Foxo3a Regulates Apoptosis by Negatively Targeting miR-21

Abstract

MicroRNAs are a class of small non-coding RNAs and participate in the regulation of apoptotic program. Although miR-21 is able to inhibit apoptosis, its expression regulation and downstream targets remain to be fully elucidated. Here we report that the transcriptional factor Foxo3a initiates apoptosis by transcriptionally repressing miR-21 expression. Our results showed that doxorubicin could simultaneously induce the translocation of Foxo3a to the cell nuclei and a reduction in miR-21 expression. Knockdown of Foxo3a resulted in an elevation in miR-21 levels, whereas enforced expression of Foxo3a led to a decrease in miR-21 expression. In exploring the molecular mechanism by which Foxo3a regulates miR-21, we observed that Foxo3a bound to the promoter region of miR-21 and suppressed its promoter activity. These results indicate that Foxo3a can transcriptionally repress miR-21 expression. In searching for the downstream targets of miR-21 in apoptosis, we found that miR-21 suppressed the translation of Fas ligand (FasL), a pro-apoptotic factor. Furthermore, Foxo3a was able to up-regulate FasL expression through down-regulating miR-21. Our data suggest that Foxo3a negatively regulates miR-21 in initiating apoptosis. miRNAs 2 are a class of small non-coding RNAs that mediate post-transcriptional gene silencing. Recently, the work on miRNAs renovates our understanding about apoptotic regulation. They can be classified as either pro-or anti-apoptotic miRNAs (1, 2). For example, miR-1 participates in the initiation of apoptosis (3), whereas miR-21 is able to inhibit apoptosis (4). miRNAs are expressed at a constant level under physiological condition, and their functions depend on their expression levels. It remains a challenging question as to how their expression is regulated in the apoptotic program.The forkhead family of transcription factors is characterized by the presence of a conserved 100-amino acid DNA binding domain and participate in regulating diverse cellular functions such as apoptosis, differentiation, metabolism, proliferation, and survival (5). Foxo3a is a substrate of protein kinase Akt, and its transcriptional output is controlled via phosphorylation. In the absence of cellular stimulation and when Akt is inactive, Foxo3a is localized within the nucleus where it performs transcription of target genes. However, upon phosphorylation by Akt at Thr-32, Ser-253, and Ser-315, it binds to 14-3-3 and cannot exert the transcriptional function (6).Fas ligand (FasL) is a potential transcriptional target of Foxo3a, but the transcriptional output can be either activation or suppression. It has been reported that Foxo3a can stimulate FasL expression, thereby triggering apoptosis (6). However, there is also evidence showing that Foxo3a decreases the expression of FasL (7). The molecular mechanism by which Foxo3a regulates FasL expression remains further elusive. miR-21 has been shown to regulate apoptosis by targeting a variety of apoptotic factors. It contributes to glioma malignancy by down-regulating matrix meta...

Up-regulation of miR-21 Mediates Resistance to Trastuzumab Therapy for Breast Cancer

“…This is inconsistent with the increased homodimerization of EGFR or heterodimerization of EGFR/HER3 and the elevated levels of phosphorylated AKT after continuous culture of previously sensitive cells in trastuzumab-containing medium, which are suggested as potential molecular mechanisms for acquired trastuzumab resistance (36 -37). Because the miR-21 promoter contains highly conserved regions with consensus binding sites for several transcription factors, including STAT3 (38, 39), AP-1 (40), and factors of the forkhead family (FOXO), such as FOXO3a (41), transcription of primary miR-21 (pri-miR-21) can be regulated via various pathways. Among them, activation of the RAS/ MEK/MAPK pathway due to increased homodimerization of EGFR or heterodimerization of EGFR/HER3 potentially may lead to enhanced transcription of pri-miR-21 via AP-1 (42,43).…”

Mitofusin 1 Is Negatively Regulated by MicroRNA 140 in Cardiomyocyte Apoptosis

“…A luciferase assay was performed as we described previously (43). In brief, cells were cotransfected with the plasmid constructs, 150 ng/well of pGL3-Mfn1-3=UTR and 300 ng/well of miR-140, by using Lipofectamine 2000 (Invitrogen).…”

FoxO3a and disease progression

“…A recent piece of evidence suggests that the FoxO family is also regulated by microRNA. mir155, mir96 and mir21 are thought to directly regulate FoxO3a, while mir205 regulates FoxO3a via its upstream target PTEN [39][40][41][42][43] . FoxO3a is also known to be regulated by a transcription factor.…”