Correspondence

C/EBPα in Asthma

N Engl J Med 2004; 351:2236-2237November 18, 2004

Article

To the Editor:

Roth et al. (Aug. 5 issue)1 report that glucocorticoids do not inhibit the proliferation of bronchial smooth-muscle cells from patients with asthma and propose that these cells lack expression of the CCAAT/enhancer binding protein α (C/EBPα), a transcription factor. However, Figure 4A in their article shows reactivity of an anti-C/EBPα antibody to a protein band about 4 kD below what is seen for patients with emphysema. This raises the possibility that a different isoform of C/EBPα is expressed, possibly as a result of alternate RNA splicing, aberrant glycosylation, or up-regulation of a cleavage enzyme of C/EBPα. The dual-banding pattern seen when the bronchial smooth-muscle cells from patients with asthma were transiently transfected with C/EBPα (Figure 4C) suggests that a proteolytic enzyme is active.

Is it possible that the bronchial smooth-muscle cells of patients with asthma actually do express C/EBPα, but that the glucocorticoid-receptor-–binding region is cleaved off? Such a possibility has important mechanistic implications.

Daniel I. Levy, M.D., Ph.D.
University of Chicago School of Medicine, Chicago, IL 60637
dlevy@medicine.bsd.uchicago.edu

1 References

1. 1

   Roth M, Johnson PR, Borger P, et al. Dysfunctional interaction of C/EBPα and the glucocorticoid receptor in asthmatic bronchial smooth-muscle cells. N Engl J Med 2004;351:560-574
   Full Text | Web of Science | Medline

Author/Editor Response

We agree with Dr. Levy that the expression of small C/EBPα bands (Figure 4A of our article) indicates the presence of small isoforms of C/EBPα in a subgroup of patients with asthma. It has to be emphasized that the full-length C/EBPα protein is missing in all 20 asthmatic cell lines. The expression of C/EBPα isoforms is not regulated by differential splicing but depends on alternative translation-initiation sites in C/EBPα messenger RNA.1,2 Four alternative translation-initiation sites have been described in human C/EBPα.1 The dual bands observed in bronchial smooth-muscle cells from patients with asthma after transfection with C/EBPα may be explained by these different translation-initiation sites, and transfected genes are not naturally processed. We agree that cleaving of the glucocorticoid-binding region would abrogate binding of C/EBPα to the glucocorticoid receptor and to its DNA consensus sequence.3,4 The antibody used in our experiments detects all known C/EBPα isoforms; therefore, it is unlikely that we missed C/EBPα variants in asthmatic cells.

Michael Roth, Ph.D.
Peter Borger, Ph.D.
Michael Tamm, M.D.
University Hospital Basel, CH-4031 Basel, Switzerland
michaelr@med.usyd.edu.au

4 References

1. 1

   Calkhoven CF, Bouwman PR, Snippe L, Ab G. Translation start multiplicity of the CCAAT/enhancer binding protein alpha mRNA is dictated by a small 5' open reading frame. Nucleic Acids Res 1999;22:554-557
   

2. 2

   Calkhoven CF, Muller C, Leutz A. Translational control of C/EBPalpha and C/EBPbeta isoform expression. Genes Dev 2000;14:1920-1932
   Web of Science | Medline

3. 3

   Ramos RA, Meilandt WJ, Wang EC, Firestone GL. Dysfunctional glucocorticoid receptor with a single point mutation ablates the CCAAT/enhancer binding protein-dependent growth suppression response in a steroid-resistant rat hepatoma cell variant. FASEB J 1999;13:169-180
   Web of Science | Medline

4. 4

   Roth M, Johnson PRA, Rudiger JJ, et al. Interaction between glucocorticoids and β2 agonists on bronchial airway smooth muscle cells through synchronised cellular signalling. Lancet 2002;360:1293-1299
   CrossRef | Web of Science | Medline

Citing Articles (1)

Citing Articles

1. 1

   Sandra Verstraelen, Inge Nelissen, Jef Hooyberghs, Hilda Witters, Greet Schoeters, Paul Van Cauwenberge, Rosette Van Den Heuvel. (2009) Gene profiles of a human bronchial epithelial cell line after in vitro exposure to respiratory (non-)sensitizing chemicals: Identification of discriminating genetic markers and pathway analysis. Toxicology 255:3, 151-159
   CrossRef