Induction of tissue-specific proline-rich protein multigene families in rat and mouse parotid glands by isoproterenol. Unusual strain differences of proline-rich protein mRNAs

Salivary Gland Adaptation to Dietary Inclusion of Hydrolysable Tannins in Boars

Simple Summary

Tannins have traditionally been avoided in animal nutrition due to their anti-nutritive effects. However, recent studies reported hydrolysable tannins as beneficial additives that have antimutagenic, anticarcinogenic, antidiarrheal, and antiulcerogenic effects on animals. In a study testing the inclusion of hydrolysable tannins as a potential nutritive factor to reduce boar taint in entire males, significant enlargement of the parotid glands (parotidomegaly) was observed. In this study, we aimed to determine the morphological and immunohistochemical basis for the observed parotidomegaly. We discovered that enlargement of glandular lobules and acinar area, an increased ratio between the nucleus and cytoplasm of serous cells, and increased excretion of proline-rich proteins (PRPs) were characteristic of the experimental group that received the highest dietary tannin intake. The mandibular salivary gland, on the other hand, did not show significant morphological changes among the experimental groups. This suggests increased functional activity of the parotid salivary glands as the first and most important line of defense against high dietary tannin and its potential negative effects.

Abstract

The ingestion of hydrolysable tannins as a potential nutrient to reduce boar odor in entire males results in the significant enlargement of parotid glands (parotidomegaly). The objective of this study was to characterize the effects of different levels of hydrolysable tannins in the diet of fattening boars (n = 24) on salivary gland morphology and proline-rich protein (PRP) expression at the histological level. Four treatment groups of pigs (n = 6 per group) were fed either a control (T0) or experimental diet, where the T0 diet was supplemented with 1% (T1), 2% (T2), or 3% (T3) of the hydrolysable tannin-rich extract Farmatan^®. After slaughter, the parotid and mandibular glands of the experimental pigs were harvested and dissected for staining using Goldner’s Trichrome method, and immunohistochemical studies with antibodies against PRPs. Morphometric analysis was performed on microtome sections of both salivary glands, to measure the acinar area, the lobular area, the area of the secretory ductal cells, and the sizes of glandular cells and their nuclei. Histological assessment revealed that significant parotidomegaly was only present in the T3 group, based on the presence of larger glandular lobules, acinar areas, and their higher nucleus to cytoplasm ratio. The immunohistochemical method, supported by color intensity measurements, indicated significant increases in basic PRPs (PRB2) in the T3 and acidic PRPs (PRH1/2) in the T1 groups. Tannin supplementation did not affect the histo-morphological properties of the mandibular gland. This study confirms that pigs can adapt to a tannin-rich diet by making structural changes in their parotid salivary gland, indicating its higher functional activity.

Altering salivary protein profile can decrease aversive oromotor responding to quinine in rats

Bitter taste is often associated with toxins, but accepting some bitter foods, such as green vegetables, can be an important part of maintaining a healthy diet. It has previously been shown that animals exposed to quinine upregulate a set of salivary proteins (SPs), and those with upregulated SPs have increased rates of feeding on a quinine diet as well as increased brief-access licking to and higher detection thresholds for quinine. These studies suggest that SPs alter orosensory feedback; however, they rely on SPs upregulated by diet exposure and cannot control for the role of learning. Here, we use taste reactivity to determine if SPs can alter bitter taste in animals with no previous bitter diet experience. First, saliva with proteins stimulated by injections of isoproterenol and pilocarpine was collected from anesthetized rats; this "donor saliva" was analyzed for protein concentration and profile. Bitter-naïve rats were implanted with oral catheters and infused with taste stimuli dissolved in saliva that contained all of the SPs from the donors, saliva that was filtered of SPs, water, or artificial saliva. Their orofacial movements were recorded and quantified. We found that presence of quinine increased movements associated with aversive stimuli, but adding SPs to the infusion was sufficient to reduce aversive oromotor responding to quinine. The effect was dependent on the total protein concentration of the saliva, as protein concentration increased aversive responses decreased. Additionally, infusions of whole saliva altered aversive responding to quinine, but not other stimuli (citric acid, NaCl, sucrose). Our work suggests that effect of these SPs is specific and the presence of SPs is sufficient to decrease aversive orosensory feedback to bitter stimuli.

Exocyst subunits are involved in isoproterenol-induced amylase release from rat parotid acinar cells

Exocytosis of secretory granules in parotid acinar cells requires multiple events: tethering, docking, priming, and fusion with a luminal plasma membrane. The exocyst complex, which is composed of eight subunits (Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70, and Exo84) that are conserved in yeast and mammalian cells, is thought to participate in the exocytotic pathway. However, to date, no exocyst subunit has been identified in salivary glands. In the present study, we investigated the expression and function of exocyst subunits in rat parotid acinar cells. The expression of mRNA for all eight exocyst subunits was detected in parotid acinar cells by RT-PCR, and Sec6 and Sec8 proteins were localized on the luminal plasma membrane. Sec6 interacted with Sec8 after 5 min of stimulation with isoproterenol. In addition, antibodies to-Sec6 and Sec8 inhibited isoproterenol-induced amylase release from streptolysin O-permeabilized parotid acinar cells. These results suggest that an exocyst complex of eight subunits is required for amylase release from parotid acinar cells.

Green tea ingestion by rats does not affect iron absorption but does alter the composition of the saliva proteome

We tested the hypothesis that rats adapt to the iron absorption inhibitory effects of tea by modifying the expression of salivary proteins. Thirty-six weanling rats were allocated into 6 groups. Two control groups were fed a semipurified diet containing 20 mg Fe(2+)/kg diet. Two groups were fed spray dried green tea infusion mixed into the diet (28.6 g tea/kg diet) and 2 groups were fed the control diet with a twice daily gavage of a tea solution (0.25 g tea/mL). Saliva samples were collected in 3 groups (control, gavage, and oral) on day 8 (acute) and in the remaining groups on day 31 (chronic). Iron absorption was assessed using a (58)Fe(3+) tracer administered on day 1 (acute) and day 24 (chronic). 2D gel electrophoresis and mass spectrometry were used to assess the composition of the saliva proteome. There was no significant difference in iron absorption between the 3 groups on either day 1 or day 24. Salivary proline-rich proteins and submandibular gland secretory protein increased to a greater extent in the oral group than in the gavage group, when compared to control, within the same exposure time period. Amylase, chitinase, deoxyribonuclease, cysteine-rich secretory protein 1, and parotid secretory protein all decreased to a greater extent in the oral tea group, compared to the control, within the same exposure time period. Our results show that green tea did not decrease iron absorption in rats but it did have a marked effect on the saliva proteome when given orally.

Experimental periodontitis induces a cAMP-dependent increase in amylase activity in parotid glands from male rats

It is known that subjects with periodontitis show enhanced amylase concentration in saliva. Our purpose was to analyze the release of amylase in parotid glands from rats with experimental periodontitis and controls. We present evidence that periodontitis induces an increase in resting amylase activity and release without changes in isoproterenol-induced amylase secretion. Changes in amylase were reverted by the inhibition of the adenylyl cyclase by SQ 22536, the cyclooxygenase type 1 by FR 122047 and by blocking the vasoactive intestinal peptide (VIP) receptor with VIP 6-28. Parotid glands from rats with periodontitis showed an increase in cAMP levels that was also reverted in the presence of SQ 22536, FR 122047 and VIP 6-28. We concluded that both PGE(2) and VIP are produced in parotid glands from rats with periodontitis and, by activating their own receptors in acinar cells, induce cAMP accumulation leading to an increase in amylase basal secretion.

Salivary secretion, taste and hyposalivation

Saliva has many essential functions. As the first digestive fluid in the alimentary canal, saliva is secreted in response to food, assisting intake and initiating the digestion of starch and lipids. During this process, saliva acts as a solvent of taste substances and affects taste sensitivity. Clinically, a more important role is in the maintenance of oral health, including the protection of teeth and mucosa from infections, maintenance of the milieu of taste receptors, and communication ability through speech. Variations in salivary flow can be affected, reversibly or irreversibly, by numerous physiological and pathological factors. Decreased salivary flow results in clinically significant oral discomfort that may manifest as increased caries, susceptibility to oral candidiasis, altered taste sensation or as a host of other problems. Hyposalivation is a condition that is frequently encountered in dental practice. The most common cause is the use of certain systemic medications, which put the elderly at greater risk because they are usually more medicated. Other causes include high doses of radiation and certain diseases such as Sjögren's syndrome. This article reviews the mechanism of salivary secretion, effect of saliva on taste, importance of saliva in oral health, and hyposalivation in relation to ageing, medicine and/or disease and management of hyposalivation.

Induction of salivary polypeptides associated with parotid hypertrophy by gallotannins administered topically into the mouse mouth

Isoproterenol-induced salivary polypeptides (IISP), a group of proline-rich proteins synthesized by mouse parotids, have been considered as markers for isoproterenol-induced parotid hypertrophy. Rodents fed diets containing high-tannin cereals (sorghum), also develop parotid hypertrophy. To test whether tannins are directly involved in provoking sialotrophic growth, we studied the effect of intraperitoneal and topical oral administrations of tannic acid (TA) on the induction of IISP polypeptides in endogamic mice (A/Snell). TA was characterized by HPLC chromatography and spectral analysis and shown to be composed solely of gallotannins, a complex family of glucose and gallic acid esters. IISP polypeptides were monitored in saliva by SDS-polyacrylamide gel electrophoresis during 36 h after ending TA stimulation. Single daily intraperitoneal administrations of TA for 3 consecutive days (0.033 mg/g bw/day), at variance of parallel administrations of isoproterenol (0.042 mg/g bw/day) failed to induce IISP polypeptides. However, repeated topical applications of TA into the mouse mouths (1.21 mg/g bw divided into three equal doses given at 4-h intervals within a single day) resulted in unequivocal induction of IISP polypeptides. That response was clearly intensified by increasing the stimulation frequency to eight equivalent doses given at 1.5-h intervals within a single day (corresponding to 3.23 mg/g bw) and even further by repeating this protocol for 3 days. Under these productive schemes of stimulations by TA, electrophoretic fractionation of parotid homogenates showed new polypeptide bands migrating in parallel to salivary IISP. These results suggest that topically administered gallotannins are effective inducers of trophic growth in mouse parotids.

Codominant expression of genes coding for different sets of inducible salivary polypeptides associated with parotid hypertrophy in two inbred mouse strains

Experimental mouse parotid hypertrophy has been associated with the expression of a number of isoproterenol-induced salivary proline-rich polypeptides (IISPs). Mouse salivary proline-rich proteins (PRPs) have been mapped both to chromosomes 6 and 8. Recently, mice of two inbred strains (A/Snell and A. Swiss) have been found to differ drastically in the IISPs. In this study, mice of both strains were used for cross-breeding experiments addressed to define the pattern of inheritance of the IISP phenotype and to establish whether the IISPs are coded on a single or on several chromosomes. The IISP phenotype of individual mice was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of whole saliva collected after three daily stimulations by isoproterenol. Parental A/Snell and A. Swiss mice were homogeneous for distinctive strain-associated IISP-patterns. First filial generation (F1) mice obtained from the cross of A/Snell with A. Swiss mice expressed with no exception both the A/Snell and A. Swiss IISPs (coexpression). In the second filial generation (F2) both parental IISP phenotypes reappeared together with a majority of mice expressing the F1-hybrid phenotype (1:2:1 ratio). Backcrosses of F1 x A/Snell and F1 x A. Swiss produced offsprings displaying the F1 and the corresponding parental phenotypes with a 1:1 ratio. No recombinants were observed among F2 mice or among mice resulting from backcrosses. Thus, genes coding for the IISPs that are expressed differentially in both mouse strains are located on the same chromosome, probably at the same locus (alleles) or at quite closely linked loci (nonalleles).

Androgen regulation of SMR2 gene expression in rat submandibular gland: evidence for a graded but not a binary response

Expression of SMR2, a member of the gene family encoding salivary glutamine/glutamic acid-rich proteins, is regulated by androgens in rat submandibular gland acinar cells. To further characterize SMR2 regulation, we analyzed SMR2 expression during submandibular gland postnatal development and rat puberty at both a global and a single-cell level. Using in situ detection of mature and primary SMR2 transcripts, we show that SMR2 expression is heterogeneous among acinar cells. However, only one cell population with various amounts of mRNAs can be defined. The number of high-expressing cells increases in males during puberty and in females up to 6 weeks of age, suggesting that some factor in addition to acinar differentiation might be important for SMR2 expression in female rats. Involvement of the beta-adrenergic system in regulating SMR2 expression was tested in rats exposed daily to isoproterenol for 4 days. Under these conditions we found an increase in SMR2 expression in female rats, associated with an increase in SMR2 mRNA levels in most acinar cells. This suggests that a signaling cascade, elicited by beta-adrenergic stimuli, might act in concert with androgens to regulate SMR2 expression.

Differential expression of isoproterenol-induced salivary polypeptides in two mouse strains that are congenic for the H-2 histocompatibility gene complex

Two inbred mouse strains, A/Snell and A.Swiss, which were produced as congenic with regard to the H-2 histocompatibility gene complex, are homozygous for two different groups of isoproterenol-induced salivary polypeptides (IISP). These polypeptides, which have been considered as markers of the hypertrophic growth of the parotid acinar cells, are members of the complex family of salivary proline-rich proteins (PRP) on the basis of both their massive accumulation in the parotid acinar cells in response to chronic isoproterenol, secretory character, high solubility in trichloroacetic acid and metachromatic staining by Coomassie blue. IISP expressed in both mouse strains were identified by unidimensional SDS-polyacrylamide electrophoresis and Coomassie blue staining both in parotid gland homogenates and in whole salivas obtained from mice repeatedly stimulated at 24-h intervals with isoproterenol. Parotid glands from 40 mice (20 A/Snell and 20 A.Swiss) and salivas from 270 mice (200 A/Snell and 70 A.Swiss) were analyzed. One of the congenic strains (A/Snell) expressed five IISP (Mr 65, 61, 51.5, 38, and 37 kDa) and the other strain (A.Swiss) expressed six IISP (Mr 59, 57, 54.5, 46, 36, and 34 kDa). No inter-individual intra-strain variations were observed, thus defining strain-associated patterns of IISP (PRP).

Analysis of isoproterenol-induced changes in parotid gland gene expression

Parotid gland acinar cells undergo marked hypertrophy and hyperplasia upon systemic exposure to the beta-adrenergic agonist, isoproterenol. This glandular enlargement is accompanied by substantial cellular changes including DNA synthesis, an increase in glandular protein synthesis, and differential changes in RNA transcription. To gain a more detailed understanding of the underlying changes induced by isoproterenol, we have examined the parotid gland gene expression profile of mice up to 24 h post-isoproterenol injection using high-density oligonucleotide arrays. Depending upon the exposure time, between 22 and 48 of the approximately 6,500 mouse genes and expressed sequence tags (ESTs) analyzed displayed significant changes in expression patterns. Genes that were previously shown to be repressed (alpha-amylase) or activated (proline-rich proteins) following isoproterenol exposure were found to be similarly affected in this experiment, validating this technique. This study demonstrates that the oligonucleotide array technology is a useful tool for examining isoproterenol-induced salivary gland gene expression changes. Using this as a starting point, we can begin to dissect the specific pathways involved in mediating isoproterenol action within the parotid gland.

Targeted disruption of the Nhe1 gene fails to inhibit beta(1)-adrenergic receptor-induced parotid gland hypertrophy

Chronic beta(1)-adrenergic receptor activation results in hypertrophy and hyperplasia of rodent salivary gland acinar cells. Na(+)/H(+) exchanger isoform 1 (NHE1) regulates cell volume and the induction of cell proliferation in many tissues. To investigate the relationship between NHE1 and the response of parotid glands to beta(1)-adrenergic agonists, we examined by Northern blot analysis NHE1 expression in saline-treated mice and mice 30 min and 2, 6, and 24 h after isoproterenol injection. NHE1 transcripts increased approximately 50% by 2 h, and a more than twofold increase was noted at 24 h. Isoproterenol did not acutely increase Na(+)/H(+) exchanger activity; however, exchanger activity was significantly elevated by 24 h. To test whether NHE1 activity is essential for inducing salivary gland hypertrophy in vivo, mice with targeted disruption of Nhe1 were treated with isoproterenol. Na(+)/H(+) exchanger activity was absent in acinar cells from Nhe1(-/-) mice, nevertheless, the lack of NHE1 failed to inhibit isoproterenol-induced hypertrophy. These data directly demonstrate that acinar cell hypertrophy induced by chronic beta(1)-adrenergic receptor stimulation occurs independently of NHE1 activity.

Autonomic regulation of cystatin S gene expression in rat submandibular glands

Innervation of rat submandibular and parotid glands by the autonomic nervous system regulates saliva volume, its rate of secretion and its composition. The autonomic nervous system also plays a regulatory role in the differentiation and growth of salivary glands, and in the expression of specific sets of genes. Rat cystatin S, a member of family 2 of the cysteine proteinase inhibitor superfamily, is expressed in submandibular and parotid glands of human and rat. In the rat, cystatin S gene expression is tissue- and cell type-specific, is temporally regulated during postnatal development, and not observed in adult animals. The beta-adrenergic agonist isoproterenol (IPR) induces hypertrophic and hyperplastic enlargements of rat salivary glands and the expression of a number of genes including cystatin S. Sympathectomy reduces, but does not completely block, IPR-induced expression of the cystatin S gene in submandibular glands of adult female rats, indicating the participation of sympathetic factor(s) in its regulation. Bilateral parasympathectomy also reduces IPR-induced cystatin S gene expression, suggesting a role of the parasympathetic nervous system in its regulation. Experiments described in this paper suggest that similar factor(s) arising from both the sympathetic and parasympathetic branches of the autonomic nervous system simultaneously participate in IPR-induced cystatin S gene expression in submandibular glands.

Isoproterenol-induced expression of the cystatin S gene in submandibular glands of parasympathectomized rats

Parasympathetic innervation of rat submandibular and parotid glands regulates saliva volume, its rate of secretion and its composition. It also has a regulatory role in hypertrophy and hyperplasia of salivary glands, and in the expression of specific sets of genes. Rat cystatin S is a member of family 2 of the cysteine proteinase inhibitor superfamily. Cystatin S gene expression is tissue- and cell type-specific, temporally regulated during postnatal development, and not observed in adult animals. Isoproterenol (IPR), a beta-adrenergic agonist, induces hypertrophic and hyperplastic enlargement of rat salivary glands and expression of a number of genes including cystatin S. Sympathectomy reduces, but does not completely block IPR-induced expression of the cystatin S gene in the submandibular glands of adult female rats, indicating the participation of sympathetic factor(s) in this regulation. Since both sympathetic and parasympathetic branches of the autonomic nervous system act in parallel in the submandibular gland, it is possible that parasympathetic nerve terminals also provide factor(s) that play a role in regulation of cystatin S gene expression. Experiments described in this paper were designed to test the hypothesis that the parasympathetic nervous system participates in IPR-induced cystatin S gene expression. Bilateral parasympathectomy reduced IPR-induced cystatin S gene expression, suggesting a role of the parasympathetic nervous system in its regulation. Unilateral parasympathectomy in contrast, had no effect on IPR-induced cystatin S gene expression, suggesting that the presence of an intact parasympathetic innervation in the contralateral side permits the 'normal' IPR-induced expression of the cystatin S gene in the parasympathectomized gland.

Acinar cells are target cells for androgens in mouse submandibular glands

The variable coding sequence (VCS) multigene family encodes diverse salivary proteins, such as the SMR1 prohormone and the PR-VB1 proline-rich protein in the rat. In situ hybridization was used to study the cell-specific expression of two new mouse VCS genes, Vcs1 and Vcs2. We show that the Vcs1 transcripts, which code for a proline-rich protein, MSG1, are highly abundant in male and female parotid glands, in which they are specifically detected in acinar cells. No expression was seen in the submandibular or sublingual glands. In contrast, Vcs2 transcripts were found only in the acinar cells of the submandibular glands (SMGs) of male mice, in which they are expressed in response to androgens. Expression was found to be heterogeneous within acinar structures. No Vcs2 transcripts were detected in the SMGs of females or castrated males by Northern blot, RNase protection, or in situ hybridization. Androgen administration to females or castrated males induced expression at a level comparable to that of intact males. The Vcs2 gene is the first example of a mouse androgen-regulated gene that is expressed in SMG acinar cells. This result, in addition to our previous observation on SMR1 expression in rats, demonstrates that both acinar cells and granular convoluted tubule (GCT) cells are target cells for androgen action in rodent SMG.

Effect of sympathectomy on isoproterenol-induced expression of the cysteine proteinase inhibitor gene, cystatin S, in rat submandibular glands

The autonomic nervous system regulates the secretory function of salivary glands. The volume, rate of secretion and composition of saliva are regulated by both sympathetic (alpha 1-, alpha 2 and beta 1-adrenergic) and parasympathetic (muscarinic and cholinergic) receptor systems. The rat cystatin S gene, a member of family 2 of the cysteine proteinase inhibitor superfamily, has a very defined pattern of expression during the postnatal development of the rat submandibular gland. Its expression is not detected in the fetus or in rats up to three weeks of age. After this time, the amount of cystatin S mRNA increases, reaching a conspicuously high concentration at 28 days, and then it declines to a barely detectable level at 32 days of age; cystatin S mRNA is not detectable in the glands of adult animals. However, the beta-adrenoreceptor agonist isoproterenol (IPR) induces high concentrations of cystatin S mRNA in the submandibular gland in vivo. This paper reports experiments analysing the participation of the sympathetic nervous system in the IPR-induced expression of the cystatin S gene. Sympathetic denervation (unilateral and bilateral) by removing the superior cervical ganglion 14 days before a single injection of IPR reduced the expression of the cystatin S gene. Chemical denervation by reserpine (a drug that depletes neurotransmitters in sympathetic nerve terminals) also reduced IPR-induced expression of the gene. Morphological analyses of sympathectomized and reserpine-treated glands showed that the structure of the gland was similar to that of glands of intact animals and to those not treated with reserpine. The hypertrophic response to IPR was less obvious in the sympathectomized glands, but was similar in reserpine treated animals. Collectively, these data suggest that even in the presence of a functional beta 1-adrenergic receptor pathway, factor(s) from the sympathetic nervous system may be required for IPR-induced expression of the cystatin S gene.

Differential expression pattern of Rab-GDI isoforms during the parotid gland secretion cycle

Rab GDP dissociation inhibitor (GDI) plays an important role in regulating the GDP/GTP cycle of small GTP binding proteins of the Rab family. It also regulates their association to membranes. The small family of Rab-GDI consists of several closely related isoforms, the functional differences between which are still unknown. Here we show that multiple GDI isoforms are expressed in rat parotid gland and that the individual GDI isoforms have a characteristic expression both at the RNA and at the protein level, during the parotid secretory cycle. GDIalpha, the major isoform in brain, is expressed throughout the secretory process and is equally distributed between cytoplasmic and membranous fractions. In contrast, an isoform related to, but different from GDIbeta is found predominantly in the cytoplasmic fraction and its expression is detected only after beta-adrenergic stimulation of the gland, at the end of the secretion phase, when exocytosis is already completed. The induction of such a GDI isoform at the beginning of the recovery stage correlates with the expression pattern of Rab1 and Rab5, but not Rab2 and Rab4. Our results suggest different functional roles for multiple GDI isoforms along the secretion and recovery phases in rat parotid gland.

Involvement of nuclear orphan receptor NGFI-B in transcriptional activation of salivary-specific R15 gene by cAMP

Proline-rich proteins (PRPs) are selectively expressed in the acinar cells of the salivary glands and are inducible by beta-agonist isoproterenol and dietary tannins. In the previous studies of rat PRP gene, R15, the 5'-flanking region up to -1.7 kilobase pairs (kb), which was thought to contain the necessary proximal regulatory elements, failed to confer the catecholamine isoproterenol and dietary tannin inducibility to the transgene expression in the salivary glands of transgenic mice. Here we analyzed distal 5'-flanking region of R15 in order to understand the mechanisms of tissue-specific and inducible gene regulation. An upstream regulatory region located between -2.4 and -1.7 kb of the R15 5'-flanking region is demonstrated to be indispensable for the salivary-specific and inducible reporter gene expression in vivo, by transgenic approach. Element(s) within the 0.7-kb (-2.4 to -1.7) region that is able to cis-activate the expression of a heterologous reporter gene expression is further elucidated by transient transfection assays in vitro. Three distinct nuclear orphan receptor NGFI-B regulatory sequences are identified within a 184-base pair (bp) minimal control region extended from -1995 to -1812 nucleotides relative to the transcription start site. When reporter gene containing this 184-bp control region and heterologous promoter was cotransfected with the NGFI-B expression construct, a transactivation that mimics the effect of cAMP is observed in the parotid cells. Finally, mutations on all three identified NGFI-B binding sites and coexpression of a dominant negative mutant construct, pCMV-NGFI-B(Delta25-195), abolish this transactivation mediated by NGFI-B. In summary, these data suggest that the inducible nuclear orphan receptor NGFI-B may participate in the regulation of salivary acinar cell-specific and inducible expression of the rat R15 gene via three distinct distal NGFI-B sites.

Isoprenaline induces biosynthesis of proline-rich proteins in the salivary glands of rat but not in sheep

Long-term treatment of rats with isoprenaline resulted in induction of proline-rich proteins (PRPs) in the salivary glands, which were subsequently purified by TCA solubility and column chromatography. When rats were removed from beta-agonist regimen, then these proteins were no longer observed. Treatment of sheep with isoprenaline did not reveal the induction of PRPs.

Purification and characterization of rat parotid glycosylated, basic and acidic proline-rich proteins

A unique family of proline-rich proteins (PRPs) is induced in rats following prolonged isoproterenol treatment. PRPs can be divided into glycosylated (GPRP), basic (BPRP) and acidic (APRP) proline-rich proteins based on their physicochemical characteristics. Inducible rat parotid PRPs were isolated from aqueous extracts of parotid glands of isoproterenol-treated animals by sequential chromatography on columns of DEAE-Sepharose CL-6B, Sephadex G-100 and FPLC on Suprose-12 column. The GPRP showed a single homogeneous band on sodium dodecylpolyacrylamide gel electrophoresis with an estimated molecular weight of approximately 220,000. Compositional analysis of GPRP revealed that this protein contained 19.7% glutamic acid/glutamine, 28.2% proline and 9.5% glycine, and 44% carbohydrate, consisting of fucose (2.81g/100g), mannose (9.78g/100g), galactose (9.29g/100g), N-acetylglucosamine (18.03g/100g) and N-acetylgalactosamine (3.90g/100g). Basic PRPs consisted of a family of proteins with estimated molecular masses ranging from 14-45 kDa. These proteins contained 42.6% proline, 20.65% glutamic acid/glutamine and 21.33% glycine. Acidic PRPs also comprised of a family of metachromatically stained ladder of 40-60 kDa containing 29.1% proline, 21.5% glutamic acid/glutamine and 17.8% glycine. APRP were heavily glycosylated containing N-acetylglucosamine (6.34g/100g), N-acetylgalactosamine (19.04g/100g) and glucuronic acid (38.08g/100g).

Quantitative immunocytochemical study of secretory protein expression in parotid glands of rats chronically treated with isoproterenol

Chronic treatment of mice and rats with isoproterenol (IPR) causes marked hypertrophy and hyperplasia of the salivary glands, and alters the expression of several secretory proteins. We used quantitative postembedding immunogold labeling to study the cellular responses in the rat parotid gland during daily (up to 10 days) injections of IPR and during recovery (up to 14 days) after cessation of IPR treatment. Labeling densities of acinar cell secretory granules with antibodies to amylase and protein SMG-B1 (cross-reactive with the rat homologue of Parotid Secretory Protein, PSP) fell to 10% of control levels after 8-10 IPR injections, then increased during recovery, paralleling previous biochemical determinations of changes in protein and mRNA levels. With antibodies to proline-rich proteins (PRP), labeling densities initially fell, then subsequently showed considerable variability, but never exceeded control levels. These results contrast with biochemical determinations showing a marked induction of PRP synthesis, and may have both immunological and structural explanations. Occasional intercalated duct cells located close to the acini underwent differentiation toward an acinar-like phenotype as a result of IPR treatment. After 1-2 IPR injections, the secretory granules of these cells labeled with antibodies to amylase and PRP. Subsequently, the granules appeared electron-lucent and were increased in size and number. These observations support earlier work, suggesting that intercalated duct cells may differentiate into other gland cell types.

Stimulation of RNA synthesis in rat parotid lobules by phorbol myristate acetate

Incubation of parotid lobules with phorbol 12-myristate 13-acetate (PMA) transiently stimulated the incorporation of [3H]uridine into RNA. At 100 nM PMA, the rate of RNA synthesis during the first hour was 30% above control rates. The Ca2+ ionophore A23187 had no effect on either basal or PMA-stimulated RNA synthesis. Stimulation by 100 nM PMA was reversed by the protein kinase C inhibitor staurosporin (10 nM). When PMA was added together with isoproterenol or okadaic acid, both of which are potent activators of RNA synthesis, the increase in RNA synthesis was additive rather than synergistic. The results suggest that in the rat parotid gland, protein kinase C induces the rapid transcription of certain cellular genes by a mechanism that is independent of the beta-adrenergic receptor-activated pathway.

Adrenergic regulation of RNA synthesis in the rat parotid gland

Adrenergic regulation of RNA synthesis by in vivo stimulated parotid glands and dispersed parotid lobules was studied by a combination of in vivo and in vitro methods. Following a single intraperitoneal injection of isoproterenol, [3H]uridine incorporation into RNA was increased by 50% after the first hour. Amylase mRNA content was also elevated within 1 hr and was 2-3-fold higher than control values at 4 hr. An increase in the rate of total protein synthesis was detectable after 2 hr, and maximal rates were achieved 6 hr after isoproterenol administration. In dispersed parotid lobules, both isoproterenol and epinephrine stimulated [3H]uridine incorporation and at optimal concentrations increased incorporation by almost 200%. Phenylephrine (10 microM) caused a slight increase of about 20% whereas methoxamine (10 microM) had no effect. Stimulation by epinephrine was reversed by propranolol, but not by either phentolamine or prazosin. The increase in RNA synthesis induced by isoproterenol or epinephrine was dose dependent and half-maximal stimulation required 5.0 x 10(-8) M isoproterenol and 7.9 x 10(-7) M epinephrine. Dibutyryl cyclic AMP also stimulated [3H]uridine incorporation, whereas 8-bromo cyclic GMP, A23187 and phorbol myristate acetate had no effect. The importance of protein phosphorylation in mediating the observed stimulation was evaluated using protein kinase and phosphatase inhibitors. N-[2-(Methylamino)ethyl]-5-isoquinolinesulphonamide, an inhibitor of cyclic nucleotide-dependent protein kinases, substantially diminished the isoproterenol-induced stimulation. Okadaic acid treatment of lobules increased [3H]uridine incorporation. Furthermore, okadaic acid synergistically potentiated the stimulatory effect of a suboptimal concentration of isoproterenol. The results demonstrate that activation of the beta-adrenergic receptor induces the synthesis of certain RNA species in the parotid gland and that protein phosphorylation by a cyclic AMP-dependent protein kinase is a key event in the signal transduction pathway.

Salivary proline-rich proteins: biochemistry, molecular biology, and regulation of expression

The proline-rich proteins (PRPs) in mammalian salivary glands are encoded by tissue-specific multigene families whose members have diverged with respect to structure and regulation of expression. PRPs are expressed constitutively in humans, and comprise about [70%] of the total salivary proteins. Families of similar proteins are dramatically increased or induced in parotid and submandibular glands of rats, mice and hamsters by treatment with the [beta-] agonist isoproterenol. Feeding tannins to rats and mice mimics the effects of isoproterenol on the parotid glands. Salivary PRPs may constitute a defense mechanism against tannins and other polyhydroxylated phenols ingested. Putative transcriptional regulatory sequences have been identified in mouse PRP genes.

Sorting and secretion of salivary proteins

Most salivary proteins are stored in secretion granules prior to export from acinar cells in response to neural stimuli. A small subset of these proteins undergo unstimulated secretion without apparent storage. This pathway probably comprises vesicles that bud from maturing storage granules and carries proteins that do not aggregate efficiently at the storage site. Expression of a parotid proline-rich protein (and deletion mutants) in pituitary AtT-20 cells has shown that an N-terminal domain is necessary for storage in secretion granules. Evidence suggests that self-aggregation of proline-rich protein mediated by this domain may function in both efficient intracellular transport and storage. Thus selective aggregation may be an important secretory sorting mechanism.

Characteristics of protooncogene expression in A5 cells

Stimulation of beta-adrenergic receptors by isoproterenol or addition of 8-BrcAMP rapidly and transiently induces the expression of the protooncogenes, c-fos, and jun B, but not that of c-jun in A5 cells. These results indicate that different intracellular pathways may operate within the same cell for the induction of this group of early response genes. The inducibility of c-fos and jun B genes by either isoproterenol of 8-BrcAMP is transcriptionally regulated and accompanied by increases in their respective products. Furthermore, both c-fos and jun B mRNA levels are elevated at G0/G1 phase of the A5 cell cycle and are inducible by isoproterenol or 8-BrcAMP at the different phases of the cell cycle. These data further suggest a possible role of c-fos and jun B in A5 cell cycle.

Reciprocal expression of c-jun, proline-rich protein and amylase genes during rat parotid salivary gland development

We have investigated the temporal expression and cellular localization of the c-jun proto-oncogene and two major rat parotid gland secretory protein genes, PRP (proline-rich protein) and amylase, during postnatal development. c-jun mRNA steady-state levels increased at days 1, 7 and 14 after birth and decreased to basal levels at 21 days and older. PRP mRNA was first detected at 14 days and abruptly increased to adult levels at day 21. Amylase transcripts were first seen at day 7 and progressively increased to adult levels by 28 days. In situ hybridization demonstrated c-jun mRNA accumulation in the differentiating acinar cells and the ducts. The c-jun mRNA accumulation with time corresponds with the proliferative activity reported to occur in these two cellular populations. PRP transcripts were present exclusively in the well differentiated acinar cells while the accumulation of amylase mRNA corresponded to the progressive commitment of parotid cells to acinar differentiation. Our data suggest that during the postnatal development of the rat parotid gland: (a) c-jun expression associates with parotid gland proliferation and precedes the expression of PRP and amylase genes, and (b) activation of PRP and amylase genes is not concomitant and apparently occurs only in differentiating acinar cells.

Structure, organization and regulation of a rat cysteine proteinase inhibitor-encoding gene

During postnatal development, submandibular glands of rats produce the secretory protein, cystatin S (CysS), which belongs to family 2 of the mammalian cysteine proteinase inhibitor superfamily. While the rat CysS gene is not expressed in the salivary glands of adult rats, it can be induced by isoproterenol (IPR), which acts via beta-adrenergic receptor/adenylate cyclase/cyclic AMP (cAMP) mechanisms. In addition, IPR-induction of CysS mRNA in submandibular glands is more pronounced in females than in males, at both prepuberal and mature ages. These results suggest that sex hormones may participate in the regulation of the rat CysS gene via estrogen-responsive elements (ERE), and IPR induction of this gene supports the hypothesis that cAMP-responsive elements (CRE) may also play a role in regulating CysS gene expression. We have isolated, sequenced and characterized the complete gene. The CysS gene contains three exons interrupted by two intervening sequences, with consensus splice junctions. The transcription start point is 73 nucleotides upstream from the start codon which is surrounded by a typical Kozak sequence. CCAAT and TATA boxes are present in the 5'-flanking region of the CysS gene. This region also contains several possible regulatory elements that resemble those of other eukaryotic genes, i.e., ERE, CRE, and glucocorticoid-responsive elements. The first intron sequence contains other potential CRE highly homologous to those found in the IPR-inducible mouse and hamster proline-rich-protein-encoding genes.

Characteristics of c-fos and jun B gene expression in A5 cells after beta-adrenoreceptor stimulation and during the cell cycle

The beta-adrenoreceptor agonist isoproterenol elevates cAMP concentrations in the A5 rat salivary epithelial cell line and rapidly and transiently induces the expression of c-fos and jun B at 30 and 60 min following continuous stimulation of these cells. The induction of both genes is mediated by cAMP. We show here that the inducibility of these genes by isoproterenol or 8-BrcAMP is transcriptionally regulated and short (5 min) incubations of A5 cells with either agent is sufficient to trigger the induction of c-fos and jun B. We also have investigated the expression and inducibility of these genes during the A5 cell cycle. Both c-fos and jun B mRNA are elevated at the early phase of the cell cycle and are detectable throughout the cycle. At different stages of the cell cycle in synchronous A5 cells, both genes are as highly induced by isoproterenol or 8-BrcAMP as in asynchronous A5 cells. These studies provide the first evidence for the transcriptional regulation of c-fos and jun B by beta-adrenergic receptor stimulation or cAMP in an epithelial cell line (A5) and demonstrate the coordinate expression and inducibility of these genes at the different stages of the A5 cell cycle.

Cellular characteristics of long-term cultured rat parotid acinar cells

We have successfully maintained and biochemically characterized differentiated rat parotid acinar cells cultured for long periods (6 mo.). The cells were cultured on a reconstituted basement membrane matrix in a medium containing a variety of agents that promote cellular proliferation and differentiation. The cultured cells retain the characteristics of the parental parotid acinar cells. They exhibit both secretory granules and abundant cellular organelles required for protein synthesis and secretion. In situ hybridization and immunocytochemistry demonstrate high levels of proline-rich protein mRNA and protein, and lower levels of amylase mRNA and protein, in their cytoplasm. These findings suggest that rat parotid acinar cells can be maintained in a differentiated state in vitro for long periods, and can serve as a useful model system for studying the regulation of exocrine secretory processes.

Molecular characterization of rat multigene family encoding proline-rich proteins

Three members of the rat proline-rich protein multigene family have been characterized. Each of these genes, RP4, RP13, and RP15, contains three exons and they are approximately 4.8, 5.7, and 5.4 kb, respectively. The DAN sequences of RP4 and RP13 are greater than 93% homologous in the 3.1-kb segment extending from the 5'-upstream region (approximately nucleotide -930) to 238 nucleotides after the second exon/intron junction; however, regions further downstream, intron II and exon III, share less than 43% identity. In contrast, exon III from RP15, RP13, and the previously sequenced mouse PRP gene MP2 are more than 73% conserved. These analyses suggest that the duplication of the ancestral genes to RP13 and RP4 occurred prior to the divergence of the rat PRP genes. The results also indicate that in the past 21.5 million years, multiple recombination events have resulted in a very high degree of divergence among intron II and exon III of RP4 and RP13. This divergence is due in part to the insertion of members of the rat long interspersed repeat DNA family at -930 bp upstream from the transcription initiation site and within intron II of RP13. Comparisons of the nucleotide sequences and organization of exon I with the genomic organization of PRP and glutamic acid/glutamine-rich protein genes in this and previous studies reveal striking resemblance among these genes. These observations are consistent with the notion that this super multigene family arose from duplication of progenitor genes via unequal crossing over events. In addition, the results suggest that concerted evolution has occurred within the tandemly repeated motif of exon II.

Genetic regulation of salivary proteins in rodents

The presence of a protein in the cell is the result of a complex pathway that is known by the term gene expression. In this article we review the existing literature on the structure and expression of representative salivary gland genes and their regulated expression during development and upon extracellular stimulation. The expression of one of the "nuclear" protooncogenes, c-fos, in rat parotid glands is also discussed. Finally, we present some suggestions for future studies that will help to understand the mechanisms leading to gene regulation in rat salivary glands.

mRNAs for PRPs, statherin, and histatins in von Ebner's gland tissues

A search was made for expression of genes for proline-rich proteins (PRPs) and other salivary-type proteins, including statherin and histatins, in taste-bud tissues of mice and primates because of previous genetic findings in mice (Azen et al., 1986) that Prp and taste genes for certain bitter substances are either the same or closely linked. Taste-bud tissues and other tissues were tested for specific mRNAs with labeled DNA probes by Northern blotting and in situ hybridization. It was found that PRP mRNAs were present in von Ebner's glands of mice and macaques, and that there was a much greater degree of PRP mRNA induction in mouse parotid (16-fold) than in von Ebner's gland (two-fold) after in vivo isoproterenol stimulation. This difference may be due, in part, to differences in autonomic nerve innervation. Statherin and histatin mRNAs were found in macaque taste-bud tissues containing von Ebner's gland, and statherin protein was found in human von Ebner's gland by immunohistochemistry. The finding of PRP gene expression in von Ebner's gland, whose secretions have been suggested to play a role in taste stimulation, adds further support to a possible function of PRPs in bitter tasting. The possible functions of statherin and histatins in von Ebner's gland secretions may be related to statherin's regulation of salivary calcium and histatins' antibacterial and antifungal properties.

Two novel c-abl mRNAs are expressed in rat parotid salivary glands during in vivo beta-adrenergic receptor stimulation

The c-abl proto-oncogene is transcribed in most cell lines and tissues into two mRNAs of 6.5 and 5.3 kb, which have different 5' ends and encode two 150 kDa proteins that are largely colinear, but have different N-termini. We show here that two unusually short and abundant c-abl-related mRNAs of 1.5 and 1.3 kb appear in rat parotid salivary glands, within 1 day of in vivo administration of the beta-adrenergic receptor agonist isoproterenol. These transcripts are not found in the submandibular salivary gland or in the heart and they are too short to encode the known c-abl proteins. RNA blot, S1 nuclease protection and primer extension analysis suggest that the isoproterenol inducible parotid gland mRNAs do not contain the kinase domain, but represent part of the C-terminal segment of the abl reading frame.

Regulation of proto-oncogenes and salivary gland cell proliferation

The proto-oncogenes c-fos and c-jun express proteins targeted into the nucleus. The fos and jun proteins form a heterodimeric complex that binds to regulatory elements in the promoter region of specific genes to influence their transcription. Through such a mechanism, the fos and jun proteins have been suggested to link extracellular stimuli to short- and long-term functional changes in cells. Recently we have shown that beta-adrenergic receptor stimulation of rat parotid acinar cells in vitro or addition of 8-BrcAMP in a rat submandibular cell line (RSMT-A5) increases the expression of the c-fos gene in a time-dependent manner. Maximal responses were found at 60 min. The expression of the c-fos gene did not correlate with DNA synthesis in either cell type, and c-fos transcripts were undetectable in the glands of animals treated for eight days with isoproterenol. The new data presented here extended our observations to c-jun gene expression in both salivary cell types where a similar pattern of expression for this proto-oncogene was seen. Conversely, treatment of rats with isoproterenol for nine days resulted in the appearance of two c-abl mRNAs of unique size, in addition to the known 5.3-kb c-abl transcripts. The data suggest that beta-adrenergic receptor stimulation or exposure to 8-BrcAMP induces the early expression of the "nuclear proto-oncogenes" c-fos and c-jun before changes are noted in salivary epithelial cell proliferation. Differences in c-abl mRNA size, occurring later, may be associated with the morphological and biochemical changes known to occur in rat parotid glands after chronic beta-adrenoreceptor stimulation.

Interaction of saliva and taste

In spite of the coexistence of saliva and taste in the oral cavity, an understanding of their interactions is still incomplete. Saliva has modulating effects on sour, salt, and the monosodium-glutamate-induced savory or umami taste. It has a diminishing effect on sour taste as a result of the buffering by salivary bicarbonate. It probably also contributes to the umami taste with endogenous salivary glutamate levels. Salt taste is detected only when above salivary sodium-chloride concentrations; thus saliva influences salt taste threshold levels. It also provides the ionic environment for taste cells, probably critical in signal transduction. Salivary flow rate and composition are influenced by the type of taste stimuli. In general, sour taste, elicited by citric acid or sour food, induces the highest flow rate and Na+ concentrations, while salt gives rise to high protein and Ca2+ concentrations. Stimulation with the four basic taste modalities (sour, sweet, salty, and bitter), however, does not increase the relative proportion of any of the salivary proteins. This review examines the literature on the interactions of saliva with taste, and the effect of taste on salivary composition. The possible role of the von Ebner's salivary glands and the role of saliva as a chemical cue are also discussed.

Separation of the proteins of rat parotid saliva by hydrophobic-interaction chromatography

Parasympathetically-evoked rat parotid salivary proteins were resolved into 11 peaks by hydrophobic-interaction chromatography. SDS-PAGE indicated that most peaks contained a single major component and that only one major protein was not chromatographed. The findings indicate that hydrophobic-interaction chromatography can be used in the quantification of changes in individual salivary proteins under different experimental conditions, and in the purification of individual salivary proteins.

Transglutaminase expression in rat parotid gland after isoproterenol stimulation

Transglutaminases (E.C. 2.3.2.13) are calcium-dependent enzymes that catalyze the covalent cross-linking of proteins, and occur in multiple molecular forms in a variety of tissues. Distribution of each form of transglutaminase varies with different tissues. Studies were undertaken to characterize the form of transglutaminase expressed in rat parotid gland, and to examine a possible physiological role for the enzyme. It was found that chronic treatment of rats with the beta-adrenergic agonist isoproterenol (IPR) resulted in the induction of parotid transglutaminase activity. The properties of this transglutaminase appeared to be distinct from those of the well-characterized guinea pig liver cytosol transglutaminase (TGase C). The findings that protein polymerization (observed on SDS-PAGE) and incorporation of radioactive putrescine, a polyamine, into protein occur in the presence of exogenous transglutaminase and calcium indicated that certain rat parotid salivary proteins are or could be substrates for this enzyme. Analysis of proteolytic digests of rat parotid salivary proteins on an amino acid analyzer and by high-performance liquid chromatography also indicated that these salivary proteins contain gamma-glutamyl derivatives of primary amines (e.g., polyamines or lysine), post-translational products of transglutaminase catalysis. The possible physiological function of this enzyme in the oral cavity might be stabilization of proteinaceous structures during normal oral homeostasis and/or woundhealing.

Effects of isoproterenol treatment on gustatory neural responses in three inbred strains of mice

1. Treatment of a beta-agonist, isoproterenol, for 5 days reduced chorda tympani responses to sucrose by about 40% of the control without affecting responses to other taste stimuli, such as NaCl, HCl and quinine HCl, in balb CrSlc mice whereas such reduction of sucrose responses was not observed in C57BL/6-CrSlc and C3H/HeSlc mice, although in the latter two strains long-lasting off-responses to quinine HCl appeared after the treatment. 2. In BALB mice, the magnitude of reduction of sucrose responses by isoproterenol increased with prolonging the treatment from 1 to 5 days, although it reached almost its maximum level by the 3 days treatment. 3. BALB mice with the removal of the submandibular glands showed slightly greater control responses of the chorda tympani nerve to sucrose than BALB mice with the sham-operation or the removal of the sublingual glands, and showed no significant reduction of sucrose responses by isoproterenol treatment. 4. These results suggest that isoproterenol probably did not act directly on sweetener receptors of taste cell membranes but affect them through the submandibular salivary system.

Isolation and characterization of six proteins from rabbit parotid saliva belonging to a unique family of proline-rich proteins

Proline-rich proteins are major components of salivary secretion from humans non-human primates, rats, hamsters and rabbits. They are also synthesized in mice in response to chronic stimulation by beta agonists. This study to provide an understanding of the structural and genetic relationships within these families of proteins to determine the possible function of the proline-rich proteins. Rabbit parotid saliva was collected and proline-rich proteins were affinity purified using goat antibodies to human proline-rich proteins. Purification was achieved by repeated cation exchange chromatography on a Mono S column a Fast Protein Liquid Chromatography system. Six basic proline-rich proteins were purified. The apparent molecular weights were between 75,000 and 125,000, based on their mobilities in sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Glycine, glutamine (and glutamate) and proline accounted for 79-87% of total amino acids in all proteins, but proline was present in smaller amounts (17-21%) than in proline-rich proteins from other species. All proteins were glycosylated but not phosphorylated. Circular dichroism of two proline-rich proteins, MS7A and MS5B, indicated the absence of secondary structure. The N-terminal sequences of three proteins electro-eluted after preparative gel electrophoresis were determined. A high degree of similarity was found in various regions of mouse, rat, monkey and human proline-rich proteins. Rabbits thus synthesize constitutively a family of proteins that are immununologically and structurally related to proline-rich proteins other species.