RXR antagonism induces G0 /G1 cell cycle arrest and ameliorates obesity by up-regulating the p53-p21(Cip1) pathway in adipocytes

Chronic kidney disease and aging: dissecting the p53/p21 pathway as a therapeutic target

Chronic kidney diseases (CKD) are a group of multi-factorial disorders that markedly impair kidney functions with progressive renal deterioration. Aging contributes to age-specific phenotypes in kidneys, which undergo several structural and functional alterations, such as a decline in regenerative capacity and increased fibrosis, inflammation, and tubular atrophy, all predisposing them to disease and increasing their susceptibility to injury while impeding their recovery. A central feature of these age-related processes is the activation of the p53/p21 pathway signaling. The pathway is a key player in cellular senescence, apoptosis, and cell cycle regulation, which are all key to maintaining the health of the kidney. P53 is a transcription factor and a tumor suppressor protein that responds to cell stress and damage. Persistent activation of cell p53 can lead to the expression of p21, an inhibitor of the cell cycle known as a cyclin-dependent kinase. This causes cells to cease dividing and leads to senescence, where cells can no longer increase. The accumulation of senescent cells in the aging kidney impairs kidney function by altering the microenvironment. As the number of senescent cells increases, the capacity of the kidney to recover from injury decreases, accelerating the progression of end-stage renal disease. This article review extensively explores the relationship between the p53/p21 pathway and cellular senescence within an aging kidney and the emerging therapeutic strategies that target it to overcome the impacts of cellular senescence on CKD.

The Emerging Role of p21 in Diabetes and Related Metabolic Disorders

In the context of cell cycle inhibition, anti-proliferation, and the dysregulation observed in certain cancer pathologies, the protein p21 assumes a pivotal role. p21 links DNA damage responses to cellular processes such as apoptosis, senescence, and cell cycle arrest, primarily functioning as a regulator of the cell cycle. However, accumulating empirical evidence suggests that p21 is both directly and indirectly linked to a number of different metabolic processes. Intriguingly, recent investigations indicate that p21 significantly contributes to the pathogenesis of diabetes. In this review, we present a comprehensive evaluation of the scientific literature regarding the involvement of p21 in metabolic processes, diabetes etiology, pancreatic function, glucose homeostasis, and insulin resistance. Furthermore, we provide an encapsulated overview of therapies that target p21 to alleviate metabolic disorders. A deeper understanding of the complex interrelationship between p21 and diabetes holds promise for informing current and future therapeutic strategies to address this rapidly escalating health crisis.

Hepatic retinaldehyde deficiency is involved in diabetes deterioration by enhancing PCK1- and G6PC-mediated gluconeogenesis

Type 2 diabetes (T2D) is often accompanied with an induction of retinaldehyde dehydrogenase 1 (RALDH1 or ALDH1A1) expression and a consequent decrease in hepatic retinaldehyde (Rald) levels. However, the role of hepatic Rald deficiency in T2D progression remains unclear. In this study, we demonstrated that reversing T2D-mediated hepatic Rald deficiency by Rald or citral treatments, or liver-specific Raldh1 silencing substantially lowered fasting glycemia levels, inhibited hepatic glucogenesis, and downregulated phosphoenolpyruvate carboxykinase 1 (PCK1) and glucose-6-phosphatase (G6PC) expression in diabetic db/db mice. Fasting glycemia and Pck1/G6pc mRNA expression levels were strongly negatively correlated with hepatic Rald levels, indicating the involvement of hepatic Rald depletion in T2D deterioration. A similar result that liver-specific Raldh1 silencing improved glucose metabolism was also observed in high-fat diet-fed mice. In primary human hepatocytes and oleic acid-treated HepG2 cells, Rald or Rald + RALDH1 silencing resulted in decreased glucose production and downregulated PCK1/G6PC mRNA and protein expression. Mechanistically, Rald downregulated direct repeat 1-mediated PCK1 and G6PC expression by antagonizing retinoid X receptor α, as confirmed by luciferase reporter assays and molecular docking. These results highlight the link between hepatic Rald deficiency, glucose dyshomeostasis, and the progression of T2D, whilst also suggesting RALDH1 as a potential therapeutic target for T2D.

The Molecular Mechanisms of Protective Role of Se on the G0/G1 Phase Arrest Caused by AFB1 in Broiler's Thymocytes

This research was designed to explore the protective effects of sodium selenite on G₀/G₁ phase arrest induced by AFB₁ in thymocytes of broilers. Two hundred eighty-eight Cobb broilers were divided into control group, + Se group (0.4 mg/kg Se), AFB₁ group (0.6 mg/kg AFB₁), and AFB₁ + Se group (0.6 mg/kg AFB₁ + 0.4 mg/kg Se). The results revealed that 0.4 mg/kg Se supplement in diets could improve the AFB₁-induced histological lesions in the thymus consisting of the more vacuoles and nuclear debris in thymic cortical area. The results of flow cytometric detect showed that 0.4 mg/kg Se relieved the G₀/G₁ phase arrest caused by AFB₁ in thymocytes. The results of transcription levels of ATM, p53, p21, p27, p15, p16, CyclinD₁, CyclinE, Cdk6, Cdk2, and PCNA genes by qRT-PC, and protein expression level of PCNA by immunohistochemistry demonstrated that 0.4 mg/kg Se could reduce the adverse effects of AFB₁ on these parameters. In conclusion, Se could relieve AFB₁-induced G₀/G₁ phase arrest by p15 (or p16)-CyclinD₁/Cdk6, ATM-p53-p21-CyclinE/Cdk2, p27-CyclinE/Cdk2 pathways.

Paternal hyperglycemia induces transgenerational inheritance of susceptibility to hepatic steatosis in rats involving altered methylation on Pparα promoter

OBJECTIVE

Diabetes exerts adverse effects on the initiation or progression of diabetes and metabolic syndrome in the next generation. In past studies, limited attention has been given to the fathers' role in shaping the metabolic landscape of offspring. Our study was designed to investigate how paternal hyperglycemia exerts an intergenerational effect in mammals as well as the underlying mechanisms.

METHODS

Hyperglycemia was introduced in male rats by intraperitoneally injected streptozotocin and these males were bred with healthy females to generate offspring. The metabolic profiles of the progeny were assessed; DNA methylation profiles and gene expression were investigated. Mutagenesis constructs of the Ppara promoter region, and a luciferase reporter assay were used to determine transcription factor binding sites (TFBSs) and the effects of hypermethylation on Ppara transcription.

RESULTS

Paternal hyperglycemia induced increased liver weight, and plasma TC, TG, LDL, accumulation of triglycerides in the liver. We discovered that CpG 13 in the amplified promoter region (-852 to -601) of Ppara was hypermethylated in adult offspring liver and expression of Ppara, Acox1, Cpt-1α, and Cd36 was down regulated. Hypermethylation of CpG site 13 in the Ppara promoter inhibited the gene transcription, probably through abrogation of SP1 binding. The same epigenetic alteration was discovered in the fetus (E16.5) liver of hyperglycemic father's progeny.

CONCLUSIONS

Paternal hyperglycemia may induce epigenetic modification of Ppara in offspring's liver, probably through interaction with SP1 binding, causing impaired lipid metabolism. Our investigation may have implications for the understanding of father-offspring interactions with the potential to account for metabolic syndromes.

Ureaplasma spp. lipid-associated membrane proteins induce human monocyte U937 cell cycle arrest through p53-independent p21 pathway

Ureaplasma spp. are known to be associated with human genitourinary tract diseases and perinatal diseases and Ureaplasma spp. Lipid-associated membrane proteins (LAMPs) play important roles in their related diseases. However, the exact mechanism underlying pathogenesis of Ureaplasma spp. LAMPs is largely unknown. In this study, we explored the pathogenic mechanisms of Ureaplasma spp. LAMPs by elucidating their role in modulating the cell cycle and related signaling pathways in human monocytic cell U937, which is highly related to the inflammatory and protective effect in infectious diseases. We utilized the two ATCC reference strains (Ureaplasma parvum serovar 3 str. ATCC 27,815 (UPA3) and Ureaplasma urealyticum serovar 8 str. ATCC 27,618 (UUR8)) and nine clinical isolates which including both UPA and UUR to study the effects of Ureaplasma spp. LAMPs on U937 in vitro. We found that LAMPs derived from UUR8 and both UPA and UUR of clinical strains markedly inhibited the cell proliferation, while UPA3 LAMPs suppressed slightly. Besides, the result of flow cytometry analysis indicated all the Ureaplasma spp. LAMPs could arrest U937 cells in G1 phase. Next, we found that the cell cycle arrest was associated with increased levels of p53 and p21, and a concomitant decrease in the levels of CDK2, CDK4, CDK6 and cyclin E1 at both transcriptional and translational levels after treatment with LAMPs derived from UUR8 or clinical strains, while only cyclin E1 was down-regulated after treatment with UPA3 LAMPs. Further study showed that p53 down-regulation had almost no effect on the distribution of cell cycle and the expression of p21. In conclusion, this study demonstrated that LAMPs derived from UUR8 and clinical strains could inhibit the proliferation of U937 cells by inducing G1 cell cycle arrest through increasing the p21 expression in a p53-independent manner, while UPA3 LAMPs could induce the cell cycle arrest slightly. Our study could contribute to the understanding of Ureaplasma spp. pathogenesis, which has potential value for the treatment of Ureaplasma spp. infections.

Is p53 Involved in Tissue-Specific Insulin Resistance Formation?

p53 constitutes an extremely versatile molecule, primarily involved in sensing the variety of cellular stresses. Functional p53 utilizes a plethora of mechanisms to protect cell from deleterious repercussions of genotoxic insults, where senescence deserves special attention. While the impressive amount of p53 roles has been perceived solely by the prism of antioncogenic effect, its presence seems to be vastly connected with metabolic abnormalities underlain by cellular aging, obesity, and inflammation. p53 has been found to regulate multiple biochemical processes such as glycolysis, oxidative phosphorylation, lipolysis, lipogenesis, β-oxidation, gluconeogenesis, and glycogen synthesis. Notably, p53-mediated metabolic effects are totally up to results of insulin action. Accumulating amount of data identifies p53 to be a factor activated upon hyperglycemia or excessive calorie intake, thus contributing to low-grade chronic inflammation and systemic insulin resistance. Prominent signs of its actions have been observed in muscles, liver, pancreas, and adipose tissue being associated with attenuation of insulin signalling. p53 is of crucial importance for the regulation of white and brown adipogenesis simultaneously being a repressor for preadipocyte differentiation. This review provides a profound insight into p53-dependent metabolic actions directed towards promotion of insulin resistance as well as presenting experimental data regarding obesity-induced p53-mediated metabolic abnormalities.

Hypoxia-Inducible Factor 1-Alpha Regulates Cancer-Inhibitory Effect of Human Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) have been shown to be able to inhibit cancer cells growth. In this study, we investigate the role and the molecular mechanism of hypoxia-inducible factor 1-alpha (HIF-1α) in inhibition of cancer cell proliferation by human MSCs through depletion and overexpression of HIF-1α in human MSCs. We found that the cell culture medium from HIF-1α-depleted Z3 cells significantly promotes breast cancer MCF-7 cell proliferation and colony formation. The expression of p21 is increased in MCF-7 cells, but p53 level remains unchanged. In contrast, the cultured medium from HIF-1α-overexpressed Z3 cells dramatically inhibits MCF-7 cell proliferation and colony formation. The expression of p21 is inhibited in MCF-7 cells, but p53 does not change. We conclude HIF-1α promotes inhibitory effect of human MCSs on breast cancer cell proliferation and colony formation. This process is tightly correlated with cell cycle protein p21 level in cancer cells.

HIF-α Promotes Chronic Myelogenous Leukemia Cell Proliferation by Upregulating p21 Expression

We sought to determine the expression levels of hypoxia-inducible factor-1α (HIF-1α) in the bone marrow chronic myelogenous leukemia (CML) patients. We also tried to determine the roles HIF-1α in the proliferation of CML cells by small interfering RNA (siRNA) knockdown. Real-time PCR was performed to determine the expression levels of HIF-1α in the bone marrows of CML patients and healthy volunteers. HIF-1α knockdown by siRNA in K562 cells was confirmed by RT-PCR. Proliferation and colony formation of the treated cells were determined by CCK8 after HIF-1α knockdown. RT-PCR and western blotting were performed to detect mRNA and protein levels of p21 and p53 in K562 cells. HIF-1α mRNA expression in the bone marrow of CML patients was significantly higher than that in the control, which was statistically significant (P < 0.05). HIF-1α knockdown dramatically reduced the proliferation of K562 cells, which was also statistically significant (P < 0.05). HIF-1α knockdown markedly reduced the colony formation ability of K562 cells, which was also statistically significant (P < 0.05). The mRNA and protein expression of p21 were significantly reduced in K562 cell after HIF-1α knockdown with affecting the mRNA and protein levels of p53. HIF-α promotes chronic CML cell proliferation by up-regulating p21 expression.

The role of the p53 tumor suppressor in metabolism and diabetes

In the context of tumor suppression, p53 is an undisputedly critical protein. Functioning primarily as a transcription factor, p53 helps fend off the initiation and progression of tumors by inducing cell cycle arrest, senescence or programmed cell death (apoptosis) in cells at the earliest stages of precancerous development. Compelling evidence, however, suggests that p53 is involved in other aspects of human physiology, including metabolism. Indeed, recent studies suggest that p53 plays a significant role in the development of metabolic diseases, including diabetes, and further that p53's role in metabolism may also be consequential to tumor suppression. Here, we present a review of the literature on the role of p53 in metabolism, diabetes, pancreatic function, glucose homeostasis and insulin resistance. Additionally, we discuss the emerging role of genetic variation in the p53 pathway (single-nucleotide polymorphisms) on the impact of p53 in metabolic disease and diabetes. A better understanding of the relationship between p53, metabolism and diabetes may one day better inform the existing and prospective therapeutic strategies to combat this rapidly growing epidemic.

Down-regulation of dihydrofolate reductase inhibits the growth of endothelial EA.hy926 cell through induction of G1 cell cycle arrest via up-regulating p53 and p21(waf1/cip1) expression

Folic acid supplementation may meliorate cardiovascular disease risk by improving vascular endothelial structure and function. However, the underlying mechanisms are still lack of a global understanding. To be used, folic acid must be converted to 7,8-dihydrofolate by dihydrofolate reductase to generate one-carbon derivatives serving as important cellular cofactors in the synthesis of nucleotides and amino acids required for cell growth. Therefore, this study explored the effect of dihydrofolate reductase knockdown on endothelial EA.hy926 cell growth and the mechanism involved. We found that down-regulation of dihydrofolate reductase inhibited EA.hy926 cell proliferation, and induced G1 phase arrest. Meanwhile, the expression of regulators necessary for G1/S phase transition, such as cyclin-dependent kinases CDK2, CDK4 and CDK6, were remarkably down-regulated; by contrast, the cell cycle inhibitors p21^waf/cip1, p27^Kip1 and p53 were significantly up-regulated after dihydrofolate reductase knockdown. Furthermore, supplementation of 5-methyltetrahydrofolate to the dihydrofolate reductase knockdown cells could weaken the inhibitory effect of dihydrofolate reductase knockdown on cell proliferation, simultaneously, inducing the expression of p53 and p21^waf/cip1 falling back moderately. Our findings suggest that attenuating dihydrofolate reductase may cause imbalanced expression of cell cycle regulators, especially up-regulation of p53-p21^waf/cip1 pathway, leading to G1 cell cycle arrest, thereby inhibiting the growth of endothelial EA.hy926 cells.

Characterization of the hypothalamic transcriptome in response to food deprivation reveals global changes in long noncoding RNA, and cell cycle response genes

The hypothalamus integrates energy balance information from the periphery using different neuronal subtypes within each of the hypothalamic areas. However, the effects of prandial state on global mRNA, microRNA and long noncoding (lnc) RNA expression within the whole hypothalamus are largely unknown. In this study, mice were given either a 24-h fast, or ad libitum access to food. RNA samples were analyzed by microarray, and then a subset was confirmed using quantitative real-time PCR (QPCR). A total of 540 mRNAs were either up- or down-regulated with food deprivation. Since gene ontology enrichment analyses identified several categories of mRNAs related to cell cycle processes, ten cell-cycle-related genes were further analyzed using QPCR with six confirmed to be significantly up-regulated and one down-regulated in response to 24-h fasting. While 22 independent microRNAs were differentially expressed by microarray, secondary analysis by QPCR failed to confirm significant changes with fasting. There were 622 lncRNAs identified as differentially expressed, and of three tested by QPCR, two were confirmed. Overall, this is the first time that expression of hypothalamic lncRNAs has been shown to be responsive to food deprivation. In addition, this study is the first to identify a list of lncRNAs with high expression in RNA extracted from hypothalamus. Individual contributions from specific miRNA, lncRNA and mRNAs to the food deprivation response can now be further studied at the physiological and biochemical levels.

Retinoid X receptor ligands: a patent review (2007 - 2013)

INTRODUCTION

Retinoid X receptors (RXRs) are nuclear receptors that act as ligand-dependent transcription factors. RXRs function as homodimers or as heterodimers with other nuclear receptors, such as retinoic acid receptors, PPARs, liver X receptors, farnesoid X receptor, vitamin D receptor or thyroid hormone receptors. RXR ligands (agonists or antagonists) show various physiological effects, depending on their partner receptors. RXR agonist bexarotene (Targretin®) is used for the treatment of cutaneous T-cell lymphoma in clinical practice. RXR agonists were also reported to be useful for treatment of type 2 diabetes, autoimmune disease and Alzheimer's disease. RXR antagonists were also reported to be effective in type 2 diabetes treatment.

AREAS COVERED

Here patent applications (2007 - 2013) concerning RXR ligands are summarized, and the usefulness of RXR ligands as pharmaceutical agents is discussed.

EXPERT OPINION

RXR agonists show a wide variety of biological effects. However, they cause serious side effects, such as blood triglyceride elevation, hypothyroidism and others. Thus, for clinical application of RXR agonists, abrogation of these side effects is required. RXR heterodimer-selective agonists and RXR partial agonists exhibiting desired effects without side effects are expected to find clinical application.

Eriocitrin ameliorates diet-induced hepatic steatosis with activation of mitochondrial biogenesis

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.

Activation of retinoid receptor-mediated signaling ameliorates diabetes-induced cardiac dysfunction in Zucker diabetic rats

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 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.

P53 and cellular glucose uptake

OBJECTIVES

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.

DISCUSSION

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.