Curcumin protects against fructose-induced podocyte insulin signaling impairment through upregulation of miR-206.

SCOPE: Fructose consumption can induce insulin resistance and metabolic syndrome, which are associated with glomerular podocyte dysfunction and proteinuria. This study investigated whether fructose caused insulin signaling impairment in podocyte dysfunction and injury, and whether curcumin reduced these disturbances. METHODS AND RESULTS: Rats were fed with 10% fructose for 6 weeks and then orally cotreated with curcumin for next 6 weeks. Metabolic syndrome, podocyte injury, microRNA expression, and insulin signaling were evaluated. Curcumin significantly alleviated fructose-induced podocyte injury and proteinuria, miR-206 low-expression, protein tyrosine phosphatase 1B (PTP1B) overexpression, as well as downregulation of insulin receptor, insulin receptor substrate 1, caveolin-1, protein kinase B, and extracellular signal-regulated kinases 1 and 2 phosphorylation in kidney cortex or glomeruli of fructose-fed rats. These effects were further confirmed in cultured differentiated podocytes exposed to 5 mM fructose in the presence or absence of curcumin, PTP1B siRNA, lentivirus-mediated PTP1B recombinant overexpression, miR-206 mimic, or miR-206 inhibitor transfection, showing that miR-206 upregulation may contribute to improve insulin signaling through regulating PTP1B expression. CONCLUSION: Curcumin is suggested to activate miR-206 expression to downregulate PTP1B, and then improve insulin signaling, protect against fructose-induced glomerular podocyte injury, and proteinuria, which may provide new evidence regarding curcumin's effects on fructose-associated podocyte injury.

Wuling San protects kidney dysfunction by inhibiting renal TLR4/MyD88 signaling and NLRP3 inflammasome activation in high fructose-induced hyperuricemic mice.

ETHNOPHARMACOLOGICAL RELEVANCE: Wuling San, a famous prescription in Chinese medicine, is composed of Polyporus, Poria, Alismatis rhizoma, Cinnamomi cortex and Atractylodis macrocephalae rhizoma, and promotes kidney function and diuresis. The main purpose of this study was to investigate its renal protective effect in high fructose-induced hyperuricemic mice. MATERIALS AND METHODS: ICR mice were fed with 30% fructose in drinking water for 6 weeks to induce hyperuricemia and renal dysfunction. Then mice were orally administrated for other 6 weeks with Wuling San (987, 1316, 1755 and 2340mg/kg), allopurinol (5mg/kg) and water daily, respectively. Serum and urine levels of uric acid, creatinine and blood urea nitrogen (BUN) were measured. Hematoxylin and eosin staining was used to assess renal histological changes. Renal interleukin (IL)-1ß concentrations were measured using ELISA kit. Renal protein levels of organic ion transporters, as well as toll-like receptor 4 (TLR4)/myeloid differentiation factor 88 (MyD88) signaling and pyrin domain containing 3 (NLRP3) inflammasome were determined by Western blot assay. RESULTS: Wuling San significantly decreased serum uric acid, creatinine and BUN levels, increased fractional excretion of uric acid (FEUA) in fructose-fed mice. It restored fructose-induced dysregulation of renal urate transporter 1 (URAT1), glucose transporter 9 (GLUT9), ATP-binding cassette subfamily G member 2 (ABCG2) and organic anion transporter 1 (OAT1), as well as organic cation transporter 1 (OCT1) and OCT2 in mice. Wuling San obviously alleviated infiltration of inflammation cells in kidney glomerulus of fructose-fed mice. Moreover, Wuling San suppressed the activation of TLR4/ MyD88 signaling to inhibit nuclear factor κB (NF-κB) signaling and mitogen-activated protein kinases (MAPKs) activation in fructose-fed mice. Additionally, Wuling San decreased NLRP3 inflammasome activation and IL-1ß secretion in the kidney of fructose-fed mice. CONCLUSION: Wuling San exerts renal protective effect by modulating renal organic ion transporters in fructose-induced hyperuricemic mice. The molecular mechanism of its action may be associated with the suppression of TLR4/MyD88 signaling and NLRP3 inflammasome activation to reduce IL-1ß production in high fructose-induced hyperuricemic mice.

Pterostilbene and allopurinol reduce fructose-induced podocyte oxidative stress and inflammation via microRNA-377.

High dietary fructose is an important causative factor in the development of metabolic syndrome-associated glomerular podocyte oxidative stress and injury. Here, we identified microRNA-377 (miR-377) as a biomarker of oxidative stress in renal cortex of fructose-fed rats, which correlated with podocyte injury and albuminuria in metabolic syndrome. Fructose feeding increased miR-377 expression, decreased superoxide dismutase (SOD) expression and activity, and caused O2(-) and H2O2 overproduction in kidney cortex or glomeruli of rats. This reactive oxygen species induction increased p38 MAPK phosphorylation and thioredoxin-interacting protein (TXNIP) expression and activated the NOD-like receptor pyrin domain-containing 3 (NLRP3) inflammasome to produce interleukin-1ß in kidney glomeruli of fructose-fed rats. These pathological processes were further evaluated in cultured differentiated podocytes exposed to 5mM fructose, or transfected with miR-377 mimic/inhibitor and TXNIP siRNA, or co-incubated with p38 MAPK inhibitor, demonstrating that miR-377 overexpression activates the O2(-)/p38 MAPK/TXNIP/NLRP3 inflammasome pathway to promote oxidative stress and inflammation in fructose-induced podocyte injury. Antioxidants pterostilbene and allopurinol were found to ameliorate fructose-induced hyperuricemia, podocyte injury, and albuminuria in rats. More importantly, pterostilbene and allopurinol inhibited podocyte miR-377 overexpression to increase SOD1 and SOD2 levels and suppress the O2(-)/p38 MAPK/TXNIP/NLRP3 inflammasome pathway activation in vivo and in vitro, consistent with the reduction of oxidative stress and inflammation. These findings suggest that miR-377 plays an important role in glomerular podocyte oxidative stress, inflammation, and injury driven by high fructose. Inhibition of miR-377 by antioxidants may be a promising therapeutic strategy for the prevention of metabolic syndrome-associated glomerular podocyte injury.

Quercetin inhibits AMPK/TXNIP activation and reduces inflammatory lesions to improve insulin signaling defect in the hypothalamus of high fructose-fed rats.

Fructose is a nutritional composition of fruits and honey. Its excess consumption induces insulin resistance-associated metabolic diseases. Hypothalamic insulin signaling plays a pivotal role in controlling whole-body insulin sensitivity and energy homeostasis. Quercetin, a natural flavonoid, has been reported to ameliorate high fructose-induced rat insulin resistance and hyperlipidemia. In this study, we investigated its regulatory effects on the hypothalamus of high fructose-fed rats. Rats were fed 10% fructose in drinking water for 10 weeks. After 4 weeks, these animals were orally treated with quercetin (50 and 100 mg/kg), allopurinol (5 mg/kg) and water daily for the next 6 weeks, respectively. Quercetin effectively restored high fructose-induced hypothalamic insulin signaling defect by up-regulating the phosphorylation of insulin receptor and protein kinase B. Furthermore, quercetin was found to reduce metabolic nutrient sensors adenosine monophosphate-activated protein kinase (AMPK) activation and thioredoxin-interacting protein (TXNIP) overexpression, as well as the glutamine-glutamate cycle dysfunction in the hypothalamus of high fructose-fed rats. Subsequently, it ameliorated high fructose-caused hypothalamic inflammatory lesions in rats by suppressing the activation of hypothalamic nuclear factor κB (NF-κB) pathway and NOD-like receptor 3 (NLRP3) inflammasome with interleukin 1ß maturation. Allopurinol had similar effects. These results provide in vivo evidence that quercetin-mediated down-regulation of AMPK/TXNIP and subsequent inhibition of NF-κB pathway/NLRP3 inflammasome activation in the hypothalamus of rats may be associated with the reduction of hypothalamic inflammatory lesions, contributing to the improvement of hypothalamic insulin signaling defect in this model. Thus, quercetin with the central activity may be a therapeutic for high fructose-induced insulin resistance and hyperlipidemia in humans.