Rat Intermediate Pituitary Cells Research Articles

Profiling Signaling Peptides in Single Mammalian Cells Using Mass Spectrometry

The peptide content of individual mammalian cells is profiled using matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. Both enzymatic and nonenzymatic procedures, including a glycerol cell stabilization method, are reported for the isolation of individual mammalian cells in a manner compatible with MALDI MS measurements. Guided microdeposition of MALDI matrix allows samples to be created with suitable analyte-to-matrix ratios. More than 15 peptides are observed in individual rat intermediate pituitary cells. The combination of accurate mass data, expected cleavages by proteolytic enzymes, and postsource decay sequencing allows identification of 14 of these peptides as pro-opiomelanocortin prohormone-derived molecules. These protocols permit the classification of individual mammalian cells by peptide profile, the elucidation of cell-specific prohormone processing, and the discovery of new signaling peptides on a cell-to-cell basis in a wide variety of mammalian cell types.

Dopamine actions on calcium currents, potassium currents and hormone release in rat melanotrophs.

1. Intracellular and whole-cell recordings were made from primary cultures of rat intermediate pituitary cells; beta-endorphin secretion was also measured by radioimmunoassay. The effects of dopamine receptor activation on hormone secretion, calcium currents and resting potassium conductance were compared. 2. Spontaneous sodium-dependent action potentials occurred in 82% of cells recorded with intracellular microelectrodes and 64% of cells recorded with whole-cell patch electrodes; the same proportion of cells showed spontaneous calcium-dependent depolarizations in the presence of tetrodotoxin. 3. Calcium currents recorded from holding potentials of -90 or -70 mV showed transient and sustained components, both of which activated at -40 mV and had similar current-voltage relations. Bay K 8644 (1 microM) increased both components by about 130% while nifedipine (1-10 microM) decreased them by a maximum of 30%. Nickel (500 microM) inhibited transient and sustained components by 68 and 50%; cadmium (100 microM) abolished the current. omega-Conotoxin (1 microM) reversibly inhibited the transient component by 26%. 4. The dopamine D2 receptor agonist, quinpirole (0.1-10 microM) inhibited transient and sustained components in all cells by a maximum of 40 and 25% respectively. Quinpirole did not alter the time course of the current. 5. Quinpirole (1-100 nM) hyperpolarized 90% of cells from which intracellular recordings were made and 55% of cells recorded from with whole-cell patch pipettes. Maximum hyperpolarization of 16 +/- 4 mV from a resting potential of -44 +/- 5 mV was observed with 100 nM-quinpirole; concentration producing half-maximal effect was 3 nM. The hyperpolarization resulted from an increase in potassium conductance. 6. Quinpirole (1-100 nM) decreased basal beta-endorphin secretion by 55% and abolished secretion stimulated by Bay K 8644 or isoprenaline; concentrations producing half-maximal inhibitions were 5-10 nM. Tetrodotoxin (1 microM), nifedipine (1 microM), nickel (500 microM) and cadmium (100 microM) did not alter basal or stimulated secretion although higher concentrations of cadmium did inhibit stimulated hormone release. 7. Pertussis toxin pre-treatment prevented all actions of quinpirole. 8. Thus, concentrations of quinpirole that abolished stimulated hormone secretion did not alter calcium currents; conversely, concentrations of calcium channel blockers that partially or completely inhibited calcium currents did not alter basal or stimulated secretion. These results may indicate that calcium influx through the voltage-dependent calcium channels measured in these experiments does not contribute significantly to hormone release from melanotrophs.(ABSTRACT TRUNCATED AT 400 WORDS)

Proadrenocorticotropin/Endorphin Production and Messenger Ribonucleic Acid Levels in Primary Intermediate Pituitary Cultures: Effects of Serum, Isoproterenol, and Dibutyryl Adenosine 3′,5′-Monophosphate*

Long term primary rat intermediate pituitary cells cultured in complete serum-free medium (CSFM) were previously shown to exhibit a low basal secretory rate and maintain a stable cellular level of immunoactive pro-ACTH/endorphin (PAE)-related peptides. The present studies used biosynthetic labeling techniques to demonstrate that the biosynthesis of PAE-derived peptides declined as a function of the time the cultures were maintained in CSFM. Compared to hormone biosynthesis in freshly isolated cells, the ability of melanotropes maintained in CSFM to synthesize hormone declined approximately 3-fold after 5 days and 7-fold after 10 days. The endoproteolytic processing of PAE was not changed. This diminution in biosynthetic rate was not observed in serum-supplemented cultures; the rates of hormone secretion and hormone biosynthesis were both substantially higher in cultures maintained in serum-containing medium, suggesting that stimulatory factors were present in serum supplements. Chronic treatment of intermediate pituitary cultures in CSFM with 100 microM (Bu)2cAMP or 100 nM isoproterenol mimicked the effects of serum on total PAE production, hormone biosynthesis, and PAE mRNA levels. The diminished biosynthetic ability of cultures maintained in CSFM for several days could be restored by subsequent chronic (B mu)2cAMP or isoproterenol treatment. Taken together with previous information, these results suggest that melanotropes maintained in CSFM exist in a functionally basal state and that stimulatory agents, in addition to inhibitory inputs such as dopamine, exert significant regulatory control over intermediate pituitary lobe function in vivo.

The role of a low pH intracellular compartment in the processing, storage, and secretion of ACTH and endorphin.

To determine whether a low pH intracellular "sorting" step is required to route peptides into secretory granules, the effects of pH altering drugs on the biosynthesis and secretion of peptides by AtT-20 mouse corticotrope tumor cells and rat intermediate pituitary cells were examined. Doses of each drug maintaining normal protein synthesis and cell morphology, while obliterating the intracellular pH gradients detected by acridine orange fluorescence, were experimentally determined. Regions of the cell rich in secretory granules were localized by immunocytochemistry and were found to coincide with organelles with a low internal pH. Biosynthetic labeling experiments were coupled with immunoprecipitation and sodium dodecyl sulfate polyacrylamide gel analyses to examine the biosynthesis and secretion of corticotropin (ACTH(1-39], alpha-melanotropin, ACTH(18-39), beta-endorphin, gamma-melanotropin, alpha-amidated joining peptide, and the NH2-terminal region of pro-ACTH/endorphin. Chloroquine (20-40 microM) and a mixture of NH4Cl and methylamine (2-5 mM each) dissipated pH gradients but had no effect on the synthetic rate of pro-ACTH/endorphin, the extent and rate of precursor processing to smaller peptides, the rate of basal secretion of the various peptides, or the extent to which secretion of each of the peptides could be stimulated by secretagogues. Monensin (0.1-1 microM) had no discernible effect on intracellular pH gradients yet totally blocked proteolytic processing of pro-ACTH/endorphin. Thus, a monensin-blockable step occurs in peptide processing, presumably in the trans Golgi region; however, a low pH chloroquine-sensitive sorting step is not required for processing or for routing peptides to a stable storage form which can be released in response to secretagogues.

The characterization of the ascorbic acid-mediated alpha-amidation of alpha-melanotropin in cultured intermediate pituitary lobe cells.

Previous studies have shown that cultured rat intermediate pituitary lobe cells lose the ability to form ACTH-(1-13)NH2-related molecules (alpha MSH) and instead produce ACTH-(1-14)-related peptides. In vitro studies have shown that peptidylglycine alpha-amidating monooxygenase the enzyme responsible for the conversion of ACTH-(1-14) to ACTH-(1-13)NH2, requires ascorbate, CuSO4, and molecular oxygen as cofactors. In the present study we have demonstrated that cultured intermediate pituitary lobe cells require long term supplementation of the medium with ascorbate for continued production of immunoactive alpha-amidated alpha MSH. When the relative quantities of alpha-amidated and COOH-terminally glycine-extended forms of alpha MSH were assessed in biosynthetic labeling experiments, it was shown that either L-ascorbate or an epimer of the vitamin, D-isoascorbate, was capable of supporting cellular alpha-amidation. However, the potency of isoascorbate was approximately 4- to 5-fold lower than that of ascorbate. The ascorbate-mediated reestablishment of alpha-amidation ability was shown to be dependent on the presence of sodium in the medium; at physiological levels of ascorbate (35 microM), the EC50 for sodium was about 40 mM. Time-course experiments indicated that the time of exposure of the cultured cells to ascorbate could be decreased to as little as 30 min. However, a total incubation time of 6 h was required after such an exposure to convert biosynthetically labeled ACTH-(1-14)-related peptides to labeled ACTH-(1-13)NH2-related peptides. The time course of the effects of ascorbic acid on the reestablishment of alpha-amidation, as well as the relative stereospecificity for L-ascorbic acid and the sodium requirement, are all consistent with the hypothesis that ascorbic acid must be transported into the intermediate pituitary lobe cells to participate in the peptidyl alpha-amidation reaction. Moreover, it is apparent that the length of time required to reestablish alpha-amidation ability (6 h) is a function of transport of the cofactor into the granules, the rate of the peptidylglycine alpha-amidating monooxygenase-catalyzed reaction, or both.

Long term culture of primary rat pituitary adrenocorticotropin/endorphin-producing cells in serum-free medium.

Insulin, transferrin, and serum albumin were found to be essential additives for maintenance of primary rat pituitary ACTH/endorphin cells in complete serum-free defined medium (CSFM). Primary anterior pituitary cultures maintained in CSFM exhibited a 3- to 5-fold increase in cell content of ACTH/endorphin-related peptide during the first 2 weeks in culture, and this level remained stable for at least the next week. Immunocytochemical and morphometric studies indicated that the number of corticotropes increased only slightly, so that hormone content per corticotrope increased. Anterior pituitary cultures maintained in CSFM exhibited a basal secretory rate of 0.3-0.4% of cell hormone content/h throughout the 3-week period in culture, and total hormone production increased 6-fold. Primary anterior pituitary cultures maintained chronically in CSFM containing 10 nM CRF demonstrated a 15- to 20-fold increase in total ACTH/endorphin production over 3 weeks in culture. Chronic treatment with CRF brought about a sustained 6-fold increase in secretory rate (2.5% of cell hormone content/h), and corticotrope content of hormone was diminished 3-fold. Corticotropes maintained chronically in CSFM containing CRF did not increase in number and exhibited a rim of immunocytochemically identifiable hormone around the cell periphery. Anterior pituitary cultures maintained chronically in CSFM containing 100 nM dexamethasone (DEX) exhibited decreased cell hormone content and an unaltered secretory rate. In the DEX-treated cultures the number of immunocytochemically identifiable corticotropes declined, as did the staining intensity per corticotrope. Primary cultures of rat intermediate pituitary cells maintained in CSFM exhibited a 1.5- to 2-fold increase in hormone content after 1-2 weeks in culture, maintained a constant basal secretory rate of 0.4-0.5% cell content/h, and were not responsive to chronic treatment with CRF or DEX.

The alpha-amidation of alpha-melanocyte stimulating hormone in intermediate pituitary requires ascorbic acid.

Rat intermediate pituitary cells in primary culture display a time-dependent loss of the ability to produce COOH-terminally alpha-amidated alpha MSH (Glembotski, C.C., Eipper, B.A., and Mains, R.E. (1983) J. Biol. Chem. 258, 7299-7304). Instead of des-, mono-, and diacetyl-adrenocorticotropic hormone(ACTH) (1-13)NH2, the cells produce des-, mono-, and diacetyl-ACTH(1-14)OH. Since the pituitary secretory granule-associated alpha-amidation enzyme requires copper and ascorbic acid for optimal activity (Eipper, B.A., Mains, R.E., and Glembotski, C. C. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 5144-5148), these cofactors were added to cultures of intermediate pituitary cells in an attempt to reverse the loss of peptide alpha-amidation ability. When the cultures were supplemented with up to 100 microM cooper (II) there was very little change in the ability to alpha-amidate alpha-melanotropin. Ascorbic acid at concentrations of up to 500 microM resulted in a dramatic increase in the ability of the cells to form the alpha-amidated peptide. Various combinations of ascorbic acid and copper additions indicated that a relatively short exposure (hours) to ascorbic acid produced the maximal response. Ascorbic acid displayed a dose-dependent effect on the alpha-amidation ability with a half-optimal concentration of about 25 microM. Pulse-chase labeling experiments demonstrated the ascorbic acid-dependent conversion of labeled ACTH(1-14)OH-related peptides to ACTH(1-13)NH2-related peptides. These results correlate with the ascorbic acid requirement of the pituitary alpha-amidation enzyme and demonstrate that the direct precursors to ACTH(1-13)NH2-related peptides are ACTH(1-14)OH-related peptides. Combined with our previous data, the present studies support the notion that a wide range of neuro- and endocrine peptides become alpha-amidated in a similar ascorbic acid-dependent manner.

Selective loss of alpha-melanotropin-amidating activity in primary cultures of rat intermediate pituitary cells.

The production of alpha-melanotropin (alpha-N-acetyl-ACTH(1-13)NH2 (alpha MSH) in rat intermediate pituitary was investigated using reverse phase high performance liquid chromatography, immunoassays, and biosynthetic labeling. Extracts of rat intermediate pituitary contain primarily alpha-N,O-diacetyl-ACTH(1-13)NH2, with smaller amounts of des- and monoacetyl-ACTH(1-13)NH2. No significant amount of ACTH(1-14)OH- or ACTH(1-13)OH-related material was observed. Analysis of the newly synthesized alpha MSH-sized peptides produced by freshly prepared rat intermediate pituitary cell suspensions revealed labeled peptides co-migrating primarily with des-, mono-, and diacetyl-ACTH(1-13)NH2. In contrast, when extracts of 2-week-old intermediate pituitary cultures were analyzed, the major peak of material detected with the general NH2-terminal ACTH antiserum eluted at the position expected for alpha-N,O-diacetyl-ACTH(1-14)OH; smaller amounts of material co-migrating with des- and monoacetyl-ACTH(1-14)OH were also detected. Analysis of the newly synthesized alpha MSH-sized peptides produced by 16-day-old cultures of intermediate pituitary cells revealed labeled peptides co-migrating with des-, mono-, and diacetyl-ACTH(1-14)OH. Thus, rat intermediate pituitary cells in culture maintain the ability to perform several major proteolytic cleavages of pro-ACTH/endorphin as well as the ability to alpha-N-acetylate alpha MSH and beta-endorphin, but selectively lose the ability to alpha-amidate the carboxyl terminus of alpha MSH.

Adrenocorticotropin(1-14)OH-related molecules in primary cultures of rat intermediate pituitary cells. Identification and role in the biosynthesis of alpha-melanotropin.

The structures of the altered alpha-melanotropin (alpha MSH or alpha-N-acetyl-ACTH(1-13)NH2)-related molecules produced by cultured rat intermediate pituitary cells were investigated. Peptide analyses demonstrated that the alpha MSH-related molecules produced by acutely prepared intermediate pituitary cells were primarily des-, mono-, and diacetylated ACTH(1-13)NH2; in contrast, longer term cultures produced primarily des-, mono-, and diacetylated ACTH(1-14)OH. No significant amount of labeled ACTH(1-13)OH-related material was observed under any incubation conditions. Intermediate pituitary cells in culture stopped producing alpha-amidated alpha MSH-related molecules with a half-time of approximately 15 to 18 h; instead, ACTH(1-14)OH-related molecules were synthesized. In several pulse-chase experiments, performed under conditions where cultured intermediate pituitary cells were capable of synthesizing alpha MSH-related molecules terminating in -Val13-NH2, labeled molecules ending in -Val13-Gly14-OH were not found to give rise to major amounts of labeled molecules ending in -Val13-NH2. This failure to observe conversion of glycine-extended molecules into alpha-amidated products was not due to selective secretion from the cells, since the acetylation and amidation states of labeled molecules that were secreted under basal conditions reflected those of the molecules stored in the cells, and the basal rate of secretion was very low.

Further analysis of post-translational processing of beta-endorphin in rat intermediate pituitary.

Rat intermediate pituitary cells maintained in culture synthesize the same forms of beta-endorphin observed in intermediate pituitary extracts. Biosynthetically labeled intermediate pituitary beta-endorphin-sized material was fractionated by ion exchange chromatography on sulfopropyl-Sephadex and the identities of the major peaks were determined by co-chromatography with synthetic marker peptides, gel filtration, and analysis of pronase, chymotrypsin, and trypsin digests. Peaks of alpha-N-acetyl-beta-endorphin(1-27), alpha-N-acetyl-beta-endorphin(1-31), and beta-endorphin(1-31) were identified and a fourth peak (eluting from the sulfopropyl-Sephadex column at 0.18 M NaCl) was tentatively identified as alpha-N-acetyl-beta-endorphin(1-26). Analysis of beta-endorphin synthesized in the presence of [35S]methionine and [3H]histidine confirmed the absence of His in the material eluting at 0.18 M NaCl. Based on both steady labeling and pulse-chase incubations, beta-endorphin(1-31) was the first form of labeled beta-endorphin-sized material to appear in cell extracts. This molecule was quickly N-acetylated on its NH2-terminal tyrosine residue and was then more slowly converted to alpha-N-acetyl-beta-endorphin(1-27) and then to alpha-N-acetyl-beta-endorphin(1-26).

Synthesis and secretion of corticotropins, melanotropins, and endorphins by rat intermediate pituitary cells.

The synthesis and secretion of various intermediate pituitary proteins was studied by using dispersed intermediate pituitary cell suspensions. Control studies indicated that the isolated cells were obtained in good yield and that after more than 24 h in culture the isolated cells continued to synthesize a collection of proteins similar to those found in freshly extracted intermediate pituitary tissue. Rat intermediate pituitary cells synthesized a molecule (Mr = 30,000; called 30K) that contained antigenic determinants for beta-endorphin, gamma-lipotropin, corticotropin (ACTH), and 16K fragment (the NH2-terminal region of mouse tumor cell pro-ACTH/endorphin). This 30K molecule, two high molecular weight forms of ACTH(13K and 20K), and 16K fragment were all shown to be glycoproteins. Continuous labeling and pulse-chase incubations were used to define the intracellular biosynthetic processing of the 30K molecule. After a 15-min pulse incubation the 30K molecule was the only labeled protein containing antigenic determinants for beta-endorphin, gamma-lipotropin, ACTH, or 16K fragment. A beta-lipotropin-like molecule served as a biosynthetic intermediate in the production of proteins similar to beta-endorphin and gamma-lipotropin. Methionine-enkephalin and alpha-endorphin were not major products in the intermediate lobe cells. Molecules similar to alpha-melanocyte-stimulating hormone and corticotropin-like intermediate lobe peptide (ACTH(18-39)) were also derived from the same 30K molecule; 20K ACTH served as a biosynthetic intermediate in this conversion. In rat intermediate pituitary cells ACTH(1-39) was not a major final product of the intracellular biosynthetic processing of the 30K molecule. The 30K molecule also served as a precursor to a protein similar to mouse tumor cell 16K fragment and related smaller proteins. With rat intermediate pituitary cells, pulse-chase experiments utilizing [35S]methionine demonstrated almost quantitative conversion of the 30K precursor into labeled proteins similar to beta-endorphin and alpha-melanocyte-stimulating hormone. In the absence of added secretagogues, small amounts of all of the smaller proteins derived from the 30K precursor were secreted coordinately into the culture medium.

Calcitionin is not contained within the common precursor to corticotropin and endorphin in the rat

The suggestion that calcitonin is contained within the structure of the common precursor to ACTH and endorphin was examined. Immunohistochemical staining demonstrated calcitonin in thyroid parafollicular cells, and ACTH and 16K fragment in ACTH/endorphin cells of pituitary. No 16K fragment immunostaining was detected in thyroid parafollicular cells; no calcitonin staining was detected in pituitary. Immunoprecipitation of [ 35S]methionine-labeled molecules synthesized by rat intermediate pituitary cells demonstrated that neither 30K precursor, 16K fragment nor any other major labeled cell product was recognized by calcitonin antiserum. Analyses of tryptic peptides of 30K precursor indicated that peptides expected from calcitonin were not present in 30K precursor.