Expression of beta1 integrins in glomerular tissue of Streptozotocin-induced diabetic rats

Based upon the importance of integrins as receptors for extracellular matrix components as well as transducers of extracellular signals, and since major alterations take place in the renal extracellular matrix during diabetes, it is important to study the role played by integrins in the development of the diabetic glomerulosclerosis. Expression of the beta1 subunit by renal glomerular cells was evaluated by biochemical and morphological means in short- and long-term diabetic rats. Western blots of isolated rat renal glomeruli demonstrated that the expression of beta1 increases along with age as well as with the hyperglycaemic state. These changes were significant as early as 6 weeks of hyperglycaemia. This was further demonstrated by immunocytochemistry, which revealed the presence of the beta1 subunit at the level of the plasma membranes of endothelial, epithelial, and mesangial cells. Quantitation of the immunolabelings confirmed the increased expression of beta1 under diabetic conditions. Further to this, expression of the focal adhesion kinase (FAK) was evaluated by immunoblotting showing little increase in diabetic conditions. On the other hand, testing the tyrosine phosphorylation of FAK, revealed significant increases in diabetes. To recover the fraction of FAK associated with the beta1 subunit, immunoprecipitation of isolated glomeruli homogenates was carried out with the anti-beta1 antibody. This demonstrated that the amounts of FAK co-precipitated with beta1, as well as its tyrosine-phosphorylation, are in fact reduced in diabetic conditions. Since the changes reported were observed at time points prior to any morphological alteration of the renal extracellular matrix, it appears that modifications in integrins and in their intracellular relays constitute early events that precede the onset of the diabetic nephropathy and must then be associated with the hyperglycaemic condition.