The mechanisms involved in the activation of transcription factors and BNP gene expression in loaded heart

Abstract

Cardiac hypertrophy is an adaptive response of the heart to a variety of mechanical, hemodynamic, neurohumoral, and pathologic stimuli. Prolonged pathophysiological load leads to development of left ventricular hypertrophy and ultimately to heart failure. The natriuretic peptides including the B-type natriuretic peptide (BNP) provide the physiological feedback mechanism to suppress the load signal. The aim of the present study was to evaluate the cis elements within the BNP promoter that mediate the cardiac load responses in vivo, and to study the involvement of paracrine factors, such as endothelin-1 (ET-1) and angiotensin II (Ang II) in activating these transcription factors.

In this study, cardiac overload was produced by bilateral nephrectomy, and infusions of arginine⁸-vasopressin (AVP) or Ang II. In isolated perfused rat heart, the direct wall stretch was achieved by inflating the left ventricular balloon. To identify the cis elements within the BNP promoter that mediate hemodynamic overload response, the approach of DNA injection into the myocardium was used. Mutation or deletion of proximal BNP GATA sites abrogated the response to nephrectomy. AVP-induced acute pressure overload increased left ventricular BNP mRNA and peptide levels. In gel mobility shift assays, pressure overload produced rapid activation of transcription factor GATA4 DNA binding, which was completely inhibited by the ET-1 receptor antagonist bosentan. Both ET-1 and Ang II receptor antagonism inhibited the wall stretch-induced increases in left ventricular GATA4 and AP-1 binding activities in isolated perfused heart preparation. BNP promoter activity and BNP mRNA and peptide levels were regulated distinctly in chronic hemodynamic overload produced by Ang II.

In conclusion, GATA4 appears to be necessary and sufficient to confer transcriptional activation of BNP gene during hemodynamic stress in vivo. ET-1 is a signaling molecule mediating the cardiac response to acute pressure overload in vivo. In isolated rat heart, Ang II and ET-1 are required for the stimulation of GATA4 and AP-1 binding activity in response to direct left ventricular wall stretch. Finally, posttranscriptional mechanisms play an important role in the regulation of BNP gene expression in pressure overload produced by Ang II in vivo.