Transcriptional regulation of alpha IIb integrin gene expression during megakaryocytic differentiation of K562 cells. Role of a silencer element.

A portion of the 5'-flanking region of the glycoprotein IIb (alpha IIb) integrin gene extending from -598 to +32 base pairs was isolated. This DNA segment is capable of driving low level base-line transcription in undifferentiated K562 cells. It also contains elements which direct the markedly increased expression observed following megakaryocytic differentiation of K562 cells with phorbol dibutyrate. Analysis of hybrid alpha IIb-chloramphenicol acetyltransferase reporter gene constructs indicates that at least three regions within the -598 to +32 region control differentiation-dependent alpha IIb transcription. Two enhancer elements as well as a silencer domain all regulate chloramphenicol acetyltransferase transcriptional activity in K562 cells. Gel mobility shift experiments revealed that nuclear binding proteins are able to interact with all three DNA regions. A small region lying between -124 and -99 bases is able to bind to nuclear proteins in undifferentiated cells but not in differentiated cells as evidenced by gel mobility shift and foot-printing studies and corresponds to the silencer element identified in the functional studies. Therefore, the tissue-specific expression of alpha IIb may be controlled transcriptionally by both positive and negative factors with the silencer element playing a major role in regulating differentiation-dependent expression.