The Homocysteine-inducible Endoplasmic Reticulum Stress Protein Counteracts Calcium Store Depletion and Induction of CCAAT Enhancer-binding Protein Homologous Protein in a Neurotoxin Model of Parkinson Disease

The endoplasmic reticulum (ER) is a key organelle regulating intracellular Ca(2+) homeostasis. Oxidants and mitochondria-derived free radicals can target ER-based Ca(2+) regulatory proteins and cause uncontrolled Ca(2+) release that may contribute to protracted ER stress and apoptosis. Several ER stress proteins have been suggested to counteract the deregulation of ER Ca(2+) homeostasis and ER stress. Here we showed that knockdown of Herp, an ubiquitin-like domain containing ER stress protein, renders PC12 and MN9D cells vulnerable to 1-methyl-4-phenylpyridinium-induced cytotoxic cell death by a mechanism involving up-regulation of CHOP expression and ER Ca(2+) depletion. Conversely, Herp overexpression confers protection by blocking 1-methyl-4-phenylpyridinium-induced CHOP upregulation, ER Ca(2+) store depletion, and mitochondrial Ca(2+) accumulation in a manner dependent on a functional ubiquitin-proteasomal protein degradation pathway. Deletion of the ubiquitin-like domain of Herp or treatment with a proteasomal inhibitor abolished the central function of Herp in ER Ca(2+) homeostasis. Thus, elucidating the underlying molecular mechanism(s) whereby Herp counteracts Ca(2+) disturbances will provide insights into the molecular cascade of cell death in dopaminergic neurons and may uncover novel therapeutic strategies to prevent and ameliorate Parkinson disease progression

Regulation of HAX-1 anti-apoptotic protein by Omi/HtrA2 protease during cell death

Omi/HtrA2 is a nuclear-encoded mitochondrial serine protease that has a pro-apoptotic function in mammalian cells. Upon induction of apoptosis, Omi translocates to the cytoplasm and participates in caspase-dependent apoptosis by binding and degrading inhibitor of apoptosis proteins. Omi can also initiate caspase-independent apoptosis in a process that relies entirely on its ability to function as an active protease. To investigate the mechanism of Omi-induced apoptosis, we set out to isolate novel substrates that are cleaved by this protease. We identified HS1-associated protein X-1 (HAX-1), a mitochondrial anti-apoptotic protein, as a specific Omi interactor that is cleaved by Omi both in vitro and in vivo. HAX-1 degradation follows Omi activation in cells treated with various apoptotic stimuli. Using a specific inhibitor of Omi, HAX-1 degradation is prevented and cell death is reduced. Cleavage of HAX-1 was not observed in a cell line derived from motor neuron degeneration 2 mice that carry a mutated form of Omi that affects its proteolytic activity. Degradation of HAX-1 is an early event in the apoptotic process and occurs while Omi is still confined in the mitochondria. Our results suggest that Omi has a unique pro-apoptotic function in mitochondria that involves removal of the HAX-1 antiapoptotic protein. This function is distinct from its ability to activate caspase-dependent apoptosis in the cytoplasm by degrading inhibitor of apoptosis proteins

Mitochondrial uncoupling protein-4 regulates calcium homeostasis and sensitivity to store depletion-induced apoptosis in neural cells

An increase in the cytoplasmic-free Ca2+ concentration mediates cellular responses to environmental signals that influence a range of processes, including gene expression, motility, secretion of hormones and neurotransmitters, changes in energy metabolism, and apoptosis. Mitochondria play important roles in cellular Ca2+ homeostasis and signaling, but the roles of specific mitochondrial proteins in these processes are unknown. Uncoupling proteins (UCPs) are a family of proteins located in the inner mitochondrial membrane that can dissociate oxidative phosphorylation from respiration, thereby promoting heat production and decreasing oxyradical production. Here we show that UCP4, a neuronal UCP, influences store-operated Ca2+ entry, a process in which depletion of endoplasmic reticulum Ca2+ stores triggers Ca2+ influx through plasma membrane store-operated channels. PC12 neural cells expressing human UCP4 exhibit reduced Ca2+ entry in response to thapsigargin-induced endoplasmic reticulum Ca2+ store depletion. The elevations of cytoplasmic and intramitochondrial Ca2+ concentrations and mitochondrial oxidative stress induced by thapsigargin were attenuated in cells expressing UCP4. The stabilization of Ca2+ homeostasis and preservation of mitochondrial function by UCP4 was correlated with reduced mitochondrial reactive oxygen species generation, oxidative stress, and Gadd153 up-regulation and increased resistance of the cells to death. Reduced Ca2+-dependent cytosolic phospholipase A2 activation and oxidative metabolism of arachidonic acid also contributed to the stabilization of mitochondrial function in cells expressing human UCP4. These findings demonstrate that UCP4 can regulate cellular Ca2+ homeostasis, suggesting that UCPs may play roles in modulating Ca2+ signaling in physiological and pathological conditions

Omi/Htra2 Protease Mediates Cisplatin-Induced Cell Death In Renal Cells

Omi/HtrA2 is a mitochondrial proapoptotic serine protease that is able to induce both caspase-dependent and caspase-independent cell death. After apoptotic stimuli, Omi is released to the cytoplasm where it binds and cleaves inhibitor of apoptosis proteins. In this report, we investigated the role of Omi in renal cell death following cisplatin treatment. Using primary mouse proximal tubule cells, as well as established renal cell lines, we show that the level of Omi protein is upregulated after treatment with cisplatin. This upregulation is followed by the release of Omi from mitochondria to the cytoplasm and degradation of XIAP. Reducing the endogenous level of Omi protein using RNA interference renders renal cells resistant to cisplatin-induced cell death. Furthermore, we show that the proteolytic activity of Omi is necessary and essential for cisplatin-induced cell death in this system. When renal cells are treated with Omi\u27s specific inhibitor, ucf-101, they become significantly resistant, to cisplatin-induced cell death. Ucf-101 was also able to minimize cisplatin-induced nephrotoxic injury in animals. Our results demonstrate that Omi is a major mediator of cisplatin-induced cell death in renal cells and suggest a way to limit renal injury by specifically inhibiting its proteolytic activity

Near Best Tree Approximation

Journal PaperTree approximation is a form of nonlinear wavelet approximation that appears naturally in applications such as image compression and entropy encoding. The distinction between tree approximation and the more familiar <i>n</i>-term wavelet approximation is that the wavelets appearing in teh appromant are required to align themselves in a certain connected tree structure. This makes their positions easy to encode. Previous work [CDGO], [CDDD] has established upper bounds for the error of tree approximation for certain (Besov) classes of functions. The present paper, in contrast, studies tree approximation of individual functions with the aim of characterizing those functions with a rpescribed approximation error. This accomplished in the case that the approximation error is measure in <i>L₂</i>, or in the case <i>p</i> not equal to 2, in the Besove spaces, which is close to (but not the same as) <i>L_p</i>. Our characterization of functions with a prescribed approximation order in these cases is given in terms of a certain maximal function applied to the wavelet coefficients.Office of Naval ResearchArmy Research OfficeNational Science Foundatio

Moderate Exercise-Induced Energy Expenditure Does Not Alter Leptin Levels In Sedentary Obese Men

OBJECTIVE: The purpose of the study was to determine whether exercise-induced increases in energy expenditure (EE) alter circulating leptin levels in obese individuals. DESIGN: Participants were randomized to an exercise intervention group (n = 8) or nonexercising control (n = 7). SETTING: All data were collected on an outpatient basis at the exercise physiology laboratory at the University of Central Florida. PATIENTS: Fifteen healthy obese males (24.9 ± 1.4 years old, body mass index 33.4 ± 0.7 kg • m). INTERVENTIONS: Members of the intervention group underwent a single exercise session of moderate intensity (58.4 ± 1.3% of VO2max) for 60 minutes. MAIN OUTCOME MEASUREMENTS: Postexercise, 24 hour postexercise, and 48 hour postexercise levels of leptin, insulin, and ghrelin. RESULTS: The exercise session elicited an EE of 567 ± 25 Kcal. No significant main effect or time-by-group interactions for leptin or ghrelin were observed immediately after the exercise bout or in the days following the intervention. CONCLUSIONS: These preliminary data suggest that a bout of acute exercise of moderate intensity and duration does not affect leptin concentration. It is possible that a higher level of EE is required to elicit substantial changes. © 2007 Lippincott Williams & Wilkins, Inc

Hyperleptinemia Is Associated With Crp But Not Apolipoprotein E And Is Reduced By Exercise Training

The purpose of this study was to examine whether leptin levels affect the response of leptin to exercise training (ET) and whether this is also affected by C-reactive protein (CRP) or the three common Apolipoprotein E genotypes (APOE). Ninety-seven (male = 45, female = 52) sedentary individuals underwent 6 months of supervised ET. Blood was sampled before the initiation of ET, and again 24 and 72 hr after completion of the final training session. ET resulted in a small reduction in body mass (80.47 ± 18.03 vs 79.42 ± 17.34 kg, p \u3c .01). Leptin was reduced 24 hr after the final exercise session (p \u3c .01), but returned to normal after 72 hr (p \u3e .05)-Pre: 13.51 ± 12.27, 24hr: 12.14 ± 12.34, 72hr: 12.98 ± 11.40 ng/ml. The most hyperleptinemic individuals had a greater initial response, which was sustained through to 72 hr after the final session in the pooled study population (p \u3c .01), and in both males (p \u3c .05) and females (p \u3c .05) separately. CRP was related to leptin independently of body weight and positively related to the reductions in leptin. APOE genotype was not related to leptin levels and did not affect the response to ET. Leptin levels may only be reduced by ET in those with hyperleptinemia. In addition, both the initial extent of hyperleptinemia and the subsequent reduction in leptin may be related to low grade chronic systemic inflammation