Homocysteine regulates expression of Herp by DNA methylation involving the AARE and CREB binding sites.
Herp (homocysteine-induced endoplasmic reticulum protein) is an ER-resident membrane protein, which has a ubiquitin-like domain at its N-terminus. Expression of Herp protein is up-regulated in response to ER stress, including homocysteine. Herp stabilizes neuronal Ca(2+) homeostasis and is involved in improving the balance of the folding capacity and protein loading in the ER. In patients with alcoholism, we observed a significant decrease in Herp mRNA expression, and an increase of Herp promoter DNA methylation, which was associated with elevated homocysteine levels. Therefore, we studied the mechanism of Herp CpG islands regulation by luciferase assays and mRNA analysis in neuronal SH-SY5Y (human neuroblastoma cell line) and HEK 293T (human embryonic kidney 293T) cells. Acute homocysteine treatment caused transient demethylation of the Herp promoter and an increase in Herp mRNA level. Global DNA methylation was increased over the following 48 h period. We identified the transcription factor binding site AARE (amino acid response element) by mutational analysis involved in Herp induction in SH-SY5Y cells, and the more significant role of the CREB binding site (cyclic AMP response element-binding protein) compared to AARE in HEK 293T cells. Stimulation with SAM (S-adenosyl methionine) and homocysteine led to an increase in Herp promoter methylation, which correlated to an acute decrease in luciferase expression in SAM, but not in homocysteine stimulated cells. Complete methylation of the CpG islands resulted in suppressed gene expression.