Coordinate transactivation of the interleukin-2 CD28 response element by c-Rel and ATF-1/CREB2.

The interleukin-2 CD28 response element (CD28RE) acts as a composite enhancer, in conjunction with a 3'-12-O-tetradecanoylphorbol-13-acetate response element (TRE)-like element, to confer CD28 receptor-dependent inducibility to the interleukin-2 promoter in T-cells. When inserted as a single copy upstream of a basal promoter, this composite enhancer, termed the CD28RE-TRE, is both highly active and CD28-inducible in transactivation assays. A multicomponent nuclear protein complex that binds the CD28RE-TRE was isolated by DNA affinity chromatography from nuclear extracts of mitogen- and CD28 receptor-costimulated human T-cells. Immunological and biochemical analyses of this complex reveal the presence of c-Rel, ATF-1, and CREB2 as major DNA-binding components. Coexpression of c-Rel in combination with ATF-1, CREB2, or ATF-1/CREB2 leads to synergistic transactivation of a CD28RE-TRE reporter plasmid in quiescent Jurkat T-cells. Furthermore, CD28-dependent transactivation of the CD28RE-TRE is specifically inhibited by cAMP response element-binding protein (CREB) dominant-negative expression vectors. Moreover, mutant promoter constructs in which the internal 5'-CD28RE and 3'-TRE-like sequences have been topologically positioned 180 degrees out of phase with one another show loss of mitogen- and CD28-dependent inducibility. Finally, the addition of the CREB-binding transcriptional coactivator p300 leads to a dramatic CREB-dependent increase in both mitogen- and CD28-mediated transactivation of the CD28RE-TRE. These findings demonstrate that full physiological responsiveness to CD28 receptor stimulation in T-cells is dependent on topologically linked sequences within the CD28RE-TRE composite enhancer and provide strong support of a direct role for the CREB family of transcription factors and p300/CREB-binding protein coactivator proteins in cytokine gene induction during T-cell activation.