Molecular mechanism analysis of apoptosis induced by silk fibroin peptides

Silk fibroin derived from silkworm cocoons exhibits excellent mechanical properties, good biocompatibility, and low immunogenicity. Previous studies showed that silk fibroin had an inhibitory effect on cells, suppressing proliferation and inducing apoptosis. However, the source of the toxicity and the mechanism of apoptosis induction are still unclear. In this study, we hypothesized that the toxicity of silk fibroin might originate from the crystalline region of the heavy chain of silk fibroin. We then verified the hypothesis and the specific induction mechanism. A target peptide segment was obtained from α-chymotrypsin. The potentially toxic mixture of silk fibroin peptides (SFPs) was separated by ion exchange, and the toxicity was tested by an MTT assay. The results showed that SFPs obtained after 4 h of enzymatic hydrolysis had significant cytotoxicity, and SFPs with isoelectric points of 4.0-6.8 (SFPα II) had a significant inhibitory effect on cell growth. LC-MS/MS analysis showed that SFPα II contained a large number of glycine-rich and alanine-rich repetitive sequence polypeptides from the heavy-chain crystallization region. A series of experiments showed that SFPα II mediated cell death through the apoptotic pathway by decreasing the expression of Bcl-2 protein and increasing the expression of Bax protein. SFPα II mainly affected the p53 pathway and the AMPK signaling pathway in HepG2 cells. SFPα II may indirectly increase the expression of Cers2 by inhibiting the phosphorylation of EGFR, which activated apoptotic signaling in the cellular mitochondrial pathway and inhibited the Akt/NF-κB pathway by increasing the expression of PPP2R2A.

Undescribed glucosylceramide, flavonol triglycoside, and oleanane saponin from the halophyte Agathophora alopecuroides: Promising candidates for stimulating ceramide synthesis

The phytochemical study of Agathophora alopecuroides (Chenopodiaceae) led to the isolation of previously undescribed glucosylceramide, flavonol triglycoside, and triterpene oleanane saponin, together with eight known compounds. Their structures were elucidated using NMR analysis and HR-MS as (2'R, 12E) N-[(2S, 3S, 4R)-1-(β-D-glucopyranosyloxy)-3,4-dihydroxy-octadec-2-yl]-2-hydroxytetracos-12-enamide, namely Agathophamide A; isorhamnetin-3-O-[β-D-xylopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→6)]-β-D-galactopyranoside, namely Agathophoroside A; and 3-O-[4'-(β-D-xylopyranosyl)-β-D-glucuronopyranosyl]-28-O-β-D-glucopyranosyl-olean-12-en-3β-ol-28-oic acid, namely Solysaponin A. We evaluated the effect of extract and isolates on ceramide levels via the up-regulated expression of the enzyme for ceramide synthesis in HaCaT keratinocytes. Interestingly, the study results revealed that the methanol extract of A. alopecuroides, together with some isolated compounds significantly up-regulated the mRNA expression of ceramide synthase-3 by 1.2- to 4.3-fold compared with the control in HaCaT cells. These findings indicate that the halophyte A. alopecuroides is a promising source of candidate compounds that can contribute to ceramide synthesis via the up-regulated expression levels of ceramide synthase-3 in the ceramide synthesis pathway.

γ-Tocotrienol induces apoptosis in pancreatic cancer cells by upregulation of ceramide synthesis and modulation of sphingolipid transport

Background

Ceramide synthesis and metabolism is a promising target in cancer drug development. γ-tocotrienol (GT3), a member of the vitamin E family, orchestrates multiple effects that ensure the induction of apoptosis in both, wild-type and RAS-mutated pancreatic cancer cells. Here, we investigated whether these effects involve changes in ceramide synthesis and transport.

Methods

The effects of GT3 on the synthesis of ceramide via the de novo pathway, and the hydrolysis of sphingomyelin were analyzed by the expression levels of the enzymes serine palmitoyl transferase, ceramide synthase-6, and dihydroceramide desaturase, and acid sphingomyelinase in wild-type RAS BxPC3, and RAS-mutated MIA PaCa-2 and Panc 1 pancreatic cancer cells. Quantitative changes in ceramides, dihydroceramides, and sphingomyelin at the cell membrane were detected by LCMS. Modulation of ceramide transport by GT3 was studied by immunochemistry of CERT and ARV-1, and the subsequent effects at the cell membrane was analyzed via immunofluorescence of ceramide, caveolin, and DR5.

Results

GT3 favors the upregulation of ceramide by stimulating synthesis at the ER and the plasma membrane. Additionally, the conversion of newly synthesized ceramide to sphingomyelin and glucosylceramide at the Golgi is prevented by the inhibition of CERT. Modulation ARV1 and previously observed inhibition of the HMG-CoA pathway, contribute to changes in membrane structure and signaling functions, allows the clustering of DR5, effectively initiating apoptosis.

Conclusions

Our results suggest that GT3 targets ceramide synthesis and transport, and that the upregulation of ceramide and modulation of transporters CERT and ARV1 are important contributors to the apoptotic properties demonstrated by GT3 in pancreatic cancer cells.

Insights into abnormal sphingolipid metabolism in multiple sclerosis: targeting ceramide biosynthesis as a unique therapeutic strategy

Multiple sclerosis (MS) is a fatal demyelinating disease that primarily affects axons leading to massive neurodegeneration. Many studies have reported the causes and drawn the conclusions that multiple factors such as recurrent viral infections, hereditary link, and environmental condition are involved in the pathogenesis of MS. In essence, all these reports indicate a severe change in the biochemical milieu in the central nervous system (CNS) leading to inflammation and neurodegeneration. Recent studies in our laboratory revealed aberrant sphingolipid metabolism and accumulation of toxic sphingosine in the CNS tissues in MS patients. An elevation in sphingosine in MS brain white matter and plaque indicated that sphingosine toxicity might mediate degeneration of oligodendrocytes contributing to demyelination. An intermittent increase in ceramide followed by sphingosine accumulation in spinal cords from Lewis rats with experimental autoimmune encephalitis (EAE) and also stimulation of serine-palmitoyltransferase (SPT) activity correlated with induction of apoptosis in the lumbar spinal cord in EAE animals. Cytokine-stimulated ceramide elevation in cultured human oligodendrocytes was almost completely blocked by myriocin, an inhibitor of SPT. Myriocin exposure also protected oligodendrocytes from induction of apoptosis. Sphingosine toxicity via ceramide biosynthesis contributed to oligodendrocyte degeneration in both EAE and MS. Although many clinical trials are being conducted for MS, to the best of our knowledge, there is still no sphingolipid-targeted therapy available. Hence, we propose that sphingosine toxicity via ceramide generation may be a potential therapeutic target in both EAE and MS.

Hyaluronan tetrasaccharides stimulate ceramide production through upregulated mRNA expression of ceramide synthesis-associated enzymes

It has been reported that hyaluronan has different physiological functions as suggested by variation in molecular weight. In addition, it has also been reported that CD44, the major hyaluronan receptor, was demonstrated to induce keratinocyte differentiation and lipid synthesis of cholesterol. We focus attention on the hyaluronan tetrasaccharides (HA4) which is the smallest unit of hyaluronan. We previously reported that HA4 induced keratinocyte differentiation and that CD44 may be involved. For the purpose of clarifying the influence of HA4 on ceramide synthesis, we evaluated both of these factors in keratinocytes in vitro and in vivo. The mRNA expression of ceramide synthesis-associated enzymes and intracellular ceramide content were evaluated after HA4 treatment in normal human epidermal keratinocytes. In addition, the ceramide increasing effect of HA4 on skin in UVA-irradiated hairless mice was assessed by water content of stratum corneum (SC) and transepidermal water loss (TEWL) methods. The mRNA expression of ceramide synthesis-associated enzymes and intracellular ceramide content after HA4 treatment were increased compared with the control. Furthermore, HA4 treatment increased water content of SC and decreased TEWL. These findings suggest that HA4 affected ceramide synthesis and is involved in the improvement of UV-induced skin damage.

Sortilin deficiency improves the metabolic phenotype and reduces hepatic steatosis of mice subjected to diet-induced obesity

BACKGROUND & AIMS

Sortilin traffics newly synthesized molecules from the trans-Golgi apparatus along secretory pathways to endosomes, lysosomes or to the cell surface. Sortilin trafficking of acid sphingomyelinase (aSMase) may regulate ceramide levels, a major modulator of insulin signalling. We therefore tested whether sortilin deficiency reduces hepatic and adipose tissue aSMase activity, improving insulin sensitivity in diet-induced obesity (DIO).

METHODS

DIO in C57BL/6 (WT) and sortilin(-/-) mice was induced by high-fat diet feeding for 10 weeks.

RESULTS

Sortilin(-/-) mice gained less body weight and less visceral fat, despite similar food intake compared to WT type mice and had enhanced glucose uptake in insulin tolerance tests, which was further corroborated by enhanced hepatic pAkt expression. Sortilin deficiency led to attenuated hepatic steatosis, reduced expression of genes involved in lipogenesis, ceramide synthesis and inflammatory cytokine production and reduced activity of ceramide synthase 5/6 (CerS5/6). Sortilin(-/-) mice had reduced hepatic aSMase activity under both steady-state and DIO. Likewise, sortilin(-/-) hepatocytes displayed hypersensitivity to insulin, due to enhanced insulin receptor downstream signalling. In adipose tissue, sortilin(-/-) mice exhibited lower expression of inflammatory cytokines and lower expression and activity of CerS5/6. As in liver, adipose tissue displayed increased insulin signalling, accompanied by attenuated aSMase activity.

CONCLUSIONS

Sortilin deficiency induces a beneficial metabolic phenotype in liver and adipose tissue upon DIO, mediated in part by reduced aSMase activity.

Role of lipids in the formation and maintenance of the cutaneous permeability barrier

The major function of the skin is to form a barrier between the internal milieu and the hostile external environment. A permeability barrier that prevents the loss of water and electrolytes is essential for life on land. The permeability barrier is mediated primarily by lipid enriched lamellar membranes that are localized to the extracellular spaces of the stratum corneum. These lipid enriched membranes have a unique structure and contain approximately 50% ceramides, 25% cholesterol, and 15% free fatty acids with very little phospholipid. Lamellar bodies, which are formed during the differentiation of keratinocytes, play a key role in delivering the lipids from the stratum granulosum cells into the extracellular spaces of the stratum corneum. Lamellar bodies contain predominantly glucosylceramides, phospholipids, and cholesterol and following the exocytosis of lamellar lipids into the extracellular space of the stratum corneum these precursor lipids are converted by beta glucocerebrosidase and phospholipases into the ceramides and fatty acids, which comprise the lamellar membranes. The lipids required for lamellar body formation are derived from de novo synthesis by keratinocytes and from extra-cutaneous sources. The lipid synthetic pathways and the regulation of these pathways are described in this review. In addition, the pathways for the uptake of extra-cutaneous lipids into keratinocytes are discussed. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Ceramide synthesis in the epidermis

The epidermis and in particular its outermost layer the stratum corneum provides terrestrial vertebrates with a pivotal defensive barrier against water loss, xenobiotics and harmful pathogens. A vital demand for this epidermal permeability barrier is the lipid-enriched lamellar matrix that embeds the enucleated corneocytes. Ceramides are the major components of these highly ordered intercellular lamellar structures, in which linoleic acid- and protein-esterified ceramides are crucial for structuring and maintaining skin barrier integrity. In this review, we describe the fascinating diversity of epidermal ceramides including 1-O-acylceramides. We focus on epidermal ceramide biosynthesis emphasizing its metabolic and topological requirements and discuss enzymes that may be involved in α- and ω-hydroxylation. Finally, we turn to epidermal ceramide regulation, highlighting transcription factors and liposensors recently described to play crucial roles in modulating skin lipid metabolism and epidermal barrier homeostasis. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier.