AMPK上游激酶——肿瘤抑制因子LKB1

AMPK是哺乳动物细胞中高度保守的蛋白质,是细胞的“代谢感受器”。AMPK的活化需要上游激酶(AMPKK)对AMPKα亚基活化环上Thr172进行磷酸化来完成。最近的研究发现,肿瘤抑制因子LKB1可以磷酸化Thr172进而激活AMPK,因此认为它是AMPKK家族的成员。     

Inhibition of Connexin 26 by the AMP-Activated Protein Kinase

Connexins provide intercellular connections that allow passage of ions and small organic molecules. They clamp the cell membrane potential and cellular ion composition to that of neighboring cells. The cell membrane potential and ion composition of an energy-depleted cell could thus be maintained despite its compromised Na⁺/K⁺ activity. By the same token, however, the breakdown of ion gradients in that cell imposes an additional challenge to the neighboring cells, which may jeopardize their survival. Thus, timely closure of connexins may be critically important for the survival of those cells. Energy depletion stimulates the AMP-activated protein kinase (AMPK), a serine/threonine kinase that senses energy depletion and stimulates several cellular mechanisms to enhance energy production and to limit energy utilization. The present study explored whether AMPK regulates connexin 26. To this end, cRNA encoding connexin 26 was injected into Xenopus oocytes with and without additional injection of wild-type AMPK (α1β1γ1), of the constitutively active ^γR70QAMPK (α1β1γ1[R70Q]) or of the inactive mutant ^αK45RAMPK (α1[K45R]β1γ1). Connexin 26 activity was determined in dual-electrode voltage-clamp experiments. Moreover, connexin 26 abundance was determined in the oocyte cell membrane by chemiluminescence and confocal microscopy. As a result, connexin 26-mediated current and connexin 26 protein abundance were significantly decreased by coexpression of ^γR70QAMPK and, to a lower extent, by wild-type AMPK but not by ^αK45RAMPK. In conclusion, AMPK is a potent regulator of connexin 26.     

Raloxifene Induces Autophagy-Dependent Cell Death in Breast Cancer Cells via the Activation of AMP-Activated Protein Kinase

Raloxifene is a selective estrogen receptor modulator (SERM) that binds to the estrogen receptor (ER), and exhibits potent anti-tumor and autophagy-inducing effects in breast cancer cells. However, the mechanism of raloxifene-induced cell death and autophagy is not well-established. So, we analyzed mechanism underlying death and autophagy induced by raloxifene in MCF-7 breast cancer cells.Treatment with raloxifene significantly induced death in MCF-7 cells. Raloxifene accumulated GFP-LC3 puncta and increased the level of autophagic marker proteins, such as LC3-II, BECN1, and ATG12-ATG5 conjugates, indicating activated autophagy. Raloxifene also increased autophagic flux indicators, the cleavage of GFP from GFP-LC3 and only red fluorescence-positive puncta in mRFP-GFP-LC3-expressing cells. An autophagy inhibitor, 3-methyladenine (3-MA), suppressed the level of LC3-II and blocked the formation of GFP-LC3 puncta. Moreover, siRNA targeting BECN1 markedly reversed cell death and the level of LC3-II increased by raloxifene. Besides, raloxifene-induced cell death was not related to cleavage of caspases-7, -9, and PARP. These results indicate that raloxifene activates autophagy-dependent cell death but not apoptosis. Interestingly, raloxifene decreased the level of intracellular adenosine triphosphate (ATP) and activated the AMPK/ULK1 pathway. However it was not suppressed the AKT/mTOR pathway. Addition of ATP decreased the phosphorylation of AMPK as well as the accumulation of LC3-II, finally attenuating raloxifene-induced cell death.Our current study demonstrates that raloxifene induces autophagy via the activation of AMPK by sensing decreases in ATP, and that the overactivation of autophagy promotes cell death and thereby mediates the anti-cancer effects of raloxifene in breast cancer cells.     

AMP-Activated Kinase Restricts Rift Valley Fever Virus Infection by Inhibiting Fatty Acid Synthesis

The cell intrinsic innate immune responses provide a first line of defense against viral infection, and often function by targeting cellular pathways usurped by the virus during infection. In particular, many viruses manipulate cellular lipids to form complex structures required for viral replication, many of which are dependent on de novo fatty acid synthesis. We found that the energy regulator AMPK, which potently inhibits fatty acid synthesis, restricts infection of the Bunyavirus, Rift Valley Fever Virus (RVFV), an important re-emerging arthropod-borne human pathogen for which there are no effective vaccines or therapeutics. We show restriction of RVFV both by AMPK and its upstream activator LKB1, indicating an antiviral role for this signaling pathway. Furthermore, we found that AMPK is activated during RVFV infection, leading to the phosphorylation and inhibition of acetyl-CoA carboxylase, the first rate-limiting enzyme in fatty acid synthesis. Activating AMPK pharmacologically both restricted infection and reduced lipid levels. This restriction could be bypassed by treatment with the fatty acid palmitate, demonstrating that AMPK restricts RVFV infection through its inhibition of fatty acid biosynthesis. Lastly, we found that this pathway plays a broad role in antiviral defense since additional viruses from disparate families were also restricted by AMPK and LKB1. Therefore, AMPK is an important component of the cell intrinsic immune response that restricts infection through a novel mechanism involving the inhibition of fatty acid metabolism.     

The MDMX Acidic Domain Uses Allovalency to Bind Both p53 and MDMX

Autoinhibition of p53 binding to MDMX requires two short-linear motifs (SLiMs) containing adjacent tryptophan (WW) and tryptophan-phenylalanine (WF) residues. NMR spectroscopy was used to show the WW and WF motifs directly compete for the p53 binding site on MDMX and circular dichroism spectroscopy was used to show the WW motif becomes helical when it is bound to the p53 binding domain (p53BD) of MDMX. Binding studies using isothermal titration calorimetry showed the WW motif is a stronger inhibitor of p53 binding than the WF motif when they are both tethered to p53BD by the natural disordered linker. We also investigated how the WW and WF motifs interact with the DNA binding domain (DBD) of p53. Both motifs bind independently to similar sites on DBD that overlap the DNA binding site. Taken together our work defines a model for complex formation between MDMX and p53 where a pair of disordered SLiMs bind overlapping sites on both proteins.     

A Cell-Autonomous Molecular Cascade Initiated by AMP-Activated Protein Kinase Represses Steroidogenesis

Steroid hormones regulate essential physiological processes, and inadequate levels are associated with various pathological conditions. In testosterone-producing Leydig cells, steroidogenesis is strongly stimulated by luteinizing hormone (LH) via its receptor leading to increased cyclic AMP (cAMP) production and expression of the steroidogenic acute regulatory (STAR) protein, which is essential for the initiation of steroidogenesis. Steroidogenesis then passively decreases with the degradation of cAMP into AMP by phosphodiesterases. In this study, we show that AMP-activated protein kinase (AMPK) is activated following cAMP-to-AMP breakdown in MA-10 and MLTC-1 Leydig cells. Activated AMPK then actively inhibits cAMP-induced steroidogenesis by repressing the expression of key regulators of steroidogenesis, including Star and Nr4a1. Similar results were obtained in Y-1 adrenal cells and in the constitutively steroidogenic R2C cells. We have also determined that maximum AMPK activation following stimulation of steroidogenesis in MA-10 Leydig cells occurs when steroid hormone production has reached a plateau. Our data identify AMPK as a molecular rheostat that actively represses steroid hormone biosynthesis to preserve cellular energy homeostasis and prevent excess steroid production.     

Opposing Activity Changes in AMP Deaminase and AMP-Activated Protein Kinase in the Hibernating Ground Squirrel

Hibernating animals develop fatty liver when active in summertime and undergo a switch to a fat oxidation state in the winter. We hypothesized that this switch might be determined by AMP and the dominance of opposing effects: metabolism through AMP deaminase (AMPD2) (summer) and activation of AMP-activated protein kinase (AMPK) (winter). Liver samples were obtained from 13-lined ground squirrels at different times during the year, including summer and multiples stages of winter hibernation, and fat synthesis and β-fatty acid oxidation were evaluated. Changes in fat metabolism were correlated with changes in AMPD2 activity and intrahepatic uric acid (downstream product of AMPD2), as well as changes in AMPK and intrahepatic β-hydroxybutyrate (a marker of fat oxidation). Hepatic fat accumulation occurred during the summer with relatively increased enzymes associated with fat synthesis (FAS, ACL and ACC) and decreased enoyl CoA hydratase (ECH1) and carnitine palmitoyltransferase 1A (CPT1A), rate limiting enzymes of fat oxidation. In summer, AMPD2 activity and intrahepatic uric acid levels were high and hepatic AMPK activity was low. In contrast, the active phosphorylated form of AMPK and β-hydroxybutyrate both increased during winter hibernation. Therefore, changes in AMPD2 and AMPK activity were paralleled with changes in fat synthesis and fat oxidation rates during the summer-winter cycle. These data illuminate the opposing forces of metabolism of AMP by AMPD2 and its availability to activate AMPK as a switch that governs fat metabolism in the liver of hibernating ground squirrel.     

Regulation of Protein Kinase A Activity by p90 Ribosomal S6 Kinase 1

Previously, we reported that the catalytic subunit of cAMP-dependent protein kinase (PKAc) binds to the active p90 ribosomal S6 kinase 1 (RSK1) (Chaturvedi, D., Poppleton, H. M., Stringfield, T., Barbier, A., and Patel, T. B. (2006) Mol. Cell. Biol. 26, 4586–4600). Herein, by overexpressing hemagglutinin-tagged RSK1 fragments in HeLa cells we have identified the region of RSK1 that is responsible for the interaction with PKAc. PKAc bound to the last 13 amino acids of RSK1, which overlaps the Erk1/2 docking site. This interaction between PKAc and RSK1 required the phosphorylation of Ser-732 in the C terminus of RSK1. Depending upon its phosphorylation status, RSK1 switched interactions between Erk1/2 and PKAc. In addition, a peptide corresponding to the last 13 amino acids of RSK1 with substitution of Ser-732 with Glu (peptide E), but not Ala (peptide A), decreased interactions between endogenous active RSK1 and PKAc. RSK1 attenuated the ability of cAMP to activate PKA in vitro and this modulation was abrogated by peptide E, but not by peptide A. Similarly, in intact cells, cAMP-mediated phosphorylation of Bcl-xL/Bcl-2-associated death promoter on Ser-115, the PKA site, was reduced when RSK1 was activated by epidermal growth factor, and this effect was blocked by peptide E, but not by peptide A. These findings demonstrate that interactions between endogenous RSK1 and PKAc in intact cells regulate the ability of cAMP to activate PKA and identify a novel mechanism by which PKA activity is regulated by the Erk1/2 pathway.     

Protein Kinase A Rescues Microtubule Affinity-regulating Kinase 2-induced Microtubule Instability and Neurite Disruption by Phosphorylating Serine 409

Microtubule affinity-regulating kinase 2 (MARK2)/PAR-1b and protein kinase A (PKA) are both involved in the regulation of microtubule stability and neurite outgrowth, but whether a direct cross-talk exists between them remains unclear. Here, we found the disruption of microtubule and neurite outgrowth induced by MARK2 overexpression was blocked by active PKA. The interaction between PKA and MARK2 was confirmed by coimmunoprecipitation and immunocytochemistry both in vitro and in vivo. PKA was found to inhibit MARK2 kinase activity by phosphorylating a novel site, serine 409. PKA could not reverse the microtubule disruption effect induced by a serine 409 to alanine (Ala) mutant of MARK2 (MARK2 S409A). In contrast, mutation of MARK2 serine 409 to glutamic acid (Glu) (MARK2 S409E) did not affect microtubule stability and neurite outgrowth. We propose that PKA functions as an upstream inhibitor of MARK2 in regulating microtubule stability and neurite outgrowth by directly interacting and phosphorylating MARK2.     

Berberine Promotes Glucose Consumption Independently of AMP-Activated Protein Kinase Activation

Berberine is a plant alkaloid with anti-diabetic action. Activation of AMP-activated protein kinase (AMPK) pathway has been proposed as mechanism for berberine’s action. This study aimed to examine whether AMPK activation was necessary for berberine’s glucose-lowering effect. We found that in HepG2 hepatocytes and C2C12 myotubes, berberine significantly increased glucose consumption and lactate release in a dose-dependent manner. AMPK and acetyl coenzyme A synthetase (ACC) phosphorylation were stimulated by 20 µmol/L berberine. Nevertheless, berberine was still effective on stimulating glucose utilization and lactate production, when the AMPK activation was blocked by (1) inhibition of AMPK activity by Compound C, (2) suppression of AMPKα expression by siRNA, and (3) blockade of AMPK pathway by adenoviruses containing dominant-negative forms of AMPKα1/α2. To test the effect of berberine on oxygen consumption, extracellular flux analysis was performed in Seahorse XF24 analyzer. The activity of respiratory chain complex I was almost fully blocked in C2C12 myotubes by berberine. Metformin, as a positive control, showed similar effects as berberine. These results suggest that berberine and metformin promote glucose metabolism by stimulating glycolysis, which probably results from inhibition of mitochondrial respiratory chain complex I, independent of AMPK activation.     

Activation of AMP-Activated Protein Kinase Revealed by Hydrogen/Deuterium Exchange Mass Spectrometry

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MDMX regulation of p53 response to ribosomal stress

Ribosomal stress such as disruption of rRNA biogenesis activates p53 by release of ribosomal proteins from the nucleoli, which bind to MDM2 and inhibit p53 degradation. We found that p53 activation by ribosomal stress requires degradation of MDMX in an MDM2-dependent fashion. Tumor cells overexpressing MDMX are less sensitive to actinomycin D-induced growth arrest due to formation of inactive p53-MDMX complexes. Knockdown of MDMX increases sensitivity to actinomycin D, whereas MDMX overexpression abrogates p53 activation and prevents growth arrest. Furthermore, MDMX expression promotes resistance to the chemotherapeutic agent 5-fluorouracil (5-FU), which at low concentrations activates p53 by inducing ribosomal stress without significant DNA damage signaling. Knockdown of MDMX abrogates HCT116 tumor xenograft formation in nude mice. MDMX overexpression does not accelerate tumor growth but increases resistance to 5-FU treatment in vivo. Therefore, MDMX is an important regulator of p53 response to ribosomal stress and RNA-targeting chemotherapy agents.     

DNA damage induces MDMX nuclear translocation by p53-dependent and -independent mechanisms

The MDM2 homolog MDMX is an important regulator of p53 activity during embryonic development. MDMX inactivation in mice results in embryonic lethality in a p53-dependent fashion. The expression level of MDMX is not induced by DNA damage, and its role in stress response is unclear. We show here that ectopically expressed MDMX is mainly localized in the cytoplasm. DNA damage promotes nuclear translocation of MDMX in cells with or without p53. Coexpression of MDM2 or p53 is sufficient to induce MDMX nuclear translocation, suggesting that activation of p53 and induction of MDM2 expression can contribute to this process. Stable transfection of MDMX into U2OS cells does not alter p53 level but results in reduced p53 DNA-binding activity and reduced MDM2 expression. The ability of ARF (alternate reading frame of INK4a) to activate p53 is also significantly inhibited by expression of MDMX. These results suggest that MDMX function may be regulated by DNA damage. Furthermore, MDMX may complement MDM2 in regulating p53 during embryonic development due to its ability to inhibit p53 in the presence of ARF.