Herpud1 impacts insulin-dependent glucose uptake in skeletal muscle cells by controlling the Ca2+-calcineurin-Akt axis.

Skeletal muscle plays a central role in insulin-controlled glucose homeostasis. The molecular mechanisms related to insulin resistance in this tissue are incompletely understood. Herpud1 is an endoplasmic reticulum membrane protein that maintains intracellular Ca2+ homeostasis under stress conditions. It has recently been reported that Herpud1-knockout mice display intolerance to a glucose load without showing altered insulin secretion. The functions of Herpud1 in skeletal muscle also remain unknown. Based on these findings, we propose that Herpud1 is necessary for insulin-dependent glucose disposal in skeletal muscle. Here we show that Herpud1 silencing decreased insulin-dependent glucose uptake, GLUT4 translocation to the plasma membrane, and Akt Ser473 phosphorylation in cultured L6 myotubes. A decrease in insulin-induced Akt Ser473 phosphorylation was observed in soleus but not in extensor digitorum longus muscle samples from Herpud1-knockout mice. Herpud1 knockdown increased the IP3R-dependent cytosolic Ca2+ response and the activity of Ca2+-dependent serine/threonine phosphatase calcineurin in L6 cells. Calcineurin decreased insulin-dependent Akt phosphorylation and glucose uptake. Moreover, calcineurin inhibition restored the insulin response in Herpud1-depleted L6 cells. Based on these findings, we conclude that Herpud1 is necessary for adequate insulin-induced glucose uptake due to its role in Ca2+/calcineurin regulation in L6 myotubes.

Mitochondria in Structural and Functional Cardiac Remodeling.

The heart must function continuously as it is responsible for both supplying oxygen and nutrients throughout the entire body, as well as for the transport of waste products to excretory organs. When facing either a physiological or pathological increase in cardiac demand, the heart undergoes structural and functional remodeling as a means of adapting to increased workload. These adaptive responses can include changes in gene expression, protein composition, and structure of sub-cellular organelles involved in energy production and metabolism. Mitochondria are essential for cardiac function, as they supply the ATP necessary to support continuous cycles of contraction and relaxation. In addition, mitochondria carry out other important processes, including synthesis of essential cellular components, calcium buffering, and initiation of cell death signals. Not surprisingly, mitochondrial dysfunction has been linked to several cardiovascular disorders, including hypertension, cardiac hypertrophy, ischemia/reperfusion and heart failure. The present chapter will discuss how changes in mitochondrial cristae structure, fusion/fission dynamics, fatty acid oxidation, ATP production, and the generation of reactive oxygen species might impact cardiac structure and function, particularly in the context of pathological hypertrophy and fibrotic response. In addition, the mechanistic role of mitochondria in autophagy and programmed cell death of cardiomyocytes will be addressed. Here we will also review strategies to improve mitochondrial function and discuss their cardioprotective potential.

Calcium and mitochondrial metabolism in ceramide-induced cardiomyocyte death.

Ceramides are important intermediates in the biosynthesis and degradation of sphingolipids that regulate numerous cellular processes, including cell cycle progression, cell growth, differentiation and death. In cardiomyocytes, ceramides induce apoptosis by decreasing mitochondrial membrane potential and promoting cytochrome-c release. Ca(2+) overload is a common feature of all types of cell death. The aim of this study was to determine the effect of ceramides on cytoplasmic Ca(2+) levels, mitochondrial function and cardiomyocyte death. Our data show that C2-ceramide induces apoptosis and necrosis in cultured cardiomyocytes by a mechanism involving increased Ca(2+) influx, mitochondrial network fragmentation and loss of the mitochondrial Ca(2+) buffer capacity. These biochemical events increase cytosolic Ca(2+) levels and trigger cardiomyocyte death via the activation of calpains.

Calpains and proteasomes mediate degradation of ryanodine receptors in a model of cardiac ischemic reperfusion.

Type-2 ryanodine receptors (RyR2)--the calcium release channels of cardiac sarcoplasmic reticulum--have a central role in cardiac excitation-contraction coupling. In the heart, ischemia/reperfusion causes a rapid and significant decrease in RyR2 content but the mechanisms responsible for this effect are not fully understood. We have studied the involvement of three proteolytic systems--calpains, the proteasome and autophagy--on the degradation of RyR2 in rat neonatal cardiomyocyte cultures subjected to simulated ischemia/reperfusion (sI/R). We found that 8h of ischemia followed by 16h of reperfusion decreased RyR2 content by 50% without any changes in RyR2 mRNA. Specific inhibitors of calpains and the proteasome prevented the decrease of RyR2 caused by sI/R, implicating both pathways in its degradation. Proteasome inhibitors also prevented the degradation of calpastatin, the endogenous calpain inhibitor, hindering the activation of calpain induced by calpastatin degradation. Autophagy was activated during sI/R as evidenced by the increase in LC3-II and beclin-1, two proteins involved in autophagosome generation, and in the emergence of GFP-LC3 containing vacuoles in adenovirus GFP-LC3 transduced cardiomyocytes. Selective autophagy inhibition, however, induced even further RyR2 degradation, making unlikely the participation of autophagy in sI/R-induced RyR2 degradation. Our results suggest that calpain activation as a result of proteasome-induced degradation of calpastatin initiates RyR2 proteolysis, which is followed by proteasome-dependent degradation of the resulting RyR2 fragments. The decrease in RyR2 content during ischemia/reperfusion may be relevant to the decrease of heart contractility after ischemia.

Polycystin-1 is required for insulin-like growth factor 1-induced cardiomyocyte hypertrophy.

Cardiac hypertrophy is the result of responses to various physiological or pathological stimuli. Recently, we showed that polycystin-1 participates in cardiomyocyte hypertrophy elicited by pressure overload and mechanical stress. Interestingly, polycystin-1 knockdown does not affect phenylephrine-induced cardiomyocyte hypertrophy, suggesting that the effects of polycystin-1 are stimulus-dependent. In this study, we aimed to identify the role of polycystin-1 in insulin-like growth factor-1 (IGF-1) signaling in cardiomyocytes. Polycystin-1 knockdown completely blunted IGF-1-induced cardiomyocyte hypertrophy. We then investigated the molecular mechanism underlying this result. We found that polycystin-1 silencing impaired the activation of the IGF-1 receptor, Akt, and ERK1/2 elicited by IGF-1. Remarkably, IGF-1-induced IGF-1 receptor, Akt, and ERK1/2 phosphorylations were restored when protein tyrosine phosphatase 1B was inhibited, suggesting that polycystin-1 knockdown deregulates this phosphatase in cardiomyocytes. Moreover, protein tyrosine phosphatase 1B inhibition also restored IGF-1-dependent cardiomyocyte hypertrophy in polycystin-1-deficient cells. Our findings provide the first evidence that polycystin-1 regulates IGF-1-induced cardiomyocyte hypertrophy through a mechanism involving protein tyrosine phosphatase 1B.

Targeting Mitochondrial Iron Metabolism Suppresses Tumor Growth and Metastasis by Inducing Mitochondrial Dysfunction and Mitophagy.

Deferoxamine (DFO) represents a widely used iron chelator for the treatment of iron overload. Here we describe the use of mitochondrially targeted deferoxamine (mitoDFO) as a novel approach to preferentially target cancer cells. The agent showed marked cytostatic, cytotoxic, and migrastatic properties in vitro, and it significantly suppressed tumor growth and metastasis in vivo. The underlying molecular mechanisms included (i) impairment of iron-sulfur [Fe-S] cluster/heme biogenesis, leading to destabilization and loss of activity of [Fe-S] cluster/heme containing enzymes, (ii) inhibition of mitochondrial respiration leading to mitochondrial reactive oxygen species production, resulting in dysfunctional mitochondria with markedly reduced supercomplexes, and (iii) fragmentation of the mitochondrial network and induction of mitophagy. Mitochondrial targeting of deferoxamine represents a way to deprive cancer cells of biologically active iron, which is incompatible with their proliferation and invasion, without disrupting systemic iron metabolism. Our findings highlight the importance of mitochondrial iron metabolism for cancer cells and demonstrate repurposing deferoxamine into an effective anticancer drug via mitochondrial targeting. SIGNIFICANCE: These findings show that targeting the iron chelator deferoxamine to mitochondria impairs mitochondrial respiration and biogenesis of [Fe-S] clusters/heme in cancer cells, which suppresses proliferation and migration and induces cell death. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/9/2289/F1.large.jpg.

Complex Mitochondrial Dysfunction Induced by TPP+-Gentisic Acid and Mitochondrial Translation Inhibition by Doxycycline Evokes Synergistic Lethality in Breast Cancer Cells.

The mitochondrion has emerged as a promising therapeutic target for novel cancer treatments because of its essential role in tumorigenesis and resistance to chemotherapy. Previously, we described a natural compound, 10-((2,5-dihydroxybenzoyl)oxy)decyl) triphenylphosphonium bromide (GA-TPP+C10), with a hydroquinone scaffold that selectively targets the mitochondria of breast cancer (BC) cells by binding to the triphenylphosphonium group as a chemical chaperone; however, the mechanism of action remains unclear. In this work, we showed that GA-TPP+C10 causes time-dependent complex inhibition of the mitochondrial bioenergetics of BC cells, characterized by (1) an initial phase of mitochondrial uptake with an uncoupling effect of oxidative phosphorylation, as previously reported, (2) inhibition of Complex I-dependent respiration, and (3) a late phase of mitochondrial accumulation with inhibition of α-ketoglutarate dehydrogenase complex (αKGDHC) activity. These events led to cell cycle arrest in the G1 phase and cell death at 24 and 48 h of exposure, and the cells were rescued by the addition of the cell-penetrating metabolic intermediates l-aspartic acid ß-methyl ester (mAsp) and dimethyl α-ketoglutarate (dm-KG). In addition, this unexpected blocking of mitochondrial function triggered metabolic remodeling toward glycolysis, AMPK activation, increased expression of proliferator-activated receptor gamma coactivator 1-alpha (pgc1α) and electron transport chain (ETC) component-related genes encoded by mitochondrial DNA and downregulation of the uncoupling proteins ucp3 and ucp4, suggesting an AMPK-dependent prosurvival adaptive response in cancer cells. Consistent with this finding, we showed that inhibition of mitochondrial translation with doxycycline, a broad-spectrum antibiotic that inhibits the 28 S subunit of the mitochondrial ribosome, in the presence of GA-TPP+C10 significantly reduces the mt-CO1 and VDAC protein levels and the FCCP-stimulated maximal electron flux and promotes selective and synergistic cytotoxic effects on BC cells at 24 h of treatment. Based on our results, we propose that this combined strategy based on blockage of the adaptive response induced by mitochondrial bioenergetic inhibition may have therapeutic relevance in BC.

Mitochondrial fragmentation, elevated mitochondrial superoxide and respiratory supercomplexes disassembly is connected with the tamoxifen-resistant phenotype of breast cancer cells.

Tamoxifen resistance remains a clinical obstacle in the treatment of hormone sensitive breast cancer. It has been reported that tamoxifen is able to target respiratory complex I within mitochondria. Therefore, we established two tamoxifen-resistant cell lines, MCF7 Tam5R and T47D Tam5R resistant to 5 µM tamoxifen and investigated whether tamoxifen-resistant cells exhibit mitochondrial changes which could help them survive the treatment. The function of mitochondria in this experimental model was evaluated in detail by studying i) the composition and activity of mitochondrial respiratory complexes; ii) respiration and glycolytic status; iii) mitochondrial distribution, dynamics and reactive oxygen species production. We show that Tam5R cells exhibit a significant decrease in mitochondrial respiration, low abundance of assembled mitochondrial respiratory supercomplexes, a more fragmented mitochondrial network connected with DRP1 Ser637 phosphorylation, higher glycolysis and sensitivity to 2-deoxyglucose. Tam5R cells also produce significantly higher levels of mitochondrial superoxide but at the same time increase their antioxidant defense (CAT, SOD2) through upregulation of SIRT3 and show phosphorylation of AMPK at Ser 485/491. Importantly, MCF7 ρ0 cells lacking functional mitochondria exhibit a markedly higher resistance to tamoxifen, supporting the role of mitochondria in tamoxifen resistance. We propose that reduced mitochondrial function and higher level of reactive oxygen species within mitochondria in concert with metabolic adaptations contribute to the phenotype of tamoxifen resistance.

Herpud1 negatively regulates pathological cardiac hypertrophy by inducing IP3 receptor degradation.

Cardiac hypertrophy is an adaptive response triggered by pathological stimuli. Regulation of the synthesis and the degradation of the Ca2+ channel inositol 1,4,5-trisphosphate receptor (IP3R) affects progression to cardiac hypertrophy. Herpud1, a component of the endoplasmic reticulum-associated degradation (ERAD) complex, participates in IP3R1 degradation and Ca2+ signaling, but the cardiac function of Herpud1 remains unknown. We hypothesize that Herpud1 acts as a negative regulator of cardiac hypertrophy by regulating IP3R protein levels. Our results show that Herpud1-knockout mice exhibit cardiac hypertrophy and dysfunction and that decreased Herpud1 protein levels lead to elevated levels of hypertrophic markers in cultured rat cardiomyocytes. In addition, IP3R levels were elevated both in Herpud1-knockout mice and Herpud1 siRNA-treated rat cardiomyocytes. The latter treatment also led to elevated cytosolic and nuclear Ca2+ levels. In summary, the absence of Herpud1 generates a pathological hypertrophic phenotype by regulating IP3R protein levels. Herpud1 is a novel negative regulator of pathological cardiac hypertrophy.

HERPUD1 protects against oxidative stress-induced apoptosis through downregulation of the inositol 1,4,5-trisphosphate receptor.

Homocysteine-inducible, endoplasmic reticulum (ER) stress-inducible, ubiquitin-like domain member 1 (HERPUD1), an ER resident protein, is upregulated in response to ER stress and Ca(2+) homeostasis deregulation. HERPUD1 exerts cytoprotective effects in various models, but its role during oxidative insult remains unknown. The aim of this study was to investigate whether HERPUD1 contributes to cytoprotection in response to redox stress and participates in mediating stress-dependent signaling pathways. Our data showed that HERPUD1 protein levels increased in HeLa cells treated for 30 min with H2O2 or angiotensin II and in aortic tissue isolated from mice treated with angiotensin II for 3 weeks. Cell death was higher in HERPUD1 knockdown (sh-HERPUD1) HeLa cells treated with H2O2 in comparison with control (sh-Luc) HeLa cells. This effect was abolished by the intracellular Ca(2+) chelating agent BAPTA-AM or the inositol 1,4,5-trisphosphate receptor (ITPR) antagonist xestospongin B, suggesting that the response to H2O2 was dependent on intracellular Ca(2+) stores and the ITPR. Ca(2+) kinetics showed that sh-HERPUD1 HeLa cells exhibited greater and more sustained cytosolic and mitochondrial Ca(2+) increases than sh-Luc HeLa cells. This higher sensitivity of sh-HERPUD1 HeLa cells to H2O2 was prevented with the mitochondrial permeability transition pore inhibitor cyclosporine A. We concluded that the HERPUD1-mediated cytoprotective effect against oxidative stress depends on the ITPR and Ca(2+) transfer from the ER to mitochondria.

Organelle communication: signaling crossroads between homeostasis and disease.

Cellular organelles do not function as isolated or static units, but rather form dynamic contacts between one another that can be modulated according to cellular needs. The physical interfaces between organelles are important for Ca2+ and lipid homeostasis, and serve as platforms for the control of many essential functions including metabolism, signaling, organelle integrity and execution of the apoptotic program. Emerging evidence also highlights the importance of organelle communication in disorders such as Alzheimer's disease, pulmonary arterial hypertension, cancer, skeletal and cardiac muscle dysfunction. Here, we provide an overview of the current literature on organelle communication and the link to human pathologies.

Cardiomyocyte ryanodine receptor degradation by chaperone-mediated autophagy.

AIMS: Chaperone-mediated autophagy (CMA) is a selective mechanism for the degradation of soluble cytosolic proteins bearing the sequence KFERQ. These proteins are targeted by chaperones and delivered to lysosomes where they are translocated into the lysosomal lumen and degraded via the lysosome-associated membrane protein type 2A (LAMP-2A). Mutations in LAMP2 that inhibit autophagy result in Danon disease characterized by hypertrophic cardiomyopathy. The ryanodine receptor type 2 (RyR2) plays a key role in cardiomyocyte excitation-contraction and its dysfunction can lead to cardiac failure. Whether RyR2 is degraded by CMA is unknown. METHODS AND RESULTS: To induce CMA, cultured neonatal rat cardiomyocytes were treated with geldanamycin (GA) to promote protein degradation through this pathway. GA increased LAMP-2A levels together with its redistribution and colocalization with Hsc70 in the perinuclear region, changes indicative of CMA activation. The inhibition of lysosomes but not proteasomes prevented the loss of RyR2. The recovery of RyR2 content after incubation with GA by siRNA targeting LAMP-2A suggests that RyR2 is degraded via CMA. In silico analysis also revealed that the RyR2 sequence harbours six KFERQ motifs which are required for the recognition Hsc70 and its degradation via CMA. Our data suggest that presenilins are involved in RyR2 degradation by CMA. CONCLUSION: These findings are consistent with a model in which oxidative damage of the RyR2 targets it for turnover by presenilins and CMA, which could lead to removal of damaged or leaky RyR2 channels.

Endoplasmic reticulum and the unfolded protein response: dynamics and metabolic integration.

The endoplasmic reticulum (ER) is a dynamic intracellular organelle with multiple functions essential for cellular homeostasis, development, and stress responsiveness. In response to cellular stress, a well-established signaling cascade, the unfolded protein response (UPR), is activated. This intricate mechanism is an important means of re-establishing cellular homeostasis and alleviating the inciting stress. Now, emerging evidence has demonstrated that the UPR influences cellular metabolism through diverse mechanisms, including calcium and lipid transfer, raising the prospect of involvement of these processes in the pathogenesis of disease, including neurodegeneration, cancer, diabetes mellitus and cardiovascular disease. Here, we review the distinct functions of the ER and UPR from a metabolic point of view, highlighting their association with prevalent pathologies.