Investigating the oncogenic function of FUBP1 in glioblastoma cell lines

Data obtained in our laboratory established FUBP1 (Fuse Binding Protein 1) as an important oncoprotein overexpressed in HCC (Hepatocellular Carcinoma) that induces tumor propagation through direct or indirect repression of cell cycle inhibitors and pro-apoptotic target genes. However, Prof. Kinzler’s laboratory (Ludwig Center for Cancer Genetics and Howard Hughes Medical Institutions, USA) found that inactivating mutations in the genes CIC and FUBP1 contribute to human oligodendroglioma. Their study enhanced our interest in investigating the expression levels of FUBP1 in different glioma cells. We detected low expression of FUBP1 in astrocytes and demonstrated elevated levels of FUBP1 in glioblastoma, the most common malignant brain tumor in humans. Among the challenges in the treatment of glioblastoma are the frequency of recurrence, the aggressiveness and the infiltrative behavior of the tumor. In contrast to oligodendroglioma, glioblastoma seem to require significant FUBP1 levels. The glioblastoma cell lines LNT229 and U87-MG were used to explore the oncogenic function of FUBP1 in glioblastoma, and further functional assays were performed. The results from the proliferation and apoptosis assays revealed less proliferation and more apoptosis in LNT229 FUBP1 knockdown cells, which co-relates with results obtained with HCC cell lines. In contrast to LNT229, U87-MG cells showed more proliferation and less apoptosis upon FUBP1 knockdown. Interestingly, FUBP1-deficient U87-MG cells showed enhanced xenograft tumor growth. Affymetrix array results revealed a significant upregulation of pro-apoptotic genes in FUBP1 knockdown LNT229 cells in contrast to FUBP1 knockdown U87-MG cells. In the latter, pro-proliferative and pro-angiogenic genes were significantly upregulated. qRT-PCR analysis showed a significant increase in TGFβ-1, MMP2 and AKT in U87-MG FUBP1-deficient cells while these genes were downregulated in LNT229 FUBP1-deficient cells. Interestingly, the knockdown of MMP2 in U87-MG cells led to a significant increase in FUBP1 protein levels. Further, APAF-1 mRNA levels were investigated in LNT229 and U87-MG cells. To our surprise, APAF1 levels were high in FUBP1-deficient LNT229 cells compared to control LNT229 cells. However, we detected no change in APAF1 levels between control and FUBP1-deficient U87-MG cells. We speculate that different signaling pathways are activated in LNT229 and U87-MG cells upon FUBP1 knockdown. Therefore, we hypothesize that FUBP1 might fulfill a special important function as an oncogene in glioblastomas which might differ from its role in HCC