Regulation of adrenocorticotrophin and its precursors in small cell lung cancer

The ACTH precursor gene, pro-opiomelanocortin (POMC) is expressed in many non-pituitary tumours. In this thesis, small cell lung cancer (SCLC) cell lines have been studied as an in vitro model of the regulation of human, non-pituitary POMC. In contrast to the aggressive growth of this tumour in patients, SCLC cell lines are slow growing and difficult to establish and maintain in culture. ACTH and its precursors were quantitated by two-site immunoradiometric assays based on monoclonal antibodies. Ten cell lines (56%) secreted predominantly ACTH precursors (POMC and pro-ACTH) in the range 37-1405 pmol/1, with very little, if any, processing to ACTH. POMC RNA was demonstrated in 5 cell lines and a major transcript of approximately approximately 1350 bases was identified by Northern blot analysis in the cell line, COR L103, slightly larger than normal human pituitary (approximately 1100 bases). Incubation of SCLC cells for 10 days with hydrocortisone (500 and 1000 nM) did not inhibit intracellular POMC mRNA or ACTH precursor secretion, while this protocol produced significant inhibition in pituitary adenoma cells (AtT20). However, the dopamine agonist, bromocriptine (10 μg/ml) markedly suppressed ACTH precursor secretion following 24h and 7 days incubation. Corticotrophin releasing factor (CRF, 1000 ng/ml) had little effect on SCLC cells but significantly stimulated AtT20 and primary cultures of human pituitary cells. ACTH precursor secretion by SCLC cells was increased by dibutyryl cyclic AMP indicating that this intracellular signalling pathway is functional. These results are consistent with the clinical features of the ectopic ACTH syndrome, where ACTH circulates in precursor forms and is relatively resistant to glucocorticoid inhibition or stimulation with CRF. Thus the POMC gene is regulated in SCLC but it appears that the mechanisms differ from those operating in the anterior pituitary.