Quantification and modulation of thrombomodulin activity in isolated rat and human glomeruli

Thrombomodulin (TM), the endothelial cell surface receptor for thrombin-mediated activation of protein C and of its anticoagulant system, is involved in maintaining vascular nonthrombogenicity, and depressed TM activity may induce intravascular fibrin formation. TM antigen was previously found by immunohistochemical methods in rabbit glomeruli. We therefore attempted to identify the corresponding TM activity in isolated detergent-solubilized rat and human glomeruli. Like purified lung TM, rat glomeruli extracts accelerated the hydrolysis by activated protein C of the chromogenic substrate S-2238 in the presence of 10 nM thrombin, as determined by spectrophotometry. One mg glomerular protein promoted the formation of 681 +/- 115 nmol activated protein C, the equivalent of the amount generated by 845 ng of purified rabbit TM. TM activity correlated with the protein content of the glomerular extracts (r = 0.94). These extracts prolonged rat plasma activated partial thromboplastin time. Incubation of glomeruli with tumor necrosis factor-alpha (TNF) or E. coli lipopolysaccharide depressed their TM-like activity in a dose and time dependent manner. Incubation with TNF suppressed their anticoagulant activity. In human glomeruli, TM activity was also found at a level which corresponded to their TM antigen content, and was determined by ELISA with mouse monoclonal antibody. These results indicate that measurement of glomerular TM activity might help to clarify the mechanisms of intraglomerular fibrin deposition in renal diseases.