Studies on the cellular function of the uPA/uPAR system
The uPA receptor, uPAR, is GPI-linked and therefore contains no transmembrane domains. Yet, uPA:uPAR binding can initiate cell signalling and may involve adaptor proteins, such as integrins. Various responses to uPA binding have been described, but the exact mechanisms remain undefined. To identify potential downstream changes caused by uPA binding, MCF7 cells were treated ± uPA or ATF (the uPAR-binding part of uPA) for various times. Gene expression was studied using custom cDNA microarrays, containing genes relating to a range of cellular processes. Changes in expression were normalised to 0 min and buffer-only controls, and genes with a fold change less than 2.5-fold were discounted. Gene expression changes that were observed in two or more treatments were shortlisted, from which thirteen were selected fro verification by real-time RT-PCR. These experiments were done on cDNA from a further cell treatment with uPA. The results did not reflect those of the microarrays; no changes in expression were observed. The microarray data were then analysed in a different, more stringent, way, but not target genes were identified. This study identified no clear candidate genes but gene expression after uPA:uPAR binding remains an important question. uPAR is present in lipid rafts and these may allow its association with signalling molecules. This study investigated whether potential uPAR adaptor proteins, αv or β1 integrins, moved into rafts when cells bound uPA. Mouse cells transfected with human uPAR were treated ±uPA, and lipid rafts were extracted from the cells. uPAR was readily detectable in lipid rafts under al conditions, but the integrins were not found to associate with lipid rafts even after uPA treatment.