Activation of the Signal Transducer and Activator of Transcription Signaling Pathway in Renal Proximal Tubular Cells by Albumin

Abstract

Abstract. Renal proximal tubular cells activated by reabsorption of protein are thought to play significant roles in the progression of kidney diseases. It was hypothesized that the signal transducer and activator of transcription (STAT) proteins may be activated by proteinuria in proximal tubular cells. To test this hypothesis, murine proximal tubular cells were treated with albumin (30 mg/ml medium) for various lengths of time. The results showed that albumin could activate Stat1 and Stat5 within 15 min in proximal tubular cells. The activation of STATs was mediated mostly by Jak2 and required no protein synthesis. In addition, activation of Stat1 occurred even after neutralization of IFN-␥. The activation of STATs was inhibited by N-acetyl-L-cysteine, a precursor of glutathione and a reactive oxygen species (ROS) scavenger, and fluorescence-activated cell sorter analysis showed upregulation of intracellular ROS after albumin overloading, suggesting that albumin per se could generate ROS in proximal tubular cells. The activation of STATs occurred by way of the ROS generating system, and especially through the membrane-bound NADPH oxidase system. Reduced activities of glutathione peroxidase and catalase could also be responsible for the accumulation of intracellular ROS. Hence, not only the ROS generating system, but also the ROS scavenging system may contribute to the induction of ROS by albumin. These findings support the hypothesis that proximal tubular cells are activated and generate ROS by reabsorption of abundant urinary proteins filtered through the glomerular capillaries, and as a consequence, various IFN-␥-inducible proteins are synthesized through IFN-␥-independent activation of STAT signaling.

Enhanced Expression of Janus Kinase–Signal Transducer and Activator of Transcription Pathway Members in Human Diabetic Nephropathy

“…This phenomenon appears to be relatively general since ROS mediate induction of Jak-2 activation in tissues such as cardiac myocytes and vascular smooth muscle cells (34 -36) as well as kidney cells (12,14,15,32). We did not test whether ROS could further augment the signaling to Stat-3 in our Jak-2-overexpressing cells but we did show that Jak-2 overexpression induced ROS production in mesangial cells, especially those cultured in high glucose medium.…”

“…We focused on these results because the activation of Jak/Stat signaling pathways can be implicated in both tubulointerstitial fibrosis and epithelial to mesenchymal transition in several conditions, including diabetes, in animal models (11)(12)(13). Similarly, Jak/Stat activation is reported in rat glomerular cells exposed to high glucose (14,15) and may be important in the glomerular transforming growth factor-␤ activation and fibronectin accumulation critical for extracellular matrix deposition in early diabetic nephropathy (15).…”

How Does Proteinuria Cause Progressive Renal Damage?

“…A link also has been made between induction of NF-B activity by protein load and mitogen-activated protein kinases, including p38 (25) and extracellular signal-regulated kinase 1 and 2 (ERK1/ERK2) (29) that are involved in chemokine synthesis. In support of the notion of protein overload as a key activator of signaling in proximal tubule is the finding that albumin activated the signal transducer and activator of transcription (STAT) proteins in cultured proximal tubular cells (30). Because the STAT pathway is the principal mechanism that converts the signal from a wide array of cytokines and growth factors into gene expression programs that regulate cell proliferation, differentiation, survival, and apoptosis (31), it was suggested that albumin may stimulate proximal tubular cells in the manner of a cytokine (32).…”

Redox Control of Renal Function and Hypertension

“…In vitro treatment of PTC with albumin leads to activation of the JAK/STAT pathway and inflammation, cell differentiation, fibrosis, and apoptosis (230). Furthermore, albumintreated PTCs demonstrate reduced levels of antioxidants catalase and GPx, and levels were brought back to normal with antioxidant (NAC) treatment.…”