Hypertension is a major risk factor for patients with type II diabetes. It has been proposed that hypertension and type II diabetes are co-morbid disease states and that the development of hypertension exacerbates the progression of diabetic nephropathy to ESRD. An inflammatory response has been identified that contributes to this progression. This sub-acute inflammatory response is characterized by an increase in the infiltration of pro-inflammatory cells into the kidney and an increase in the expression of pro-inflammatory genes [2
]. The sEH has been identified as a target for the treatment of hypertension and inflammation. Inhibitors of sEH have been reported to decrease blood pressure in hypertension and decrease hypertension-induced renal damage [13
]. In addition, sEH inhibitors have demonstrated anti-inflammatory properties and were reported to decrease inflammation-induced tissue damage [22
]. In the present study, we investigated the hypothesis that sEH inhibition protects the kidney from renal damage associated with hypertension and type II diabetes. Hypertension induced in the spontaneously type II diabetic Goto-Kakizaki rat accelerated the development of renal damage and the infiltration of pro-inflammatory cells into the kidney. AUDA treatment decreased renal damage independent of an effect on blood pressure. In addition, AUDA treatment inhibited the infiltration of pro-inflammatory cells into the kidney and decreased the expression of pro-inflammatory genes.
In the present studies, mean arterial blood pressure increased significantly in Goto-Kakizaki rats infused with angiotensin and fed a high fat diet. The oral administration of AUDA to hypertensive Goto-Kakizaki rats had no effect on mean arterial pressure. This result was of particular interest because the administration of AUDA or other sEH inhibitors were reported to decrease blood pressure in animal models of hypertension [13
]. Our laboratory has demonstrated that AUDA treatment decreased blood pressure in hypertensive Sprague-Dawley rats. In these experiments hypertension was induced with either angiotensin II infusion or angiotensin II infusion with a high salt diet [25
]. The urinary excretion of AUDA averaged 121 ± 61 ng/d for the AUDA treated angiotensin II with high salt diet-induced hypertensive group. In the current study, the urinary excretion of AUDA averaged only 28 ± 9 ng/day for the AUDA treated hypertensive Goto-Kakizaki group [25
]. Therefore, it is possible that higher AUDA levels than those attained in the current study are required to elicit effects on blood pressure. Although AUDA did not alter mean arterial pressure, AUDA treatment did have beneficial effects on the hypertensive Goto-Kakizaki rats. Therefore the data presented here suggest that the renal protective effects afforded by AUDA occur via a mechanism that is independent of its effects on blood pressure.
Hypertension induced in the Goto-Kakizaki rat resulted in an increase in blood pressure and was accompanied by a measured increase in heart rate during the second week. It was reported that angiotensin II and/or salt sensitive hypertension resulted in a transient decrease in heart rate that returns to control levels by the end of the first week [25
]. Therefore, the ability to control heart rate in the spontaneously type II diabetic Goto-Kakizaki rat after the induction of hypertension using angiotensin II and high salt diet is not the same as rats with hypertension alone. The specific contribution of increased heart rate to the development of renal damage in this animal model of hypertension and type II diabetes is not known.
The progression of renal damage in the spontaneously diabetic Goto-Kakizaki rats was accelerated by hypertension. This was evidenced by and increase in urinary albumin excretion, an indicator of renal damage. In addition, hypertension induced morphological changes in the kidney that are characteristic to the development of nephropathy such as tubular dilation and fibroid necrosis of interstitial arterioles. Similar changes were observed in other models of hypertension induced in the Goto-Kakizaki rat. Salt-sensitive hypertension resulted in a 50% increase in albuminuria and administration of deoxycorticosterone acetate salt to Goto-Kakizaki rats, a model of mineralcorticoid-induced experimental hypertension, induced a 4-fold increase in proteinuria [26
]. Likewise, heminephrectomized Goto-Kakizaki rats have increased macrophage infiltration and accelerated renal damage [28
]. Treatment of hypertensive Goto-Kakizaki rats with AUDA inhibited hypertension-induced increases in albumin excretion and prevented the development of many of the morphological changes induced by hypertension. These renal protective effects resulting from epoxide hydrolase inhibition are consistent with data generated in our laboratory using in
vivo models of hypertension. Zhao et al. reported that chronic administration of the epoxide hydrolase inhibitor 1-cyclohexyl-3-dodecylurea (CDU) to angiotensin II-induced hypertensive Sprague-Dawley rats decreased urinary albumin excretion and protected the kidney from hypertension-induced damage [13
]. Similarly, it was reported that AUDA, decreased urinary albumin excretion and renal damage in both angiotensin II-induced and angiotensin II-induced salt-sensitive hypertensive Sprague-Dawley rats [25
]. Taken together, these data provide support for the hypothesis that AUDA protects the kidney from damage associated with hypertension and type II diabetes.
An inflammatory component has been identified that contributes to the progression of nephropathy to end stage renal disease. This sub-acute inflammatory component is characterized by an increase in the infiltration of inflammatory cells into the kidney and an increase in the expression of pro-inflammatory genes. In the present study, we demonstrated that hypertension exacerbated monocyte/macrophage infiltration in the Goto-Kakizaki kidney. This increased infiltration of inflammatory cells into the kidney cortex was accompanied by an increase in urinary MCP-1 excretion and an increase in MCP-1 gene expression in the kidney cortex. This hypertension-induced increase in the sub-acute inflammatory response corresponds with the observed increase in urinary albumin excretion and renal morphological damage. These data are consistent with renal damage that occurs in other experimental models of hypertension and heminephrectomy in the Goto-Kakizaki rat [26
]. Cheng et al. reported an increase in monocyte/macrophage infiltration into the kidney with a corresponding increase in immunostaining for the intracellular adhesion molecule-1 in Goto-Kakizaki rats with salt-sensitive hypertension [26
]. In addition, administration of deoxycorticosterone acetate salt to Goto-Kakizaki rats resulted in an increase in macrophage infiltration into the kidney and was accompanied by an increase in renal immunohistochemical staining for MCP-1 [28
]. Interestingly, treatment of hypertensive Goto-Kakizaki rats with AUDA inhibited the infiltration of monocyte/macrophages into the kidney and decreased gene expression and urinary excretion of MCP-1. Taken together, these data suggest that the development of hypertension in an animal model of type II diabetes exacerbates the progression of renal injury at least in part by inducing a sub-acute inflammatory response and that this response can be attenuated with AUDA treatment.
The sEH enzyme has been identified as a target for the treatment of hypertension and inflammation [9
]. In animal models of hypertension, sEH inhibitors are reported to decrease blood pressure and thereby inhibit hypertension-induced renal damage [13
]. The sEH inhibitors 1-cyclohexyl-3-dodecylurea and AUDA lowered blood pressure in animal models of hypertension and reduced hypertension-induced renal damage [13
]. In animal models of systemic inflammation sEH inhibitors are reported to decrease the infiltration of pro-inflammatory cells into tissue and decrease the expression of pro-inflammatory genes. The sEH inhibitor 12-(3-adamantane-1-yl-ureido)-dodecanoic acid n-butyl ester (AUDA-nBE), was reported to inhibit tobacco smoke-induced lung inflammation [22
]. In addition, the sEH inhibitors AUDA-nBE and 1-adamantan-3-(5-(2-(2-ethyl-ethoxy)ethoxy)pentyl)urea decreased liposaccharide-induced increases in plasma concentrations of pro-inflammatory cytokines like TNF-alpha, IL-6 and MCP-5 in mice [23
]. In these animal models, the protective effects of the sEH inhibitors are attributed to a decrease in EET metabolism. Although increased epoxides is the likely mechanism, it has been reported that AUDA in addition to sEH inhibition can activate the peroxisome proliferator-activated receptor-α (PPAR-α); however the concentration of AUDA required for PPAR-α activation is much higher than the concentrations obtained in the current study [29
]. Also, PPAR-α activation would be expected to decrease triglyceride levels, which were unchanged in AUDA treated hypertensive Goto-Kakizaki rats. sEH inhibitors have also been reported to increase the incorporation and retention of the EETs into endothelial phospholipids and enhance the shuttling of the EETs into alternate metabolic pathways [30
]. The contribution of these alternate actions of the sEH inhibitors in the current study are not known and therefore might contribute to the renal-protective effects observed.
In the present study, we demonstrated that hypertension, induced in a model of type II diabetes, exacerbates the development of renal damage. In addition, we provide evidence that this progression involves a sub-acute inflammatory response. We demonstrated that sEH inhibition protects the kidney from the development of renal damage independent of any effects on blood pressure. A possible mechanism by which AUDA treatment attenuates the development of renal damage in this model of hypertension and type II diabetes is via inhibition of the sub-acute inflammatory response. Taken together, the data presented provides support for the hypothesis inhibition of the epoxide hydrolase ameliorates the inflammatory component of nephropathy associated with hypertension and type II diabetes.