Adaptational response in transcription factors during development of myocardial hypertrophy.

Cardiac hypertrophy is characterized, among others, by the molecular events which selectively activate the expression of genes for contractile proteins within individual myocardial cells. As such, myosin light chain 2 (MLC-2), which is upregulated in the hypertrophic state in both rat and human, serves as a marker for hypertrophy. In an attempt to investigate the gene regulatory mechanisms of this phenomenon, we tested the hypothesis that certain transcription factors are directly involved in the development of cardiac hypertrophy by demonstrating the presence of cardiac tissue-specific regulatory elements in the 5'-flanking region of the MLC-2 promoter and testing them in the gel mobility shift assay for their binding activity to nuclear proteins from hypertrophied and normal cardiac tissue. In nuclear extracts from the ventricular tissues of the spontaneously hypertensive rat (SHR), distinctive changes in two families of activator proteins, the A/T-rich DNA-binding transcription factors, myocyte enhancer factor (MEF-2) and CArG-binding factor, manifested in a developmentally dictated manner paralleling the evolution of cardiac hypertrophy in these animals. Extracts isolated from brains and skeletal muscle tissues from the same animals did not exhibit the changes in binding activity. Also, the changes were not apparent when a distal negative regulatory element (CSS), which confers cardiac-specific expression, was tested in gel mobility shift assays. The ubiquitous TATA-binding proteins remained unchanged in comparing SHR with the control strain WKY in the same assay. These data support the notion that the expression of specific transcription factors is modulated in response to hypertrophy related signals which execute changes at the gene level effecting the enrichment of certain contractile proteins in an effort discrete and estranged from the basal transcription machinery.

Regulation of proteolysis in erythroid cells.

Reticulocytes contain non-lysosomal ATP dependent proteolytic activity which appears important in degrading abnormal proteins as well as certain organelles. ATP acts to repress an endogenous protease inhibitor activity by a process requiring the polypeptide ubiquitin. Free epsilon amino groups on substrates are important for recognition by the full ATP-dependent system but do not appear necessary for hydrolysis per se suggesting that ubiquitin-substrate conjugates repress the inhibitor. ATP-dependent proteolysis declines markedly with reticulocyte maturation and decreases further to negligible levels with aging of erythrocytes. With cell maturation the inhibitor and protease(s) remain but the ubiquitin-containing fraction is less effective in repressing the inhibitor. Finally, additional control mechanisms selective for certain proteins may exist.