Interaction between transcription factors Iroquois proteins 4 and 5 controls cardiac potassium channel Kv4.2 gene transcription

The homeobox transcription factor, Iroquois protein 5 (Irx5), plays an essential role in the generation of region-selective expression of Kv4.2 gene across the left ventricular wall of rodent hearts. Here, we analyse molecular mechanisms underlying the Irx5-induced regulation of the rat Kv4.2 promoter. The mRNA levels for Irx members in various heart regions were assessed by RT-PCR. A luciferase reporter gene with the rat Kv4.2 promoter was used to test the effects of Irx members on channel promoter activity. Irx3 and Irx5 mRNAs were differentially distributed across the left ventricular wall, whereas Irx4 message was equally abundant in various ventricular regions. Irx5, but not Irx3 or Irx4, increased Kv4.2 promoter activity in 10T1/2 fibroblasts, whereas the transcription factor decreased promoter activity in neonatal ventricular myocytes. These effects were mediated by the C-terminal portion of Irx5. Irx4 appeared to inhibit the Irx5-induced increase in channel promoter activity in 10T1/2 cells. The N-terminal region of Irx4 was necessary and sufficient for this inhibition. Furthermore, when endogenous Irx4 expression was suppressed with siRNA, Irx5 increased channel promoter activity in neonatal myocytes. These results indicate that Irx5 possesses the ability to activate the Kv4.2 promoter. The abundant Irx4 expression throughout the rat ventricle may play a role in the inverse relationship between Irx5 and Kv4.2 levels across the left ventricular wall.