Elaboration of Nonsomatostatin Peptides Containing the Amino-Terminal Portion of Prosomatostatin in a Somatostatin-Secreting Human Medullary Thyroid Carcinoma CellLine*

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Abstract
We identified a system, the TT human medullary thyroid carcinoma cell line which contained 74.5 +/- 26.0 ng (mean +/- SD) immunoreactive SRIH/mg protein, for studying the products of SRIH gene expression. The close homology between man and rat for the entire proSRIH and complete homology for a portion of the amino terminus (proSRIH-NTP) permitted the application of a RIA directed toward the amino terminus of rat proSRIH. Immunohistochemical studies of the human TT cell line showed that SRIH and proSRIH-NTP immunoreactivities were present in the same cells. RIA of gel filtration fractions showed that the major cellular proSRIH-NTP-containing species lack SRIH and had an apparent mol wt of 8000. A 10,000-dalton SRIH-containing species coeluting with proSRIH-NTP immunoreactivity, putative proSRIH, also was found. Similar species were found in culture medium. The major cellular and secreted form of SRIH immunoreactivity had a mol wt of 1600 (SRIH). High pressure liquid chromatography revealed that both cellular and secreted 8000-dalton proSRIH-NTP-containing material consisted of a major and several minor species. None of the species contained SRIH. The precise structure of these proSRIH-NTP-containing species and their physiological roles are not known. The fact that proSRIH-NTP-containing species are secreted suggests that they may be functional. Since the processing of proSRIH and consequently the final products appear to be tissue specific, alterations in the relative presence of different forms might reflect specific pathological processes.

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