Somatostatin-like polypeptides in the plasma and tissues of a patient with a somatostatin-producing tumour.

Abstract

The existence of multiple molecular forms of the polypeptide hormones and probably of all secretory proteins is a consequence of their mode of biosynthesis. Polypeptides larger than somatostatin (mol.wt. 1600) that possess somatostatin-like immunoreactivity have been detected in extracts of hypothalamus, extrahypothalamic brain, stomach, pancreas and gut. Analysis of newly synthesized proteins in pancreatic islets of the anglerfish (Noe et al., 1979) and rat (Patzelt et al., 1979) has indicated a mol.wt. of approx. 12000 for prosomatostatin. Release into the circulation of multiple forms of a hormone is a characteristic of endocrine neoplasms and has been extensively documented in the case of islet tumours. The clinical and morphological features of a patient with a somatostatinproducing tumour of the pancreas and multiple liver metastases has recently been described (Krejs et al., 1979). In the present study, the distribution of molecular forms of the somatostatin-like polypeptides in extracts of the primary tumour of this patient are compared with those released by the primary tumour and by the metastases into plasma. Blood samples, collected from a peripheral vein by the method of Harris et al. (1978), were taken from the patient in the fasted state and after a 4.2MJ (1000 calorie) mixed meal. Further samples were taken, in the fasted state, after resection of the primary tumour. Tissue was extracted by the method of Conlon et al. (1978a). Plasma samples and tissue extracts were chromatographed on columns of Biogel P-10 and Sephadex G-200 and somatostatin-like polypeptides in the efRuent fractions were detected by radioimmunoassay (Harris et al., 1978). Samples were diluted at least 20-fold before assay so that degradation of the radio-labelled tracer and non-specific interference by plasma proteins in the assay were negligible. Amounts of immunoreactivity equivalent to 9-25 ng of somatostatin/ml were measured in plasma samples in the fasted state (value in normal subjects < 100pg/ml) and increased 8-fold after a mixed meal. Three samples of tissue from the primary tumour contained immunoreactivity equivalent to 0.8, 1.2 and 4.6pg of somatostatin/mg wet wt. of tissue (the normal pancreas content was 3 1440pg/mg wet wt. of tissue; McIntosh et al., 1978). The somatostatin-like polypeptides in plasma samples and tissue extracts were resolved by gel filtration into three main peaks in the 10000-15000, 250Ck3500 and 1500-2000 mol.wt. zones. The relative amounts of these components were estimated from the area under the peaks and are shown in Table 1. Plasma samples, but not tissue extracts, contained low amounts (< 1% of total immunoreactivity) of somatostatin-like immunoreactive material, which was eluted from Sephadex G-200 columns in the 150000-200000 mol.wt. zone and may represent smaller immunoreactive peptides bound non-covalently to plasma proteins (Conlon et al., 1978b). The amounts of the higher molecular-weight components relative to the somatostatin-sized component are higher in plasma than in tissue extracts, which probably reflects a lower rate of clearance of the larger peptides from the circulation. The neoplasms responded to a nutrient stimulus with an increased release of all three components, but the rate of release of the 1500-2000-mol.wt. component was much greater than that of the larger peptides. The marked increase in the relative amounts of the 1000015 000and 2500-3500-mol.wt. polypeptides in the postoperative plasma is evidence for an increased rate of release of these components from the metastases compared with the primary tumour. Somatostatin-like polypeptides of mol.wts. approx. 12000, 3000 and 1600 have been identified in extracts of normal dog pancreas (Conlon et al., 1978a), but only the 1600-mol.wt. component was detected in dog plasma and in the plasma-free efAuent of the isolated dog pancreas (Conlon et al., 19786). Proteolytic cleavage at sites of pairs of basic amino acids represents the predominant, but not exclusive, means of processing of prohormones. In the light of evidence from biosynthetic studies, it is proposed that the component with mol.wt. approx. 12000 in normal and tumour tissue represents prosomatostatin, which is specifically cleaved to the approx. 1600-mol.wt. component. The approx. 3000-mol.wt. polypeptide may be a product of non-specific cleavage of the prohormone. In normal pancreatic tissue, the rate of processing of prosomatostatin is rapid so that concentrations of the prohormone and the approx. 3000-mol.wt. component are low and only the 1600-mol.wt. component is released into plasma. In the tumours, however, the rate of conversion of the prohormone into hormone is much slower, so that high concentrations of prosomatostatin and its non-specific cleavage products accumulate in the cells and are subsequently released into plasma. The rate of processing of prosomatostatin appears to be less in the metastases than in the primary tumour.