AIMS: 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 analyze molecular mechanisms underlying the Irx5-induced regulation of rat Kv4.2
promoter. METHODS: The
mRNA levels for Irx members in various
heart regions were assessed by
RT-PCR. A
luciferase reporter gene with rat Kv4.2
promoter was used to test the effects of Irx members on channel
promoter activity. RESULTS: 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 the
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. CONCLUSIONS: These results indicate that Irx5 possesses the ability to activate 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.
