**7.1 Astrocyte Elevated Gene-1**

Some newly reported show that elevation in expression level of astrocyte elevated gene-1(AEG-1, also known as Metadherin and lyric) in human breast cancer dramatically

Breast Cancer from Molecular Point of View: Pathogenesis and Biomarkers 115

Fig. 4. MTDH/AEG-1 promotes tumor progression through the integration of multiple signaling pathways. Oncogenic Ha-Ras increases *MTDH/AEG-1* expression through the activation of the PI3K/Akt pathway, which phosphorylates and inactivates GSK3β, and subsequently enhances the stabilization and binding of c-Myc to the *MTDH/AEG-1* promoter. MTDH/AEG-1 can activate AKT, NFκB, and Wnt/β-catenin pathways to promote proliferation, survival, and invasion. Activation of NFκB signaling is in part mediated by the direct interaction of MTDH/AEG-1 with p65 and CBP, a general

transcriptional co-activator. MTDH/AEG-1 activates the Wnt/β-catenin pathway through increasing the activity of MAPK kinases ERK and p38, which phosphorylates GSK3β and stabilized β-catenin. Furthermore, MTDH/AEG-1 increases the expression of LEF-1, a transcriptional cofactor for β-catenin. The prometastasis function of MTDH/AEG-1 is mediated by the interaction of the LHD of MTDH/AEG-1 with an unknown receptor in endothelial cells. The broad spectrum chemoresistance function of MTDH/AEG-1 is mediated by a number of downstream genes that promote the resistance to multiple

enhanced cell proliferation and their ability of anchorage-independent growth of breast cancer cells. These proliferative effects were significantly related to attenuation of two key cell-cycle inhibitors, p27Kip1 and p21Cip1, via Akt/ FOXO1 signaling pathway. FOXO1 is a transcription factor belonging to the Forkhead box-containing class O (FOXO) subfamily. Many biological functions have been shown to be related with FOXO1 including cell-cycle control, differentiation, stress response and apoptosis (37). FOXO proteins could act as tumor suppressors through induction of CDK inhibitors, including p21 Cip1, p27Kip1and p57 (38). Overexpression of *AEG*-1 increases migration and invasion of human glioma cells because of the presence of a lung-homing domain which facilitates breast tumor metastasis to lungs. Recent observations indicate that *AEG*-1 play this role by activating NF-κB pathway. Our recent observations indicate that, AEG-1 facilitates IκBa degradation, resulting in an increase in NF- κB DNA binding activity and NF- κB promoter activity in reporter assays These valuable findings are strengthen the idea which recommend *AEG*-1 as a crucial regulator of tumor progression and metastasis (39).

Another considerable role attributed to AEG-1 is mediating a broad-spectrum chemoresistance. In vitro and in vivo studies showed that knocking down AEG-1 makes several different breast cancer cell lines more sensetive to paclitaxel, doxorubicin, cisplatin, 4-hydroxy cylco phosphamide, hydrogen peroxide, and UV radiation mediated by the prosurvival pathways such as PI3K and NFκ B, or through other downstream genes of MTDH/AEG-1 that directly regulate chemoresistance. AEG-1 has also resulted in chemoresistance neuroblastoma and prostate cancer. In fact, MTDH/AEG-1does not affect the uptake or retention of chemotherapy a. Instead,

it enhances chemoresistance by increasing cell survival after chemotherapy. Data gathered from Microarray analysis of breast cancer cells showed reduction of expression of chemoresistance genes ALDH3A1, MET, HSP90, and HMOX1, and increased expression of pro-apoptotic genes BNIP3 and TRAIL after MTDH/AEG-1knocking down. Among these genes, ALDH3A1 and MET were established to partially be associated with the chemoresistance role of MTDH/AEG-1 in MDA-MB-231 breast cancer cells. Some other genes also contribute to chemoresistance including drug-metabolizing enzymes for different chemotherapeutic agents, such as dihydropyrimidine dehydrogenase (DPYD), cytochrome P4502B6 (CYP2B6), dihydrodiol dehydrogenase (AKR1C2), and the ATP-binding cassette transporter ABCC11 for drug efflux (40). Roles of MTDH/AEG-1 have been simplified in figure 4.

There are some studies which suggest that Activated protein C (APC), an anticoagulant serine protease, is related to cell survival, cell migration, angiogenesis and breast cancer invasion. APC recruits EPCR, PAR-1, and EGFR in extracellular matrix in order to increase the invasive properties of MDA-MB-231 cells. Other mechanisms include activation of matrix metalloprotease (MMP) -2 and/or -9 and activation of ERK, Akt, and NF-κB (but not the JNK) pathways. APC does not employ the endogenous plasminogen activation system to increase invasion (41).
