Role of coxsackie- and adenovirus receptor (CXADR/CAR) in the regulation of cell plasticity in cancer and inflammation

Abstract: The coxsackie- and adenovirus receptor (CXADR) is a transmembrane protein, which localizes at tight junctions (TJ) in epithelial cells. As highlighted by its name, CXADR was initially identified as a receptor for type C adenoviruses and group B coxsackieviruses. Subsequently, CXADR has been shown to mediate cell-cell adhesion, immune cell activation and cellular signaling. Unlike other TJ components, CXADR is vital for the early stages of development. Deregulation of CXADR is frequently observed in pathological conditions including cancer and chronic inflammation. However, mechanistic insight into the role of CXADR in pathophysiology has been lacking. The overall aim of this thesis was therefore to study the role of CXADR in cancer progression and inflammatory diseases. In Paper I, we show that CXADR regulates the capacity of breast cancer cells to undergo epithelial-mesenchymal transition (EMT) in response to the cytokine TGF-β1. The mechanism was traced to a previously unidentified role of CXADR in acting as a negative regulator of the AKT signaling pathway by forming a signalosome with, PTEN and PHLPP2. Through lossand gain-of-function experiments we showed that by regulating the stability of the signalosome at tight junction, CXADR controls AKT activity and epithelial-mesenchymal plasticity in breast cancer cells. Moreover, we found that loss of CXADR correlated with loss of PTEN and PHLPP2, and poor prognosis in luminal A breast cancer. In Paper II, we found that CXADR expression is significantly induced during the formation of atherosclerotic plaques in arterial walls. Macrophages were identified as a previously unknown cellular source of CXADR in both murine and human atherosclerotic plaques. A combination of gene expression profiling, mass spectrometric analysis and in vitro studies using human monocytes (THP1 cells), revealed that the induction of CXADR expression is linked to monocyte-macrophage differentiation and further polarization into M1 subtype, and foam cells. Intriguingly, we also found a significant correlation between CXADR and receptors for other viruses, associated with atherosclerosis in human plaques. In Paper III, inspired by the results from Paper I, we show that CXADR also regulates the metabolic arm downstream of AKT. We found that CXADR controls glucose uptake in various types of cells by regulating the expression and localization of the glucose transporter GLUT-1. Further studies revealed that CXADR expression is upregulated in heart and liver tissues in a mouse model of type 2 diabetes (T2D). In line with this, we found that CXADR expression is induced by IL-6, an inflammatory cytokine which is known to play a role in T2D. In conclusion, the results presented in this thesis provide a novel and mechanistic insight into the role of CXADR as a pathogenic factor in breast cancer progression and suggest that CXADR contributes to the progression of chronic inflammatory diseases including atherosclerosis and T2D. This may offer new possibilities for using CXADR as a target to develop novel diagnostic tools and therapeutic strategies in cancer and inflammation.