; JNK, c-Jun NH2-terminal kinase; PGE2, prostaglandin E2; MAP, mitogen-activated protein; RT-PCR, reverse transcription-PCR. ![]()
Received 12/18/02. Accepted 3/ 4/03.
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M. A. Hull, S. C.W. Ko, and G. Hawcroft Prostaglandin EP receptors: Targets for treatment and prevention of colorectal cancer? Mol. Cancer Ther., August 1, 2004; 3(8): 1031 - 1039. [Abstract] [Full Text] [PDF] |
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N. Wakisaka, S. Kondo, T. Yoshizaki, S. Murono, M. Furukawa, and J. S. Pagano Epstein-Barr Virus Latent Membrane Protein 1 Induces Synthesis of Hypoxia-Inducible Factor 1{alpha} Mol. Cell. Biol., June 15, 2004; 24(12): 5223 - 5234. [Abstract] [Full Text] [PDF] |
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N. Miki, M. Ikuta, and T. Matsui Hypoxia-induced Activation of the Retinoic Acid Receptor-related Orphan Receptor {alpha}4 Gene by an Interaction between Hypoxia-inducible Factor-1 and Sp1 J. Biol. Chem., April 9, 2004; 279(15): 15025 - 15031. [Abstract] [Full Text] [PDF] |
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N. E. Sounni, C. Roghi, V. Chabottaux, M. Janssen, C. Munaut, E. Maquoi, B. G. Galvez, C. Gilles, F. Frankenne, G. Murphy, et al. Up-regulation of Vascular Endothelial Growth Factor-A by Active Membrane-type 1 Matrix Metalloproteinase through Activation of Src-Tyrosine Kinases J. Biol. Chem., April 2, 2004; 279(14): 13564 - 13574. [Abstract] [Full Text] [PDF] |
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G. Hopfl, O. Ogunshola, and M. Gassmann HIFs and tumors--causes and consequences Am J Physiol Regulatory Integrative Comp Physiol, April 1, 2004; 286(4): R608 - R623. [Abstract] [Full Text] [PDF] |
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G.-i. Kusakai, A. Suzuki, T. Ogura, S.'i. Miyamoto, A. Ochiai, M. Kaminishi, and H. Esumi ARK5 Expression in Colorectal Cancer and Its Implications for Tumor Progression Am. J. Pathol., March 1, 2004; 164(3): 987 - 995. [Abstract] [Full Text] [PDF] |
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K. Kasuno, S. Takabuchi, K. Fukuda, S. Kizaka-Kondoh, J. Yodoi, T. Adachi, G. L. Semenza, and K. Hirota Nitric Oxide Induces Hypoxia-inducible Factor 1 Activation That Is Dependent on MAPK and Phosphatidylinositol 3-Kinase Signaling J. Biol. Chem., January 23, 2004; 279(4): 2550 - 2558. [Abstract] [Full Text] [PDF] |
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T J Jang, S K Min, J D Bae, K H Jung, J I Lee, J R Kim, and W S Ahn Expression of cyclooxygenase 2, microsomal prostaglandin E synthase 1, and EP receptors is increased in rat oesophageal squamous cell dysplasia and Barrett's metaplasia induced by duodenal contents reflux Gut, January 1, 2004; 53(1): 27 - 33. [Abstract] [Full Text] [PDF] |
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N Raghunand, R A Gatenby, and R J Gillies Microenvironmental and cellular consequences of altered blood flow in tumours Br. J. Radiol., December 1, 2003; 76(suppl_1): S11 - S22. [Abstract] [Full Text] [PDF] |
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