Function of the PDLIM2 protein in oncogenic Wnt signalling pathways

Aberrant regulation of the Wnt signalling pathway is a recurrent theme in cancer biology. Hyper activation due to oncogenic mutations and paracrine activity has been found in both colon cancer and breast cancer, and continues to evolve as a central mechanism in oncogenesis. PDLIM2, a cytoskeletal PDZ protein, is an IGF-1 regulated gene that is highly expressed in cancer cell lines derived from metastatic tumours. Suppression of PDLIM2 inhibits polarized cell migration, reverses the Epithelial to Mesenchymal transition (EMT) phenotype, suppresses the transcription of β-catenin target genes, and regulates gene expression of key transcription factors in EMT. This thesis investigates the mechanism by which PDLIM2 contributes to the maintenance of Wnt signalling in cancer cells. Here we show that PDLIM2 is a critical regulator of the Wnt pathway by regulating β-catenin at the adherens juctions, as also its transcriptional activity by the interaction of PDLIM2 with TCF4 at the nucleus. Evaluation of PDLIM2 in macrophages and co-culture studies with cancer cells and fibroblasts showed the influence exerted on PDLIM2 by paracrine cues. Thus, PDLIM2 integrates cytoskeleton signalling with gene expression by modulating the Wnt signalling pathway and reconciling microenvironmental cues with signals in epithelial cells. Negative correlation of mRNA and protein levels in the triple negative breast cancer cell BT549 suggests that PDLIM2 is part of a more complex mechanism that involves transcription and posttranslational modifications. GST pulldown studies and subsequent mass spectrometry analysis showed that PDLIM2 interacts with 300 proteins, with a high biological function in protein biosynthesis and Ubiquitin/proteasome pathways, including 13 E3 ligases. Overall, these data suggest that PDLIM2 has two distinct functions depending of its location. Located at the cytoplasm mediates cytoskeletal re-arrangements, whereas at the nucleus PDLIM2 acts as a signal transduction adaptor protein mediating transcription and ubiquitination of key transcription factors in cancer development.

Study of insulin-like growth factor signaling in mitochondrial function

Insulin-like Growth Factor-1 (IGF-1) signalling promotes cell growth and is associated with cancer progression, including metastasis, epithelial-mesenchymal transition (EMT), and resistance to therapy. Mitochondria play an essential role in cancer cell metabolism and accumulating evidence demonstrates that dysfunctional mitochondria associated with release of mitochondrial reactive oxygen species (ROS) can influence cancer cell phenotype and invasive potential. We previously isolated a mitochondrial UTP carrier (PNC1/SLC25A33) whose expression is regulated by IGF-1, and which is essential for mitochondrial maintenance. PNC1 suppression in cancer cells results in mitochondrial dysfunction and acquisition of a profound ROS-dependent invasive (EMT) phenotype. Moreover, over-expression of PNC1 in cancer cells that exhibit an EMT phenotype is sufficient to suppress mitochondrial ROS production and reverse the invasive phenotype. This led us to investigate the IGF-1-mitochondrial signalling axis in cancer cells. We found that IGF-1 signalling supports increased mitochondrial mass and Oxphos potential through a PI3K dependant pathway. Acute inhibition of IGF-1R activity with a tyrosine kinase inhibitor results in dysfunctional mitochondria and cell death. We also observed an adaptive response to IGF-1R inhibition upon prolonged exposure to the kinase inhibitor, where increased expression of the EGF receptor can compensate for loss of mitochondrial mass through activation of PI3K/mTOR signalling. However, these cells exhibit impaired mitochondrial biogenesis and mitophagy. We conclude that the IGF-1 is required for mitochondrial maintenance and biogenesis in cancer cells, and that pharmacological inhibition of this pathway may induce mitochondrial dysfunction and may render the cells more sensitive to glycolysis-targeted drugs.