Transcription factor BETA2 acts cooperatively with E2A and PDX1 to activate the insulin gene promoter

The insulin gene is efficiently expressed only in pancreatic beta cells. Us ing reverse transcriptase-polymerase chain reaction analysis, we show that insulin mRNA levels are at least 10(5)-fold higher in beta cells than non-b eta cells. To examine the underlying mechanisms, we expressed beta cell tra nscription factors by transfection of non-beta cells. Separate expression o f BETA2, E2A or PDX1 led to modest (<10-fold) activation of the insulin pro moter, whereas co-expression of the three proteins produced synergistic, hi gh level activation (160-fold), This level of activity is similar to 25% th at observed in transfected beta cell lines. Of the three factors studied, B ETA2 appears to play a dominant role. Efficient transcription required a C- terminal activation domain of BETA2 and an N-terminal region, which does no t function as an independent activation domain, The myogenic basic helix-lo op-helix (bHLH) protein MyoD was unable to bind and activate the promoter, even when its DNA binding region was replaced with that of BETA2. Our resul ts demonstrate the central importance of BETA2 in insulin gene transcriptio n and the importance of sequences outside the canonical DNA binding domain in permitting efficient DNA binding and cell-specific activity of the insul in gene promoter.