Alpha macroglobulins and the low-density-lipoprotein-related protein/alpha-2-macroglobulin receptor in experimental renal fibrosis.

In this study, we evaluated the location of non-specific proteinase inhibitors and their receptor in experimental glomerular and interstitial fibrosis. The alpha macroglobulins alpha-2-macroglobulin (alpha2M) and alpha-1-inhibitor 3 (alpha1I3) are proteinase inhibitors, including metalloproteinases and serine proteases. Using immunohistochemistry, we detected alpha1I3 in the glomerular mesangium in control rats. In acute and chronic fibrosis, the alpha1I3 protein expression was dramatically increased throughout the glomerulus and at sites of increased extracellular matrix deposition in the interstitium. The presence of alpha1I3 in normal and nephrotic kidneys was confirmed by Western blotting. Under chemically reducing conditions, we found that, in contrast to native alpha1I3, kidney-derived alpha1I3 has reacted upon by proteinases, thereby revealing a functional role for this macroglobulin under normal and pathological conditions. Double staining revealed that high amounts of glomerular alpha1I3 were present in sclerotic lesions. alpha2M was absent in glomeruli and interstitium from control rats, but present in small amounts in glomerular mesangial areas of acute nephrotic rats. alpha2M was also present in significant amounts in glomeruli from rats with chronic fibrosis. The receptor mediating the uptake of proteinase inhibitor-proteinase complexes, the low-density-lipoprotein-related protein/alpha-2-macroglobulin receptor, was found in the glomerular mesangium and tubulo-interstitium from control rats. Significant increments in receptor expression were found in glomeruli and interstitium of rats with chronic fibrosis, with a preferential localization in fibrotic areas. Interstitial staining for low-density-lipoprotein-related protein/alpha-2-macroglobulin receptor was attributed to fibroblasts, since double staining ruled out dendritic cells and macrophages. In conclusion, these studies demonstrate the intrarenal presence of plasma-derived proteinase inhibitors together with their receptors. These findings may point to a novel mechanism for renal fibrosis wherein matrix-degrading proteinases are inhibited, resulting in renal fibrosis.