The influence of sex steroid hormones on ganglioside biosynthesis in rat kidney

Associations between metabolomic-identified changes of biomarkers and arterial stiffness in subjects progressing to impaired fasting glucose

OBJECTIVE

We investigated correlations between age-related changes in circulating metabolites and arterial stiffness in impaired fasting glucose (IFG).

DESIGN, SUBJECTS, MEASUREMENT

This prospective cohort study included 602 healthy, normal fasting glucose (NFG) subjects (30-65 years old) who underwent triennial medical evaluation. After 3 years, 9·3% of subjects developed IFG (n = 56). Age, gender, BMI and fasting glucose were used to match the remaining NFG subjects (n = 546) that were included for the control group (NFG group, n = 80).

RESULTS

After 3 years, levels of fasting glucose, insulin and malondialdehyde, and brachial-ankle pulse wave velocity (baPWV) were significantly greater in the IFG group than in the NFG group after adjusting for baseline values. The IFG group had a greater increase in lactosylceramide (P = 0·001, q < 0·05) and a greater reduction in phosphatidylcholine (PC) (18:0/20:4) than the NFG group. Multiple linear regression analysis showed that the change in baPWV was independently and positively associated with changes in fasting glucose and lactosylceramide. In all subjects, lactosylceramide levels positively correlated with changes in baPWV and fasting glucose, while premenopausal women were not shown, and negatively correlated with changes in PC and LDL particle size.

CONCLUSIONS

This study indicates that age-related increase in circulating lactosylceramide is an independent predictor of increased arterial stiffness in subjects with impaired fasting glucose.

β4GalT6 is involved in the synthesis of lactosylceramide with less intensity than β4GalT5

Glycosphingolipids are expressed on the cell membrane and act as important factors in various events that occur across the plasma membrane. Lactosylceramide (LacCer) is synthesized from glucosylceramide and is a common precursor of various glycosphingolipids existing in whole body. Based on the enzyme purification, β1,4-galactosyltransferase 6 (B4galt6) cDNA was isolated as a LacCer synthase-coding gene in the rat brain. We generated B4galt6 gene knockout (KO) mice and analyzed their phenotypes to examine roles of β4GalT6. B4galt6 KO mice were born and grew up apparently normal. LacCer synthase activity and the composition of acidic glycosphingolipids in the brain were almost equivalent or minimally different between wild-type and KO mice. Studies by mouse embryonic fibroblasts (MEFs) revealed that the silencing of B4galt5 gene resulted in the marked reduction in LacCer synthase activity and this reduction was more severe in MEFs derived from B4galt6 KO mice than those from wild-type mice. These results suggested that β4GalT6 plays a role as a LacCer synthase, whereas β4GalT5 acts as a main enzyme for LacCer biosynthesis in these tissues and cells.

The interplay between bioactive sphingolipids and steroid hormones

Steroid hormones regulate various physiological processes including development, reproduction, and metabolism. These regulatory molecules are synthesized from cholesterol in endocrine organs - such as the adrenal glands and gonads - via a multi-step enzymatic process that is catalyzed by the cytochrome P450 superfamily of monooxygenases and hydroxysteroid dehydrogenases. Steroidogenesis is induced by trophic peptide hormones primarily via the activation of a cAMP/protein kinase A (PKA)-dependent pathway. However, other signaling molecules, including cytokines and growth factors, control the steroid hormone biosynthetic pathway. More recently, sphingolipids, including ceramide, sphingosine-1-phosphate, and sphingosine, have been found to modulate steroid hormone secretion at multiple levels. In this review, we provide a brief overview of the mechanisms by which sphingolipids regulate steroidogenesis. In addition, we discuss how steroid hormones control sphingolipid metabolism. Finally, we outline evidence supporting the emerging role of bioactive sphingolipids in various nuclear processes and discuss a role for nuclear sphingolipid metabolism in the control of gene transcription.

Multiple roles for sphingolipids in steroid hormone biosynthesis

Steroid hormones are essential regulators of a vast number of physiological processes. The biosynthesis of these chemical messengers occurs in specialized steroidogenic tissues via a multi-step process that is catalyzed by members of the cytochrome P450 superfamily of monooxygenases and hydroxysteroid dehydrogenases. Though numerous signaling mediators, including cytokines and growth factors control steroidogenesis, trophic peptide hormones are the primary regulators of steroid hormone production. These peptide hormones activate a cAMP/cAMP-dependent kinase (PKA) signaling pathway, however, studies have shown that crosstalk between multiple signal transduction pathways and signaling molecules modulates optimal steroidogenic capacity. Sphingolipids such as ceramide, sphingosine, sphingosine-1-phosphate, sphingomyelin, and gangliosides have been shown to control the steroid hormone biosynthetic pathway at multiple levels, including regulating steroidogenic gene expression and activity as well as acting as second messengers in signaling cascades. In this review, we provide an overview of recent studies that have investigated the role of sphingolipids in adrenal, gonadal, and neural steroidogenesis.

Androgens induce CD-9 in human prostate tissue

Based on microarray analyses of LNCaP and LNCaP-r prostatic cell-lines we tentatively identified CD-9 as an androgen sensitive protein. This prompted us to characterize the androgen sensitivity and the correlation to malignancy of CD-9 at the protein level. Using Western blot, RT-PCR and immunohistochemistry the expression of CD-9 was analysed in LNCaP cells stimulated during increasing time by the synthetic androgen R1881 and also in 88 specimens of human prostate cancer tissues. Expression of CD-9 was induced by R1881 in LNCaP. CD-9 was immunolocalized in human prostate tissue sections representing non-malignant tissue as well as tumour areas. In non-malignant glands CD-9 immunoreactivity was observed at the apical and lateral cell borders of luminal epithelial cells. Basal epithelial cells were largely unstained. In tumour areas CD-9 staining intensity was variable and apparently not related to primary Gleason grade. In prostate tissue from a patient under androgen ablation therapy no staining was observed in luminal epithelial cells or in the tumour areas, but some staining was observed in basal epithelial cells. CD-9 is regulated by androgens in LNCaP and present in human prostate specimens. The expression is variable and changes in expression levels. These and earlier studies using other tissues indicate that CD-9 and its cellular localization could have an important role in prostate cancer cell development.

Decreased expression of alpha2,8 sialyltransferase and increased expression of beta1,4 N-acetylgalactosaminyltransferase in gastrointestinal cancers

Gangliosides such as GD3, GM2, and GD2 are abundantly expressed on the cell surfaces of various malignant cells, suggesting the potential for anti-ganglioside antibody therapy for tumors. Anti-ganglioside GD2 antibody treatment is currently undergoing clinical trials for melanoma and neuroblastoma. We previously reported high in vivo antitumor effects of anti-GM2 ganglioside antibody against lung cancer. To determine whether anti-GM2 antibody may be clinically indicated for gastrointestinal cancers, we evaluated the mRNA expression of alpha2,8 sialyltransferase, a GD3 synthase, and beta1,4 N-acetylgalactosaminyltransferase (beta1,4 GalNAc-T), a GM2/GD2 synthase, in gastrointestinal cancers. We performed modified semi-quantitative RT-PCR, which reduces complexity incidental to radiolabeling on samples taken from small surgically removed clinical specimens. Stomach (19/22) and colorectal (21/30) cancers showed decreased expression of alpha2,8 sialyltransferase as compared with respective normal tissues (P < 0.05). In contrast, increased expression of beta1,4 GalNAc-T was detected in both types of tumors. Clinicopathological analysis revealed significantly higher expression level of alpha2,8 sialyltransferase in the poorly differentiated than in the well-differentiated stomach cancer group (P < 0.05). Furthermore, the expression level of alpha2,8 sialyltransferase was significantly decreased in male as compared with female colorectal cancer patients (P < 0.05). These results suggest that expression level of GM2 ganglioside is elevated in gastrointestinal cancer, and that anti-GM2 antibody may be applicable to its treatment.

Specific ganglioside changes in extraneural tissues of adult rats with hypothyroidism

Adults rats with hypothyroidism were prepared by administration of 6-propyl-2-thiouracil (PTU) or methimazole, and the tissues were examined for their gangliosides through methods including glycolipid-overlay techniques. Normal thyroid tissue contained GM3, GD3, and GD1a as the major gangliosides, with GM1, GD1b, GT1b, and GQ1b in lesser amounts. The goitrous tissue of PTU-induced hypothyroid rats had higher concentrations of GM1 and GD1a with a concomitant decrease of GM3. The amount of GT3 in thyroid tissue was increased in hypothyroid animals. While normal liver tissue had a complex ganglioside pattern with a- and b-series gangliosides, the PTU-induced hypothyroid tissue showed a simpler ganglioside profile that consisted mainly of a-series gangliosides with almost undetectable amounts of b-series gangliosides. The expression of c-series gangliosides was suppressed in the hypothyroid liver tissue. Heart tissue had higher contents of GM3 and GT3 than control. No apparent change was observed in the compositions of major and c-series gangliosides in other extraneural tissues (i.e., kidney, lung, spleen, thymus, pancreas, testis, skeletal muscle, and eye lenses), and neural tissues (i.e., cerebrum and cerebellum) from PTU-induced hypothyroid rats. The ganglioside changes of thyroid, liver, and heart tissues were reproduced in corresponding tissues of methimazole-induced hypothyroid rats. These results suggest that hypothyroid conditions affect the biosynthesis and expression of gangliosides in specific tissue and cell types.