AMPKalpha1 deletion shortens erythrocyte life span in mice: role of oxidative stress

Serum pantetheinase/vanin levels regulate erythrocyte homeostasis and severity of malaria

Tissue pantetheinase, encoded by the VNN1 gene, regulates response to stress, and previous studies have shown that VNN genes contribute to the susceptibility to malaria. Herein, we evaluated the role of pantetheinase on erythrocyte homeostasis and on the development of malaria in patients and in a new mouse model of pantetheinase insufficiency. Patients with cerebral malaria have significantly reduced levels of serum pantetheinase activity (PA). In mouse, we show that a reduction in serum PA predisposes to severe malaria, including cerebral malaria and severe anemia. Therefore, scoring pantetheinase in serum may serve as a severity marker in malaria infection. This disease triggers an acute stress in erythrocytes, which enhances cytoadherence and hemolysis. We speculated that serum pantetheinase might contribute to erythrocyte resistance to stress under homeostatic conditions. We show that mutant mice with a reduced serum PA are anemic and prone to phenylhydrazine-induced anemia. A cytofluorometric and spectroscopic analysis documented an increased frequency of erythrocytes with an autofluorescent aging phenotype. This is associated with an enhanced oxidative stress and shear stress-induced hemolysis. Red blood cell transfer and bone marrow chimera experiments show that the aging phenotype is not cell intrinsic but conferred by the environment, leading to a shortening of red blood cell half-life. Therefore, serum pantetheinase level regulates erythrocyte life span and modulates the risk of developing complicated malaria.

Determination of RBC Survival in C57BL/6 and C57BL/6-Tg(UBC-GFP) Mice

Although several methods for determining erythrocyte lifespan are used in research studies that involve mice, all involve the alteration of RBC to allow for its tracking over time, which may affect overall RBC survival. The aims of this study were to determine 1) whether sex affects RBC survival; 2) whether RBC survival differs between the biotin method and an alternative method that uses GFP; and 3) whether repeat exposure of mice to biotin results in an antibiotin antibody response or decreased RBC survival. The results suggest no difference in the RBC half-life between male and female C57BL/6 mice (22.9 ± 1.2 and 22.4 ± 0.9 d, respectively). In addition, RBC half-life did not differ between the biotin- and GFP-based methods (20.5 ± 2.1 d and 22.7 ± 2.1 d, respectively). Finally, retransfusion of mice 90 d after an initial transfusion with biotin-labeled RBC did not induce detectable antibiotin antibodies nor alter the half-life of transfused biotin-labeled RBC (initial transfusion, 22.0 ± 1.2 d; subsequent transfusion, 23.4 ± 1.4 d, respectively).

The effects of short-term and long-term testosterone supplementation on blood viscosity and erythrocyte deformability in healthy adult mice

Testosterone treatment induces erythrocytosis that could potentially affect blood viscosity and cardiovascular risk. We thus investigated the effects of testosterone administration on blood viscosity and erythrocyte deformability using mouse models. Blood viscosity, erythrocyte deformability, and hematocrits were measured in normal male and female mice, as well as in females and castrated males after short-term (2 wk) and long-term (5-7 mo) testosterone intervention (50 mg/kg, weekly). Castrated males for long-term intervention were studied in parallel with the normal males to assess the effect of long-term testosterone deprivation. An additional short-term intervention study was conducted in females with a lower testosterone dose (5 mg/kg). Our results indicate no rheological difference among normal males, females, and castrated males at steady-state. Short-term high-dose testosterone increased hematocrit and whole-blood viscosity in both females and castrated males. This effect diminished after long-term treatment, in association with increased erythrocyte deformability in the testosterone-treated mice, suggesting the presence of adaptive mechanism. Considering that cardiovascular events in human trials are seen early after intervention, rheological changes as potential mediator of vascular events warrant further investigation.

PRKAA1/AMPKα1 is required for autophagy-dependent mitochondrial clearance during erythrocyte maturation

AMP-activated protein kinase α1 knockout (prkaa1(-/-)) mice manifest splenomegaly and anemia. The underlying molecular mechanisms, however, remain to be established. In this study, we tested the hypothesis that defective autophagy-dependent mitochondrial clearance in prkaa1(-/-) mice exacerbates oxidative stress, thereby enhancing erythrocyte destruction. The levels of ULK1 phosphorylation, autophagical flux, mitochondrial contents, and reactive oxygen species (ROS) were examined in human erythroleukemia cell line, K562 cells, as well as prkaa1(-/-) mouse embryonic fibroblasts and erythrocytes. Deletion of Prkaa1 resulted in the inhibition of ULK1 phosphorylation at Ser555, prevented the formation of ULK1 and BECN1- PtdIns3K complexes, and reduced autophagy capacity. The suppression of autophagy was associated with enhanced damaged mitochondrial accumulation and ROS production. Compared with wild-type (WT) mice, prkaa1(-/-) mice exhibited a shortened erythrocyte life span, hemolytic destruction of erythrocytes, splenomegaly, and anemia, all of which were alleviated by the administration of either rapamycin to activate autophagy or Mito-tempol, a mitochondria-targeted antioxidant, to scavenge mitochondrial ROS. Furthermore, transplantation of WT bone marrow into prkaa1(-/-) mice restored mitochondrial removal, reduced intracellular ROS levels, and normalized hematologic parameters and spleen size. Conversely, transplantation of prkaa1 (-/-) bone marrow into WT mice recapitulated the prkaa1(-/-) mouse phenotypes. We conclude that PRKAA1-dependent autophagy-mediated clearance of damaged mitochondria is required for erythrocyte maturation and homeostasis.

AMPK is involved in mediation of erythropoietin influence on metabolic activity and reactive oxygen species production in white adipocytes

Erythropoietin, discovered for its indispensable role during erythropoiesis, has been used in therapy for selected red blood cell disorders in erythropoietin-deficient patients. The biological activities of erythropoietin have been found in animal models to extend to non-erythroid tissues due to the expression of erythropoietin receptor. We previously demonstrated that erythropoietin promotes metabolic activity and white adipocytes browning to increase mitochondrial function and energy expenditure via peroxisome proliferator-activated receptor alpha and Sirtuin1. Here we report that AMP-activated protein kinase was activated by erythropoietin possibly via Ca(2+)/calmodulin-dependent protein kinase kinase in adipocytes as well as in white adipose tissue from diet induced obese mice. Erythropoietin increased cellular nicotinamide adenine dinucleotide via increased AMP-activated protein kinase activity, possibly leading to Sirtuin1 activation. AMP-activated protein kinase knock down reduced erythropoietin mediated increase in cellular oxidative function including the increased oxygen consumption rate, fatty acid utilization and induction of key metabolic genes. Under hypoxia, adipocytes were found to generate more reactive oxygen species, and erythropoietin reduced the reactive oxygen species and increased antioxidant gene expression, suggesting that erythropoietin may provide protection from oxidative stress in adipocytes. Erythropoietin also reversed increased nicotinamide adenine dinucleotide by hypoxia via increased AMP-activated protein kinase. Additionally, AMP-activated protein kinase is found to be involved in erythropoietin stimulated increase in oxygen consumption rate, fatty acid oxidation and mitochondrial gene expression. AMP-activated protein kinase knock down impaired erythropoietin stimulated increases in antioxidant gene expression. Collectively, our findings identify the AMP-activated protein kinase involvement in erythropoietin signaling in regulating adipocyte cellular redox status and metabolic activity.

High molecular weight kininogen binds phosphatidylserine and opsonizes urokinase plasminogen activator receptor-mediated efferocytosis

Phagocytosis of apoptotic cells (efferocytosis) is essential for regulation of immune responses and tissue homeostasis and is mediated by phagocytic receptors. In this study, we found that urokinase plasminogen activator receptor (uPAR) plays an important role in internalization of apoptotic cells and also characterized the underlying mechanisms. In a flow cytometry-based phagocytic assay, uPAR-deficient macrophages displayed significant defect in internalization but not tethering of apoptotic cells. When uPAR-deficient mice were challenged with apoptotic cells, they exhibited pronounced splenomegaly resulting from accumulation of abundant apoptotic cells in spleen. Overexpression of uPAR in HEK-293 cells enhanced efferocytosis, which was inhibited by Annexin V and phosphatidylserine (PS) liposome, suggesting that uPAR-mediated efferocytosis is dependent on PS. In serum lacking high m.w. kininogen (HK), a uPAR ligand, uPAR-mediated efferocytosis was significantly attenuated, which was rescued by replenishment of HK. As detected by flow cytometry, HK selectively bound to apoptotic cells, but not viable cells. In purified systems, HK was specifically associated with PS liposome. HK binding to apoptotic cells induced its rapid cleavage to the two-chain form of HK (HKa) and bradykinin. Both the H chain and L chain of HKa were associated with PS liposome and apoptotic cells. HKa has higher binding affinity than HK to uPAR. Overexpression of Rac1/N17 cDNA inhibited uPAR-mediated efferocytosis. HK plus PS liposome stimulated a complex formation of CrkII with p130Cas and Dock-180 and Rac1 activation in uPAR-293 cells, but not in control HEK-293 cells. Thus, uPAR mediates efferocytosis through HK interaction with PS on apoptotic cells and activation of the Rac1 pathway.

Redox regulation of endothelial cell fate

Endothelial cells (ECs) are present throughout blood vessels and have variable roles in both physiological and pathological settings. EC fate is altered and regulated by several key factors in physiological or pathological conditions. Reactive nitrogen species and reactive oxygen species derived from NAD(P)H oxidases, mitochondria, or nitric oxide-producing enzymes are not only cytotoxic but also compose a signaling network in the redox system. The formation, actions, key molecular interactions, and physiological and pathological relevance of redox signals in ECs remain unclear. We review the identities, sources, and biological actions of oxidants and reductants produced during EC function or dysfunction. Further, we discuss how ECs shape key redox sensors and examine the biological functions, transcriptional responses, and post-translational modifications evoked by the redox system in ECs. We summarize recent findings regarding the mechanisms by which redox signals regulate the fate of ECs and address the outcome of altered EC fate in health and disease. Future studies will examine if the redox biology of ECs can be targeted in pathophysiological conditions.

Red blood cell oxidative stress impairs oxygen delivery and induces red blood cell aging

Red Blood Cells (RBCs) need to deform and squeeze through narrow capillaries. Decreased deformability of RBCs is, therefore, one of the factors that can contribute to the elimination of aged or damaged RBCs from the circulation. This process can also cause impaired oxygen delivery, which contributes to the pathology of a number of diseases. Studies from our laboratory have shown that oxidative stress plays a significant role in damaging the RBC membrane and impairing its deformability. RBCs are continuously exposed to both endogenous and exogenous sources of reactive oxygen species (ROS) like superoxide and hydrogen peroxide (H2O2). The bulk of the ROS are neutralized by the RBC antioxidant system consisting of both non-enzymatic and enzymatic antioxidants including catalase, glutathione peroxidase and peroxiredoxin-2. However, the autoxidation of hemoglobin (Hb) bound to the membrane is relatively inaccessible to the predominantly cytosolic RBC antioxidant system. This inaccessibility becomes more pronounced under hypoxic conditions when Hb is partially oxygenated, resulting in an increased rate of autoxidation and increased affinity for the RBC membrane. We have shown that a fraction of peroxyredoxin-2 present on the RBC membrane may play a major role in neutralizing these ROS. H2O2 that is not neutralized by the RBC antioxidant system can react with the heme producing fluorescent heme degradation products (HDPs). We have used the level of these HDP as a measure of RBC oxidative Stress. Increased levels of HDP are detected during cellular aging and various diseases. The negative correlation (p < 0.0001) between the level of HDP and RBC deformability establishes a contribution of RBC oxidative stress to impaired deformability and cellular stiffness. While decreased deformability contributes to the removal of RBCs from the circulation, oxidative stress also contributes to the uptake of RBCs by macrophages, which plays a major role in the removal of RBCs from circulation. The contribution of oxidative stress to the removal of RBCs by macrophages involves caspase-3 activation, which requires oxidative stress. RBC oxidative stress, therefore, plays a significant role in inducing RBC aging.

Role of AMPK-mediated adaptive responses in human cells with mitochondrial dysfunction to oxidative stress

BACKGROUND

Mitochondrial DNA (mtDNA) mutations are an important cause of mitochondrial diseases, for which there is no effective treatment due to complex pathophysiology. It has been suggested that mitochondrial dysfunction-elicited reactive oxygen species (ROS) plays a vital role in the pathogenesis of mitochondrial diseases, and the expression levels of several clusters of genes are altered in response to the elevated oxidative stress. Recently, we reported that glycolysis in affected cells with mitochondrial dysfunction is upregulated by AMP-activated protein kinase (AMPK), and such an adaptive response of metabolic reprogramming plays an important role in the pathophysiology of mitochondrial diseases.

SCOPE OF REVIEW

We summarize recent findings regarding the role of AMPK-mediated signaling pathways that are involved in: (1) metabolic reprogramming, (2) alteration of cellular redox status and antioxidant enzyme expression, (3) mitochondrial biogenesis, and (4) autophagy, a master regulator of mitochondrial quality control in skin fibroblasts from patients with mitochondrial diseases.

MAJOR CONCLUSION

Induction of adaptive responses via AMPK-PFK2, AMPK-FOXO3a, AMPK-PGC-1α, and AMPK-mTOR signaling pathways, respectively is modulated for the survival of human cells under oxidative stress induced by mitochondrial dysfunction. We suggest that AMPK may be a potential target for the development of therapeutic agents for the treatment of mitochondrial diseases.

GENERAL SIGNIFICANCE

Elucidation of the adaptive mechanism involved in AMPK activation cascades would lead us to gain a deeper insight into the crosstalk between mitochondria and the nucleus in affected tissue cells from patients with mitochondrial diseases. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Hemoglobin redox reactions and red blood cell aging

SIGNIFICANCE

The physiological mechanism(s) for recognition and removal of red blood cells (RBCs) from circulation after 120 days of its lifespan is not fully understood. Many of the processes thought to be associated with the removal of RBCs involve oxidative stress. We have focused on hemoglobin (Hb) redox reactions, which is the major source of RBC oxidative stress.

RECENT ADVANCES

The importance of Hb redox reactions have been shown to originate in large parts from the continuous slow autoxidation of Hb producing superoxide and its dramatic increase under hypoxic conditions. In addition, oxidative stress has been shown to be associated with redox reactions that originate from Hb reactions with nitrite and nitric oxide (NO) and the resultant formation of highly toxic peroxynitrite when NO reacts with superoxide released during Hb autoxidation.

CRITICAL ISSUES

The interaction of Hb, particularly under hypoxic conditions with band 3 of the RBC membrane is critical for the generating the RBC membrane changes that trigger the removal of cells from circulation. These changes include exposure of antigenic sites, increased calcium leakage into the RBC, and the resultant leakage of potassium out of the RBC causing cell shrinkage and impaired deformability.

FUTURE DIRECTIONS

The need to understand the oxidative damage to specific membrane proteins that result from redox reactions occurring when Hb is bound to the membrane. Proteomic studies that can pinpoint the specific proteins damaged under different conditions will help elucidate the cellular aging processes that result in cells being removed from circulation.

Glutathione S-transferases interact with AMP-activated protein kinase: evidence for S-glutathionylation and activation in vitro

AMP-activated protein kinase (AMPK) is a cellular and whole body energy sensor with manifold functions in regulating energy homeostasis, cell morphology and proliferation in health and disease. Here we apply multiple, complementary in vitro and in vivo interaction assays to identify several isoforms of glutathione S-transferase (GST) as direct AMPK binding partners: Pi-family member rat GSTP1 and Mu-family members rat GSTM1, as well as Schistosoma japonicum GST. GST/AMPK interaction is direct and involves the N-terminal domain of the AMPK β-subunit. Complex formation of the mammalian GSTP1 and -M1 with AMPK leads to their enzymatic activation and in turn facilitates glutathionylation and activation of AMPK in vitro. GST-facilitated S-glutathionylation of AMPK may be involved in rapid, full activation of the kinase under mildly oxidative physiological conditions.

AMP-activated protein kinase α1 protects against diet-induced insulin resistance and obesity

AMP-activated protein kinase (AMPK) is an essential sensor of cellular energy status. Defects in the α2 catalytic subunit of AMPK (AMPKα1) are associated with metabolic syndrome. The current study investigated the role AMPKα1 in the pathogenesis of obesity and inflammation using male AMPKα1-deficent (AMPKα1(-/-)) mice and their wild-type (WT) littermates. After being fed a high-fat diet (HFD), global AMPKα1(-/-) mice gained more body weight and greater adiposity and exhibited systemic insulin resistance and metabolic dysfunction with increased severity in their adipose tissues compared with their WT littermates. Interestingly, upon HFD feeding, irradiated WT mice that received the bone marrow of AMPKα1(-/-) mice showed increased insulin resistance but not obesity, whereas irradiated AMPKα1(-/-) mice with WT bone marrow had a phenotype of metabolic dysregulation that was similar to that of global AMPKα1(-/-) mice. AMPKα1 deficiency in macrophages markedly increased the macrophage proinflammatory status. In addition, AMPKα1 knockdown enhanced adipocyte lipid accumulation and exacerbated the inflammatory response and insulin resistance. Together, these data show that AMPKα1 protects mice from diet-induced obesity and insulin resistance, demonstrating that AMPKα1 is a promising therapeutic target in the treatment of the metabolic syndrome.

Inactivation of AMPKα1 induces asthenozoospermia and alters spermatozoa morphology

AMP-activated protein kinase (AMPK), a key regulator of cellular energy homeostasis, is present in metabolic tissues (muscle and liver) and has been identified as a modulator of the female reproductive functions. However, its function in the testis has not yet been clearly defined. We have investigated the potential role of AMPK in male reproduction by using transgenic mice lacking the activity of AMPK catalytic subunit α1 gene [α1AMPK knockout (KO)]. In the testis, the α1AMPK subunit is expressed in germ cells and also in somatic cells (Sertoli and Leydig cells). α1AMPK KO male mice show a decrease in fertility, despite no clear alteration in the testis morphology or sperm production. However, in α1AMPK(-/-) mice, we demonstrate that spermatozoa have structural abnormalities and are less motile than in control mice. These spermatozoa alterations are associated with a 50% decrease in mitochondrial activity, a 60% decrease in basal oxygen consumption, and morphological defects. The α1AMPK KO male mice had high androgen levels associated with a 5- and 3-fold increase in intratesticular cholesterol and testosterone concentrations, respectively. High concentrations of proteins involved in steroid production (3β-hydroxysteroid dehydrogenase, cytochrome steroid 17 alpha-hydroxylase/17,20 lysate, and steroidogenic acute regulatory protein) were also detected in α1AMPK(-/-) testes. In the pituitary, the LH and FSH concentrations tended to be lower in α1AMPK(-/-) male mice, probably due to the negative feedback of the high testosterone levels. These results suggest that total α1AMPK deficiency in male mice affects androgen production and quality of spermatozoa, leading to a decrease in fertility.

Red blood cell distribution width is associated with poor clinical outcome in acute cerebral infarction

Increased red blood cell distribution width (RDW), which is a marker of anisocytosis, is associated with mortality and cardiovascular events in the general population and in patients with heart failure or coronary heart disease. We investigated whether RDW in acute cerebral infarction is predictive of functional outcome and mortality. A total of 847 consecutive patients with first-ever acute cerebral infarction who presented to the emergency department within seven days of symptom onset were enrolled in this study. We investigated the association of RDW with poor functional outcome (modified Rankin Scale >2) and all-cause mortality at three months, as well as survival time for one year after stroke onset. Multivariate logistic regression revealed that higher RDW was independently associated with poor functional outcome (adjusted odds ratio [OR], 1.222 per 1% increment in RDW, 95% confidence interval [CI] 1.059-1.409, p=0.006) and all-cause death (adjusted OR, 1.395 per 1% increment in RDW, 95% CI 1.168-1.665, p<0.001) at three months after stroke onset. RDW was an independent predictor of survival in multivariate Cox-proportional regression model (adjusted hazard ratio, 1.328 per 1% increment in RDW, 95%CI 1.178-1.498, p<0.001). The addition of RDW to a survival model significantly increased predictability for survival across the entire follow-up period (weighted average of the area-under the curves, 0.858 vs. 0.841, p<0.05). In conclusion, higher RDW measured in cases of acute stage cerebral infarction was associated with poor functional outcome and mortality. RDW may be used as a biomarker for the prediction of long-term outcomes in patients with acute cerebral infarction.

Regulation of NAD(P)H oxidases by AMPK in cardiovascular systems

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are ubiquitously produced in cardiovascular systems. Under physiological conditions, ROS/RNS function as signaling molecules that are essential in maintaining cardiovascular function. Aberrant concentrations of ROS/RNS have been demonstrated in cardiovascular diseases owing to increased production or decreased scavenging, which have been considered common pathways for the initiation and progression of cardiovascular diseases such as atherosclerosis, hypertension, (re)stenosis, and congestive heart failure. NAD(P)H oxidases are primary sources of ROS and can be induced or activated by all known cardiovascular risk factors. Stresses, hormones, vasoactive agents, and cytokines via different signaling cascades control the expression and activity of these enzymes and of their regulatory subunits. But the molecular mechanisms by which NAD(P)H oxidase is regulated in cardiovascular systems remain poorly characterized. Investigations by us and others suggest that adenosine monophosphate-activated protein kinase (AMPK), as an energy sensor and modulator, is highly sensitive to ROS/RNS. We have also obtained convincing evidence that AMPK is a physiological suppressor of NAD(P)H oxidase in multiple cardiovascular cell systems. In this review, we summarize our current understanding of how AMPK functions as a physiological repressor of NAD(P)H oxidase.

AMP-activated protein kinase, stress responses and cardiovascular diseases

AMPK (AMP-activated protein kinase) is one of the key players in maintaining intracellular homoeostasis. AMPK is well known as an energy sensor and can be activated by increased intracellular AMP levels. Generally, the activation of AMPK turns on catabolic pathways that generate ATP, while inhibiting cell proliferation and biosynthetic processes that consume ATP. In recent years, intensive investigations on the regulation and the function of AMPK indicates that AMPK not only functions as an intracellular energy sensor and regulator, but is also a general stress sensor that is important in maintaining intracellular homoeostasis during many kinds of stress challenges. In the present paper, we will review recent literature showing that AMPK functions far beyond its proposed energy sensor and regulator function. AMPK regulates ROS (reactive oxygen species)/redox balance, autophagy, cell proliferation, cell apoptosis, cellular polarity, mitochondrial function and genotoxic response, either directly or indirectly via numerous downstream pathways under physiological and pathological conditions.

Mature erythrocyte membrane homeostasis is compromised by loss of the GATA1-FOG1 interaction

GATA1 plays essential roles in erythroid gene expression. The N-terminal finger of GATA1 (GATA1-Nf) is important for association with FOG1. Substitution mutations in GATA1-Nf, such as GATA1V205M that diminish the GATA1-FOG1 association, have been identified in human thrombocytopenia and anemia cases. A mouse model of human thrombocytopenia has been established using a transgenic complementation rescue approach; GATA1-deficient mice were successfully rescued from embryonic lethality by excess expression of GATA1V205G, but rescued adult mice suffered from severe thrombocytopenia. In this study, we examined GATA1-deficient mice rescued with GATA1V205G at a comparable level to endogenous GATA1. Mice rescued with this level of GATA1V205G rarely survive to adulthood. Rescued newborns suffered from severe anemia and jaundice accompanied with anisocytosis and spherocytosis. Expression of Slc4a1, Spna1, and Aqp1 genes (encoding the membrane proteins band-3, α-spectrin, and aquaporin-1, respectively) were strikingly diminished, whereas expression of other canonical GATA1-target genes, such as Alas2, were little affected. Lack of these membrane proteins provoked perturbation of membrane skeleton. Importantly, the red cells exhibited increased reactive oxygen species accumulation. These results thus demonstrate that the loss of the GATA1-FOG1 interaction causes a unique combination of membrane protein deficiency and disturbs the function of GATA1 in maintaining erythroid homeostasis.

Physiologically aged red blood cells undergo erythrophagocytosis in vivo but not in vitro

BACKGROUND

The lifespan of red blood cells is terminated when macrophages remove senescent red blood cells by erythrophagocytosis. This puts macrophages at the center of systemic iron recycling in addition to their functions in tissue remodeling and innate immunity. Thus far, erythrophagocytosis has been studied by evaluating phagocytosis of erythrocytes that were damaged to mimic senescence. These studies have demonstrated that acquisition of some specific individual senescence markers can trigger erythrophagocytosis by macrophages, but we hypothesized that the mechanism of erythrophagocytosis of such damaged erythrocytes might differ from erythrophagocytosis of physiologically aged erythrocytes.

DESIGN AND METHODS

To test this hypothesis we generated an erythrocyte population highly enriched in senescent erythrocytes by a hypertransfusion procedure in mice. Various erythrocyte-aging signals were analyzed and erythrophagocytosis was evaluated in vivo and in vitro.

RESULTS

The large cohort of senescent erythrocytes from hypertransfused mice carried numerous aging signals identical to those of senescent erythrocytes from control mice. Phagocytosis of fluorescently-labeled erythrocytes from hypertransfused mice injected into untreated mice was much higher than phagocytosis of labeled erythrocytes from control mice. However, neither erythrocytes from hypertransfused mice, nor those from control mice were phagocytosed in vitro by primary macrophage cultures, even though these cultures were able to phagocytose oxidatively damaged erythrocytes.

CONCLUSIONS

The large senescent erythrocyte population found in hypertransfused mice mimics physiologically aged erythrocytes. For effective erythrophagocytosis of these senescent erythrocytes, macrophages depend on some features of the intact phagocytosing tissue for support.

Inhibition of AMP-activated protein kinase α (AMPKα) by doxorubicin accentuates genotoxic stress and cell death in mouse embryonic fibroblasts and cardiomyocytes: role of p53 and SIRT1

Doxorubicin, an anthracycline antibiotic, is widely used in cancer treatment. Doxorubicin produces genotoxic stress and p53 activation in both carcinoma and non-carcinoma cells. Although its side effects in non-carcinoma cells, especially in heart tissue, are well known, the molecular targets of doxorubicin are poorly characterized. Here, we report that doxorubicin inhibits AMP-activated protein kinase (AMPK) resulting in SIRT1 dysfunction and p53 accumulation. Spontaneously immortalized mouse embryonic fibroblasts (MEFs) or H9C2 cardiomyocyte were exposed to doxorubicin at different doses and durations. Cell death and p53, SIRT1, and AMPK levels were examined by Western blot. In MEFs, doxorubicin inhibited AMPK activation, increased cell death, and induced robust p53 accumulation. Genetic deletion of AMPKα1 reduced NAD(+) levels and SIRT1 activity and significantly increased the levels of p53 and cell death. Pre-activation of AMPK by 5-aminoimidazole-4-carboxamide ribonucleoside or transfection with an adenovirus encoding a constitutively active AMPK (AMPK-CA) markedly reduced the effects of doxorubicin in MEFs from Ampkα1 knock-out mice. Conversely, pre-inhibition of Ampk further sensitized MEFs to doxorubicin-induced cell death. Genetic knockdown of p53 protected both wild-type and Ampkα1(-/-) MEFs from doxorubicin-induced cell death. p53 accumulation in Ampkα1(-/-) MEFs was reversed by SIRT1 activation by resveratrol. Taken together, these data suggest that AMPK inhibition by doxorubicin causes p53 accumulation and SIRT1 dysfunction in MEFs and further suggest that pharmacological activation of AMPK might alleviate the side effects of doxorubicin.

PDGF-BB modulates hematopoiesis and tumor angiogenesis by inducing erythropoietin production in stromal cells

The platelet-derived growth factor (PDGF) signaling system contributes to tumor angiogenesis and vascular remodeling. Here we show in mouse tumor models that PDGF-BB induces erythropoietin (EPO) mRNA and protein expression by targeting stromal and perivascular cells that express PDGF receptor-β (PDGFR-β). Tumor-derived PDGF-BB promoted tumor growth, angiogenesis and extramedullary hematopoiesis at least in part through modulation of EPO expression. Moreover, adenoviral delivery of PDGF-BB to tumor-free mice increased both EPO production and erythropoiesis, as well as protecting from irradiation-induced anemia. At the molecular level, we show that the PDGF-BB-PDGFR-bβ signaling system activates the EPO promoter, acting in part through transcriptional regulation by the transcription factor Atf3, possibly through its association with two additional transcription factors, c-Jun and Sp1. Our findings suggest that PDGF-BB-induced EPO promotes tumor growth through two mechanisms: first, paracrine stimulation of tumor angiogenesis by direct induction of endothelial cell proliferation, migration, sprouting and tube formation, and second, endocrine stimulation of extramedullary hematopoiesis leading to increased oxygen perfusion and protection against tumor-associated anemia.

5'-Adenosine monophosphate-activated protein kinase--mammalian target of rapamycin axis as therapeutic target for age-related macular degeneration

Age-related macular degeneration (AMD) is the most common reason for blindness in developed countries. AMD essentially involves chronic oxidative stress, increased accumulation of lipofuscin in retinal pigment epithelial (RPE) cells, and extracellular drusen formation, as well as presence of chronic inflammation in the retina. The capacity to prevent the accumulation of cellular cytotoxic protein aggregates is decreased in senescent cells, which may evoke lipofuscin accumulation into lysosomes in postmitotic RPE cells. The formation of lipofuscin, in turn, decreases the lysosomal enzyme activity and impairs the autophagic clearance of damaged proteins destined for cellular removal. 5'-Adenosine monophosphate-activated protein kinase (AMPK) is a well-known inhibitor of mammalian target of rapamycin (mTOR) that subsequently evokes induction of autophagy. This review examines the novel potential therapeutic targets on the AMPK-mTOR axis and the ways in which autophagy clearance can suppress or prevent RPE degeneration and development of AMD.

The transient receptor potential (TRP) channel TRPC3 TRP domain and AMP-activated protein kinase binding site are required for TRPC3 activation by erythropoietin

Modulation of intracellular calcium ([Ca(2+)](i)) by erythropoietin (Epo) is an important signaling pathway controlling erythroid proliferation and differentiation. Transient receptor potential (TRP) channels TRPC3 and homologous TRPC6 are expressed on normal human erythroid precursors, but Epo stimulates an increase in [Ca(2+)](i) through TRPC3 but not TRPC6. Here, the role of specific domains in the different responsiveness of TRPC3 and TRPC6 to erythropoietin was explored. TRPC3 and TRPC6 TRP domains differ in seven amino acids. Substitution of five amino acids (DDKPS) in the TRPC3 TRP domain with those of TRPC6 (EERVN) abolished the Epo-stimulated increase in [Ca(2+)](i). Substitution of EERVN in TRPC6 TRP domain with DDKPS in TRPC3 did not confer Epo responsiveness. However, substitution of TRPC6 TRP with DDKPS from TRPC3 TRP, as well as swapping the TRPC6 distal C terminus (C2) with that of TRPC3, resulted in a chimeric TRPC6 channel with Epo responsiveness similar to TRPC3. Substitution of TRPC6 with TRPC3 TRP and the putative TRPC3 C-terminal AMP-activated protein kinase (AMPK) binding site straddling TRPC3 C1/C2 also resulted in TRPC6 activation. In contrast, substitution of the TRPC3 C-terminal leucine zipper motif or TRPC3 phosphorylation sites Ser-681, Ser-708, or Ser-764 with TRPC6 sequence did not affect TRPC3 Epo responsiveness. TRPC3, but not TRPC6, and TRPC6 chimeras expressing TRPC3 C2 showed significantly increased plasma membrane insertion following Epo stimulation and substantial cytoskeletal association. The TRPC3 TRP domain, distal C terminus (C2), and AMPK binding site are critical elements that confer Epo responsiveness. In particular, the TRPC3 C2 and AMPK site are essential for association of TRPC3 with the cytoskeleton and increased channel translocation to the cell surface in response to Epo stimulation.

Dietary epicatechin promotes survival of obese diabetic mice and Drosophila melanogaster

The lifespan of diabetic patients is 7-8 y shorter than that of the general population because of hyperglycemia-induced vascular complications and damage to other organs such as the liver and skeletal muscle. Here, we investigated the effects of epicatechin, one of the major flavonoids in cocoa, on health-promoting effects in obese diabetic (db/db) mice (0.25% in drinking water for 15 wk) and Drosophila melanogaster (0.01-8 mmol/L in diet). Dietary intake of epicatechin promoted survival in the diabetic mice (50% mortality in diabetic control group vs. 8.4% in epicatechin group after 15 wk of treatment), whereas blood pressure, blood glucose, food intake, and body weight gain were not significantly altered. Pathological analysis showed that epicatechin administration reduced the degeneration of aortic vessels and blunted fat deposition and hydropic degeneration in the liver caused by diabetes. Epicatechin treatment caused changes in diabetic mice that are associated with a healthier and longer lifespan, including improved skeletal muscle stress output, reduced systematic inflammation markers and serum LDL cholesterol, increased hepatic antioxidant glutathione concentration and total superoxide dismutase activity, decreased circulating insulin-like growth factor-1 (from 303 ± 21 mg/L in the diabetic control group to 189 ± 21 mg/L in the epicatechin-treated group), and improved AMP-activated protein kinase-α activity in the liver and skeletal muscle. Consistently, epicatechin (0.1-8 mmol/L) also promoted survival and increased mean lifespan of Drosophila. Therefore, epicatechin may be a novel food-derived, antiaging compound.

Inhibition of the AMP-activated protein kinase-α2 accentuates agonist-induced vascular smooth muscle contraction and high blood pressure in mice

The aim of the present study was to determine the effects and molecular mechanisms by which AMP-activated protein kinase (AMPK) regulates smooth muscle contraction and blood pressure in mice. In cultured human vascular smooth muscle cells, we observed that activation of AMPK by 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside inhibited agonist-induced phosphorylation of myosin light chain (MLC) and myosin phosphatase targeting subunit 1 (MYPT1). Conversely, AMPK inhibition with pharmacological or genetic means potentiated agonist-induced the phosphorylation of MLC and MYPT1, whereas it inhibited both Ras homolog gene family member A and Rho-associated kinase activity. In addition, AMPK activation or Rho-associated kinase inhibition with Y27632 abolished agonist-induced phosphorylation of MLC and MYPT1. Gene silencing of p190-guanosine triphosphatase-activating protein abolished the effects of AMPK activation on MLC, MYPT1, and Ras homolog gene family member A in human smooth muscle cells. Ex vivo analyses revealed that agonist-induced contractions of the mesenteric artery and aortas were stronger in both AMPKα1(-/-) and AMPKα2(-/-) knockout mice than in wild-type mice. Inhibition of Rho-associated kinase with Y27632 normalized agonist-induced contractions of AMPKα1(-/-) and AMPKα2(-/-) vessels. AMPKα2(-/-) mice had higher blood pressure along with decreased serine phosphorylation of p190-guanosine triphosphatase-activating protein. Finally, inhibition of the Ras homolog gene family member A/Rho-associated kinase pathway with Y27632, which suppressed MYPT1 and MLC phosphorylation, lowered blood pressure in AMPKα2(-/-) mice. In conclusion, AMPK decreases vascular smooth muscle cell contractility by inhibiting p190-GTP-activating protein-dependent Ras homolog gene family member A activation, indicating that AMPK may be a new therapeutic target in lowering high blood pressure.

Isoform-selective 5'-AMP-activated protein kinase-dependent preconditioning mechanisms to prevent postischemic leukocyte-endothelial cell adhesive interactions

We previously demonstrated that preconditioning induced by ethanol consumption at low levels [ethanol preconditioning (EPC)] or with 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside (AICAR-PC) 24 h before ischemia-reperfusion prevents postischemic leukocyte-endothelial cell adhesive interactions (LEI) by a mechanism that is initiated by nitric oxide formed by endothelial nitric oxide synthase. Recent work indicates that 1) ethanol increases the activity of AMP-activated protein kinase (AMPK) and 2) AMPK phosphorylates endothelial nitric oxide synthase at the same activation site seen following EPC (Ser1177). In light of these observations, we postulated that the heterotrimeric serine/threonine kinase, AMPK, may play a role in triggering the development of the anti-inflammatory phenotype induced by EPC. Ethanol was administered to C57BL/6J mice by gavage in the presence or absence of AMPK inhibition. Twenty-four hours later, the numbers of rolling and adherent leukocytes in postcapillary venules of the small intestine were recorded using an intravital microscopic approach. Following 45 min of ischemia, LEI were recorded after 30 and 60 min of reperfusion or at equivalent time points in control animals. Ischemia-reperfusion induced a marked increase in LEI relative to sham-operated control mice. The increase in LEI was prevented by EPC, an effect that was lost with AMPK inhibition during the period of ethanol exposure. Studies conducted in AMPK α(1)- and α(2)-knockout mice suggest that the anti-inflammatory effects of AICAR are not dependent on which isoform of the catalytic α-subunit is present because a deficiency of either isoform results in a loss of protection. In sharp contrast, EPC appears to be triggered by an AMPK α(2)-isoform-dependent mechanism.

Maintenance of red blood cell integrity by AMP-activated protein kinase alpha1 catalytic subunit

AMP-activated protein kinase (AMPK) plays a pivotal role in regulating cellular energy metabolism. We previously showed that AMPKalpha1-/- mice develop moderate anemia associated with splenomegaly and high reticulocytosis. Here, we report that splenectomy of AMPKalpha1-/- mice worsened anemia supporting evidence that AMPKalpha1-/- mice developed a compensatory response through extramedullary erythropoiesis in the spleen. Transplantation of bone marrow from AMPKalpha1-/- mice into wild-type recipients recapitulated the hematologic phenotype. Further, AMPKalpha1-/- red blood cells (RBC) showed less deformability in response to shear stress limiting their membrane flexibility. Thus, our results highlight the crucial role of AMPK to preserve RBC integrity.