Protection of the kidney by thiazolidinediones: An assessment from bench to bedside

Thiazolidinediones provide better renoprotection than insulin in an obese, hypertensive type II diabetic rat model

Hyperinsulinemia has been implicated in the development of diabetic nephropathy. In the present study we compared the renoprotective effects of the thiazolidinedione, pioglitazone (PGZ), to that of insulin in a hypertensive, obese, type II diabetic rat model. PGZ aggravated obesity and gave less glycemic control than insulin. However, renoprotection was markedly better with PZG compared to insulin as shown by lower proteinuria, improved renal function, and less histological evidence of diabetic glomerular and tubulointerstitial lesions. PZG and insulin both reduced renal accumulation of pentosidine and oxidative stress to a similar extent. In contrast, PGZ but not insulin suppressed enhanced transforming growth factor-beta (TGF-beta) expression. We further confirmed in cultured rat proximal tubular cells that insulin enhanced TGF-beta mRNA expression and protein production. Our results identify hyperinsulinemia and the attendant increase of TGF-beta expression as potential therapeutic targets in diabetes independent of glycemic control. This confirms prior clinical evidence that PZG provides renoprotection in obese, diabetic patients with nephropathy.

Role of altered renal lipid metabolism in the development of renal injury induced by a high-fat diet

Metabolic syndrome is associated with increased risk of chronic kidney disease, and the renal injury in patients with metabolic syndrome may be a result of altered renal lipid metabolism. We fed wild-type or insulin-sensitive heterozygous peroxisome proliferator-activated receptor gamma-deficient (PPARgamma(+/-)) mice a high-fat diet for 16 weeks. In wild-type mice, this diet induced core features of metabolic syndrome, subsequent renal lipid accumulation, and renal injury including glomerulosclerosis, interstitial fibrosis, and albuminuria. Renal lipogenesis accelerated, determined by increased renal mRNA expression of the lipogenic enzymes fatty acid synthase and acetyl-CoA carboxylase (ACC) and by increased ACC activity. In addition, renal lipolysis was suppressed, determined by reduced mRNA expression of the lipolytic enzyme carnitine palmitoyl acyl-CoA transferase 1 and by reduced activity of AMP-activated protein kinase. In PPARgamma(+/-) mice, renal injury, systemic metabolic abnormalities, renal accumulation of lipids, and the changes in renal lipid metabolism were attenuated. Thus, a high-fat diet leads to an altered balance between renal lipogenesis and lipolysis, subsequent renal accumulation of lipid, and renal injury. We suggest that renal lipid metabolism could serve as a new therapeutic target to prevent chronic kidney disease in patients with metabolic syndrome.

Pioglitazone shift circadian rhythm of blood pressure from non-dipper to dipper type in type 2 diabetes mellitus

BACKGROUND

Insulin resistance significantly correlated with a non-dipper type of essential hypertension. Thiazolidinediones (TZD), oral hypoglycaemic agents that act as insulin sensitizers, have been demonstrated in multiple in vivo and in vitro studies to possess antihypertensive properties. This study examined the efficacy of TZD therapy with pioglitazone at transforming the circadian rhythms of blood pressure from a non-dipper to a dipper type.

MATERIALS

We examined 31 patients with type 2 diabetes mellitus during both a baseline period and a period of treatment with pioglitazone. Patients received 15 mg day(-1) pioglitazone for four weeks and 30 mg day(-1) for 12 weeks. Twenty-four hour ambulatory blood pressure monitoring (ABPM) and laboratory data (blood tests for cardiovascular risk factors) were obtained at the beginning and end of the study.

RESULTS

In non-dippers (n = 16), but not dippers (n = 15), we observed a significant interaction between pioglitazone therapy and nocturnal falls in systolic and diastolic blood pressure. This examination indicated that the magnitude of the nocturnal blood pressure fall was affected by pioglitazone therapy. In non-dippers, but not dippers, a significant correlation was observed between the percent decrease in nocturnal BP and the homeostasis model assessment (HOMA) index (r = 0.774, P = 0.0007).

CONCLUSIONS

The present study demonstrated that pioglitazone can restore the nocturnal BP declines in parallel to reductions in the HOMA index, suggesting that insulin resistance may play an important role in the genesis of circadian BP rhythms. TZD-based treatment may thus have the additional therapeutic advantage of reducing the risk of cardiovascular complications by transforming the circadian rhythm of BP.

Lipoprotein metabolism in chronic renal insufficiency

Chronic renal insufficiency (CRI) is associated with a characteristic dyslipidemia. Findings in children with CRI largely parallel those in adults. Moderate hypertriglyceridemia, increased triglyceride-rich lipoproteins (TRL) and reduced high-density lipoproteins (HDL) are the most usual findings, whereas total and low-density lipoprotein cholesterol (LDL-C) remain normal or modestly increased. Qualitative abnormalities in lipoproteins are common, including small dense LDL, oxidized LDL, and cholesterol-enriched TRL. Measures of lipoprotein lipase and hepatic lipase activity are reduced, and concentrations of apolipoprotein C-III are markedly elevated. Still an active area of research, major pathophysiological mechanisms leading to the dyslipidemia of CRI include insulin resistance and nonnephrotic proteinuria. Sources of variability in the severity of this dyslipidemia include the degree of renal impairment and the modality of dialysis. The benefits of maintaining normal body weight and physical activity extend to those with CRI. In addition to multiple hypolipidemic pharmaceuticals, fish oils are also effective as a triglyceride-lowering agent, and the phosphorous binding agent sevelamer also lowers LDL-C. Emerging classes of hypolipidemic agents and drugs affecting sensitivity to insulin may impact future treatment. Unfortunately, cardiovascular benefit has not been convincingly demonstrated by any trial designed to study adults or children with renal disease. Therefore, it is not possible at this time to endorse general recommendations for the use of any agent to treat dyslipidemia in children with chronic kidney disease.

Rosiglitazone increases PPARgamma in renal tubular epithelial cells and protects against damage by hydrogen peroxide

BACKGROUND/AIMS

Thiazolidinediones (TZD) are ligands known to bind to and activate the nuclear peroxisome proliferator-activated receptor gamma (PPARgamma), and are currently used as insulin sensitizers in type 2 diabetes. Recently, several studies have shown that TZD may have a role in renal protection in various experimental models. However, the precise mechanisms by which TZD may possibly affect tubular cell survival after injury remain unclear. We studied the influence of the TZD rosiglitazone on PPARgamma expression and cell function with cellular damage induced by increasing hydrogen peroxide (H2O2) concentrations in bovine renal tubular epithelial cells (bEPC) to determine whether rosiglitazone is cytoprotective under these conditions.

METHODS

bEPC were cultured in the presence of H2O2 after pretreatment with or without 25 microM rosiglitazone. The expression of PPARgamma mRNA and protein were determined using RT-PCR or Western blots, respectively, after 6 and 24 h. Some cells also received actinomycin D or cycloheximide and PPARgamma protein expression was tested. Proliferation rates of cultures were compared after 15 h and after a recovery phase of 6 days. Apoptosis was assessed by DNA fragmentation. Nuclear PPARgamma activity was evaluated by electrophoretic mobility shift assay (EMSA), and the cellular location was detected using immunofluorescence.

RESULTS

Incubation of bEPC with H2O2 concentrations up to 0.75 mM did not induce apoptosis as tested by DNA fragmentation assay, but significantly and dose-dependently reduced proliferation 15 h after injury as measured by [3H]thymidine incorporation. 25 microM rosiglitazone alone also reduced proliferation and failed to attenuate the H2O2-mediated inhibition of proliferation. However, rosiglitazone facilitates recovery of tubular cells 6 days after H2O2-induced injury. Rosiglitazone (25 microM) increased PPARgamma mRNA and protein expression in bEPC in the absence of H2O2. Rosiglitazone failed to increase PPARgamma mRNA in cells with oxidative stress, but Western blots revealed an increase in cellular PPARgamma protein content in the presence of rosiglitazone and increasing concentrations of H2O2. This increase in PPARgamma protein content was almost totally abolished in the presence of 1 microg/ml cycloheximide, but was only marginally reduced by 0.1 microg/ml actinomycin D. EMSA showed a robust increase in nuclear PPARgamma protein binding in vitro to its consensus site after rosiglitazone whereas H2O2 treatment reduced PPARgamma activation. Rosiglitazone treatment of cells with oxidative stress preserved nuclear transactivation of PPARgamma.

CONCLUSIONS

Rosiglitazone increases the PPARgamma content in bEPC after H2O2-induced injury by a posttranscriptional mechanism. Activation of PPARgamma facilitates the long-term recovery of tubular cells 6 days after oxidative injury, but had no effect on the attenuated proliferation shortly after injury. TZD cannot prevent oxidative injury to tubular cells, but may be important mediators to enhance cellular recovery after oxidative stress.

Protektion der Nierenfunktion bei Diabetikern

The course of diabetic nephropathy is affected by several factors that can be manipulated. In primary prevention, near normal metabolic control beginning at the time of the diagnosis of diabetes diminishes the risk of microalbuminuria to an extent depending on the HbA1c level attained. Hypertensive diabetics should be consistently treated with renin-angiotensin system (RAS)-blocking agents. In secondary prevention, a multifactorial therapy is able to stop or retard further progression. Its components are a sustained decrease of blood pressure in the normotensive range using RAS-blocking agents (normotensive patients should also be treated) as well as near normal metabolic control that takes into account the changing pharmacokinetic and pharmacodynamic properties of blood glucose-lowering drugs in renal insufficiency. Consideration of further factors (smoking, protein intake, anemia) and several general nephroprotective measures complete the treatment spectrum. Therapy for dyslipidemia and the administration of aspirin are important in view of the high cardiovascular morbidity. It is essential to monitor kidney function and the therapeutic components at short intervals.

Notes on the kidney and its diseases for the neurologist

To save their patients from dialysis and transplantation, neurologists need simply remain alert to the possibility of renal failure, particularly in the context of systemic disease, diabetes, sepsis and drugs. Of the numerous territories shared by our respective specialities, we outline a pragmatic approach to the diagnosis and treatment of the vasculitides, underpinned by knowing which questions to ask, equally importantly when to ask them, and in the art of obtaining a tissue diagnosis. We consider the current evolving trial evidence that directs the usage of a growing arsenal of therapies in the induction and maintenance stages of vasculitis treatment, and extend this consideration to Lupus and Sjogren's.

Clinical trials in nephrology: success or failure

PURPOSE OF REVIEW

A large amount of clinical and epidemiologic evidence has been gathered supporting the importance of blood pressure control in reducing chronic kidney disease progression. Suppression of the rennin-angiotensin-aldosterone system should also be considered in any patient with chronic kidney disease, in particular if albuminuria is present.

RECENT FINDINGS

Analysis of renal outcome by estimating glomerular filtration rate in trials primarily devoted to cardiovascular protection in hypertensive patients, in particular the Antihypertensive and Lipid-Lowering treatment to prevent Heart Attack Trial trial, has come to question the effectiveness of suppressing the rennin-angiotensin-aldosterone system in controlling blood pressure and thus protecting the kidney in hypertensive patients. This subject is particularly interesting because the existence of an increased cardiovascular risk associated with renal function decline has been demonstrated in many different clinical conditions including arterial hypertension. The increase in global cardiovascular risk accompanying chronic kidney disease would necessitate the use of drugs suppressing the rennin-angiotensin-aldosterone system for cardiovascular protection irrespective of the influence on renal outcome.

SUMMARY

This review includes the most recent data evaluating renal endpoints in clinical trials primarily devoted to renal function as well as those dedicated to arterial hypertension and its cardiovascular consequences.

Insulin resistance, hyperinsulinemia, and renal injury: mechanisms and implications

Most of the basic components of the metabolic syndrome, namely type 2 diabetes mellitus, hypertension, obesity, or low high-density lipoprotein cholesterol levels, apart from being major risk factors for cardiovascular disease have been also associated with an increased risk of chronic kidney disease. However, several epidemiologic studies conducted over the past years suggest that the central component of the syndrome, insulin resistance, as well as compensatory hyperinsulinemia are independently associated with an increased prevalence of chronic kidney disease. In addition, background studies support the existence of several pathways linking insulin resistance and hyperinsulinemia with kidney damage. Insulin per se promotes the proliferation of renal cells and stimulates the production of other important growth factors such as insulin-like growth factor-1 and transforming growth factor beta. Insulin also upregulates the expression of angiotensin II type 1 receptor in mesangial cells, thus enhancing the deleterious effects of angiotensin II in the kidney, and stimulates production and renal action of endothelin-1. Moreover, insulin resistance and hyperinsulinemia are associated with decreased endothelial production of nitric oxide and increased oxidative stress which have been also implicated in the progression of diabetic nephropathy. This review analyzes the above and other potential mechanisms, through which insulin resistance and hyperinsulinemia can contribute to renal injury.