Abstract B27: TAZ as a regulator of mesenchymal transformation and clinical aggressiveness in gliomas

Background: Glioblastoma (GBM) is a highly malignant and infiltrative brain tumor. Our laboratory has performed extensive microarray studies on GBM and has shown that a subset of these tumors have a gene expression signature characterized by those associated with the mesenchyme. Tumors which overexpress these genes (mesenchymal tumors) are associated with poor outcome and resistance to treatment. In an effort to identify the molecular mechanisms by which this mesenchymal shift occurs, we have identified TAZ (transcriptional coactivator with PDZ-binding motif) as a transcription co-activator whose expression level is tightly associated with the mesenchymal change (elevated TAZ is positively associated with higher expression of key mesenchymal genes). High expression of TAZ also correlated with higher grade glioma as well as poorer patient outcome. These data lead to our main hypothesis, that TAZ activation is critically important in the mesenchymal transition and aggressive clinical behavior in GBM. Results: To further discern TAZ9s role in GBM, we stably silenced or overexpressed this gene in glioma stem cell lines depending on their basal levels of TAZ. Loss of TAZ did not affect proliferation, but did decrease invasion and dramatically decreased tumor formation when injected intracranially into severe combined immunodeficiency (SCID) mice. Five of 5 mice injected with control stem cell cultures developed tumors, while only 2 of 10 TAZ-knockdown cultures resulted in tumor formation. Furthermore, the 2 tumors that formed were of lower histologic grade compared to controls. Cells expressing a constitutively active form of TAZ were injected into SCID mice and 5 of the 5 mice injected with the overexpression clone died less than 60 days post-injection, while the mice injected with the vector control (low TAZ levels) survived an average of 120 days post-injection. Overexpression of TAZ increased invasion, and induced osteogenesis as evidenced by alizarin red staining, but did not alter proliferation. Further TAZ activation regulates the expression of signature mesenchymal genes, as shown by Affymetrix profiling of TAZ-transfected and TAZ-knockdown constructs. These data support the hypothesis that TAZ is a central regulator of mesenchymal gene expression in glioma. To further characterize TAZ function in human glioma, we examined the expression of its binding partners (TEAD1-4), as well as a downstream target (FN1) in human tumor specimens. Western analyses showed that TEAD4 was specifically upregulated in grade IV/GBM compared to lower grade gliomas, and TAZ/TEAD4 expression tightly paralleled FN1 expression, a marker of mesenchymal phenotype. Inspection of the TAZ promoter region revealed a CpG island in the promoter region that was methylated in most lower grade tumors (grade II/III gliomas), but not in grade IV GBMs implying that methylation may be one means by which TAZ is regulated and appears to be epigenetically silenced in tumors with a more favorable outcome. To confirm this, we found that treatment of TAZ-methylated glioma stem cell lines with a demethylation agent resulted in an increase in TAZ expression in both the mRNA and protein levels. Discussion: We have identified TAZ as a critical regulator in the mesenchymal transition in gliomas. Since mesenchymal differentiation is associated with gliomagenesis and tumor aggressiveness, strategies to target TAZ and its downstream targets may be warranted in alternative treatment options for patients. This study may also provide a rationale for developing inhibitors which directly target TAZ. Citation Information: Clin Cancer Res 2010;16(14 Suppl):B27.