ANALYSIS OF THE ROLE OF E2A-ENCODED PROTEINS IN INSULIN GENE-TRANSCRIPTION

Pancreatic beta-cell type-specific transcription of the insulin gene i s mediated, in part, by factors in the basic helix-loop-helix (bHLH) f amily that act on a site within the insulin enhancer, termed the E1-bo x. Expression from this element is regulated by a heteromeric protein complex containing ubiquitous (i.e. the E2A- and HEB-encoded proteins) and islet-enriched members of the bHLH family. Recent studies indicat e that the E2A- and HEB-encoded proteins contain a transactivation dom ain, termed AD2, that functions more efficiently in transfected beta-c ell lines. In the present report, we extend this observation by demons trating that expression of full-length E2A proteins (E47, E12, and E2/ 5) activates insulin E element-directed transcription in a beta-cell l ine-selective manner. Stimulation required functional interactions wit h other key insulin gene transcription factors, including its islet bH LH partner as well as those that act on the RIPE3b1 and RIPE3a2 elemen ts of the insulin gene enhancer. The conserved AD2 domain in the E2A p roteins was essential in this process. The effect of the E2A- and HEB- encoded proteins on insulin gene expression was also analyzed in mice lacking a functional E2A or HEB gene. There was no apparent difference in insulin production between wild type, heterozygote, and homozygous mutant E2A or HEB mice. These results suggest that neither the E2A- o r HEB-encoded proteins are essential for insulin transcription and tha t one factor can substitute for the other to impart normal insulin E1 activator function in mutant animals.