Binding of antibodies against human prealbumin to intestinal and bronchial carcinoids and to pancreatic endocrine tumours

Classic and recent special stains used in differential diagnosis of endocrine tumors

During the twentieth century, stains for endocrine cells and tumors were developed from empiric cytologic procedures aimed at modern cytochemical methods. Before the 1970s, endocrine stains were mainly based on silver reaction, although other reactions were also proposed; however, the chemical basis of most of these reactions is still unclear. The development of fluorescence procedures for detecting biogenic amines at the cellular level provided information about endocrine cell function of normal cells and related tumors. However, the application of immunocytochemical reactions brought greater and more definitive insights. Several immunocytochemical markers are now available. Some are specific for a definite cell type, while others detect endocrine differentiation in general. Some of these "pan-endocrine" markers are highly specific, and others are highly sensitive but less specific. They all play a definite role in diagnostic pathology. The use of molecular procedures to detect specific mRNA or genetic mutations of diagnostic interest in endocrine pathology should complement immunophenotyping, especially in some problematic fields, such as that of "poorly differentiated" tumors.

Transthyretin expression in medulloblastomas and medulloblastoma cell lines

Transthyretin is a protein crucial to the transport of lipophilic molecules such as thyroid hormones and retinoids. In the central nervous system, large amounts of transthyretin are synthesized by the choroid plexus and are secreted into the cerebrospinal fluid. The choroid plexus is the only site of transthyretin synthesis in the brain. Transthyretin is expressed by most benign and malignant choroid plexus tumours while gliomas and meningiomas do not express transthyretin. Other major sites of transthyretin synthesis are the retinal pigment epithelium and hepatocytes. Medulloblastoma is the prototypical primitive neuroectodermal tumour of the cerebellum and can show multiple lines of differentiation, including the expression of retinal markers. In this study, we examined transthyretin expression both at the RNA and protein level in four medulloblastomas and six medulloblastoma cell lines using Northern and Western blot analysis, reverse transcription polymerase chain reaction (PCR), RNA in situ hybridization, and immunohistochemistry. All four medulloblastomas and five of the six medulloblastoma cell lines expressed transthyretin-mRNA as demonstrated by reverse PCR and in situ hybridization while three medulloblastomas and one cell line were positive on Northern blot. The medulloblastoma with the most abundant RNA expression was transthyretin-immunoreactive on cryosections and the medulloblastoma cell line that was positive on Northern blot also expressed transthyretin at levels detectable by Western blot. No transthyretin-immunoreactivity was seen in 16 additional medulloblastomas studied on paraffin sections. These findings indicate that low-level expression of transthyretin-mRNA is common in medulloblastomas and medulloblastoma cell lines. Expression of transthyretin protein occurs rarely but can reach significant levels. Transthyretin expression in medulloblastoma is consistent with retinal pigment epithelium differentiation in medulloblastomas and reflects their pluripotential nature. Furthermore, the potential for transthyretin-immunoreactivity in medulloblastoma should be kept in mind when performing immunohistochemical studies on poorly differentiated cerebellar tumours.

Prealbumin in the diagnosis of bronchopulmonary carcinoid tumours

The reliability of prealbumin as a diagnostic marker was studied in 60 cases of bronchopulmonary carcinoid tumours. There were differences in the incidence of positivity between typical and atypical carcinoids (well differentiated neuroendocrine carcinomas). Seventy five per cent of the carcinoid tumours were positive for prealbumin; (86.7% typical and 63.3% atypical carcinoids). In 15 cases, which were Grimelius negative, 10 were prealbumin positive. Only 8.3% carcinoids were negative with both prealbumin and Grimelius stains. Ten squamous, 10 adeno- and 10 small cell carcinomas showed only occasional scattered prealbumin positive cells. It is concluded that prealbumin is a useful marker for bronchopulmonary carcinoid tumours. It is cheap, readily available, and should be considered part of routine diagnostic procedures for the diagnosis of carcinoid tumours.

Immunohistochemical localization of factor X-like antigen in pancreatic islets and their tumours

The authors have investigated by immunohistochemistry the distribution of factor X-like antigen in normal pancreatic islets and in a series of 46 pancreatic endocrine tumours. It was found that both glucagon-producing (A) cells and pancreatic polypeptide-producing (PP) cells are immunoreactive for the antigen. Benign glucagonomas and PP-omas presented the highest concentrations of immunoreactive material whose intracellular distribution was consistent with localization within cell secretory granules. Some benign insulinomas also presented factor X immunostaining in spite of the absence of the antigen in normal insulin-producing B cells. Although malignant tumours usually exhibited very low or no immunostaining, two of three malignant glucagonomas showed scattered, intensely immunoreactive cells. The factor X-like antigen identified in this study was found to differ from chromogranin A and B. The possible implications of the present findings for coagulative disorders associated with glucagonomas or diabetes are discussed.

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.

Comparative study of seven neuroendocrine markers in pancreatic endocrine tumours

A comparative immunocytochemical investigation was performed on a series of 59 pancreatic endocrine tumours using a panel of seven markers for neuroendocrine neoplasms: neurone specific enolase (NSE), PGP 9.5, chromogranin A (CgA), PHE5, prealbumin (Pa), HISL-19, and alpha-subunit of human chorionic gonadotropin (alpha-HCG). Most markers can be separated into two groups characterized by an identical immunoreactive cellular compartment and substantial overlapping in the immunohistochemical results. The first group comprises soluble cytoplasmic proteins such as NSE and PGP 9.5 and is characterized by a diffuse, homogeneous staining of the cell cytoplasm that is not related to the type of hormone produced or the degree of cell differentiation. The second group includes antigens located in the cell secretory granules such as CgA, PHE5, Pa and HISL-19 and is characterized by a heterogenous, often polarized cell staining. The latter markers strongly react with benign glucagonomas and PP-omas and, in contrast with those of the former group, are strictly neuroendocrine-specific. However, they often are less effective in staining insulinomas and malignant tumours. An additional, distinctive and useful characteristic of the HISL-19 antibody was its ability to label the Golgi complex also in tumours with absent granular staining. Finally, alpha-HCG was found in 9 of 16 malignant tumours (mostly glucagonomas and insulinomas) and in 4 of 43 benign neoplasms (all insulinomas). The latter finding is not in accordance with the reputed specificity of the alpha-HCG expression by pancreatic endocrine tumours as a marker for tumour malignancy.

Calcitonin-like immunoreactivity of amyloid fibrils in medullary thyroid carcinomas. An immunoelectron microscope study

Using 3 polyclonal antisera directed against synthetic human calcitonin, we investigated at the electron microscope level the intra-or-extra-cellular fibrillar/filamentous aggregates found in 4 amyloid-rich medullary thyroid carcinomas (MTC) and in a number of other endocrine polypeptide tumours with or without demonstrable amyloid deposition. The antisera were applied by the immunogold procedure on ultrathin sections of glutaraldehyde-fixed, usually osmium-postfixed, tissues. In MTC cases, a strong labelling was present over two types of aggregates: one composed of rigid, criss-crossing fibrils 7-10 nm in diameter, suggestive of amyloid, and the other consisting of loosely arranged fibrils, 4-7 nm in width, often wavy or poorly defined. In both cases, the labelling was closely associated with that part of the sectioned fibril exposed to the antiserum. Amorphous material was sometimes present adjacent to the later aggregates, but did not bind the calcitonin antibodies. In contrast, no labelling occurred over the amyloid deposits found in two non-calcitonin-producing endocrine tumours of the pancreas, nor over the cytoskeletal filaments stored in various endocrine polypeptide tumours. The specific value of the labelling for calcitonin-like immunoreactivity was assessed by control tests, such as absorption of the antiserum by excess calcitonin and comparative use of normal serum and antisera directed against human IgG and P component. No immunoreactivity of the MTC amyloid fibrils was found using antibodies directed against katacalcin and human prealbumin. We conclude that in tumour tissues conventionally processed for electron microscopy, MTC amyloid fibrils of varying morphology can be selectively and specifically labelled for calcitonin-like immunoreactivity.

Immunohistochemical localization of plasma retinol-binding protein and prealbumin in human pancreatic islets

This study was undertaken, employing the immunoenzyme method, to confirm the presence of retinol-binding protein in human pancreatic islets, and to compare its distribution with that of prealbumin, insulin, glucagon, somatostatin and pancreatic polypeptide. It was found that most islet cells contained retinol-binding protein, although centrally located cells showed stronger reactivity than those in the peripheral region. The distribution of each of the five polypeptides differed from that of retinol-binding protein, indicating that these peptides did not cross-react with anti-retinol-binding protein antibody. Islet cells which contained prealbumin, on the other hand, were mostly classified as A cells. Further studies are necessary to confirm whether the islet cells produce retinol-binding protein or only store it.

Plasma and cellular retinoid-binding proteins and transthyretin (prealbumin) are all localized in the islets of Langerhans in the rat

The immunohistochemical localization of plasma retinol-binding protein (RBP), cellular retinol-binding protein (CRBP), and transthyretin (TTR) was studied in rat pancreas. The studies employed antibodies purified by immunosorbent affinity chromatography, permitting the specific staining and localization of each antigen by the unlabeled peroxidase-antiperoxidase method. Specific immunostaining for each of these three proteins was found localized to the islets of Langerhans. Both RBP and CRBP were localized in cells that were peripherally distributed within the islets, with an anatomic distribution that resembled that of the glucagon-containing A cells. Immunoreactive TTR was localized in cells that were more centrally distributed in the islets, with an anatomic distribution that resembled that of the insulin-containing B cells. These findings were confirmed by radioimmunoassay of a homogenate of isolated rat islets. By using sensitive and specific radioimmunoassays for each antigen, unusually high levels of CRBP, RBP, TTR, and cellular retinoic acid-binding protein (CRABP) were found in rat islets. The physiological significance of the localization of RBP, CRBP, CRABP, and TTR in the islets is not known. The findings suggest that retinoids and their binding proteins may play important metabolic roles within islet cells, and hence that they may be involved in some way in the biological, endocrine, function of the islets.