Ectopic Production of Somatostatin-Like Immuno- and Bioactivity by Cultured Human Pulmonary Small Cell Carcinoma*

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Abstract
Continuous human pulmonary small cell carcinoma cells cultures (11) [including DMS 53 cells, DMS 55 cells, DMS 79 cells, DMS 92 cells, DMS 114 cells, DMS 153 cells, DMS 187 cells, DMS 235 cells and DMS 273 cells] were examined, and 8 were shown to secrete quantities of somatostatin-like immunoreactivity (SRIF-LI) ranging from 0.07-27 ng/ml culture medium per 4 days. SRIF-LI was found in a 2-N acetic acid extract of 1 of 3 human pulmonary small cell carcinomas obtained at autopsy as well as in the extract of a solid tumor resulting from inoculation of nude, athymic mice with SRIF-LI-producing, cultured small cell carcinoma cells. The SRIF-LI produced by 1 continuous cell line, DMS 53, was characterized in terms of its immunological, chromatographic and biological properties. SRIF-LI from DMS 53 culture media and lysed cells was heat stable and exhibited parallel displacement to synthetic SRIF standard in a double antibody radioimmunoassay. DMS 53 SRIF-LI was quantitatively retained on an immunoaffinity column of sheep anti-SRIF-Sepharose 4B under neutral conditions and could be eluted with 2 N acetic acid. Gel filtration chromatography of immunoaffinity-purified SRIF-LI revealed multiple MW forms, the largest of which had an apparent MW of 10,000-12,000 daltons and may represent a precursor form. This high MW SRIF-LI form was resistant to exposure to denaturing conditions (8 M urea or 4 M urea plus 0.5% mercaptoethanol), suggesting the absence of noncovalent and/or disulfide linkages. A low MW form coeluted with synthetic SRIF. Additional evidence for the identity of this form with the tetradecapeptide was provided by highly specific reverse phase high performance liquid chromatography. The rate of degradation of high MW SRIF-LI by the cultures was markedly reduced in comparison to that of the SRIF monomer, resulting in a preferential accumulation of high MW SRIF-LI in 4-day culture medium. Bioactivity of DMS 53 SRIF-LI was assessed in 4-day primary monolayer cultures of rat adenohypophyseal cells where 10-10-10-9 M synthetic SRIF elicited a linear log-dose suppression of 5 .times. 10-4 M dibutyryl cAMP-stimulated rat growth hormone release. Immunoaffinity-purified SRIF-LI from DMS 53 lysed cells and 1 h serum-free incubation medium, which consisted predominantly of monomeric SRIF, was equipotent to synthetic SRIF. SRIF-LI from 4-day culture medium consisted mostly of the high MW form and exhibited a reduced bioassay potency ratio relative to synthetic SRIF of 0.73 (95% confidence limits, 0.99-0.53). Chromatographically purified high MW SRIF-LI had significant bioactivity with a bioassay to immunoassay ratio of 0.19 (95% confidence limits, 0.33-0.09). The demonstration of ectopic SRIF production by human pulmonary small cell carcinoma is consistent with the proposed derivation of this tumor from a cell type in the amine precursor uptake and decarboxylation cell series.