Although the diverse roles of AEG-1 in tumor progression in multiple cancers are now being elucidated, investigations of its function in brain tumorigenesis remains unclear. We presently provide definitive evidence that elevated AEG-1 expression is a common event in brain tumors of diverse origin, including GBM. Moreover, knockdown of AEG-1 expression significantly inhibits the growth of human and rat glioma cells in vitro and significantly suppresses the invasive ability of glioma cells by modulating MMP-2 and MMP-9 activity. Additionally, AEG-1 inhibition significantly abrogates intracranial tumor growth in nude mice. Taken together, these results indicate that AEG-1 overexpression is a frequent alteration in brain tumors of multiple origins and overexpression of AEG-1 correlates with increased glioma cell proliferation and tumorigenicity. Moreover, inhibiting AEG-1 expression translates into suppression of transformation-associated properties and decreased intracranial tumor growth in vivo suggesting that AEG-1 could be a novel target for the therapy of malignant glioma and other types of brain cancers.
Recent studies have implicated AEG
-1 overexpression as an important event in multiple types of cancer. Aberrant AEG-1
expression has been observed in a number of solid tumors including breast, prostate, esophageal carcinoma, and neuroblastoma (15
). In addition, we showed that AEG
-1 expression is increased with the stages and grades of the disease in more than 90% of the hepatocellular carcinoma samples analyzed (16
). Additionally, AEG
-1 can enhance survival of normal cells under conditions of serum-starvation, induce an aggressive tumorigenic phenotype in immortal cloned rat embryo fibroblasts and promote tumor angiogenesis (29
). All these observations suggest that AEG
-1 may potentially act as an oncogene in various cancer types. To investigate whether the upregulation of AEG
-1 is also related to the progression of malignant glioma, we presently performed immunohistochemistry and Western blot analysis to characterize AEG-1 expression in commercially available tissue microarrays and in clinical archival astrocytoma and glioma tissues. These results demonstrate that AEG-1 is expressed at higher levels in glioma tissues as compared with astrocytoma tissues. These studies are now being expanded with a larger number of clinical samples to validate the prognostic significance of AEG-1 in glioma.
In addition to vasculature remodeling and destruction of the surrounding normal brain tissue, local invasive infiltration and growth are key features of GBM. The invasive character of glioma appears to depend partly on the proteolytic destruction of the estracellular matrix. Several studies have indicated that proteases are involved in tumor growth and invasion at the primary and metastatic sites (33
). Previous observations suggest that MMP-2, MMP-9 and MT1-MMP are present at elevated levels in several glioma cell lines and surgical specimens. In the present study we demonstrate that elevated AEG-1 expression in glioma specimens is associated with increased expression of MMP-2 and MMP-9. Additionally, we found that AEG
-1 siRNA significantly inhibits MMP-2 and MMP-9 activation blocking glioma invasion. These data provide a direct link between induction of these proteolytic enzymes by AEG
-1 and the induction of invasive properties in GBM cells.
Previously we have reported that AEG-1
is a significant positive regulator of nuclear factor kappa-β (NF–κB) and activation of NF-κB by AEG-1
represents a key molecular mechanism by which AEG-1
promotes anchorage-independent growth and invasion in malignant glioma cells (13
). Aberrant or constitutive activation of NF-κB has been documented in human malignant gliomas (36
). The observation that in many glioma cells NF–κB is constituvely active emphasizes a vital role of AEG-1 in the induction of NF-κB and in regulation of aggressive properties of malignant gliomas. When AEG
-1 was inhibited by siRNA, p65 could associate with the IL-8 promoter, whereas the association of AEG-1 as well as CBP was significantly reduced indicating that AEG
-1 might function as a bridging molecule between p65 NF-κB and CBP and the basal transcription machinery, thus facilitating transcriptional activation of NF-κB downstream genes necessary for migration and invasion (14
might play an important role in glioma cell growth and survival, we explored the potential therapeutic role of AEG-1
in combination with a chemotherapeutic drug. We found that knockdown of AEG-1
significantly enhanced the cytotoxicity of docetaxel in human glioma cells. Docetaxel is a tubulin-stabilizing agent currently used for the therapy of metastatic breast cancer, prostate cancer and non-small cell lung cancer (39
). Our results suggest that docetaxel chemotherapy could be efficacious in glioma cells when administered in combination with AEG-1 siRNA. This possibility remains to be determined, since docetaxel has not been used clinically in GBM. Studies are in progress to ascertain if there are beneficial effects of AEG
-1 inhibition when combined with temozolomide, a chemotherapeutic widely used in malignant glioma cells.
A key finding of the present study is that knockdown of AEG-1 by a lentivirus-based siRNA approach significantly inhibits GBM12 cell growth in vivo. Further studies using lentivirus or adenovirus expressing AEG-1 siRNA will be necessary in established tumors in the brain of nude and syngeneic animals to confirm potential therapeutic efficacy of this approach. Targeting AEG-1 by lentivirus-based siRNA approaches or by small molecule inhibitors in combination with radiotherapy and chemotherapy might produce extended survival benefits in malignant glioma patients and significantly improve the outcome of patients afflicted with this aggressive and invariably fatal disease.