Differential binding of an SRF/NK-2/MEF2 transcription factor complex in normal versus neoplastic smooth muscle tissues

The malignant potential of smooth muscle tumors correlates strongly with th e disappearance of gamma -smooth muscle isoactin, a lineage-specific marker of smooth muscle development. In this paper, we identify a 36-base pair re gulatory motif containing an AT-rich domain, CArG box, and a non-canonical NK-2 homeodomain-binding site that has the capacity to regulate smooth musc le-specific gene expression in cultured intestinal smooth muscle cells. Ser um-response factor associates with an NK-2 transcription factor via protein -protein interactions and binds to the core CArG box element. Our studies s uggest that the NK-2 transcription factor that associates with serum-respon se factor during smooth muscle differentiation is Nkx2-3. Myocyte-specific enhancer factor 2 binding to this regulatory complex was also observed but limited to uterine smooth muscle tissues. Smooth muscle neoplasms displayed altered transcription factor binding when compared with normal myometrium. Differential nuclear accessibility of serum-response factor protein during smooth muscle differentiation and neoplastic transformation was also obser ved. Thus, we have identified a unique regulatory complex whose differentia l binding properties and nuclear accessibility are associated with modulati ng gamma -smooth muscle isoactin-specific gene expression in both normal an d neoplastic tissues.