Characterization of rat somatotroph and mammotroph secretory granules. Presence of sulfated molecules

Growth Hormone(s), Testosterone, Insulin-Like Growth Factors, and Cortisol: Roles and Integration for Cellular Development and Growth With Exercise

Hormones are largely responsible for the integrated communication of several physiological systems responsible for modulating cellular growth and development. Although the specific hormonal influence must be considered within the context of the entire endocrine system and its relationship with other physiological systems, three key hormones are considered the "anabolic giants" in cellular growth and repair: testosterone, the growth hormone superfamily, and the insulin-like growth factor (IGF) superfamily. In addition to these anabolic hormones, glucocorticoids, mainly cortisol must also be considered because of their profound opposing influence on human skeletal muscle anabolism in many instances. This review presents emerging research on: (1) Testosterone signaling pathways, responses, and adaptations to resistance training; (2) Growth hormone: presents new complexity with exercise stress; (3) Current perspectives on IGF-I and physiological adaptations and complexity these hormones as related to training; and (4) Glucocorticoid roles in integrated communication for anabolic/catabolic signaling. Specifically, the review describes (1) Testosterone as the primary anabolic hormone, with an anabolic influence largely dictated primarily by genomic and possible non-genomic signaling, satellite cell activation, interaction with other anabolic signaling pathways, upregulation or downregulation of the androgen receptor, and potential roles in co-activators and transcriptional activity; (2) Differential influences of growth hormones depending on the "type" of the hormone being assayed and the magnitude of the physiological stress; (3) The exquisite regulation of IGF-1 by a family of binding proteins (IGFBPs 1-6), which can either stimulate or inhibit biological action depending on binding; and (4) Circadian patterning and newly discovered variants of glucocorticoid isoforms largely dictating glucocorticoid sensitivity and catabolic, muscle sparing, or pathological influence. The downstream integrated anabolic and catabolic mechanisms of these hormones not only affect the ability of skeletal muscle to generate force; they also have implications for pharmaceutical treatments, aging, and prevalent chronic conditions such as metabolic syndrome, insulin resistance, and hypertension. Thus, advances in our understanding of hormones that impact anabolic: catabolic processes have relevance for athletes and the general population, alike.

Prolactin and growth hormone aggregates in secretory granules: the need to understand the structure of the aggregate

Prolactin and GH form reversible aggregates in the trans-Golgi lumen that become the dense cores of secretory granules. Aggregation is an economical means of sorting, because self-association removes the hormones from other possible pathways. Secretory granules containing different aggregates show different behavior, such as the reduction in stimulated release of granules containing R183H-GH compared with release of those containing wild-type hormone. Aggregates may facilitate localization of membrane proteins necessary for transport and exocytosis of secretory granules, and therefore understanding their properties is important. Three types of self-association have been characterized: dimers of human GH that form with Zn(2+), low-affinity self-association of human prolactin caused by acidic pH and Zn(2+) with macromolecular crowding, and amyloid fibers of prolactin. The best candidate for the form in most granules may be low-affinity self-association because it occurs rapidly at Zn(2+) concentrations that are likely to be in granules and reverses rapidly in neutral pH. Amyloid may form in older granules. Determining differences between aggregates of wild type and those of R183H-GH should help to understand why granules containing the mutant behave differently from those containing wild-type hormone. If reversible aggregation of other hormones, including those that are proteolytically processed, is the crucial act in forming granules, rather than use of a sorting signal, then prohormones should form reversible aggregates in solution in conditions that resemble those of the trans-Golgi lumen, including macromolecular crowding.

Sulfur-containing macromolecules in cultured monocyte-like cells

The monocytic cell line U-937 was cultured in vitro in the absence or presence of phorbol myristate acetate, and agent known to induce differentiation of these cells along the monocyte/macrophage lineage. The cells were fixed, prepared for, and subjected to transmission scanning electron microscopy. The intracellular structures of the cells were compared with those of mastocytoma cells. The latter cell type, known to produce heparin, was shown to contain numerous electron dense granules; by X-ray micro-analyses shown to contain significant amounts of sulfur. In contrast, neither control nor PMA-treated U-937 cells contained such granules. Both control and PMA-treated U-937 cells were pulsed with 35S-sulfate for 60 min and chased. The amount of 35S-proteoglycan in the medium of both cell cultures was found to increase in a time-dependent manner, suggesting that these products are destined for release and not intracellular storage under in vitro conditions.

Presence of glycoconjugates in prolactin granules of male rats

Prolactin granules in the anterior pituitary glands of male rats contain densely stained materials at the periphery of the matrix. These occur in both small spherical and large polymorphic types of granules. The presence of densely stained materials around secretory granules may be a useful criterion for identification of prolactin cells since the dense structure was observed in 95% of these cells after conventional staining by uranyl acetate and lead citrate. The localization of glycoconjugates in the prolactin granules was examined by applying concanavalin A (Con A) on the ultrathin sections. HRP-Con A or ferritin-conjugated Con A bound specifically to the densely stained materials in the peripheral region of the prolactin granule matrix, indicating that this densely stained matrix contains glycoconjugates; the significance thereof is discussed with reference to the concentration and packaging of secretory product.

Accumulation of mercury in the anterior pituitary of rats following oral or intraperitoneal administration of methyl mercury

A sensitive histochemical technique has been used to visualize the ultrastructural localization of mercury in the anterior pituitary of rats which have been exposed to methyl mercury. After administration of methyl mercury in the drinking water (20 mg X l-1 methyl mercury in distilled water) or intraperitoneally (daily dose 100 ug or 200 ug methyl mercury) intracellular accumulations of mercury were found in the lysosomes and granules of secretory cells (somatotrophs, thyrotrophs and corticotrophs). In non-secretory cells (follicular cell and marginal layer cells) mercury deposits were found in lysosomes. In orally treated rats, the number of mercury deposits increased significantly with time up to day 21. In rats exposed intraperitoneally, a continuous increase was seen in intracellular mercury accumulation. Apart from vacuolation of lysosomes, no structural damage was observed in the cells containing mercury.

Specific and reversible degranulation of pituitary mammotrophs

When dissociated anterior pituitary cells were cultured over a layer of gelatin, electron microscopic observation showed the mammotrophs in the culture to be essentially degranulated and to have massively dilated rough endoplasmic reticulum (RER) and Golgi apparatus (GA), while there were no discernible effects on any of the other cell types. PRL synthesis in the gelatin-cultured cells occurred at twice the rate of that in the control cells and the gelatin-cultured cells contained 3% of the normal amount of PRL. Transfer of the cells to a more traditional culture situation reversed most of the morphological changes within 24 h. The mammotrophs were responsive to thyrotropin-releasing hormone (10(-7) M) both immediately after gelatin culture (3-fold control) and after reversal (5-fold control). As placement of the gelatin-cultured cells into traditional culture synchronizes the onset of granulation, we conclude that mammotrophs cultured in this fashion will be useful in the further study of PRL synthesis, packaging, storage and degradation.

Pathways of protein secretion in eukaryotes

Protein secretion from cells can take several forms. Secretion is constitutive if proteins are secreted as fast as they are synthesized. In regulated secretion newly synthesized proteins destined for secretion are stored at high concentration in secretory vesicles until the cell receives an appropriate stimulus. When both constitutive and regulated protein secretion can take place in the same cell a mechanism must exist for sorting the correct secretory protein into the correct secretory vesicle. The secretory vesicle must then be delivered to the appropriate region of plasma membrane. Transfection of DNA encoding foreign secretory proteins into regulated secretory cells has provided insight into the specificity of sorting into secretory vesicles.

The major tyrosine-sulfated protein of the bovine anterior pituitary is a secretory protein present in gonadotrophs, thyrotrophs, mammotrophs, and corticotrophs

The anterior pituitary is a complex secretory tissue known to contain several sulfated macromolecules. In the present study, we identified the major tyrosine-sulfated protein of the bovine anterior pituitary and investigated its cellular and subcellular localization. This protein consisted of two tyrosine-sulfated polypeptides of molecular weight 86,000 and 84,000 that were highly homologous to each other. In agreement with previous biochemical studies, the tyrosine-sulfated protein of Mr 86,000/84,000 was found to be secretory, as it was observed in the matrix of secretory granules by immunoelectron microscopy. Immunofluorescence studies indicated that the tyrosine-sulfated, secretory protein of Mr 86,000/84,000, referred to as TSP 86/84, was present in all endocrine cells except for some somatotrophic cells. Higher levels of immunoreactivity for TSP 86/84 were observed in gonadotrophic and thyrotrophic than in mammotrophic and corticotrophic cells. This appeared to result from the occurrence of TSP 86/84 in all secretory granules of the former cells and in only some secretory granules of the latter cells. We discuss the possibility that TSP 86/84 may have a role in the packaging of several distinct peptides hormones into secretory granules. One, though not the only, possible function of tyrosine sulfation may concern the sorting of this protein in the Golgi complex.

Sorting and secretion of adrenocorticotropin in a pituitary tumor cell line after perturbation of the level of a secretory granule-specific proteoglycan

A mouse anterior pituitary tumor cell line (AtT-20) that secretes adrenocorticotropin and beta endorphin sorts the proteins it transports to the surface into two exocytotic pathways. AtT-20 cells also synthesize a secretory granule-specific sulfated molecule and secrete it on stimulation (Moore, H.-P., B. Gumbiner, and R. B. Kelly, 1983, J. Cell Biol., 97:810-817). We show here that this molecule is sensitive to proteolysis and that the residual sulfated material co-migrates with a chondroitin sulfate standard on thin-layer electrophoresis. Furthermore, this sulfated molecule is completely sensitive to chondroitinase ABC digestion. Thus the secretory granule-specific sulfated molecule is a proteoglycan with chondroitin sulfate side chains. We examined the role of proteoglycans in the sorting and secretion of adrenocorticotropin in AtT-20 cells by severely decreasing the amount of this vesicle-specific proteoglycan in two ways. First, a xyloside was used to inhibit proteoglycan biosynthesis; second, a variant of the AtT-20 cell line was isolated that synthesized little of the sulfated proteoglycan. In neither case was the sorting or secretion of adrenocorticotropin detectably altered, suggesting that the proteoglycan is not required for these processes.

A subclass of proteins and sulfated macromolecules secreted by AtT-20 (mouse pituitary tumor) cells is sorted with adrenocorticotropin into dense secretory granules

The AtT-20 cell, a mouse pituitary tumor line that secretes adrenocorticotropin and beta-endorphin, sorts the proteins it externalizes into two exocytotic pathways. Cells that are labeled with [35S]methionine or [35S]sulfate can be shown to transport three acidic polypeptides (65,000, 60,000, and 37,000 mol wt) and at least two sulfated macromolecules into storage secretory granules. When the cells are stimulated by the secretagogue 8-bromo-cAMP, these polypeptides are coordinately secreted with mature adrenocorticotropin into the culture medium. In contrast, a completely different set of secreted polypeptides and sulfated macromolecules does not enter a storage form and is transported to the cell surface more rapidly. Their secretion from the cells is constitutive and does not require the presence of secretagogues. These molecules, like a viral membrane glycoprotein described previously (Gumbiner, B., and R. B. Kelly, 1982, Cell, 28:51-59) are not found in isolated secretory granules and therefore must reach the cell surface in a different exocytotic vesicle. The segregation of a subclass of secretory macromolecules into the secretory granules, despite the existence of another potential secretory pathway, suggests that these molecules have specific functions related to regulated hormone secretion or storage. Presumably all of the proteins secreted by the regulated secretory granule pathway share some common property that targets them to the secretory granule.

Characterization of adenohypophysial polypeptides by two-dimensional gel electrophoresis. II. Sulfated and glycosylated polypeptides

Adenohypophysial sulfated and glycosylated polypeptides were studied by high-resolution two-dimensional polyacrylamide-gel electrophoresis followed by fluorography. The preparations analyzed were the following: (a) homogenates from cow and rat anterior pituitary slices labeled in vitro either with [35S]sulfate or D-[6-3H]glucosamine; (b) materials released from bovine adenohypophysis slices pulse labeled with [35S]sulfate; and (c) purified fractions of bovine prolactin granules stripped by detergent treatment of their limiting membrane. A heterogeneous family of sulfated components, almost all glycosylated, differing in their peptide moieties as well as in their isoelectric points, was revealed in the glandular tissue. The major of these components (apparent Mr approximately 70 000; pI approximately 4.8), which was also highly labeled by L-[3H]-leucine (Zanini, A., and Rosa, P. (1981) Mol. Cell. Endocrinol. 24), might be a secretory protein because it accumulates in the medium during chase incubation of bovine pituitary slices in vitro. This sulfated component, which was more concentrated in the bovine than in the rat gland, was present in purified bovine prolactin granules stripped of their limiting membrane. However, the available evidence suggests that this might not be the only subcellular location of the sulfated polypeptide in the pituitary tissue.

Isolation of rat somatotroph and mammotroph secretory granules by equilibrium density centrifugation in a linear metrizamide gradient

A procedure has been developed for separating rat somatotroph and mammotroph secretory granules from other membranous organelles. The approach devised is a 6-step procedure involving filtration and equilibrium and rate density-gradient centrifugation in Metrizamide gradients of low ionic strength. The use of Metrizamide minimizes disruption and solubilization of the contents of these granules. The majority of the somatotroph and mammotroph granules can be isolated in a gradient fraction virtually free of any mitochondria, microsomes or other secretory granules.