CYCLOSPORINE TREATMENT ALTERS PROTEIN-PHOSPHORYLATION IN KIDNEY MEMBRANES

Phosphorylation, protein carboxyl methylation, and ADP-ribosylation were assayed in renal basolateral membranes and brush border membranes isolated from rats treated by subcutaneous administration of 5 or 10 mg/(kg day) of cyclosporin A (CsA) for 10 days to investigate potential alterations in signal transduction in kidney cortex. Protein carboxyl methylation of class II measured in membranes and in cytosolic fraction was not affected by CsA treatment. ADP-ribosylation performed in the presence of pertussis or cholera toxin was also similar in control and treated rats. However, changes in phosphorylation of endogenous substrates were observed in membranes and cytosol isolated from rats treated with 10 mg/(kg day) of CsA. Phosphorylation was increased for two brush border membrane proteins (56 and 77 kilodaltons (kDa)) by 47 and 24% and for two basolateral membrane proteins (51 and 80 kDa) by 28 and 29%, respectively. In the cytosolic fraction, phosphorylation of two proteins (31 and 65 kDa) was increased by 37% and that of 25- and 43-kDa proteins was reduced by 29%. Protein kinase A, protein kinase C, and tyrosine protein kinase activities were also determined in membranes. Increases in protein kinase C and tyrosine protein kinase activities were observed in basolateral membranes, but not in brush border membranes after cyclosporin A administration. Endogenous substrates for tyrosine kinase were also detected with an antiphosphotyrosine (PY20) monoclonal antibody. Densitometric analysis indicated that tile phosphorylation of three proteins of high molecular masses (61, 132, and 183 kDa) was stimulated by CsA in basolateral membranes. Immunodetection of calcineurin with a monoclonal antibody directed against the beta-subunit was similar in control and treated rats. These results suggest that cyclosporin A treatment may bring about changes in covalent modifications as an adaptative response in signal transduction in the kidney proximal tubule.