Distribution and regulation of proconvertases PC1 and PC2 in human pituitary adenomas

Secretory granule protein chromogranin B (CHGB) forms an anion channel in membranes

The CHGB subfamily of secretory granule proteins forms a new family of anion-selective channels by interacting with membranes via two amphipathic α-helices. The channel exhibits higher anion selectivity, larger conductance, higher DIDS-binding affinity, and higher Cl⁻ sensitivity than other known anion channels.

Regulated secretion is an intracellular pathway that is highly conserved from protists to humans. Granin family proteins were proposed to participate in the biogenesis, maturation and release of secretory granules in this pathway. However, the exact molecular mechanisms underlying the intracellular functions of the granin family proteins remain unclear. Here, we show that chromogranin B (CHGB), a secretory granule protein, inserts itself into membrane and forms a chloride-conducting channel. CHGB interacts strongly with phospholipid membranes through two amphipathic α helices. At a high local concentration, CHGB insertion in membrane causes significant bilayer remodeling, producing protein-coated nanoparticles and nanotubules. Fast kinetics and high cooperativity for anion efflux from CHGB vesicles suggest that CHGB tetramerizes to form a functional channel with a single-channel conductance of ∼125 pS (150/150 mM Cl⁻). The CHGB channel is sensitive to an anion channel blocker and exhibits higher anion selectivity than the other six known families of Cl⁻ channels. Our data suggest that the CHGB subfamily of granin proteins forms a new family of organelle chloride channels.

Granin-derived peptides

The granin family comprises altogether 7 different proteins originating from the diffuse neuroendocrine system and elements of the central and peripheral nervous systems. The family is dominated by three uniquely acidic members, namely chromogranin A (CgA), chromogranin B (CgB) and secretogranin II (SgII). Since the late 1980s it has become evident that these proteins are proteolytically processed, intragranularly and/or extracellularly into a range of biologically active peptides; a number of them with regulatory properties of physiological and/or pathophysiological significance. The aim of this comprehensive overview is to provide an up-to-date insight into the distribution and properties of the well established granin-derived peptides and their putative roles in homeostatic regulations. Hence, focus is directed to peptides derived from the three main granins, e.g. to the chromogranin A derived vasostatins, betagranins, pancreastatin and catestatins, the chromogranin B-derived secretolytin and the secretogranin II-derived secretoneurin (SN). In addition, the distribution and properties of the chromogranin A-derived peptides prochromacin, chromofungin, WE14, parastatin, GE-25 and serpinins, the CgB-peptide PE-11 and the SgII-peptides EM66 and manserin will also be commented on. Finally, the opposing effects of the CgA-derived vasostatin-I and catestatin and the SgII-derived peptide SN on the integrity of the vasculature, myocardial contractility, angiogenesis in wound healing, inflammatory conditions and tumors will be discussed.

Genetics of the first seven proprotein convertase enzymes in health and disease

Members of the substilisin/kexin like proprotein convertase (PCSK) protease family cleave and convert immature pro-proteins into their biologically active forms. By cleaving for example prohormones, cytokines and cell membrane proteins, PCSKs participate in maintaining the homeostasis in a healthy human body. Conversely, erratic enzymatic function is thought to contribute to the pathogenesis of a wide variety of diseases, including obesity and hypercholestrolemia. The first characterized seven PCSK enzymes (PCSK1-2, FURIN, PCSK4-7) process their substrates at a motif made up of paired basic amino acid residues. This feature results in a variable degree of biochemical redundancy in vitro, and consequently, shared substrate molecules between the different PCSK enzymes. This redundancy has confounded our understanding of the specific biological functions of PCSKs. The physiological roles of these enzymes have been best illustrated by the phenotypes of genetically engineered mice and patients that carry mutations in the PCSK genes. Recent developments in genome-wide methodology have generated a large amount of novel information on the genetics of the first seven proprotein convertases. In this review we summarize the reported genetic alterations and their associated phenotypes.

On the cutting edge of proprotein convertase pharmacology: from molecular concepts to clinical applications

There is increasing interest in the therapeutic targeting of proteases for the treatment of important diseases. Additionally new protein-based therapeutic strategies have the potential to widen the available treatments against these pathologies. In the last decade, accumulated evidence has confirmed that the family of proteases known as proprotein convertases (PCs) are potential targets for viral infections, osteoarthritis, cancer and cardiovascular disease, among others. Nevertheless, there are still many unanswered questions about the relevance of targeting PCs in a therapeutic context, especially regarding the anticipated secondary effects of treatment, considering the observed embryonic lethality of some PC knockout mice. In this review, the benefits of PCs as pharmacological targets will be discussed, with focus on concepts and strategies, as well as on the state of advancement of actual and future inhibitors.

Possible relevance between prohormone convertase 2 expression and tumor growth in human adrenocorticotropin-producing pituitary adenoma

CONTEXT

Methods for preoperative diagnosis of prohormone convertase 2 (PC2)-positive ACTH-producing pituitary adenomas (APPAs) have not been established. Also, their characteristics are not evident.

OBJECTIVE

This study was designed to understand the meaning of plasma alphaMSH levels and the role of cell proliferation-signaling molecules in PC2-positive APPAs.

PATIENTS AND MAIN OUTCOME MEASURES

Nineteen human APPAs (four males and 15 females) were examined for the expression of PC2, phosphorylated ERK1/2, phosphorylated Akt1/2/3 (p-Akt) and receptor tyrosine kinases. alphaMSH was measured in extracted plasma from 17 APPA patients and 30 healthy volunteers.

RESULTS

Nine adenomas (47.4%) were immunopositive for PC2 and were large and invasive in nature. In all normal controls and eight PC2-negative cases, plasma alphaMSH was undetectable, whereas in four PC2-positive cases, it was detected at abnormally higher levels. Eight adenomas (42.1%) were immunopositive for both PC2 and p-Akt, and seven others (36.8%) were immunonegative for both, suggesting significant coexpression of PC2 and p-Akt in tumors. Quantitative RT-PCR revealed that PC2 expression is associated with phosphorylation of Akt but not with its gene expression. Most APPAs expressed receptor tyrosine kinases, but membrane-bound receptors could not be identified.

CONCLUSIONS

Our study suggests that PC2 expression and Akt phosphorylation are related at the molecular level, resulting in a change in cell cycle and an increase in pituitary adenoma size. An elevation of plasma alphaMSH could conjecture the activation of the phosphatidylinositol 3/Akt cascade in PC2-positive APPAs and may become a valuable clinical marker of tumor growth in Cushing's disease.

Defective expression of prohormone convertase 1/3 in silent corticotroph adenoma

Silent corticotroph adenoma (SCA) is defined as an ACTH-producing pituitary tumor not associated with clinical and endocrine feartures of Cushing's syndrome, but its underlying molecular mechanism(s) remains unknown thus far. We tested the hypothesis that reduced expression of prohormone convertase (PC) 1/3 responsible for proteolytic processing of proopiomelanocortin (POMC) in SCA may lead to production of unprocessed, biologically inactive POMC and/or precursor of ACTH. Among 30 non-functioning pituitary macroadenomas (NFA) examined, we found 6 SCAs by immunohistochemical study using anti-ACTH antibody. Preoperative endocrine and diagnostic image tests did not reveal any differences between SCA and the remaining NFA except for the higher recurrence rate of SCA. While steady-state PC1/3 mRNA levels determined by RT-PCR were almost comparable between SCAs and NFAs, immunohistochemical study showed negative immunostaining for PC1/3 in all 6 SCAs. Our data suggest that defective PC1/3 expression may lead to preferential production of unprocessed, biologically inactive ACTH variants in SCA.

Immunohistochemical properties of silent corticotroph adenoma and Cushing's disease

Proopiomelanocortin processing in corticotroph cells is known to be operated by prohormone convertase (PC) 1/3 which is activating several pro-proteins and prohormones by intracellular limited proteolysis processing. In this study, we hypothesized that PC1/3 expression differs between Cushing's disease (CD) and silent corticotroph adenoma (SCA), and investigated whether PC1/3 expression is involved in the adrenocorticotropin (ACTH) silence of SCA. We performed immunohistochemical analysis of pituitary adenoma specimens for six adenohypophysial hormones, PC1/3 and chromogranin A (CgA). Subjects for this study consisted of 12 anterior pituitary adenomas of CD (1 male, 11 female; 14-70 years old) and 31 non-functioning adenomas (23 male, 8 female; 32-71 years old).ACTH immunoreactivity was observed in all of CD and three of 31 non-functioning adenomas. The three cases diagnosed as SCA were also positive for growth hormone and follicle-stimulating hormone. Cushing's adenomas and SCAs were all positive for PC1/3. PC1/3-positive cells did not always colocalize with ACTH but some of them colocalized with CgA in SCAs. Even if PC1/3 is not present in corticotroph cells, PC1/3 immunoreactivity in SCA may originate from CgA-positive cells. We conclude that immunohistochemistry for PC1/3 is not helpful for differential diagnosis between CD and SCA in clinical practice, though the regulation of PC1/3 expression is likely to be an important etiological factor in ACTH silence of SCA. The diversity of immunohistochemical properties of SCA leads us to speculate that it is not a single entity and may be a general diagnostic term for adenomas of varying etiology.

Galectin-4 expression in carcinoid tumors

Galectins (Gal) are an evolutionarily conserved family of 15 carbohydrate-binding proteins (lectins) that are widely distributed in normal and neoplastic cells in a wide range of organisms. They have roles in inflammation, cell adhesion, tumor progression, and metastasis. The function and distribution of Gal-3 and Gal-1 are well characterized, but less information is available about Gal-4. Recent studies have localized Gal-4 in the enterochromaffin cells of the porcine and murine small intestine. We examined the expression of Gal-4 in primary and metastatic human ileal carcinoid tumors as well as in carcinoid tumors of the stomach, lung, and rectum. A total of 44 primary and 42 ileal metastatic carcinoid tumors were examined by immunohistochemistry using tissue microarrays (TMA) with monoclonal antibodies to Gal-4, Gal-3, and Gal-1. Pulmonary (n = 7), rectal (n = 6), and gastric (n = 6) carcinoids were examined with larger tissue sections. A total of 18 pancreatic neuroendocrine tumors were also examined with larger tissue sections. Western blots of three ileal carcinoids were also done. Gal-4 was most highly expressed in the ileal carcinoids and the levels of expression tended to be higher in primary ileal carcinoids compared to the metastatic tumors (p = 0.069). All 18 pancreatic neuroendocrine tumors were negative for Gal-1, Gal-3, and Gal-4. Western blot showed a 32 kDa band for Gal-4 in the ileal carcinoids. Gal-3 and Gal-1 were not detected in the metastatic ileal carcinoids by Western blotting. Gastric carcinoids also expressed Gal-4, but very few pulmonary or rectal carcinoids were positive for Gal-4 (p = 0.002). Lower levels of Gal-1 and Gal-3 expression were present in ileal carcinoids compared to primary pulmonary and rectal tumors. These results show a differential distribution of Gal-4 in carcinoid tumors in different locations of the gastrointestinal tract and the lungs.

Melanotrope cells as a model to understand the (patho)physiological regulation of hormone secretion

Regulation of hormone secretion is a complex process that comprises the sequential participation of numerous subcellular mechanisms. Hormone secretion is dictated by extracellular stimuli that are transduced intracellularly into activation/deactivation of different mechanisms, such as hormone expression, processing and exocytosis, which will ultimately determine the precise availability of hormone to be secreted. Malfunction in any of these steps may result in deficient or excessive hormone release and the subsequent appearance of endocrine disorders. Given the complexity of this system, it is difficult to find appropriate cellular models wherein to investigate the multiple components of the secretory process in a physiologically relevant, experimentally manipulable setting. In this review, we present recent evidence on the use of the intermediate lobe (IL) of the pituitary as a powerful tool to understand different aspects of the regulated secretory pathway. IL is composed of a single endocrine cell type, alpha-melanocyte stimulating hormone (alpha-MSH)-producing melanotropes, a fact that greatly facilitates its study. Furthermore, melanotropes can be separated using classic cell separation techniques into two cell subtypes showing opposite morphophysiological phenotypes of hypo- and hypersecretory cells. Comparison of their gene expression fingerprints has unveiled the existence of certain genes preferentially expressed in each melanotrope subtype. Because of their direct participation in the secretory pathway, we postulate that characterization of these gene products in an endocrine cell type may represent novel and useful markers for reliably determining the general secretory status in an endocrine gland, as well as a valuable new tool to further investigate this complex process.

Role of subtilisin-like convertases in cadherin processing or the conundrum to stall cadherin function by convertase inhibitors in cancer therapy

Cadherins are a family of intercellular adhesion receptors. Produced as inactive precursors, they become functional adhesion molecules after proteolytic cleavage by subtilisin-like pro-protein convertases (PCs). Owing to their activation and assembly into multiprotein adhesion complexes at sites of cell contacts, adhesion-competent cadherins are prerequisite for tissue integrity. In recent years evidence has accumulated that intercellular junctions not only provide mechanical linkage, but in addition are potent modulators of signalling cascades. This infers a biological role to intercellular adhesion complexes that is significantly more complex and powerful. Currently, the broad implications of disturbances in somatic tissue adhesion components are only just beginning to emerge. Prominent examples of adhesion defects include autoimmune diseases, or tumour invasion and metastasis and malignant transformation. This review reports on our current knowledge of cadherin function and their maturation by pro-protein convertases, and puts special emphasis on the consequences of pro-protein convertase inhibition for epithelial tissue homeostasis.

Chromogranin A transcription and gene expression in Folliculostellate (TtT/GF) cells inhibit cell growth

Folliculostellate (FS) cells are present in the anterior pituitary and have important regulatory functions including controlling hormone release from other anterior pituitary cells. FS cells do not usually express neuroendocrine genes such as chromogranin A (CgA). We analyzed transcriptional regulation and gene expression in the TtT/GF FS cell line to better understand the role of FS cells in anterior pituitary function. After transient transfection with a human (h) CgA promoter sequence linked to a luciferase reporter, there was basal level of transcriptional activity, which was two- to fourfold less than that observed in the anterior pituitary neuroendocrine cell lines HP75 and GH3. The transcriptional activity was decreased in all cell lines when a mutant hCgA promoter cyclic AMP response element (CRE) was used for transfection. Sodium butyrate treatment increased the transcriptional activity in all cell lines, but remained two- to fourfold higher in the HP75 and GH3 cell lines than in the TtT/GF cells. Stable transfection of a plasmid expressing bovine (b) CgA in the TtT/GF cells led to inhibition of cell growth as measured by 3H-thymidine incorporation, Ki-67 labeling index, and growth curve analysis. CgA protein and mRNA could be readily demonstrated in the cloned cells but not in the parental cell line or vector control cells. When the CgA expressing cloned cells were injected into SCID mice, there was a decrease in the rate of tumor growth compared to the vector control in vivo. These results indicate that the TtT/GF FS cells are fibroblast-like compared to the neuroendocrine anterior pituitary secretory cells when analyzed by transcriptional activity with a transiently transfected CgA promoter. In TtT/GF cells with a stably transfected bCgA plasmid, CgA has a direct regulatory effect on tumor cell proliferation.

Pituitary cell lines and their endocrine applications

The pituitary gland is an important component of the endocrine system, and together with the hypothalamus, exerts considerable influence over the functions of other endocrine glands. The hypothalamus either positively or negatively regulates hormonal productions in the pituitary through its release of various trophic hormones which act on specific cell types in the pituitary to secrete a variety of pituitary hormones that are important for growth and development, metabolism, reproductive and nervous system functions. The pituitary is divided into three sections-the anterior lobe which constitute the majority of the pituitary mass and is composed primarily of five hormone-producing cell types (thyrotropes, lactotropes, corticotropes, somatotropes and gonadotropes) each secreting thyrotropin, prolactin, ACTH, growth hormone and gonadotropins (FSH and LH) respectively. There is also a sixth cell type in the anterior lobe-the non-endocrine, agranular, folliculostellate cells. The intermediate lobe produces melanocyte-stimulating hormone and endorphins, whereas the posterior lobe secretes anti-diuretic hormone (vasopressin) and oxytocin. Representative cell lines of all the six cell types of the anterior pituitary have been established and have provided valuable information on genealogy of the various cell lineages, endocrine feedback control of hormone synthesis and secretions, intrapituitary interactions between the various cell types, as well as the role of specific transcription factors that determine each differentiated cell phenotype. In this review, we will discuss the morphology and function of the cell types that make up the anterior pituitary, and the characteristics of the various functional anterior pituitary cell systems that have been established to be representative of each anterior pituitary cell lineage.

Regulation of VEGF-A, VEGFR-I, thrombospondin-1, -2, and -3 expression in a human pituitary cell line (HP75) by TGFbeta1, bFGF, and EGF

Pituitary tumors are highly vascular neoplasms, which suggest an important role of angiogenesis in pituitary tumor growth. We used the human pituitary cell line (HP75) to examine the effects of the growth factors TGFbeta1, bFGF, and EGF on cell growth, and on the regulation of the pro-angiogenic growth factor VEGF-A and the VEGFR-I and the anti-angiogenic molecules thrombospondin (TSP) TSP-1 and TSP-2 along with TSP-3. Real-time RT-PCR was used to measure mRNA levels, and Western blot was used to analyze TSP-1 and TSP-2 protein levels. TGFbeta1 treatment (1 x 10(-9) M) increased VEGF-A mRNA levels significantly (p < 0.05) after 4 and 24 h of treatment. TGF beta1 treatment decreased VEGF-R mRNA levels after 96 h of treatment (p < 0.05). After 96 h of treatment, TSP-1 and TSP-2 mRNA levels were significantly increased (p < 0.05) by TGFbeta1 treatment, which also inhibited HP75 cell growth. Basic FGF also increased TSP-1 mRNA levels after 96 h of treatment, but did not regulate growth of the pituitary tumor cells. Basic FGF and EGF did not modulate changes in VEGF-A mRNA levels after 4 and 24 h of treatment, but EGF increased VEGF-A significantly (p < 0.05) after 96 h of treatment. These results indicate that TGFbeta1 treatment may regulate angiogenesis in pituitary cells by initially increasing levels of pro-angiogenic VEGF-A and then stimulating the anti-angiogenic molecules TSP-1 and TSP-2 levels.

Chromogranin a processing in human pituitary adenomas and carcinomas: analysis with region-specific antibodies

The expression of various chromogranin A (CgA) peptide fragments was examined with region-specific antisera in benign and malignant pituitary tumors. Analysis of the proconvertases responsible for proteolytic processing of CgA, prohormone convertase 1/3 (PC1/3), and PC2 was also performed. Adenomas were studied using tissue microarrays, and a larger tissue section of a subset of the prolactin (PRL) adenomas was used to compare to the tissue microarray analysis. Carcinomas were analyzed using larger tissue sections. There were differences in CgA proteolytic products detected between the functional (PRL, adrenocorticotropic hormone [ACTH], and growth hormone tumors and the nonfunctional (gonadotroph and null cell) tumors, with the former group expressing lower levels of many peptides. These differences were most notable in the PRL adenomas and carcinomas in which the region-specific antisera against vasostatin I and vasostatin II detected these fragments in the lowest percentage of tumors and/or had the weakest immunoreactivity. The CgA peptide fragment detected by CgA 176-195 (chromacin) antiserum was expressed by the highest percentage of most functional and nonfunctional benign and malignant pituitary tumors. ACTH carcinomas (n = 3) were more strongly immunoreactive compared to the ACTH adenomas. These results show that there is differential expression of CgA peptide fragments and PC1/3 among different types of pituitary tumors and that ACTH pituitary carcinomas have higher levels of immunoreactive CgA peptide fragments compared to ACTH adenomas. This study also shows the utility of tissue microarrays in the analysis of a large group of tumors with regionspecific antisera.

Proprotein convertases in tumor progression and malignancy: novel targets in cancer therapy

The mammalian subtilisin/kexin-like proprotein convertase (PC) family has been implicated in the activation of a wide spectrum of proteins. These proteins are usually synthesized as inactive precursors before their conversion to fully mature bioactive forms. A large majority of these active proteins such as matrix metalloproteases, growth factors, and adhesion molecules are crucial in the processes of cellular transformation, acquisition of the tumorigenic phenotype, and metastases formation. Inhibition of PCs significantly affects the malignant phenotype of various tumor cells. In addition to direct tumor cell proliferation and migration blockade, PC inhibitors can also be used to target tumor angiogenesis. In this Review article we discuss a number of recent findings on the clinical relevance of PCs in cancer patients, their implication in the regulation of multiple cellular functions that impact on the invasive/metastatic potential of cancer cells. Thus, PC inhibitors may constitute new promising agents for the treatment of multiple tumors and/or in adjuvant therapy to prevent recurrence.