Expression of pro-opiomelanocortin gene and quantification of adrenocorticotropic hormone-like immunoreactivity in human normal peripheral mononuclear cells and lymphoid and myeloid malignancies.

Using Northern blotting with a human genomic DNA probe for the pro-opiomelanocortin (POMC) gene, we have shown specific mRNA in normal human peripheral mononuclear cells (PBMC); the presence of specific mRNA was also observed in a T lymphocyte cell line derived from a patient with lymphoma. We then demonstrated that PBMC translate the message into protein. Thus, using a radioimmunoassay with an antibody for ACTH, a median of 29 pg of ACTH-like immunoreactivity (ACTH-LIR) was found in 10(7) PBMC. ACTH-LIR was also detected in seven different cell lines derived from patients with lymphoid and myeloid malignancies, two of them JM and U937 showing the highest values 135 and 108 pg/10(7) cells, respectively. The chromatographic characterization of this ACTH-LIR showed, at least, three molecular forms of immunoreactive ACTH with molecular weights of the order of 31,000 POMC, 22,000 ACTH, and 4,500 ACTH, in addition to high-molecular-weight material (greater than 43,000). We conclude that PBMC produce ACTH-LIR which may act as a paracrine immunomodulator in a similar way to lymphokines and/or may signal the adrenal gland to secrete glucocorticoids.

In vitro and in vivo analysis of the processing and fate of the peptide products of the short proopiomelanocortin mRNA.

Many peripheral tissues express the proopiomelanocortin (POMC) gene as an 800-base mRNA that lacks the 5' end of the 1200-base pituitary transcript. The missing region encodes the peptide signal sequence, and thus, it is unlikely that any translation product would be secreted. We have found that a RNA transcript equivalent to this short message, generated by transcription in vitro from a T7 polymerase promoter, is translatable in a rabbit reticulocyte lysate, generating peptides of 27.5, 22.5, and 15.5 kD. None of these peptides appears to be processed or protected from proteinase-K digestion by a microsomal membrane fraction. In vivo studies were undertaken by transfecting into GH3 cells one of two expression vectors containing sequences that would produce either a full-length mRNA or a short (800-base) mRNA. The neomycin resistance gene was cotransfected with these plasmids, and 30 permanent cell lines were produced after selection in G418. Cell lines containing the full-length RNA secreted large quantities of ACTH and beta-endorphin immunoreactivity, whereas those expressing the short transcript secreted neither of these peptides. However extractable peptide was present in this latter type of cell line, thereby suggesting that the 800-base mRNA was translated, and that no peptide reached the secretory vesicle. These findings raise important questions about the role of peripheral POMC gene expression.

Defective glucocorticoid regulation of proopiomelanocortin gene expression and peptide secretion in a small cell lung cancer cell line.

A human small cell lung cancer cell line (COR L103) that actively expresses the proopiomelanocortin (POMC) gene has been used as a model of extrapituitary ACTH-secreting tumors to investigate the phenomenon of resistence of ACTH production to glucocorticoids. After both short term (24 h) and long term (10 days) exposure to hydrocortisone at concentrations of 500 and 1000 nM, the accumulation of intracellular POMC mRNA, ACTH, and ACTH precursor peptides in the culture medium was not suppressed. These finding contrast with those in the pituitary corticotroph cell line AtT20, in which POMC mRNA, ACTH, and ACTH precursors were suppressed under the same conditions. Two other genes that are regulated by glucocorticoids in other cell types, the tyrosine amino transferase gene and the glucocorticoid receptor gene, were expressed in COR L103 cells. However, neither gene appeared to be regulated by hydrocortisone in this small cell lung cancer cell line. Further studies demonstrated that glucocorticoid receptor binding could be detected in the nucleus and cytoplasm, with a Kd of 5 X 10(-9) M. It is concluded that nonsuppression of POMC by glucocorticoids is probably part of a more global defect of glucocorticoid signaling in these cells, but that this defect lies distal to steroid binding in the nucleus.

Pro-opiomelanocortin mRNA size heterogeneity in ACTH-dependent Cushing's syndrome.

As an approach to understanding the abnormalities of pro-opiomelanocortin (POMC) gene regulation in human ACTH-secreting tumours, we have analysed the POMC mRNA content of nine such tumours using the Northern blot technique. Most of the tumours and normal human pituitary contained easily detectable quantities of POMC mRNA. The length of this message in most tumours was similar to, or slightly larger than, that in the normal pituitary (1150-1200 bases). Ribonuclease H studies suggested that the origin of any size heterogeneity was a longer poly(A) tail in the tumour RNA. Some tumours, however, expressed a short POMC mRNA (800 bases) which may lack the first two exons of the POMC gene as has been described. A third POMC mRNA size variant (1500 bases) was also seen in low levels in two cases, and as the principal mRNA species in one case. Primer extension and S1 nuclease protection studies suggested that most transcripts in the tumours analysed originated from the conventional promoter, and thus the use of an alternative promoter is not an adequate explanation for the expression of this gene in ectopic ACTH-secreting tumours.

Pro-opiomelanocortin gene expression and peptide secretion in human small-cell lung cancer cell lines.

Expression of the RNA coding for the ACTH-beta-lipotrophin precursor, pro-opiomelanocortin (POMC), has been demonstrated in five human small-cell lung cancer (SCLC) cell lines. Using Northern and slot-blot hybridization analysis of RNA and a bovine POMC cDNA as probe, the processed POMC RNA from SCLC cells was found to be approximately 1350 nucleotides in length, which is larger than that found in the normal human pituitary. Expression of the POMC gene was confirmed by measurement of ACTH precursors secreted by the cells, using a novel two-site immunoradiometric assay based on monoclonal antibodies, which directly quantifies both POMC and pro-ACTH but does not recognize ACTH. Levels of POMC in medium accumulated throughout the growth of the cells, in contrast to POMC RNA which showed a relatively constant level of expression. We conclude that human SCLC cell lines are valuable models for studying the aberrant expression and regulation of the human POMC gene.