Characterization of human NIPK (TRB3, SKIP3) gene activation in stressful conditions

The neuronal cell death-inducible putative kinase (NIPK) gene is upregulated in several cell types under stressful conditions. In order to understand the molecular control of the human (h) NIPK gene (also known as TRB3 and SKIP3), we mapped the transcriptional start sites of the gene in HepG2 cells treated with thapsigargin, the inhibitor of endoplasmic reticular Ca2+-ATPase, and determined the promoter region of the gene which is essential for endoplasmic reticulum and arsenite stress responses. The analysis of cDNA clones revealed the presence of several hNIPK mRNA isoforms, differing in their 5′ regions upstream of the hNIPK translation initiation codon as a result of alternative transcription initiation and alternative splicing. The induction of hNIPK gene in response to thapsigargin and arsenite treatments is mediated by a promoter segment consisting of tandemly arranged 33-bp repeats that contain a regulatory element similar to C/EBP-ATF composite site of the Chop gene promoter. ATF4, whose level is upregulated in the cells exposed to thapsigargin or arsenite, is able to bind to the 33-bp repeat and activate the hNIPK promoter. The coexpression of hNIPK inhibits activation of hNIPK promoter in response to the stress-inducing agents and to overexpressed ATF4, and thus NIPK may function as a negative feedback regulator of ATF4.