Functional characterization of Cytoplasmic Polyadenylation Element Binding proteins in the developing and diseased brain

Abstract

Gliomas are the most common primary brain tumors with aggressive progression and devastating prognosis. Therefore identification of new therapeutic and diagnostic factors is necessary to improve the dramatic situation of glioma patients. Thus far, cancer-related genes were thoroughly analyzed by transcript-based microarrays (van den Boom et al., 2003; Phillips et al., 2006; Rickman et al., 2001). However, recent research shed light on the importance of post-transcriptional modifications of mRNAs that alter gene expression under physiological conditions, but also gives rise to the pathology of gliomas. <br /> The main goal of the present study was to investigate CPEB expression in human glioma specimens. CPEBs are auxiliary regulators associating with consensus sequences present in 3’UTRs of mRNAs, which activate or repress their translation. Via this mechanism CPEBs regulate essential cellular processes, such as development (Groisman et al., 2002; Novoa et al., 2010), memories formation (Theis et al., 2003) and progression of cancer (Ortiz-Zapater et al., 2012). In the current work aberrant CPEB expression was found to be a frequent phenomenon in both, low- and high-grade gliomas. Decreased CPEB1 expression was associated with the rising grade of tumor malignancy, suggesting it being a putative tumor suppressor. One of the mechanisms potentially underlying transcriptional silencing of cancer-related genes might be DNA methylation. However, despite hypermethylation of the CPEB1 gene, DNA methylation proved not to be directly responsible for its downregulation in gliomas. Thus, the underlying mechanism remains elusive. Abundant expression of CPEB 2-4 was detected in numerous human glioma specimens. CPEB2 expression in endothelial tumor cells suggested that CPEB2-mediated protein synthesis takes place in the close proximity of blood vessels within tumor tissue. On the other hand, CPEB4 expression appeared to support tumorigenic properties by its putative entanglement into local protein synthesis in transformed cells. Intriguingly, only CPEB3 expression was correlated positively with tumor progression. Phosphorylation of CPEB3 within the alternatively spliced region was negatively correlated with tumor malignancy. The loss of CPEB3 activity in high-grade gliomas is likely caused by the expression of alternatively spliced variants. This suggests that a substantial part of the CPEB3 in GBM specimens does not undergo phosphorylation and indicates phosphorylation to be a biologically important mechanism regulating CPEB3 activity. Furthermore, a significant correlation between CPEB3 activity and survival was observed in less aggressive low-grade astrocytomas considered to be sGBM precursors. Consequently, CPEB3 may be considered as an attractive therapeutic target in gliomas.<br /> To further investigate the relationship between CPEB activity, growth properties and cancer-relevant parameters an in vitro overexpression study was performed. This revealed a striking link between CPEB1, enhanced apoptotic activity and enhanced migration. One explanation could be that through rapid changes in the cytoskeleton of migrating cell the chance of mistakes rises, turning GBM cells towards an apoptotic pathway. On the other hand, CPEB2 overexpression decreased apoptotic activity, enhanced migration and by this additionally strengthened the cancerous properties of the cells. Importantly, CPEBs were also found to regulate various cancer pathways, including the TGF-β signaling cascade. However, further studies are needed to fully understand the interaction between CPEBs translational control and the pathophysiology of cultured GMB cells and human gliomas. <br /> Finally, the study revealed that CPEB2 is expressed in different cellular populations, brain regions, and stages of development, which indicates that this protein plays an important role in regulation of local protein synthesis, synaptic plasticity, and neuronal morphogenesis.<br /> The present study does not only increase our understanding of the function of CPEBs but also shows the importance of post-transcriptional modifications of mRNAs as a pathophysiological mechanism in gliomas and potentially other cancers. Therefore, these results may serve as a valuable basis for the identification of new therapeutic and diagnostic factors in cancer treatment.