Intercellular junctional proteins as targets for protein kinase c-mediated signalling pathways

Intercellular junctions are dynamic structures responding to a variety of physiological and pathological stimuli. They are particularly well developed in epithelia and endothelia, which function as barriers between different body compartments. This barrier function is dependent on intercellular tight junctions, which prevent unrestricted movement of ions and other solutes between the cells. The integrity of the tight junctions is dependent on a second intercellular junction, the adherens junction, which is the site of cadherin-mediated Ca2+-dependent cell-cell adhesion. Little is known of the molecular mechanisms by which intercellular junctions are dynamically regulated. Activation of protein kinase C (PKC) is one route by which modulation of junctional integrity can be achieved. I have investigated whether the proteins which make up intercellular junctions are targets of PKC-mediated signal transduction pathways. p120, a component of the adherens junction, was found to be a target of a PKC signalling pathway. Activation of PKC by phorbol-12,13-dibutyrate (PDB) led to dephosphorylation of p120 and the closely related p100 protein in epithelial and endothelial cell lines, p100 and p120 are capable of cycling between lesser and higher phosphorylated forms, subject to the action of a serine/threonine phosphatase (or phosphatases) and serine/threonine kinase(s). The pathway by which PKC regulates pl00/pl20 phosphorylation was investigated, as was the effect of dephosphorylation on the stability and cellular distribution of p100/p120. Tyrosine phosphorylation of junctional proteins has been correlated with disruption of adhesion. A number of cellular proteins were found to become phosphorylated on tyrosine in response to PDB. The tyrosine phosphorylation of these proteins correlates with increased permeability across epithelial cell monolayers; inhibition of phosphorylation using a tyrosine kinase inhibitor blocks the ability of PKC to increase paracellular permeability. These data demonstrate the presence of novel PKC-mediated signal transduction pathways that may be involved in the regulation of intercellular adhesion