RXR antagonism induces G 0/G 1 cell cycle arrest and ameliorates obesity by up-regulating the p53-p21 Cip1 pathway in adipocytes
Department of Medicine and Clinical Science, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Japan. The Journal of Pathology
(Impact Factor: 7.43).
04/2012; 226(5):784-95. DOI: 10.1002/path.3001
The peroxisome proliferator activated receptor-γ (PPARγ) agonist, pioglitazone (PIO), exerts anti-diabetic properties associated with increased fat mass, whereas the retinoid X receptor (RXR) antagonist HX531 demonstrates anti-obesity and anti-diabetic effects with reduced body weight and fat pad mass. The cell cycle abnormality in adipocytes has not been well-investigated in obesity or during treatment with modulators of nuclear receptors. We therefore investigated cell size and cell cycle distributions of adipocytes in vivo and examined the expression of cell cycle regulators in cultured human visceral preadipocytes. The cell size distribution and cell cycle analyses of in vivo adipocytes derived from OLETF rats demonstrated that HX531 brought about G0/G1 cell cycle arrest associated with the inhibition of cellular hypertrophy, which resulted in the reduction of fat pad mass. In contrast, PIO promoted proliferation activities associated with the increase in M + late M:G0 + G1 ratio and the appearance of both small and hypertrophied adipocytes. In cultured human visceral preadipocytes HX531 up-regulated cell cycle regulators, p53, p21(Cip1), cyclin D1, Fbxw7 and Skp2, which are known contributors towards G0 /G1 cell cycle arrest. The knockdown of p53 with a shRNA lentivirus reversed the HX531-induced up-regulation of p21(Cip1), which is one of the major p53-effector molecules. We conclude that HX531 exerts anti-obesity and anti-diabetes properties by up-regulating the p53-p21(Cip1) pathway, resulting in G0/G1 cell cycle arrest and the inhibition of cellular hypertrophy of adipocytes.
Available from: Liqing Zang
- "PPARA regulates genes involved in lipid metabolism via heterodimerization with RXR as an obligate partner4243, suggesting interaction between eriocitrin and RXR, especially RXR-α. RXR antagonist HX531 has been reported to ameliorate obesity4445; therefore, we hypothesize that eriocitrin acts as an RXR antagonist in liver adiposity. In fact, daidzein (a flavonoid found in soybeans) has been shown to suppress RXR-α expression46 and improve lipid metabolism47, which is in accordance with the prediction of RXR enrolment in eriocitrin pathways. "
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ABSTRACT: Lemon (Citrus limon) contains various bioactive flavonoids, and prevents obesity and obesity-associated metabolic diseases. We focused on eriocitrin (eriodictyol 7-rutinoside), a powerful antioxidative flavonoid in lemon with lipid-lowering effects in a rat model of high-fat diet. To investigate the mechanism of action of eriocitrin, we conducted feeding experiments on zebrafish with diet-induced obesity. Oral administration of eriocitrin (32 mg/kg/day for 28 days) improved dyslipidaemia and decreased lipid droplets in the liver. DNA microarray analysis revealed that eriocitrin increased mRNA of mitochondrial biogenesis genes, such as mitochondria transcription factor, nuclear respiratory factor 1, cytochrome c oxidase subunit 4, and ATP synthase. In HepG2 cells, eriocitrin also induced the corresponding orthologues, and reduced lipid accumulation under conditions of lipid loading. Eriocitrin increased mitochondrial size and mtDNA content, which resulted in ATP production in HepG2 cells and zebrafish. In summary, dietary eriocitrin ameliorates diet-induced hepatic steatosis with activation of mitochondrial biogenesis.
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Tumor protein p53 is a transcription factor involved with cellular responses to stressors including limited glucose availability. We hypothesized that modulating p53 levels would affect cellular glucose uptake.
Methods and results:
Transfecting cultured primary mouse hepatocytes with p53 siRNA suppressed p53 mRNA expression >90%. Control hepatocytes (transfected with non-targeting siRNA) increased glucose uptake (2.28 ± 1.02-fold vs basal, p 0.009) in response to 100 nM insulin, but p53 siRNA-treated hepatocytes had a blunted response (0.92 ± 0.11-fold vs basal; between group difference p 0.0012). In adipocytes differentiated from the pre-adipocyte line 3T3-L1, knockdown of p53 had no effect on insulin-stimulated glucose uptake. There were no differences in Glut 1 or Glut 2 expression in the plasma membrane fraction or in the levels of phosphorylated AKT in cell lysates between primary hepatocytes transfected with p53 siRNA or control siRNA. Glycemic responses to insulin tolerance, glucose tolerance, and pyruvate tolerance tests did not differ between p53 knockout and wild type mice.
Thus, inhibition of p53 has pleiotropic effects, inhibiting glucose uptake in the liver but having no effect on adipocytes. Knockout of p53 has no apparent effect on glucose homeostasis in intact lean mice. An explanation for the association between p53 expression and hepatocyte glucose uptake remains to be elucidated.
Available from: Amar Bahadur Singh
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ABSTRACT: Diabetic cardiomyopathy (DCM) is a significant contributor to the morbidity and mortality associated with diabetes and metabolic syndrome. Retinoids, through activation of retinoic acid receptor (RAR) and retinoid x receptor (RXR), have been linked to control of glucose and lipid homeostasis, with effects on obesity and diabetes. However, the functional role of RAR and RXR in the development of DCM remains unclear. Zucker diabetic fatty (ZDF) and lean rats were treated with Am580 (RARα agonist) or LGD1069 (RXR agonist) for 16 weeks, and cardiac function and metabolic alterations were determined. Hyperglycemia, hyperlipidemia and insulin resistance were observed in ZDF rats. Diabetic cardiomyopathy was characterized in ZDF rats by increased oxidative stress, apoptosis, fibrosis, inflammation, activation of MAP kinases and NF-κB signaling and diminished Akt phosphorylation, along with decreased glucose transport and increased cardiac lipid accumulation, and ultimately diastolic dysfunction. Am580 and LGD1069 attenuated diabetes-induced cardiac dysfunction and the pathological alterations, by improving glucose tolerance and insulin resistance; facilitating Akt activation and glucose utilization, and attenuating oxidative stress and interrelated MAP kinase and NF-κB signaling pathways. Am580 inhibited body weight gain, attenuated the increased cardiac fatty acid uptake, β-oxidation and lipid accumulation in the hearts of ZDF rats. However, LGD1069 promoted body weight gain, hyperlipidemia and cardiac lipid accumulation. In conclusion, our data suggest that activation of RAR and RXR may have therapeutic potential in the treatment of diabetic cardiomyopathy. However, further studies are necessary to clarify the role of RAR and RXR in the regulation of lipid metabolism and homeostasis.
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