Caspase-2, a novel lipid sensor under the control of sterol regulatory element binding protein 2

Maternal supplementation with n-3 fatty acids affects placental lipid metabolism, inflammation, oxidative stress, the endocannabinoid system, and the neonate cytokine concentrations in dairy cows

BACKGROUND

The placenta plays a crucial role in supporting and influencing fetal development. We compared the effects of prepartum supplementation with omega-3 (n-3) fatty acid (FA) sources, flaxseed oil (FLX) and fish oil (FO), on the expression of genes and proteins related to lipid metabolism, inflammation, oxidative stress, and the endocannabinoid system (ECS) in the expelled placenta, as well as on FA profile and inflammatory response of neonates. Late-pregnant Holstein dairy cows were supplemented with saturated fat (CTL), FLX, or FO. Placental cotyledons (n = 5) were collected immediately after expulsion, and extracted RNA and proteins were analyzed by RT-PCR and proteomic analysis. Neonatal blood was assessed for FA composition and concentrations of inflammatory markers.

RESULTS

FO increased the gene expression of fatty acid binding protein 4 (FABP4), interleukin 10 (IL-10), catalase (CAT), cannabinoid receptor 1 (CNR1), and cannabinoid receptor 2 (CNR2) compared with CTL placenta. Gene expression of ECS-enzyme FA-amide hydrolase (FAAH) was lower in FLX and FO than in CTL. Proteomic analysis identified 3,974 proteins; of these, 51-59 were differentially abundant between treatments (P ≤ 0.05, |fold change| ≥ 1.5). Top canonical pathways enriched in FLX vs. CTL and in FO vs. CTL were triglyceride metabolism and inflammatory processes. Both n-3 FA increased the placental abundance of FA binding proteins (FABPs) 3 and 7. The abundance of CNR1 cannabinoid-receptor-interacting-protein-1 (CNRIP1) was reduced in FO vs. FLX. In silico modeling affirmed that bovine FABPs bind to endocannabinoids. The FLX increased the abundance of inflammatory CD44-antigen and secreted-phosphoprotein-1, whereas prostaglandin-endoperoxide synthase 2 was decreased in FO vs. CTL placenta. Maternal FO enriched neonatal plasma with n-3 FAs, and both FLX and FO reduced interleukin-6 concentrations compared with CTL.

CONCLUSION

Maternal n-3 FA from FLX and FO differentially affected the bovine placenta; both enhanced lipid metabolism and modulated oxidative stress, however, FO increased some transcriptional ECS components, possibly related to the increased FABPs. Maternal FO induced a unique balance of pro- and anti-inflammatory components in the placenta. Taken together, different sources of n-3 FA during late pregnancy enhanced placental immune and metabolic processes, which may affect the neonatal immune system.

PIDDosome-SCAP crosstalk controls high-fructose-diet-dependent transition from simple steatosis to steatohepatitis

Sterol deficiency triggers SCAP-mediated SREBP activation, whereas hypernutrition together with ER stress activates SREBP1/2 via caspase-2. Whether these pathways interact and how they are selectively activated by different dietary cues are unknown. Here, we reveal regulatory crosstalk between the two pathways that controls the transition from hepatosteatosis to steatohepatitis. Hepatic ER stress elicited by NASH-inducing diets activates IRE1 and induces expression of the PIDDosome subunits caspase-2, RAIDD, and PIDD1, along with INSIG2, an inhibitor of SCAP-dependent SREBP activation. PIDDosome assembly activates caspase-2 and sustains IRE1 activation. PIDDosome ablation or IRE1 inhibition blunt steatohepatitis and diminish INSIG2 expression. Conversely, while inhibiting simple steatosis, SCAP ablation amplifies IRE1 and PIDDosome activation and liver damage in NASH-diet-fed animals, effects linked to ER disruption and preventable by IRE1 inhibition. Thus, the PIDDosome and SCAP pathways antagonistically modulate nutrient-induced hepatic ER stress to control non-linear transition from simple steatosis to hepatitis, a key step in NASH pathogenesis.

A long way to go: caspase inhibitors in clinical use

Caspases are an evolutionary conserved family of cysteine-dependent proteases that are involved in many vital cellular processes including apoptosis, proliferation, differentiation and inflammatory response. Dysregulation of caspase-mediated apoptosis and inflammation has been linked to the pathogenesis of various diseases such as inflammatory diseases, neurological disorders, metabolic diseases, and cancer. Multiple caspase inhibitors have been designed and synthesized as a potential therapeutic tool for the treatment of cell death-related pathologies. However, only a few have progressed to clinical trials because of the consistent challenges faced amongst the different types of caspase inhibitors used for the treatment of the various pathologies, namely an inadequate efficacy, poor target specificity, or adverse side effects. Importantly, a large proportion of this failure lies in the lack of understanding various caspase functions. To overcome the current challenges, further studies on understanding caspase function in a disease model is a fundamental requirement to effectively develop their inhibitors as a treatment for the different pathologies. Therefore, the present review focuses on the descriptive properties and characteristics of caspase inhibitors known to date, and their therapeutic application in animal and clinical studies. In addition, a brief discussion on the achievements, and current challenges faced, are presented in support to providing more perspectives for further development of successful therapeutic caspase inhibitors for various diseases.

Upregulation of miR-96-5p by bone marrow mesenchymal stem cells and their exosomes alleviate non-alcoholic steatohepatitis: Emphasis on caspase-2 signaling inhibition

Non-alcoholic steatohepatitis (NASH) has evolved as the most common and devastating chronic liver disease. This study aimed to explore the underlined mechanism for the therapeutic potentials of bone marrow mesenchymal stem cells (BM-MSCs) and their derived exosomes (BM-MSCs-Exo) in an experimental model of high fat diet (HFD) induced NASH. Rats were fed with HFD for 12 weeks. At the seventh week, BM-MSCs were given at a dose of 1x10⁶ cell i.v., per rat. A total of three doses of BM-MSCs were given per each rat in six weeks. BM-MSCs-Exo were given at a dose of 15, 30 and 120 µg/kg i.v., twice per week for six weeks. Perfect homing to the liver was detected. Beneficial effects were reported to BM-MSCs or BM-MSCs-Exo cotreatment; where the highest anti-steatotic effects were attributed to BM-MSCs-Exo (120 µg/kg) showing significant downregulation of fatty acid synthesis (SREB1, 2, ACC), downregulation in lipid uptake (CD36); accompanied by significant upregulation in fatty acid oxidation (PPARα, CPT1). These events were associated with abrogation of hepatic steatosis and ballooning in HFD-induced NASH. BM-MSCs or BM-MSCs-Exo cotreatment exerted significant anti-apoptotic effects mediated by significant decrease in Bax/Bcl2 ratio. Besides, significant increase in mitochondrial mitophagy genes (Parkin, PINK1, ULK1, BNIP3L, ATG5, ATG7, ATG12) were detected in BM-MSCs or BM-MSCs-Exo cotreated groups. These findings are thought to be modulated through upregulation of miRNA-96-5p which leads to downregulation of its downstream target caspase-2. Being a critical player in NASH development, caspase-2 targeting by miRNA-96-5p could be a promising therapeutic modality to treat NASH.

SREBF2 triggers endoplasmic reticulum stress and Bax dysregulation to promote lipopolysaccharide-induced endothelial cell injury

An increased lipopolysaccharide (LPS) level in patients with cirrhosis induced the dysregulation of sterol regulatory element-binding transcription factor 2 (SREBF2), which participated in the modulation of tumor inflammatory microenvironment. However, the role of SREBF2 in the LPS-induced injury of portal vein endothelium was scarcely reported. This study aimed to investigate the effects of SREBF2 on the LPS-induced injury to endothelial cells (ECs) in vitro and in vivo and explore the underlying mechanism. In this study, we found that LPS increased SREBF2 expression through activating the TLR4/JNK/c-Jun pathway and suppressed UBE2I-mediated SREBF2 sumoylation to enhance its transcriptional activity. The dysregulation of SREBF2 induced ER stress by increasing the intracellular cholesterol level and facilitated Bax expression to cause additional damage to LPS-induced ECs. As a potential intervention, miR590-3p negatively regulated SREBF2 expression and upregulated UBE2I expression by targeting TLR4, thus alleviating LPS-induced injury. These results suggest that LPS-induced SREBF2 triggered ER stress and promoted Bax expression to injure ECs, which was reversed by miR590-3p. The mechanisms of SREBF2 mediated LPS-induced endothelial injury of portal vein, which might be the therapeutic target for PVT development in cirrhosis patients. 1. LPS promoted SREBF2 expression by activating the TLR4/JNK/c-Jun pathway and suppressed UBE2I-mediated SREBF2 sumoylation to upregulate SREBF2 transcriptional activity 2. SREBF2-mediated ER stress and Bax expression involved in LPS-induced EC injury 3. miR590-3p decreased SREBF2 expression by targeting TLR4 and mitigated LPS-induced EC injury.

Caspase-2 Substrates: To Apoptosis, Cell Cycle Control, and Beyond

Caspase-2 belongs to the caspase family of proteins responsible for essential cellular functions including apoptosis and inflammation. Uniquely, caspase-2 has been identified as a tumor suppressor, but how it regulates this function is still unknown. For many years, caspase-2 has been considered an “orphan” caspase because, although it is able to induce apoptosis, there is an abundance of conflicting evidence that questions its necessity for apoptosis. Recent evidence supports that caspase-2 has non-apoptotic functions in the cell cycle and protection from genomic instability. It is unclear how caspase-2 regulates these opposing functions, which has made the mechanism of tumor suppression by caspase-2 difficult to determine. As a protease, caspase-2 likely exerts its functions by proteolytic cleavage of cellular substrates. This review highlights the known substrates of caspase-2 with a special focus on their functional relevance to caspase-2’s role as a tumor suppressor.

Uncovering the PIDDosome and caspase-2 as regulators of organogenesis and cellular differentiation

The PIDDosome is a multiprotein complex that drives activation of caspase-2, an endopeptidase originally implicated in apoptosis. Yet, unlike other caspases involved in cell death and inflammation, caspase-2 seems to exert additional versatile functions unrelated to cell death. These emerging roles range from control of transcription factor activity to ploidy surveillance. Thus, caspase-2 and the PIDDosome act as a critical regulatory unit controlling cellular differentiation processes during organogenesis and regeneration. These newly established functions of the PIDDosome and its downstream effector render its components attractive targets for drug-development aiming to prevent fatty liver diseases, neurodegenerative disorders or osteoporosis.

Prognostic significance of mRNA expression of CASPs in gastric cancer

Current studies suggest that the cysteinyl aspartate specific proteinase (caspase/CASP) family may be closely associated with apoptosis. Scientists have suggested that caspases may be a key to the development of more effective anti-cancer therapies. However, the prognostic value of CASP expression in gastric cancer (GC) remains unclear. Using a Kaplan-Meier plotter online database, the predictive prognostic significance of the expression of 12 CASPs genes (CASP1, CASP2, CASP3, CASP4, CASP5, CASP6, CASP7, CASP8, CASP9, CASP10, CASP12 and CASP14) to overall survival (OS) in different clinicopathological features, including Lauren classification, pathological stages, therapies employed and differentiation in gastric cancer patients was explored. The present study revealed that higher CASP1, 2, 3, 4, 5, 6, 7 and 8 mRNA expression was associated with better OS, whereas higher expression of CASP9, 10, 12 and 14 showed an unfavorable OS in all GC patients. Moreover, CASP1 to 8 were all associated with favorable OS in intestinal type and diffuse type classified by Lauren classification. Therefore, the results of the present study suggested that the CASP family may function as new prognostic indicators in GC and may be helpful in making treatment decisions.

Caspase-2 and p75 neurotrophin receptor (p75NTR) are involved in the regulation of SREBP and lipid genes in hepatocyte cells

Lipid-induced toxicity is part of several human diseases, but the mechanisms involved are not fully understood. Fatty liver is characterized by the expression of different growth and tissue factors. The neurotrophin, nerve growth factor (NGF) and its pro-form, pro-NGF, are present in fatty liver together with p75 neurotrophin receptor (p75NTR). Stimulation of human Huh7 hepatocyte cells with NGF and pro-NGF induced Sterol-regulator-element-binding protein-2 (SREBP2) activation and increased Low-Density Lipoprotein Receptor (LDLR) expression. We observed that phosphorylation of caspase-2 by p38 MAPK was essential for this regulation involving a caspase-3-mediated cleavage of SREBP2. RNA sequencing showed that several genes involved in lipid metabolism were altered in p75NTR-deficient mouse liver. The same lipogenic genes were downregulated in p75NTR gene-engineered human Huh7 cells and reciprocally upregulated by stimulation of p75NTRs. In the knock-out mice the serum cholesterol and triglyceride levels were reduced, suggesting a physiological role of p75NTRs in whole-body lipid metabolism. Taken together, this study shows that p75NTR signaling influences a network of genes involved in lipid metabolism in liver and hepatocyte cells. Modulation of p75NTR signaling may be a target to consider in various metabolic disorders accompanied by increased lipid accumulation.

ER Stress Drives Lipogenesis and Steatohepatitis via Caspase-2 Activation of S1P

Nonalcoholic fatty liver disease (NAFLD) progresses to nonalcoholic steatohepatitis (NASH) in response to elevated endoplasmic reticulum (ER) stress. Whereas the onset of simple steatosis requires elevated de novo lipogenesis, progression to NASH is triggered by accumulation of hepatocyte-free cholesterol. We now show that caspase-2, whose expression is ER-stress inducible and elevated in human and mouse NASH, controls the buildup of hepatic-free cholesterol and triglycerides by activating sterol regulatory element-binding proteins (SREBP) in a manner refractory to feedback inhibition. Caspase-2 colocalizes with site 1 protease (S1P) and cleaves it to generate a soluble active fragment that initiates SCAP-independent SREBP1/2 activation in the ER. Caspase-2 ablation or pharmacological inhibition prevents diet-induced steatosis and NASH progression in ER-stress-prone mice. Caspase-2 inhibition offers a specific and effective strategy for preventing or treating stress-driven fatty liver diseases, whereas caspase-2-generated S1P proteolytic fragments, which enter the secretory pathway, are potential NASH biomarkers.

Caspases in metabolic disease and their therapeutic potential

Caspases, a family of cysteine-dependent aspartate-specific proteases, are central to the maintenance of cellular and organismal homoeostasis by functioning as key mediators of the inflammatory response and/or apoptosis. Both metabolic inflammation and apoptosis play a central role in the pathogenesis of metabolic disease such as obesity and the progression of nonalcoholic steatohepatisis (NASH) to more severe liver disease. Obesity and nonalcoholic fatty liver disease (NAFLD) are the leading global health challenges associated with the development of numerous comorbidities including insulin resistance, type-2 diabetes and early mortality. Despite the high prevalence, current treatment strategies including lifestyle, dietary, pharmaceutical and surgical interventions, are often limited in their efficacy to manage or treat obesity, and there are currently no clinical therapies for NAFLD/NASH. As mediators of inflammation and cell death, caspases are attractive therapeutic targets for the treatment of these metabolic diseases. As such, pan-caspase inhibitors that act by blocking apoptosis have reached phase I/II clinical trials in severe liver disease. However, there is still a lack of knowledge of the specific and differential functions of individual caspases. In addition, cross-talk between alternate cell death pathways is a growing concern for long-term caspase inhibition. Evidence is emerging of the important cell-death-independent, non-apoptotic functions of caspases in metabolic homoeostasis that may be of therapeutic value. Here, we review the current evidence for roles of caspases in metabolic disease and discuss their potential targeting as a therapeutic strategy.

Chitosan oligosaccharides enhance lipid droplets via down-regulation of PCSK9 gene expression in HepG2 cells

Chitosan oligosaccharides (COS), linear polymers of N-acetyl-D-glucosamine and deacetylated glucosamine, exhibit diverse pharmacological effects such as antimicrobial, antitumor, antioxidant and anti-inflammatory activities. Here, we explored their hypocholesterolemic effects in vivo and the molecular mechanisms of COS in hepatic cells. Our in vivo study of dyslipidemic ApoE^-/- male mice showed that COS treatment of 500 mg kg^-1 d^-1 for 4 weeks clearly reduced the lipid deposits in the aorta and significantly decreased the hepatic proprotein convertase subtilisin/kexin type 9 (PCSK9) protein levels versus HFD groups (p < 0.05). To elucidate the mechanisms behind these effects, the HepG2 cell line was treated with COS. We found that COS (200 μg/ml) increased the amount of cell-surface low-density lipoprotein receptor (LDLR) and enhanced the lipid droplets in HepG2 cells (p < 0.05). The mRNA levels of LDLR and HMG-CoA protein levels were not altered, and the mRNA levels of PCSK9 were down-regulated by COS treatment for 24 h. We also observed that the expression levels of SREBP-2 (125 kD) and HNF-1α were increased in total cell lysates, but nuclear SREBP-2 (nSREBP-2, 68 kD, the active subunit of SREBP-2) levels were decreased and FOXO3a levels increased in nuclear lysates after COS treatment for 24 h. We demonstrated that one of the reasons for regulation of lipid transfer with COS is that FOXO3a levels are up-regulated by COS, leading to a reduction in the PCSK9 promoter binding capacity of HNF-1α and thus suppressing PCSK9 gene expression, up-regulating LDLR levels, and enhancing the lipid droplets in HepG2 cells. In addition, decreased expression of the PCSK9 gene was also contributed to by down-regulation of SREBP-2 by COS. We further confirmed the effect of suppression of PCSK9 expression by COS by utilizing RNA interference to silence HNF-1α and SREBP-2. Finally, to the best of our knowledge, we are the first to demonstrate that PCSK9 expression and LDLR activity are synergistically changed by a combination of HNF-1α and SREBP-2 after COS treatment. Our findings indicate that COS may regulate PCSK9 to modulate hepatic LDLR abundance and activity.

SREBP inhibition ameliorates renal injury after unilateral ureteral obstruction

Tubulointerstitial fibrosis is a major feature associated with declining kidney function in chronic kidney disease of diverse etiology. No effective means as yet exists to prevent the progression of fibrosis. We have shown that the transcription factor sterol-regulatory element-binding protein 1 (SREBP-1) is an important mediator of the profibrotic response to transforming growth factor-β (TGF-β) and angiotensin II, both key cytokines in the fibrotic process. Here, we examined the role of SREBP in renal interstitial fibrosis in the unilateral ureteral obstruction (UUO) model. The two isoforms of SREBP (-1 and -2) were activated by 3 days after UUO, with SREBP-1 showing a more sustained activation to 21 days. We then examined whether SREBP1/2 inhibition with the small-molecule inhibitor fatostatin could attenuate fibrosis after 14 days of UUO. SREBP activation was confirmed to be inhibited by fatostatin. Treatment decreased interstitial fibrosis, TGF-β signaling, and upregulation of α-smooth muscle actin (SMA), a marker of fibroblast activation. Fatostatin also attenuated inflammatory cell infiltrate and apoptosis. Associated with this, fatostatin preserved proximal tubular mass. The significant increase in atubular glomeruli observed after UUO, known to correlate with irreversible renal functional decline, was also decreased by treatment. In cultured primary fibroblasts, TGF-β1 induced the activation of SREBP-1 and -2. Fatostatin blocked TGF-β1-induced α-SMA and matrix protein upregulation. The inhibition of SREBP is thus a potential novel therapeutic target in the treatment of fibrosis in chronic kidney disease.

Sex-specific alterations in glucose homeostasis and metabolic parameters during ageing of caspase-2-deficient mice

Gender-specific differences are commonly found in metabolic pathways and in response to nutritional manipulation. Previously, we identified a role for caspase-2 in age-related glucose homeostasis and lipid metabolism using male caspase-2-deficient (Casp2^−/−) mice. Here we show that the resistance to age-induced glucose tolerance does not occur in female Casp2^−/− mice and it appears to be independent of insulin sensitivity in males. Using fasting (18 h) as a means to further investigate the role of caspase-2 in energy and lipid metabolism, we identified sex-specific differences in the fasting response and lipid mobilization. In aged (18–22 months) male Casp2^−/− mice, a significant decrease in fasting liver mass, but not total body weight, was observed while in females, total body weight, but not liver mass, was reduced when compared with wild-type (WT) animals. Fasting-induced lipolysis of adipose tissue was enhanced in male Casp2^−/− mice as indicated by a significant reduction in white adipocyte cell size, and increased serum-free fatty acids. In females, white adipocyte cell size was significantly smaller in both fed and fasted Casp2^−/− mice. No difference in fasting-induced hepatosteatosis was observed in the absence of caspase-2. Further analysis of white adipose tissue (WAT) indicated that female Casp2^−/− mice may have enhanced fatty acid recycling and metabolism with expression of genes involved in glyceroneogenesis and fatty acid oxidation increased. Loss of Casp2 also increased fasting-induced autophagy in both male and female liver and in female skeletal muscle. Our observations suggest that caspase-2 can regulate glucose homeostasis and lipid metabolism in a tissue and sex-specific manner.

DAPK2 Downregulation Associates With Attenuated Adipocyte Autophagic Clearance in Human Obesity

Adipose tissue dysfunction in obesity has been linked to low-grade inflammation causing insulin resistance. Transcriptomic studies have identified death-associated protein kinase 2 (DAPK2) among the most strongly downregulated adipose tissue genes in human obesity, but the role of this kinase is unknown. We show that mature adipocytes rather than the stromal vascular cells in adipose tissue mainly expressed DAPK2 and that DAPK2 mRNA in obese patients gradually recovered after bariatric surgery-induced weight loss. DAPK2 mRNA is also downregulated in high-fat diet-induced obese mice. Adenoviral-mediated DAPK2 overexpression in 3T3-L1 adipocytes did not affect lipid droplet size or cell viability but did increase autophagic clearance in nutrient-rich conditions, dependent on protein kinase activity. Conversely, DAPK2 inhibition in human preadipocytes by small interfering RNA decreased LC3-II accumulation rates with lysosome inhibitors. This led us to assess autophagic clearance in adipocytes freshly isolated from subcutaneous adipose tissue of obese patients. Severe reduction in autophagic flux was observed in obese adipocytes compared with control adipocytes, inversely correlated to fat cell lipids. After bariatric surgery, adipocyte autophagic clearance partially recovered proportional to the extent of fat cell size reduction. This study links adipocyte expression of an autophagy-regulating kinase, lysosome-mediated clearance and fat cell lipid accumulation; it demonstrates obesity-related attenuated autophagy in adipocytes, and identifies DAPK2 dependence in this regulation.

A nonapoptotic role for CASP2/caspase 2: modulation of autophagy

CASP2/caspase 2 plays a role in aging, neurodegeneration, and cancer. The contributions of CASP2 have been attributed to its regulatory role in apoptotic and nonapoptotic processes including the cell cycle, DNA repair, lipid biosynthesis, and regulation of oxidant levels in the cells. Previously, our lab demonstrated CASP2-mediated modulation of autophagy during oxidative stress. Here we report the novel finding that CASP2 is an endogenous repressor of autophagy. Knockout or knockdown of CASP2 resulted in upregulation of autophagy in a variety of cell types and tissues. Reinsertion of Caspase-2 gene (Casp2) in mouse embryonic fibroblast (MEFs) lacking Casp2 (casp2(-/-)) suppresses autophagy, suggesting its role as a negative regulator of autophagy. Loss of CASP2-mediated autophagy involved AMP-activated protein kinase, mechanistic target of rapamycin, mitogen-activated protein kinase, and autophagy-related proteins, indicating the involvement of the canonical pathway of autophagy. The present study also demonstrates an important role for loss of CASP2-induced enhanced reactive oxygen species production as an upstream event in autophagy induction. Additionally, in response to a variety of stressors that induce CASP2-mediated apoptosis, casp2(-/-) cells demonstrate a further upregulation of autophagy compared with wild-type MEFs, and upregulated autophagy provides a survival advantage. In conclusion, we document a novel role for CASP2 as a negative regulator of autophagy, which may provide important insight into the role of CASP2 in various processes including aging, neurodegeneration, and cancer.

Age-related proteostasis and metabolic alterations in Caspase-2-deficient mice

Ageing is a complex biological process for which underlying biochemical changes are still largely unknown. We performed comparative profiling of the cellular proteome and metabolome to understand the molecular basis of ageing in Caspase-2-deficient (Casp2^−/−) mice that are a model of premature ageing in the absence of overt disease. Age-related changes were determined in the liver and serum of young (6–9 week) and aged (18–24 month) wild-type and Casp2^−/− mice. We identified perturbed metabolic pathways, decreased levels of ribosomal and respiratory complex proteins and altered mitochondrial function that contribute to premature ageing in the Casp2^−/− mice. We show that the metabolic profile changes in the young Casp2^−/− mice resemble those found in aged wild-type mice. Intriguingly, aged Casp2^−/− mice were found to have reduced blood glucose and improved glucose tolerance. These results demonstrate an important role for caspase-2 in regulating proteome and metabolome remodelling during ageing.

High-fat feeding does not induce an autophagic or apoptotic phenotype in female rat skeletal muscle

Apoptosis and autophagy are critical in normal skeletal muscle homeostasis; however, dysregulation can lead to muscle atrophy and dysfunction. Lipotoxicity and/or lipid accumulation may promote apoptosis, as well as directly or indirectly influence autophagic signaling. Therefore, the purpose of this study was to examine the effect of a 16-week high-fat diet on morphological, apoptotic, and autophagic indices in oxidative and glycolytic skeletal muscle of female rats. High-fat feeding resulted in increased fat pad mass, altered glucose tolerance, and lower muscle pAKT levels, as well as lipid accumulation and reactive oxygen species generation in soleus muscle; however, muscle weights, fiber type-specific cross-sectional area, and fiber type distribution were not affected. Moreover, DNA fragmentation and LC3 lipidation as well as several apoptotic (ARC, Bax, Bid, tBid, Hsp70, pBcl-2) and autophagic (ATG7, ATG4B, Beclin 1, BNIP3, p70 s6k, cathepsin activity) indices were not altered in soleus or plantaris following high-fat diet. Interestingly, soleus muscle displayed small increases in caspase-3, caspase-8, and caspase-9 activity, as well as higher ATG12-5 and p62 protein, while both soleus and plantaris muscle showed dramatically reduced Bcl-2 and X-linked inhibitor of apoptosis protein (XIAP) levels. In conclusion, this work demonstrates that 16 weeks of high-fat feeding does not affect tissue morphology or induce a global autophagic or apoptotic phenotype in skeletal muscle of female rats. However, high-fat feeding selectively influenced a number of apoptotic and autophagic indices which could have implications during periods of enhanced muscle stress.

Transcriptomic analysis unveils correlations between regulative apoptotic caspases and genes of cholesterol homeostasis in human brain

Regulative circuits controlling expression of genes involved in the same biological processes are frequently interconnected. These circuits operate to coordinate the expression of multiple genes and also to compensate dysfunctions in specific elements of the network. Caspases are cysteine-proteases with key roles in the execution phase of apoptosis. Silencing of caspase-2 expression in cultured glioblastoma cells allows the up-regulation of a limited number of genes, among which some are related to cholesterol homeostasis. Lysosomal Acid Lipase A (LIPA) was up-regulated in two different cell lines in response to caspase-2 down-regulation and cells silenced for caspase-2 exhibit reduced cholesterol staining in the lipid droplets. We expanded this observation by large-scale analysis of mRNA expression. All caspases were analyzed in terms of co-expression in comparison with 166 genes involved in cholesterol homeostasis. In the brain, hierarchical clustering has revealed that the expression of regulative apoptotic caspases (CASP2, CASP8 CASP9, CASP10) and of the inflammatory CASP1 is linked to several genes involved in cholesterol homeostasis. These correlations resulted in altered GBM (Glioblastoma Multiforme), in particular for CASP1. We have also demonstrated that these correlations are tissue specific being reduced (CASP9 and CASP10) or different (CASP2) in the liver. For some caspases (CASP1, CASP6 and CASP7) these correlations could be related to brain aging.

A nonapoptotic role for CASP2/caspase 2: modulation of autophagy

CASP2/caspase 2 plays a role in aging, neurodegeneration, and cancer. The contributions of CASP2 have been attributed to its regulatory role in apoptotic and nonapoptotic processes including the cell cycle, DNA repair, lipid biosynthesis, and regulation of oxidant levels in the cells. Previously, our lab demonstrated CASP2-mediated modulation of autophagy during oxidative stress. Here we report the novel finding that CASP2 is an endogenous repressor of autophagy. Knockout or knockdown of CASP2 resulted in upregulation of autophagy in a variety of cell types and tissues. Reinsertion of Caspase-2 gene (Casp2) in mouse embryonic fibroblast (MEFs) lacking Casp2 (casp2(-/-)) suppresses autophagy, suggesting its role as a negative regulator of autophagy. Loss of CASP2-mediated autophagy involved AMP-activated protein kinase, mechanistic target of rapamycin, mitogen-activated protein kinase, and autophagy-related proteins, indicating the involvement of the canonical pathway of autophagy. The present study also demonstrates an important role for loss of CASP2-induced enhanced reactive oxygen species production as an upstream event in autophagy induction. Additionally, in response to a variety of stressors that induce CASP2-mediated apoptosis, casp2(-/-) cells demonstrate a further upregulation of autophagy compared with wild-type MEFs, and upregulated autophagy provides a survival advantage. In conclusion, we document a novel role for CASP2 as a negative regulator of autophagy, which may provide important insight into the role of CASP2 in various processes including aging, neurodegeneration, and cancer.

Phospho-ΔNp63α/SREBF1 protein interactions: bridging cell metabolism and cisplatin chemoresistance

Tumor protein (TP)-p53 family members (TP63, TP63 and TP73) are guardians of the genome and key players in orchestrating the cellular response to cisplatin treatment. Cisplatin-induced phosphorylation of ΔNp63α was shown to have a role in regulating intracellular ΔNp63α protein levels. We previously found that squamous cell carcinoma (SCC) cells exposed to cisplatin displayed the ATM-dependent phosphorylation of ΔNp63α (p-ΔNp63α), which is critical for the transcriptional regulation of specific downstream mRNAs and microRNAs and is likely to underlie the chemoresistance of SCC cells. However, SCC cells expressing non-p-ΔNp63α became more cisplatin-resistant. We also found that p-ΔNp63α forms complexes with a number of proteins involved in cell death response through regulation of cell cycle arrest, apoptosis, autophagy, RNA splicing and chromatin modifications. Here, we showed that p-ΔNp63α induced ARG1, GAPDH, and CPT2 gene transcription in cisplatin-sensitive SCC cells, while non-p-ΔNp63α increased a transcription of CAD, G6PD and FASN genes in cisplatin-resistant SCC cells. We report that the p-ΔNp63α-dependent regulatory mechanisms implicated in the modulation of plethora of pathways, including amino acid, carbohydrate, lipid and nucleotide metabolisms, thereby affect tumor cell response to cisplatin-induced cell death, suggesting that the ATM-dependent ΔNp63α pathway plays a role in the resistance of tumor cells to platinum therapy.

ER stress contributes to renal proximal tubule injury by increasing SREBP-2-mediated lipid accumulation and apoptotic cell death

Renal proximal tubule injury is induced by agents/conditions known to cause endoplasmic reticulum (ER) stress, including cyclosporine A (CsA), an immunosuppressant drug with nephrotoxic effects. However, the underlying mechanism by which ER stress contributes to proximal tubule cell injury is not well understood. In this study, we report lipid accumulation, sterol regulatory element-binding protein-2 (SREBP-2) expression, and ER stress in proximal tubules of kidneys from mice treated with the classic ER stressor tunicamycin (Tm) or in human renal biopsy specimens showing CsA-induced nephrotoxicity. Colocalization of ER stress markers [78-kDa glucose regulated protein (GRP78), CHOP] with SREBP-2 expression and lipid accumulation was prominent within the proximal tubule cells exposed to Tm or CsA. Prolonged ER stress resulted in increased apoptotic cell death of lipid-enriched proximal tubule cells with colocalization of GRP78, SREBP-2, and Ca(2+)-independent phospholipase A(2) (iPLA(2)β), an SREBP-2 inducible gene with proapoptotic characteristics. In cultured HK-2 human proximal tubule cells, CsA- and Tm-induced ER stress caused lipid accumulation and SREBP-2 activation. Furthermore, overexpression of SREBP-2 or activation of endogenous SREBP-2 in HK-2 cells stimulated apoptosis. Inhibition of SREBP-2 activation with the site-1-serine protease inhibitor AEBSF prevented ER stress-induced lipid accumulation and apoptosis. Overexpression of the ER-resident chaperone GRP78 attenuated ER stress and inhibited CsA-induced SREBP-2 expression and lipid accumulation. In summary, our findings suggest that ER stress-induced SREBP-2 activation contributes to renal proximal tubule cell injury by dysregulating lipid homeostasis.

Caspase-2 deficiency protects mice from diabetes-induced marrow adiposity

Type I (T1) diabetes is an autoimmune and metabolic disease associated with bone loss. Bone formation and density are decreased in T1-diabetic mice. Correspondingly, the number of TUNEL positive, dying osteoblasts increases in bones of T1-diabetic mice. Moreover, two known mediators of osteoblast death, TNFα and ROS, are increased in T1-diabetic bone. TNFα and oxidative stress are known to activate caspase-2, a factor involved in the extrinsic apoptotic pathway. Therefore, we investigated the requirement of caspase-2 for diabetes-induced osteoblast death and bone loss. Diabetes was induced in 16-week old C57BL/6 caspase-2 deficient mice and their wild type littermates and markers of osteoblast death, bone formation and resorption, and marrow adiposity were examined. Despite its involvement in extrinsic cell death, deficiency of caspase-2 did not prevent or reduce diabetes-induced osteoblast death as evidenced by a twofold increase in TUNEL positive osteoblasts in both mouse genotypes. Similarly, deficiency of caspase-2 did not prevent T1-diabetes induced bone loss in trabecular bone (BV/TV decreased by 30 and 50%, respectively) and cortical bone (decreased cortical thickness and area with increased marrow area). Interestingly, at this age, differences in bone parameters were not seen between genotypes. However, caspase-2 deficiency attenuated diabetes-induced bone marrow adiposity and adipocyte gene expression. Taken together, our data suggest that caspase-2 deficiency may play a role in promoting marrow adiposity under stress or disease conditions, but it is not required for T1-diabetes induced bone loss.

Adaptation to statins restricts human tumour growth in Nude mice

BACKGROUND

Statins have long been used as anti-hypercholesterolemia drugs, but numerous lines of evidence suggest that they may also bear anti-tumour potential. We have recently demonstrated that it was possible to isolate cancer cells adapted to growth in the continuous presence of lovastatin. These cells grew more slowly than the statin-sensitive cells of origin. In the present study, we compared the ability of both statin-sensitive and statin-resistant cells to give rise to tumours in Nude mice.

METHODS

HGT-1 human gastric cancer cells and L50 statin-resistant derivatives were injected subcutaneously into Nude mice and tumour growth was recorded. At the end of the experiment, tumours were recovered and marker proteins were analyzed by western blotting, RT-PCR and immunohistochemistry.

RESULTS

L50 tumours grew more slowly, showed a strong decrease in cyclin B1, over-expressed collagen IV, and had reduced laminin 332, VEGF and CD34 levels, which, collectively, may have restricted cell division, cell adhesion and neoangiogenesis.

CONCLUSIONS

Taken together, these results showed that statin-resistant cells developed into smaller tumours than statin-sensitive cells. This may be reflective of the cancer restricting activity of statins in humans, as suggested from several retrospective studies with subjects undergoing statin therapy for several years.

Nelfinavir induces liposarcoma apoptosis through inhibition of regulated intramembrane proteolysis of SREBP-1 and ATF6

PURPOSE

We previously reported that nelfinavir (NFV) induces G(1) cell-cycle block and apoptosis selectively in liposarcoma cell lines due to increased SREBP-1 (sterol regulatory element binding protein-1) expression in the absence of increased transcription. We postulate that NFV interferes with regulated intramembrane proteolysis of SREBP-1 and ATF6 (activating transcription factor 6).

EXPERIMENTAL DESIGN

Time-lapse, confocal microscopic studies show that NFV inhibits the nuclear translocation of full-length SREBP-1-EGFP and ATF6-EGFP fusion proteins. siRNA-mediated knockdown of site-1 protease (S1P) and/or site-2 protease (S2P) leads to inhibition of SREBP-1 intracellular trafficking to the nucleus and reduces liposarcoma cell proliferation. Treatment of LiSa-2 liposarcoma cells with 3,4-dichloroisocoumarin, a serine protease inhibitor of S1P, did not affect SREBP-1 processing. In contrast, 1,10-phenanthroline, an S2P-specific inhibitor, reproduces the molecular and biological phenotypes observed in NFV-treated cells, which implicates S2P as a target of NFV. In vivo evaluation of NFV in a murine liposarcoma xenograft model leads to inhibition of tumor growth without significant toxicity.

RESULTS

NFV-induced upregulation of SREBP-1 and ATF6 results from inhibition of S2P, which together with S1P mediates regulated intramembrane proteolysis from their precursor to their transcriptionally active forms. The resulting endoplasmic reticulum (ER) stress and concurrent inhibition of the unfolded protein response induce caspase-mediated apoptosis.

CONCLUSIONS

These results provide new insight into the mechanism of NFV-mediated induction of ER stress and cell death in liposarcomas and are the first to report targeting S2P for cancer therapy.

CELF proteins regulate CFTR pre-mRNA splicing: essential role of the divergent domain of ETR-3

Cystic fibrosis is a prominent genetic disease caused by mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Among the many disease-causing alterations are pre-mRNA splicing defects that can hamper mandatory exon inclusion. CFTR exon 9 splicing depends in part on a polymorphic UG(m)U(n) sequence at the end of intron 8, which can be bound by TDP-43, leading to partial exon 9 skipping. CELF proteins, like CUG-BP1 and ETR-3, can also bind UG repeats and regulate splicing. We show here that ETR-3, but not CUG-BP1, strongly stimulates exon 9 skipping, although both proteins bind efficiently to the same RNA motif as TDP-43 and with higher affinity. We further show that the skipping of this exon may be due to the functional antagonism between U2AF65 and ETR-3 binding onto the polymorphic U or UG stretch, respectively. Importantly, we demonstrate that the divergent domain of ETR-3 is critical for CFTR exon 9 skipping, as shown by deletion and domain-swapping experiments. We propose a model whereby several RNA-binding events account for the complex regulation of CFTR exon 9 inclusion, with strikingly distinct activities of ETR-3 and CUG-BP1, related to the structure of their divergent domain.

Caspase-2: controversial killer or checkpoint controller?

The caspases are an evolutionarily conserved family of cysteine proteases, with essential roles in apoptosis or inflammation. Caspase-2 was the second caspase to be cloned and it resembles the prototypical nematode caspase CED-3 more closely than any other mammalian protein. An absence of caspase-2-specific reagents and the subtle phenotype of caspase-2-deficient mice have hampered definition of the physiological role of caspase-2 and identification of factors regulating its activity. Although some data implicate caspase-2 in apoptotic pathways, a link with apoptosis has been less firmly established for caspase-2 than for some other caspases. Emerging evidence suggests that caspase-2 regulates the cell cycle and may act as a tumour suppressor. This article critically reviews the current state of knowledge regarding the biochemistry and biology of this controversial caspase.

Human caspase 7 is positively controlled by SREBP-1 and SREBP-2

Statins are lipid-lowering drugs that may help limit cancer occurrence in humans. They drive blockage of the mevalonate pathway, trigger cancer cell apoptosis in vitro and reduce tumour incidence in animals. We have shown in the present study that statins induced apoptosis in HGT-1 human gastric cancer cells, and this was prevented by intermediates of the cholesterol synthetic pathway. In addition, similarly to what we have reported previously for caspase 2 [Logette, Le Jossic-Corcos, Masson, Solier, Sequeira-Legrand, Dugail, Lemaire-Ewing, Desoche, Solary and Corcos (2005) Mol. Cell. Biol. 25, 9621-9631], caspase 7 may also be induced by statins and is under the positive control of SREBP (sterol-regulatory-element-binding protein)-1 and -2, major activators of cholesterol and fatty acid synthesis genes, in HGT-1 cells. Knocking down these proteins strongly reduced caspase 7 mRNA and protein expression, and chromatin immunoprecipitation analyses showed that the proximal promoter region of the CASP7 gene could bind either SREBP-1 or -2. Strikingly, cells selected to grow in the continuous presence of statins showed increased expression of caspase 7 mRNA and protein, which was maintained in the absence of statins for several weeks, suggesting that high expression of this caspase might participate in adaptation to blunting of the mevalonate pathway in this model. Taken together, our results show that caspase 7, as an SREBP-1/2 target, can be induced under mevalonate-restricting conditions, which might help overcome its shortage.

Dynamical modeling of the cholesterol regulatory pathway with Boolean networks

BACKGROUND

Qualitative dynamics of small gene regulatory networks have been studied in quite some details both with synchronous and asynchronous analysis. However, both methods have their drawbacks: synchronous analysis leads to spurious attractors and asynchronous analysis lacks computational efficiency, which is a problem to simulate large networks. We addressed this question through the analysis of a major biosynthesis pathway. Indeed the cholesterol synthesis pathway plays a pivotal role in dislypidemia and, ultimately, in cancer through intermediates such as mevalonate, farnesyl pyrophosphate and geranyl geranyl pyrophosphate, but no dynamic model of this pathway has been proposed until now.

RESULTS

We set up a computational framework to dynamically analyze large biological networks. This framework associates a classical and computationally efficient synchronous Boolean analysis with a newly introduced method based on Markov chains, which identifies spurious cycles among the results of the synchronous simulation. Based on this method, we present here the results of the analysis of the cholesterol biosynthesis pathway and its physiological regulation by the Sterol Response Element Binding Proteins (SREBPs), as well as the modeling of the action of statins, inhibitor drugs, on this pathway. The in silico experiments show the blockade of the cholesterol endogenous synthesis by statins and its regulation by SREPBs, in full agreement with the known biochemical features of the pathway.

CONCLUSION

We believe that the method described here to identify spurious cycles opens new routes to compute large and biologically relevant models, thanks to the computational efficiency of synchronous simulation. Furthermore, to the best of our knowledge, we present here the first dynamic systems biology model of the human cholesterol pathway and several of its key regulatory control elements, hoping it would provide a good basis to perform in silico experiments and confront the resulting properties with published and experimental data. The model of the cholesterol pathway and its regulation, along with Boolean formulae used for simulation are available on our web site http://Bioinformaticsu613.free.fr. Graphical results of the simulation are also shown online. The SBML model is available in the BioModels database http://www.ebi.ac.uk/biomodels/ with submission ID: MODEL0568648427.

Midregion PTHrP regulates Rip1 and caspase expression in MDA-MB231 breast cancer cells

It was previously reported that the midregion PTHrP domain (38-94)-amide restrains growth and invasion "in vitro", causes striking toxicity and accelerates death of some breast cancer cell lines, the most responsive being MDA-MB231 whose tumorigenesis was also attenuated "in vivo". In addition, we have demonstrated that midregion PTHrP is imported in the nucleoplasm of cultured MDA-MB231 cells, and that "in vitro" it can bind chromatin of metaphase spread preparations and also an isolated 20-mer oligonucleotide, thereby appearing endowed with a putative transcription factor-like DNA-binding ability. Here, we examined whether PTHrP (38-94)-amide was able to modulate the expression of genes encoding for apoptosis factors and caspases. Employing a combination of conventional and semi-quantitative multiplex PCR techniques, antisense oligonucleotide (asODN) transfections, proliferation/invasion assays and protein analyses, here we report that PTHrP treatment induces the up-regulation of Bcl-xS, Bad and Rip1 and switches-on the expression of caspase-2, -5, -6, -7 and -8 in MDA-MB231 cells. Moreover, we demonstrate for the first time that asODN-induced under-expression of Rip1 can lead to a more pronounced up-regulation of some caspases due, at least in part, to JNK inactivation, thus providing a new example of factor involved in the transcriptional regulation of the apoptotic enzymes.

Effects of caspase inhibitors (z-VAD-fmk, z-VDVAD-fmk) on Nile Red fluorescence pattern in 7-ketocholesterol-treated cells: investigation by flow cytometry and spectral imaging microscopy

BACKGROUND

The 7-ketocholesterol (7KC)-induced cell death has some characteristics of apoptosis and is associated with polar lipid accumulation. So, we investigated the effects of the broad-spectrum caspase inhibitor z-VAD-fmk and of the caspase-2 inhibitor z-VDVAD-fmk on lipid profile evaluated by staining with Nile Red (NR).

METHODS

The 7KC-treated human monocytic U937 cells were cultured in the absence or in the presence of the caspase inhibitors z-VAD-fmk or z-VDVAD-fmk. When staining with NR is performed, neutral and polar lipids have yellow and orange/red emission, respectively, and fluorescence was then analyzed by flow cytometry (FCM) and by confocal laser scanning microscopy (CLSM) combined with subsequent image processing. The 3D-image sequences were obtained by means of CLSM using spectral analysis, and were analyzed by the factor analysis of medical image sequences algorithm to differentiate spectra inside mixed fluorescence emission and get corresponding specific images.

RESULTS

By FCM, comparatively to untreated cells, higher percentages of red fluorescent cells were identified in 7KC-treated cells. Factor curves and images reveal orange and red fluorescence emissions in 7KC-treated cells and show yellow, orange, and red fluorescence emissions in 7KC-treated cells cultured in the presence of z-VAD-fmk or z-VDVAD-fmk.

CONCLUSIONS

Our data support that investigation by FCM and by spectral analysis in CLSM associated with subsequent image processing provides useful tools to determine the effect of caspase inhibitors on lipid content evaluated with NR. They also favor the hypothesis of relationships between caspase activity and polar lipid accumulation.

SREBP in signal transduction: cholesterol metabolism and beyond

The last few years have seen important advances in defining the mechanisms that cells use to monitor changes in cholesterol levels and regulate lipid metabolism. This work has unraveled a feedback system that enables cholesterol and certain sterol intermediates to regulate the proteolysis and transport of specific membrane proteins. The sterol regulatory element-binding protein (SREBP) family of transcription factors is at the center of this feedback system. These membrane-embedded proteins are activated by ER-to-Golgi transport followed by limited proteolysis. In addition, both the activation of the SREBPs and the stability of the rate-limiting enzyme in cholesterol synthesis are regulated by the ubiquitin-proteasome system in a sterol-dependent manner. The ubiquitin-proteasome system also regulates the degradation of active SREBPs. Recent work also highlights the important role of this regulatory system in several organisms, ranging from yeast to humans. In addition, the SREBP pathway has been found to regulate a diverse set of cellular processes, including phagocytosis, cell cycle progression, oxygen sensing and survival in response to bacterial infection. These advances illustrate the wide-ranging roles that SREBPs and membrane biogenesis have in cell biology.

Cytotoxic oxysterols induce caspase-independent myelin figure formation and caspase-dependent polar lipid accumulation

Oxysterols, mainly those oxidized at the C7 position, induce a complex mode of cell death exhibiting some characteristics of apoptosis associated with a rapid induction of lipid rich multilamellar cytoplasmic structures (myelin figures) observed in various pathologies including atherosclerosis. The aim of this study was to determine the relationships between myelin figure formation, cell death, and lipid accumulation in various cell lines (U937, THP-1, MCF-7 [caspase-3 deficient], A7R5) treated either with oxysterols (7-ketocholesterol [7KC], 7beta-hydroxycholesterol, cholesterol-5alpha,6alpha-epoxide, cholesterol-5beta,6beta-epoxide, 25-hydroxycholesterol) or cytotoxic drugs (etoposide, daunorubicin, tunicamycin, rapamycin). Cell death was assessed by the measurement of cellular permeability with propidium iodide, characterization of the morphological aspect of the nuclei with Hoechst 33342, and identification of myelin figures by transmission electron microscopy. Nile Red staining (distinguishing neutral and polar lipids) was used to identify lipid content by flow cytometry and spectral imaging microscopy. Whatever the cells considered, myelin figures were only observed with cytotoxic oxysterols (7KC, 7beta-hydroxycholesterol, cholesterol-5beta, 6beta-epoxide), and their formation was not inhibited by the broad spectrum caspase inhibitor z-VAD-fmk. When U937 cells were treated with oxysterols or cytotoxic drugs, polar lipid accumulation was mainly observed with 7KC and 7beta-hydroxycholesterol. The highest polar lipid accumulation, which was triggered by 7KC, was counteracted by z-VAD-fmk. These findings demonstrate that myelin figure formation is a caspase-independent event closely linked with the cytotoxicity of oxysterols, and they highlight a relationship between caspase activity and polar lipid accumulation.

Caspase-1 activation of lipid metabolic pathways in response to bacterial pore-forming toxins promotes cell survival

Many pathogenic organisms produce pore-forming toxins as virulence factors. Target cells however mount a response to such membrane damage. Here we show that toxin-induced membrane permeabilization leads to a decrease in cytoplasmic potassium, which promotes the formation of a multiprotein oligomeric innate immune complex, called the inflammasome, and the activation of caspase-1. Further, we find that when rendered proteolytic in this context caspase-1 induces the activation of the central regulators of membrane biogenesis, the Sterol Regulatory Element Binding Proteins (SREBPs), which in turn promote cell survival upon toxin challenge possibly by facilitating membrane repair. This study highlights that, in addition to its well-established role in triggering inflammation via the processing of the precursor forms of interleukins, caspase-1 has a broader role, in particular linking the intracellular ion composition to lipid metabolic pathways, membrane biogenesis, and survival.

Essential fatty acids: biochemistry, physiology and pathology

Essential fatty acids (EFAs), linoleic acid (LA), and alpha-linolenic acid (ALA) are essential for humans, and are freely available in the diet. Hence, EFA deficiency is extremely rare in humans. To derive the full benefits of EFAs, they need to be metabolized to their respective long-chain metabolites, i.e., dihomo-gamma-linolenic acid (DGLA), and arachidonic acid (AA) from LA; and eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from ALA. Some of these long-chain metabolites not only form precursors to respective prostaglandins (PGs), thromboxanes (TXs), and leukotrienes (LTs), but also give rise to lipoxins (LXs) and resolvins that have potent anti-inflammatory actions. Furthermore, EFAs and their metabolites may function as endogenous angiotensin-converting enzyme and 3-hdroxy-3-methylglutaryl coenzyme A reductase inhibitors, nitric oxide (NO) enhancers, anti-hypertensives, and anti-atherosclerotic molecules. Recent studies revealed that EFAs react with NO to yield respective nitroalkene derivatives that exert cell-signaling actions via ligation and activation of peroxisome proliferator-activated receptors. The metabolism of EFAs is altered in several diseases such as obesity, hypertension, diabetes mellitus, coronary heart disease, schizophrenia, Alzheimer's disease, atherosclerosis, and cancer. Thus, EFAs and their derivatives have varied biological actions and seem to be involved in several physiological and pathological processes.

Mitochondria in hematopoiesis and hematological diseases

Mitochondria are involved in hematopoietic cell homeostasis through multiple ways such as oxidative phosphorylation, various metabolic processes and the release of cytochrome c in the cytosol to trigger caspase activation and cell death. In erythroid cells, the mitochondrial steps in heme synthesis, iron (Fe) metabolism and Fe-sulfur (Fe-S) cluster biogenesis are of particular importance. Mutations in the specific delta-aminolevulinic acid synthase (ALAS) 2 isoform that catalyses the first and rate-limiting step in heme synthesis pathway in the mitochondrial matrix, lead to ineffective erythropoiesis that characterizes X-linked sideroblastic anemia (XLSA), the most common inherited sideroblastic anemia. Mutations in the adenosine triphosphate-binding cassette protein ABCB7, identified in XLSA with ataxia (XLSA-A), disrupt the maturation of cytosolic (Fe-S) clusters, leading to mitochondrial Fe accumulation. In addition, large deletions in mitochondrial DNA, whose integrity depends on a specific DNA polymerase, are the hallmark of Pearson's syndrome, a rare congenital disorder with sideroblastic anemia. In acquired myelodysplastic syndromes at early stage, exacerbation of physiological pathways involving caspases and the mitochondria in erythroid differentiation leads to abnormal activation of a mitochondria-mediated apoptotic cell death pathway. In contrast, oncogenesis-associated changes at the mitochondrial level can alter the apoptotic response of transformed hematopoietic cells to chemotherapeutic agents. Recent findings in mitochondria metabolism and functions open new perspectives in treating hematopoietic cell diseases, for example various compounds currently developed to trigger tumor cell death by directly targeting the mitochondria could prove efficient as either cytotoxic drugs or chemosensitizing agents in treating hematological malignancies.

Thrombin-induced endothelial microparticle generation: identification of a novel pathway involving ROCK-II activation by caspase-2

Thrombin exerts pleiotropic effects on endothelial cells, including the release of microparticles (EMPs) that disseminate and exchange information with vascular cells. Nevertheless, the mechanisms leading to their generation are not elucidated. We performed microarray analysis to identify genes involved in EMP release by the endothelial cell line HMEC-1 in response to thrombin. We identified a group of genes linked to the cytoskeleton reorganization family. Among these, the Rho-kinase ROCK-II presented a high transcription rate. ROCK-I, another Rho-kinase isoform, was not modulated by thrombin. Pharmacologic inhibition of Rho-kinases or specific depletion of ROCK-II by short interfering (si) RNA inhibited thrombin-induced EMP release. In contrast, ROCK-I mRNA silencing did not modify EMP generation by thrombin. Exposure of HMEC-1 to thrombin in presence of the caspase-2 selective inhibitor Z-VDVAD-FMK prevented ROCK-II cleavage and inhibited the thrombin-induced EMP release. These events were observed in absence of cell death. Our data clearly identified ROCK-II as a target of thrombin in EMP generation. They indicated that the 2 Rho-kinases did not share identical functions. The involvement of caspase-2 in ROCK-II activation independently of cell death points out a novel signaling pathway that emphasizes the proteolytic activity of caspase in EMP generation in response to cell activation.

DnaJA4 is a SREBP-regulated chaperone involved in the cholesterol biosynthesis pathway

Using subtractive hybridization technique in 3T3-L1 adipocytes overexpressing constitutively active SREBP2, we have identified a DnaJ/Hsp40 chaperone, DnaJA4, as a new SREBP-responsive gene. SREBP2 regulation was demonstrated by changes in DnaJA4 mRNA under conditions of altered sterol status that were strictly parallel to that of well-characterized SREBP targets (LDL receptor and HMG-CoA reductase). The role of SREBP2 was further established using adenoviral overexpression of a dominant negative SREBP2, which abolished cholesterol-regulated changes in DnaJA4 expression. To determine the functional significance of this regulation, DnaJA4 was overexpressed in COS cells, which induced a specific increase in the synthesis of cholesterol from acetate. We also observed that DnaJA4 overexpression increased the activity and the protein content of HMG-CoA reductase, the rate limiting enzyme in this pathway. At the molecular level, DnaJA4 overexpression did not alter HMG-CoA reductase stability or mRNA levels, suggesting a co-translational effect of the chaperone. In the DnaJ/Hsp40 family, DnaJA4 uniquely exhibited SREBP-regulated expression, and also responded to heat shock. Through its responsiveness to SREBP, and its stimulatory effect on cholesterol synthesis, the DnaJA4 chaperone can be viewed as a new player in cholesterol synthesis. These data suggest a link between molecular chaperones, heat stress and cholesterol synthesis.