In this study, we show that NEAT1 expression is repressed by PML-RARα. In addition, we provide evidence that NEAT1 expression is involved in the differentiation of APL cells.
PML-RARα is a potent transcriptional repressor in APL cells, and it blocks promyelocyte differentiation. An interesting characteristic of this oncogenic protein is that its transcriptional repression effects can be reverted with pharmacologic doses of ATRA, resulting in the reactivation of genes essential for definitive myeloid differentiation [
2,
3,
25]. Recently, lncRNAs have been shown to be dysregulated in various cancers, and several lncRNAs have been functionally linked to cancer and cell differentiation [
13,
15]. In this study, our findings of particular low NEAT1 levels in APL cells indicate that NEAT1 transcription is further repressed by PML-RARα.Indeed, we found that NEAT1 was repressed by PML-RARα, and this repression could be relieved by ATRA. These results suggest that NEAT1 expression is regulated by PML-RARα. Unlike ATRA, arsenic trioxide induces limited transcriptional changes of NEAT1 in APL cells (Additional file
3: Figure S3A). This result may due to arsenic trioxide induces significant cell death but only very limited differentiation [
26,
27], and upregulation of NEAT1 in ATRA-treated APL cells is a consequence of the relief of PML-RARα-mediated transcriptional repression. Notably, previous studies have indicated PML-RARα is able to interact with many other transcription factors, such as PU.1, providing the potential for the oncoprotein to target genes primarily regulated by other transcription factors [
7,
28]. However, due to the structural and functional complexity of PML-RARα, whether this regulation is by direct DNA binding or by other transcription factors requires further investigation. More importantly, this lncRNA is involved in the differentiation of APL cells, suggesting an interesting characteristic for the PML-RARα oncogenic protein in the regulation of gene expression by NEAT1. The mammalian nucleus is highly organized and contains several membraneless subcompartment nuclear bodies, including nucleoli, paraspeckles, PML bodies and speckles, which are thought to be involved in gene regulation [
29]. NEAT1 is a critical component of the paraspeckle structure, and paraspeckles have been proposed to be involved in the regulation of gene expression through the control of the nuclear retention of mRNAs containing long inverted repeats, which are capable of forming intramolecular double-stranded RNAs subject to adenosine-to-inosine editing [
17]. More recently, NEAT1 was also shown to be involved in transcriptional regulation by sequestrating a transcriptional regulator [
21]. Previous studies have demonstrated that PML-RARα was capable of promoting leukemic transformation by impairing the formation of functional PML nuclear bodies [
30]. Thus, PML-RARα may influence the nuclear retention of structured mRNAs or gene transcription, which is indispensable for myeloid differentiation, through NEAT1. This finding is the first evidence that a lncRNA cooperates with a fusion protein and plays a critical role in the response to treatment. However, the underlying molecular mechanisms of NEAT1 and potential roles of paraspeckles in APL require further investigation.
In conclusion, we report abnormally decreased expression of the nuclear long noncoding RNA NEAT1, which is responsible for the differentiation block in blast cells in APL. These findings provide a more comprehensive understanding of APL pathogenesis. Because NEAT1 plays an important role in the posttranscriptional and transcriptional regulation of gene expression and we found that the expression of this nuclear long non-coding RNA was regulated by PML-RARα, it remains an interesting open question whether subsets of genes essential for myeloid differentiation are also modulated by perturbation of NEAT1 expression.