Letter by Goetze et al Regarding Article, “B-Type Natriuretic Peptide Signal Peptide Circulates in Human Blood: Evaluation as a Potential Biomarker of Cardiac Ischemia”
To the Editor:
Siriwardena and colleagues from the Christchurch Cardioendocrine Research Group report on the signal peptide from the B-type natriuretic peptide (BNP) preprohormone as a separate entity in cardiac tissue and circulation.1 The report is of general interest in mammalian peptide biology because the findings challenge the prevailing concepts of peptide biosynthesis and secretion. Moreover, signal peptide measurement in plasma has no prior examples in clinical practice. The data even imply that other regulatory peptide precursors should be reexamined because signal peptide fragments have not been considered of relevance in circulation. These hydrophobic, usually not evolutionary, conserved N-terminal presequences target the prohormones to the endoplasmic reticulum during translation and onward to posttranslational maturation in the secretory pathway.
The biosynthesis and cellular secretion of the BNP precursor are still areas of surprise and discovery. The first model of proBNP maturation consisted of only 1 endoproteolytical cleavage before secretion, which splits the precursor into an N-terminal fragment and a C-terminal bioactive peptide. Since then, the molecular heterogeneity has proven highly complex, including variable glycosylation, N-terminal trimming, and enzymatic cleavages.2 Taken together, there is indisputable evidence that proBNP enters the secretory pathway and thus is located to the endoplasmic reticulum, which requires the signal peptide as a “molecular guidewire.”
Given the general relevance of the report, we believe that the data need to be scrutinized with some critical evaluation before cellular signal peptide release can be pursued as a new concept in biological thinking. First, we note that the identified C-terminus of the signal peptide is not unique but a structural part of other human proteins. Immunologic measurement of the C-terminal signal peptide sequence can therefore not readily be related only to the proBNP signal peptide without further data on epitope recognition. Often, epitope binding of polyclonal antisera cannot be appreciated only by the sequence used for immunization.3 An exercise would be to test the assay in tissue and plasma from other species because the C-terminal region in the signal peptide differs.4 Second, the validation of the immunoassay leaves the reader with concerns regarding sensitivity and precision. In fact, the reported sensitivity (no apparent sensitivity <50 pmol/L; Figure 1B in the article by Siriwardena et al) challenges the validity of the plasma measurements, which are reported to be ≈15 pmol/L in healthy persons (Figure 1D in the article by Siriwardena et al). Finally, the cellular release of the peptide fragment is ultrarapid (Figure 4A in the article by Siriwardena et al), which contrasts the current data on cardiac BNP expression as being regulated at the transcriptional level with a constitutive release pattern. Specific cellular storage of only the signal peptide to be released during cellular necrosis before other intracellular substances, including myoglobin, seems unlikely.
We are intrigued by the identification of a sequence resembling the decapeptide fragment from the preproBNP signal peptide in circulation. If the present findings can be validated, we will have to extend our thoughts on regulatory peptides to also include signal peptides as independent entities in tissue and circulation. This would be a major paradigmatic shift in mammalian protein biology and biogenetics.
Jens P. Goetze, MD, DMSc
Anders H. Johnsen, MSc, DSc
Jens F. Rehfeld, MD, DMSc
Department of Clinical Biochemistry
Rigshospitalet, University of Copenhagen
- © 2011 American Heart Association, Inc.
- Siriwardena M,
- Kleffmann T,
- Ruygrok P,
- Cameron VA,
- Yandle TG,
- Nicholls MG,
- Richards AM,
- Pemberton CJ
- Goetze JP