Abstract 3010: MiRNA-26 Plays Essential Role in Myocyte Survival and Hypertrophy by Regulating GATA4
MicroRNAs are posttranscriptional regulators of gene expression, which are involved in organogenesis and pathogenesis. We have previously reported that among the miRNAs that are differentially regulated during cardiac hypertrophy, miR-26 was significantly downregulated. Northern blot analysis confirmed that it is downregulated by 40 % after one week of pressure overload on the heart. Downregulation of a miRNA implies that the translation of its mRNA targets will be increased. The target prediction software TargetscanS identified the transcription factor GATA4 as a target for miR-26. GATA4 is upregulated during hypertrophy and plays an essential role in both myocytes hypertrophy and survival. To address whether miR-26 had a role in regulation of this protein we supplemented the myocytes with various doses of an adenovirus expressing the miR-26 gene or a nonsense control. Cells were then stimulated with endothelin-1 (ET-1) for 24 h. This resulted in an increase in GATA4 that was inhibited by miR-26 in a dose-dependent fashion, with complete inhibition observed at the higher doses. Further increase in the miR-26 dose was associated with a reduction in the basal levels of GATA4 and cell death. In contrast, knockdown of miR-26 using an adenovirus expressing a tandem repeat of antisense miR-26 resulted in a dose-dependent increase in cell size (200±55 % maximum). Paradoxically, further knockdown of endogenous miR-26 (>50 %) resulted in cell death. This suggests that below a certain concentration other targets of miR-26 that may play a role in cell death become uninhibited. One possibility is the predicted target phosphatase and tensin homolog (PTEN), which we are currently investigating. The data emphasizes the importance of taking into account the stoichiometry of miRNA to target mRNA when manipulating its concentration in a given cell, in order to determine its targets and functions. Thus, very precise downregulation of miR-26 plays a positive role in cell survival through upregulating GATA4. On the other hand, a minimum level of miR-26 is also required for cell survival through suppression of other, as-of-yet unconfirmed, anti-survival targets.