Chromogranin A messenger RNA expression in the rat anterior pituitary is permissively regulated by the adrenal gland

Neuroendocrine inhibition of glucose production and resistance to cancer in dwarf mice

Pit1 null (Snell dwarf) and Proph1 null (Ames dwarf) mutant mice lack GH, PRL and TSH. Snell and Ames dwarf mice also exhibit reduced IGF-I, resistance to cancer and a longer lifespan than control mice. Endogenous glucose production during fasting is reduced in Snell dwarf mice compared to fasting control mice. In view of cancer cell dependence on glucose for energy, low endogenous glucose production may provide Snell dwarf mice with resistance to cancer. We investigated whether endogenous glucose production is lower in Snell dwarf mice during feeding. Inhibition of endogenous glucose production by glucose injection was enhanced in 12 to 14 month-old female Snell dwarf mice. Thus, we hypothesize that lower endogenous glucose production during feeding and fasting reduces cancer cell glucose utilization providing Snell dwarf mice with resistance to cancer. The elevation of circulating adiponectin, a hormone produced by adipose tissue, may contribute to the suppression of endogenous glucose production in 12 to 14 month-old Snell dwarf mice. We compared the incidence of cancer at time of death between old Snell dwarf and control mice. Only 18% of old Snell dwarf mice had malignant lesions at the time of death compared to 82% of control mice. The median ages at death for old Snell dwarf and control mice were 33 and 26 months, respectively. By contrast, previous studies showed a high incidence of cancer in old Ames dwarf mice at the time of death. Hence, resistance to cancer in old Snell dwarf mice may be mediated by neuroendocrine factors that reduce glucose utilization besides elevated adiponectin, reduced IGF-I and a lack of GH, PRL and TSH, seen in both Snell and Ames dwarf mice. Proteomics analysis of pituitary secretions from Snell dwarf mice confirmed the absence of GH and PRL, the secretion of ACTH and elevated secretion of Chromogranin B and Secretogranin II. Radioimmune assays confirmed that circulating Chromogranin B and Secretogranin II were elevated in 12 to 14 month-old Snell dwarf mice. In summary, our results in Snell dwarf mice suggest that the pituitary gland and adipose tissue are part of a neuroendocrine loop that lowers the risk of cancer during aging by reducing the availability of glucose.

Effects of sex steroids on secretory granule formation in gonadotropes of castrated male rats with respect to granin expression

Pituitary gonadotropes show sex-related differences in their ultrastructure. Typical gonadotropes of male rats exhibit both large granules, which contain chromogranin A (CgA), and small granules, which contain secretogranin II (SgII). In contrast, typical female rat gonadotropes show only a very few large granules among the numerous small granules. To clarify the nature of the biogenesis of these secretory granules and the effects of sex steroids, the ultrastructural and immunocytochemical changes in gonadotropes were examined in castrated male rats supplied with a testosterone or estradiol implant. In castrated rats, pituitary expression and plasma levels of LH increased drastically, but the pituitary content of CgA decreased. The majority of gonadotropes then showed features of "castration cells" containing many small secretory granules. A testosterone implant to castrated rats remarkably suppressed the expression and circulating levels of LH and increased the CgA content in the pituitary to near-normal levels. In this situation, immunocytochemical studies demonstrated that gonadotropes again exhibited large and small secretory granules with the respective localization of CgA and SgII. On the contrary, in castrated rats supplied with an estradiol implant, the expression and content of CgA in the pituitary were remarkably suppressed, and large secretory granules disappeared from gonadotropes. These results suggest that the expression of CgA in gonadotropes is regulated differently by male and female sex steroids. These different effects of androgen and estrogen on the expression level of CgA are closely associated with the sex-related differences in the ultrastructure of secretory granules within gonadotropes.

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

Direct estradiol down-regulation of secretogranin II and chromogranin A mRNA levels in rat pituitary cells

Estradiol (E2) has been previously shown to negatively regulate, in vivo, the secretogranin (SgII) and chromogranin A (CgA) mRNA levels in the rat pituitary. Using cultured pituitary cell aggregates, experiments were undertaken to discriminate between direct or indirect effects of E2 on SgII and CgA levels. SgII, CgA and gonadotropin alpha- and beta-subunit mRNA levels were determined by Northern blotting. SgII and CgA protein levels were quantitated by Western blotting, and by immunoprecipitation of radioactive SgII after [35S]methionine labeling. Increasing concentrations of E2 (from 10(-12) M to 10(-8) M) in the culture medium promoted a decrease of SgII and CgA mRNA levels to 30% and 50% of the control, respectively, after 72.h treatment. By contrast, none of the gonadotropin subunit mRNAs exhibited changes in concentration. A 24 h treatment with 10(-8) M E2 was sufficient to promote such a decrease in SgII and CgA mRNAs. Quantitation of the proteins after Western blotting revealed that 10(-8) M E2 lowered by 30% in CgA content of aggregates (P < 0.05 vs. control) while SgII content remained unaffected. Moreover, quantitation of the newly synthesized SgII by immunoprecipitation of the 35S-labeled SgII gave evidence for a lack of E2 effect. These data demonstrate: (1) a direct effect of E2 on the pituitary cells to down-regulate SgII and CgA mRNA steady-state levels; (2) though contained within the same secretory granules, a distinct pathway for negative E2 regulation of the gonadotropins and both granins; and (3) a differential effect of E2 on cell SgII and CgA contents as was previously demonstrated in vivo.

Regulatory Effects of Estrogens on the Pituitary Content of Secretogranin II and Chromogranin A

Modulation of pituitary content of secretogranin II and chromogranin A has been compared to that of luteinizing hormone (LH) in adult female rats following gonadectomy and subsequent estradiol therapy. Both proteins were quantified using two specific antisera, BS 487 and anti-CAP-14, respectively. Ovariectomy promoted an increase in the concentration of secretogranin II and chromogranin A, though to a lesser degree than that of LH. Moreover, intracellular processing of secretogranin II was significantly enhanced. Daily estradiol injection further increased pituitary secretogranin II and LH contents while that of chromogranin A was reduced to sham-operated control values. These data 1) demonstrate that both granins are differentially regulated through ovarian factors, and 2) confirm that regulation of secretogranin II production may be closely related to that of LH.

Chromogranins in rat brain: characterization, topographical distribution and regulation of synthesis

The properties and distribution of chromogranins A, B and secretogranin II in rat brain were analyzed by quantitative immunoblotting. In contrast to endocrine tissues brain contains a significant amount of the proteoglycan form of chromogranin A. For secretogranin II a significant degree of endogenous proteolytic processing is apparent. Chromogranin A and secretogranin II had a similar topographical distribution with the highest concentrations found in the hypothalamus, amygdala/piriform cortex and hippocampus, whereas for chromogranin B by far the highest concentration was found in the cerebellum. Compared with adrenal medulla the concentration of all three proteins is low, however, secretogranin II appears relatively enriched. The synthesis of chromogranin A in brain does not depend on glucocorticoids since neither adrenalectomy nor dexamethasone treatment changed its levels. This is in contrast to adrenal medulla and to the anterior pituitary. Three days after kainic acid-induced seizures the levels of chromogranin A in frontal cortex and hippocampus were significantly elevated. For frontal cortex there was also an increase of the respective mRNA. This result establishes that the synthesis of chromogranin A can be regulated like that of neuropeptides.