Chymase: An alternative Pathway of Angiotensin II-Mediated Diabetic and Hypertensive Kidney Disease
Hui Y Lan, M.D., Ph.D.
Associate Professor of Medicine and Pathology
Baylor College of Medicine. Houston, Texas. USA
Diabetic and hypertensive nephropathy is a leading cause of end-stage renal disease (ESRD).Over the last decade, many experimental and clinical studies have demonstrated that the intrarenal renin-angiotensin system (RAS), particularly the effector molecule angiotensin II (Ang II), plays a pivotal role in progressive renal injury (1,2). This is supported by the findings that blockade of Ang II with either angiotensin-converting enzyme (ACE) inhibitor or an angiotensin type 1 (AT1) receptor antagonist can prevent or slow the progression of renal injury associated with diabetes (3-5).
The conventional view that Ang II formation is solely dependent on ACE has been recently challenged with the demonstration of alternative pathways to the ACE for Ang II generation in the variety of tissues including heart, arteries, and kidney in human, monkey, dog, hamster, and rat (6,7). Of them, the chymase-dependent Ang II generating pathway is the most important one (6,7), as Ang II formation is substantially blocked by chymase inhibitors such as chymostatin and NK3201 (8, 9).
Indeed, more than 60-80% of Ang II formation in the human cardiovascular system appears to be chymase-dependent and full blockade of RAS requires both ACE and chymase inhibition in chronic heart failure and coronary heart disease (10). However, it remains completely unknown whether the chymase pathway is activated in diabetic and hypertensive nephropathy.
Chymase and its functional roles in cardiovascular disease
Chymases are serine proteinases without enzymatic activity in the normal state but are activated immediately upon release into the extracellular matrix (8). There are two forms of mammalian chymase, a and b, which differ in species and have different functions (6,7,11). In human, only a -chymase is found, while dogs, rats, and mice have both a - and b -chymases(6,7,11). In the human heart, chymase is synthesized and stored in mast cells, endothelial cells, and mesenchymal cells and is secreted directly to the interstitium, contributing up to 80% of Ang II generation (12). It has been shown that a -chymase convert Ang I to Ang II, while not degrading Ang II. In addition, the known functional role of chymase includes conversion of big-endothelin-1 to endothelin-1 (13), inactivation of bradykinin and kallidin (14).
Chymase has also been shown to play an important role in cardiovascular tissue remodeling through their ability to activate progelatinase B (MMP-9) and to directly degrade matrix proteins (15), and to promote angiogenesis by upregulating VEGF and basic fibroblast growth factor (16,17). A number of chymase inhibitors are now available. The use of the chymase inhibitors, TEI-E-548 or BCEAB, has been shown to improve survival of hamsters with myocardial infarction despite no effect on the infarction size and improve LV function in cardiomyopathy (18,19).
In addition, inhibition of chymase by NK-3201 has been shown to suppress intimal hyperplasia after balloon injury or to prevent peritoneal adhesions after intraabdominal trauma (9, 20). The ability of chymase inhibitors to improve cardiovascular injury indicates that chymase may play a critical role in cardiovascular disease. However, the functional role of chymase in the kidney disease is not known.
Chymase, an alternative pathway of Angiotensin-generating pathway within the kidney
Direct evidence for chymase-dependent Ang II-generating pathway within the kidney comes from a recent study in ACE-knockout mice (21). It has shown that local Ang II generation within the kidney in ACE-knockout mice is unchanged. This is due to a 14-fold increase in the chymase activity (21). Hollenberg et al. also reported that in the kidney virtually all Ang II generation is renin-dependent, which is inhibited by renin inhibitors, but at least 40% of Ang I is converted to Ang II by pathways other than ACE (22).
We found that chymase is constitutively expressed by glomerular mesangial cells and vascular smooth muscle cells (VSMC), but absent in glomerular and tubular epithelial cells in the normal human kidney (23), which may contribute to the ACE-independent Ang II formation within the kidney. In contrast, expression of chymase by both mesangial cells and VSMC is markedly upregulated in diabetic and hypertensive kidney, suggesting that chymase-dependent Ang II generating system is activated within the diseased kidney. A similar result has also been reported in the rejected kidney in which increased expression of chymase is found in mast cells (24).
Chymase, a mediator in diabetic and hypertensive kidney disease
While the importance of chymase in cardiovascular diseases has been well documented, little is known about a role of chymase in kidney disease. In the human rejected kidneys, increased expression of chymase in mast cells has been shown to correlate with the severity of interstitial fibrosis (24).By using a combination of microwave antigen retrieval method with a sensitive immunohistochemical technique, we are able to provide the first evidence that chymase was markedly upregulated by glomerular mesangial cells and VSMC, but not by glomerular epithelial cells and tubular epithelial cells in the diabetic kidney with hypertension (23).
Importantly, strong chymase expression was also found in fibrotic tissues with rich collagen matrix accumulation, particularly in the mesangial region with diffuse or nodular sclerosis, in vascular walls with arteriosclerosis, and thickened Bowman capsular and tubular basement membrane. This indicates activation of chymase-dependent Ang II generating system within the diabetic and hypertensive kidney. Interestingly, compared to chymase, ACE is expressed primarily by glomerular and tubular epithelial cells, and interstitial mononuclear cells and fibroblasts.
The identification of differential upregulation of ACE and chymase within the diabetic kidneys demonstrate both ACE-dependent and chymase-dependent Ang II generating pathways exist in diabetic and hypertensive nephropathy.
Future approach to study chymase in kidney disease
The mechanisms by which chymase is induced and the functional role for chymase in the progression of kidney disease are largely unknown and required much further attentions. Without such information available, it is difficult to evaluate the importance of the chymase-dependent Ang II generating pathway in progressive renal injury.
Our recent finding of differential upregulation of ACE and chymase during the progression of diabetic nephropathy will stimulate the studies in this potential important aspect and supports the notion that a therapy combining an AT1 receptor blocker and ACE inhibitor produces additional benefit to patients with or without diabetes (25-30). These findings also provide a rationale for applying this combination regimen in the prevention and treatment of hypertensive and diabetic nephropathy.
This work is supported by grants from the Juvenile Diabetes Research Foundation (JDRF 1-2001-596) and NIH/NIDDK (P50 DK064232-1).