Regulation of HO-2
Predicted secondary structure of 610 bp 3' untransleated sequence of human HO-2.
Brackets indicate polyadenylation signals. The arrow indicates the alternative
cleavage/polyadenylation site that yields the shorter form of the mRNA. The structure
in the inset is that of the shorter form.
The heme oxygenase-2 protein is also ubiquitously expressed, although the highest levels appear to be in the testis and the brain. In the course of our studies with BVR, it was found that treatment of cardiomyocytes with siRNA against BVR resulted in down-regulation of HO-2, at two levels. Prolonged treatment with BVR siRNA led to loss of HO-2 mRNA from cells, and there was also a loss of protein, which was found to be due to increased ubiquitination of HO-2, and subsequent proteasomal degradation. The HO-2 gene uses two alternative promoters that transcribe alternative untranslated 5' exons, and give a means of transcriptional control of gene expression. The ubiquitously expressed exon 1 is located ˜30 kb from exon 2, which contains the protein initiation codon. The alternate promoter, used in testis and to a lesser extent brain, transcribes a 5' exon located within intron 1 of the ubiquitous transcript.
To date, only glucocorticoids activate transcription of HO-2. Transcripts from both promoters then use a common set of exons to yield HO-2 mRNA. Two alternative poly(A) signals are located at the 3' end; use of the first site yields an ˜1.3 kb mRNA, while use of the second yields an ˜1.9 kb species. It was observed several years ago that the 1.3 kb mRNA is actively translated, whereas the 1.9 kb sequence was not, and that there was a selective loss of the 1.3 kb species in hypoxia. We are focusing our attention on miRNA and identification of proteins that bind to the 3' HO-2 stem- loops to define the mechanisms that modulate he relative levels of the two mRNAs, and vent translation of the longer form. In addition, we are very interested in exploring the mechanism and signaling pathways through which hBVR stabilizes HO-2 mRNA and protein.
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