AMPK protects proximal tubular cells from stress-induced apoptosis by an ATP-independent mechanism: potential role of Akt activation

Metabolic Rewiring and Communication: An Integrative View of Kidney Proximal Tubule Function

The kidney proximal tubule is a key organ for human metabolism. The kidney responds to stress with altered metabolite transformation and perturbed metabolic pathways, an ultimate cause for kidney disease. Here, we review the proximal tubule's metabolic function through an integrative view of transport, metabolism, and function, and embed it in the context of metabolome-wide data-driven research. Function (filtration, transport, secretion, and reabsorption), metabolite transformation, and metabolite signaling determine kidney metabolic rewiring in disease. Energy metabolism and substrates for key metabolic pathways are orchestrated by metabolite sensors. Given the importance of renal function for the inner milieu, we also review metabolic communication routes with other organs. Exciting research opportunities exist to understand metabolic perturbation of kidney and proximal tubule function, for example, in hypertension-associated kidney disease. We argue that, based on the integrative view outlined here, kidney diseases without genetic cause should be approached scientifically as metabolic diseases.

Diabetes mellitus and Parkinson's disease: dangerous liaisons between insulin and dopamine

The relationship between diabetes mellitus and Parkinson's disease has been described in several epidemiological studies over the 1960s to date. Molecular studies have shown the possible functional link between insulin and dopamine, as there is strong evidence demonstrating the action of dopamine in pancreatic islets, as well as the insulin effects on feeding and cognition through central nervous system mechanism, largely independent of glucose utilization. Therapies used for the treatment of type 2 diabetes mellitus appear to be promising candidates for symptomatic and/or disease-modifying action in neurodegenerative diseases including Parkinson's disease, while an old dopamine agonist, bromocriptine, has been repositioned for the type 2 diabetes mellitus treatment. This review will aim at reappraising the different studies that have highlighted the dangerous liaisons between diabetes mellitus and Parkinson's disease.

The regulatory networks of the Hippo signaling pathway in cancer development

The Hippo signaling pathway is a relatively young tumor-related signaling pathway. Although it was discovered lately, research on it developed rapidly. The Hippo signaling pathway is closely relevant to the occurrence and development of tumors and the maintenance of organ size and other biological processes. This manuscript focuses on YAP, the core molecule of the Hippo signaling pathway, and discussion the upstream and downstream regulatory networks of the Hippo signaling pathway during tumorigenesis and development. It also summarizes the relevant drugs involved in this signaling pathway, which may be helpful to the development of targeted drugs for cancer therapy.

Reducing NADPH Synthesis Counteracts Diabetic Nephropathy through Restoration of AMPK Activity in Type 1 Diabetic Rats

Diabetic nephropathy (DN) is a major complication of diabetes mellitus and a primary cause of end-stage renal failure. Clinical studies indicate that metabolic surgery improves DN; however, the mechanism remains unclear. Here, we report that Roux-en-Y Gastric Bypass (RYGB) surgery significantly blocked and reversed DN without affecting the insulin signaling pathway. This protective role of RYGB surgery is almost blocked by either inhibition or knockout of 5'AMP-activated protein kinase (AMPK) in podocytes. Furthermore, mRNA microarray data reveal that RYGB surgery obviously reduced the gene expression involved in nicotinamide adenine dinucleotide phosphate (NAPDH) synthesis. The expression of a key NADPH synthase, hexose-6-phosphate dehydrogenase (H6PD), was inhibited by the low plasma corticosterone level after surgery. In addition, blocking NAPDH synthesis by knocking down H6PD mimicked the beneficial role of RYGB surgery through activation of AMPK in podocytes. Therefore, this study demonstrates that reducing NADPH production is critical for renal AMPK activation in response to RYGB surgery.

Relieving lipid accumulation through UCP1 suppresses the progression of acute kidney injury by promoting the AMPK/ULK1/autophagy pathway

Rationale: Acute kidney injury (AKI) is a serious clinical emergency with an acute onset, rapid progression, and poor prognosis. Recent evidence suggests that AKI is accompanied by significant metabolic abnormalities, including alterations in lipid metabolism. However, the specific changes in lipids in AKI, and their role and regulation mechanisms are currently unclear.

Methods: Quantitative metabolomics was performed in AKI models to reveal the differences of lipid metabolism-related products. Regulated pathway was detected by western blot, qRT-PCR, immunoblot analysis and immunohistochemistry.

Results: The present study systematically analyzes the changes in lipid composition in AKI for the first time and find that the degree of lipid accumulation was highly correlated with uncoupling protein 1 (UCP1). Importantly, relieving lipid accumulation in AKI by upregulating UCP1 can significantly inhibit the progression of AKI through promoting AMPK/ULK1/autophagy pathway.

Conclusions: The present findings suggest that lipid accumulation in AKI is directly regulated by UCP1, which can activate cell autophagy and thus significantly inhibit disease progression. It will provide new ideas and targets for the treatment of AKI.

Compound C Protects Against Cisplatin-Induced Nephrotoxicity Through Pleiotropic Effects

AICAR (Acadesine/AICA riboside) as an activator of AMPK, can protect renal tubular cells from cisplatin induced apoptosis. But in our experiment, the dorsomorphin (compound C, an inhibitor of AMPK) also significantly reduced cisplatin induced renal tubular cells apoptosis. Accordingly, we tested whether compound C can protect cisplatin-induced nephrotoxicity and the specific mechanism. Here, we treated Boston University mouse proximal tubular cells (BUMPT-306) with cisplatin and/or different dosages of AICAR (Acadesine/AICA riboside) or compound C to confirm the effect of AICAR and compound C in vitro. The AMPK-siRNA treated cells to evaluate whether the protective effect of compound C was through inhibiting AMPK. Male C57BL/6 mice were used to verify the effect of compound C in vivo. Both compound C and AICAR can reduce renal tubular cells apoptosis in dose-dependent manners, and compound C decreased serum creatinine and renal tubular injury induced by cisplatin. Mechanistically, compound C inhibited P53, CHOP and p-IREα during cisplatin treatment. Our results demonstrated that compound C inhibited AMPK, but the renal protective effects of compound C were not through AMPK. Instead, compound C protected cisplatin nephrotoxicity by inhibiting P53 and endoplasmic reticulum (ER) stress. Therefore, compound C may protect against cisplatin-induced nephrotoxicity through pleiotropic effects.

Lithium targeting of AMPK protects against cisplatin-induced acute kidney injury by enhancing autophagy in renal proximal tubular epithelial cells

Autophagy has been demonstrated to be vital for kidney homeostasis and is centrally implicated in the pathogenesis of cisplatin-induced acute kidney injury (AKI). Lithium is a potent autophagy inducer in a number of cell types. However, it remains uncertain whether its autophagic activity is associated with a beneficial effect on renal tubular cells in AKI. This study aimed to examine the effect of lithium on renal autophagy in cisplatin-induced AKI. Mice or renal proximal tubular epithelial cells in culture were exposed to cisplatin-induced acute injury in the presence or absence of lithium treatment. AKI or tubular cell injury was evaluated, and cell signaling associated with autophagy was examined. Lithium pretreatment prominently ameliorated acute renal tubular damage in mice exposed to cisplatin insult, associated with enhanced autophagy in renal tubules, as assessed by measuring microtubule-associated protein 1A/1B-light chain 3 (LC3)BII/I expression and autophagosome formation. Consistently, in cisplatin-injured renal tubular cells in vitro, lithium enhanced autophagic activities, improved cell viability, and attenuated cell death. Mechanistically, lithium triggered AMPK-α phosphorylation and activation, which in turn positively correlated with the induced expression of autophagy-related molecules, like mammalian target of rapamycin and LC3BII/I. AMPK-α activation is likely required for lithium-induced tubular cell autophagy and protection in cisplatin-induced AKI because blockade of AMPK-α phosphorylation by compound C markedly abrogated lithium-induced autophagosome formation and mitigated the protective effect of lithium on AKI. Our findings suggest that lithium represents a promising therapeutic strategy for protecting renal tubular cells against cisplatin-induced AKI by enhancing autophagy via AMPK-α activation.-Bao, H., Zhang, Q., Liu, X., Song, Y., Li, X., Wang, Z., Li, C., Peng, A., Gong, R. Lithium targeting of AMPK protects against cisplatin-induced acute kidney injury by enhancing autophagy in renal proximal tubular epithelial cells.

Activation of vitamin D receptor attenuates high glucose-induced cellular injury partially dependent on CYP2J5 in murine renal tubule epithelial cell

AIMS

Vitamin D and its receptor, vitamin D receptor (VDR), have renoprotection effect against diabetic nephropathy (DN). But the exact mechanism has not been fully elucidated. Epoxyeicosatrienoic acids (EETs) are cytochrome P450 (CYP) epoxygenase-derived metabolites of arachidonic acid, protecting against diabetes and DN. Herein, we hypothesized that activation of VDR attenuated high glucose-induced cellular injury in renal tubular epithelial cells partially through up-regulating CYP2J5 expression.

MAIN METHODS

Streptozotocin (STZ) was injected to induce diabetic in wild type and Vdr^-/- mice. The effects of VDR knockout and an activator of VDR, paricalcitol, on the renal injury were detected. In vitro, a murine kidney proximal tubule epithelial cell line BU.MPT induced by high glucose were treated with or without paricalcitol (30 mM) for 12 h or 24 h.

KEY FINDINGS

The expression of CYP2J5 was significantly decreased both in wild type and Vdr^-/- diabetic mice induced by STZ. The STZ-induced kidney architecture damage and apoptosis rate in Vdr^-/- mice were more severe. In vitro, high glucose treatment strongly reduced the CYP2J5 expression and the synthesis of 14,15-EET in BU.MPT cells. Supplement of 14,15-EET significantly reduced the lactate dehydrogenase (LDH) release induced by high glucose in BU.MPT cells. Furthermore, treatment with paricalcitol attenuated cellular injury and restored the expression of CYP2J5 reduced by high glucose in BU.MPT cells.

SIGNIFICANCE

We conclude that activation of VDR attenuates high glucose-induced cellular injury partially dependent on CYP2J5 in murine renal tubule epithelial cells and paricalcitol may represent a potential therapy for DN.

AMPK-mediated activation of Akt protects renal tubular cells from stress-induced apoptosis in vitro and ameliorates ischemic AKI in vivo

We have reported that preconditioning renal tubular cells (RTCs) with A-769662 [a pharmacological activator of AMP-activated protein kinase (AMPK)] reduces apoptosis of RTCs induced by subsequent stress and ameliorates the severity of ischemic acute kidney injury (AKI) in mice. In the present study, we examined the role of the phosphoinositide 3-kinase (PI3K)/Akt pathway in mediating these effects. Using shRNA, we developed knockdown (KD) RTCs to confirm that any novel effects of A-769662 are mediated specifically by AMPK. We reduced expression of the total β-domain of AMPK in KD RTCs by >80%. Control RTCs were transfected with “scrambled” shRNA. Preconditioning control RTCs with A-769662 increased both the phosphorylation (activity) of AMPK and survival of these cells when exposed to subsequent stress, but neither effect was observed in KD cells. These data demonstrate that activation of AMPK by A-769662 is profoundly impaired in KD cells. A-769662 activated PI3K and Akt in control but not KD RTCs. These data provide novel evidence that activation of the PI3K/Akt pathway by A-769662 is mediated specifically through activation of AMPK and not by a nonspecific mechanism. We also demonstrate that, in control RTCs, Akt plays a role in mediating the antiapoptotic effects of A-769662. In addition, we provide evidence that AMPK ameliorates the severity of ischemic AKI in mice and that this effect is also partially mediated by Akt. Finally, we provide evidence that AMPK activates PI3K by inhibiting mechanistic target of rapamycin complex 1 and preventing mechanistic target of rapamycin complex 1-mediated inhibition of insulin receptor substrate-1-associated activation of PI3K.

Morin reduces inflammatory responses and alleviates lipid accumulation in hepatocytes

Morin (MO), a natural bioflavinoid, exists in many herbs. Previous studies have acclaimed MO's anti-inflammatory, antidiabetic, antioxidant, antifibrotic, anticancer, and antihyperglycemic biological effects. This study aimed to assess the molecular mechanism of MO involved in the oleic acid (OA)-induced inflammatory damage and lipid accumulation in HepG2 cell and tyloxapol (Ty)-induced hyperlipidemia in mice. We found that MO can efficaciously mitigate reactive tumor necrosis factor-α (TNF-α) level and triglyceride (TG) accumulation in OA-induced HepG2 cell and in tyloxapol-induced mice. Next, the study testified that MO apparently suppressed OA-excited nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinases (MAPKs) signaling pathways in HepG2 cell. In addition, MO distinctly upregulated the expression of peroxisome proliferator-activated receptor α (PPARα) and decreased the expression of sterol regulatory element-binding protein 1c (SREBP-1c) in OA-induced HepG2 cell and in tyloxapol-induced mice, both of which are dependent upon the phosphorylation of acetyl-CoA carboxylase (ACC), adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK), and protein kinase B (AKT). In conclusion, these results suggest that MO has protective potential against hyperlipidemia and steatosis, and the potential mechanism may have a close relation with activation of PPARα and inhibition of SREBP-1c.

Magnolol Alleviates Inflammatory Responses and Lipid Accumulation by AMP-Activated Protein Kinase-Dependent Peroxisome Proliferator-Activated Receptor α Activation

Magnolol (MG) is a kind of lignin isolated from Magnolia officinalis, which serves several different biological functions, such as antifungal, anticancer, antioxidant, and hepatoprotective functions. This study aimed to evaluate the protective effect of MG against oleic acid (OA)-induced hepatic steatosis and inflammatory damage in HepG2 cells and in a tyloxapol (Ty)-induced hyperlipidemia mouse model. Our findings indicated that MG can effectively inhibit OA-stimulated tumor necrosis factor α (TNF-α) secretion, reactive oxygen species generation, and triglyceride (TG) accumulation. Further study manifested that MG significantly suppressed OA-activated mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-κB) signaling pathways and that these inflammatory responses can be negated by pretreatment with inhibitors of extracellular regulated protein kinase and c-Jun N-terminal kinase (U0126 and SP600125, respectively). In addition, MG dramatically upregulated peroxisome proliferator-activated receptor α (PPARα) translocation and reduced sterol regulatory element-binding protein 1c (SREBP-1c) protein synthesis and excretion, both of which are dependent upon the phosphorylation of adenosine monophosphate (AMP)-activated protein kinase (AMPK), acetyl-CoA carboxylase, and AKT kinase (AKT). However, MG suspended the activation of PPARα expression and was thus blocked by pretreatment with LY294002 and compound c (specific inhibitors of AKT and AMPK). Furthermore, MG clearly alleviated serum TG and total cholesterol release; upregulated AKT, AMPK, and PPARα expression; suppressed SREBP-1c generation; and alleviated hepatic steatosis and dyslipidemia in Ty-induced hyperlipidemia mice. Taken together, these results suggest that MG exerts protective effects against steatosis, hyperlipidemia, and the underlying mechanism, which may be closely associated with AKT/AMPK/PPARα activation and MAPK/NF-κB/SREBP-1c inhibition.

Preconditioning mice with activators of AMPK ameliorates ischemic acute kidney injury in vivo

This study had two objectives: 1) to determine whether preconditioning cultured proximal tubular cells (PTCs) with pharmacological activators of AMP-activated protein kinase (AMPK) protects these cells from apoptosis induced by metabolic stress in vitro and 2) to assess the effects of preconditioning mice with these agents on the severity of ischemic acute renal kidney injury (AKI) in vivo. We demonstrate that preconditioning PTCs with 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) or A-769662 reduces apoptosis of PTCs induced by subsequent stress. We also show that the reduction in cell death during metabolic stress associated with pretreatment by AMPK activators is associated with an increase in the cytosolic level of ATP, which is mediated by an increase in the rate of glycolysis. In addition, we provide evidence that the effect of AMPK activators on glycolysis is mediated, at least in part, by an increased uptake of glucose, and by the induction of hexokinase II (HK II) expression. Our data also show that the increased in HK II expression associated with preconditioning with AMPK activators is mediated by the activation (phosphorylation) of the cAMP-response element binding protein (CREB). We also provide entirely novel evidence that that A-79662 is substantially more effective than AICAR in mediating these alterations in PTCs in vitro. Finally, we demonstrate that preconditioning mice with AICAR or A-769662 substantially reduces the severity of renal dysfunction and tubular injury in a model of ischemic AKI in vivo and that the efficacy of AICAR and A-768662 in ameliorating ischemic AKI in vivo is comparable.

Metformin Protects Cells from Mutant Huntingtin Toxicity Through Activation of AMPK and Modulation of Mitochondrial Dynamics

Huntington's disease (HD) is a devastating neurodegenerative disease caused by the pathological elongation of the CAG repeats in the huntingtin gene. Caloric restriction (CR) has been the most reproducible environmental intervention to improve health and prolong life span. We have demonstrated that CR delayed onset and slowed disease progression in a mouse model of HD. Metformin, an antidiabetic drug, mimics CR by acting on cell metabolism at multiple levels. Long-term administration of metformin improved health and life span in mice. In this study, we showed that metformin rescued cells from mutant huntingtin (HTT)-induced toxicity, as indicated by reduced lactate dehydrogenase (LDH) release from cells and preserved ATP levels in cells expressing mutant HTT. Further mechanistic study indicated that metformin activated AMP-activated protein kinase (AMPK) and that inhibition of AMPK activation reduced its protective effects on mutant HTT toxicity, suggesting that AMPK mediates the protection of metformin in HD cells. Furthermore, metformin treatment prevented mitochondrial membrane depolarization and excess fission and modulated the disturbed mitochondrial dynamics in HD cells. We confirmed that metformin crossed the blood-brain barrier after oral administration and activated AMPK in the mouse brain. Our results urge further evaluation of the clinical potential for use of metformin in HD treatment.

Apoptotic cells activate AMP-activated protein kinase (AMPK) and inhibit epithelial cell growth without change in intracellular energy stores

Apoptosis plays an indispensable role in the maintenance and development of tissues. We have shown that receptor-mediated recognition of apoptotic target cells by viable kidney proximal tubular epithelial cells (PTECs) inhibits the proliferation and survival of PTECs. Here, we examined the effect of apoptotic targets on PTEC cell growth (cell size during G1 phase of the cell cycle). Using a cell culture model, we show that apoptotic cells potently activate AMP-activated protein kinase (AMPK), a highly sensitive sensor of intracellular energy stores. AMPK activation leads to decreased activity of its downstream target, ribosomal protein p70 S6 kinase (p70S6K), and concomitant inhibition of cell growth. Importantly, these events occur without detectable change in intracellular levels of AMP, ADP, or ATP. Inhibition of AMPK, either pharmacologically by compound C or molecularly by shRNA, diminishes the effects of apoptotic targets and largely restores p70S6K activity and cell size to normal levels. Apoptotic targets also inhibit Akt, a second signaling pathway regulating cell growth. Expression of a constitutively active Akt construct partially relieved cell growth inhibition but was less effective than inhibition of AMPK. Inhibition of cell growth by apoptotic targets is dependent on physical interaction between apoptotic targets and PTECs but independent of phagocytosis. We conclude that receptor-mediated recognition of apoptotic targets mimics the effects of intracellular energy depletion, activating AMPK and inhibiting cell growth. By acting as sentinels of environmental change, apoptotic death may enable nearby viable cells, especially nonmigratory epithelial cells, to monitor and adapt to local stresses.

Synthesis of a novel, sequentially active-targeted drug delivery nanoplatform for breast cancer therapy

Breast cancer is the leading cause of cancer deaths among women. Paclitaxel (PTX), an important breast cancer medicine, exhibits reduced bioavailability and therapeutic index due to high hydrophobicity and indiscriminate cytotoxicity. PTX encapsulation in one-level active targeting overcomes such barriers, but enhances toxicity to normal tissues with cancer-similar expression profiles. This research attempted to overcome this challenge by increasing selectivity of cancer cell targeting while maintaining an ability to overcome traditional pharmacological barriers. Thus, a multi-core, multi-targeting construct for tumor specific delivery of PTX was fabricated with (i) an inner-core prodrug targeting the cancer-overexpressed cathepsin B through a cathepsin B-cleavable tetrapeptide that conjugates PTX to a poly(amidoamine) dendrimer, and (ii) the encapsulation of this prodrug (PGD) in an outer core of a RES-evading, folate receptor (FR)-targeting liposome. Compared to traditional FR-targeting PTX liposomes, this sequentially active-targeted dendrosome demonstrated better prodrug retention, an increased cytotoxicity to cancer cells (latter being true when FR and cathepsin B activities were both at moderate-to-high levels) and higher tumor reduction. This research may eventually evolve a product platform with reduced systemic toxicity inherent with traditional chemotherapy and localized toxicity inherent to single-target nanoplatforms, thereby allowing for better tolerance of higher therapeutic load in advanced disease states.

Metformin as a cellular protector; a synoptic view of modern evidences

Due to limited knowledge and chemical class effect assimilation the biguanide metformin has long been considered as a useful but risky treatment for type 2 diabetes treatment. The worldwide long-term experience of clinical use of this compound and the growing knowledge about its mechanisms of action have, however, reversed this reputation to the point that nowadays it is not only considered as relatively harmless but even increasingly as a cellular protector. The present mini-review simply aims at giving a brief overview of the evidences accumulated overt recent periods and to provide the reader with information as to mechanistic hypotheses, knowing that there remains a lot to be done to better understand the pleiotropic behavior of this drug and its possible future new therapeutic applications. Data are shown at a glance for the kidney but also for other various organs and cell types corroborating this new notion for an old drug and paradox.

Tongxinluo decreases apoptosis of mesenchymal stem cells concentration-dependently under hypoxia and serum deprivation conditions through the AMPK/eNOS pathway

Tongxinluo (TXL), a traditional Chinese medicine, is widely used to treat cardiovascular diseases in China. Our previous study has demonstrated the pro-survival role of TXL on mesenchymal stem cells (MSCs) in vivo. But whether TXL could decrease apoptosis of MSCs in vitro, and the underlying mechanism are still unknown. Moreover, AMPK/eNOS pathway is crucial in regulating cell apoptosis. Therefore, we designed the study to investigate whether TXL could decrease MSCs apoptosis under hypoxia and serum deprivation (H/SD) conditions and to determine the role of AMPK/eNOS pathway. To test the hypothesis, MSCs were treated with TXL (50-400 μg/mL) under H/SD for 6 hours. For inhibitor studies, the cells were preincubated with AMPK inhibitor compound C. Results indicated that TXL decreased MSCs apoptosis concentration-dependently evidenced by reduced Annexin V+/PI- cells and increased red/green ratio of JC-1. Further, TXL enhanced the phosphorylation of AMPK and eNOS. Whereas, treatment with compound C decreased the phosphorylation of AMPK and eNOS and was accompanied by attenuated anti-apoptotic effect of TXL. In conclusion, TXL protected MSCs against H/SD-induced injury at least in part through the AMPK/eNOS pathway, which provides a novel explanation for the multi-effect of TXL on cardiovascular system.

Lipid accumulation is ahead of epithelial-to-mesenchymal transition and therapeutic intervention by acetyl-CoA carboxylase 2 silence in diabetic nephropathy

OBJECTIVE

The study investigated the relationship between epithelial-to-mesenchymal transition (EMT) and lipotoxicity in diabetic nephropathy as well as the protective effect of acetyl-CoA carboxylase 2 (ACC2) silence.

METHODS

High glucose (30mmol/L) cultured human proximal tubular epithelial cells (HK-2 cells) were used. Triglyceride content, fatty acid β-oxidation rate, malonyl CoA content, and marker proteins of EMT, including E-cadherin (E-cad), α-smooth muscle actin (α-SMA) and transforming grow factor-β (TGF-β), were assessed. Silence of ACC2 was achieved by ACC2-shRNA lentivirus transfection.

RESULTS

In cultured human proximal tubular cells, high glucose induced fatty acid deposit before phenotypical and morphological changes of EMT. At 48h, more triglyceride content, more malonyl CoA content and lower fatty acid β-oxidation rate were detected. However, increased expression of TGF-β, accompanied by loss of E-cad and acquisition of α-SMA, was observed at 98h but not at 48h. The silence of ACC2 in HK-2 cells led to restored cell morphology with less lipid deposition and less malonyl-CoA content, which resulted from faster β-oxidation rate.

CONCLUSION

The progress of lipotoxicity participates in the development of diabetic nephropathy in early stage before EMT. The manipulation of lipid metabolism might act as a promising therapeutic intervention for diabetic nephropathy.

Regulation of proximal tubule vacuolar H(+)-ATPase by PKA and AMP-activated protein kinase

The vacuolar H(+)-ATPase (V-ATPase) mediates ATP-driven H(+) transport across membranes. This pump is present at the apical membrane of kidney proximal tubule cells and intercalated cells. Defects in the V-ATPase and in proximal tubule function can cause renal tubular acidosis. We examined the role of protein kinase A (PKA) and AMP-activated protein kinase (AMPK) in the regulation of the V-ATPase in the proximal tubule as these two kinases coregulate the V-ATPase in the collecting duct. As the proximal tubule V-ATPases have different subunit compositions from other nephron segments, we postulated that V-ATPase regulation in the proximal tubule could differ from other kidney tubule segments. Immunofluorescence labeling of rat ex vivo kidney slices revealed that the V-ATPase was present in the proximal tubule both at the apical pole, colocalizing with the brush-border marker wheat germ agglutinin, and in the cytosol when slices were incubated in buffer alone. When slices were incubated with a cAMP analog and a phosphodiesterase inhibitor, the V-ATPase accumulated at the apical pole of S3 segment cells. These PKA activators also increased V-ATPase apical membrane expression as well as the rate of V-ATPase-dependent extracellular acidification in S3 cell monolayers relative to untreated cells. However, the AMPK activator AICAR decreased PKA-induced V-ATPase apical accumulation in proximal tubules of kidney slices and decreased V-ATPase activity in S3 cell monolayers. Our results suggest that in proximal tubule the V-ATPase subcellular localization and activity are acutely coregulated via PKA downstream of hormonal signals and via AMPK downstream of metabolic stress.

LPS inhibits caspase 3-dependent apoptosis in RAW264.7 macrophages induced by the AMPK activator AICAR

AMP-activated kinase is a cellular energy sensor which is activated in stages of increased ATP consumption. Its activation has been associated with a number of beneficial effects such as decreasing inflammatory processes and the disease progress of diabetes and obesity, respectively. Furthermore, AMPK activation has been linked with induction of cell cycle arrest and apoptosis in cancer and vascular cells, indicating that it might have a therapeutic impact for the treatment of cancer and atherosclerosis. However, the impact of AMPK on the proliferation of macrophages, which also play a key role in the formation of atherosclerotic plaques and in inflammatory processes, has not been focused so far. We have assessed the influence of AICAR- and metformin-induced AMPK activation on cell viability of macrophages with and without inflammatory stimulation, respectively. In cells without inflammatory stimulation, we found a strong induction of caspase 3-dependent apoptosis associated with decreased mTOR levels and increased expression of p21. Interestingly, these effects could be inhibited by co-stimulation with bacterial lipopolysaccharide (LPS) but not by other proinflammatory cytokines suggesting that AICAR induces apoptosis via AMPK in a TLR4-pathway dependent manner. In conclusion, our results revealed that AMPK activation is not only associated with positive effects but might also contribute to risk factors by disturbing important features of macrophages. The fact that LPS is able to restore AMPK-associated apoptosis might indicate an important role of TLR4 agonists in preventing unfavorable cell death of immune cells.

Pharmacological AMP kinase activators target the nucleolar organization and control cell proliferation

AIMS

Phenformin, resveratrol and AICAR stimulate the energy sensor 5'-AMP activated kinase (AMPK) and inhibit the first step of ribosome biogenesis, de novo RNA synthesis in nucleoli. Nucleolar activities are relevant to human health, because ribosome production is crucial to the development of diabetic complications. Although the function of nucleoli relies on their organization, the impact of AMPK activators on nucleolar structures is not known. Here, we addressed this question by examining four nucleolar proteins that are essential for ribosome biogenesis.

METHODS

Kidney cells were selected as model system, because diabetic nephropathy is one of the complications associated with diabetes mellitus. To determine the impact of pharmacological agents on nucleoli, we focused on the subcellular and subnuclear distribution of B23/nucleophosmin, fibrillarin, nucleolin and RPA194. This was achieved by quantitative confocal microscopy at the single-cell level in combination with cell fractionation and quantitative Western blotting.

RESULTS

AMPK activators induced the re-organization of nucleoli, which was accompanied by changes in cell proliferation. Among the compounds tested, phenformin and resveratrol had the most pronounced impact on nucleolar organization. For B23, fibrillarin, nucleolin and RPA194, both agents (i) altered the nucleocytoplasmic distribution and nucleolar association and (ii) reduced significantly the retention in the nucleus. (iii) Phenformin and resveratrol also increased significantly the total concentration of B23 and nucleolin.

CONCLUSIONS

AMPK activators have unique effects on the subcellular localization, nuclear retention and abundance of nucleolar proteins. We propose that the combination of these events inhibits de novo ribosomal RNA synthesis and modulates cell proliferation. Our studies identified nucleolin as a target that is especially sensitive to pharmacological AMPK activators. Because of its response to pharmacological agents, nucleolin represents a potential biomarker for the development of drugs that diminish diabetic renal hypertrophy.

The protective role of AMP-activated protein kinase in alpha-synuclein neurotoxicity in vitro

In the present study, we investigated the role of the main intracellular energy sensor, AMP-activated protein kinase (AMPK), in the in vitro neurotoxicity of α-synuclein (ASYN), one of the key culprits in the pathogenesis of Parkinson's disease. The loss of viability in retinoic acid-differentiated SH-SY5Y human neuroblastoma cells inducibly overexpressing wild-type ASYN was associated with the reduced activation of AMPK and its activator LKB1, as well as AMPK target Raptor. ASYN-overexpressing rat primary neurons also displayed lower activity of LKB1/AMPK/Raptor pathway. Restoration of AMPK activity by metformin or AICAR reduced the in vitro neurotoxicity of ASYN overexpression, acting independently of the prosurvival kinase Akt or the induction of autophagic response. The conditioned medium from ASYN-overexpressing cells, containing secreted ASYN, as well as dopamine-modified or nitrated recombinant ASYN oligomers, all inhibited AMPK activation in differentiated SH-SY5Y cells and reduced their viability, but not in the presence of metformin or AICAR. The RNA interference-mediated knockdown of AMPK increased the sensitivity of SH-SY5Y cells to the harmful effects of secreted ASYN. AMPK-dependent protection from extracellular ASYN was also observed in rat neuron-like pheochromocytoma cell line PC12. These data demonstrate the protective role of AMPK against the toxicity of both intracellular and extracellular ASYN, suggesting that modulation of AMPK activity may be a promising therapeutic strategy in Parkinson's disease.

Signalling pathways in succinate dehydrogenase B-associated renal carcinoma

AIMS

Renal tumours have recently been described in association with mutations in the gene encoding the B subunit of succinate dehydrogenase, a mitochondrial Krebs cycle and electron transport chain enzyme (SDHB-associated renal cell carcinomas). The aim of this study was to investigate the roles of different signalling pathways in the pathogenesis of these tumours.

METHODS AND RESULTS

We used immunohistochemistry and antibodies against phospho-specific epitopes to examine the activity of three potential signalling pathways in tumour cells of three genetically confirmed cases of SDHB-associated renal cell carcinomas. We found no evidence supporting a role for either the mTOR [p-mTOR (Ser2448), p-S6 riboprotein (Ser235/236)] or hypoxia-inducible (carbonic anhydrase 9 and EGFR) pathways. However, there was immunohistochemical reactivity for phosphorylated AMP-dependent kinase (p-AMPK Thr172) and glycogen synthase kinase 3 (GSK3) phosphorylation (p-GSK3 Ser12), and nuclear expression of cyclin D1.

CONCLUSIONS

We suggest that these tumours may arise through a mechanism involving ATP depletion, activation of AMPK, and induction of cyclin D1, and that this may be a unique pathway of tumour development that has the potential for therapeutic intervention in these rare tumours.

Susceptibility to ATP depletion of primary proximal tubular cell cultures derived from mice lacking either the α1 or the α2 isoform of the catalytic domain of AMPK

BACKGROUND

The purpose of this study was to determine whether AMPK influences the survival of primary cultures of mouse proximal tubular (MPT) cells subjected to metabolic stress. Previous studies, using an immortalized MPT cell line, suggest that AMPK is activated during metabolic stress, and ameliorates stress-induced apoptosis of these cells.

METHODS

Primary MPT cells were cultured from AMPK knockout (KO) mice lacking either the α1 or the α2 isoform of the catalytic domain of AMPK. MPT cells were subjected to ATP depletion using antimycin A.

RESULTS

Surprisingly, there was no difference in the amount of death induced by metabolic stress of MPT cells from either type of AMPK KO mice compared to its WT control. Moreover, inhibition of the activity of the α1 isoform in primary MPT cells from α2-/- mice (pharmacologically, via compound C) or inhibition of the α2 isoform in primary MPT cells from α1-/- mice (molecularly, via knockdown) both decreased cell viability equivalently in response to metabolic stress. The explanation for this unexpected result appears to be an adaptive increase in expression of the non-deleted α-isoform. As a consequence, total α-domain expression (i.e. α1 + α2), is comparable in kidney cortex and in cultured MPT cells derived from either type of KO mouse versus its WT control. Importantly, each α-isoform appears able to compensate fully for the absence of the other, with respect to both the phosphorylation of downstream targets of AMPK and the amelioration of stress-induced cell death.

CONCLUSIONS

These findings not only confirm the importance of AMPK as a pro-survival kinase in MPT cells during metabolic stress, but also show, for the first time, that each of the two α-isoforms can substitute for the other in MPT cells from AMPK KO mice with regard to amelioration of stress-induced loss of cell viability.

Bortezomib enhances fatty liver preservation in Institut George Lopez-1 solution through adenosine monophosphate activated protein kinase and Akt/mTOR pathways

OBJECTIVES

The aim of this study is to investigate the protective mechanisms induced by bortezomib added to Institut George Lopez (IGL)-1 preservation solution to protect steatotic livers against cold ischaemia reperfusion injury and to examine whether these mechanisms occur through the activation of adenosine monophosphate activated protein kinase (AMPK), Akt/mTOR pathways.

METHODS

Steatotic livers from obese rats were preserved for 24 h (at 4 °C) in IGL-1 solution with or without bortezomib (100 nM) or pretreated with AMPK inhibitor adenine 9-α-D-arabinofuranoside and preserved in IGL-1 + bortezomib. Livers were then perfused for 2 h at 37 °C. Liver injury (alanine aminotransferase/aspartate aminotransferase) and function (bile production and vascular resistance) were measured. Also, Akt/mTOR, phosphorylated AMPK (pAMPK) and apoptosis were determined by Western blot analyses.

KEY FINDINGS

Bortezomib addition to IGL-1 solution significantly reduced steatotic liver injury, improved graft function and decreased liver apoptosis. These benefits were diminished by the pretreatment of obese rats with AMPK inhibitor Ara. Western blot analyses showed a significant increase in pAMPK after ischaemia and reperfusion. We also observed a significant phosphorylation of Akt in IGL-1 +bortezomib group that, in turn, induced the phosphorylation of mTOR and glycogen synthase kinase 3β.

CONCLUSIONS

Bortezomib, at low and non toxic concentration, is a promising additive to IGL-1 solution for steatotic liver preservation. Its protective effect is due to the activation of AMPK and Akt/mTOR pathways.

Decreased levels of proapoptotic factors and increased key regulators of mitochondrial biogenesis constitute new potential beneficial features of long-lived growth hormone receptor gene-disrupted mice

Decreased somatotrophic signaling is among the most important mechanisms associated with extended longevity. Mice homozygous for the targeted disruption of the growth hormone (GH) receptor gene (GH receptor knockout; GHRKO) are obese and dwarf, are characterized by a reduced weight and body size, undetectable levels of GH receptor, high concentration of serum GH, and greatly reduced plasma levels of insulin and insulin-like growth factor-I, and are remarkably long lived. Recent results suggest new features of GHRKO mice that may positively affect longevity-decreased levels of proapoptotic factors and increased levels of key regulators of mitochondrial biogenesis. The alterations in levels of the proapoptotic factors and key regulators of mitochondrial biogenesis were not further improved by two other potential life-extending interventions-calorie restriction and visceral fat removal. This may attribute the primary role to GH resistance in the regulation of apoptosis and mitochondrial biogenesis in GHRKO mice in terms of increased life span.

Long-term supplementation with a cystine-based antioxidant delays loss of muscle mass in aging

Oxidative stress increases with age and is postulated to be a major causal factor for sarcopenia in aging. Here, we examined whether the administration of a cystine-based antioxidant (F1) can alleviate/delay age-specific changes in skeletal muscles. C57BL6 male mice aged 17 months (middle aged) were fed with normal diet with or without supplementation of F1 (3 mg/kg food) for 6 months. Compared with young (5 months old) mice old mice exhibited increased markers of oxidative stress, inflammation, and muscle cell apoptosis and decreased muscle weight. These age-related changes were further associated with inactivation of adenosine-5'-monophosphate-activated protein kinase (AMPK), increased lipogenesis, activation of c-Jun NH2-terminal kinase, and decreased expression of Delta 1, phospho-Akt, and proliferating cell nuclear antigen in aged skeletal muscle. Such alterations were significantly prevented by F1. These results demonstrate the beneficial effects of F1 to attenuate loss of muscle mass associated with aging.

Coordinated time-dependent modulation of AMPK/Akt/mTOR signaling and autophagy controls osteogenic differentiation of human mesenchymal stem cells

We investigated the role of AMP-activated protein kinase (AMPK), Akt, mammalian target of rapamycin (mTOR), autophagy and their interplay in osteogenic differentiation of human dental pulp mesenchymal stem cells. The activation of various members of AMPK, Akt and mTOR signaling pathways and autophagy was analyzed by immunoblotting, while osteogenic differentiation was assessed by alkaline phosphatase staining and real-time RT-PCR/immunoblot quantification of osteocalcin, Runt-related transcription factor 2 and bone morphogenetic protein 2 mRNA and/or protein levels. Osteogenic differentiation of mesenchymal stem cells was associated with early (day 1) activation of AMPK and its target Raptor, coinciding with the inhibition of mTOR and its substrate p70S6 kinase. The early induction of autophagy was demonstrated by accumulation of autophagosome-bound LC3-II, upregulation of proautophagic beclin-1 and a decrease in the selective autophagic target p62. This was followed by the late activation of Akt/mTOR at days 3-7 of differentiation. The RNA interference-mediated silencing of AMPK, mTOR or autophagy-essential LC3β, as well as the pharmacological inhibitors of AMPK (compound C), Akt (10-DEBC hydrochloride), mTOR (rapamycin) and autophagy (bafilomycin A1, chloroquine and ammonium chloride), each suppressed mesenchymal stem cell differentiation to osteoblasts. AMPK knockdown prevented early mTOR inhibition and autophagy induction, as well as late activation of Akt/mTOR signaling, while Akt inhibition suppressed mTOR activation without affecting AMPK phosphorylation. Our data indicate that AMPK controls osteogenic differentiation of human mesenchymal stem cells through both early mTOR inhibition-mediated autophagy and late activation of Akt/mTOR signaling axis.

Inhibition of platelet-derived growth factor receptor tyrosine kinase and downstream signaling pathways by Compound C

AMP-activated protein kinase (AMPK) has been implicated in anti-proliferative actions in a range of cell systems. Recently, it was observed that Compound C, an inhibitor of AMPK, also reduced the cell viability in human diploid fibroblasts (HDFs). Compound C-induced growth arrest was associated with a decrease in the cell cycle regulatory proteins, such as proliferating cell nuclear antigen, phosphorylated pRB, cyclin-dependent protein kinases (Cdk 2 and 4), cyclins (D and E), and the Cdk inhibitors (p21, p16, and p27). Therefore, the present study examined the molecular mechanism of the antiproliferative effects of Compound C. Although Compound C inhibited serum-induced phosphorylation of Akt and its substrate, glycogen synthase kinase-3β, it did not affect the Akt activity in vitro. Compound C significantly inhibited the receptor tyrosine phosphorylation and the activity of downstream signaling molecules, such as p85 phosphoinositide 3-kinase, phospholipase C-γ1, and extracellular signal-regulated kinase 1/2, induced by platelet-derived growth factor (PDGF) but not by epidermal growth factor- and insulin-like growth factor. In vitro growth factor receptor tyrosine kinase activity profiling revealed the IC50 for PDGF receptor-β (PDGFRβ) to be 5.07μM, whereas the IC50 for the epidermal growth factor receptor and insulin-like growth factor receptor was ≥100μM. The inhibitory effect of Compound C on PDGFRβ and Akt was also observed in AMPKα1/α2-knockout mouse embryonic fibroblasts, indicating that its inhibitory effect is independent of the AMPK activity. The inhibitory effect of Compound C on cell proliferation and PDGFRβ tyrosine phosphorylation was also demonstrated in various PDGFR-expressing cells, including MRC-5, BEAS-2B, rat aortic vascular smooth muscle cells, and A172 glioblastoma cells. These results indicate that Compound C can be used as a potential antiproliferative agent for PDGF- or PDGFR-associated diseases, such as cancer, atherosclerosis, and fibrosis.

Recognition-dependent signaling events in response to apoptotic targets inhibit epithelial cell viability by multiple mechanisms: implications for non-immune tissue homeostasis

Apoptosis allows for the removal of damaged, aged, and/or excess cells without harm to surrounding tissue. To accomplish this, cells undergoing apoptosis acquire new activities that enable them to modulate the fate and function of nearby cells. We have shown that receptor-mediated recognition of apoptotic versus necrotic target cells by viable kidney proximal tubular epithelial cells (PTEC) modulates the activity of several signaling pathways critically involved in regulation of proliferation and survival. Generally, apoptotic and necrotic targets have opposite effects with apoptotic targets inhibiting and necrotic targets stimulating the activity of these pathways. Here we explore the consequences of these signaling differences. We show that recognition of apoptotic targets induces a profound decrease in PTEC viability through increased responder cell death and decreased proliferation. In contrast, necrotic targets promote viability through decreased death and increased proliferation. Both target types mediate their effects through a network of Akt-dependent and -independent events. Apoptotic targets modulate Akt-dependent viability in part through a reduction in cellular β-catenin and decreased inactivation of Bad. In contrast, Akt-independent modulation of viability occurs through activation of caspase-8, suggesting that death receptor-dependent pathways are involved. Apoptotic targets also activate p38, which partially protects responders from target-induced death. The response of epithelial cells varies depending on their tissue origin. Some cell lines, like PTEC, demonstrate decreased viability, whereas others (e.g. breast-derived) show increased viability. By acting as sentinels of environmental change, apoptotic targets allow neighboring cells, especially non-migratory epithelial cells, to monitor and potentially adapt to local stresses.

The outcome of renal ischemia-reperfusion injury is unchanged in AMPK-β1 deficient mice

Aim

Activation of the master energy-regulator AMP-activated protein kinase (AMPK) in the heart reduces the severity of ischemia-reperfusion injury (IRI) but the role of AMPK in renal IRI is not known. The aim of this study was to determine whether activation of AMPK by acute renal ischemia influences the severity of renal IRI.

Methods

AMPK expression and activation and the severity of renal IRI was studied in mice lacking the AMPK β1 subunit and compared to wild type (WT) mice.

Results

Basal expression of activated AMPK, phosphorylayed at αThr¹⁷², was markedly reduced by 96% in AMPK-β1^−/− mice. Acute renal ischaemia caused a 3.2-fold increase in α1-AMPK activity and a 2.5-fold increase in α2-AMPK activity (P<0.001) that was associated with an increase in AMPK phosphorylation of the AMPK-α subunit at Thr¹⁷² and Ser⁴⁸⁵, and increased inhibitory phosphorylation of the AMPK substrate acetyl-CoA carboxylase. After acute renal ischemia AMPK activity was reduced by 66% in AMPK-β1^−/− mice compared with WT. There was no difference, however, in the severity of renal IRI at 24-hours between AMPK-β1^−/− and WT mice, as measured by serum urea and creatinine and histological injury score. In the heart, macrophage migration inhibitory factor (MIF) released during IRI contributes to AMPK activation and protects from injury. In the kidney, however, no difference in AMPK activation by acute ischemia was observed between MIF^−/− and WT mice. Compared with the heart, expression of the MIF receptor CD74 was found to be reduced in the kidney.

Conclusion

The failure of AMPK activation to influence the outcome of IRI in the kidney contrasts with what is reported in the heart. This difference might be due to a lack of effect of MIF on AMPK activation and lower CD74 expression in the kidney.