Background— Acetyltransferase p300 is essential for cardiac development and is thought to be involved in cardiac myocyte growth through MEF2- and GATA4-dependent transcription. However, the importance of p300 in the modulation of cardiac growth in vivo is unknown. Methods and Results— Pressure overload induced by transverse aortic coarctation, postnatal physiological growth, and human heart failure were associated with large increases in p300. Minimal transgenic overexpression of p300 (1.5- to 3.5-fold) induced striking myocyte and cardiac hypertrophy. Both mortality and cardiac mass were directly related to p300 protein dosage. Heterozygous loss of a single p300 allele reduced pressure overload–induced hypertrophy by ≈50% and rescued the hypertrophic phenotype of p300 overexpressers. Increased p300 expression had no effect on total histone deacetylase activity but was associated with proportional increases in p300 acetyltransferase activity and acetylation of the p300 substrates histone 3 and GATA-4. Remarkably, a doubling of p300 levels was associated with the de novo acetylation of MEF2. Consistent with this, genes specifically upregulated in p300 transgenic hearts were highly enriched for MEF2 binding sites. Conclusions— Small increments in p300 are necessary and sufficient to drive myocardial hypertrophy, possibly through acetylation of MEF2 and upstream of signals promoting phosphorylation or nuclear export of histone deacetylases. We propose that induction of myocardial p300 content is a primary rate-limiting event in the response to hemodynamic loading in vivo and that p300 availability drives and constrains adaptive myocardial growth. Specific reduction of p300 content or activity may diminish stress-induced hypertrophy and forestall the development of heart failure.

Morphological changes in the myocardium after left ventricular hypertrophy, due to chronic experimental hypertension, require an understanding of the quantitative relationship between myocyte and nonmyocyte compartments forming the structural framework of the myocardium. Hypertension was induced by long-term low-dosage inhibition of nitric oxide synthesis by N^G-nitro-L-arginine methyl ester (L-NAME) in rats. L-NAME (12 mg/kg) was given to animals in water ad libitum during 15 weeks. After this period, systolic blood pressure increased almost 50% as compared with that in the control group. Morphological changes in control and L-NAME animals were investigated with stereology and immunohistochemistry. Comparing control and L-NAME animals, the surface density of myocytes decreased 73.7% while the mean cross-sectional area increased 97.6% in L-NAME rats. The volume density of myocytes decreased 45.9% and the volume density of the interstitium increased 71.7% in L-NAME rats. No stereological difference was found in blood vessels comparing the two groups. Remodeling of the cardiac interstitium occurred with increased deposition of both fibronectin and type III collagen. Fibronectin was seen in both early and latter responses to infarction while type III collagen was seen mainly in areas of incomplete healing among myocytes and around intramyocardial branches of the coronary arteries. The long-term low-dosage administration of an inhibitor of the NO synthase such as L-NAME causes myocyte hypertrophy and early interstitial and perivascular fibrosis without important quantitative changes in microcirculation.

This work analyzes the relationship between the number of viable cells and alteration of the cardiomyocytes growth response capacity of the hypertensive rat myocardium. Hypertension was induced in Wistar rats by means of nitric oxide synthesis blockade using NG-nitro-L-arginine methyl ester (L-NAME). L-NAME (12 mg/kg per day) was given to animals in drinking water ad lib for 15 weeks. Proliferating cell nuclear antigen (PCNA) protein expression and the disector method were used to evaluate the proliferation capacity of the cardiomyocytes and its numerical density alteration (Nv[m]), respectively. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) and monoclonal antibody to single-stranded DNA were two methods that detected the process of the apoptotic cell death. The association of the p53 expression with the apoptosis was investigated using anti-p53 antibody. The heart weight, body weight, and heart weight/body weight ratio of the control rats increased 114%, 77%, and 22%, respectively, and the Nv[m] decreased 60% (P<0.0001) relative to the L-NAME rats. The cardiomyocytes did not present PCNA labeling, indicating the absence of cellular proliferation. The decline of the Nv[m] was also associated with apoptotic cell death in the myocardium of the hypertensive rats. A p53-dependent pathway seems to mediate the programmed cell death in this model of hypertension.

ObjectiveTo characterize downstream effectors of p300 acetyltransferase in the myocardium.BackgroundAcetyltransferase p300 is a central driver of the hypertrophic response to increased workload, but its biological targets and downstream effectors are incompletely known.Methods and ResultsMice expressing a myocyte-restricted transgene encoding acetyltransferase p300, previously shown to develop spontaneous hypertrophy, were observed to undergo robust compensatory blood vessel growth together with increased angiogenic gene expression. Chromatin immunoprecipitation demonstrated binding of p300 to the enhancers of the angiogenic regulators Angpt1 and Egln3. Interestingly, p300 overexpression in vivo was also associated with relative upregulation of several members of the anti-angiogenic miR-17∼92 cluster in vivo. Confirming this finding, both miR-17-3p and miR-20a were upregulated in neonatal rat ventricular myocytes following adenoviral transduction of p300. Relative expression of most members of the 17∼92 cluster was similar in all 4 cardiac chambers and in other organs, however, significant downregulation of miR-17-3p and miR-20a occurred between 1 and 8 months of age in both wt and tg mice. The decline in expression of these microRNAs was associated with increased expression of VEGFA, a validated miR-20a target. In addition, miR-20a was demonstrated to directly repress p300 expression through a consensus binding site in the p300 3′UTR. In vivo transduction of p300 resulted in repression both of p300 and of p300-induced angiogenic transcripts.Conclusionp300 drives an angiogenic transcription program during hypertrophy that is fine-tuned in part through direct repression of p300 by miR-20a.

We studied with quantitative methods the myocardium of 32 specimens of rats divided into three age groups: embryos, fetuses and neonates. Days of gestation were counted from the day following an overnight mating that was considered day 1 of gestation and only one animal per litter was used. The hearts were fixed in Bouin’s fixative, sectioned and stained by routine methods. Stereological determinations were made on ventricular myocardium: (1) volume density of the myocytes [Vv(myocyte)] and interstitium [Vv(interstitium)], and (2) numerical density of the myocytes [Nv(myocyte) 1/mm3] calculated from fifteen optical dissector pairs per specimen. The total number of cardiac myocytes [N(myo-cyte)] was calculated as the product of Nv(myocyte) and the cardiac volume. The Nv(myocyte) increased from embryos to neonates, differences between embryos and fetuses and between embryos and neonates were statistically significant. The Vv(myocyte) increased from embryos to neonates (from 80.0 to 94.0%). During this period the Vv(interstitium) decreased from 21.0 to 5.5%. Differences of the Vv(myocyte) and Vv(interstitium) were significant comparing embryos with neonates and comparing fetuses with neonates. The N(myo-cyte) is roughly (mean ± standard error of the mean) 9,297 ± 487 in fetuses and 38,438 ± 612 in neonates. This represents an increase of about 3.1 times from fetuses to neonates while the cardiac weight increased about 2.2 times in the same period. Coefficients of error for the Nv(myocyte) estimates averaged about 7.3%, for the Vv(myocyte) estimates averaged about 1.8%, and for the Vv(inter-stitium) estimates averaged about 9.1%. These results suggest a high mitotic activity in the rat myocardium during prenatal life and after birth.

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Objective -To study the healing process of the myocardium in hypertensive rats undergoing inhibition of nitric oxide synthesis. Methods -