Implication of caspases and subcellular compartments in tert-butylhydroperoxide induced apoptosis

cGMP Signaling in Photoreceptor Degeneration

Photoreceptors in the retina are highly specialized neurons with photosensitive molecules in the outer segment that transform light into chemical and electrical signals, and these signals are ultimately relayed to the visual cortex in the brain to form vision. Photoreceptors are composed of rods and cones. Rods are responsible for dim light vision, whereas cones are responsible for bright light, color vision, and visual acuity. Photoreceptors undergo progressive degeneration over time in many hereditary and age-related retinal diseases. Despite the remarkable heterogeneity of disease-causing genes, environmental factors, and pathogenesis, the progressive death of rod and cone photoreceptors ultimately leads to loss of vision/blindness. There are currently no treatments available for retinal degeneration. Cyclic guanosine 3', 5'-monophosphate (cGMP) plays a pivotal role in phototransduction. cGMP governs the cyclic nucleotide-gated (CNG) channels on the plasma membrane of the photoreceptor outer segments, thereby regulating membrane potential and signal transmission. By gating the CNG channels, cGMP regulates cellular Ca2+ homeostasis and signal transduction. As a second messenger, cGMP activates the cGMP-dependent protein kinase G (PKG), which regulates numerous targets/cellular events. The dysregulation of cGMP signaling is observed in varieties of photoreceptor/retinal degenerative diseases. Abnormally elevated cGMP signaling interferes with various cellular events, which ultimately leads to photoreceptor degeneration. In line with this, strategies to reduce cellular cGMP signaling result in photoreceptor protection in mouse models of retinal degeneration. The potential mechanisms underlying cGMP signaling-induced photoreceptor degeneration involve the activation of PKG and impaired Ca2+ homeostasis/Ca2+ overload, resulting from overactivation of the CNG channels, as well as the subsequent activation of the downstream cellular stress/death pathways. Thus, targeting the cellular cGMP/PKG signaling and the Ca2+-regulating pathways represents a significant strategy for photoreceptor protection in retinal degenerative diseases.

Sex Steroid Regulation of Oxidative Stress in Bone Cells: An In Vitro Study

Environmental stimuli, including sex hormones and oxidative stress (OS), affect bone balance, modifying the epigenetic profiles of key osteogenic genes. Nonetheless, the interplay between sex steroids, epigenome and OS has yet be fully elucidated. This paper aims to study in vitro the role of sex steroids in OS-induced alteration in bone cells’ homeostasis, and to assess the possible contribution of epigenetic modifications. Toward this purpose, osteoblast (MC3T3-E1) and osteocyte (MLOY-4) cell lines were exposed to two different sources of free oxygen radicals, i.e., tert-butyl hydroperoxide and dexamethasone, and the protective effect of pre-treatment with androgens and estrogens was evaluated. In particular, we analyzed parameters that reflect bone cell homeostasis such as cell viability, cell migration, transcriptomic profile, transcriptional activity, and epigenetic signature. Our findings indicate that estrogens and androgens counteract OS effects. Using partially overlapping strategies, they reduce OS outcomes regarding cell viability, cell migration, the transcriptomic profile of gene families involved in bone remodeling, and epigenetic profile, i.e., H3K4me3 level. Additionally, we demonstrated that the protective effect of steroids against OS on bone homeostasis is partially mediated by the Akt pathway. Overall, these results suggest that the hormonal milieu may influence the mechanisms of age-related bone disease.

Control of stress-induced apoptosis by freezing tolerance-associated wheat proteins during cryopreservation of rat hepatocytes

Cryopreservation is used for long-term storage of cells and tissues. Cryoprotectants such as dimethyl disulfoxide (DMSO) are used to protect cells against freeze-thaw damage. Despite the use of cryoprotectants, hepatocytes are sensitive to stresses imposed by freeze and thaw processes, which cause physical damage, loss of functionality, or cell death. As an alternative, we have developed new technology using several recombinant wheat proteins as cryoprotectants: TaENO (enolase), TaBAS1 (2-Cys peroxiredoxin), and a combination of WCS120 (dehydrin) with TaIRI-2 (inhibitor of ice recrystallization). This study aims to understand the mechanisms by which these plant proteins protect rat hepatocytes against cell death incurred during cryopreservation. Our analysis revealed that for cells cryopreserved with DMSO, cell death occurred by apoptosis and necrosis. Apoptosis was detected by activation of effector caspases-3 and -7, PARP cleavage, and nuclear chromatin condensation. These apoptotic events were inhibited when hepatocytes were cryopreserved with the different plant proteins. Cryopreservation with DMSO activated apoptosis through the mitochondrial pathway: the Bax/Bcl-2 protein ratio increased, mitochondrial membrane potential decreased, and initiator caspase-9 was activated. Furthermore, the endoplasmic reticulum pathway of apoptosis was activated: levels of the chaperone Bip/GRP78 decreased, pro-apoptotic transcription factor CHOP was induced, and initiator caspase-12 was activated. Activation of the mitochondrial and endoplasmic reticulum pathways of apoptosis was attenuated when hepatocytes were cryopreserved with the different recombinant proteins. This study improves understanding of mechanisms of cryoprotection provided by these plant proteins during freezing stress. These proteins are natural products and show promising potential by decreasing cell death during cryopreservation of hepatocytes.

Melatonin protects vertebral endplate chondrocytes against apoptosis and calcification via the Sirt1-autophagy pathway

Melatonin is reportedly associated with intervertebral disc degeneration (IDD). Endplate cartilage is vitally important to intervertebral discs in physiological and pathological conditions. However, the effects and mechanism of melatonin on endplate chondrocytes (EPCs) are still unclear. Herein, we studied the effects of melatonin on EPC apoptosis and calcification and elucidated the underlying mechanism. Our study revealed that melatonin treatment decreases the incidence of apoptosis and inhibits EPC calcification in a dose-dependent manner. We also found that melatonin upregulates Sirt1 expression and activity and promotes autophagy in EPCs. Autophagy inhibition by 3-methyladenine reversed the protective effect of melatonin on apoptosis and calcification, while the Sirt1 inhibitor EX-527 suppressed melatonin-induced autophagy and the protective effects of melatonin against apoptosis and calcification, indicating that the beneficial effects of melatonin in EPCs are mediated through the Sirt1-autophagy pathway. Furthermore, melatonin may ameliorate IDD in vivo in rats. Collectively, this study revealed that melatonin reduces EPC apoptosis and calcification and that the underlying mechanism may be related to Sirt1-autophagy pathway regulation, which may help us better understand the association between melatonin and IDD.

Therapeutic Potential of Naringin for Intervertebral Disc Degeneration: Involvement of Autophagy Against Oxidative Stress-Induced Apoptosis in Nucleus Pulposus Cells

Intervertebral disc degeneration (IDD) is a major cause of lower back pain, but few efficacious medicines have been developed for IDD. Increased nucleus pulposus cells apoptosis is a dominant pathogenesis of IDD and is considered a therapeutic target. Previously, our group proved that autophagy may protect nucleus pulposus cells against apoptosis. As one of the major bioflavonoids of citrus, naringin activates autophagy. Therefore, we hypothesize that naringin may have therapeutic potential for IDD by activating autophagy in nucleus pulposus cells. In this study, we evaluated the effects of naringin on TBHP-induced oxidative stress in nucleus pulposus cells in vitro as well as in puncture-induced rat IDD model in vivo. Our results showed that naringin could reduce the incidence of oxidative stress-induced apoptosis in nucleus pulposus cells and promoted the expression of autophagy markers LC3-II/I and beclin-1. Meanwhile, inhibition of autophagy by 3-MA may partially reverse the anti-apoptotic effect of naringin, indicating that autophagy was involved in the protective effect of naringin in nucleus pulposus cells. Further study showed that autophagy regulation of naringin may be related to AMPK signaling. Also, we found that naringin treatment can regulate the expression of collagen II, aggrecan and Mmp13 to sustain the extracellular matrix. Furthermore, our in vivo study showed that naringin can ameliorate IDD in puncture-induced rat model. In conclusion, our study suggests that naringin can protect nucleus pulposus cells against apoptosis and ameliorate IDD in vivo, the mechanism may relate to its autophagy regulation.

Treatment with Caffeic Acid and Resveratrol Alleviates Oxidative Stress Induced Neurotoxicity in Cell and Drosophila Models of Spinocerebellar Ataxia Type3

Spinocerebellar ataxia type 3 (SCA3) is caused by the expansion of a polyglutamine (polyQ) repeat in the protein ataxin-3 which is involved in susceptibility to mild oxidative stress induced neuronal death. Here we show that caffeic acid (CA) and resveratrol (Res) decreased reactive oxygen species (ROS), mutant ataxin-3 and apoptosis and increased autophagy in the pro-oxidant tert-butyl hydroperoxide (tBH)-treated SK-N-SH-MJD78 cells containing mutant ataxin-3. Furthermore, CA and Res improved survival and locomotor activity and decreased mutant ataxin-3 and ROS levels in tBH-treated SCA3 Drosophila. CA and Res also altered p53 and nuclear factor-κB (NF-κB) activation and expression in tBH-treated cell and fly models of SCA3, respectively. Blockade of NF-κB activation annulled the protective effects of CA and Res on apoptosis, ROS, and p53 activation in tBH-treated SK-N-SH-MJD78 cells, which suggests the importance of restoring NF-κB activity by CA and Res. Our findings suggest that CA and Res may be useful in the management of oxidative stress induced neuronal apoptosis in SCA3.

Activation of apoptosis signalling pathways by reactive oxygen species

Reactive oxygen species (ROS) are short-lived and highly reactive molecules. The generation of ROS in cells exists in equilibrium with a variety of antioxidant defences. At low to modest doses, ROS are considered to be essential for regulation of normal physiological functions involved in development such as cell cycle progression and proliferation, differentiation, migration and cell death. ROS also play an important role in the immune system, maintenance of the redox balance and have been implicated in activation of various cellular signalling pathways. Excess cellular levels of ROS cause damage to proteins, nucleic acids, lipids, membranes and organelles, which can lead to activation of cell death processes such as apoptosis. Apoptosis is a highly regulated process that is essential for the development and survival of multicellular organisms. These organisms often need to discard cells that are superfluous or potentially harmful, having accumulated mutations or become infected by pathogens. Apoptosis features a characteristic set of morphological and biochemical features whereby cells undergo a cascade of self-destruction. Thus, proper regulation of apoptosis is essential for maintaining normal cellular homeostasis. ROS play a central role in cell signalling as well as in regulation of the main pathways of apoptosis mediated by mitochondria, death receptors and the endoplasmic reticulum (ER). This review focuses on current understanding of the role of ROS in each of these three main pathways of apoptosis. The role of ROS in the complex interplay and crosstalk between these different signalling pathways remains to be further unravelled during the coming years.

Acylated and unacylated ghrelin protect MC3T3-E1 cells against tert-butyl hydroperoxide-induced oxidative injury: pharmacological characterization of ghrelin receptor and possible epigenetic involvement

Increasing evidence suggests a role for oxidative stress in age-related decrease in osteoblast number and function leading to the development of osteoporosis. This study was undertaken to investigate whether ghrelin, previously reported to stimulate osteoblast proliferation, counteracts tert-butyl hydroperoxide (t-BHP)-induced oxidative damage in MC3T3-E1 osteoblastic cells as well as to characterize the ghrelin receptor (GHS-R) involved in such activity. Pretreatment with ghrelin (10(-7)-10(-11)M) significantly increased viability and reduced apoptosis of MC3T3-E1 cells cultured with t-BHP (250 μM) for three hours at the low concentration of 10(-9)M as shown by MTT assay and Hoechst-33258 staining. Furthermore, ghrelin prevented t-BHP-induced osteoblastic dysfunction and changes in the cytoskeleton organization evidenced by the staining of the actin fibers with Phalloidin-FITC by reducing reactive oxygen species generation. The GHS-R type 1a agonist, EP1572 (10(-7)-10(-11)M), had no effect against t-BHP-induced cytotoxicity and pretreatment with the selective GHS-R1a antagonist, D-Lys(3)-GHRP-6 (10(-7)M), failed to remove ghrelin (10(-9) M)-protective effects against oxidative injury, indicating that GHS-R1a is not involved in such ghrelin activity. Accordingly, unacylated ghrelin (DAG), not binding GHS-R1a, displays the same protective actions of ghrelin against t-BHP-induced cytotoxicity. Preliminary observations indicate that ghrelin increased the trimethylation of lys4 on histones H3, a known epigenetic mark activator, which may regulate the expression of some genes limiting oxidative damage. In conclusion, our data demonstrate that ghrelin and DAG promote survival of MC3T3-E1 cell exposed to t-BHP-induced oxidative damage. Such effect is independent of GHS-R1a and is likely mediated by a common ghrelin/DAG binding site.

Biocompatibility evaluation of porous ceria foams for orthopedic tissue engineering

Ceria ceramics have the unique ability to protect cells from free radical-induced damage, making them materials of interest for biomedical applications. To expand upon the understanding of the potential of ceria as a biomaterial, porous ceria, fabricated via direct foaming, was investigated to assess its biocompatibility and its ability to scavenge free radicals. A mouse osteoblast (7F2) cell line was cultured with the ceria foams to determine the extent of the foams' toxicity. Toxicity assessments indicate that mouse osteoblasts cultured directly on the ceria scaffold for 72 h did not show a significant (p > 0.05) increase in toxicity, but rather show comparable toxicity to cells cultured on porous 45S5 Bioglass. The in vitro inflammatory response elicited from porous ceria foams was measured as a function of tumor necrosis factor alpha (TNF-α) secreted from a human monocytic leukemia cell line. Results indicate that the ceria foams do not cause a significant inflammatory response, eliciting a response of 27.1 ± 7.1 pg mL(-1) of TNF-α compared to 36.3 ± 5.8 pg mL(-1) from cells on Bioglass, and 20.1 ± 2.9 pg mL(-1) from untreated cells. Finally, we report cellular toxicity in response to free radicals from tert-butyl hydroperoxide with and without foamed ceria. Our preliminary results show that the foamed ceria is able to decrease the toxic effect of induced oxidative stress. Collectively, this study demonstrates that foamed ceria scaffolds do not activate an inflammatory response, and show potential free radical scavenging ability, thus they have promise as an orthopedic biomaterial.

Exploring the effects of tert-butylhydroperoxide induced liver injury using proteomic approach

Tert-butyl hydroperoxide (t-BHP), an organic lipid hydroperoxide analog, has been demonstrated to exert pro-oxidant effects to evaluate mechanisms involving oxidative stress in hepatocyte cells and rat liver. Herein, we present an investigation of the event of molecular mechanism of t-BHP related acute liver injury. A proteomic approach was used to identify proteins which are differentially expressed in liver cells following t-BHP treatment and the mechanism of its action in apoptotic and endoplasmic reticulum stress pathways. Our results demonstrate that the t-BHP treatment of liver cells increased cell cytoxicity and apoptosis. t-BHP dose-dependent induction of cell apoptosis and stained liver sections relieved the acute rat liver injury were accompanied by sustained phosphorylation of JNK1/2 and p65. In addition, there were 13 differentially displayed proteins between the t-BHP-induced and untreated were assayed and validated in vivo. Furthermore, we demonstrated that t-BHP induced human Chang liver cell viability and apoptosis properties by up-regulating the levels of ETFA (electron transfer flavoprotein subunit alpha). This study demonstrated that there was an increase in the cellular levels of ETFA in the t-BHP induction in viability and apoptosis via the activation of JNK1/2 and NFκB signaling modules. NAC administration and shRNA ETFA conferred resistance to t-BHP-increased ETFA and CHOP expression via IRE1-alpha/TRAF2 complex formation, activation of JNK1/2 and p50. We concluded that the mechanism of t-BHP-induced an apoptosis cascade and endoplasmic reticulum stress in hepatocyte cells by up-regulation of ETFA, providing a new mechanism for liver injury.

Calpain mediates caspase-dependent apoptosis initiated by hydrogen peroxide in pancreatic acinar AR42J cells

Several studies have shown that oxidative stress induces apoptosis in many cellular systems including pancreatic acinar cells. However, the exact molecular mechanisms leading to apoptosis remain partially understood. This study aimed to investigate the role of the cytosolic cysteine protease calpain in H2O2-induced apoptosis in pancreatic AR42J cells. Apoptosis was evaluated using flow cytometric analysis of sub-G1 DNA populations, electron-microscopic analysis, caspase-3-specific αII-spectrin breakdown, and measuring the proteolytic activities of the initiator caspase-12 and caspase-8, and the executioner caspase-3. H2O2 induced an increase in the calpain proteolytic activity immediately after starting the experiments that tended to return to a nearly normal level after 8 h and could be attributed to m-calpain. Whereas no caspase-12, caspase-8 and caspase-3 activations could be detected within the first 0.5 h, significantly increased proteolytic activities were observed after 8 h compared with the control. At the same time, the cells showed first ultrastructural hallmarks of apoptosis and a decreased viability. In addition, αII-spectrin fragmentation was identified using immunoblotting that could be attributed to both calpain and caspase-3. Calpain inhibition reduced the activities of caspase-12, caspase-8, and caspase-3 leading to a decrease in the number of apoptotic cells. Immunoblotting analyses of caspase-12 and caspase-8 indicate that calpain may be involved in the activation process of both proteases. The results suggest that H2O2-induced apoptosis of AR42J cells requires activation of m-calpain initiating the endoplasmic reticulum stress-induced caspase-12 pathway and a caspase-8-dependent pathway. The findings also suggest that calpain may be involved in the execution phase of apoptosis.

Altered HepG2 cell models using etomoxir versus tert-butylhydroperoxide

Energetic failure which occurs in both ischemia/reperfusion and acute drug-induced hepatotoxicity is frequently associated with oxidative stress. This study displays the setting of a new cell culture model for hepatic energetic failure, i.e., HepG2 models modified by etomoxir [ETO] addition [0.1 mM to 1 mM] and compares the cell impact versus tert-butylhydroperoxide [TBOOH; 0.2 mM], an oxidative stress inducer. As it was observed with Minimum Essential Medium (MEM) without any interfering agent, decreasing temperature drastically lowered adenosine triphosphate (ATP) levels, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) viability test, and protein content, compared to 37 °C (p=0.02, p<0.001 and p<0.001, respectively), but to a larger extent in the presence of ETO or TBOOH. The alteration was generally highly dependent on the ETO concentration, time, and temperature. At 37 °C 24 h after (T24h), regarding ETO concentration, R² correlation ratio was 0.65 (p<0.001), 0.70 (p<0.001), and 0.89 (p<0.001) for ATP levels, protein content, and viability, respectively. The lowest ETO concentration producing a significant effect was 0.25 mM. Concerning time dependency (i.e., T24h versus after 5 h (T5h)), at 37 °C with ETO, ATP level continued to significantly decrease between T5h and T24h. In a similar way, at 37 °C, the MTT viability test decrease was accelerated only between T5h and T24h for ETO concentrations higher than 0.5 mM (p=0.016 and p=0.0001 for 0.75 and 1 mM, respectively). On the contrary, with TBOOH, comparing T24h versus T5h, cellular indicators were improved but generally remained lower than MEM without any interfering agent at T24h, suggesting that TBOOH action was time limited probably in relation with its oxidation in cell medium. This study confirms the interest of altered ETO cell model to screen agents (or formulation) prone to prevent or treat energetic depletion in relation with oxidative stress.

Opposing responses of apoptosis and autophagy to moderate compression in skeletal muscle

AIM

The molecular mechanism that contributes to the pathogenesis of deep pressure ulcer remains to be elucidated. This study tested the hypotheses that: (1) apoptosis and autophagy are activated in compression-induced muscle pathology and (2) apoptotic and autophagic changes precede pathohistological changes in skeletal muscle in response to prolonged moderate compression.

METHODS

  Adult Sprague-Dawley rats were subjected to an experimental model of pressure-induced deep tissue injury. Static pressure of 100 mmHg was applied to an area of 1.5 cm(2) over the mid-tibialis region of right limb of rats for one single session of 6-h compression (1D) or two sessions of 6-h compression over two consecutive days with rats sacrificed one day (2D) or immediately after (2D-IM) the compression. The left uncompressed limb served as the intra-animal control. Muscle tissues underneath compression region were collected for analysis.

RESULTS

Our histological analysis indicated that pathohistological characteristics including rounding contour of myofibres and massive nuclei accumulation were apparently demonstrated in muscles of 2D and 2D-IM. In contrast, these pathohistological changes were generally not found in muscle following 1D. Apoptotic DNA fragmentation, terminal dUTP nick-end labelling index and caspase-3 protease activity were significantly elevated in compressed muscles of all groups. Caspase-9 enzymatic activity was found to be significantly increased in compressed muscles of 2D and 2D-IM whereas increase in caspase-8 activity was exclusively found in compressed muscle of 1D. According to our immunoblot analysis, FoxO3 was significantly reduced in compressed muscles of all groups whereas Beclin-1 was decreased only in 2D. LC3-I was significantly reduced in compressed muscles of all groups while LC3-II was decreased in 2D and 1D. No significant differences were found in the protein abundance of Akt and phospho-Akt in muscles among all groups.

CONCLUSION

These data demonstrate the opposing responses of apoptosis and autophagy to moderate compression in muscle. Moreover, our findings suggest that cellular changes in apoptosis and autophagy have already taken place in the very early stage in which apparent histopathology has yet to develop in the process of compression-induced muscle pathology.

Contribution of endoplasmic reticulum stress to hepatocyte apoptosis in alcoholic liver disease

Endoplasmic reticulum stress represents a subcellular pathological state caused by a disorder of physiological function of the endoplasmic reticulum. Endoplasmic reticulum stress is not only a self-protective mechanism of the body, but also an important factor to promote cell apoptosis. In alcoholic liver disease, endoplasmic reticulum stress induced by several factors such as hyperhomocysteinemia can lead to hepatocyte apoptosis. In this article, we will review the contribution of endoplasmic reticulum stress to hepatocyte apoptosis in alcoholic liver disease.

Antitumor effects of dehydroxymethylepoxyquinomicin, a novel nuclear factor-kappaB inhibitor, in human liver cancer cells are mediated through a reactive oxygen species-dependent mechanism

Activation of the nuclear transcription factor-kappaB (NF-kappaB) has been implicated in liver tumorigenesis. We evaluated the effects of a novel NF-kappaB inhibitor, dehydroxymethylepoxyquinomicin (DHMEQ), in two human liver cancer cell lines HA22T/VGH and HuH-6. DHMEQ treatment dose dependently decreased the DNA-binding capacity of the NF-kappaB p65 subunit, inhibited cell growth and proliferation, and increased apoptosis as shown by caspase activation, release of cytochrome c, poly(ADP-ribose) polymerase cleavage, and down-regulation of survivin. DHMEQ also induced a dose-dependent activation of mitogen-activated protein kinase kinase/extracellular signal-regulated kinase signaling, and inhibition of this pathway significantly reduced cell growth. It is noteworthy that we observed that DHMEQ stimulated reactive oxygen species (ROS) production in a dose-dependent manner and that pretreatment of the cells with the antioxidant N-acetyl-L-cysteine (NAC) significantly reduced DHMEQ-induced ROS generation. Accordingly, NAC completely reversed the DHMEQ-induced growth inhibition, caspase activation, and cell death. DHMEQ-treated cells exhibited DNA damage, as evaluated by accumulation in nuclear foci of phospho-H2AX, which was completely reversed by NAC. Moreover, DHMEQ induced the expression of genes involved in the endoplasmic reticulum stress response (GRP78, CHOP, TRB3) and promoted the splicing of XBP1 mRNA in a dose-dependent fashion in both cell lines, which was reversed in the presence of NAC. Knockdown of TRB3 mRNA expression by small interference RNA significantly decreased DHMEQ-induced cell growth inhibition. These data suggest that DHMEQ antitumor effects are primarily mediated through ROS generation. Thereby, considering that cancer cells are under increased ER stress and oxidative stress conditions, DHMEQ may greatly improve various anticancer strategies.

Oxidants and Ca+2 induce PGC-1alpha degradation through calpain

Peroxisome proliferator activator receptor gamma coactivator 1alpha (PGC-1alpha) is a transcriptional coactivator known to mediate mitochondrial biogenesis. Whereas PGC-1alpha transcription is regulated by a variety of signaling cascades, the mechanisms of PGC-1alpha degradation have received less investigation. Thus, we investigated the mechanisms responsible for PGC-1alpha degradation in renal proximal tubular cells (RPTC). Amino acid sequence analysis of the PGC-1alpha protein revealed three PEST sequence-rich regions, predictive of proteolysis by calpains and/or the proteasome. Under basal conditions, treatment with the protein synthesis inhibitor cycloheximide resulted in rapid degradation of PGC-1alpha (t(1/2)=38 min), which was blocked by the proteasome inhibitor epoxomicin, but not the calpain inhibitor calpeptin. Oxidant exposure resulted in the degradation of both endogenous and adenovirally over-expressed PGC-1alpha, which was inhibited by calpeptin but not epoxomicin. Thapsigargin-induced release of ER Ca(2+) also stimulated calpain-dependent, epoxomicin-independent degradation of PGC-1alpha. Finally, Ca(2+) addition to lysates of RPTC over-expressing PGC-1alpha resulted in calpeptin-sensitive, epoxomicin-insensitive degradation of PGC-1alpha. In summary, we suggest two distinct mechanisms regulate PGC-1alpha: basal PGC-1alpha turnover by proteasome degradation and oxidant- and Ca(2+)-mediated PGC-1alpha degradation through calpain.