Protein and 28S ribosomal RNA fractional turnover rates in the rat heart after abdominal aortic stenosis

Myocytes positive for in situ markers for DNA breaks in human hearts which are hypertrophic, but neither failed nor dilated: a manifestation of cardiac hypertrophy rather than failure

The significance of DNA breaks reported in failing hearts is controversial, although they may suggest myocyte apoptosis and may thus be responsible for the progression of heart failure. This study attempted to check the validity of the in situ markers for DNA breaks for detecting myocyte death and to evaluate separately two factors, failure or hypertrophy, crucial for DNA breaks in pathological human hearts. In the autopsy study, myocytes showed positivity for in situ nick end-labelling (TUNEL) and of Taq and Pfu polymerase-based in situ ligation assays not only in dilated cardiomyopathy (DCM, n = 9) with failure, but also in hypertrophic cardiomyopathy (HCM, n = 8) and hypertensive heart disease (HHD, n = 4) without failure. There was a significant correlation between each in situ marker and heart weight. The incidence of TUNEL-positive myocytes always exceeded that seen in in situ ligation assays. In addition, there were significant correlations between the in situ markers and the expression of the proliferating cell nuclear antigen (PCNA) and of the spliceosome component of 35 kD (SC-35). Similarly, in the left ventricular biopsy study using 23 DCM, 21 HCM, 11 HHD, and 13 non-hypertrophic hearts, the incidence of the in situ markers showed significant correlations with the left ventricular mass index and myocyte size, but not with cardiac function and dilatation. Positivity of myocytes for in situ markers for DNA breaks, such as TUNEL and in situ ligation assays, may be an epiphenomenon accompanying cardiac hypertrophy, but not myocyte death in pathological human hearts.

Molecular mechanisms of myocardial remodeling

"Remodeling" implies changes that result in rearrangement of normally existing structures. This review focuses only on permanent modifications in relation to clinical dysfunction in cardiac remodeling (CR) secondary to myocardial infarction (MI) and/or arterial hypertension and includes a special section on the senescent heart, since CR is mainly a disease of the elderly. From a biological point of view, CR is determined by 1 ) the general process of adaptation which allows both the myocyte and the collagen network to adapt to new working conditions; 2) ventricular fibrosis, i.e., increased collagen concentration, which is multifactorial and caused by senescence, ischemia, various hormones, and/or inflammatory processes; 3) cell death, a parameter linked to fibrosis, which is usually due to necrosis and apoptosis and occurs in nearly all models of CR. The process of adaptation is associated with various changes in genetic expression, including a general activation that causes hypertrophy, isogenic shifts which result in the appearance of a slow isomyosin, and a new Na+-K+-ATPase with a low affinity for sodium, reactivation of genes encoding for atrial natriuretic factor and the renin-angiotensin system, and a diminished concentration of sarcoplasmic reticulum Ca2+-ATPase, beta-adrenergic receptors, and the potassium channel responsible for transient outward current. From a clinical point of view, fibrosis is for the moment a major marker for cardiac failure and a crucial determinant of myocardial heterogeneity, increasing diastolic stiffness, and the propensity for reentry arrhythmias. In addition, systolic dysfunction is facilitated by slowing of the calcium transient and the downregulation of the entire adrenergic system. Modifications of intracellular calcium movements are the main determinants of the triggered activity and automaticity that cause arrhythmias and alterations in relaxation.

Alteration of growth responses in established cardiac pressure overload hypertrophy in rats with aortic banding

We examined the acute effects of elevated wall stress, norepinephrine, and angiotensin II on cardiac protein synthesis as well as protooncogene expression in hearts with established pressure overload left ventricular hypertrophy. Isolated rat hearts with chronic hypertrophy (LVH) were studied 12 wk after ascending aortic banding when systolic function was fully maintained. New protein synthesis (incorporation of [3H]phenylalanine [Phe]) was analyzed in isolated perfused rat hearts after a 3-h protocol; c-fos, c-jun, c-myc, and early growth response gene-1 (EGR-1) mRNA levels (Northern blot) were studied over a time course from 15 to 240 min of perfusion. Under baseline conditions (i.e., before mechanical or neurohormonal stimulation), [3H]-Phe-incorporation (280 nmoles/gram protein/h) and protooncogene mRNA levels were similar in age-matched control and LVH hearts. However, hearts with chronic LVH were characterized by a markedly blunted or absent [3H]-Phe-incorporation after acute imposition of isovolumic systolic load (90 mmHg/gram left ventricle), as well as norepinephrine (10(-6)M), or angiotensin II infusion (10(-8)M plus prazosin 10(-7)M) compared with nonhypertrophied control hearts. Similarly, stimulation of LVH hearts with acute systolic load or norepinephrine was associated with a significantly blunted increase of protooncogene mRNA levels relative to control hearts. The blunted induction of c-fos mRNA in LVH hearts was not due to feedback inhibition, since cycloheximide perfusion of hearts exposed to elevated wall stress further increased the differences between age-matched control and LVH hearts. The data suggest that acute molecular growth responses to mechanical or neurohormonal stimulation are altered in rat hearts with established LVH relative to nonhypertrophied control hearts. This alteration of molecular adaptations in hearts with compensatory hypertrophy may prevent inappropriate excess cardiac growth in response to mechanical and neurohormonal stimuli.

Antithetical accumulation of myosin heavy chain but not alpha-actin mRNA isoforms during early stages of pressure-overload-induced rat cardiac hypertrophy

Myocardial response to a hemodynamic overload involves changes in the expression of isogenes encoding myosin heavy chain (MHC) and actin: beta-MHC/alpha-MHC and skeletal/cardiac alpha-actin mRNA isoform ratios are increased. It is not known whether these changes are due to increased accumulations of the two neosynthesized transcripts, beta-MHC and skeletal alpha-actin, or whether the mRNA isoforms normally present, alpha-MHC and cardiac alpha-actin, are concomitantly decreased. To answer these questions, using dot-blot hybridizations, primer extension, and exonuclease VII mapping assays, we have analyzed the content of sarcomeric MHC and actin mRNAs in the poly(A+) RNA in left ventricles of 23-24-day-old rats 18 and 24 hours after a pressure overload induced by stenosis of the thoracic aorta. The results showed a 1.9-fold increase in poly(A+) RNA after the stenosis. Skeletal/cardiac alpha-actin mRNA isoforms were already increased fivefold (from 0.19 to 0.99) at 18 hours, and this was exclusively due to a 5.5-fold increase in skeletal alpha-actin mRNA. At 24 hours, this ratio was increased ninefold (from 0.14 to 1.22), and this was due to a 4.3-fold increase in the level of skeletal alpha-actin mRNAs (p < 0.001) and a 1.9-fold decrease of cardiac alpha-actin mRNA (p < 0.001), restoring the same proportion of sarcomeric actin mRNA in sham-operated and operated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization and regulation of c-fos and c-jun protooncogene induction by systolic wall stress in normal and hypertrophied rat hearts

The effect of changes in left ventricular (LV) systolic force generation on cardiac c-fos and c-jun protooncogene expression was studied by using isolated beating hearts from male Wistar rats. An isovolumic buffer-perfused heart preparation was utilized in which coronary flow and heart rate were held constant and increments in LV balloon volume were used to generate defined levels of LV systolic wall stress. Using Northern and slot-blot analyses, we found that LV tissue from control hearts that generated high levels of LV systolic wall stress expressed 3- to 4.4-fold higher c-fos and c-jun mRNA levels in comparison with tissue from the respective flaccid right ventricles, and in comparison with LV tissue from hearts that generated minimal LV systolic wall stress. To distinguish the role of passive LV diastolic wall stretch from active LV force generation, we found that distension of the LV balloon per se did not have a significant effect on protooncogene induction in hearts perfused with 2,3-butanedione monoxime, which prevents systolic cross-bridge cycling and force generation. In additional hearts studied at a constant LV balloon volume to generate an LV end-diastolic pressure of 10 mm Hg, c-fos mRNA levels were proportional to the magnitude of peak LV systolic wall stress (r = 0.823, P less than 0.05). In these protocols, Fos protein was localized by immunohistochemistry in myocyte nuclei with minimal staining in fibroblasts and vascular smooth muscle. When c-fos and c-jun mRNA expression was compared in hearts with chronic LV hypertrophy due to ascending aortic banding and age-matched control hearts that generated similar incremental levels of LV systolic wall stress, significantly lower levels of c-fos and c-jun mRNA were measured in the hypertrophied hearts. However, there was no difference in protooncogene mRNA expression in response to stimulation by the Ca2+ ionophore A23187. These data suggest that, in this isolated isovolumic beating heart preparation, the active generation of an acute increment in LV systolic force independent of passive diastolic myocardial stretch causes a rapid induction of both c-fos and c-jun, which is down-regulated in the presence of established LV hypertrophy.

Mechanogenetic regulation of transcription

In many biological systems mechanical forces regulate gene expression: in bacteria changes in turgor pressure cause a deformation of the membrane and induce the expression of osmoregulatory genes; in plants gravity regulates cell growth ('geotropism'); in mammals stretching a muscle induces hypertrophy which is accompanied by qualitative changes in protein synthesis. Consequently, the term 'mechanogenetic control' seems to be a suitable common name for all these processes. The mechanism by which mechanical factors modulate transcriptional activity is still unknown. The purpose of this review is to bring together data from different fields in order to obtain a better understanding of the mechanogenetic control of cell growth.

Function of the sarcoplasmic reticulum and expression of its Ca2(+)-ATPase gene in pressure overload-induced cardiac hypertrophy in the rat

The reduction in Ca2+ concentration during diastole and relaxation occurs differently in normal hearts and in hypertrophied hearts secondary to pressure overload. We have studied some possible molecular mechanisms underlying these differences by examining the function of the sarcoplasmic reticulum and the expression of the gene encoding its Ca2(+)-ATPase in rat hearts with mild and severe compensatory hypertrophy induced by abdominal aortic constriction. Twelve sham-operated rats and 31 operated rats were studied 1 month after surgery. Eighteen animals exhibited mild hypertrophy (left ventricular wt/body wt less than 2.6) and 13 animals severe hypertrophy (left ventricular wt/body wt greater than 2.6). During hypertrophy we observed a decline in the function of the sarcoplasmic reticulum as assessed by the oxalate-stimulated Ca2+ uptake of homogenates of the left ventricle. Values decreased from 12.1 +/- 1.2 nmol Ca2+/mg protein/min in sham-operated rats to 9.1 +/- 1.5 and 6.7 +/- 1.1 in rats with mild and severe hypertrophy, respectively (p less than 0.001 and p less than 0.001, respectively, vs. shams). This decrease was accompanied by a parallel reduction in the number of functionally active CA2(+)-ATPase molecules, as determined by the level of Ca2(+)-dependent phosphorylated intermediate: 58.8 +/- 7.4 and 48.1 +/- 13.5 pmol P/mg protein in mild and severe hypertrophy, respectively, compared with 69.7 +/- 8.2 in shams (p less than 0.05 and p less than 0.01, respectively, vs. shams). Using S1 nuclease mapping, we observed that the Ca2(+)-ATPase messenger RNA (mRNA) from sham-operated and hypertrophied hearts was identical. Finally, the relative level of expression of the Ca2(+)-ATPase gene was studied by dot blot analysis at both the mRNA and protein levels using complementary DNA clones and a monoclonal antibody specific to the sarcoplasmic reticulum Ca2(+)-ATPase. In mild hypertrophy, the concentrations of Ca2(+)-ATPase mRNA and protein in the left ventricle were unchanged when compared with shams (mRNA, 93.8 +/- 10.6% vs. sham, NS; protein, 105.5 +/- 14% vs. sham, NS). in severe hypertrophy, the concentration of Ca2(+)-ATPase mRNA decreased to 68.7 +/- 12.9% and that of protein to 80.1 +/- 15.5% (p less than 0.001 and p less than 0.05, respectively), whereas the total amount of mRNA and enzyme per left ventricle was either unchanged or slightly increased. The slow velocity of relaxation of severely hypertrophied heart can be at least partially explained by the absence of an increase in the expression of the Ca2(+)-ATPase gene and by the relative diminution in the density of the Ca2+ pumps.(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesis of stress proteins in rat cardiac myocytes 2-4 days after imposition of hemodynamic overload

Isolated adult myocytes incubated with [35S]methionine were used to study the expression of proteins in the rat heart during the first 2 wk after either pressure or volume overload. In both models an early (2-4 d) and transient expression of three major stress proteins (heat shock protein [HSP] HSP 70, HSP 68, and HSP 58) was observed together with an increased synthesis of putative ribosomal proteins. Only traces of 35S-labeled HSPs were detected in controls and sham-operated animals. The three stress proteins were identified by their migration in two-dimensional gels, by comigration with HSPs, which had been induced in myocytes by incubation at 41 degrees C and immunoblot analysis using antisera directed against the 70-kD protein. Immunohistochemical staining of HSP 70 in rod-shaped myocytes and detection by immunoblot showed that HSP 70 was equally present and distributed in both sham-operated and overloaded hearts, and provided no evidence for a subpopulation of myocytes acutely involved in the increased expression of HSP 70. It is suggested that the transient expression of HSPs that occurs during the early adaptation of the myocardial cells to overload could confer some degree of protection to the actively growing myocytes.