Inducible Regulation of the Human Brain Natriuretic Peptide Promoter in Transgenic Mice
Brain natriuretic peptide (BNP), a cardiac hormone, is rapidly elevated in the infarcted heart, and plasma BNP levels reflect the degree of left ventricular dysfunction. Our previous work indicated that the proximal promoter of the hBNP gene was active when injected into adult rat hearts. To study tissue-specific and inducible regulation of the hBNP gene in vivo, we generated transgenic mice containing the hBNP promoter (-408 to +100) coupled to a luciferase reporter gene. Cardiac function was measured by 2D echocardiography and transgene expression was measured by luciferase assay and tissue section immunostaining. Luciferase activity was measured in the heart, brain, kidney, liver, lung and skeletal muscle of 4 lines of mice. High levels of luciferase activity were only detected in the heart, with ventricular expression higher than atrial expression (LV=RV>RA>>LA). Immunostaining with a luciferase antibody showed staining in myocytes (line 203). There were no differences in the blood pressure, heart rate, heart size and function of transgenic mice compared with controls (line 203). To test whether the transgene responded to a pathophysiological stimulus, mice had coronary artery ligation to produce infarction. Luciferase activity was measured after 48 hr and 1, 3 and 4 weeks. Luciferase activity driven by the hBNP promoter was 4 to 41-fold higher in the infarcted left ventricle vs sham-operated animals (lines 203, 83 and 86) at 48hr, 3.1-fold higher at 1 week (line 203), and 2.5-fold at 3 and 4 weeks (line 203). There was a clear increase in heart weight to body weight ratio and a decrease in cardiac output and ejection fraction 4 weeks post-infarction. Endogenous BNP mRNA was increased in infarcted hearts from a separate group of non-transgenic mice at 48 hr. We concluded that: 1) the hBNP promoter is preferentially expressed in the heart; 2) 408hBNPLuc transgenic mice have normal blood pressure and cardiac function; and 3) the hBNP promoter is stimulated by ischemic injury following infarction, as is the endogenous mouse BNP gene. These data suggest that we have a valid model for the study of both basal and inducible regulation of the hBNP gene in vivo.