Transthyretin immunoreactivity in human and porcine liver, choroid plexus, and pancreatic islets

Transthyretin expression evolved more recently in liver than in brain

1. Transthyretin was found to be synthesized and secreted by choroid plexus from rats, echidnas, and lizards, but not toads. 2. Transthyretin was observed in blood from placental mammals, birds, and marsupials, but not reptiles and monotremes. 3. The obtained data suggest that transthyretin synthesis by the liver evolved independently in the lineage leading to the placental mammals and marsupials and in that leading to the birds. 4. It is proposed that transthyretin gene expression in mammalian liver appeared about 200 million years later than its first occurrence in the choroid plexus of the stem reptiles.

Cystatin C in the human pancreas and gut: an immunohistochemical study of normal and neoplastic tissues

The occurrence of immunoreactive cystatin C (CC) in normal and neoplastic cells of the human pancreas and gut was investigated using an indirect streptavidin-biotin method on formaldehyde-fixed and paraffin-embedded tissues. Virtually all pancreatic islet cells and many neuroendocrine cells throughout the gastrointestinal tract showed strong CC immunoreactivity and a granular cytoplasmic staining pattern. All 14 endocrine pancreatic tumours (insulinomas, glucagonomas, gastrinomas and non-producing tumours), as well as 16 of 17 gut carcinoid tumours, were also strongly CC immunoreactive. In addition, non-endocrine epithelial cells of pancreatic ducts and the gastrointestinal mucosa and 20 of the 24 adenocarcinomas from these sites showed weak CC immunoreactivity. Thus, CC cannot be used as a reliable immunohistochemical marker for endocrine gastro-entero-pancreatic tumours despite the fact that the protein is strongly expressed in a majority of such tumours.

Distribution of retinol-binding protein in the human digestive tract

By employing polyclonal antibodies for retinol-binding protein (RBP), its distribution in the human pancreas and digestive tract mucosa was compared with those of transthyretin (TTR) and various peptide hormones. The materials used included surgically removed pancreas, esophagus, stomach, small and large intestines. Paraffin sections were stained by the indirect immunoenzyme method. The results indicate that RBP-containing cells are found in the pancreas and the gastrointestinal mucosa, but most frequently in the gastric antrum and duodenum. In the pancreas, RBP-containing cells are found in the islets and among acinar and ductal epithelial cells, and consistently stain for chromogranin A. RBP-containing cells in the gastrointestinal mucosa showed typical features of endocrine cells and also stained for chromogranin A. The distribution of TTR in these tissue sites resembled that of RBP, but the immunoreactive intensities of both peptides altered independently. Comparison of the distribution of RBP, TTR, and various gastrointestinal peptide hormones revealed that the distribution of RBP coincided with none of the other peptides, although some of the RBP-containing cells stained for most of the peptides examined and vice versa. These results suggest that RBP may be a consistent component of gastrointestinal endocrine cells.

Neuroendocrine regulatory mechanisms in the choroid plexus-cerebrospinal fluid system

The CSF is often regarded as merely a mechanical support for the brain, as well as an unspecific sink for waste products from the CNS. New methodology in receptor autoradiography, immunohistochemistry and molecular biology has revealed the presence of many different neuroendocrine substances or their corresponding receptors in the main CSF-forming structure, the choroid plexus. Both older research on the sympathetic nerves and recent studies of peptide neurotransmitters in the choroid plexus support a neurogenic regulation of choroid plexus CSF production and other transport functions. Among the endocrine substances present in blood and CSF, 5-HT, ANP, vasopressin and the IGFs have high receptor concentrations in the choroid plexus and have been shown to influence choroid plexus function. Finally, the choroid plexus produces the growth factor IGF-II and a number of transport proteins, most importantly transthyretin, that might regulate hormone transport from blood to brain. These studies suggest that the choroid plexus-CSF system could constitute an important pathway for neuroendocrine signalling in the brain, although clearcut evidence for such a role is still largely lacking.

Studies on the synthesis and secretion of transthyretin by the human hepatoma cell line Hep G2

Transthyretin (TTR) is a circulatory protein which plays an important role in the transport of both thyroid hormone and retinol. Hep G2 cells, a human hepatoma-derived cell line, have been used extensively in studies of protein secretion by liver cells. The original description of this cell line indicated that this line, unlike primary hepatocytes, does not secrete TTR. We now report studies which reexamine the ability of Hep G2 cells to synthesize and secrete TTR. For this purpose, total RNA was isolated from Hep G2 cells grown on both uncoated and collagen-coated plastic plates and was examined for TTR expression by Northern blot analysis. TTR mRNA was found to be present in nearly equal amounts in Hep G2 cells cultured in either condition. When Hep G2 cells were cultured in [35S]methionine-containing medium, the cells were found both to synthesize and to secrete immunoprecipitable [35S]TTR. Hep G2 cells were found, by sensitive and specific radioimmunoassay, to contain 142 +/- 91 ng TTR/10(6) cells and to secrete TTR into the medium at a nearly constant rate for at least 24 h after medium change. Our data demonstrate that Hep G2 cells do synthesize and secrete TTR and suggest that this cell line might be useful for studies of the secretion of TTR.

Clinicopathologic correlations in epithelial choroid plexus neoplasms: a study of 52 cases

Sixty-seven tumor specimens of epithelial choroid plexus neoplasms obtained by 60 biopsies and 7 autopsies from 52 patients were investigated. Diagnoses of the first operations were choroid plexus papilloma (PP; 32 cases), choroid plexus papilloma with histological atypies (atypical PP; 6 cases), and choroid plexus carcinoma (PC; 14 cases). Carcinoembryonic antigen was expressed by 2 of the 3 biopsies autoptically recognized as metastatic carcinomas and by 2 autopsy cases of PC, while it was absent in all biopsies of true choroid plexus tumors. Tumor cells positive for transthyretin (TTR, prealbumin), S-100 protein (S100), and glial fibrillary acidic protein (GFAP) were detected in 39, 46 and 13, respectively, of the 49 cases of true choroid plexus tumors. Fourth ventricle tumors expressed more S100 (number of positive tumor cells) than lateral ventricle tumors, PP more S100 and TTR than atypical PP/PC. Tumors from patients 20 years of age and older expressed more GFAP and TTR than tumors from younger patients. Of the 30 patients with complete follow-up 19 were alive 2 to 11 years after surgery, including 7 recurrencies. Eleven died from the tumor 4 months to 7 years after surgery. The following histopathologic features (in order of decreasing significance) were correlated with poor prognosis (recurrency or fatal outcome): less than 50% of the tumor cells heavily positive for S100, presence of mitoses, absence of TTR-positive cells, brain invasion by cell nests, absence of marked stromal edema, and presence of necrotic areas. Our results indicate that some histologic features correlate significantly with poor prognosis and that immunohistochemical results correlate with tumor localization, age, and malignancy.

Localization of transthyretin-mRNA and of immunoreactive transthyretin in the human fetus

Five human fetuses at mid-term (16-20 weeks) and one with a gestational age of 8 weeks were investigated. The cellular localization of transthyretin (TTR)-mRNA in different organs was demonstrated by in situ hybridization with a 35S-labelled, single-stranded RNA probe. Immunoreactive TTR (TTR-IR) was localized with a monoclonal antibody to TTR. In all fetuses, choroid plexus epithelial cells demonstrated intense labelling for TTR-mRNA as well as strong TTR-IR. Hepatocytes, on the other hand, showed weak in situ labelling and weak or, in some cases, non-demonstrable TTR-IR. In all mid-term fetuses, but not in the 8 week fetus, TTR-mRNA and TTR were also expressed in pancreatic endocrine A-cells. The degree of in situ labelling in these A-cells was moderate, whereas that of TTR-IR was strong. Despite negative findings for TTR-mRNA in the gut and the kidney, endocrine cells of the gut and epithelial cells of the renal proximal convoluted tubules showed TTR-IR in some of the fetuses. The investigation provides evidence for TTR synthesis in the human fetal choroid plexus, liver and endocrine pancreas. However, further studies are required to demonstrate TTR synthesis in the gut and the kidney.

In situ localization of transthyretin-mRNA in the adult human liver, choroid plexus and pancreatic islets and in endocrine tumours of the pancreas and gut

In situ hybridization with 35S-labeled single stranded RNA probes was used on sections from formaldehyde-fixed and paraffin-embedded tissue specimens to provide semiquantitative data on the occurrence of transthyretin(TTR)-mRNA in human liver, choroid plexus and pancreatic islets as well as in 15 endocrine tumours of the pancreas and gut. A monoclonal antibody to TTR was used for immunocytochemical identification of the protein in consecutive sections. The amount of TTR-mRNA in hepatocytes was found to be much less than that in epithelial cells of the choroid plexus. Glucagon cells of the pancreatic islets were also specifically labeled and the level of TTR-mRNA in these cells was intermediate between that of hepatocytes and choroid plexus epithelial cells. Four glucagonomas, one malignant insulinoma and two midgut carcinoids were shown to contain TTR-mRNA. The 'in situ' labeled cells were also found to be TTR immunoreactive. These findings present the first conclusive evidence for TTR synthesis in pancreatic islets and in endocrine tumours. They also establish that the high serum concentration of TTR found in some patients with endocrine tumours (notably glucagonomas) is most likely due to tumour production of TTR.