Rekha Singh

Mechanism of Increased Angiotensin II Levels in Glomerular Mesangial Cells Cultured in High Glucose

Previous studies have shown that glucose increases angiotensin II (AngII) levels in rat glomerular mesangial cells and that AngII mediates the inhibitory effects of high glucose on matrix degradation in these cells. The present study addresses the following questions: (1) What are the mechanisms for the generation of AngII in mesangial cells? (2) What are the effects of glucose on AngII generation by these mechanisms? Experiments employed primary mesangial cells from normal Sprague-Dawley rats. The levels of immunoreactive angiotensinogen (AGT), angiotensin I (AngI), and angiotensin II (AngII) were measured by ELISA. AGT mRNA expression was determined by Northern blot analysis. Incubation of cells for 24 h in high glucose (30 mM) increased AGT levels by 1.5-fold and increased AGT mRNA expression; this was accompanied by a 1.5-fold increment in AngI and 1.7-fold increment in AngII levels. Renin activity (measured as AngI generation in the presence of excess AGT) and ACE levels and activity were not altered by high glucose. In further experiments, the effect of high glucose on formation of Ang peptides from exogenous AngI in mesangial cell extracts was examined using HPLC. Exogenous AngI was converted into various Ang peptides, including AngII, Ang(1-9), Ang(1-7), and Ang(3-8). A significant increase in formation of AngII from AngI was observed in cells incubated in high glucose. In addition, AngII production from exogenous Ang(1-9) in cell extracts was also stimulated by high glucose. These findings demonstrate that glucose increases mesangial AngII levels via an increase in AGT and AngI. In addition, this study provides new information that Ang(1-9) is produced by mesangial cells, can be converted to AngII, and that this conversion is also stimulated under high-glucose conditions.

Glomerular renin angiotensin system in streptozotocin diabetic and Zucker diabetic fatty rats.

Substantial evidence suggests that the intrarenal renin-angiotensin system (RAS) plays a role in the pathogenesis of diabetic nephropathy. Although the glomerular RAS is activated in the streptozotocin (STZ)-diabetic rat, the status of the glomerular RAS in the Zucker diabetic fatty (ZDF) rat, which is a commonly used genetic model of diabetes, is not known. Angiotensinogen (AGT), angiotensin II (Ang II), angiotensin converting enzyme (ACE), and angiotensin converting enzyme 2 (ACE2) were measured in glomeruli isolated from 4-week-old STZ-diabetic rats and 32-week-old ZDF rats. Glomerular injury was evaluated by histopathologic methods. Both STZ-diabetic and ZDF rats exhibited marked hyperglycemia and renal hypertrophy, but only ZDF rats demonstrated proteinuria and glomerulosclerosis. Glomerular AGT and Ang II levels were increased significantly in STZ-diabetic compared with nondiabetic control rats, accompanied by a reduction in ACE2 activity. In contrast, glomerular AGT, Ang II, and ACE2 were similar in ZDF rats and lean controls. ACE levels were not affected by diabetes in either diabetic model. In conclusion, the glomerular RAS is activated in the STZ diabetic rat but not in the ZDF rat despite a similar degree of hyperglycemia. The mechanism of nephropathy in the ZDF rat may involve factors other than hyperglycemia and RAS activation, such as hypertension and hyperlipidemia.

Effect of High Glucose on Superoxide in Human Mesangial Cells: Role of Angiotensin II

Background/Aims: Reactive oxygen species, and especially superoxide (O2·–),have been implicated in diabetic nephropathy. O2·– accumulation in cells is dependent on O2·– production (by NADH/NADPH oxidase) as well as scavenging by superoxide dismutase (SOD) activity. This study was designed to investigate the effects of high glucose (HG) on O2·– accumulation and SOD activity in human mesangial cells (HMC) and to determine if these effects are mediated by angiotensin II (Ang II). Methods: HMC were incubated in media containing 10 mMglucose (control, C), 30 mM glucose (HG), 10 mM glucose + either 20 mM 2-deoxy-D-glucose (2-DG) or 20 mM mannitol (high mannitol, HM) (osmotic controls), or Ang II (10–5M). Ang II action was antagonized by employing 10–4M of Ang II receptor antagonists (losartan or irbesartan) or 10–4M of NADH/NADPH oxidase inhibitors [diphenyleneiodonium chloride (DPI) or apocynin]. Superoxide and total SOD activity were assayed using chemiluminescence of lucigenin. Results: Incubation of HMC in HG resulted in a 1.6-fold increase in Ang I (p < 0.05) and a 1.4-fold increase in Ang II levels (p < 0.05) in cell lysates. These changes were accompanied by a >2-fold increase in O2·– accumulation (p < 0.01), which was inhibited by losartan and irbesartan. Exogenous Ang II increased net O2·– accumulation by 2.7-fold (p < 0.01), which was normalized by losartan and irbesartan. DPI and apocynin blocked the HG and Ang II-induced increases in O2·– (p < 0.01). HG but not exogenous Ang II inhibited total SOD activity by 30%, which was not affected by losartan. Conclusion: High glucose increases O2–· accumulation in HMC primarily by increasing its production via the Ang II-NADH/NADPH oxidase system.

Inhibition of intracellular Angiotensin II formation blocks high glucose effect on mesangial matrix

High glucose causes increased matrix synthesis by glomerular mesangial cells and angiotensin II (Ang II) has been shown to mediate this effect of glucose. These studies investigate whether inhibition of Ang II formation can block high glucose-induced increase in mesangial matrix. Human mesangial cells were incubated with 25 mM glucose (HG) along with captopril, an ACE inhibitor, to block Ang II formation. In other experiments, cells were nucleofected with siRNA to knockdown angiotensinogen (Agt), the precursor of Ang II, and then exposed to high glucose. Captopril blocked high glucose-induced increase in Ang II levels in the cell media (extracellular) but failed to inhibit it in the cell lysate (intracellular). Moreover, captopril treatment did not block the stimulatory effect of high glucose on TGF-beta1 and fibronectin. In contrast, knockdown of the Agt gene prevented high glucose-induced increase in both extracellular and intracellular Ang II levels, and was accompanied by normalization of TGF-beta1 and fibronectin. These data suggest that intracellular Ang II may play an important role in the mediation of the high glucose effect on matrix and that ACE inhibitors may not be effective in blocking intracellular Ang II formation in mesangial cells.

Angiotensin II and Its Receptors in the Pathogenesis of Diabetic Nephropathy

Diabetic nephropathy (DN) is characterized by accumulation of extracellular matrix (ECM) in the kidney. Glomerular mesangial expansion and tubulo-interstitial fibrosis eventually leads to renal failure. The mediators of renal injury in this disease have not been fully identified. The peptide angiotensin (Ang) II has many hemodynamic and biochemical effects that could contribute to DN (Table 1). A prominent role for Ang II has been suggested by experimental and clinical evidence indicating that angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs) have renoprotective effects and that these agents can attenuate the progression of glomerulosclerosis (1). In clinical studies, as well as studies conducted in experimental diabetic animals, it is difficult to separate hemodynamic from nonhemodynamic effects of Ang II. On the other hand, in vitro studies using cultured cells allow study of the specifically nonhemodynamic effects of Ang II and its inhibition (2). These nonhemodynamic effects of Ang II include stimulation of transforming growth factor (TGF)-β1, activation of matrix protein synthesis, and inhibition of matrix degradation (3,4). Ang II also increases generation of reactive oxygen species (ROS) in mesangial cells (MCs) (5) and may contribute to oxidant-induced renal injury.

Bitter Melon Extract Promotes Granulation Tissue Growth and Angiogenesis in the Diabetic Wound.

OBJECTIVE: Bitter melon is a plant fruit that has been shown to exert a hypoglycemic effect when used systemically in patients with diabetes. This study was designed to investigate the topical effect of bitter melon on diabetic wounds using the wound chamber model in rats. DESIGN: Two bilateral wound chambers were implanted subcutaneously in the thoracic-lumbar region of male Sprague-Dawley rats. Diabetes was induced with streptozotocin 7 days after implantation of wound chambers. After 24 hours of induction of diabetes, aqueous extract of bitter melon was injected into 1 wound chamber, and saline (0.9% sodium chloride solution) was injected into the contralateral chamber once daily for 3 days. Wound fluid was collected on day 4 for analysis, following which rats were euthanized. The granulation tissue encapsulating the wound chamber was removed and processed for histology. Controls included diabetic rats with wound chambers injected with saline (instead of bitter melon) and nondiabetic rats with wound chambers injected with bitter melon. RESULTS: In rats with diabetes, wound granulation tissue treated with bitter melon was well formed, with distinct cellular layers, whereas the saline-treated granulation tissue showed a severe loss of tissue organization and blood vessels. Moreover, the bitter melon treatment increased angiogenesis in the diabetic granulation tissue, marked by abundant microvessels and large blood vessels. In nondiabetic rats, no differences in wound granulation tissues were observed between saline- and bitter melon-treated groups. Bitter melon treatment had no effect on systemic blood glucose levels or insulin receptor substrate 1, suggesting that its stimulatory effect on diabetic granulation tissue was not due to alteration of systemic blood glucose levels. CONCLUSIONS: When applied locally to diabetic wounds, bitter melon extract prevents regression of granulation tissue and blood vessels, thus accelerating and improving wound healing.