The atypical cadherin FAT1 is a novel regulator of STAT1, driving its pro-tumorigenic effect via the STAT1/PDCD4 axis in glioblastoma

FAT1 is an atypical cadherin that has been shown to act both as an oncogene and a tumour suppressor gene (TSG) in different tumor types. We have earlier shown that upregulated FAT1 acts as an oncogene in glial tumors by promoting pro-tumorigenic inflammation and EMT in primary human glioblastoma and in cell lines. One effect was through the suppression of the Tumor Suppressor Gene (TSG), Programmed Cell Death 4 (PDCD4). Here, we have studied how, in glioblastoma, upregulated FAT1 affects downstream events that control PDCD4 expression. In silico analysis of the PDCD4 promoter revealed multiple STAT1 binding sites. We also found a positive correlation in mRNA levels of STAT1 and FAT1 in the Glioblastoma databases, as well as in resected patient derived tumor samples by qPCR. Increased FAT1 as well as STAT1 were associated with poor prognosis in these data bases. In the glioblastoma cell lines LN229 and U87MG, FAT1 knockdown resulted in decreased STAT1 expression. Also, STAT1 knockdown resulted in increased PDCD4 expression, implying that STAT1 may mediate FAT1s role in suppressing PDCD4. Further, ChIP experiments showed that STAT1 protein binds to the PDCD4 promoter and upon FAT1 knockdown, STAT1 binding to the PDCD4 promoter reduces. As for FAT1, STAT1 knockdown also reduces the expression of pro-inflammatory cytokines and EMT markers, also migration and invasion of glioma derived cell lines. This work identifies STAT1 as a novel downstream mediator of FAT1 which mediates its pro-tumorigenic action in suppressing the TSG, PDCD4.