Blockade of Tumor Growth Due to Matrix Metalloproteinase-9 Inhibition Is Mediated by Sequential Activation of β1-Integrin, ERK, and NF-κB*

We previously showed that matrix metalloproteinase (MMP)-9 inhibition using an adenovirus-mediated delivery of MMP-9 small interfering RNA (Ad-MMP-9), caused senescence in medulloblastoma cells. Regardless of whether or not Ad-MMP-9 would induce apoptosis, the possible signaling mechanism is still obscure. In this report, we demonstrate that Ad-MMP-9 induced apoptosis in DAOY cells as determined by propidium iodide and terminal deoxynucleotidyltransferase-mediated nick end labeling staining. Ad-MMP-9 infection induced the release of cytochrome c, activation of caspase-9 and -3, and cleavage of poly(ADP-ribose) polymerase. Ad-MMP-9 infection stimulated ERK, and electrophoretic mobility shift assay indicated an increase in NF-κB activation. ERK inhibition, using a kinase-dead mutant for ERK, ameliorated NF-κB activation and caspase-mediated apoptosis in Ad-MMP-9-infected cells. β1-Integrin expression in Ad-MMP-9-infected cells also increased, and this increase was reversed by the reintroduction of MMP-9. We found that the addition of β1 blocking antibodies inhibited Ad-MMP-9-induced ERK activation. Taken together, our results indicate that MMP-9 inhibition induces apoptosis due to altered β1-integrin expression in medulloblastoma. In addition, ERK activation plays an active role in this process and functions upstream of NF-κB activation to initiate the apoptotic signal.

is composed of serine/threonine kinases, is one such modulator. MAPKs are mediators of intracellular signals that respond to various stimuli. The importance of MAPK signaling pathways in regulating apoptosis during conditions of stress has been widely investigated. MAPKs include extracellular signalregulated kinase (ERK), c-Jun N-terminal protein kinase, and p38 MAPK. Each MAPK is activated through a specific phosphorylation cascade. The ERK pathway is activated by mitogenic stimuli, including growth factors, cytokines, and phorbol esters, and plays a major role in regulating cell growth and differentiation (2,3). Many such studies have supported the general view that activation of the ERK pathway delivers a survival signal that counteracts proapoptotic effects associated with c-Jun N-terminal protein kinase and p38 activation (4 -7). However, the proapoptotic influence of ERK activation has also been demonstrated (8 -11).
Matrix metalloproteinases (MMPs) are capable of digesting various components of the extracellular matrix and other molecules, such as growth factors, cell surface receptors, and cell adhesion molecules. MMPs play an important role in tissue repair, tumor invasion, and metastasis (12,13). The generation and analysis of transgenic and knock-out mice for both MMPs and tissue inhibitors of MMPs have revealed that MMPs also play key roles in the process of carcinogenesis (14). As such, the inhibition of MMPs seems to be an ideal solution to control tumor growth. However, the enthusiasm generated by a large number of in vitro and in vivo studies has dramatically diminished in recent years due to the failure of MMP inhibitors to block tumor progression in clinical trials (15). To better target MMPs, an appreciation of their many extracellular and intracellular roles in cell death is required. To this effect, we have constructed an adenovirus capable of expressing siRNA targeting the human MMP-9 gene (Ad-MMP-9). We demonstrated that MMP-9 inhibition induced senescence in medulloblastoma cells in vitro and regressed pre-established tumor growth in an intracranial model (16). The aims of the present study were to further delineate the role of MMP-9 in medulloblastoma tumorigenesis and to evaluate the mechanisms underlying the apoptotic induction caused by MMP-9 inhibition. Molecular dissection of the signaling pathways that activate the apoptotic cell death machinery is critical for both our understanding of cell death events and the development of novel cancer therapeutic agents. We show that MMP-9 inhibition induced apoptosis in medulloblastoma in vitro and in vivo. Our data also suggest that up-regulation of the ERK pathway is critical for NF-B activation and caspase-mediated cell death.
Daoy Cell Culture-Daoy cells were cultured in advanced minimal essential medium supplemented with 5% fetal bovine serum, 2 mM/liter L-glutamine, 100 units/ml penicillin, and 100 g/ml streptomycin. Cells were maintained in a humidified atmosphere containing 5% CO 2 at 37°C.
Adenoviral siRNA Constructs and Infection-The adenoviral siRNA for MMP-9 (Ad-MMP-9) and scrambled vector (Ad-SV) were constructed and amplified as described previously (16). Viral titers were quantified as plaque-forming units/ml following infection of 293 cells. We obtained the following titers for the viruses: Ad-SV (7.6 ϫ 10 11 plaque-forming units/ml) and Ad-MMP-9 (5.0 ϫ 10 11 plaque-forming units/ml). The amount of infective adenoviral vector per cell (plaque-forming units/cell) in culture media was expressed as multiplicity of infection (MOI). Virus constructs were diluted in serum-free culture media to the desired concentration, added to cells, and incubated at 37°C for 1 h. The necessary amount of complete medium was then added, and cells were incubated for the desired time periods.
Transfection with Plasmids-All transfection experiments were performed with fuGene HD transfection reagent according to the manufacturer's protocol (Roche Applied Science). Daoy cells were transfected with plasmid constructs containing ERK dominant negative mutant (Dn-ERK) (17), MMP-9-expressing cDNA (pcMMP-9) construct, or commercial MMP-9 siRNA (25 and 50 l of 10 mM). Briefly, plasmid containing either Dn-ERK or pcMMP-9 was mixed with fuGene HD reagent (1:3 ratio) in 500 l of serum-free medium and left for 0.5 h for complex formation. The complex is then added to the plate, which had 2.5 ml of serum-free medium (2 g of plasmid/ml of medium). After 6 h of transfection, complete medium was added and kept for 24 h and used for further experiments.
Western Blotting-Western blot analysis was performed as described previously (16). Briefly, 48 h after infection with mock, 100 MOI of Ad-SV, or various MOI of Ad-MMP-9, Daoy cells were collected and lysed in radioimmunoprecipitation assay buffer, and protein concentrations were measured using BCA protein assay reagents (Pierce). Equal amounts of proteins were resolved on SDS-PAGE and transferred onto a polyvinyli-dene difluoride membrane. The blot was blocked and probed overnight with a 1:1000 dilution of primary antibodies followed by horseradish peroxidase-conjugated secondary antibodies. An ECL system was used to detect chemiluminescent signals. All blots were reprobed with glyceraldehyde-3-phosphate dehydrogenase antibody for measuring equal loading.
Isolation of Cytosol and Mitochondrial Fractions-Cells were infected as described above. 48 h later, cells were collected and resuspended in 1 ml of lysis buffer A containing 20 mM HEPES-KOH, pH 7.5, 10 mM KCl, 1.5 mM MgCl 2 , 1 mM EDTA, 1 mM EGTA, 1 mM phenylmethylsulfonyl fluoride, 10 g/ml leupeptin, 10 g/ml aprotinin, and 250 mM sucrose. The cells were homogenized with a 26-gauge needle syringe 4 -6 times and centrifuged at 750 ϫ g for 10 min at 4°C to remove nuclei and unbroken cells. Then the supernatant was centrifuged at 10,000 ϫ g for 15 min at 4°C, and the resulting supernatant was collected (i.e. the cytosolic extract). The pellet with mitochondria was lysed in lysis buffer B containing 50 l of 20 mM Tris, pH 7.4, 100 mM NaCl, 1 mM phenylmethylsulfonyl fluoride, 10 g/ml leupeptin, 10 g/ml aprotinin and centrifuged at 10,000 ϫ g for 20 min at 4°C. The supernatant was collected for the mitochondrial fraction. The protein content of the fractions was determined by the BCA method. Equal amounts of lysates were subjected to Western blot analysis as described above and probed for cytochrome c.
FACS Analysis-FACS analysis was performed as described earlier (16). Briefly, cells were infected as described above for 48 h and collected. Cells were washed three times with ice-cold phosphate-buffered saline (PBS), stained with propidium iodide (2 mg/ml) in 4 mM/liter sodium citrate containing 3% (w/v) Triton X-100 and RNase A (0.1 mg/ml) (Sigma) and were analyzed with the FACSCalibur system (BD Biosciences). The percentages of cells undergoing apoptosis were assessed using Cell Quest software (BD Biosciences).
Treatment with NF-B Inhibitor II (JSH-23)-Daoy cells were infected with Ad-MMP-9 as described above. After 36 h of infection, the cells were treated with 50 M JSH-23 (NF-B inhibitor) for 6 h. After the treatment, the cells were collected, and nuclear extractions were prepared and immunoblotted.
Binding reaction was performed with 5 g of nuclear protein in a total volume of 20 l containing 40 ng of poly(dI-dC), 4 l of 5ϫ binding buffer (1ϫ binding buffer: 20 mM HEPES, pH 7.9, 50 mM KCl, 5 mM MgCl 2 , 1 mM EDTA, 1 mM dithiothreitol, 10% glycerol). For supershift, protein extracts were incubated with 6 g of p65 or p50 monoclonal antibody or isotype control before the addition of the 32 P-labeled probe. DNA-protein complexes were resolved on 5% PAGE in Tris/glycine buffer at 4°C. The double-stranded oligonucleotides (Santa Cruz Biotechnology, Inc., Santa Cruz, CA) used in this study for NF-B were as follows: 5Ј-AGT TGA GGG GAC TTT CCC AGG C-3Ј and 5Ј-GCC TGG GAA AGT CCC CTC AAC T-3Ј. Oligonucleotides were end-labeled with 40 Ci (1480 MBq) of [␥-32 P]ATP using T4 polynucleotide kinase and were purified on NAP-5 Sephadex G-25 DNA grade columns.
Intracranial Tumor Model and Immunohistochemistry-Daoy cells were stereotactically implanted as described previously (16). Two weeks after tumor cell implantation, treatments were given as described earlier (16). Animals losing Ͼ20% of body weight or having trouble ambulating, feeding, or grooming were sacrificed. Animals were monitored for 180 days, the designated termination point of the experiment. Excised brains were fixed in 10% formalin and embedded in paraffin. Tissue sections (5 mm thick) were subjected to immunostaining with antibodies for either active caspase-3 or pERK. Protein expression was detected using 3,3-diaminobenzidine solution (Sigma). Sections were counterstained with hematoxylin, and negative control slides were obtained by nonspecific IgG. Sections were washed and mounted with mounting solution and analyzed with an inverted microscope.  JANUARY 18, 2008 • VOLUME 283 • NUMBER 3

JOURNAL OF BIOLOGICAL CHEMISTRY 1547
TUNEL Assay-To evaluate the apoptotic response of Ad-MMP-9, we have performed the terminal deoxynucleotide transferase (TdT)-mediated biotin-dUTP nick end labeling technique using the commercially available in situ cell death detection kit fluorescence (Roche Applied Science). Briefly, 20,000 cells were seeded onto the 8-well chamber slides and infected with mock, 100 MOI of Ad-SV, or various MOIs of Ad-MMP-9. After 60 h of infection, the cells were washed and fixed with 4% buffered paraformaldehyde and permeabilized with freshly prepared 0.1% Triton X-100, 0.1% sodium citrate solution. These cells were then incubated with TUNEL reaction mixture for 1 h at 37°C in a humidified chamber. The slides were washed three times with PBS, and the incorporated biotin-dUTP was detected under a fluorescence microscope. For the paraffin-embedded tissue sections, slides were dewaxed and fixed according to standard protocols and then were treated as described above.
Statistical Analysis-The significance of differences between experimental conditions was determined using the two-tailed Student's t test.

MMP-9 Inhibition Induces Apoptosis in Medulloblastoma Cells-
We have previously shown that MMP-9 inhibition mediated by adenoviral delivery of siRNA against the human MMP-9 gene caused specific inhibition of MMP-9 and induced senescence in medulloblastoma cells and decreased medulloblastoma tumor growth in vivo (16). To examine the ability of Ad-MMP-9 to induce apoptosis in medulloblastoma cells, Daoy cells were infected with various doses of Ad-MMP-9 and subjected to FACS analysis after propidium iodide staining and TUNEL staining. Ad-MMP-9 caused apoptosis in Daoy cells in a dose-dependent manner, with a concentration of 100 MOI resulting in the death of more than 75% of the cell population compared with mock and Ad-SV. There was no major difference in the number of apoptotic cells in cells infected with mock (PBS control) and scrambled vector (Ad-SV). MMP-9 inhibition significantly increased the number of apoptotic cells (fraction of subdiploid) as determined by FACS analysis (Fig.  1A). Also, the number of apoptotic cells increased from ϳ4% (cells infected with mock and Ad-SV) to 56.45% in cells infected with 50 MOI and 78.05% in cells infected with 100 MOI of Ad-MMP-9 as determined by TUNEL analysis (Fig. 1B). It is known that the translocation of cytochrome c from the mitochondria to the cytosol is an important step in the apoptotic signaling pathway, linking mitochondrial changes to the activation of caspases (19). Once located in the cytosol, cytochrome c, together with Apaf-1 and procaspase-9, forms a multiprotein complex, which initiates the activation of caspase-3, leading to cell apoptosis (20). Cytochrome c levels in the cytoplasm increased in response to Ad-MMP-9 treatment, and this finding correlated with the cleavage of both caspase-9 and -3. Consistent with the activation of caspase-3, the degradation of PARP, which is a caspase-3 substrate, was also observed (Fig. 1, C and D).
Ad-MMP-9 Infection Activates the MEK/ERK Signaling Pathway-It is well known that the MAPK cascade plays an essential role in controlling cellular proliferation, differentiation, and apoptosis (21,22). To elucidate the mechanism mediating the apoptotic effect, cell lysates of Ad-MMP-9-infected Daoy cells were subjected to Western blot analysis using antibodies against phospho-ERK and total ERK. Ad-MMP-9 infec- h. An electrophoretic mobility shift assay was performed with 32 P-labeled NF-B oligonucleotide using 5 g of protein from nuclear extracts as described under "Experimental Procedures." A solid arrow indicates the specific NF-B complexes, and open arrows indicate antibody supershift. D, nuclear and cytoplasmic proteins were prepared as described under "Experimental Procedures." Equal amounts of proteins were resolved on an SDS-polyacrylamide gel and blotted onto polyvinylidene difluoride membrane. Signals were detected by using antibodies specific for IB␣, p65, and p50. The data represent a typical experiment conducted three times with similar results. tion increased phospho-ERK levels in a dose-dependent manner as compared with mock and Ad-SV controls ( Fig. 2A). Densitometric analysis indicated that phospho-ERK levels increased by 1.33-and 1.88-fold in 25 and 50 MOI, respectively, when normalized to total ERK levels of the controls (Fig. 2B).
Ad-MMP-9 Induces NF-B Activation and Degradation of IB␣-To investigate the effect of Ad-MMP-9 on NF-B activation, Daoy cells were infected with the indicated MOI of Ad-MMP-9. For the assessment of NF-B activation, 5 g of protein from the nuclear extracts were used to perform the electrophoretic mobility shift assay. Fig. 2C indicates that Ad-MMP-9 infection induces NF-B activation. Supershift assays with antibodies directed against various members of the Rel family indicated that the bands contained both the p50 and p65 components. To examine the effect of Ad-MMP-9 on IB␣, p65, and p50 levels, nuclear and cytoplasmic extracts from Ad-MMP-9-infected Daoy cells were subjected to Western blot analysis using antibodies against IB␣, p50, and p65. Western blot analysis revealed that Ad-MMP-9 induced nuclear translocation of p50 and p65 proteins and degradation of IB␣ (Fig. 2D).
ERK Suppression Blocks Ad-MMP-9-induced NF-B Activation and Apoptosis-With the demonstration that Ad-MMP-9 activated caspases, enhanced cytochrome c release, activated NF-B, and stimulated ERK, we examined the hierarchical relationship among these signaling molecules. To evaluate whether expression of ERK activity is required for the NF-B-mediated induction of apoptosis in Ad-MMP-9 infection, we transiently transfected Daoy medulloblastoma cells with a dominant negative mutant of ERK (Dn-ERK; kinase-dead mutants of ERK-1) prior to Ad-MMP-9 infection and analyzed its effect on Ad-MMP-9induced apoptosis. As expected, the increase of phosphorylated ERK1/2 level was inhibited by Dn-ERK transfection in Ad-MMP-9-infected cells (Fig. 3A). ERK inhibition decreased the number of apoptotic cells, as determined by TUNEL staining (Fig. 3B), and cleavage of caspase-3 and PARP, which is a biochemical feature of apoptosis (Fig.  3C). Furthermore, Dn-ERK transfection suppressed Ad-MMP-9-induced NF-B activation (Fig. 4A) and blocked p65 translocation to the nucleus (Fig. 4B). These data confirmed the requirement of activation of ERK for NF-B activation. Because cytochrome c release into the cytosol precedes the activation of caspase-3, we examined the role of ERK signal transduction in Ad-MMP-9-induced cytochrome c release. Dn-ERK transfection drastically reduced the release of cytochrome c into the cytosol by Ad-MMP-9 infection (Fig. 4C). These data suggest that cytochrome c is a key factor in Ad-MMP-9-induced apoptosis in Daoy cells and that its release may relate to the activation of caspase-9 and -3, which subsequently triggers the cleavage of PARP and the appearance of apoptosis. Fig. 4D indicated that the NF-B inhibitor reversed Ad-MMP-9-mediated apoptosis but not in phospho-ERK levels under these conditions. These data indicate that NF-B activation is required for Ad-MMP-9-mediated apoptosis and that ERK activation is required for NF-B activation and cytochrome c release.
ERK Activation Mediates through ␤1-Integrin in Ad-MMP-9-treated Cells-Ad-MMP-9 infection increases integrin levels, ERK phosphorylation, and cell adhesion to various matrices (16). Because integrin-mediated cell adhesion has been shown to strongly activate MAPK, we next determined whether Ad-MMP-9-mediated alterations of integrin expression are responsible for ERK phosphorylation. We found that blocking ␤1-integrin signaling using a function-blocking ␤1-integrin antibody diminished the Ad-MMP-9-mediated stimulatory effect on ERK phosphorylation (Fig. 5A). To further elucidate the role of ␤1-integrin in ERK phosphorylation in Ad-MMP-9induced apoptosis, we determined whether ectopic expression of MMP-9 reversed the increase in ␤1-integrin levels in Ad-MMP-9-infected cells. Gelatin zymography for MMP-9 activity (Fig. 5B) indicated that transient transfection with an expression vector carrying MMP-9 expressing cDNA vector, prior to Ad-MMP-9 infection, reversed the MMP-9 inhibition. We also assessed ␤1-integrin expressions under these conditions. We observed an increase in ␤1 expression in Ad-MMP-9-infected cells compared with mock (PBS) and Ad-SV-infected cells. Ad-MMP-9-induced ␤1-integrin levels were reversed upon the reintroduction of MMP-9 expression (Fig. 5,  C and D). These results suggest that MMP-9-mediated alteration in ␤1-integrin expression induces ERK phosphorylation.
Characterization of Tumors from Mice Treated with Ad-MMP-9-We have previously shown that Ad-MMP-9 inhibits medulloblastoma tumor growth in vivo in an intracranial model (16). A TUNEL assay was performed in established tumors from mice implanted with Daoy medulloblastoma cells and treated with mock, Ad-SV or Ad-MMP-9. The results show a clear increase in apoptosis in tumor sections from Ad-MMP-9-treated mice as compared with sections from mock-and Ad-SV-treated animals (Fig. 6A). To determine whether ERK phosphorylation mediates Ad-MMP-9-mediated apoptosis in vivo, phosphorylation of ERK1/2 (pERK) and cleaved caspase-3 were measured by immunohistochemical analysis. Consistent with the TUNEL results, an increase in cleaved caspase-3 was found in Ad-MMP-9-treated tumors (Fig. 6B). Extensive phosphorylation of ERK in tumors from mice treated with Ad-MMP-9 was observed compared with tumors from mice that received mock and Ad-SV treatments (Fig. 6C).

DISCUSSION
Several previous studies have shown the ability of MMP inhibitors to block tumor growth and induce apoptosis. However, no studies have shown that apoptosis is directly related to MMP inhibition or whether or not it is a property of a given inhibitor or the cell system used. Moreover, the functional mechanism by which MMP inhibition induces apoptotic cell death is unclear. We have previously reported that MMP-9 inhibition caused senescence in medulloblastoma cells (16). In this study, we demonstrate that MMP-9 inhibition mediated by adenoviral delivery of MMP-9 siRNA (Ad-MMP-9) caused programmed cell death in medulloblastoma in vitro and in vivo. Further investigations of the signaling mechanism indicated that Ad-MMP-9-induced apoptosis in medulloblastoma cells was preceded by activation of the ERK pathway. We also determined the effect of altered integrin expression in Ad-MMP-9-infected cells on ERK activation. Thus, our data identify a novel mechanism whereby MMP inhibition-mediated alterations of integrin expression induced apoptosis following the activation of the ERK pathway.
MMP-9 inhibition caused apoptosis in medulloblastoma in vitro and in vivo as demonstrated by TUNEL staining, FACS analysis, and caspase-3 activation. We demonstrate that the apoptosis-inducing ability of Ad-MMP-9 depends on NF-B activation. Recent studies have focused on the proapoptotic or antiapoptotic role of NF-B that mediates cell survival. Regulation of apoptotic behavior by NF-B either in a proapoptotic or antiapoptotic manner is determined by the nature of the apoptotic stimuli. Inhibition of inducible or constitutive NF-B activation confers sensitivity to apoptosis-inducing therapies, such as tumor necrosis factor ␣, in some cancer cell types (23). However, although less frequently observed than prosurvival functions, NF-B activation triggers apoptosis (24,25). Several examples of NF-B-induced death have been reported in neuronal cell types. For example, dopamine-induced death of pheochromocytoma cells and neuronal death in response to ischemia require NF-B (26,27). Similarly, doxorubicininduced cell death in neuroblastoma cells is mediated by IB␣ degradation and an increase in the DNA binding of NF-B p65/p50 heterodimer, and specific inhibition of NF-B renders cells resistant to Dox (28). Aspirin prevents neural cell death via inhibition of NF-B activation that indicates a proapoptotic role for NF-B in neural cells (29).
Two major distinct apoptosis pathways have been described for mammalian cells. One involves caspase-8, which is recruited by the adapter molecule Fas/APO-1-associated death domain protein to death receptors upon extracellular ligand binding (30). The other involves cytochrome c release-dependent activation of caspase-9 (20). We did not observe any change in either Fas or FasL expression in Ad-MMP-9-infected Daoy cells (data not shown). We did, however, observe increased levels of cytochrome c in the cytoplasm of Ad-MMP-9-treated cells relative to control and Ad-SV-treated cells, suggesting that cytochrome c release plays a role in mediating Ad-MMP-9-induced apoptosis. The enhanced cytochrome c release into cytosol and the activation of caspase-9 and -3 in Daoy cells suggest that Ad-MMP-9 induces apoptosis by mitochondrial dysfunction mechanisms.
To gain further insight into the mechanisms by which Ad-MMP-9 promotes apoptosis in medulloblastoma cells, we looked at the early signaling events. Integrins act as important regulators of cell function through their ability to mediate adhesion to extracellular matrices, to induce cytoskeletal rearrangements, and to activate intracellular signaling pathways (31,32). Our previous studies indicated that Ad-MMP-9 infection caused increased expression of integrins (16). In this study, we show that ectopic expression of MMP-9 in Ad-MMP-9treated cells reversed the increase in ␤1-integrin level, which was originally caused by Ad-MMP-9 infection, indicating that the ␤1 expression is regulated by MMP-9. Many intracellular signaling molecules are activated by integrin engagement, including components of the Ras/Raf/MEK/Erk pathway (33). We show that blocking ␤1-integrin, using blocking antibodies, decreased Ad-MMP-9-induced ERK phosphorylation. We have presented much evidence suggesting that ERK activation plays a key role in Ad-MMP-9-mediated induction of apoptosis. We demonstrate that ERK activation plays a central role in Ad-MMP-9-mediated apoptosis using Dn-ERK plasmid. Daoy cells transfected with Dn-ERK prior to Ad-MMP-9 infection indicated that the ERK signaling pathway is involved in the activation of NF-B and downstream caspase activation. The ability of ERK inhibition to diminish cytochrome c release suggests that the ERK signaling pathway functions upstream of cytochrome c release in the induction of caspase-mediated cell death. Furthermore, the inhibition of ERK also inhibited caspase-3 cleavage. A link between ERK and caspase-3 activation has been described in leptin-induced apoptosis in bone marrow cells, although the mechanism of this link is not fully understood (34). It was reported that PARP cleavage was suppressed by a MEK inhibitor, demonstrating that the caspase cascade is downstream of the MEK/ERK pathway (35). Consistent with this observation, our studies also indicate that ERK inhibition decreased PARP cleavage, suggesting that ERK activation is necessary for the caspase cascade. Our results clearly indicate ERK inhibition in Ad-MMP-9-treated cells, decreased cytochrome c release, NF-B activation, and caspase-3 cleavage. Generally, ERK inhibits rather than activates caspase-3; this inhibition has been associated with NF-B activation (36) or with direct phospho- Lysates were electrophoresed, blotted, and probed with antibodies against the phospho-ERK and an anti-ERK antibody as a loading control. B and C, Daoy cells were transfected with 2 g/ml pcMMP-9 plasmid expressing MMP-9 for 16 h and infected with mock, Ad-SV, and Ad-MMP-9. Shown are results from gelatin zymography for MMP-9 in media (B) and Western blot analysis for ␤1-integrin expression in cell lysates (C). D, the band intensities of ␤1-integrin expression were quantified by densitometry and normalized with the intensity of the mock band as shown in the corresponding bar graph (means Ϯ S.E.) (p Ͻ 0.01) from three independent experiment measurements of ␤1 expression. *, difference between mock and Ad-MMP-9 treatment (p Ͻ 0.05); **, difference between Ad-MMP-9 treatment and pcMMP-9 plus Ad-MMP-9 (p Ͻ 0.05). GAPDH, glyceraldehyde-3-phosphate dehydrogenase. FIGURE 6. MMP-9 inhibition induces apoptosis in vivo. Brain tumor sections from mice that received mock, Ad-SV, and Ad-MMP-9 treatments were analyzed. A, apoptotic cells were visualized by TUNEL assay. Paraffin-embedded sections of tumors were stained with either active caspase-3 (B) or pERK (C) antibody (inset; ϫ40 magnification). Results are representative of multiple tumors taken from five separate mice in each treatment group.
rylation of caspase-9 and subsequent inhibition of caspase-3 activation (37). Determining whether NF-B activation or inhibition of phosphorylation of caspase-9 is responsible for the opposing effects of ERK activation in cell survival in Ad-MMP-9-infected cells will be an area for future attention.
In summary, our study suggests that MMP-9 takes considerable part in the regulation of apoptosis in human medulloblastoma cells. We propose a model in which Ad-MMP-9-mediated alterations in integrin expression potentiate ERK-induced apoptosis in medulloblastoma (Fig. 7). Although the potential cross-talk between NF-B and caspase activation pathways still needs to be investigated, we show evidence that ERK activation is required for NF-B activation and subsequent downstream caspase activation. The ERK MAPK pathway has been the subject of intense research scrutiny, leading to the development of pharmacologic inhibitors for the treatment of cancer (38). Our findings indicate the need for caution when targeting this pathway with respect to the generality of this approach. On the other hand, our findings indicate that strategies targeting activation of this cascade could be employed to enhance the therapeutic effectiveness of MMP-9 inhibition in medulloblastoma.