Expression of the gene for the atrial natriuretic peptide in cardiac myocytes in vitro

Atrial natriuretic peptide modulates auditory brainstem response of rat

CONCLUSION

These findings suggest that Atrial natriuretic peptide (ANP) exhibits an inhibitory effect on auditory brainstem response (ABR) and is involved in the neuromodulation of the auditory nervous system.

OBJECTIVES

ANP may alter electrophysiological properties of the cochlea and play a role in auditory action.

METHODS

This study was undertaken to examine and clarify the role of ANP in the rat auditory system using ABR audiometry. The mean ABR thresholds and the latencies for wave II at the ABR threshold altered at given frequencies throughout the study.

RESULTS

Intra-arterial infusion of ANP (0.1 mg/kg, 4 mg/kg, and 8 mg/kg; bolus injection) resulted in a significant increase in ABR thresholds. A significant shift in the ABR wave II latency was observed at lower frequency (1 kHz and 2 kHz). There was a little change in latency at 20 kHz. Increased amount of ANP significantly altered the ABR in rats.

Dual role for glucocorticoids in cardiomyocyte hypertrophy and apoptosis

Glucocorticoids and their synthetic derivatives are known to alter cardiac function in vivo; however, the nature of these effects and whether glucocorticoids act directly on cardiomyocytes are poorly understood. To explore the role of glucocorticoid signaling in the heart, we used rat embryonic H9C2 cardiomyocytes and primary cardiomyocytes as model systems. Dexamethasone (100 nm) treatment of cardiomyocytes caused a significant increase in cell size and up-regulated the expression of cardiac hypertrophic markers, including atrial natriuretic factor, β-myosin heavy chain, and skeletal muscle α-actin. In contrast, serum deprivation and TNFα exposure triggered cardiomyocyte apoptosis, and these apoptotic effects were inhibited by dexamethasone. Both the hypertrophic and anti-apoptotic actions of glucocorticoids were abolished by the glucocorticoid receptor (GR) antagonist RU486 and by short hairpin RNA-mediated GR depletion. Blocking the activity of the mineralocorticoid receptor had no effect on these glucocorticoid-dependent cardiomyocyte responses. Aldosterone (1 μm) activation of GR also promoted cardiomyocyte hypertrophy and cell survival. To elucidate the mechanism of the dual glucocorticoid actions, a genome-wide microarray was performed on H9C2 cardiomyocytes treated with vehicle or dexamethasone in the absence or presence of serum. Serum dramatically influenced the transcriptome regulated by GR, revealing potential glucocorticoid signaling mediators in both cardiomyocyte hypertrophy and apoptosis. These studies reveal a direct and dynamic role for glucocorticoids and GR signaling in the modulation of cardiomyocyte function.

Glucocorticoid signaling in cardiac disease

As major mediators of stress regulation, glucocorticoids have an essential role in maintaining cardiovascular homeostasis under both physiological and pathological conditions. The release of glucocorticoids into the peripheral circulation is adjusted by the hypothalamic-pituitary-adrenal axis in response to various pathological challenges such as sepsis, starvation, and psychological stress. Clinically, dysregulation of the glucocorticoid-mediated signaling as a result of either excess ligand or receptor hypersensitivity is connected with the progression of unfavorable cardiovascular events such as cardiac hypertrophy, atherosclerosis, and coronary artery disease. The direct effects of glucocorticoids on cardiac tissues are mediated by two steroid receptors, the glucocorticoid receptor and mineralocorticoid receptor, which are both expressed by cardiomyocytes. Although each receptor has some shared responses to glucocorticoids, each receptor also has unique effects on cardiac functions. Elucidating the selective actions of each receptor is critical for determining the proper pharmaceutical targets in cardiovascular diseases.

Ligand-based gene expression profiling reveals novel roles of glucocorticoid receptor in cardiac metabolism

Recent studies have documented various roles of adrenal corticosteroid signaling in cardiac physiology and pathophysiology. It is known that glucocorticoids and aldosterone are able to bind glucocorticoid receptor (GR) and mineralocorticoid receptor, and these ligand-receptor interactions are redundant. It, therefore, has been impossible to delineate how these nuclear receptors couple with corticosteroid ligands and differentially regulate gene expression for operation of their distinct functions in the heart. Here, to particularly define the role of GR in cardiac muscle cells, we applied a ligand-based approach involving the GR-specific agonist cortivazol (CVZ) and the GR antagonist RU-486 and performed microarray analysis using rat neonatal cardiomyocytes. We indicated that glucocorticoids appear to be a major determinant of GR-mediated gene expression when compared with aldosterone. Moreover, expression profiles of these genes highlighted numerous roles of glucocorticoids in various aspects of cardiac physiology. At first, we identified that glucocorticoids, via GR, induce mRNA and protein expression of a transcription factor Kruppel-like factor 15 and its downstream target genes, including branched-chain aminotransferase 2, a key enzyme for amino acid catabolism in the muscle. CVZ treatment or overexpression of KLF15 decreased cellular branched-chain amino acid concentrations and introduction of small-interfering RNA against KLF15 cancelled these CVZ actions in cardiomyocytes. Second, glucocorticoid-GR signaling promoted gene expression of the enzymes involved in the prostaglandin biosynthesis, including cyclooxygenase-2 and phospholipase A2 in cardiomyocytes. Together, we may conclude that GR signaling should have distinct roles for maintenance of cardiac function, for example, in amino acid catabolism and prostaglandin biosynthesis in the heart.

Hypertension and left ventricular hypertrophy: is drug therapy beneficial?

Left ventricular hypertrophy (LVH) is a consequence of long-standing hypertension and is considered to be an independent risk factor for cardiovascular morbidity and mortality. Several antihypertensive agents are capable of inducing regression of LVH, but it is not known which class of drugs is most effective. The impact of drug-induced reversal of hypertrophy on ventricular function remains a controversial issue. Furthermore, the long-term clinical benefits of LVH regression have yet to be documented. Controversies also exist regarding the clinical outcomes associated with drug-induced LVH regression.

Divergent regulation of the human atrial natriuretic peptide gene by c-jun and c-fos

Employing transient transfection analysis in neonatal rat cardiocytes, we have demonstrated that overexpression of c-jun results in a dose-dependent induction of the human atrial natriuretic peptide (hANP) gene promoter. Studies using a series of mutations in the hANP gene promoter identified a TRE-like, cis-acting regulatory sequence which conferred c-jun sensitivity. This same region was shown to interact with the c-jun/c-fos complex in an in vitro gel mobility shift assay. Selective mutation of this site suppressed basal activity of the hANP promoter and significantly reduced c-jun-dependent activation. Overexpression of c-fos had a biphasic effect on hANP gene promoter activity. At low levels, in concert with c-jun, it activated, while at higher levels it suppressed, transcription from the hANP gene promoter. This inhibition was both cell and promoter specific. hANP gene promoter sequences which mediate c-fos-dependent inhibition appear to be separable from those responsible for the induction. In addition, the protein domains on c-fos responsible for transcriptional activation and repression can be segregated topographically, with the inhibitory activity being localized to the carboxy-terminal domain. Thus, c-fos can activate or repress hANP gene expression through two separate functional domains that act on distinct regulatory elements in the hANP gene promoter. These data imply that the ANP gene may be a physiological target for c-fos- and c-jun-dependent activity in the heart and suggest a potential mechanism linking environmental stimuli to its expression.

Divergent regulation of the human atrial natriuretic peptide gene by c-jun and c-fos

Employing transient transfection analysis in neonatal rat cardiocytes, we have demonstrated that overexpression of c-jun results in a dose-dependent induction of the human atrial natriuretic peptide (hANP) gene promoter. Studies using a series of mutations in the hANP gene promoter identified a TRE-like, cis-acting regulatory sequence which conferred c-jun sensitivity. This same region was shown to interact with the c-jun/c-fos complex in an in vitro gel mobility shift assay. Selective mutation of this site suppressed basal activity of the hANP promoter and significantly reduced c-jun-dependent activation. Overexpression of c-fos had a biphasic effect on hANP gene promoter activity. At low levels, in concert with c-jun, it activated, while at higher levels it suppressed, transcription from the hANP gene promoter. This inhibition was both cell and promoter specific. hANP gene promoter sequences which mediate c-fos-dependent inhibition appear to be separable from those responsible for the induction. In addition, the protein domains on c-fos responsible for transcriptional activation and repression can be segregated topographically, with the inhibitory activity being localized to the carboxy-terminal domain. Thus, c-fos can activate or repress hANP gene expression through two separate functional domains that act on distinct regulatory elements in the hANP gene promoter. These data imply that the ANP gene may be a physiological target for c-fos- and c-jun-dependent activity in the heart and suggest a potential mechanism linking environmental stimuli to its expression.