Why fibrosis after exposure to low frequency noise?

Abstract

Introduction: The cardiovascular effects of the exposure to low frequency noise (LFN) (<500 Hz) are not fully understood. Previous experimental work confirmed an increase in collagens I and III and ultrastructural changes in the intercalated discs of cardiomyocytes from rats exposed to LFN. Our work aims at characterizing the alterations in gene expression in the heart of rats exposed for 13 weeks to LFN.

Materials and methods: 8 adult Wistar rats assigned randomly in equal number to continuous LFN exposure or silent conditions for control purposes were sacrificed after 13 weeks. The heart apexes were frozen and preserved at -80 ºC until RNA extraction. Gene expression was analysed using GeneChip® Rat Gene 1.0 ST (Affymetrix). The set of genes was submitted to Ingenuity Pathway Analysis (IPA) software to predict relevant outcomes. For immunohistochemistry we have used samples of heart apexes fixed in formalin and embedded in paraffin and for immunofluorescence the samples were fixed in PFA 4%. In both methodologies we have incubated the samples with rabbit Hemoglobin α polyclonal antibodies (H80).

Results: Microarray analysis showed that 19,353 genes were expressed in heart, from a total of 27,342 well-annotated genes of the chip, covering the whole transcript. The LFN exposed group showed 186 genes with significant fold change above 1.5, mostly (115 genes) down-regulated. The gene with highest variation was haemoglobin alpha, adult chain 2, 6-fold down-regulated. Its expression in rat cardiomyocytes was confirmed by immunostaining and immunofluorescence. The integrated view of gene expression, performed by IPA, assigned a subset of 19 genes to fibrosis in heart. Overall their expression variation indicated an increase in fibrosis, though not significant (z-score 0.9 < 2). Analysis of Z-scores predicted several biological functions significantly altered namely transport of molecule, concentration of cyclic nucleotides, hydrolysis of acylglycerol, olfaction, cell death and permeability transition of mitochondria.

Discussion and conclusions: We have found an unexpected significant change in haemoglobin alpha, adult chain 2 expression in LFN-exposed hearts and were able to confirm the gene expression results by immunohistochemistry and immunofluorescence. Although there is increasing evidence of the presence of haemoglobin in tissues other than erythrocytes, its role is not yet clarified. Despite the well-documented occurrence of tissue fibrosis in LFN-exposed cardiomyocytes, we did not found significant differences in expression of collagen-coding genes. However, the expression of a subset of genes associated with cardiac fibrosis indicated a possible increase. Our results showed both an increase in olfaction and a decrease in cell death processes, which are compatible with a response to the chronic stress to which the rats were submitted.

Acknowledgements

The authors would like to address a posthumous thanks to Professor José Martins dos Santos for contributing to the works related to low frequency noise; The authors acknowledge the funding by Egas Moniz, CRL.