MACROPHAGES PROMOTE PROSCLEROTIC RESPONSES IN CULTURED RAT MESANGIAL CELLS - A MECHANISM FOR THE INITIATION OF GLOMERULOSCLEROSIS

Glomerulosclerosis is the final outcome of a number of different cause s of glomerular injury during which the structures of the glomerulus a re obliterated by extracellular matrix. Accumulating evidence suggests that infiltrating macrophages play a pivotal role in the progression to glomerulosclerosis. The present study defines the role played by ma crophages at both cellular and molecular levels in the initiation of t he sclerotic process in cultured rat mesangial cells. Macrophage-condi tioned medium (MPCM) generated from thioglycollate-elicited, lipopolys accharide-stimulated macrophages upregulated mesangial cell fibronecti n production in a dose- and time-dependent manner, independently of ce ll proliferation. Immunoprecipitation of metabolically labeled S-35-fi bronectin confirmed that the matrix protein was synthesized de novo. T he genes for fibronectin and the matrix proteins laminin and collagen IV were also found to be upregulated 2.86 +/- 0.24-, 4.94 +/- 0.17-, a nd 3.03 +/- 0.31-fold over controls, respectively (P < 0.001). Macroph age modulation of matrix turnover was suggested. by an upregulation of both transin and tissue inhibitor of metalloproteinase-l gene transcr iption. Transforming growth factor (TGF) beta(1), platelet-derived gro wth factor, tumor necrosis factor (TNF) alpha, or interleukin (IL)-1 b eta could not be detected in the MPCM per se; however, TGF beta(1) and platelet-derived growth factor AB were found to be secreted into mesa ngial cell culture supernatants. Secretion was augmented 1.69 +/- 0.16 - and 2.28 +/- 0.28-fold, respectively (both P < 0.001), in response t o MPCM. Northern blot analysis demonstrated that protein secretion had been preceded by upregulation of the genes for these cytokines (2.2 /- 0.4-fold [P < 0.001] and 5.7 +/- 1.2-fold [P < 0.004], respectively ). Incubation of MPCM with either neutralizing antibody or the growth factor receptor antagonist suramin demonstrated that TGF beta(1) playe d a significant, although minor, role in MPCM-stimulated fibronectin p roduction. In conclusion, this study provides compelling evidence for a direct role of macrophages in the progression to glomerulosclerosis.