Regulation of troponin C synthesis in primary culture of chicken cardiac muscle cells

Slow troponin C gene expression in chicken heart and liver is regulated by similar enhancers

Two isoforms of troponin C (TnC) are encoded by distinct single copy genes. Expression of fast TnC is restricted to the skeletal muscle, whereas the slow isoform is expressed in both skeletal and cardiac muscle. Chicken slow TnC (cTnC) gene is also expressed in some non-muscle tissues like the liver and the brain. Expression of cTnC gene is regulated by two distinct enhancers in cardiac and skeletal muscles. The cardiac specific enhancer is located in the immediate 5' flanking region (bp-124 to -79) of the murine cTnC gene whereas the skeletal enhancer is located within the first intron (bp 997 to 1141). In the present study we have examined how cTnC gene expression is regulated in the chicken liver. Transient transfection of liver cells with CTnC-CAT reporter constructs containing various regions of the murine cTnC gene showed that its expression in chicken liver is regulated by the cardiac specific enhancer. Furthermore, electrophoretic mobility shift assays using synthetic oligonucleotides corresponding to both CEF-1 and CEF-2 regions of the murine cardiac enhancer revealed formation of specific DNA-protein complexes. Ultraviolet light induced covalent linking of nuclear proteins to CEF-1 and CEF-2 oligomers were used to examine the nature of the cardiac enhancer binding polypeptides; one polypeptide of 48 kDa appeared to bind to both CEF-1 and CEF-2 sequences.

Regulation of translation and stability of an mRNA coding for a 40-kDa polypeptide in rat L6 muscle cells

The mRNA coding for a 40-kDa polypeptide (P-40) was previously cloned and sub-cellular distribution of this mRNA was examined in rat L6 myoblast cells under different conditions [Pramanik, S. & Bag, J. (1987) Eur. J. Biochem. 170, 59-67]. The translation of this mRNA was found to be regulated during differentiation of myoblasts. This mRNA was translated in proliferating myoblasts but not in the non-dividing differentiated myotubes. We have further examined whether the mRNA present in the polysomal fraction of myoblasts and that in the free non-polysomal fraction in myotubes was identical by nuclease S1 mapping. The coding strand of the 600-base-pair PstI fragment of the recombinant clone was 3'-end-labeled with cordycepin 5'-[alpha-32P]triphosphate and hybridized with RNA from either myoblasts or myotubes. The results of these studies have shown that RNA from both preparations was fully able to hybridize with the probe DNA and, therefore, protected the 600-nucleotide-long fragment from nuclease S1 digestion, thus suggesting that the sequence of 600 nucleotides at 3' ends of both translationally active polysomal mRNA of myoblasts and repressed free mRNA of myotubes are identical. These results also confirmed the results of our earlier studies on the subcellular distribution of this mRNA by Northern blot analysis. Further studies were also performed to determine whether withdrawal of muscle cells from the cell cycle during differentiation to form myotubes alone was responsible for regulating translation of P-40 mRNA. The results of the subcellular distribution of this mRNA in proliferating myoblasts following inhibition of DNA synthesis by cytosine arabinoside have shown that translation of P-40 mRNA continued in absence of DNA synthesis. This observation suggests that an additional signal is necessary to block the translation of P-40 mRNA in myotubes. The relationship between the translation of P-40 mRNA and its stability was examined. Two different methods were used to determine the stability of mRNAs. The first approach was by determining the steady-state levels of this mRNA following inhibition of RNA synthesis by actinomycin D. In the second method, we have determined the amount of 3H-labeled P-40 mRNA during pulse and chase experiments. Both methods produced similar results. It was found that the stability of P-40 mRNA was not altered during differentiation of rat L6 cells. The results of pulse and chase studies have also shown that P-40 mRNA was synthesized in both myoblasts and myotubes.(ABSTRACT TRUNCATED AT 400 WORDS)