Expression of nuclear-acting early-response genes in the rat heart : implications for cardiac hypertrophy

Post-natal growth of the mammalian heart is characterized by an increase in size of pre-existing cardiomyocytes (hypertrophy) rather than an increase in their number (hyperplasia). The primary stimuli for this growth process are not clearly understood but seem to involve both hemodynamic and hormonal factors. A major challenge to researchers has been to clearly define the signals that activate and regulate cardiac hypertrophy and to elucidate the intracellular transducing mechanisms which couple the hypertrophic stimuli to the long term changes in cardiac phenotype and function. Of the most likely candidate molecular signals the nuclear-acting early-response genes are of particular interest since their protein products are thought to play key roles in linking extracellular signals with terminal patterns of gene expression during growth and differentiation. The work in this thesis has examined the ability of various hypertrophic stimuli to modulate the expression of nuclear acting early response genes in the rat heart both in vivo and in vitro. A single injection of norepinephrine (2.5 ug/kg to 2.5 mg/kg) transiently increased mRNA levels of the nuclear acting early-response genes c-myc, c-fos, c-jun, fra-1 and fra-2 in the rat heart. Similar responses were also observed following chronic infusion of norepinephrine (100 ug/kg/h) but not in response to treatment with the hypertrophic hormone triiodo-L-thyroxine. Hybridization histochemistry and immunocytochemistry techniques were used to localize early response genes to particular cell types and regions of the heart. Following norepinephrine administration (2.5 mg/kg) Fos protein transiently accumulated in the cardiac myocytes and to a much lesser extent other cell types. In direct contrast, little Myc immunostaining was observed in the cardiac myocytes with greatest expression being localized to the cardiac non-myocyte population, presumably fibroblasts and cells of the vasculature system. The observed responses for both genes was not uniform but appeared greatest in the left atrium and left ventricle with lesser expression elsewhere. In order to differentiate the complex systemic interactions of norepinephrine from its direct actions upon the heart an isolated perfused heart system was employed. Both elevated perfusion pressure (60-120 mmHg) and the inclusion of norepinephrine (1 nM to 1 uM) in the perfusion buffer (60 mmHg) led to elevated mRNA levels of c-myc , c-fos, fra-1 and fra-2. These findings demonstrate the utitlity of the isolated perfused heart system as a model to study separately the effects of pressure load and NE on gene expression during the early stages of cardiac hypertrophy. Taken together with the in vivo results they lend further support to the notion that the products of early-response genes structurally or functionally related to c-fos may mediate the hypertrophic actions of norepinephrine and pressure overload. In contrast, c-myc expression may be associated with the proliferation of cardiac non-myocyte cells which occurs concomitant with cardiac hypertrophy.