Plasma membrane-associated sialidase (NEU3) regulates progression of prostate cancer to androgen-independent growth through modulation of androgen receptor signaling

Bioisosteres at C9 of 2-Deoxy-2,3-didehydro-N-acetyl Neuraminic Acid Identify Selective Inhibitors of NEU3

Human neuraminidases play critical roles in many physiological and pathological processes. Humans have four isoenzymes of NEU, making selective inhibitors important tools to investigate the function of individual isoenzymes. A typical scaffold for NEU inhibitors is 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (DANA) where C9 modifications can be critical for potency and selectivity against human NEU. To design improved DANA analogues, we generated a library of compounds with either a short alkyl chain or a biphenyl substituent linked to the C9 position through one of six amide bioisosteres. Bioisostere linkers included triazole, urea, thiourea, carbamate, thiocarbamate, and sulfonamide groups. Within this library, we identified a C9 biphenyl carbamate derivative (963) that showed high selectivity and potency for NEU3 (Ki = 0.12 ± 0.01 μM). In contrast, NEU1 and NEU4 isoenzymes preferred amide and triazole linkers, respectively. Finally, analogues with urea, sulfonamide, and amide linkers showed enhanced inhibitory activity for a bacterial NEU, NanI from Clostridium perfringens.

Regulation of sialic acid metabolism in cancer

Sialic acid, the terminal structure of cell surface glycans, has essential functions in regulating immune response, cell-to-cell communication, and cell adhesion. More importantly, an increased level of sialic acid, termed hypersialylation, has emerged as a commonly observed phenotype in cancer. Therefore, targeting sialic acid ligands (sialoglycans) and their receptors (Siglecs) may provide a new therapeutic approach for cancer immunotherapy. We highlight the complexity of the sialic acid metabolism and its involvement in malignant transformation within individual cancer subtypes. In this review, we focus on the dysregulation of sialylation, the intricate nature of sialic acid synthesis, and clinical perspective. We aim to provide a brief insight into the mechanism of hypersialylation and how our understanding of these processes can be leveraged for the development of novel therapeutics.

Exploration of sialic acid receptors as a potential target for cancer treatment: A comprehensive review

The potential to target anticancer drugs directly to cancer cells is the most difficult challenge in the current scenario. Progressive works are being done on multifarious receptors and are on the horizon, expected to facilitate tailored treatment for cancer. Among several receptors, one is the sialic acid (SA) receptor by which cancer cells can be targeted directly as hyper sialylation is one of the most distinguishing characteristics of cancer cells. SA receptors have shown tremendous potential for tumor targeting because of their elevated expression in a range of human malignancies including prostate, breast, gastric cells, myeloid leukemia, liver, etc. This article reviews the overexpression of SA receptors in various tumors and diverse strategies for targeting these receptors to deliver drugs, enzymes, and genes for therapeutic applications. It also summarizes the diagnostic applications of SA-grafted nanoparticles for imaging various SA-overexpressing cancer cells and technological advances that are propelling sialic acid to the forefront of cancer therapy.

The Ying and Yang of Ganglioside Function in Cancer

The plethora of information about the expression of cancer cell-associated gangliosides, their role(s) in signal transduction, and their potential usefulness in the development of cancer treatments makes this an appropriate time to review these enigmatic glycosphingolipids. Evidence, reflecting the work of many, indicates that (1) expression of specific gangliosides, not generally found in high concentrations in most normal human cells, can be linked to certain types of cancer. (2) Gangliosides can affect the ability of cells to interact either directly or indirectly with growth factor receptors, thereby changing such things as a cell's mobility, rate of proliferation, and metastatic ability. (3) Anti-ganglioside antibodies have been tested, with some success, as potential treatments for certain cancers. (4) Cancer-associated gangliosides shed into the circulation can (a) affect immune cell responsiveness either positively or negatively, (b) be considered as diagnostic markers, and (c) be used to look for recurrence. (5) Cancer registries enable investigators to evaluate data from sufficient numbers of patients to obtain information about potential therapies. Despite advances that have been made, a discussion of possible approaches to identifying additional treatment strategies to inhibit metastasis, responsible for the majority of deaths of cancer patients, as well as for treating therapy-resistant tumors, is included.

Inhibitors of the Sialidase NEU3 as Potential Therapeutics for Fibrosis

Fibrosing diseases are a major medical problem, and are associated with more deaths per year than cancer in the US. Sialidases are enzymes that remove the sugar sialic acid from glycoconjugates. In this review, we describe efforts to inhibit fibrosis by inhibiting sialidases, and describe the following rationale for considering sialidases to be a potential target to inhibit fibrosis. First, sialidases are upregulated in fibrotic lesions in humans and in a mouse model of pulmonary fibrosis. Second, the extracellular sialidase NEU3 appears to be both necessary and sufficient for pulmonary fibrosis in mice. Third, there exist at least three mechanistic ways in which NEU3 potentiates fibrosis, with two of them being positive feedback loops where a profibrotic cytokine upregulates NEU3, and the upregulated NEU3 then upregulates the profibrotic cytokine. Fourth, a variety of NEU3 inhibitors block pulmonary fibrosis in a mouse model. Finally, the high sialidase levels in a fibrotic lesion cause an easily observed desialylation of serum proteins, and in a mouse model, sialidase inhibitors that stop fibrosis reverse the serum protein desialylation. This then indicates that serum protein sialylation is a potential surrogate biomarker for the effect of sialidase inhibitors, which would facilitate clinical trials to test the exciting possibility that sialidase inhibitors could be used as therapeutics for fibrosis.

Sialylated glycoproteins as biomarkers and drivers of progression in prostate cancer

Sialic acids have been implicated in cancer initiation, progression, and immune evasion in diverse human malignancies. Sialylation of terminal glycans on cell surface and secreted glycoproteins is a long-recognized feature of cancer cells. Recently, immune checkpoint inhibitor immunotherapy has tremendously improved the outcomes of patients with various cancers. However, available immunotherapy approaches have had limited efficacy in metastatic castration-resistant prostate cancer. Sialic acid modified glycoproteins in prostate cancers and their interaction with Siglec receptors on tumor infiltrating immune cells might underlie immunosuppressive signaling in prostate cancer. Here, we summarize the function of sialic acids and relevant glycosynthetic enzymes in cancer initiation and progression. We also discuss the possible uses of sialic acids as biomarkers in prostate cancer and the potential methods for targeting Siglec-sialic acid interactions for prostate cancer treatment.

Aberrant Sialylation in Cancer: Therapeutic Opportunities

The surface of every eukaryotic cell is coated in a thick layer of glycans that acts as a key interface with the extracellular environment. Cancer cells have a different ‘glycan coat’ to healthy cells and aberrant glycosylation is a universal feature of cancer cells linked to all of the cancer hallmarks. This means glycans hold huge potential for the development of new diagnostic and therapeutic strategies. One key change in tumour glycosylation is increased sialylation, both on N-glycans and O-glycans, which leads to a dense forest of sialylated structures covering the cell surface. This hypersialylation has far-reaching consequences for cancer cells, and sialylated glycans are fundamental in tumour growth, metastasis, immune evasion and drug resistance. The development of strategies to inhibit aberrant sialylation in cancer represents an important opportunity to develop new therapeutics. Here, I summarise recent advances to target aberrant sialylation in cancer, including the development of sialyltransferase inhibitors and strategies to inhibit Siglecs and Selectins, and discuss opportunities for the future.

Endothelial Glycocalyx

The glycocalyx is a polysaccharide structure that protrudes from the body of a cell. It is primarily conformed of glycoproteins and proteoglycans, which provide communication, electrostatic charge, ionic buffering, permeability, and mechanosensation-mechanotransduction capabilities to cells. In blood vessels, the endothelial glycocalyx that projects into the vascular lumen separates the vascular wall from the circulating blood. Such a physical location allows a number of its components, including sialic acid, glypican-1, heparan sulfate, and hyaluronan, to participate in the mechanosensation-mechanotransduction of blood flow-dependent shear stress, which results in the synthesis of nitric oxide and flow-mediated vasodilation. The endothelial glycocalyx also participates in the regulation of vascular permeability and the modulation of inflammatory responses, including the processes of leukocyte rolling and extravasation. Its structural architecture and negative charge work to prevent macromolecules greater than approximately 70 kDa and cationic molecules from binding and flowing out of the vasculature. This also prevents the extravasation of pathogens such as bacteria and virus, as well as that of tumor cells. Due to its constant exposure to shear and circulating enzymes such as neuraminidase, heparanase, hyaluronidase, and matrix metalloproteinases, the endothelial glycocalyx is in a continuous process of degradation and renovation. A balance favoring degradation is associated with a variety of pathologies including atherosclerosis, hypertension, vascular aging, metastatic cancer, and diabetic vasculopathies. Consequently, ongoing research efforts are focused on deciphering the mechanisms that promote glycocalyx degradation or limit its syntheses, as well as on therapeutic approaches to improve glycocalyx integrity with the goal of reducing vascular disease. © 2022 American Physiological Society. Compr Physiol 12: 1-31, 2022.

Review sialidase NEU3 and its pathological significance

Sialidases (EC 3.2.1.18, also called neuraminidases) catalyze the removal of α-glycosidically linked sialic acid residues from glycoproteins and glycolipids; this is the initial step in the degradation of these glycoconjugates. Sialidases of mammalian origin have been implicated in not only lysosomal catabolism but also the modulation of functional molecules involved in many biological processes. To date, four types of mammalian sialidases have been cloned and designated as Neu1, Neu2, Neu3 and Neu4. These sialidases differ in their subcellular localization and enzymatic properties, as well as their chromosomal localization, and they are expressed in a tissue-specific manner. Among the sialidases, the plasma membrane-associated sialidase Neu3 appears to play particular roles in controlling transmembrane signaling through the modulation of gangliosides, and its aberrant expression is closely related to various pathogeneses, including that of cancer. Interestingly, the human orthologue NEU3 acts in two ways, catalytic hydrolysis of gangliosides and protein interactions with other signaling molecules. Aberrant NEU3 expression can induce various pathological conditions. This review briefly summarizes recent studies, focusing on the involvement of NEU3 in various pathological phenomena.

Sphingolipid metabolism in the development and progression of cancer: one cancer's help is another's hindrance

Cancer development is a multistep process in which cells must overcome a series of obstacles before they can become fully developed tumors. First, cells must develop the ability to proliferate unchecked. Once this is accomplished, they must be able to invade the neighboring tissue, as well as provide themselves with oxygen and nutrients. Finally, they must acquire the ability to detach from the newly formed mass in order to spread to other tissues, all the while evading an immune system that is primed for their destruction. Furthermore, increased levels of inflammation have been shown to be linked to the development of cancer, with sites of chronic inflammation being a common component of tumorigenic microenvironments. In this Review, we give an overview of the impact of sphingolipid metabolism in cancers, from initiation to metastatic dissemination, as well as discussing immune responses and resistance to treatments. We explore how sphingolipids can either help or hinder the progression of cells from a healthy phenotype to a cancerous one.

KLF9 suppresses cell growth and induces apoptosis via the AR pathway in androgen-dependent prostate cancer cells

Kruppel-like factors (KLFs) play an important role in many biological processes including cell proliferation, differentiation and development. Our study showed that the level of KLF9 is lower in PCa cell lines compared to a benign prostate cell line; the androgen-independent cell line PC3 expresses significantly lower KLF9 than the androgen-dependent cell line, LNCaP. Forced overexpression of KLF9 suppressed cell growth, colony formation, and induced cell apoptosis in LNCaP cells. We also found that KLF9 expression was induced in response to apoptosis caused by flutamide, and further addition of dihydrotestosterone antagonized the action of flutamide and significantly decreased KLF9 expression. Furthermore, activation of the androgen receptor (AR) was inhibited by the overexpression of KLF9. Our research shows that KLF9 is lower in androgen-independent cell lines than in androgen-dependent cell lines; Overexpression of KLF9 dramatically suppresses the proliferation, anchorage-independent growth, and induces apoptosis in androgen-dependent cells; KLF9 inhibition on prostate cancer cell growth may be acting through the AR pathway. Our results therefore suggest that KLF9 may play a significant role in the transition from androgen-dependent to androgen-independent prostate cancer and is a potential target of prevention and therapy.

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KLF9 is lower in androgen-independent cell lines than in androgen-dependent cell lines.

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Overexpression of KLF9 suppresses the proliferation and induces apoptosis in androgen-dependent cells.

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KLF9 inhibition on androgen-dependent Pca growth may be associated with the inhibition of AR pathway.

Serum Sialylation Changes in Actinic Keratosis and Cutaneous Squamous Cell Carcinoma Patients

Cutaneous squamous cell carcinoma (cSCC), a malignant proliferation of the cutaneous epithelium, is the second most common skin cancer after basal cell carcinoma (BCC). Unlike BCC, cSCC exhibits a greater aggressiveness and the ability to metastasize to any organ in the body. Chronic inflammation and immunosuppression are important processes linked to the development of cSCC. The tumor can occur de novo or from the histological transformation of preexisting actinic keratoses (AK). Malignant cells exhibit a higher amount of sialic acid in their membranes than normal cells, and changes in the amount, type, or linkage of sialic acid in malignant cell glycoconjugates are related to tumor progression and metastasis. The aim of our study was to investigate the sialyation in patients with cSCC and patients with AK. We have determined the serum levels of total sialic acid (TSA), lipid-bound sialic acid (LSA), beta-galactoside 2,6-sialyltransferase I (ST6GalI), and neuraminidase 3 (NEU3) in 40 patients with cSCC, 28 patients with AK, and 40 healthy subjects. Data analysis indicated a significant increase in serum levels of TSA (p < 0.001), LSA (p < 0.001), ST6GalI (p < 0.001), and NEU3 (p < 0.001) in the cSCC group compared to the control group, whereas in patients with AK only the serum level of TSA was significantly higher compared to the control group (p < 0.001). When the cSCC and AK groups were compared, significant differences between the serum levels of TSA (p < 0.001), LSA (p < 0.001), ST6GalI (p < 0.001) and NEU3 (p < 0.001) were found. The rate of synthesis of sialoglycoconjugates and their rate of enzymatic degradation, expressed by the ST6GalI/NEU3 ratio, is 1.64 times lower in the cSCC group compared to the control group (p < 0.01) and 1.53 times lower compared to the AK group (p < 0.01). The tumor diameter, depth of invasion, and Ki67 were associated with higher levels of TSA and LSA. These results indicate an aberrant sialylation in cSCC that correlates with tumor aggressiveness.

Neu3 neuraminidase induction triggers intestinal inflammation and colitis in a model of recurrent human food-poisoning

Intestinal inflammation is the underlying basis of colitis and the inflammatory bowel diseases. These syndromes originate from genetic and environmental factors that remain to be fully identified. Infections are possible disease triggers, including recurrent human food-poisoning by the common foodborne pathogen Salmonella enterica Typhimurium (ST), which in laboratory mice causes progressive intestinal inflammation leading to an enduring colitis. In this colitis model, disease onset has been linked to Toll-like receptor-4-dependent induction of intestinal neuraminidase activity, leading to the desialylation, reduced half-life, and acquired deficiency of anti-inflammatory intestinal alkaline phosphatase (IAP). Neuraminidase (Neu) inhibition protected against disease onset; however, the source and identity of the Neu enzyme(s) responsible remained unknown. Herein, we report that the mammalian Neu3 neuraminidase is responsible for intestinal IAP desialylation and deficiency. Absence of Neu3 thereby prevented the accumulation of lipopolysaccharide-phosphate and inflammatory cytokine expression in providing protection against the development of severe colitis.

Gangliosides as Signaling Regulators in Cancer

At the plasma membrane, gangliosides, a group of glycosphingolipids, are expressed along with glycosphingolipids, phospholipids, and cholesterol in so-called lipid rafts that interact with signaling receptors and related molecules. Most cancers present abnormalities in the intracellular signal transduction system involved in tumor growth, invasion, and metastasis. To date, the roles of gangliosides as regulators of signal transduction have been reported in several cancer types. Gangliosides can be expressed by the exogenous ganglioside addition, with their endogenous expression regulated at the enzymatic level by targeting specific glycosyltransferases. Accordingly, the relationship between changes in the composition of cell surface gangliosides and signal transduction has been investigated by controlling ganglioside expression. In cancer cells, several types of signaling molecules are positively or negatively regulated by ganglioside expression levels, promoting malignant properties. Moreover, antibodies against gangliosides have been shown to possess cytotoxic effects on ganglioside-expressing cancer cells. In the present review, we highlight the involvement of gangliosides in the regulation of cancer cell signaling, and we explore possible therapies targeting ganglioside-expressing cancer.

Inhibiting Sialidase-Induced TGF-β1 Activation Attenuates Pulmonary Fibrosis in Mice

The active form of transforming growth factor-β1 (TGF-β1) plays a key role in potentiating fibrosis. TGF-β1 is sequestered in an inactive state by a latency-associated glycopeptide (LAP). Sialidases (also called neuraminidases (NEU)) cleave terminal sialic acids from glycoconjugates. The sialidase NEU3 is upregulated in fibrosis, and mice lacking Neu3 show attenuated bleomycin-induced increases in active TGF-β1 in the lungs and attenuated pulmonary fibrosis. Here we observe that recombinant human NEU3 upregulates active human TGF-β1 by releasing active TGF-β1 from its latent inactive form by desialylating LAP. Based on the proposed mechanism of action of NEU3, we hypothesized that compounds with a ring structure resembling picolinic acid might be transition state analogs and thus possible NEU3 inhibitors. Some compounds in this class showed nanomolar IC₅₀ for recombinant human NEU3 releasing active human TGF-β1 from the latent inactive form. The compounds given as daily 0.1-1-mg/kg injections starting at day 10 strongly attenuated lung inflammation, lung TGF-β1 upregulation, and pulmonary fibrosis at day 21 in a mouse bleomycin model of pulmonary fibrosis. These results suggest that NEU3 participates in fibrosis by desialylating LAP and releasing TGF-β1 and that the new class of NEU3 inhibitors are potential therapeutics for fibrosis. SIGNIFICANCE STATEMENT: The extracellular sialidase NEU3 appears to be a key driver of pulmonary fibrosis. The significance of this report is that 1) we show the mechanism (NEU3 desialylates the latency-associated glycopeptide protein that keeps the profibrotic cytokine transforming growth factor-β1 (TGF-β1) in an inactive state, causing active TGF-β1 release), 2) we then use the predicted NEU3 mechanism to identify nM IC₅₀ NEU3 inhibitors, and 3) these new NEU3 inhibitors are potent therapeutics in a mouse model of pulmonary fibrosis.

Desialylation of Sonic-Hedgehog by Neu2 Inhibits Its Association with Patched1 Reducing Stemness-Like Properties in Pancreatic Cancer Sphere-forming Cells

Cancer stem cells (CSCs) are crucial regulators of tumor recurrence/progression. The maintenance of CSCs is dependent on aberrant activation of various pathways, including Hedgehog. Prevalent sialylations contribute to aggressiveness in CSCs. Here, we have addressed the role of sialylation in regulating stemness-like properties of pancreatic cancer sphere-forming cells (PCS) through modulation of the Hedgehog (Hh) pathway. The status of CD133/CD44/surface-sialylation was checked by flow cytometry and effects of Neu2 overexpression in PCS were compared using qPCR, immunoblotting, co-immunoprecipitation and also by colony-formation assays. The work was also validated in a xenograft model after Neu2 overexpression. Neu2 and Shh status in patient tissues were examined by immunohistochemistry. PCS showed higher Hh-pathway activity and sialylation with reduced cytosolic-sialidase (Neu2). Neu2 overexpression caused desialylation of Shh, thereby reducing Shh-Patched1 binding thus causing decreased Hh-pathway activity with lower expression of Snail/Slug/CyclinD1 leading to reduction of stemness-like properties. Neu2-overexpression also induced apoptosis in PCS. Additionally, Neu2-overexpressed PCS demonstrated lower mTORC2 formation and inhibitory-phosphorylation of Gsk3β, reflecting a close relationship with reduced Hh pathway. Moreover, both Neu2 and Rictor (a major component of mTORC2) co-transfection reduced stem cell markers and Hh-pathway activity in PCS. Neu2-overexpressed tumors showed reduction in tumor mass with downregulation of stem cell markers/Shh/mTOR and upregulation of Bax/Caspase8/Caspase3. Thus, we established that reduced sialylation by Neu2 overexpression leads to decreased stemness-like properties by desialylation of Shh, which impaired its association with Patched1 thereby inhibiting the Hh pathway. All these may be responsible for enhanced apoptosis in Neu2-overexpressed PCS.

Isoenzyme-Selective Inhibitors of Human Neuraminidases Reveal Distinct Effects on Cell Migration

The human neuraminidase enzymes (NEU1, NEU2, NEU3, and NEU4) are a class of enzymes implicated in pathologies including cancer and diabetes. Several reports have linked neuraminidase activity to the regulation of cell migration in cancer cells. Using an in vitro cell migration assay on fibronectin (FN) coated surfaces, we have investigated the role of these enzymes in integrin-mediated cell migration. We observed that neuraminidase inhibition caused significant retardation of cell migration in breast cancer (MDA-MB-231) and prostate cancer (PC-3) cell lines when using inhibitors of NEU3 and NEU4. In contrast, inhibition of NEU1 caused a significant increase in cell migration for the same cell lines. We concluded that the blockade of human neuraminidase enzymes with isoenzyme-selective inhibitors can lead to disparate results and has significant potential in the development of anticancer or wound healing therapeutics.

HILIC-MRM-MS for Linkage-Specific Separation of Sialylated Glycopeptides to Quantify Prostate-Specific Antigen Proteoforms

Elevated serum prostate-specific antigen (PSA) levels in body fluids may indicate prostate cancer (PCa), but it is noted that the clinical performance is rather poor. Specificity and sensitivity values of 20 and 94% at a cutoff value of 4.1 ng/mL, respectively, result in overdiagnosis and unnecessary interventions. Previous exploratory studies have indicated that the glycosylation of PSA potentially leads to improved PCa diagnosis based on qualitative analyses. However, the applied methods are not suited for a quantitative evaluation or implementation in a medical laboratory. Therefore, in this proof-of-principle study, we have evaluated the use of hydrophilic interaction liquid chromatography (HILIC) in combination with targeted quantitative mass spectrometry for the sialic acid linkage-specific analysis of PSA glyco-proteoforms based on either trypsin or ArgC peptides. The efficiency of PSA proteolysis was optimized as well as the glycopeptide separation conditions (buffer type, strength, and pH). The HILIC-based analysis of PSA glyco-proteoforms presented here has the potential for the clinical validation of patient cohorts. The method shows the feasibility of the use of a HILIC stationary phase for the separation of isomeric glycopeptides to detect specific glyco-proteoforms. This is the first step toward the development and evaluation of PSA glyco-proteoforms for use in a clinical chemistry setting aiming for improved PCa diagnosis or screening.

Comprehensive Profiling of Surface Gangliosides Extracted from Various Cell Lines by LC-MS/MS

Gangliosides act as a surface marker at the outer cellular membrane and play key roles in cancer cell invasion and metastasis. Despite the biological importance of gangliosides, they have been still poorly characterized due to the lack of effective analytical tools. Herein, we performed molecular profiling and structural elucidation of intact gangliosides in various cell lines including CFPAC1, A549, NCI-H358, MCF7, and Caski. We identified and quantified a total of 76 gangliosides on cell membrane using C18 LC-MS/MS. Gangliosides found in each cell line exhibited high complexity and diversity both qualitatively and quantitatively. The most abundant species was GM3(d34:1) in CFPAC1, NCI-H358, and MCF7, while GM2(d34:1) and GM1(d34:1) were major components in A549 and Caski, respectively. Notably, glycan moieties showed more diversity between cancer cell lines than ceramide moieties. In addition, noncancerous pancreatic cell line (hTERT/HPNE) could be distinguished by gangliosides containing different levels of sialic acid compared with cancerous pancreatic cell line (CFPAC1). These results clearly demonstrated the feasibility of our analytical platform to comprehensive profile of cell surface gangliosides for identifying cell types and subgrouping cancer cell types.

Attenuated pulmonary fibrosis in sialidase-3 knockout (Neu3-/-) mice

Pulmonary fibrosis involves the formation of inappropriate scar tissue in the lungs, but what drives fibrosis is unclear. Sialidases (also called neuraminidases) cleave terminal sialic acids from glycoconjugates. In humans and mice, pulmonary fibrosis is associated with desialylation of glycoconjugates and upregulation of sialidases. Of the four mammalian sialidases, we previously detected only NEU3 in the bronchoalveolar lavage fluid from mice with bleomycin-induced pulmonary fibrosis. In this report, we show that NEU3 upregulates extracellular accumulation of the profibrotic cytokines IL-6 and IL-1β, and IL-6 upregulates NEU3 in human peripheral blood mononuclear cells, suggesting that NEU3 may be part of a positive feedback loop potentiating fibrosis. To further elucidate the role of NEU3 in fibrosis, we used bleomycin to induce lung fibrosis in wild-type C57BL/6 and Neu3^-/- mice. At 21 days after bleomycin, compared with male and female C57BL/6 mice, male and female Neu3^-/- mice had significantly less inflammation, less upregulation of other sialidases and the profibrotic cytokine active transforming growth factor β1, and less fibrosis in the lungs. Our results suggest that NEU3 participates in fibrosis and that NEU3 could be a target to develop treatments for fibrosis.

Deregulation of sialidases in human normal and tumor tissues

BACKGROUND

Aberrant sialylation is a characteristic feature associated with cancer. The four types of mammalian sialidases identified to date have been shown to behave in different manners during carcinogenesis. While NEU1, NEU2 and NEU4 have been observed to oppose malignant phenotypes, the membrane-bound sialidase NEU3 was revealed to promote cancer progression.

OBJECTIVES

With the aim of improving the knowledge about sialidases deregulation in various cancer types, we investigated the amount of NEU1, NEU3 and NEU4 transcripts in paired normal and tumor tissues from 170 patients with 11 cancer types.

METHODS

mRNA was extracted from patients' tissue specimens and retrotranscribed into cDNA, which was quantified by Real-Time PCR.

RESULTS

We found NEU1 and NEU3 to be up regulated, while NEU4 was down regulated in most cancer types. In particular, colorectal cancer tissues showed the highest increase in NEU3 expression. Both NEU1 and NEU3 showed a strong up-regulation in ovarian cancer.

CONCLUSIONS

Our data show that human sialidases are expressed at different levels in healthy tissues and are strongly deregulated in tumors. Moreover, sialidases expression in our European cohort showed significant differences from Asian populations. Some of these peculiar features open potential applications of sialidases in cancer diagnosis and therapy.

Serum sialylation changes in cancer

Cancer is a major cause of death in both developing and developed countries. Early detection and efficient therapy can greatly enhance survival. Aberrant glycosylation has been recognized to be one of the hallmarks of cancer as glycans participate in many cancer-associated events. Cancer-associated glycosylation changes often involve sialic acids which play important roles in cell-cell interaction, recognition and immunological response. This review aims at giving a comprehensive overview of the literature on changes of sialylation in serum of cancer patients. Furthermore, the methods available to measure serum and plasma sialic acids as well as possible underlying biochemical mechanisms involved in the serum sialylation changes are surveyed. In general, total serum sialylation levels appear to be increased with various malignancies and show a potential for clinical applications, especially for disease monitoring and prognosis. In addition to overall sialic acid levels and the amount of sialic acid per total protein, glycoprofiling of specific cancer-associated glycoproteins, acute phase proteins and immunoglobulins in serum as well as the measurements of sialylation-related enzymes such as sialidases and sialyltransferases have been reported for early detection of cancer, assessing cancer progression and improving prognosis of cancer patients. Moreover, sialic-acid containing glycan antigens such as CA19-9, sialyl Lewis X and sialyl Tn on serum proteins have also displayed their value in cancer diagnosis and management whereby increased levels of these factors positively correlated with metastasis or poor prognosis.

Recurrent infection progressively disables host protection against intestinal inflammation

Intestinal inflammation is the central pathological feature of colitis and the inflammatory bowel diseases. These syndromes arise from unidentified environmental factors. We found that recurrent nonlethal gastric infections of Gram-negative Salmonella enterica Typhimurium (ST), a major source of human food poisoning, caused inflammation of murine intestinal tissue, predominantly the colon, which persisted after pathogen clearance and irreversibly escalated in severity with repeated infections. ST progressively disabled a host mechanism of protection by inducing endogenous neuraminidase activity, which accelerated the molecular aging and clearance of intestinal alkaline phosphatase (IAP). Disease was linked to a Toll-like receptor 4 (TLR4)-dependent mechanism of IAP desialylation with accumulation of the IAP substrate and TLR4 ligand, lipopolysaccharide-phosphate. The administration of IAP or the antiviral neuraminidase inhibitor zanamivir was therapeutic by maintaining IAP abundance and function.

Sialidase NEU3 defines invasive potential of human glioblastoma cells by regulating calpain-mediated proteolysis of focal adhesion proteins

BACKGROUND

Glioblastoma multiforme is one of the most malignant tumors of the human central nervous system characterized by high degree of invasiveness. Focusing on this invasive nature, we investigated whether ganglioside-specific sialidase NEU3 might be involved, because gangliosides are major components of brain tissues, and cell surface sialic acids, as target residues of sialidase catalysis, are thought to be closely related to cell invasion.

METHODS

NEU3 mRNA levels of human glioblastoma specimens were evaluated by quantitative RT-PCR. Human glioblastoma cell lines, U251, A172, and T98G were used for cell invasion and migration by transwell and cell scratching assay. The molecules involved in the signaling cascade were investigated by western blot and immunofluorescent microscopy.

RESULTS

NEU3 expression was down-regulated in human glioblastoma specimens. In the human glioblastoma cell lines, NEU3 overexpression reduced invasion and migration by promoting the assembly of focal adhesions through reduced calpain-dependent proteolysis, but NEU3 silencing resulted in accelerating cell invasion via disassembly of focal adhesions. In NEU3-silenced cells, elevation of calpain activity and GM3 accumulation were observed, as results of reduced sialidase hydrolysis, localization of calpain and GM3 at the cell lamellipodium being demonstrated by immunofluorescence microscopy.

CONCLUSION

Sialidase NEU3 was found to exert a great influence on cell invasion in regulation of calpain activity and focal adhesion disassembly and consequent invasive potential of glioblastoma cells.

GENERAL SIGNIFICANCE

This first demonstration of sialidase involvement in invasive potential of gliolastoma cells may point to NEU3 as an attractive treatment target of human gliomas.

Integrin-mediated cell migration is blocked by inhibitors of human neuraminidase

Integrins are critical receptors in cell migration and adhesion. A number of mechanisms are known to regulate the function of integrins, including phosphorylation, conformational change, and cytoskeletal anchoring. We investigated whether native neuraminidase (Neu, or sialidase) enzymes which modify glycolipids could play a role in regulating integrin-mediated cell migration. Using a scratch assay, we found that exogenously added Neu3 and Neu4 activity altered rates of cell migration. We observed that Neu4 increased the rate of migration in two cell lines (HeLa, A549); while Neu3 only increased migration in HeLa cells. A bacterial neuraminidase was able to increase the rate of migration in HeLa, but not in A549 cells. Treatment of cells with complex gangliosides (GM1, GD1a, GD1b, and GT1b) resulted in decreased cell migration rates, while LacCer was able to increase rates of migration in both lines. Importantly, our results show that treatment of cells with inhibitors of native Neu enzymes had a dramatic effect on the rates of cell migration. The most potent compound tested targeted the human Neu4 isoenzyme, and was able to substantially reduce the rate of cell migration. We found that the lateral mobility of integrins was reduced by treatment of cells with Neu3, suggesting that Neu3 enzyme activity resulted in changes to integrin-co-receptor or integrin-cytoskeleton interactions. Finally, our results support the hypothesis that inhibitors of human Neu can be used to investigate mechanisms of cell migration and for the development of anti-adhesive therapies.

NEU3 inhibitory effect of naringin suppresses cancer cell growth by attenuation of EGFR signaling through GM3 ganglioside accumulation

Naringin, which is one of the flavonoids contained in citrus fruits, is well known to possess various healthy functions to humans. It has been reported that naringin suppresses cancer cell growth in vitro and in vivo, although the underlying mechanisms are not fully understood. Recently, the roles of glycoconjugates, such as gangliosides, in cancer cells have been focused because of their regulatory effects of malignant phenotypes. Here, to clarify the roles of naringin in the negative-regulation of cancer cell growth, the alteration of glycoconjugates induced by naringin exposure and its significance on cell signaling were investigated. Human cancer cells, HeLa and A549, were exposed to various concentrations of naringin. Naringin treatment induced the suppression of cell growth toward HeLa and A549 cells accompanied with an increase of apoptotic cells. In naringin-exposed cells, GM3 ganglioside was drastically increased compared to the GM3 content prior to the treatment. Furthermore, naringin inhibited NEU3 sialidase, a GM3 degrading glycosidase. Similarly, NEU3 inhibition activities were also detected by other flavanone, such as hesperidin and neohesperidin dihydrocalcone, but their aglycones showed less inhibitions. Naringin-treated cancer cells showed suppressed EGFR and ERK phosphorylation levels. These results suggest a novel mechanism of naringin in the suppression of cancer cell growth through the alteration of glycolipids. NEU3 inhibitory effect of naringin induced GM3 accumulation in HeLa and A549 cells, leading the attenuation of EGFR/ERK signaling accompanied with a decrease in cell growth.

NEU3 Sialidase Protein Interactors in the Plasma Membrane and in the Endosomes

NEU3 sialidase has been shown to be a key player in many physio- and pathological processes, including cell differentiation, cellular response to hypoxic stress, and carcinogenesis. The enzyme, peculiarly localized on the outer leaflet of the plasma membrane, has been shown to be able to remove sialic acid residues from the gangliosides present on adjacent cells, thus creating cell to cell interactions. Nonetheless, herein we report that the enzyme localization is dynamically regulated between the plasma membrane and the endosomes, where a substantial amount of NEU3 is stored with low enzymatic activity. However, under opportune stimuli, NEU3 is shifted from the endosomes to the plasma membrane, where it greatly increases the sialidase activity. Finally, we found that NEU3 possesses also the ability to interact with specific proteins, many of which are different in each cell compartment. They were identified by mass spectrometry, and some selected ones were also confirmed by cross-immunoprecipitation with the enzyme, supporting NEU3 involvement in the cell stress response, protein folding, and intracellular trafficking.

Upregulation of sialidase NEU3 in head and neck squamous cell carcinoma associated with lymph node metastasis

Regional lymph node metastasis in head and neck squamous cell carcinoma (HNSCC) is a crucial event for its progression, associated with a high rate of mortality. Sialidase, a key enzyme for the regulation of cellular sialic acids through catalyzing the initial step of degradation of glycoproteins and glycolipids, has been implicated in cancer progression. To facilitate the development of novel treatments for HNSCC, we have investigated whether sialidase is involved in the progression of this cancer. We found plasma membrane‐associated sialidase (NEU3) to be significantly upregulated in tumor compared to non‐tumor tissues; particularly, an increase in its mRNA levels was significantly associated with lymph node metastasis. To understand the mechanisms, we analyzed the NEU3‐mediated effects on the malignant phenotype using squamous carcinoma HSC‐2 and SAS cells. NEU3 promoted cell motility and invasion, accompanied by the increased expression of MMP‐9, whereas NEU3 silencing or the activity‐null mutant did not. NEU3 enhanced phosphorylation of epidermal growth factor receptor (EGFR), and an EGFR inhibitor, AG1478, abrogated the NEU3‐induced MMP9 augmentation. These findings identify NEU3 as a participant in HNSCC progression through the regulation of EGFR signaling and thus as a potential target for inhibiting EGFR‐mediated tumor progression.

Sialidase NEU3 contributes neoplastic potential on colon cancer cells as a key modulator of gangliosides by regulating Wnt signaling

The plasma membrane-associated sialidase NEU3 is a key enzyme for ganglioside degradation. We previously demonstrated remarkable up-regulation of NEU3 in various human cancers, with augmented malignant properties. Here, we provide evidence of a close link between NEU3 expression and Wnt/β-catenin signaling in colon cancer cells by analyzing tumorigenic potential and cancer stem-like characteristics. NEU3 silencing in HT-29 and HCT116 colon cancer cells resulted in significant decrease in clonogenicity on soft agar and in vivo tumor growth, along with down-regulation of stemness and Wnt-related genes. Analyses further revealed that NEU3 enhanced phosphorylation of the Wnt receptor LRP6 and consequently β-catenin activation by accelerating complex formation with LRP6 and recruitment of GSK3β and Axin, whereas its silencing exerted the opposite effects. NEU3 activity-null mutants failed to demonstrate the activation, indicating the requirement of ganglioside modulation by the sialidase for the effects. Under sphere-forming conditions, when stemness genes are up-regulated, endogenous NEU3 expression was found to be significantly increased, whereas NEU3 silencing suppressed sphere-formation and in vivo tumor incidence in NOD-SCID mice. Increased ability of clonogenicity on soft agar and sphere formation by Wnt stimulation was abrogated by NEU3 silencing. Furthermore, NEU3 was found to regulate phosphorylation of ERK and Akt via EGF receptor and Ras cascades, thought to be additionally required for tumor progression. The results indicate an essential contribution of NEU3 to tumorigenic potential through maintenance of stem-like characteristics of colon cancer cells by regulating Wnt signaling at the receptor level, in addition to tumor progression via Ras/MAPK signaling.

Potentiation of epidermal growth factor-mediated oncogenic transformation by sialidase NEU3 leading to Src activation

We previously demonstrated that sialidase NEU3, a key glycosidase for ganglioside degradation, is up-regulated in various human cancers, leading to increased cell invasion, motility and survival of cancer cells possibly through activation of EGF signaling. Its up-regulation is also important for promotion of the stage of colorectal carcinogenesis in vivo in human NEU3 transgenic mice treated with azoxymethane for the induction of aberrant crypt foci in the colon mucosa, accompanied by enhanced phosphorylation of EGF receptor (EGFR). To address whether the activation of EGF signaling by the sialidase is associated with oncogenic transformation, we here analyzed the effects of overexpression of NEU3 and EGFR in NIH-3T3 cells. When NEU3 was stably transfected with or without EGFR, it was associated with significant increases in clonogenic growth, clonogenicity on soft agar and in vivo tumor growth in nude mice either with or without the receptor overexpression in the presence of EGF, compared with the levels in their vector controls. Despite the fact that the endogenous level of EGFR is known to be extremely low in these cells, NEU3 significantly enhanced the phosphorylation of Akt and ERK, as well as that of the receptor. The NEU3-mediated activation was largely abrogated by the EGFR inhibitor AG1478 or PD153035, but significant clonogenic growth still remained. NEU3 was then found to activate Src kinase, and the clonogenicity was completely suppressed by an Src inhibitor, PP2. The activity-null mutants failed to activate Src and EGFR, indicating that ganglioside modulation by NEU3 may be necessary for the activation. NEU3 and Src were co-immunoprecipitated with EGFR in NEU3- and EGFR- transfected cells. These findings identify NEU3 as an essential participant in tumorigenesis through the EGFR/Src signaling pathway and a potential target for inhibiting EGFR-mediated tumor progression.

Increased sialidase activity in serum of cancer patients: Identification of sialidase and inhibitor activities in human serum

Aberrant sialylation in glycoproteins and glycolipids is a characteristic feature of malignancy. Human sialidases, which catalyze the removal of sialic acid residues from glycoconjugates, have been implicated in cancer progression. They have been detected in a wide variety of human cells and tissues, but few studies have focused on their existence in human serum. Among the four types identified to date, we previously demonstrated that plasma membrane-associated ganglioside sialidase (NEU3) is markedly upregulated in various human cancers, including examples in the colon and prostate. Here, using a sensitive assay method, we found a significant increase of sialidase activity in the serum of patients with prostate cancer compared with that in healthy subjects having low activity, if any. Activity was apparent with gangliosides as substrates, but only to a very limited extent with 4-methylumbelliferyl sialic acid, a good synthetic substrate for sialidases other than human NEU3. The serum sialidase was also almost entirely immunoprecipitated with anti-NEU3 antibody, but not with antibodies for other sialidases. Interestingly, sera additionally contained inhibitory activity against the sialidase and also against recombinant human NEU3. The sialidase and inhibitor activities could be separated by exosome isolation and by hydrophobic column chromatography. The serum sialidase was assessed by a sandwich ELISA method using two anti-NEU3 antibodies. The results provide strong evidence that the serum sialidase is, in fact, NEU3, and this subtype may, therefore, be a potential utility for novel diagnosis of human cancers.

Phosphatidic acid-mediated activation and translocation to the cell surface of sialidase NEU3, promoting signaling for cell migration

The plasma membrane-associated sialidase NEU3 plays crucial roles in regulation of transmembrane signaling, and its aberrant up-regulation in various cancers contributes to malignancy. However, it remains uncertain how NEU3 is naturally activated and locates to plasma membranes, because of its Triton X-100 requirement for the sialidase activity in vitro and its often changing subcellular location. Among phospholipids examined, we demonstrate that phosphatidic acid (PA) elevates its sialidase activity 4 to 5 times at 50 μM in vitro at neutral pH and promotes translocation to the cell surface and cell migration through Ras-signaling in HeLa and COS-1 cells. NEU3 was found to interact selectively with PA as assessed by phospholipid array, liposome coprecipitation, and ELISA assays and to colocalize with phospholipase D (PLD) 1 in response to epidermal growth factor (EGF) or serum stimulation. Studies using tagged NEU3 fragments with point mutations identified PA- and calmodulin (CaM)-binding sites around the N terminus and confirmed its participation in translocation and catalytic activity. EGF induced PLD1 activation concomitantly with enhanced NEU3 translocation to the cell surface, as assessed by confocal microscopy. These results suggest that interactions of NEU3 with PA produced by PLD1 are important for regulation of transmembrane signaling, this aberrant acceleration probably promoting malignancy in cancers.

Sphingolipids: key regulators of apoptosis and pivotal players in cancer drug resistance

Drug resistance elicited by cancer cells still constitutes a huge problem that frequently impairs the efficacy of both conventional and novel molecular therapies. Chemotherapy usually acts to induce apoptosis in cancer cells; therefore, the investigation of apoptosis control and of the mechanisms used by cancer cells to evade apoptosis could be translated in an improvement of therapies. Among many tools acquired by cancer cells to this end, the de-regulated synthesis and metabolism of sphingolipids have been well documented. Sphingolipids are known to play many structural and signalling roles in cells, as they are involved in the control of growth, survival, adhesion, and motility. In particular, in order to increase survival, cancer cells: (a) counteract the accumulation of ceramide that is endowed with pro-apoptotic potential and is induced by many drugs; (b) increase the synthesis of sphingosine-1-phosphate and glucosylceramide that are pro-survivals signals; (c) modify the synthesis and the metabolism of complex glycosphingolipids, particularly increasing the levels of modified species of gangliosides such as 9-O acetylated GD3 (αNeu5Ac(2-8)αNeu5Ac(2-3)βGal(1-4)βGlc(1-1)Cer) or N-glycolyl GM3 (αNeu5Ac (2-3)βGal(1-4)βGlc(1-1)Cer) and de-N-acetyl GM3 (NeuNH(2)βGal(1-4)βGlc(1-1)Cer) endowed with anti-apoptotic roles and of globoside Gb3 related to a higher expression of the multidrug resistance gene MDR1. In light of this evidence, the employment of chemical or genetic approaches specifically targeting sphingolipid dysregulations appears a promising tool for the improvement of current chemotherapy efficacy.

The epithelial calcium channel TRPV5 is regulated differentially by klotho and sialidase

The transient receptor potential vanilloid type 5 (TRPV5) Ca(2+) channel facilitates transcellular Ca(2+) transport in the distal convoluted tubule (DCT) of the kidney. The channel is glycosylated with a complex type N-glycan and it has been postulated that hydrolysis of the terminal sialic acid(s) stimulate TRPV5 activity. The present study delineates the role of the N-glycan in TRPV5 activity using biochemical assays in Human Embryonic Kidney 293 cells expressing TRPV5, isoelectric focusing and total internal reflection fluorescent microscopy. The anti-aging hormone klotho and other glycosidases stimulate TRPV5-dependent Ca(2+) uptake. Klotho was found to increase the plasma membrane stability of TRPV5, via the TRPV5 N-glycan. Sialidase mimicked this stimulatory action. However, this effect was independent of the N-glycosylation state of TRPV5, since the N-glycosylation mutant (TRPV5(N358Q)) was activated to the same extent. We showed that the increased TRPV5 activity after sialidase treatment is caused by inhibition of lipid raft-mediated internalization. In addition, sialidase modified the N-glycan of transferrin, a model glycoprotein, differently from klotho. Previous studies showed that after klotho treatment, galectin-1 binds the TRPV5 N-glycan and thereby increases TRPV5 activity. However, galectin-3, but not galectin-1, was expressed in the DCT. Furthermore, an increase in TRPV5-mediated Ca(2+) uptake was detected after galectin-3 treatment. In conclusion, two distinct TRPV5 stimulatory mechanisms were demonstrated; a klotho-mediated effect that is dependent on the N-glycan of TRPV5 and a sialidase-mediated stimulation that is lipid raft-dependent and independent of the N-glycan of TRPV5.

Identification and characterization of sialidase-like activity in the developmental stages of Amblyomma variegatum

Amblyomma variegatum F. are obligate hematophagous ectoparasites of livestock that serve as the vectors of Ehrlichia ruminantium (formerly known as Cowdria ruminantium), the causative agent of heartwater disease. In the light of the fact that they are blood-feeding, their salivary glands play prominent role in their acquisition of nutrients from the bloodmeal. Sialic acids are a major component of glycoprotein in mammalian blood fluid and cells. Sialome of hard ticks is still sparse. Here, for the first time, the possible expression of sialidase in A. variegatum was investigated. Our finding established the presence of type II sialidase-like activity in the three stages (larva, nymph, and adult) of the fed and unfed tick. There was no statistically significant difference in sialidase activity in the various stages of this ectoparasite (P > 0.05). The enzyme was purified by combination of salting out and ion exchange chromatography on DEAE--cellulose and hydroxylapatite columns. Characterization of the enzyme revealed that it is optimally active at 40 degrees C and pH 5.5, and is activated by bivalent cations Zn2+ or Fe2+. The enzyme has a Km of 0.023 mM and Vmax of 0.16 millimol/min with Fetuin as the substrate. To assess the susceptibility of some mammalian cells to the tick sialidase, we prepared erythrocyte ghost cells from different animals, which were incubated with the enzyme. Results revealed that the ruminant cells were better substrates. Our work and findings contribute to the preliminary characterization of the A. variegatum salivary proteome, and may pave way to the development of new acaricides.

Cell surface associated glycohydrolases in normal and Gaucher disease fibroblasts

Gaucher disease (GD) is the most common lysosomal disorder and is caused by an inherited autosomal recessive deficiency in β-glucocerebrosidase. This enzyme, like other glycohydrolases involved in glycosphingolipid (GSL) metabolism, is present in both plasma membrane (PM) and intracellular fractions. We analyzed the activities of CBE-sensitive β-glucosidase (GBA1) and AMP-DNM-sensitive β-glucosidase (GBA2) in total cell lysates and PM of human fibroblast cell lines from control (normal) subjects and from patients with GD clinical types 1, 2, and 3. GBA1 activities in both total lysate and PM of GD fibroblasts were low, and their relative percentages were similar to those of control cells. In contrast, GBA2 activities were higher in GD cells than in control cells, and the degree of increase differed among the three GD types. The increase of GBA2 enzyme activity was correlated with increased expression of GBA2 protein as evaluated by QRT-PCR. Activities of β-galactosidase and β-hexosaminidase in PM were significantly higher for GD cells than for control cells and also showed significant differences among the three GD types, suggesting the occurrence of cross-talk among the enzymes involved in GSL metabolism. Our findings indicate that the profiles of glycohydrolase activities in PM may provide a valuable tool to refine the classification of GD into distinct clinical types.

Disease progression mediated by egr-1 associated signaling in response to oxidative stress

When cellular reducing enzymes fail to shield the cell from increased amounts of reactive oxygen species (ROS), oxidative stress arises. The redox state is misbalanced, DNA and proteins are damaged and cellular transcription networks are activated. This condition can lead to the initiation and/or to the progression of atherosclerosis, tumors or pulmonary hypertension; diseases that are decisively furthered by the presence of oxidizing agents. Redox sensitive genes, like the zinc finger transcription factor early growth response 1 (Egr-1), play a pivotal role in the pathophysiology of these diseases. Apart from inducing apoptosis, signaling partners like the MEK/ERK pathway or the protein kinase C (PKC) can activate salvage programs such as cell proliferation that do not ameliorate, but rather worsen their outcome. Here, we review the currently available data on Egr-1 related signal transduction cascades in response to oxidative stress in the progression of epidemiologically significant diseases. Knowing the molecular pathways behind the pathology will greatly enhance our ability to identify possible targets for the development of new therapeutic strategies.

Reduced susceptibility to colitis-associated colon carcinogenesis in mice lacking plasma membrane-associated sialidase

Sialic acids are acidic monosaccharides that bind to the sugar chains of glycoconjugates and change their conformation, intermolecular interactions, and/or half-life. Thus, sialidases are believed to modulate the function of sialoglycoconjugates by desialylation. We previously reported that the membrane-associated mammalian sialidase NEU3, which preferentially acts on gangliosides, is involved in cell differentiation, motility, and tumorigenesis. The NEU3 gene expression is aberrantly elevated in several human cancers, including colon, renal, prostate, and ovarian cancers. The small interfering RNA-mediated knock-down of NEU3 in cancer cell lines, but not in normal cell-derived primary cultures, downregulates EGFR signaling and induces apoptosis. Here, to investigate the physiological role of NEU3 in tumorigenesis, we established Neu3-deficient mice and then subjected them to carcinogen-induced tumorigenesis, using a sporadic and a colitis-associated colon cancer models. The Neu3-deficient mice showed no conspicuous accumulation of gangliosides in the brain or colon mucosa, or overt abnormalities in their growth, development, behavior, or fertility. In dimethylhydrazine-induced colon carcinogenesis, there were no differences in the incidence or growth of tumors between the Neu3-deficient and wild-type mice. On the other hand, the Neu3-deficient mice were less susceptible than wild-type mice to the colitis-associated colon carcinogenesis induced by azoxymethane and dextran sodium sulfate. These results suggest that NEU3 plays an important role in inflammation-dependent tumor development.

Sialidase significance for cancer progression

Aberrant glycosylation is a characteristic feature of cancer cells. In particular, altered sialylation is closely associated with malignant properties, including invasiveness and metastatic potential. To elucidate the molecular mechanisms underlying the aberrancy, our studies have focused on mammalian sialidase, which catalyzes the removal of sialic acid residues from glycoproteins and glycolipids. The four types of mammalian sialidase identified to date show altered expression and behave in different manners during carcinogenesis. The present review briefly summarizes results on altered expression of sialidases and their possible roles in cancer progression. These enzymes are indeed factors defining cancer malignancy and thus potential targets for cancer diagnosis and therapy.

Ionizing radiations increase the activity of the cell surface glycohydrolases and the plasma membrane ceramide content

We detected significant levels of β-glucosidase, β-galactosidase, sialidase Neu3 and sphingomyelinase activities associated with the plasma membrane of fibroblasts from normal and Niemann-Pick subjects and of cells from breast, ovary, colon and neuroblastoma tumors in culture. All of the cells subjected to ionizing radiations showed an increase of the activity of plasma membrane β-glucosidase, β-galactosidase and sialidase Neu3, in addition of the well known increase of activity of plasma membrane sphingomyelinase, under similar conditions. Human breast cancer cell line T47D was studied in detail. In these cells the increase of activity of β-glucosidase and β-galactosidase was parallel to the increase of irradiation dose up to 60 Gy and continued with time, at least up to 72 h from irradiation. β-glucosidase increased up to 17 times and β-galactosidase up to 40 times with respect to control. Sialidase Neu3 and sphingomyelinase increased about 2 times at a dose of 20 Gy but no further significant differences were observed with increase of radiation dose and time. After irradiation, we observed a reduction of cell proliferation, an increase of apoptotic cell death and an increase of plasma membrane ceramide up to 3 times, with respect to control cells. Tritiated GM3 ganglioside has been administered to T47D cells under conditions that prevented the lysosomal catabolism. GM3 became component of the plasma membranes and was transformed into LacCer, GlcCer and ceramide. The quantity of ceramide produced in irradiated cells was about two times that of control cells.

Mammalian sialidases: physiological and pathological roles in cellular functions

Sialic acids are terminal acidic monosaccharides, which influence the chemical and biological features of glycoconjugates. Their removal catalyzed by a sialidase modulates various biological processes through change in conformation and creation or loss of binding sites of functional molecules. Sialidases exist widely in vertebrates and also in a variety of microorganisms. Recent research on mammalian sialidases has provided evidence for great importance of these enzymes in various cellular functions, including lysosomal catabolism, whereas microbial sialidases appear to play roles limited to nutrition and pathogenesis. Four types of mammalian sialidases have been identified and characterized to date, designated as NEU1, NEU2, NEU3 and NEU4. They are encoded by different genes and differ in major subcellular localization and enzymatic properties including substrate specificity, and each has been found to play a unique role depending on its particular properties. This review is an attempt to concisely summarize current knowledge concerning mammalian sialidases, with a special focus on their properties and physiological and pathological roles in cellular functions.