Existence of proopiomelanocortin (POMC) messenger RNA (mRNA) and related peptides in extrapituitary sites has been demonstrated in immune cells, although the particular type of immune cell has been the source of considerable debate. Specifically, double labeling studies have shown that POMC peptide expressing cells in the spleen represent a subpopulation of red pulp macrophages, while splenic lymphocyte areas are POMC negative. In addition, it has also been reported that peripheral blood leukocytes express the POMC gene. Using a sensitive solution hybridization technique with a POMC exon-1 RNA probe, we detected 70 ± 20 fg and 65 ± 5 fg POMC mRNA per microgram total RNA in whole spleen and lung, respectively, approximately 20, 000-fold lower concentrations than found in the neurointermediate lobe of the pituitary. The presence of nuclease protected full length exon-1 bands, rather than the 5' truncated POMC RNAs seen in many nonpituitary tissues, indicates transcription initiation at the normal pituitary POMC promoter site in lung and spleen. In order to localize POMC gene expression in these tissues we employed an in situ hybridization method. There was an intense signal in a small population of large mononuclear cells scattered throughout the splenic red pulp and lung parenchyma. In the lung, these cells were concentrated in the periarteriolar zone in a manner suggestive of migration from the intravascular lumen. These cells had a histomor- phology suggestive of monocyte-macrophages. POMC mRNA was undetectable in the splenic white pulp and bronchus-associated lymphoid tissue, indicating an absence of POMC gene expression in splenic and lung lymphocytes. Immunocytochemical studies suggested that POMC- positive cells made up a subpopulation of cells expressing the rat monocyte-macrophage markers EDI and ED2. Similarly, the distribution of Jenner-Giemsa stained monocyte-macrophages appeared to overlap with POMC positive cells. Studies with anti-rat β-endorphin antisera revealed scattered cells in the splenic red pulp and lung parenchyma, suggesting that the POMC mRNA is translated in these cells. In summary, POMC mRNA is expressed in a small population of monocyte-macrophage-like cells in lung and spleen but not in lympho-cytes in these tissues.
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