Characterisation of integrin splice variants and the interaction between integrins and the urokinase receptor

Sammanfattning: Integrins are a family of receptors involved in adhesion to extracellular matrix proteins and in cell-cell contacts. Each integrin consists of one a and one β subunit. Our main interest has been the β1 subunit and its splice variants.The new splice variant, named β1C-2, was identified and its transcript found to be generated as a result of utilisation of a more 3' splice acceptor-site in exon C. Hence, β1C-2 has an internal deletion of six amino acids compared to β1C-1. Exon C is part of a retrotransposable Alu element, and is thus primate specific. Low levels of β1C-1 and β1C-2 transcripts were found in all human cells and cell lines tested. Expression of β1A, β1C-1 and β1C-2 cDNAs in the mouse β1-deficient GD25 cells demonstrated that β1A integrins localised at the cell surface, while β1C-1 and β1C-2 were retained and finally degraded intracellularly. This suggests that the ER quality control system is able to recognise and prevent cell surface localisation of the β1C-1 and β1C-2 subunits.The β1B splice variant has been reported to have dominant negative effects on β1A-integrin functions. Since β1A and β3 cytoplasmic tails are highly homologous, we investigated if expression of β1B had a similar effect on αVβ3 integrin functions by expressing β1B in GD25 cells. The presence of β1B did not interfere with a Vβ3 mediated adhesion or signalling.The urokinase receptor (uPAR) has been shown to modulate integrin function. To study the effect of uPAR on αVβ3 and β1-integrins separately, GD25 and GD25Tβ1A cells were transfected with the human uPAR cDNA. Although stimulation of uPAR with its ligand (uPA) resulted in increased adhesion to vitronectin, but not to fibronectin, phosphorylation of FAK and paxillin was unchanged, while phosphorylation of p 130Cas was reduced. However, the absence or presence of β1A did not alter the intracellular signals generated by stimulation of uPAR.