Molecular regulation of MICA expression after HDAC-inhibitor treatment of Jurkat T cells

The immune system is critically dependent on the ability to recognize or sense infected, stressed and transformed cells. One of these sensing systems rely on NKG2D/NKG2D-ligand interaction, where NKG2D is an activating receptor constitutively expressed by several effector cells of the immune system and NKG2D-ligands are up-regulated on the surface of abnormal cells.

We have previously shown that cancer cells can be stimulated to express the NKG2D-ligands MICA/B after exposure to HDAC-inhibitors (HDAC-i), an occurrence that is not observed in healthy cells.

Here we characterize the molecular signal pathways that lead to MICA expression after HDAC-inhibitor treatment of Jurkat T cells. Chelating Calcium with Bapta-AM or EGTA potently inhibited HDAC-inhibitor mediated MICA/B expression. It was further observed that ER Calcium stores were depleted after HDAC-inhibitor treatment. NF-kB activity can be induced by HDAC-inhibitor treatment. However, nuclear translocation of NF-kB p65 was not observed after HDAC-inhibitor treatment of Jurkat T cells, and even though we could effectively inhibit p65 expression by siRNA, it did not modify MICA/B expression. To identify important elements in MICA regulation, we made a promoter construct consisting of ~3kb of the proximal MICA promoter in front of GFP. Deletion analysis showed that a GC-box containing a putative Sp1 site from position -113 to -93 relative to the mRNA start site, was important for HDAC-inhibitor induced promoter activity. Sp1 was subsequently shown to be important, as targeted mutation of the Sp1 binding sequence or siRNA mediated down modulation of Sp1 inhibited MICA promoter activity and surface-expression.