Identification of novel, cardiac-restricted transcription factors binding to a CACC-box within the human cardiac troponin I promoter.
OBJECTIVE
The expression of the human cardiac troponin I (hTnIc) gene is developmentally regulated and tissue-specific. In analysing the putative binding elements within the proximal promoter, a CACC-box sequence overlapping a consensus Sp1 element has been identified. The aim of this study was to characterise the factors binding to this element and to determine their importance in the transcriptional activity of the promoter.


METHODS
A combination of supershift and competition electrophoretic mobility shift assays (EMSA) were used to identify the binding of factors to the overlapping CACC-box/Sp1 consensus element. The functional importance of this element was tested by transient transfection into primary neonatal rat cardiac myocytes in culture.


RESULTS
At least four factors were able to interact with this region including the zinc finger proteins Sp1, Sp3 and two potentially novel factors. Whereas both Sp1 and Sp3 bound to the consensus Sp1 element, and to a lesser extent the CACC-box, two of the complexes required the intact CACC-box for binding. Site-directed mutagenesis of this region showed that the CACC-box is essential for hTnIc promoter-reporter activity. Further characterisation using EMSA indicated that the factors binding the hTnIc CACC-box are unlikely to be zinc finger proteins as they are insensitive to the addition of divalent cation chelating agents. They were also unable to bind to other known CACC-box elements. These factors are present in both human and rat cardiac muscle but absent from a number of cell lines including several derived from skeletal muscle.


CONCLUSIONS
The human cardiac troponin I gene promoter requires an upstream CACC-box element for full activity. This element binds at least two complexes which represent novel, tissue-restricted DNA-binding activity present in the heart which we have named HCB1 and HCB2 for heart CACC-box binding factors.
Dellow
K A
KA
Cardiothoracic Surgery, National Heart and Lung Institute, Imperial College of Science Technology and Medicine, Dovehouse Street, London SW3 6LY, UK. k.dellow@ic.ac.uk
Bhavsar
P K
PK
Brand
N J
NJ
Barton
P J
PJ
D000818:Animals; D018929:Cell Culture Techniques; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D018507:Gene Expression Regulation, Developmental; D006801:Humans; D010957:Plasmids; D011401:Promoter Regions, Genetic; D051381:Rats; D017207:Rats, Sprague-Dawley; D016329:Sp1 Transcription Factor / Q000235:genetics / Q000378:metabolism; D051705:Sp3 Transcription Factor; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014162:Transfection; D019210:Troponin I / Q000235:genetics* / Q000378:metabolism; D016335:Zinc Fingers / Q000235:genetics
D004268:DNA-Binding Proteins; C497121:SP3 protein, human; D016329:Sp1 Transcription Factor; C497123:Sp3 protein, rat; D014157:Transcription Factors; D019210:Troponin I; C085482:Zbtb10 protein, rat; D051705:Sp3 Transcription Factor