Age-related decline in protein synthesis in the rat parotid gland

The influence of ageing on the insulin signalling system in rat lacrimal and salivary glands

PURPOSE

Ageing adversely affects the structure and function of lacrimal and salivary glands (LG and SG) and leads to marked insulin resistance that correlates with reduced insulin signal transduction. The aim of this study was to investigate whether ageing affects insulin signal transduction in LG and SG in vivo.

METHODS

Male Wistar rats aged 20 months and 2 months (control group) were compared (n=8/group). Samples were removed under anaesthesia after i.v. injection of insulin, homogenized, immunoprecipitated with anti-insulin receptor (IR), Shc and STAT-1 antibodies and immunoblotted with antiphosphotyrosine antibody.

RESULTS

The 20-month-old rats were significantly hyperinsulinaemic and presented a reduced rate of blood glucose disappearance in response to insulin, compared to the 2-month-old rats. The level of phosphorylation determined by densitometry in the older group of rats showed that ageing significantly reduced insulin-induced IR phosphorylation in LG and SG and STAT-1 phosphorylation in SG, compared to in the control group, but did not alter Shc phosphorylation.

CONCLUSIONS

Ageing influences insulin signal transduction in the LG and SG of rats. Considering the major anabolic actions of insulin, these observations may help to explain the mechanisms of LG and SG dysfunctions observed in ageing.

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.

Amylase mRNA synthesis and ageing in rat parotid glands following isoproterenol-stimulated secretion

In the parotid, as well as in other exocrine glands, secretory protein synthesis declines with age. However, whether this decline in the steady-state rate of protein synthesis reflects the reduced digestive activity of the animal or actual cellular alterations that affect synthesis is unknown. Here the ability to synthesize amylase and its mRNA during the period of enhanced protein synthesis following secretion induced by isoproterenol was compared in acinar cells of 2-and 24-month-old rats. In unstimulated glands, rates of synthesis of total protein and amylase, as well as amounts of amylase mRNA, were significantly less in the older rats than in their younger counterparts. After stimulation with isoproterenol, which induced the secretion of about 50% of stored proteins, rates of synthesis of total protein, as well as amylase, were increased by about 2.5 x the unstimulated rates in both age groups. However, the amount of amylase mRNA did not increase in parallel with the increase in the rate of amylase protein synthesis in both young and old rats. The molecular size of the mRNA was the same in stimulated and unstimulated glands of both age groups. Thus, it appears that parotid acinar cells from old rats can be stimulated to synthesize secretory proteins at an increased rate. It remains to be determined what causes the reduced rate of protein synthesis in unstimulated glands in old rats.

A biochemical analysis of parotid and submandibular salivary gland function with age after simultaneous stimulation with pilocarpine and isoproterenol in female NIA Fischer 344 rats

This analysis of physiological, biochemical and molecular changes related to aging was made in 3-, 12- and 24-month-old rats. The salivary gland weight/body weight ratio and the structural membrane proteins did not change with age for either gland, but a significant age-related decline in DNA synthesis for both glands was detected, unrelated to the hormonal responsiveness at the level of the plasma membrane. There was a marked increase in the concentration of soluble proteins in adolescent parotid gland and, for the two older age groups, in submandibular gland. The saliva flow rate was different when expressed as volume per time, as volume per time and g glandular wet weight, and/or kg body weight. The concentration of secreted proteins was not affected by age in either gland. The total amount of proteins secreted over 30 min revealed no age-related perturbation for the parotid gland, but showed a significant age-related increase in submandibular saliva. Sodium dodecyl sulphate-polyacrylamide gel analysis revealed changes in the protein bands between 39 and 50 kDa in the Coomassie blue-stained gels from 12-month-old animals. Amylase showed an initial increase (12 months), followed by a marked decline in its activity in parotid saliva. The glandular supernatant had low residual cellular amylase activity after stimulation. Therefore, secretory impairment with age after pilocarpine-isoproterenol stimulation was excluded. Analysis of total RNA showed a pronounced decrease of amylase mRNA in the parotid gland between 12 and 24 months of age. No amylase mRNA was expressed in any of the submandibular samples. For epidermal growth factor, total saliva showed a decrease with age. It seemed that the submandibular gland followed the same picture with age as the parotid gland, with a specific decline in the biosynthesis of single secretory proteins.

Increased mucin levels in submandibular saliva from mice following repeated isoproterenol treatment

Mucin decreases with age in the submandibular glands of mice and can be restored by chronic administration of isoproterenol. Mice were given repeated injections of isoproterenol to determine if the increase of mucin in the submandibular gland leads to elevated levels of mucin in the submandibular saliva. Mucin was significantly elevated in both the gland and the saliva after isoproterenol. Gland contents of mucin and the concentration of mucin in their salivas were positively correlated for both the isoproterenol (p = 0.01) and the control group (p less than 0.01).

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.

Effects of age on diabetes- and insulin-induced changes in pancreatic levels of alpha-amylase and its mRNA

During aging, protein synthesis undergoes decremental changes in many organs and tissues. In the pancreas, as well as in other exocrine glands, the rate of protein synthesis declines with age. However, it is unknown whether this decline is related to intrinsic aging changes which affect the secretory cell function. In this study, we compared the ability of pancreatic acinar cells to synthesize amylase and its messenger RNA (mRNA) in response to insulin treatment of young and old rats rendered diabetic with streptozotocin (STZ). In STZ-induced diabetic rats, amylase protein and its mRNA levels were reduced drastically in the pancreas of young and old groups. Injections of these diabetic rats with insulin increased pancreatic amylase mRNA contents significantly in both young and old rats. Insulin also increased proportions of amylase protein synthesis in the pancreas of both age groups. These results indicate that the pancreatic cells remain effective during aging in their transcriptional activity for functional amylase mRNA in response to an exogenous stimulus of insulin.

The effect of selective desalivation on wound healing in mice

The effectiveness of wound licking in the acceleration of wound healing was evaluated in selectively desalivated mice. Rate of healing of experimentally induced cutaneous wounds was evaluated macroscopically by photography at 0, 2, 4, and 6 days after wounding. Sialadenectomy of submandibular and sublingual glands significantly slowed down wound healing in animals caged together compared to sham-operated controls. Separate caging as compared to caging in groups slowed down healing in sham-operated animals at day 2 but not at day 4 and 6. No effect on the rate of healing in sialadenectomized mice was observed in separate caging compared to mice caged in groups. Ligation of the parotid duct had an insignificant effect. The rate of wound healing of sublingual sialadenectomized mice was slower than that of sham-operated controls, but not as slow as those of sublingual and submandibular sialadenectomized mice. The results suggest that the rate of healing of experimentally induced cutaneous wounds of mice is slowed down when licking is prevented by separate caging which confirms previous reports. Licking with submandibular saliva seems to be more effective than sublingual saliva. Parotid saliva or minor salivary glands secretions are the least effective.

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.

Protein secretion by rat submandibular glands in response to isoproterenol, alpha-methylnoradrenaline and clonidine during aging

Changes in salivary volumes and the three types of proteins secreted by the submandibular salivary gland (SMG) of male rats at 3.5, 5.5, 8, 12, 13, 14, 15, 19, 21 and 24 months of age in response to the beta 1-, alpha 1- and alpha 2-adrenoceptor agonists, isoproterenol (IPR), alpha-methylnoradrenaline (alpha-mNA) and clonidine (Clonid), were studied and compared by measuring the weight and by isoelectric focusing electrophoresis with the Phast System on both the gradient pH 3.5-5 and 3.5-9 gels with silver staining. A protein (protein A, tentatively termed in this study) purified by FPLC from saliva elicited by IPR was also analyzed by SDS-polyacrylamide gel electrophoresis, the immuno-thermoblotting method, carbohydrate determination and neuraminidase treatment. Unexpected findings were observed that salivary volumes, but not the protein concentration, were substantially increased by Clonid-, but not IPR-, stimulation with ages up to 24 months of age and that the three types of proteins elicited by each agonist were different during aging. The gamma-type of proteins elicited by Clonid was not greatly changed during aging, whereas several proteins at about neutral pI in the alpha-type, elicited by alpha-mNA, at 5.5 to 21 months of age and a protein A in the beta-type, elicited by IPR, at 13 to 24 months of age were greatly increased. This protein A without any carbohydrate and sialic acid, located only in the acinar cells, but not in any duct system, had a molecular weight of 16,000 and a pI of 4.05. We conclude that the secretory function of the SMG in the aged animals is in general little changed.

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.

Protein synthesis and amylase messenger RNA content in rat parotid salivary glands after total or partial stimulation with isoproterenol

The rate of synthesis of secretory proteins increases significantly in rat parotid glands after stimulated discharge of stored proteins. How any difference in the amount of secretory protein discharge affects the rate of subsequent protein synthesis, and whether this post-secretory synthesis is regulated at the level of messenger RNA, was now examined. One group of rats was stimulated to secrete 97% of stored secretory proteins by an intraperitoneal injection of isoproterenol. The other group received a much smaller dose to induce the discharge of about 40% of the proteins. Despite this difference in secretion, the subsequent rates of total protein synthesis, as well as of amylase, were increased to about the same extent. The amylase messenger RNA (mRNA) was identified and quantified by hybridization with a 32P-labelled amylase complementary DNA (cDNA) probe. The amylase mRNA in stimulated and unstimulated rats was of the same molecular size (Northern blot analyses). The amount of amylase mRNA, determined by dot blot analyses, were also increased in stimulated rats, although this increase was not as great as that in the rate of amylase protein synthesis. The implications of this discrepancy concern the possibility that the mechanism of regulation of secretory protein synthesis in parotid glands is at the translational level.

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

N-Linked protein glycosylation was examined in vitro in dispersed rat parotid acinar cells from young adult (3-6 months) and aged (22-24 months) rats. A small decrease in general protein production was observed with cells from aged animals (approximately 20% lower incorporation of [14C]leucine into 10% CCl3 COOH insoluble protein during continuous pulse labeling). Incorporation of [3H]mannose into N-linked glycoproteins by aged cells was further reduced (approximately 35%). Similarly microsomal membranes from parotid glands of aged animals showed approximately 50% reduction in the synthesis of mannosylphosphoryl dolichol, a key intermediate in the dolichol pathway of protein N-glycosylation. Man-P-Dol synthase, the microsomal enzyme responsible for production of this saccharide-lipid, displayed no change in apparent Km for GDP-mannose when preparations from aged animals were utilized, but did show approximately 50% reduction in Vmax. Following beta-adrenoreceptor activation, cells from both young adult and aged glands showed increased N-linked protein glycosylation almost to the same extent (approximately 2-fold). The data suggested that in aged rat parotid cells there is a basal reduction of activity in the pathway responsible for asparagine-linked protein glycosylation, but that following exocytotic stimuli this pathway responds in a manner comparable to cells from young adult glands.

The influence of aging on lysosomal acid DNase of the rat submandibular gland

Lysosomal and cytoplasmic fractions were prepared from rat submandibular glands for investigation of the release of lysosomal acid DNase in relation to aging. It was found that the acid DNase activity ratio for cytoplasmic/lysosomal fractions in rats aged 27 months was higher than that in three-month-old rats. The release of acid DNase from the lysosomal fraction by shaking was markedly increased in the fraction from the older animals.

Immunochemical quantitation of alpha-amylase and secretory IgA in parotid saliva from people of various ages

The alpha-amylase (128 subjects) and secretory IgA (118 subjects) concentration in stimulated parotid saliva of healthy individuals aged 23-84 years, was determined. They were divided into three age groups: I, 23-39; II, 40-59; and III, 60-84 years old. The concentrations (microgram/ml) of alpha-amylase for group I = 803.6; II = 648.0; and III = 652.4; the concentrations for secretory IgA for group I = 96.2; II = 101.8; and III = 97.7. The Kruskal-Wallis test revealed no significant differences between groups for alpha-amylase or secretory IgA.

The synthesis of amylase in parotid glands of young and old rats

The age-related changes in the rate of synthesis of total and secretory proteins were examined in parotid glands of young (2 months) and old (24 months) rats. The differences in the rate of incorporation of radioactive leucine into acid-insoluble proteins of the gland indicate that the rate of protein synthesis declines with age in this gland. To determine whether the rate of synthesis of secretory proteins changes with age in this gland, the rates of incorporation of [3H]leucine into amylase, a major secretory protein of the gland, were compared by radioactivity determinations. For this comparison, amylase was precipitated with glycogen after incubating the gland slices in the presence of the labeled amino acid. The study shows that rate of synthesis of amylase declines significantly with age in this gland. The possible relationship between the decline in protein synthesis and the reduced level of secretory activity of the gland due to aging is discussed.

Lacrimal protein secretion: comparison of young and old rats

The in vitro protein secretory response of lacrimal glands from healthy 4- and 24-month-old F344 rats was measured. Tissue fragments were incubated in a 0.2 ml perifusion chamber and stimulated to secrete protein by a 4-min bolus of 0.01 mM carbachol. Fractions of the perifusate were collected and assayed for protein concentration and peroxidase activity. The response to carbachol was a well-defined peak of secreted protein that included peroxidase. The response was inhibited by 1.0 microM atropine. No differences were found between the responses of male and female glands. The peak of protein secretion by the young glands contained 263.9 +/- 71.8 micrograms g tissue-1 compared to 175.4 +/- 56.5 micrograms g tissue-1 for the old glands (P less than 0.005). While the mean activity of the secreted peroxidase was similar in the two age groups, the aged glands were significantly more variable in their response. The F344 rat may provide a useful model for studying the normal age-dependent changes that occur in lacrimal gland.

Changes in the secretory acinar cells of the rat parotid gland during aging

The secretory acinar cells of parotid glands from rats of varying ages have been examined by electron microscopy to determine what age-related changes occur in these cells. The most prominent change noted in these cells is the progressive increase in the amount of lipofuscin granules with age. Lipofuscin granules are membrane-bound structures consisting of lipids, other subcomponents, and a matrix. In addition, these cells contain lipid droplets that are not associated with any other components and tend to accumulate at the base of the cells in older rats. Also, many acinar cells in the glands of old rats contain altered secretory granules which appear to be in the process of degeneration. The accumulation of lipid and degenerating secretory granules appears to be related to the reduced level of cellular secretory activity in the glands of older rats. It is possible that these two types of inclusions contribute to the formation of lipofuscin granules. Lipofuscin and degenerating secretory granules are associated with acid phosphatase, which is demonstrated cytochemically, indicating that these granules are lysosomal structures.

Protein production and processing in young adult and aged rat submandibular gland cells in vitro

Protein production and processing were evaluated in vitro in dispersed submandibular gland cells from young adult (4-6 months) and aged (24 month) rats. A modest decrease (approx. 25%) in protein production (incorporation of radiolabeled amino acids into 10% CCl3COOH-insoluble material) was found with old cells in both continuous-pulse and pulse-chase studies. Also new protein processing, followed by gel electrophoresis and autofluorography of radiolabeled samples, showed specific, marked alterations in old cells. In particular a significant difference in the processing of a 225 000 molecular weight glycoprotein (likely the major rat submandibular mucin) was detected.

Secretory protein synthesis in parotid glands of young and old rats

To determine whether the synthesis of secretory proteins changes with age, the incorporation of [14C]-leucine into exportable proteins of the parotid gland was compared in 2- and 24-month-old rats. The proteins were separated by SDS-gel electrophoresis of the post-microsomal supernatant and identified by comparing the banding patterns in gels prepared from unstimulated glands with those from the glands stimulated to secrete. The amount of radioactivity incorporated into the bands corresponding to exportable proteins was significantly less in the older group, indicating that the synthesis of secretory proteins declines with age.

Age-related changes in EGF and protease in submandibular glands of C57BL/6J mice

The submandibular glands of male C57BL/6J mice were studied cytologically and chemically at the following ages (months): 1-1.5, 6-8, 12-13, 28-32. The relative proportion of granular convoluted tubules (GCT), as well as the size and content of secretion granules of GCT cells, progressively increased throughout the first year of life. Correspondingly, the concentration within the glands of two GCT cell products, epidermal growth factor (EGF) and protease, also steadily increased. In senescent glands, GCTs formed relatively less of the gland parenchyma and were composed of shorter cells with reduced amounts of secretory granules. The concentration of EGF was reduced to 17% of its peak value at one year, while protease activity declined to 50% of its peak value. These morphologic and chemical findings imply a functional impairment in submandibular glands of the mouse with senescence.

Changes in the synthesis of exportable and nonexportable proteins in parotid glands during aging

The age-related differences in the synthesis of exportable and nonexportable proteins of the parotid salivary gland were compared in 2- and 24-months-old rats. Parotid slices from these rats were incubated in the presence of [14C]leucine and the amount of radioactivity incorporated into the water-soluble proteins of the postmicrosomal supernatant was compared. The exportable and nonexportable proteins were identified by electrophoretic separation of these proteins by comparing the banding patterns of the gel preparations from unstimulated glands to those from the glands stimulated to secrete. The radioactivity determination in various protein bands from these rats indicated that the synthesis of exportable secretory proteins declined with age, while that of nonexportable proteins did not appear to change.

Secretion of alpha-amylase from parotid lobules of young and old rats

The rate of release of a secretory enzyme, alpha-amylase, from the parotid lobules of 2 and 24 month old rats has been compared to determine whether the secretory activity of the cells change during aging. Upon incubation in the presence of a secretogogue, isoproterenol (10(-5) M), about the same proportions of the glandular alpha-amylase are released at about the same rate from these lobules. The isoproterenol-stimulated release of the enzyme is inhibited nearly completely by preincubating the lobules with propranolol (10(-5) M) in both age groups, indicating that the enzyme secretion occurs through the stimulation of beta-receptors. When viewed in the electron microscope, the cell membranes bordering the secretory lumen in isoproterenol-incubated lobules reveal festooned appearances which suggest that the enzyme release occurs by means of exocytosis. These observations indicate that the ability of the parotid cells to release secretory products through the beta-receptor mediation does not significantly change with increasing age.

Age-related changes in the cellular level of amylase and protein synthesis in the rat parotid gland

Age-related changes in the cellular content of secretory proteins and protein synthesis were studied in parotid glands of rats of various ages. The secretory protein content (determined by measuring the level of alpha-amylase activity) and the synthesis of proteins (assayed by the rate of incorporation of 3H-leucine into acid-insoluble proteins) decline with increasing age. Morphological and radioautographic studies of the gland indicate that the decline in protein synthesis is due to the reduction in the ability of secretory cells to synthesize proteins.