Bird, Rebecca Jane
Novel areas of crosstalk between the cyclic AMP and PKC signalling pathways.
PhD thesis, University of Glasgow.
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Mediation of biological functions occurs via tightly regulated signal transduction pathways. These complex cascades often employ crosstalk with other signalling pathways to exert strict control to allow for correct cellular responses. The cyclic AMP signalling pathway is involved in a wide range of cellular processes which require tight control, including cell proliferation and differentiation, metabolism and inflammation. Protein Kinase C (PKC) signalling is also involved in the regulation of many biological functions, due to the wide range of PKC isoforms, and there is emerging evidence that there are critical points of crosstalk between these two central signalling pathways. The aims of this research, therefore, are to identify the molecular basis underlying this pivotal cross-communication.
The identification of the complex formed by Receptor for activated C Kinase 1 (RACK1), a scaffold protein for PKC, and the cyclic AMP-specific phosphodiesterase PDE4D5 demonstrated a potential area of crosstalk between the cyclic AMP and PKC signalling pathways although the function of the complex remained largely unknown. In this thesis I have outlined a role for RACK1 binding to PDE4D5 to control the enzymatic activity of the phosphodiesterase. Although RACK1 does not affect the intracellular localisation of PDE4D5, it does afford structural stability to PDE4D5, providing protection against denaturation. Furthermore, interaction with RACK1 facilitates high affinity binding of PDE4D5 to cyclic AMP and increases phosphodiesterase sensitivity to inhibition by rolipram, a PDE4-specific inhibitor that is a therapeutic treatment for depression and Alzheimer’s disease. Additionally, RACK1-bound PDE4D5 was found to be activated by PKCα, providing a route of negative regulation by PKC on cyclic AMP in HEK293 cells.
The discovery of EPAC (Exchange Protein directly Activated by Cyclic AMP) has opened up the field of cyclic AMP research, providing an alternative route for the cyclic AMP signalling originally thought to occur solely through Protein Kinase A (PKA). Recent investigations have linked cyclic AMP signalling via EPAC to the control of inflammation, through the induction of Suppressor of Cytokine Signalling 3 (SOCS-3) to inhibit IL-6 signalling. Here I have further delineated this pathway in COS1 to show that induction of SOCS-3 by EPAC requires phospholipase C (PLC) ε. Investigation into downstream effectors of PLC action lead to the identification of PKCα and PKCδ as essential components of this pathway, further elucidating a mechanism by which cyclic AMP can affect inflammation and revealing a point of crosstalk between the two signalling pathways.
Further elaborating on the identification of PKC isoforms α and δ as crucial components in the control of cytokine signalling by cyclic AMP via EPAC, investigations into the effect of cyclic AMP on PKC α and δ activation and autophosphorylation, and on downstream effectors, were carried out. It was revealed that cyclic AMP had no influence on PKCδ activity, although a role for cyclic AMP signalling through EPAC on the activation and autophosphorylation of PKCα was identified. Additionally, phosphorylation of the downstream kinase ERK was found to occur independently of PKC activation and required the presence of EPAC1 in COS1 cells.
The work presented in this thesis therefore begins to delineate a novel pathway in which the cyclic AMP and PKC pathways work together to afford cell regulation, including the regulation of gene expression, through novel areas of crosstalk.
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