Cell surface localization of proteolysis of human endothelial angiotensin I-converting enzyme. Effect of the amino-terminal domain in the solubilization process

Angiotensin-converting enzyme (ACE) belongs to the type I class of ectoproteins and is solubilized by Chinese hamster ovary cells transfected with the full-length human ACE cDNA. ACE release in Chinese hamster ovary cells involves a proteolytic cleavage occurring in the carboxyl-terminal region, between Arg-1137 and Leu-1138. The subcellular localization of ACE proteolysis was established by pulse-chase experiments, cell surface immunolabeling, and biotinylation of radiolabeled mature proteins. The proteolysis of ACE takes place primarily at the plasma membrane. The solubilization of ACE is less than 2% within 1 h, is increased 2.4-fold by phorbol esters, but is not influenced by ionophores. An ACE mutant lacking the transmembrane domain and the cytosolic part (ACE delta COOH), is secreted at a faster rate without a carboxyl-terminal cleavage, and phorbol esters or ionophores have no effect on its rate of production in the medium. Therefore, the proteolysis of ACE is dependent on the presence of the membrane anchor and suggests that the secretase(s) involved is also membrane-associated. An ACE mutant lacking the amino-terminal domain (ACECF) is secreted 10-fold faster compared with wild-type ACE. The solubilization of ACECF occurs at the plasma membrane and is stimulated 2.7-fold by phorbol esters, and the cleavage site is localized between Arg-1227 and Val-1228. The amino-terminal domain of ACE slows down the proteolysis and seems to act as a "conformational inhibitor" of the proteolytic process, possibly via interactions with the "stalk" of ACE and the secretase(s) itself.

Proteolytic release of human angiotensin-converting enzyme. Localization of the cleavage site

Angiotensin-converting enzyme (EC 3.4.15.1, ACE) is a transmembrane protein with a short carboxyl-terminal cytoplasmic domain, a 17-amino acid hydrophobic anchor domain, and a large N-terminal extracellular region containing two catalytically homologous domains. An active soluble form of ACE circulates in human plasma and is produced in culture medium of Chinese hamster ovary (CHO) cells transfected with the full-length human ACE cDNA. The mechanism of ACE release in CHO cells involves a post-translational proteolytic cleavage occurring in the carboxyl-terminal region. The carboxyl terminus of the secreted recombinant ACE, AGQR, was established by carboxyl-terminal microsequencing and corresponds to a cleavage site between Arg-1137 and Leu-1138. Two independent studies confirmed this proposed cleavage site: amino acid analysis of a carboxyl-terminal peptide derived from soluble ACE and immunocharacterization of membrane-bound and soluble ACE with antibodies raised against three peptides located along the carboxyl-terminal ACE sequence. In order to assess the importance of Arg-1137, this amino acid was mutated to a glutamine residue. This mutation did not prevent the secretion of ACE, suggesting that the solubilizing enzyme can accommodate this change or can use an alternative cleavage site. Finally, the production of soluble ACE in CHO cells appears to be proportional to the level of cellular ACE, implying that the solubilizing enzyme is not a limiting factor. In addition, the carboxyl-terminal sequence of the human plasma ACE was identified as AGQR, thus supporting the fact that a similar mechanism could operate in human vascular cells.

Contribution of immunohistochemistry, electron microscopy, and cell culture to the characterization of nonfunctioning pituitary adenomas: a study of 40 cases

We studied 40 endocrinologically inactive pituitary adenomas by immunohistochemistry, electron microscopy, and cell culture in order to determine the incidence of gonadotropic adenomas and to classify nonfunctioning adenomas. Immunohistochemical studies using a large panel of monoclonal and polyclonal antibodies identified the following nonfunctioning adenomas: 20 gonadotropic adenomas, four silent corticotropic adenomas, one plurihormonal adenoma, and 15 nonsecreting adenomas. Among nonsecreting adenomas, ultrastructural study of 13 cases identified seven null cell adenomas and six oncocytomas. Silent corticotropic adenomas were classified into subtypes I, II, and III according to Kovacs and Horvath. Most often, gonadotropic adenomas displayed a varying number of oncocytic cells, characteristic secretory granules, and a prominent Golgi apparatus. Postembedding immunoelectron microscopy was performed on eight gonadotropic or nonsecreting adenomas, but this technique did not provide any additional information. Six gonadotropic adenomas and 10 so-called nonsecreting adenomas were studied in primary cell cultures. The six gonadotropic adenomas and seven of the 10 nonsecreting adenomas released gonadotropins in the culture medium. The use of in vitro results as a supplementary diagnostic criterion allowed classification of the 40 nonfunctioning adenomas as follows: 27 gonadotropic adenomas, four silent corticotropic adenomas, one plurihormonal adenoma, and eight nonsecreting adenomas. These results demonstrate a high proportion of gonadotropic adenomas among nonfunctioning adenomas (67.5%) and the usefulness of several techniques in characterizing this type of pituitary adenoma.

Non-Hodgkin's lymphoma presenting with primary adrenal insufficiency. A disease with an underestimated frequency?

Adrenal involvement in the course of a non-Hodgkin's lymphoma (NHL) appears to occur relatively often, but only seven cases of NHL-induced adrenal insufficiency were found in a recent review of the literature. The authors report four cases of hypoadrenalism in 127 patients treated for NHL; the cases were staged and classified according to the Working Formulation and were investigated for endocrine function by the cosyntropin stimulation test. The involvement was bilateral in four patients; all of the patients had high grade, mostly widespread NHL. These observations suggest that adrenal insufficiency may be underestimated in NHL. Basal hormonal serum levels may be borderline; consequently, only stimulation tests can prove the hormonal failure. The authors suggest that such tests are essential if the staging of the NHL shows bilateral adrenal enlargement, and that the tests should be performed before chemotherapy begins because of the risk of acute adrenal insufficiency.

Presence of renin, angiotensinogen, angiotensin II in the lamb anterior pituitary gland: immunocytochemical study after cryoultramicrotomy

The presence of renin, angiotensin I-converting enzyme and angiotensin II detected by immunocytochemistry in the adult male rat anterior pituitary has suggested the existence of a pituitary renin-angiotensin system. To establish another mammalian experimental model we have investigated the presence of renin, angiotensinogen, angiotensin I-converting enzyme, and angiotensin II II in five normal lamb anterior pituitaries by immunocytochemistry after cryoultramicrotomy. Renin, angiotensinogen and angiotensin II immunoreactivities were observed only in cytoplasmic granules of lactotrophs, and the three proteins were found co-localized with prolactin in the same granules by double immunolabelling. No immunoreactive angiotensin I-converting enzyme was observed. These results suggest an activation of renin in the cytoplasmic granules of lactotrophs leading to a local synthesis of angiotensin II. Thus, the lamb anterior pituitary may provide a good experimental model for investigating the possible autocrine action of a local renin-angiotensin system on prolactin release in the human pituitary.

Immunocytochemical and biochemical evidence of renin in human lactotrophic cell cultures

Primary cell cultures from human prolactin (PRL)-secreting adenomas were used to test the ability of human lactotrophs to synthesize renin in vitro. The renin content of the culture medium and of cellular extracts was measured by enzyme-linked immunosorbent assay. The level of PRL release in the culture medium and the amount of PRL in a cellular extract were determined by radioimmunoassay. Morphologic studies included indirect immunofluorescence, pre-embedding immunoelectron microscopy using a three-layer peroxidase-antiperoxidase method and postembedding immunoelectron microscopy using protein A-gold complexes. Renin was detected in cellular extracts and was found to be absent in the culture medium, whereas PRL was extracellularly secreted. PRL and renin immunoreactivity was observed in all the cultures studied by immunofluorescence. The subcellular localization of renin was found to be similar to that of PRL and was observed in the rough endoplasmic reticulum, the Golgi apparatus, and cytoplasmic secretory granules. The results suggest that, in vitro, renin may be synthesized and intracellularly metabolized in human adenomatous lactotrophic cells rather than secreted. Cell cultures may be a useful model to further the understanding of the role of a local renin-angiotensin system in PRL secretion.

[Low levels of TSH measured by a sensitive assay: do they reflect hyperthyroidism? A critical analysis of 580 cases]

Highly sensitive TSH assays make it easier to diagnose thyroid diseases. During one year, we performed 5,300 sensitive TSH assays (normal range: 0.15-4 mU/l) in various patients. The purpose of this work was to test the value of the low TSH plasma concentrations found in 580 patients. In 99.7 percent of the cases, low TSH levels were the consequence of a thyroid disorder or a treatment by thyroid hormones; non thyroidal illnesses were detected in only 0.3 percent. However, not all TSH values below 0.15 mU/l were associated with overt or occult thyrotoxicosis. When TSH was undetectable (less than 0.04 mU/l), and excluding thyroid hormone-treated patients, thyrotoxicosis was present in 97 percent of the cases. On the other hand, when TSH values were between 0.04 and 0.15 mU/l, 41 percent of the patients failed to show any sign or symptom of hyperthyroidism, although they had functioning thyroid nodules, multinodular goitre or iodine overload, or they received thyroid hormones.