The JNK Signaling Pathway in Renal Fibrosis

Abstract

Fibrosis of the glomerular and tubulointerstitial compartments is a common feature of chronic kidney disease leading to end-stage renal failure. This fibrotic process involves a number of pathologic mechanisms, including cell death and inflammation. This review focuses on the role of the c-Jun amino terminal kinase (JNK) signaling pathway in the development of renal fibrosis. The JNK pathway is activated in response to various cellular stresses and plays an important role in cell death and inflammation. Activation of JNK signaling is a common feature in most forms of human kidney injury, evident in both intrinsic glomerular and tubular cells as well as in infiltrating leukocytes. Similar patterns of JNK activation are evident in animal models of acute and chronic renal injury. Administration of JNK inhibitors can protect against acute kidney injury and suppress the development of glomerulosclerosis and tubulointerstitial fibrosis. In particular, JNK activation in tubular epithelial cells may be a pivotal mechanism in determining the outcome of both acute kidney injury and progression of chronic kidney disease. JNK signaling promotes tubular epithelial cell production of pro-inflammatory and pro-fibrotic molecules as well as tubular cell de-differentiation toward a mesenchymal phenotype. However, the role of JNK within renal fibroblasts is less well-characterized. The JNK pathway interacts with other pro-fibrotic pathways, most notable with the TGF-β/SMAD pathway. JNK activation can augment TGF-β gene transcription, induce expression of enzymes that activate the latent form of TGF-β, and JNK directly phosphorylates SMAD3 to enhance transcription of pro-fibrotic molecules. In conclusion, JNK signaling plays an integral role in several key mechanisms operating in renal fibrosis. Targeting of JNK enzymes has therapeutic potential for the treatment of fibrotic kidney diseases.

Keywords: ASK1; SMAD3; apoptosis; kidney disease; kidney inflammation; p38 MAPK.

Figure 1

Schematic diagram of the JNK signaling pathway. Inflammatory cytokines, danger-associated molecules pattern ligands (alarmins), and pro-fibrotic growth factors can induce activation of members of the mitogen-activated protein kinase kinase kinase (MAP3K) family. Other factors such as reactive oxygen species (ROS) and osmotic stress can also lead to MAP3K activation. Individual members of the MAP3K and MAP2K families plus JNK are held in close proximity by the scaffold protein (JNK-interacting protein-1) which facilitates a rapid series of phosphorylation reactions leading to phosphorylation of the activation site of JNK. The activated JNK then dissociates from this complex and can promote mitochondria-dependent apoptosis through B cell lymphoma (Bcl-2) and Bcl-2 associated x-protein (Bax). Active JNK can promote transcription of genes involved in the inflammatory and fibrotic responses through phosphorylation of c-Jun [to promote activator protein-1 (AP-1) function], SMAD3 and ATF2. JNK activation can promote cell death, inflammation and fibrosis.

Figure 2

JNK activation in kidney injury. Detection of JNK activation by immunohistochemistry staining for c-Jun phosphorylated at Ser63 (p-c-Jun). (A–F) JNK activation in rat anti-glomerular basement membrane (GBM) glomerulonephritis (GN). (A) Normal rat glomeruli lack p-c-Jun staining. (B) Two color immunostaining for p-c-Jun (brown nuclei) and ED1+ macrophages (purple/blue) in a glomerulus on day 1 of anti-GBM disease. Arrows indicate cells stained for both ED1 and p-c-Jun, demonstrating JNK signaling in macrophages. Arrowhead shows p-c-Jun immunostaining in a podocyte-like cell. Parietal epithelial cells staining for p-c-Jun is also evident. (C) Glomerulus on day 24 of anti-GBM disease shows nuclear p-c-Jun staining of cells in the capillary tuft and in the crescent. (D) Day 24 of anti-GBM disease showing p-c-Jun staining in a multinucleated giant cell in a glomerular crescent (^*). (E) Lower power view of day 24 anti-GBM disease showing p-c-Jun+ cells in the glomerular tuft, in crescents, in many tubular epithelial cells and in some interstitial cells. (F) Treatment with the JNK inhibitor CC-401 over days 7–24 of anti-GBM disease blocks c-Jun phosphorylation. (G–I) JNK activation in the rat unilateral ureteric obstruction (UUO) model. (G) Normal rat kidney shows a lack of p-c-Jun staining in glomeruli and tubules. (H) Day 7 after UUO shows extensive JNK activation in both atrophic tubules and in tubules with normal morphology. Glomeruli do not show inflammation in this model and they lack p-c-Jun staining. (I) Treatment with the JNK inhibitor CC-401 over days 0–7 of the UUO model blocks c-Jun phosphorylation. Reproduced with permission from Flanc et al. (2007) (A,B); Ma et al. (2009) (C–F), and; Ma et al. (2007a) (G–I).

Figure 3

Schematic diagram of JNK signaling in the tubular epithelial cell response to damage. Many insults to the tubular epithelial cell, such as oxidative stress and nephrotoxins, as well as cytokines and growth factors can result in activation of the JNK pathway. JNK signaling in tubular epithelial cells can result in tubular cell damage and consequent tubulointerstitial damage via three pathways. (1) JNK activation can directly induce apoptosis and necrosis via mitochondrial dependent mechanisms. (2) JNK phosphorylates a number of transcription factors (e.g., c-Jun, SMAD3, ATF2) resulting in tubular production of pro-inflammatory and pro-fibrotic molecules that promote recruitment and activation of macrophages and myofibroblasts. (3) JNK activation is implicated TGF-β induced de-differentiation of tubular cells toward a partial mesenchymal phenotype with production of pro-fibrotic factors. This switch toward a pro-fibrotic mesenchymal phenotype can also be induced by JNK-dependent arrest at the G2/M phase of the cell cycle when tubular cells are undergoing a repair response.