Cloning and biosynthetic studies of rat somatostatin.
The predicted amino acid sequence of rat preprosomatostatin has been obtained by cloning and subsequent DNA sequence analysis of a cDNA obtained from mRNA prepared from a rat medullary thyroid carcinoma (MTC). The predicted preprosomatostatin is 116 amino acid residues in length. Somatostatin-14 is located at the C-terminus of the preprohormone and the amino terminus contains a 'signal' peptide of 24 amino acids. A somatostatin amino terminal protein of 78 residues is found between the signal peptide and somatostatin-14. Both somatostatin-14 and somatostatin-28 are observed in the thyroid tumour C-cells. A comparison of the rat and human preprosomatostatin amino acid sequences shows only 4 substitutions observed in 116 amino acids. Patients with localized MTC often exhibit high serum calcitonin levels while patients showing cellular heterogeneity in the MTC appear to have lower calcitonin levels and a virulent neoplasia with a grave prognosis. Numerous rat MTCs have been examined by two dimensional gel electrophoresis. It is possible to distinguish characteristic differences in protein profiles of tumours producing high levels of calcitonin from those showing low calcitonin and high somatostatin levels. This analysis can be done with less than 1 mg of tissue and may represent a valuable prognostic tool in evaluating the clinical variability of MTC.