A nodal regulator of endoplasmic reticulum stress is the
transcription factor, ATF6, which is activated by
ischemia and protects the heart from ischemic damage, in vivo. To explore mechanisms of ATF6-mediated protection in the heart, a whole-genome
microRNA (
miRNA) array analysis of
RNA from the hearts of ATF6 transgenic (TG) mice was performed. The array identified 13 ATF6-regulated
miRNAs, eight of which were downregulated, suggesting that they could contribute to increasing levels of their mRNAs. The down-regulated
miRNAs, including miR-455, were predicted to target 45 mRNAs that we had previously shown by microarray analysis to be up-regulated by ATF6 in the heart. One of the miR-455 targets was
calreticulin (Calr), which is up-regulated in the pathologic heart, where it modulates hypertrophic growth, potentially reducing the impact of the pathology. To validate the effects of miR-455, we showed that Calr
protein was increased by ATF6 in mouse hearts, in vivo. In cultured cardiac myocytes, treatment with the ER stressor,
tunicamycin, or with adenovirus encoding activated ATF6 decreased miR-455 and increased Calr levels, consistent with the effects of ATF6 on miR-455 and Calr, in vivo. Moreover, transfection of cultured cardiac myocytes with a synthetic precursor, premiR-455, decreased Calr levels, while transfection with an antisense, antimiR-455, increased Calr levels. The results of this study suggest that ER stress can regulate gene expression via ATF6-mediated changes in
micro-RNA levels. Moreover, these findings support the hypothesis that ATF6-mediated down-regulation of miR-455 augments Calr expression, which may contribute to the protective effects of ATF6 in the heart.