Cell signaling pathways in experimental kidney diseases

Thesis Summary Renal inflammation and fibrosis are common features in progressive forms of kidney disease. The infiltration of immune cells, such as macrophages, invariably accompanies renal inflammation and fibrosis. Inflammatory cytokines and reactive oxygen species produced by infiltrating leukocytes and damaged renal cells can activate cellular signaling pathways such as the c-Jun N-terminal kinase (JNK) pathway, extracellular signaling-regulated kinase (ERK) pathway and M-CSF/c-fms pathway. These activated signaling pathways can induce a wide range of cellular responses, including cell proliferation, survival, apoptosis and inflammation. These inflammatory events cause damage that promotes renal fibrosis, characterized by glomerulosclerosis and tubulointerstitial fibrosis, and progressive loss of renal function. In order to investigate the pathological role of these signaling pathways in renal inflammation and fibrosis and to assess the therapeutic potential of their blockade, three individual studies were undertaken. First, the functional role of the ERK signaling pathway in the development of renal interstitial fibrosis was examined using a small molecule MEK1/2 kinase inhibitor, U0126. In the mouse unilateral ureteric obstruction model, ERK phosphorylation was inhibited by U0126 treatment, and the proliferation and accumulation of interstitial F4/80+ macrophages were significantly reduced. However, ERK blockade had no effect upon renal interstitial fibrosis. Thus, interstitial accumulation of macrophages but not tubulointerstitial fibrosis in the obstructed kidney is, in part, dependent upon the MEK-ERK signaling pathway. Second, activation of the JNK signaling was identified in a rat model of mesangial proliferative nephritis (Thy-1 disease). However, administration of CC401, a well characterized JNK inhibitor, failed to modify mesangial cell apoptosis in the early phase (Day 0-3) of Thy-1 disease or to affect mesangial cell proliferation and matrix deposition in the late phase (Day 3-8) of Thy-1 disease, although target serum CC401 levels were achieved. Third, the role of macrophages in the pathogenesis of rat crescentic glomerulonephritis was examined in a rat model of rapidly progressive crescentic glomerulonephritis (anti-GBM disease) using a novel inhibitor of the M-CSF/c-fms signaling pathway termed fms-I. In the first experiment, fms-I treatment was given during the induction phase of disease (days 0 to 14). This treatment reversed an early glomerular macrophage infiltration, and prevented glomerular and tubulointerstitial damage and loss of renal function. This was associated with prevention of the upregulation of pro-inflammatory factors and dendritic cell infiltration. In the second experiment, fms-I treatment was given during established crescentic disease (day 14 to 35). fms-I treatment reversed the renal macrophage infiltrate, improved renal function, had a minor beneficial effect upon crescent formation and reduced interstitial fibrosis in terms of the deposition of collagen IV. Thus, while fms-I treatment was highly effective when given early in the disease process, it still had beneficial effects when given late in the disease. In conclusion, this thesis has identified important mechanisms by which ERK and M-CSF/c-fms signaling promote renal inflammation and fibrosis. Furthermore, blockade of M-CSF/c-fms signaling pathway may be a potential therapeutic strategy in the treatment of human rapidly progressive glomerulonephritis.