Adrenomedullin Receptor Expression in Human Lung and in Pulmonary TumorsAlfredo Martíneza, Mae Jean Millera, Kevin J. Cattb, and Frank Cuttittaaa Biomarkers and Prevention Research Branch, Division of Clinical Sciences, National Cancer Institute, National Institutes of Health, Rockville, Maryland b Endocrinology and Reproduction Research Branch, NICHD, National Institutes of Health, Bethesda, Maryland Correspondence to: Alfredo Martínez, Div. of Clinical Sciences, National Cancer Institute, NIH, 9610 Medical Center Dr., Rm. 300, Rockville, MD 20850-3300.
Adrenomedullin (AM) is a multifunctional regulatory peptide that stimulates cyclic AMP production in many target tissues and is highly expressed in the lung. Analysis of the distribution of the recently cloned AM receptor (AM-R) by non-radioactive in situ hybridization revealed abundant expression in the basal cells of the airway epithelium and Type II pneumocytes. The expression of AM-R in the two cell types involved in epithelial regeneration of the lung suggests that AM may be relevant in such functions as organ development, wound repair, and epithelial turnover. AM-Rs are also synthesized in vivo and in vitro by a variety of tumor cells that also express the ligand, suggesting the existence of an autocrine loop that may be involved in tumor growth stimulation. The present findings suggest that the AM/AM-R regulatory system plays a major role in respiratory physiology and lung carcinogenesis and that new functions for AM remain to be identified. (J Histochem Cytochem 45:159-164, 1997) Key Words: human lung, lung tumors, adrenomedullin receptor, growth regulation, in situ hybridization
Adrenomedullin (AM) is an
The biological effects of AM are mediated by elevating cellular cAMP levels through a G-protein-coupled mechanism (
Expression of the AM-R is particularly high in lung, in which Northern blot analysis reveals several mRNA species, of which the most prominent is a 1.8-KB transcript (
Tissues
Cell Culture
RT-PCR
Reverse transcription was performed using the SuperScript Preamplification System (Life Technologies; Gaithersburg, MD) per the manufacturers protocol. A Perkin-Elmer 9600 thermocycler was used in amplifying the gene products in the samples. All buffers, enzymes and nucleotides used were obtained from Applied Biosystems (Perkin-Elmer Cetus; Norwalk, CT). PCR products were analyzed electrophoretically using 1% agarose gels, and after ethidium bromide staining were visualized under UV light, followed by Southern analysis. Two PCR products (total human lung and cell line NCI-H720) obtained using the same primers have been previously cloned and sequenced, obtaining a high homology with the original rat clone (
Southern Blot Analysis
In Situ Hybridization
Immunocytochemistry
Normal Lung
Tumors
We have shown, using in situ hybridization techniques, that AM-R is highly expressed in the lung, consistent with the results of the Northern blot data presented in the original description of the AM-R (
We have previously described the distribution of AM in the normal and malignant human lung. AM is produced by the airway epithelium, bronchial glands, neurons, endothelial cells, chondrocytes, macrophages, and smooth muscle cells, and is present in many tumors (
The expression of AM-R in basal cells and Type II pneumocytes, two cell types involved in regeneration of the epithelial layers of the lung (
The specific distribution of AM and its receptor in the airway epithelium suggests a new hormonal mechanism of action that might be important in the rapid response to tissue injury. AM is stored in the apical end of the ciliated cells and is secreted into the lumen of the organ, where it acts as an antimicrobial agent (Tom Walsh, Pediatric Oncology Branch, NCI, personal communication). Ligand-specific receptors, on the other hand, are located in the basal region of the epithelium and are separated from this source of AM by the intact epithelial layer. If an epithelial injury occurs (infection, chemical or mechanical damage), cell boundaries are disrupted and the luminal pool of AM becomes available for receptor binding. Given the mitogenic potential of AM, this results in a rapid proliferative response which eventually heals the wound, restoring epithelial integrity and re-separating the ligand from the receptor. The term "traumakines" would be appropriate for peptides displaying such behavior, with AM as the first identified member of this family. During normal growth conditions (epithelial turnover), the receptors could receive the ligand from mesenchymal components, such as smooth muscle cells, or from the interstitial fluid derived from the plasma, a recognized source of AM (
The co-expression of both the ligand and the receptor in tumors suggests that AM and its receptor are implicated in an autocrine mechanism, that may regulate cell growth (
An unexpected finding was the absence of a hybridization signal in the smooth muscle cells of the blood vessels and the bronchial walls (Figure 1A and Figure 1C). Smooth muscle cells are believed to express AM-R that mediate the vasodilatatory and bronchorelaxant responses to AM. There are several reports (e.g., In conclusion, our data clearly demonstrate the expression of AM/AM-R in normal and malignant lung. The regional distribution of these molecules, along with previously reported data on their trophic action, implicates this ligand/receptor relationship as a possible growth regulatory mechanism involved with normal epithelial turnover, wound repair, and carcinogenesis.
We thank Dr Jill M. Siegfried (Department of Pharmacology, University of Pittsburgh) for providing the normal tissue, and Dr R. Ilona Linnoila (NCI) for help with the tumors. Received for publication May 29, 1996; accepted October 14, 1996.
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