Upregulation of intrarenal angiotensinogen in diabetes

The role of the local renin–angiotensin system (RAS) has been extensively studied in the progression of kidney disease. Previous research has indicated that local tissue production of angiotensin II (Ang II), which functions in an intracrine, autocrine and paracrine manner, is important in the progression of kidney disease. Clinical and animal studies have shown that treatment with angiotensin-converting enzyme inhibitors and Ang II type 1 receptor blockers reduces the progression of diabetic nephropathy, thereby resulting in effective amelioration of proteinuria and glomerular damage. However, these benefits cannot be fully explained only by the hemodynamic effects of the system causing a reduction in blood pressure.^{1, 2}

In addition to the well-characterized systemic RAS, the presence of a local intrarenal RAS has now been generally accepted. The mRNA and functional proteins of the RAS, including angiotensinogen (AGT), renin, pro-renin receptor,³ angiotensin I-converting enzyme and Ang II receptors (AT-1 and AT-2 subtypes), are expressed in rat mesangial cells,^{4, 5} mouse and rat immortalized renal proximal tubular cells^{6, 7} and rat collecting duct cells^{8, 9} (Table 1).

Table 1 RAS along the nephron

Vidotti et al.⁴ described a significant increase in Ang II generation in mesangial cells exposed to a high glucose concentration, suggesting that the mechanisms responsible for this enhancement primarily include an increase in intracellular renin activity paralleled by an increase in AGT gene expression. These increases indicate the availability of the substrate to renin. Interestingly, the increased expression of intrarenal AGT has been shown to be directly involved in disease progression in several animal models of renal injury.^{10, 11}

Brezniceanu et al.¹² demonstrated that tumor growth factor-β1 stimulates Ang gene expression in rat kidney proximal tubular cells and that its action is mediated, at least in part, by reactive oxygen species (ROS) generation, p38 MAPK activation and p53 expression. These results suggest that Ang II and tumor growth factor-β1 form a positive feedback loop to enhance their respective gene expression, leading to renal injury.

Although the signal transduction pathways involved in AGT expression have been described by several authors, the mechanism has not been completely elucidated.^{13, 14}

In this issue of Hypertension Research, Ohashi et al.¹⁵ demonstrated that the levels of glomerular AGT expression in Zucker diabetic fatty (ZDF) obese rats were higher than the levels in ZDF lean rats. In addition, they reported increased levels of immunoreactivity for 4-hydroxy-2-nonenal, urinary excretion of 8-isoprostane and markers of ROS in the ZDF obese animals. They also focused on the signal transduction pathway for glomerular AGT expression. H₂O₂ induced an increase in AGT expression in a dose-dependent and time-dependent manner, and the H₂O₂-induced upregulation of AGT was suppressed by catalase. Moreover, the H₂O₂-induced upregulation of AGT was inhibited by a mitogen-activated protein kinase (MAPK), a kinase inhibitor (MEK) and a c-Jun amino-terminal kinase (JNK) inhibitor, but was not inhibited by a p38 MAPK inhibitor.

Two aspects of their results were particularly intriguing. First, they offered further evidence for the pathological role of an activated AGT in mesangial cells found in ZDF rats with diabetic nephropathy. Second, their data demonstrated increased AGT expression in mesangial cells mediated by H₂O₂ with activation of extracellular-regulated kinase (ERK)/JNK pathways, but not p38 MAPK pathways. This result provides a new mechanism for the activation of intrarenal RAS in diabetic nephropathy. The increase of AGT could result in high levels of Ang II in these cells, which may result in intracrine actions involved in the well-known effects of diabetic nephropathy, such as induction of gene expression (particularly genes involved in cell growth and metabolism), synthesis of extracellular matrix components, and manifestations that are typical of diabetic nephropathy.

Are there clinical implications for increased intrarenal AGT?

AGT is the only known substrate for renin, which is the rate-limiting enzyme of RAS. The results described by Ohashi et al., in addition to previous studies from Kobori's group that found elevated urinary AGT levels in patients with chronic kidney disease including patients with diabetes, confirm that urinary AGT levels can function as an early biomarker for determining the status of the intrarenal RAS.

These results strengthen the role of tissue RAS acting in an autocrine, paracrine or intracrine manner in local kidney control.