Differential activation of mitogen-activated protein kinases in experimental mesangioproliferative glomerulonephritis

Multiple extracellular mitogens are involved in the pathogenesis of prolife rative forms of glomerulonephritis (GN). In vitro studies demonstrate the p ivotal role of mitogen-activated protein (MAP) kinases in the regulation of cellular proliferation. This study was conducted to examine whether these kinases, as a convergence point of mitogenic stimuli, are activated in mesa ngioproliferative GN in vivo. Therefore, anti-Thy1 GN was induced in rats u sing a monoclonal anti-Thy1.1 antibody (OX-7). Whole cortical tissue as wel l as isolated glomeruli were examined at different time points using kinase activity assays and Western blot analysis. A maximal increase in the numbe r of glomerular mitotic figures (9.7-fold) was demonstrated 6 d after injec tion of the anti-Thy1.1 antibody. In parallel with this finding, a signific ant increase in cortical, and more dramatically glomerular, activity of ext racellular signal-regulated kinase (ERK) was detected. Maximal activation o f ERK was detectable on day 6. This activation of ERK was accompanied by an increase in the expression of MEK (MAP kinase/ERK kinase), the ERK-activat ing kinase. A marked induction of glomerular apoptosis at 2 h after injecti on of the anti-Thy1.1 antibody, which subsided subsequently, was demonstrat ed using the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nic k end labeling assay as well as staining for single-stranded DNA, However, no significant activation of stress-activated protein kinase or p38 MAP kin ase, both MAP kinases that are suggested to induce apoptosis and to inhibit cellular growth, was detectable at this early time point. Rather, on day 6 a dramatic decrease in the activity of p38 MAP kinase, which might have co ntributed to the overshooting glomerular cellular proliferation, was observ ed. Treatment of rats with heparin blunted glomerular proliferation as well as ERK activation and restored p38 MAP kinase activity. These observations point to ERK and p38 MAP kinase as putative mediators of the proliferative response in mesangioproliferative GN and suggest that upregulation of MEK is involved in the long-term regulation of ERK in vivo.