Neu1 sialidase interacts with perilipin 1 on lipid droplets and inhibits lipolysis in 3T3-L1 adipocytes.

Fatty acids are stored within adipocytes in lipid droplets (LDs) as triacylglycerol (TG), which is converted to free fatty acid (FFA) and glycerol via lipolysis. Increased plasma FFA levels in obesity are associated with several clinical conditions. We previously found that Neu1 activity is aberrant in the epididymal fat and liver of obese and diabetic mice. Here, we examined involvement of Neu1 in lipolysis in 3T3-L1 adipocytes. Small interfering RNA against Neu1 was introduced into adipocytes, and glycerol concentrations were measured in the culture medium. We then assessed the effects of Neu1 knockdown on lipolytic protein expression and phosphorylation, as well as interactions between perilipin 1 (Plin1) and hormone-sensitive lipase (HSL) after isoproterenol (IS) stimulation. Interactions between Neu1 and Plin1 were analyzed by immunoprecipitation and immunofluorescent imaging using adipocytes transfected with pCMV6-mNeu1-myc-DYKDDDDK (mNeu1DDK). Neu1 knockdown increased glycerol concentrations in culture media and Plin1 phosphorylation in whole lysates of IS-stimulated cells. Neu1 knockdown increased interaction between Plin1 and HSL after IS stimulation whereas that between Neu1 and Plin1 on LD observed under basal conditions was lost. These results suggest that Neu1 inhibits lipolysis induced by ß-adrenergic stimulation in adipocytes via interactions with Plin1 on LD.

NEU1 sialidase controls gene expression and secretion of IL-6 and MCP-1 through NF-κB pathway in 3T3-L1 adipocytes.

A sialidase NEU1 that removes sialic acids from glycoconjugates has been implicated in diverse cellular functions. Aberrant NEU1 activity is associated with various pathologies including lysosomal storage disorder sialidosis, autoimmune diseases and the malignancy and metastasis of cancer cells. We recently reported that NEU1 activity increases during 3T3-L1 adipogenesis and that it is higher in the epididymal fat of obese and diabetic mice. However, the precise functions of NEU1 in adipocytes have not been elucidated. Knockdown of NEU1 using siRNA transfection in 3T3-L1 adipocytes significantly decreased the mRNA expression and protein secretion of IL-6 and MCP-1 induced by LPS. The promoter activities of both IL-6 and MCP-1 as well as nuclear factor-kappa B (NF-κB) nuclear translocation were reduced in adipocytes transfected with an siRNA sequence that targets NEU1(siNEU1). NEU1 suppression using siNEU1 affected TLR4 sialylation. These findings suggest that NEU1 is involved in the production of IL-6 and MCP-1 in adipocytes possibly through TLR4/NF-κB signalling.

Acidic sialidase activity is aberrant in obese and diabetic mice.

Mammalian sialidases (NEU1, NEU2, NEU3 and NEU4) that remove sialic acids from glycoconjugates have been implicated in diverse cellular functions. Human sialidases are involved in the development of various disease states such as cancer, diabetes and arteriosclerosis. Unregulated acidic sialidase NEU1 activity is associated with the pathogenesis of lysosomal storage disorder (LSD) sialidosis, abnormal immune responses and cancer progression. Obesity is closely related to several chronic diseases such as diabetes, cardiovascular diseases, hyperlipidemia or hypertension that are associated with metabolic syndrome. We examined fluctuations in mRNA levels and sialidase activities of NEU1 in two strains of obese and diabetic mice to assess the involvement of NEU1 in obesity. The activity of NEU1 was preferentially higher in epididymal fat and lower in the livers of two strains of obese and diabetic mice. Fluctuations in NEU1 activity might be associated with the pathological status of these tissues in obesity.

Involvement of the cannabinoid CB2 receptor and its endogenous ligand 2-arachidonoylglycerol in oxazolone-induced contact dermatitis in mice.

The possible involvement of 2-arachidonoylglycerol (2-AG), an endogenous ligand for the cannabinoid receptors (CB1 and CB2), in contact dermatitis in mouse ear was investigated. We found that the level of 2-AG was markedly elevated in the ear following a challenge with oxazolone in sensitized mice. Of note, the swelling following the challenge was suppressed by either the administration of SR144528, a CB2 receptor antagonist, immediately after sensitization, or the administration of SR144528 upon the challenge. The effect of AM251, a CB1 receptor antagonist, was marginal in either case. It seems apparent, therefore, that the CB2 receptor and its endogenous ligand 2-AG are closely involved in both the sensitization phase and the elicitation phase of oxazolone-induced contact dermatitis. In line with this, we found that Langerhans cells (MHC class II(+)) contain a substantial amount of CB2 receptor mRNA, whereas keratinocytes (MHC class II(-)) do not. We also obtained evidence that the expression of mRNAs for proinflammatory cytokines following a challenge with oxazolone was markedly suppressed by treatment with SR144528. We next examined whether the CB2 receptor and 2-AG participate in chronic contact dermatitis accompanied by the infiltration of tissues by eosinophils. The amount of 2-AG in mouse ear dramatically increased following repeated challenge with oxazolone. Importantly, treatment with SR144528 attenuated both the recruitment of eosinophils and ear swelling in chronic contact dermatitis induced by repeated challenge with oxazolone. These results strongly suggest that the CB2 receptor and 2-AG play important stimulative roles in the sensitization, elicitation, and exacerbation of allergic inflammation.