N-linked protein glycosylation in the rat parotid gland during aging

DPM1 expression as a potential prognostic tumor marker in hepatocellular carcinoma

Background

Altered glycosylation of proteins contributes to tumor progression. Dolichol phosphate mannose synthase (DPMS), an essential mannosyltransferase, plays a central role in post-translational modification of proteins, including N-linked glycoproteins, O-mannosylation, C-mannosylation and glycosylphosphatidylinositol anchors synthesis. Little is known about the function of DPMS in liver cancer.

Methods

The study explored the roles of DPMS in the prognosis of hepatocellular carcinoma using UALCAN, Human Protein Atlas, GEPIA, cBioPortal and Metascape databases. The mRNA expressions of DPM1/2/3 also were detected by quantitative real-time PCR experiments in vitro.

Results

The transcriptional and proteinic expressions of DPM1/2/3 were both over-expressed in patients with hepatocellular carcinoma. Over-expressions of DPMS were discovered to be dramatically associated with clinical cancer stages and pathological tumor grades in hepatocellular carcinoma patients. In addition, higher mRNA expressions of DPM1/2/3 were found to be significantly related to shorter overall survival in liver cancer patients. Futhermore, high genetic alteration rate of DPMS (41%) was also observed in patients with liver cancer, and genetic alteration in DPMS was associated with shorter overall survival in hepatocellular carcinoma patients. We also performed quantitative real-time PCR experiments in human normal hepatocytes and hepatoma cells to verify the expressions of DPM1/2/3 and results showed that the expression of DPM1 was significantly increased in hepatoma cells SMMC-7721 and HepG2.

Conclusions

Taken together, these results suggested that DPM1 could be a potential prognostic biomarker for survivals of hepatocellular carcinoma patients.

Dolichol phosphate mannose synthase: a Glycosyltransferase with Unity in molecular diversities

N-glycans provide structural and functional stability to asparagine-linked (N-linked) glycoproteins, and add flexibility. Glycan biosynthesis is elaborative, multi-compartmental and involves many glycosyltransferases. Failure to assemble N-glycans leads to phenotypic changes developing infection, cancer, congenital disorders of glycosylation (CDGs) among others. Biosynthesis of N-glycans begins at the endoplasmic reticulum (ER) with the assembly of dolichol-linked tetra-decasaccharide (Glc₃Man₉GlcNAc₂-PP-Dol) where dolichol phosphate mannose synthase (DPMS) plays a central role. DPMS is also essential for GPI anchor biosynthesis as well as for O- and C-mannosylation of proteins in yeast and in mammalian cells. DPMS has been purified from several sources and its gene has been cloned from 39 species (e.g., from protozoan parasite to human). It is an inverting GT-A folded enzyme and classified as GT2 by CAZy (carbohydrate active enZyme; http://www.cazy.org ). The sequence alignment detects the presence of a metal binding DAD signature in DPMS from all 39 species but finds cAMP-dependent protein phosphorylation motif (PKA motif) in only 38 species. DPMS also has hydrophobic region(s). Hydropathy analysis of amino acid sequences from bovine, human, S. crevisiae and A. thaliana DPMS show PKA motif is present between the hydrophobic domains. The location of PKA motif as well as the hydrophobic domain(s) in the DPMS sequence vary from species to species. For example, the domain(s) could be located at the center or more towards the C-terminus. Irrespective of their catalytic similarity, the DNA sequence, the amino acid identity, and the lack of a stretch of hydrophobic amino acid residues at the C-terminus, DPMS is still classified as Type I and Type II enzyme. Because of an apparent bio-sensing ability, extracellular signaling and microenvironment regulate DPMS catalytic activity. In this review, we highlight some important features and the molecular diversities of DPMS.

Cellular and animal models for high-throughput screening of therapeutic agents for the treatment of the diseases of the elderly in general and Alzheimer's disease in particular(†)

It is currently thought that the dementia of Alzheimer's disease is due to the neurotoxicity of the deposits or aggregates of amyloid-β (Aβ) in the extracellular space of the cerebral cortex. This model has been widely criticized because there is a poor correlation between deposits and dementia. Others have questioned whether Aβ is truly neurotoxic. Yet, in spite of these concerns, the search for therapeutic agents has been based on the development of mouse models transfected with mutant genes associated in humans with early onset Alzheimer's disease. A major limitation of these models is that although they exhibit many of the pathological and clinical manifestation of the human disease, the bulk of individuals who develop the dementia of Alzheimer's disease have none of these mutant genes. Furthermore, nine clinical trials of drugs that were effective in transgenic mice failed to show any benefit in patients. Finally, a major unresolved issue with the Aβ model is that since Aβ is produced in everyone, why are deposits only seen in the elderly? This issue must be resolved if we are to understand the etiology of the disease and develop test systems for both diagnosis and drug discovery. Published studies from my laboratory demonstrate that in human cerebrospinal fluid immunoreactive Aβ is only present as a complex with two chaperones, ERp57 and calreticulin and is N-glycosylated. This complex formation is catalyzed by the posttranslational protein processing system of the endoplasmic reticulum (ER). Others have reported that in plaque Aβ is present only as the naked peptide. Together these results suggest that both plaque and dementia are secondary to an age related decline in the capacity of the ER to catalyze protein, posttranslational processing. Since the synaptic membrane proteins necessary for a functioning memory are also processed in the ER, these findings would suggest that the loss of cognition is due to a decline in the capacity of the neuron to produce and maintain functioning synapses. Work from my laboratory and from others further indicate that the components of the ER, posttranslational, protein processing pathway do dramatically decline with age. These data suggest that this decline may be found in all cells and could account not only for the dementia of Alzheimer's disease, but also for many of the other manifestations of the aging process. These observations also suggest that declining ER function has a role in two well-recognized phenomena associated with aging: a loss of mitochondrial function and a decrease in myelin. Finally, based on this paradigm I propose new cellular and animals models for high-throughput screening for drug discovery.

Age- and sex-related differences in extra-hepatic low-density lipoprotein receptor

To determine whether differences in LDLr behavior in extra-hepatic tissues and whether extra-hepatic receptors could differentially contribute to cholesterol homeostasis under physiological conditions, we evaluated the presence and regulation of LDLr from both a gender and an aging perspective. We used the brain cortex, the gastrocnemius, and the heart ventricle of 3- and 12-month-old male and female rats. We observed a protein decrease of total LDLr in 12-month-old female rat brains that was completely restored by 17-β estradiol treatment. In the gastrocnemius, LDLr accumulates in the skeletal muscle in both male and female aged rats as a precursor probably due to a glycosylation impairment. In the heart, no modifications were observed in either older rats or rats of a specific gender. These data highlight a tissue-specific dysregulation of LDLr that is age- and gender-dependent.

Mannosylphosphodolichol synthase overexpression supports angiogenesis

Mannosylphospho dolichol synthase (DPMS) plays a critical role in Glc(3)Man(9)GlcNAc(2)-PP-Dol (lipid-linked oligosaccharide, LLO) biosynthesis, an essential intermediate in asparagine-linked (N-linked) protein glycosylation. We have observed earlier that phosphorylation of DPMS increases the catalytic activity of the enzyme by increasing the V(max) as well as the enzyme turnover (k(cat)) without significantly changing the K(m) for GDP-mannose. As a result, LLO biosynthesis, turnover and protein N-glycosylation are increased. This is manifested in increased proliferation of capillary endothelial cells, i.e., angiogenesis. We have then asked if the phosphorylation event or the up-regulation of the DPMS due to over production of the enzyme is a key factor in up-regulating angiogenesis? This question has been answered by isolating a stable capillary endothelial cell clone overexpressing the DPMS gene. Our results indicate that the DPMS overexpressing clone has a high level DPMS mRNA judged by QRT-PCR. The clone also expresses nearly four-times higher DPMS protein over the clone transfected with pEGFP-N1 vector only (i.e., control) as analyzed by western blotting. Most importantly, the overexpressing DPMS clone has ~108% higher DPMS activity than that of the vector control. Immunofluorescence microscopy with Texas-Red conjugated WGA indicates a high level expression of GlcNAc-beta-(1,4)-GlcNAc)1-4-beta-GlcNAc-NeuAc glycans on the external surface of the capillary endothelial cells overexpressing DPMS. Increased cellular proliferation and accelerated healing of the wound induced by a mechanical stress of the DPMS overexpressing clone unequivocally supports DPMS for angiogenesis.

The effect of aging on the chaperone concentrations in the hepatic, endoplasmic reticulum of male rats: the possible role of protein misfolding due to the loss of chaperones in the decline in physiological function seen with age

The endoplasmic reticulum (ER) chaperones are highly conserved proteins that catalyze the posttranslational processing of all secretory and membrane proteins. Our studies suggest that chaperone declines are one of the two central defects in Alzheimer's disease. We propose that similar declines in other organ systems underlie the physiological deficits of aging. Rats were maintained in a colony from age 21 days to death. Animals were killed at regular intervals, and hepatic, ER chaperone contents were determined by immunoblotting. ERp55, ERp57, ERp72, BiP, and calnexin constitutive levels declined 30%-50% with age. Calreticulin was unaffected. BiP (also known as GRP78), ERp55, and ERp57 showed marked swings with peaks occurring in midwinter and midsummer. This cyclics declined 73% with age. Considering the role of the ER chaperones in membrane and secretory protein posttranslational processing, these data support the concept that their loss could lead to many of the physiological declines associated with aging.

Structural and functional changes in salivary glands during aging

Various salivary glands in senescent humans and other animals have been examined extensively to characterize the structural and functional changes that occur during aging. Although a wide range of different structural changes, involving both the parenchymal and stromal tissues, have been described, it is unclear how any of these changes affects the function of the salivary glands. One major change in structure is the reduction in the volume of acini with a concomitant increase in the ductal volume. Despite this loss of functional acini, the salivary output and the contents seem to be unaltered, or minimally altered, due to aging. One consistent change observed in many salivary glands of aged animals is the decline in the rate of synthesis of proteins and their messenger RNA (mRNA). However, the salivary acinar cells from aged animals can synthesize secretory proteins at an elevated rate just as effectively as those from their younger counterparts in response to external stimuli, which are known to enhance the rate of protein synthesis. Thus, it appears that the salivary acinar cells, which remain structurally intact during aging, seem to retain their functional efficiency. Furthermore, these acinar cells, although reduced in number, are sufficient in quantity to carry out most of the salivary gland functions.

Age-related decline in multiple unit action potentials of CA3 region of rat hippocampus: correlation with lipid peroxidation and lipofuscin concentration and the effect of centrophenoxine

Changes in lipid peroxidation, lipofuscin concentration, and multiple unit activity (MUA recorded in conscious animals) in the CA3 region were studied in the hippocampus of male Wistar rats aged 4, 8, 16, and 24 months. The lipid peroxidation and lipofuscin concentration were increased with age. The MUA, however, declined with age. Correlational analyses were performed for the four age groups to determine the relationship between the age-associated decline in MUA with the age-related alterations in lipid peroxidation and lipofuscin concentrations. The age-related increase in lipid peroxidation correlated positively with the age-associated increase in lipofuscin concentration. The age-related increases in lipid peroxidation and lipofuscin concentration correlated negatively with the changes in MUA. Since lipid peroxidation may affect neuronal electrophysiology, our data suggested that age-related increase in lipid peroxidation may contribute to an age-associated decline in neuronal electrical activity. Centrophenoxine effects were studied on the three above-mentioned age-associated changes in the hippocampus. The drug had no effect on all three parameters in 4- and 8-month-old rats. In 16- and 24-month-old rats, however, the drug significantly increased the MUA but concomitantly decreased lipofuscin concentration and lipid peroxidation. Correlational analyses of the data on MUA, lipid peroxidation and lipofuscin concentration from the centrophenoxine-treated animals showed that the drug-induced diminution in both lipofuscin and lipid peroxidation was significantly correlated with the drug-induced increase in MUA. The differential effect of the drug in younger (4-8 months) and older (16-24 months) animals indicated that the stimulation of MUA was clearly associated with concomitant decrease in lipid peroxidation and lipofuscin concentration.

Analysis of carbohydrate composition and sialidase activity in oral secretions of patients with tumors in the upper aerodigestive tract

Total saliva and secretions from parotid and submandibular glands of patients with carcinomas in the oral cavity, oropharynx or larynx and a control group of healthy individuals were analyzed for concentrations of glycosidically bound sugars and free N-acetylneuraminic acid as well as for sialidase activity. When compared to the data obtained for normal donors, the relative amounts of the individual monosaccharides fucose, galactose, mannose, N-acetylgalactosamine, and N-acetylglucosamine as components of glycoconjugates showed variable differences to the group of tumor patients depending on the type of secretion and the location of the tumor. The percentage of glycosidically linked N-acetylneuraminic acid, however, was always higher for the healthy donors. A significant difference was found in the amount of free sialic acid, with the exception of submandibular gland secretion from a patient with an oropharyngeal carcinoma, and sialidase activity which were increased for tumor patients, independent of the type of secretion and the location of the tumor. From these results it is concluded that free sialic acid and sialidase activity may be considered as markers for carcinomas in the upper aerodigestive tract.

Age-related changes in mucins from human whole saliva

The predominant mucins in human whole saliva, MG1 and MG2, serve to protect and to lubricate the oral cavity. In this study, both unstimulated and stimulated whole salivas were collected from two groups of subjects: young (18-35 years of age) and aged (65-83 years of age). The subjects were in apparent good health. Saliva samples from each subject were analyzed by SDS-PAGE. The gels were stained with Stains-all, and both MG1 and MG2 were quantitated by video-image densitometry. The protocol gave reproducible values for each mucin. The stimulated and unstimulated salivas from aged subjects showed significant reductions in concentrations of both MG1 and MG2, as quantitated in mucin dye-binding units. Possible associations of these reductions with the aging process are discussed.

The effect of ageing on mucin contents in mouse submandibular glands

There may be deleterious effects if there is an age-dependent reduction in mucin composition or content in salivary glands. To assess whether there was an effect of age in submandibular glands from male mice aged 3 months, 10 and 20 months and 27, 29 and 30 months, mucin, protein and sialic acid were quantitated. Changes in wet weight per gland and protein per gland were highly significant (p less than 0.001), as tested by analysis of variance, and the means for both were highest in middle aged glands. While sialic acid per gland showed no significant change from middle aged to old glands, mucin per gland showed an age-related decrease (p less than 0.01) when comparing either young or middle aged glands with old glands.

Increased mucin levels in submandibular glands of aged male mice after chronic isoproterenol treatment

Mucin levels are reduced in submandibular glands of mice during ageing. Isoproterenol (IPR) was given to mice 27 and 29 months old to assess whether levels of mucin could be restored to levels similar to those in younger mice. When compared with controls, mucin levels per gland were significantly elevated (p less than 0.05) after IPR treatment, and exceeded the peak amounts in young animals. Mucin concentrations were also higher after IPR. These observations support the contention that the gland has the ability to return to or exceed pre-senescent levels of mucin. Mean sialic acid levels were elevated in each experiment after IPR treatment. There was evidence for significant diurnal variation in mucin in the aged control animals, but little evidence for such a relationship in the IPR-treated animals. After administration of pilocarpine there was a modest stimulation of sialic acid and protein levels in 20-month-old animals while mucin levels showed virtually no indication of stimulation.

Effect of ageing on adenylate cyclase activity and G-proteins in rat submandibular salivary glands

Membranes were prepared from the submandibular salivary glands of male young (3-months-old) and aged (24-months-old), Fisher 344 rats, and assayed for adenylate cyclase activity and for the ribosylation of G-proteins in the presence of [32P]-NAD+ and cholera toxin. Adenylate cyclase activity (basal, fluoride-, forskolin-, and isoproterenol-stimulated) was significantly lower (p less than 0.01) in the glands of aged rats. The pattern of ribosylation was different between the 2 groups. Two cholera toxin-specific bands (Mr 42,000 and 44,000) were ribosylated in the glands of young rats whereas in aged rats, only an Mr 42,000 band was clearly observed. The incorporation of 32P from NAD+ into the Mr 42,000 and 44,000 bands increased with the time of ribosylation (7.5-90 min). In aged rats, however, the incorporation of 32P into the Mr 42,000 band was much slower and did not increase much after 60 min of ribosylation. At each time point, the extent of ribosylation was lower in the membranes from the aged rats. These findings suggest that the decrease in adenylate cyclase activity in the submandibular salivary glands of aged rats may be due partly to the changes in their levels of Gs as a result of ageing.

Changes with senescence in the fine structure of the granular convoluted tubule of the submandibular gland of the mouse

Cells of the granular convoluted tubules (GCTs) of the submandibular gland of senescent male mice show structural changes indicative of functional decline. In order to define the nature of these age-related changes more clearly, the fine structure of GCT cells of 12- and 28-month-old males was compared. In old mice, there was cell-to-cell variation in the extent of these changes, with some cells of senescent males appearing no different from those of young adults. In affected cells the most striking alterations were seen in secretion granules and lysosomal elements. Secretion granules varied greatly in size, with some GCT cells having only very fine apical granules. Secondary lysosomes and large lipofuscin granules were frequent in the basal cytoplasm. Very large dense bodies (3-5 micron) occurred in many cells. These possibly represent intracellular pools of released secretory materials, as they were occasionally seen in continuity with the luminal contents. Structures whose appearance was intermediate between the very large dense bodies and lipofuscin granules were common, suggesting crinophagic activity. There was an apparent decrease in numbers of polysomes and in the extent of the Golgi apparatus. These fine structural changes are consistent with impairments with advanced age in synthesis and posttranslational processing of secretory products by affected GCT cells. In addition to cell-to-cell variation in any one male, there was also interanimal variation in the degree and extent of these senescent changes.