ABSTRACT
The expression of the α-myosin heavy chain (MHC) gene is restricted primarily to cardiac myocytes. To date, several positive regulatory elements and their binding factors involved in α-MHC gene regulation have been identified; however, the mechanism restricting the expression of this gene to cardiac myocytes has yet to be elucidated. In this study, we have identified by using sequential deletion mutants of the rat cardiac α-MHC gene a 30-bp purine-rich negative regulatory (PNR) element located in the first intronic region that appeared to be essential for the tissue-specific expression of the α-MHC gene. Removal of this element alone elevated (20- to 30-fold) the expression of the α-MHC gene in cardiac myocyte cultures and in heart muscle directly injected with plasmid DNA. Surprisingly, this deletion also allowed a significant expression of the α-MHC gene in HeLa and other nonmuscle cells, where it is normally inactive. The PNR element required upstream sequences of the α-MHC gene for negative gene regulation. By DNase I footprint analysis of the PNR element, a palindrome of two high-affinity Ets-binding sites (CTTCCCTGGAAG) was identified. Furthermore, by analyses of site-specific base-pair mutation, mobility gel shift competition, and UV cross-linking, two different Ets-like proteins from cardiac and HeLa cell nuclear extracts were found to bind to the PNR motif. Moreover, the activity of the PNR-binding factor was found to be increased two- to threefold in adult rat hearts subjected to pressure overload hypertrophy, where the α-MHC gene is usually suppressed. These data demonstrate that the PNR element plays a dual role, both downregulating the expression of the α-MHC gene in cardiac myocytes and silencing the muscle gene activity in nonmuscle cells. Similar palindromic Ets-binding motifs are found conserved in the α-MHC genes from different species and in other cardiac myocyte-restricted genes. These results are the first to reveal a role of the Ets class of proteins in controlling the tissue-specific expression of a cardiac muscle gene.
LIM-only protein, CRP2, switched on smooth muscle gene activity in adult cardiac myocytes
