Rat angiotensinogen is secreted only constitutively when transfected into AtT-20 cells

Adipocytes do not significantly contribute to plasma angiotensinogen

Recently, it has been reported that 25% of plasma angiotensinogen (Agt) is derived from fat. Meanwhile, liver-specific Agt knockout (KO) mice have markedly low plasma Agt, which may be due to reduced fat mass. To study the contribution of the fat to plasma Agt, we tested whether increasing fat mass can elevate plasma Agt and blood pressure in liver-Agt KO mice. Epididymal fat mass in liver-Agt KO mice fed a high-fat diet (HFD) was 4.1-fold larger than that in liver-Agt KO mice on a normal-fat diet (NFD). The liver-Agt KO mice on NFD were hypotensive with low levels of plasma Agt (on average, 0.11 vs 2.38 μg/ml). HFD slightly increased plasma Agt (0.17 μg/ml) without increase in blood pressure. To further increase fat mass, liver-Agt KO mice were fed HFD and simultaneously supplemented with low-dose angiotensin II and compared with control mice. Fat mass was comparable between the two groups. However, liver-Agt KO mice had uniformly low plasma Agt (0.09 vs 2.07 μg/ml) and systolic blood pressure (78±12 vs 111±6 mm Hg). In conclusion, adipocyte-derived Agt has essentially no contribution to the plasma concentration and no impact on blood pressure compared to liver-derived Agt.

Adipose tissue-specific increase in angiotensinogen expression and secretion in the obese (fa/fa) Zucker rat

We investigated angiotensinogen (AGT) expression in adipose tissue and liver of Zucker rats during the onset of obesity. The developmental pattern of AGT expression (protein and mRNA) in liver was similar in both genotypes. In inguinal adipose tissue, AGT cell content was similar in suckling and weaned pups in lean rats, whereas it continuously increased with age in obese rats. AGT amount in adipocytes was unaffected by the genotype until weaning. Thereafter, adipocytes from obese rats displayed a significant increase in AGT content that was strengthened with age. Compared with the cell content, the amount of secreted AGT over 24 h was higher, and a genotype effect was observed as early as 14 days of age. Using fat cell populations differing by size, we showed that this AGT oversecretion was not solely related to adipocyte hypertrophy. Our results demonstrate that the fa genotype exerts a control on the production of AGT in a tissue-specific manner, suggesting a local role of AGT in the overdevelopment of adipose tissue.

Calcium homeostasis in a clonal pituitary cell line of mouse corticotropes

Calcium homeostasis was studied following a depolarization-induced transient increase in [Ca2+]i in single cells of the clonal pituitary cell line of corticotropes, AtT-20 cells. The KCl-induced increase in [Ca2+]i was blocked in (i) extracellular calcium-deficient solutions, (ii) external cobalt (2.0 mM), (iii) cadmium (200 µM), and (iv) nifedipine (2.0 µM). The mean increase in [Ca2+]i in single cells in the presence of an uncoupler of mitochondrial function [carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, FCCP, 1 µM] was 54 ± 13 nM (n = 9). The increase in [Ca2+]i produced by FCCP was greater either during or following a KCl-induced [Ca2+]i load. However, FCCP did not significantly alter the clearance of calcium during a KCl-induced rise in [Ca2+]i. Fifty percent of the cells responded to caffeine (10 mM) with an increase in [Ca2+]i (191 ± 24 nM; n = 21) above resting levels; this effect was blocked by ryanodine (10 µM). Thapsigargin (2 µM) and 2,5 di(-t-butyl)-1,4 hydroquinone (BuBHQ, 10 µM) produced increases in [Ca2+]i (47 ± 11 nM, n = 6 and 22 ± 4 nM, n = 8, respectively) that increased cell excitability. These results support a role for mitochondria and sarco-endoplasmic reticulum calcium stores in cytosolic [Ca2+]i regulation; however, none of these organelles are primarily responsible for the return of [Ca2+]i to resting levels following this KCl-induced [Ca2+]i load.Key words: calcium homeostasis, intracellular calcium stores, anterior pituitary cells, mitochondria.

Rat proximal tubule cell line transformed with origin-defective SV40 DNA: autocrine ANG II feedback

The renal proximal tubule (PT) is a major site for a complete tissue renin-angiotensin system (RAS) and produces endogenous angiotensin II (ANG II). The present studies demonstrate autocrine RAS feedback in a line of origin-defective SV40 plasmid transformed immortalized rat PT cells (IRPTC) designated as line 93-p-2-1, which are highly differentiated and express all RAS components. Receptor competition assays and Southern blot following RT-PCR demonstrated that these IRPTC express AT1 and AT2 angiotensin receptor subtypes. Autocrine RAS feedback was examined following exposure to ANG II (10(-8) M), and it was noted that angiotensinogen mRNA increases significantly by 1 h and remains elevated through 24 h. The AT1 blocker losartan prevents this increase. Moreover, ANG II upregulates expression of ANG II receptor mRNA (both AT1 and AT2). Thus the present studies demonstrate positive ANG II feedback with angiotensinogen and ANG II receptors in PTC, suggesting that the main site of such intrarenal feedback in vivo is within PT. ANG II secreted by line 93-p-2-1 is increased by isoproterenol, suggesting beta-adrenergic regulation in IRPTC.

Autocrine/paracrine determinants of strain-activated brain natriuretic peptide gene expression in cultured cardiac myocytes

The application of mechanical strain leads to activation of human brain natriuretic peptide gene promoter activity, a marker of hypertrophy, in cultured neonatal rat ventricular myocytes. We have used a combination of transient transfection analysis and reverse transcriptase-polymerase chain reaction to examine the role of locally produced factors in contributing to this activation. Conditioned media from strained, but not static, cultures led to a dose-dependent increase in human brain natriuretic peptide gene promoter activity. This increase was completely blocked by losartan or BQ-123, implying a role for angiotensin and endothelin as autocrine/paracrine mediators of the response to strain. Inclusion of the same antagonists in the cultures themselves led to only partial inhibition (approximately 60%), whereas inclusion of exogenous endothelin or angiotensin II resulted in amplification of the strain response. Angiotensin II and endothelin appear to be arrayed in series in the regulatory circuitry; the angiotensin response was blocked by BQ-123, whereas the endothelin response was unaffected by losartan. Mechanical strain was also shown to stimulate expression of the endogenous angiotensinogen, angiotensin-converting enzyme, and endothelin genes in this system. Collectively, these data indicate that locally generated angiotensin II and endothelin, acting in series, play an important autocrine/paracrine role in mediating strain-dependent activation of cardiac-specific gene expression.

Angiotensinogen, prorenin, and renin are Co-localized in the secretory granules of all glandular cells of the rat anterior pituitary: an immunoultrastructural study

In addition to the circulating renin-angiotensin system (RAS), a local system has been postulated in the anterior pituitary because immunodetection of its components in various mammalian species. However, different cell types appear to be involved in different species, and there is no general consensus on the subcellular localization of prorenin, renin and angiotensinogen. In this ultrastructural study, we investigated and quantified the presence of these components using double or triple immunogold labeling methods, in all the immunologically identified glandular cell types of the rat anterior pituitary. In contrast to previous reports, all these components were identified not only in lactotropes and gonadotropes but also in somatotropes, corticotropes, and thyrotropes. The highest levels were detected in lactotropes and gonadotropes, and renin gave the greatest signal. Angiotensinogen, prorenin, and renin were co-localized in the secretory granules of all rat pituitary glandular cell types. The simultaneous detection of the substrate (angiotensinogen) and both its specific cleavage enzyme and its proenzyme within the same granule suggests intragranular processing of this component. Moreover, the localization of these three constituents in the secretory granules also suggests that, in the rat anterior pituitary, they follow the regulated secretory pathway.

Tissue angiotensinogen gene expression induced by lipopolysaccharide in hypertensive rats

There is now convincing evidence that various tissues express their own tissue renin-angiotensin system, which may be regulated independently of the systemic renin-angiotensin system. However, little information is available on the regulation of the tissue renin-angiotensin system. We investigated the regulation of tissue angiotensinogen gene expression with respect to the development of hypertension. We measured basal and lipopolysaccharide-stimulated plasma angiotensinogen concentrations by radioimmunoassay and examined the expression of tissue angiotensinogen by Northern blot analysis in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) at 4 and 13 weeks of age. Basal plasma angiotensinogen concentration in SHR was comparable to that in WKY at 4 weeks of age and was significantly higher than that in WKY at 13 weeks of age. Lipopolysaccharide induced a significant increase in plasma angiotensinogen concentration in both WKY and SHR at 4 and 13 weeks of age. At 4 weeks of age, the basal levels of angiotensinogen mRNA in the liver, fat, adrenal, and aorta were higher in WKY than in SHR. At 13 weeks of age, the basal levels of angiotensinogen mRNA in the fat, adrenal, aorta, spleen, and kidney were higher in WKY than in SHR, while that in the liver did not differ significantly between the two strains. At 4 weeks of age, pretreatment with lipopolysaccharide increased the angiotensinogen mRNA levels in the liver, fat, adrenal, and aorta in both WKY and SHR. At 13 weeks of age, pretreatment with lipopolysaccharide increased the angiotensinogen mRNA levels in the liver, aorta, and adrenal; decreased those in the spleen; and had no effect in the kidney in both WKY and SHR. Interestingly, lipopolysaccharide increased the angiotensinogen mRNA level in fat only in SHR, with no effect in WKY, at 13 weeks of age. Lipopolysaccharide stimulated tumor necrosis factor-a mRNA expression in fat of WKY and SHR, and the increase in tumor necrosis factor-alpha mRNA level in SHR was significantly greater than that in WKY. Therefore, the increased tumor necrosis factor-alpha mRNA expression may be involved in the increased lipopolysaccharide-induced expression of angiotensinogen gene in fat of SHR at 13 weeks of age. These data suggest that the transcriptional and probably posttranscriptional regulation of angiotensinogen mRNA differs between SHR and WKY, that the regulation of angiotensinogen gene expression is tissue-specific, and that the altered expression of the angiotensinogen gene may be involved in the development of hypertension.

Novel perspectives on pituitary and brain angiotensinogen

All the angiotensin peptides originate from angiotensinogen, a glycoprotein synthesized by several tissues, including the brain and the anterior pituitary. In the rat, immunohistochemistry has been used to localize angiotensinogen in gonadotropes and in uncharacterized cells surrounding sinusoids. Both cell types are capable of secreting angiotensinogen in cell culture; only the gonadotropes contain angiotensin II (AngII) and are capable of secreting it in culture. It has been asserted that the perisinusoidal cells are the only source of angiotensinogen for the generation of AngII by gonadotropes. Our current data favor the existence of a complete intracellular renin-angiotensin system (RAS) in gonadotropes and a separate extracellular system which utilizes the high concentration of angiotensinogen from perisinusoidal cells. Furthermore, we postulate that gonadotrope AngII serves mainly reproductive functions, while the proximity of angiotensinogen-secreting cells to folliculostellate cells, and their access to the intercellular sinusoidal and follicular spaces, places the extracellular RAS in a strategic position to affect pituitary growth and the mediation of acute-phase immune responses. In the rat brain, angiotensinogen is expressed by the 16-18th day of fetal life and by areas generally concerned with vasopressor, electrolyte, and fluid homeostasis. Antisense deoxyoligonucleotides to angiotensinogen mRNA lower blood pressure in hypertensive rats and inhibit in vitro growth of neuroblastoma cells, indicating a significant role for angiotensinogen in mitogenic and homeostatic functions. It is commonly agreed that astrocytes express angiotensinogen. Neuronal angiotensinogen has also been demonstrated by immunohistochemistry, as a secretion from neuronal cell cultures, and by reverse-transcriptase polymerase chain reaction. The fate of secreted astrocytic and neuronal angiotensinogen remains obscure. Angiotensinogen is regulated in a tissue-specific manner with smaller or absent responses observed for brain tissue. By using astrocyte and neuronal cultures the actions on angiotensinogen production of growth hormone, IGF-1, inflammatory lipopolysaccharide, and phorbol ester have been examined. Recent observations show that angiotensinogen is regulated positively or negatively by glucocorticoids and that a positive synergism between cAMP and glucocorticoids exists. On the basis of analogous systems for other proteins, a scheme involving glucocorticoid receptors, CREB, and AP-1 transcription factors is formulated to explain glucocorticoid-cAMP interactions. These transcriptional interactions may form a significant functional link between the RAS and adrenergic mechanisms.

Secretory granule content proteins and the luminal domains of granule membrane proteins aggregate in vitro at mildly acidic pH

A major unresolved issue in the field of secretory granule biogenesis is the extent to which the aggregation of granule content proteins is responsible for the sorting of regulated from constitutively secreted proteins. The aggregation process is postulated to take place in the trans-Golgi network and immature secretory granules as the proteins encounter mildly acidic pH and high calcium concentrations. We have developed in vitro assays that reconstitute the precipitation out of solution of secretory granule content proteins of anterior pituitary gland and adrenal medulla. In the assays, all of the major granule content polypeptides form a precipitate as the pH is titrated below 6.5, and this precipitate can be recovered in the pellet fraction after centrifugation. Addition of calcium is required for the aggregation of chromaffin granule content. In contrast to the proteins secreted by the regulated pathway, the constitutively secreted proteins IgG, albumin, and angiotensinogen, when added to the assays, remain predominantly in the supernatant. Among the individual proteins tested, prolactin is found to aggregate homophilically under these conditions and can drive the co-aggregation of other proteins, such as the chromogranins. Soluble forms of granule membrane proteins, including dopamine beta-hydroxylase and peptidyl glycine alpha-amidating enzyme also co-aggregated with granule content proteins. The results are consistent with the idea that spontaneous aggregation of proteins occurring under ionic conditions similar to those at the sites of granule formation is a property restricted to those proteins packaged in secretory granules. In addition, the association of luminal domains of membrane proteins with content proteins in vitro raises the possibility that analogous interactions between membrane-bound and content proteins also occur during granule formation in intact cells.

Location and secretion of brain angiotensinogen

Angiotensinogen is a glycoprotein with intriguing structural similarities to the serine proteinase inhibitors but with only one known function: to act as a substrate in the enzymatic generation of angiotensin peptides. It is expressed as a constitutive protein by the liver and various other tissues, including the brain. It is in this tissue that the expression of angiotensinogen attains its most complex and controversial manifestations. In late gestation, an unfolding of cellular expression occurs, starting at an epicentre in the eppendymal and astroglia cells of the hypothalamus, which rapidly and sequentially spreads to sub-cortical and then cortical regions, concentrating at sites of electrolyte, fluid and pressure regulation. This initial burgeoning of astroglial angiotensinogen is trailed by a wave of neuronal expression in various limbic and sensorimotor regions of the brain. The predominance of AT2 receptors in these regions suggests that the RAS actions are mediated by AT2 receptors. The angiotensinogen found in the CSF and secreted by cultures of glia and neurones is similar to the two major molecular sizes found in plasma. However, by electrophoretic separation on the basis of charge imparted by differential glycosylation, it can be shown that glia and neurones secrete distinct forms. The expression of different forms is under hormonal regulation. If these structural forms are shown to affect function, then the resulting ramifications may extend to pathological conditions, such as hypertension. Primary cell cultures of astrocytes secrete angiotensinogen constitutively and in a region-specific manner related to the size of the sub-population of secretory cells. Neurone cultures secrete angiotensinogen at about 25% the rate of hypothalamic astrocytes. The use of RT-PCR shows that both cell types express angiotensinogen mRNA. There is still an unresolved mismatch between these data and in situ hybridization histochemistry which shows expression limited to astrocytes but it is suggested that changes to more appropriate techniques will resolve any outstanding discrepancies.

An N-terminal hydrophobic peak is the sorting signal of regulated secretory proteins

Endocrine and exocrine cells each contain a regulated and constitutive secretory pathway. The presence of two distinct secretory pathways in the same cell type requires a sorting step to direct secretory proteins to the correct pathway. It is thought that regulated secretory proteins contain a specific sorting signal. However, this signal has not been identified. Amino acid sequence comparisons have not revealed any significant similarity between different regulated secretory proteins, suggesting that the sorting signal does not consist of a conserved primary sequence. In the present report, we have analyzed the predicted secondary structures of regulated secretory proteins and identified an N-terminal hydrophobic peak (NHP) which is located approximately from amino acids 9-26, overlaps with a predicted alpha-helix and contains charged amino acid residues. This signal is present in regulated secretory proteins that exhibit an N-terminal sorting sequence, but it is absent from constitutively secreted proteins and proteins where the sorting sequence is not located near the N-terminus. It appears that the NHP is both necessary and sufficient for sorting of many secretory proteins to the regulated secretory pathway.

Angiotensinogen: hormonal regulation and relative importance in the generation of angiotensin II

The production of angiotensinogen is controlled mainly by hormones that affect the concentration of its mRNA in tissues. Accordingly, hormones that act upon gene transcription play a prominent role. However, other factors may modulate the transcriptional effects of hormones, and these should be considered to appreciate the final effects of hormones on the secretion of angiotensinogen. The most important role played by hormones in the regulation of angiotensinogen may be to maintain its production in the face of rapid consumption by high levels of renin. However, elevated levels of angiotensinogen may become a risk factor in situations where the normal feedback regulation of renin does not operate normally. Finally, the synthesis of angiotensinogen in tissues may be regulated differentially than that in liver, although the exact importance of these observations is still poorly understood.

Angiotensinogen is secreted by pure rat neuronal cell cultures

Previous studies are divided between those which support a neuroglial (astrocyte) source for brain angiotensinogen and those which indicate that both astrocytes and neurones synthesize the precursor of angiotensin II. In this study, separate cultures of astrocytes and neuronal cells were prepared and established as being essentially pure by appropriate immunocytochemical cell markers. Angiotensinogen production by these cultures, as measured by a direct radioimmunoassay, was 20.74 +/- 3.62 ng angiotensinogen/10(6) cells/24 h (mean +/- S.D., n = 8) for astrocytes and 4.39 +/- 0.94 ng/10(6) cells/24 h (mean +/- S.D., n = 29) for neurones. Angiotensinogen secretion from both cell types was unaffected by treatments which stimulate the regulatory secretory pathway by modulating intracellular cAMP levels. In contrast, it was reduced from 23.20 +/- 2.14 to 8.14 +/- 1.31 ng/10(6) cells/24 h (S.E.M., n = 7) in astrocyte cultures by the constitutive pathway inhibitor, monensin. Angiotensinogen secreted by astrocytes and neurones was compared to pure angiotensinogen and that in plasma and cerebrospinal fluid (CSF) by cation-exchange mono S column chromatography. Pure angiotensinogen eluted as two separate peaks corresponding to the major forms of plasma angiotensinogen, whereas angiotensinogen in CSF and culture media coeluted with a third minor form of plasma angiotensinogen. It was concluded that neuronal cells as well as astrocytes secrete angiotensinogen which is distinctly different from plasma angiotensinogen.

The renin-angiotensin system in the pituitary gland

There is evidence that angiotensin II, in addition to being generated in the circulating blood, is synthesized in the anterior pituitary lobe and other endocrine tissues. Angiotensin II produced locally may act on pituitary cell receptors to modulate or mediate the action of other hormonal factors. However, tissue angiotensins may be synthesized by a different mechanism than most other neuroendocrine peptides. A precise understanding of the mode of formation of local angiotensin II is necessary for the comprehension of its physiologic role in the pituitary gland.

Expression and sorting of rat plasma kallikrein in POMC-producing AtT-20 cells

A vaccinia virus (VV) vector was used to express rat plasma kallikrein (rPK) in the constitutively secreting cells, BSC-40, and in the endocrine regulated cells, AtT-20. Using a specific rPK antibody and a fluorogenic substrate, Phe-Phe-Arg-AMC, we demonstrated that in both cell lines VV infections resulted in the synthesis of an immunoreactive enzyme predominantly present as a zymogen which can be activated with trypsin. Stimulation of VV:rPK-infected AtT-20 cells with either 5mM 8-bromo-cAMP or 56 mM KCl resulted in a different pattern of rPK and ACTH secretion, strongly suggesting that rPK follows the constitutive secretory pathway. Finally, the 10% rPK activity found within AtT-20 cell extracts had no effect on pro-opiomelanocortin (POMC) processing either intracellularly or extracellularly. The above data show that the biosynthetic machinery of both cell lines analyzed does not allow the efficient activation of plasma prekallikrein. Finally, despite the PK's demonstrated ability to cleave various hormone precursors in vitro at pairs of basic residues, in vivo, we did not obtain evidence that this hepatic enzyme can also act as an intracellular pro-protein processing enzyme.