Ctla-4 transcriptional activation: regulation of induced expression

The costimulatory molecule CTLA-4 functions as an immunomodulator generally associated with suppression of T cell proliferation. Though structurally similar to CD28, which is expressed constitutively on T cells, transcription of CTLA-4 is highly regulated. CTLA-4 is elevated in cutaneous T cell lymphoma (CTCL), which may contribute to suppression of anti-tumor response during disease progression. The transcriptional regulator GATA3 is also over-expressed in CTCL, but its significance in CTLA-4 is unclear. We find that both transcript and protein levels of GATA3 are augmented by proteasome inhibition. We show by polyubiquitin immunoblot that the proteasome pathway is dysregulated in CTCL. Here we demonstrate a role for GATA3 in transcriptional regulation of CTLA-4 using the proteasome inhibitor bortezomib, a compound which reversibly binds to and inactivates the catalytic core of the proteasome through its boron atom. Bortezomib treatment leads to a dose-dependent increase in both GATA3 and CTLA-4 expression in normal CD4 T cells at both the transcript and protein level. We also detect an increase in phospho-GATA3, the activated form, with bortezomib. Flow cytometric analysis confirms the elevated CTLA-4 is properly trafficked to the cell surface. Overexpression of GATA3 into Jurkat T cells by transfection stimulates a