2604 - Functional Interaction between Tribbles1 (TRIB1) Pseudokinase and PTEN Tumor Suppressor in GBM Tumorigenesis

K. Singh¹, C. Han¹, J. L. Fleming¹, J. McElroy², C. A. Showalter¹, H. Manring¹, J. Haque¹, and A. Chakravarti¹; ¹The Ohio State University Comprehensive Cancer Center, Columbus, OH, ²The Ohio State University, Center for Biostatistics, Department of Biomedical Informatics, Columbus, OH

Purpose/Objective(s): Glioblastoma (GBM, WHO Grade 4) is associated with a low 5-year survival rate and a highly aggressive phenotype. PTEN is a tumor suppressor gene that is known to act as an inhibitor of the PI3K-Akt pathway. It is mutated in about 36-44% of GBM patients and its deficiency positively correlates with poor survival. We have previously shown that TRIB1 activates the Akt and ERK pathways in GBM providing survival advantage to GBM cells in response to radiation and chemotherapy treatments. Previous studies in prostate cancer have shown that TRIB1 mRNA expression is inversely correlated with PTEN gene expression. In this study we evaluated the relationship between TRIB1 and PTEN in the context of GBM. Materials/

Methods: The publicly available TCGA GBM cohort was analyzed for TRIB1 gene expression in PTEN wildtype (WT) and mutant patients. Furthermore, correlative analysis was done between TRIB1 and PTEN gene expression in TCGA GBM and CGGA cohorts. Transient transfection was used to overexpress PTEN. Patient derived xenograft (PDX) cell lines stably overexpressing TRIB1 were created by antibiotic selection. Protein levels were detected by western blotting.
Results: CGGA cohorts (Table 1). Western blot data revealed that after overexpression of TRIB1, PTEN protein levels were decreased in the T08-387 PDX cell line (50% PTEN allelic loss) and LN18 cells (PTEN WT). We then overexpressed PTEN in U87 cells (PTEN null) overexpressing TRIB1 transgene and observed the same effect on PTEN protein levels. However, the Akt and ERK phosphorylation were also reduced. Table 1: Correlation between TRIB1 and PTEN gene expression in various patient cohorts.
Conclusion: TRIB1 and PTEN gene expression have a direct correlation in TCGA GBM database. Also, TRIB1 mRNA levels are not dictated by PTEN mutation status. TRIB1 affects PTEN protein levels in respective cell lines however the mechanism is not known. The reduction in Akt and ERK phosphorylation in U87 cells may be attributed to TP53 status of these cells and the fact that addition of WT PTEN over WT TP53 may enhance tumor suppression in these cells. In conclusion, further studies are needed to delineate the relationship between TRIB1 and PTEN, as the mutation status of PTEN (WT, heterozygous loss and complete loss) has differing tumor-initiation effects.

Cohort	Group	Number of patients	Correlation (R)	P-value
TCGA	PTEN WT	217	0.4643	5.325e-13
TCGA	PTEN MUT	110	0.5232	4.515e-09
CGGA	Cohort 1	140	0.28	3.71e-03
CGGA	Cohort 2	85	0.239	2.82e-02