Intracellular processing and secretion of parathyroid gland proteins

Biosynthesis and secretion of parathyroid hormone are sensitive to proteasome inhibitors in dispersed bovine parathyroid cells

Preproparathyroid hormone (prepro-PTH) is one of the proteins abundantly synthesized by parathyroid chief cells; yet under normal growth conditions, little or no prepro-PTH can be detected in these cells. Although this may be attributed to effective cotranslational translocation and proteolytic processing, proteasome-mediated degradation of PTH precursors may be important in the regulation of the levels of these precursors and hence PTH secretion. The effects of N-acetyl-Leu-Leu-norleucinal, N-acetyl-Leu-Leu-methional, carbobenzoxy-Leu-Leu-leucinal (MG132), benzyloxycarbonyl-Ile-Glu(t-butyl)-Ala-leucinal (proteasome inhibitor I), and lactacystin on the biosynthesis and secretion of PTH were examined in dispersed bovine parathyroid cells. We demonstrate that treatment of these cells with proteasome inhibitors caused the accumulation of prepro-PTH and pro-PTH. Compared with mock-treated cells, the processing of pro-PTH to PTH was delayed, and the secretion of intact PTH decreased in proteasome inhibitor-treated cells. Relieving the inhibition of the proteasome by chasing MG132-treated cells in medium without the inhibitor led to the rapid disappearance of the accumulated prepro-PTH, and the rate of PTH secretion was restored to levels comparable to those in mock-treated cells. Furthermore, overexpression of the Hsp70 family of molecular chaperones was observed in proteasome inhibitor-treated cells, and we show that PTH/PTH precursors interact with these molecular chaperones. These data suggest the involvement of parathyroid cell proteasomes in the quality control of PTH biosynthesis.

Quantitative and three-dimensional aspects of the rat parathyroid gland in normo-, hypo-, and hypercalcemia

The ultrastructure of the rat parathyroid has been under study for more than 35 years, but controversies still exist, especially regarding structure-function relationships. The present review focuses on recent morphological parathyroid research on rats under normal conditions and in various states of disturbed calcium metabolism. To facilitate discussions on functional aspects, current biochemical data, particularly those dealing with the regulation of parathyroid hormone synthesis and release, are also considered. Our results from quantitative studies and from investigations employing serial sectioning form the basis for the discussions. A central issue is whether the parathyroid secretory cells undergo secretory cycles. Prompted by results obtained from improved fixation procedures and serial sectioning, we question the basis for the theory of secretory cycles. Since the rat parathyroid secretory cell is polar, a single section is not an appropriate sample for estimating functional activity and for comparing the structure and distribution of intracellular components of adjacent cells. The heterogeneity in ultrastructural appearance of intracellular vesicles calls for the use of specific markers in relating the structure of the vesicular compartment to intracellular processing of hormone. The importance of unbiased quantitative techniques is illustrated in discussions on cell number and size for estimating the response of the parathyroid gland to different functional states or disorders demanding changes in secretion of parathyroid hormone, e.g., hyper- and hypocalcemia.

Chromogranin-A secretion from individual parathyroid cells: effects of 1,25-(OH)2 vitamin D3 and calcium

Chromogranin-A (CgA) is a 50-kDa protein located in and secreted by most endocrine and neuroendocrine cells along with native hormone. In the parathyroid gland, CgA is cosecreted with parathyroid hormone (PTH). Although these peptides are secreted together, recent evidence has suggested that they are processed differently in response to stimuli, such as 1,25-(OH)2 vitamin D3 and calcium. We have validated a reverse hemolytic plaque assay for studying CgA release from parathyroid cells. This assay allows for the detection of quantitative changes in hormone secretion from individual parathyroid cells. Bovine parathyroid cells were mixed with protein-A-linked ovine erythrocytes (oRBC) and plated in a monolayer in the presence of CgA antiserum. After incubation, complement was added to the cells to induce cell lysis. Lysis of oRBC around a parathyroid cell indicated the release of CgA from a cell. Results showed that plaque formation was dependent on assay reagents and that serial dilution of the antibody reduced plaque formation. CgA secretion was inhibited by increasing concentrations of calcium and stimulated by increasing concentrations of 1,25-(OH)2vitamin D3.

Effects of pancreastatin and chromogranin A on insulin release stimulated by various insulinotropic agents

The effects of porcine pancreastatin on insulin release stimulated by insulinotropic agents, glucagon, cholecystokinin-octapeptide (CCK-8), gastric inhibitory polypeptide (GIP) and L-arginine, were compared to those of bovine chromogranin A (CGA) using the isolated perfused rat pancreas. Pancreastatin significantly potentiated glucagon-stimulated insulin release (first phase: 12.5 +/- 0.9 ng/8 min; second phase: 34.5 +/- 1.6 ng/25 min in controls; 16.5 +/- 1.1 ng/8 min and 44.0 +/- 2.2 ng/25 min in pancreastatin group), whereas CGA was ineffective. The first phase of L-arginine-stimulated insulin release was also potentiated by pancreastatin (6.9 +/- 0.5 ng/5 min in controls, 8.4 +/- 0.6 ng/5 min in pancreastatin group), but not by CGA. Pancreastatin did not affect CCK-8 or GIP-stimulated insulin release. Similarly, CGA did not affect insulin release stimulated by CCK-8 or GIP. These findings suggest that pancreastatin stimulates insulin release in the presence of glucagon. Because pancreastatin can have multiple effects on insulin release, which are dependent upon the local concentration of insulin effectors, pancreastatin may participate in the fine tuning of insulin release from B cells.

Parathyroid cell number and size in hypercalcemic rats: a stereologic study employing modern unbiased estimators

Recently developed stereologic methods for unbiased estimations of particle number and size were employed to study parathyroid growth in normal and hypercalcemic young rats. Thus, the parathyroid cell number and size of parathyroid secretory cells were estimated by both the disector method and the volume-weighted mean volume method. The glandular volume was calculated from serial sections, and the volume density of secretory cells was estimated by conventional stereologic techniques. Three groups of animals were studied: normal rats at 3 weeks of age, hypercalcemic rats at 7 weeks of age, and age-matched controls. Hypercalcemia was induced by feeding the animals a purified diet that was nutritionally adequate except for low amounts of phosphate (0.02%) from 3 weeks of age. During the period from 3 to 7 weeks of age, the number of parathyroid secretory cells increased by 100%, whereas the mean cell volume increased by 20%. However, when calculated per gram body weight the volume and number of cells were larger in the younger animals. The phosphate-depleted animals grew slowly and developed severe hypercalcemia. Their parathyroid secretory cells were smaller, and each gland contained fewer cells than in age-matched controls. The lower cell number and cell volume, however, were proportional to the reduced body weight. Data from the 3-week-old animals indicate that the reduced cell number and size in hypercalcemic rats reflected growth arrest rather than atrophy.

Interaction of calcium with porcine adrenal chromogranin A (secretory protein-I) and chromogranin B (secretogranin I)

Secretory granules of endocrine cells contain one or more of the acidic secretory proteins chromogranin A (secretory protein-I), chromogranin B (secretogranin I), and secretogranin II (chromogranin C). It has been proposed that these proteins play a role in the packaging of secretory products. In the present study, lysates of purified porcine adrenal chromaffin granules containing chromogranins A and B and a putative chromogranin B fragment bound calcium and formed aggregates in the presence of 10-20 mM calcium at pH 5-6 and at 100 mM or less KCl, NaCl, or norepinephrine. The precipitates contained virtually all of the chromogranin B and the chromogranin B fragment and about one-third of the chromogranin A. The aggregates did not form or were dissociated at the pH and salt concentration of the extracellular fluid. Calcium precipitated purified chromogranin A and chromogranin B from pure solution to the same extent as from the granule lysates. Parathormone, added to the lysates, was incorporated in the precipitates, whereas the acidic secretory protein ovalbumin and norepinephrine were not. These findings suggest that secretory protein-I and secretogranin can exist in situ as aggregates that may include selected secretory products.

The parathyroid glands in the rat as seen by ultrathin step and serial sectioning

Parathyroid cells exhibit ultrastructural heterogeneity. This has been interpreted as an indication that the cells are undergoing secretory cycles, varying in synthetic activity. By electron microscopy we examined step sections at 1 and 2 micron intervals and serial sections of parathyroids from normocalcemic rats of three different age groups. The secretory cells in all three age groups exhibit structural polarity: nuclei and secondary lysosomes are generally located towards the interstitial tissue, whereas the Golgi is confined mainly to regions in the interior of the cell sheets. Cell surfaces facing the interstitial tissue are smooth, whereas plasma membranes in the interior of the cell sheets form tortuous areas. Consequently, an individual cell profile may show quite different appearance depending on section level. Our results do not support the concept that the observed heterogeneity is necessarily due to differences in organelle contents between the cells. The present investigation thus offers an alternative explanation to the ultrastructural variability among secretory parathyroid cells in the rat.

Stereological studies of the parathyroids in the young rat with hypercalcemia induced by severe phosphate depletion

In order to suppress the parathyroid glands by inducing hypercalcemia, young rats were fed a diet containing a low (0.02%) phosphate content. After 28 days blood samples were taken for estimation of serum calcium, phosphate and immunoreactive parathyroid hormone levels. Both parathyroids from each animal were subjected to serial sectioning so that the total glandular volume could be calculated by light microscopy. Volume and surface densities of cells and organelles were measured according to conventional stereological principles, so that the total volumes and surface areas could be estimated. Phosphate depletion caused marked growth retardation. The animals also developed hypophosphatemia, but in spite of pronounced hypercalcemia the levels of circulating immunoreactive parathyroid hormone remained unchanged. The volume of the parathyroids was reduced, but only to an extent commensurate with the reduced body mass. In the experimental group the volume density of cells was unchanged, but that of nuclei was increased; the volume density of Golgi complexes was reduced. The densities of the other cell components measured, namely the volume density of mitochondria and the surface densities of secretory cells, nuclear membranes and rough endoplasmic reticulum were unchanged. When the volumes and surfaces were expressed in absolute terms and related to total body mass, no differences between the groups were apparent.(ABSTRACT TRUNCATED AT 250 WORDS)

Opposite effects of 1,25(OH)2D3 on synthesis and release of PTH compared with secretory protein I

Aggregates of bovine parathyroid cells (organoids) were cultured with or without 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and examined for effects on the synthesis, cellular content, and release of parathyroid hormone (PTH) and secretory protein I. Organoids cultured with 120 pM 1,25(OH)2D3 at 1.35 mM calcium contained 27 +/- 2% (mean +/- SE) less and released 26 +/- 7% less PTH and contained 21 +/- 2% more and released 49 +/- 16% more secretory protein I than untreated organoids. After 15-min incubations with radioactive leucine, treated organoids incorporated 25 +/- 2% less radioactivity into precipitable cellular proteins than did untreated organoids, indicating that the overall synthetic rate of proteins was decreased by 1,25(OH)2D3 at 1.35 mM calcium. After separation of cellular proteins by gel electrophoresis, analysis of individual protein bands indicated that the mean treated-control ratio for proparathyroid hormone radioactivity was 0.88 +/- 0.06 (P less than 0.10) while that for secretory protein I was 1.56 +/- 0.17 (P less than 0.01), respectively. Experiments were performed at 1.2, 12, 120, and 1,200 pM 1,25(OH)2D3 and both 1.4 and 1.8 mM calcium. In both cases, increasing levels of 1,25(OH)2D3 progressively decreased PTH synthesis and release and progressively increased that of secretory protein I, significant effects being observed at 1.2 pM 1,25(OH)2D3. We conclude that at physiological calcium levels, 1.2-1,200 pM 1,25(OH)2D3 suppresses the synthesis and secretion of PTH, increases that of secretory protein I, and at similar concentrations decreases the overall protein synthetic rate compared with untreated controls.

Primary structure of bovine pituitary secretory protein I (chromogranin A) deduced from the cDNA sequence

Secretory protein I (SP-I), also referred to as chromogranin A, is an acidic glycoprotein that has been found in every tissue of endocrine and neuroendocrine origin examined but never in exocrine or epithelial cells. Its co-storage and co-secretion with peptide hormones and neurotransmitters suggest that it has an important endocrine or secretory function. We have isolated cDNA clones from a bovine pituitary lambda gt11 expression library using an antiserum to parathyroid SP-I. The largest clone (SP4B) (approximately equal to 1.6 kilobases) hybridized to a transcript of 2.1 kilobases in RNA from parathyroid, pituitary, and adrenal medulla. Immunoblots of bacterial lysates derived from SP4B lysogens demonstrated specific antibody binding to an SP4B/beta-galactosidase fusion protein (160 kDa) with a cDNA-derived component of 46 kDa. Radioimmunoassay of the bacterial lysates with SP-I antiserum yielded parallel displacement curves of 125I-labeled SP-I by the SP4B lysate and authentic SP-I. SP4B contains a cDNA of 1614 nucleotides that encodes a 449-amino acid protein (calculated mass, 50 kDa). The nucleotide sequences of the pituitary SP-I cDNA and adrenal medullary SP-I cDNAs are nearly identical. Analysis of genomic DNA suggests that pituitary, adrenal, and parathyroid SP-I are products of the same gene.