Long noncoding RNA TUNAR regulates β-cell proliferation

The discovery that long noncoding RNA TUNAR regulates β-cell proliferation may be important in designing new treatments for diabetes.
Many long noncoding RNAs (lncRNAs) are enriched in pancreatic islets and several lncRNAs are linked to type 2 diabetes (T2D). Although they have emerged as potential players in β-cell biology and T2D, little is known about their functions and mechanisms in human β-cells. We identified an islet-enriched lncRNA, TUNAR (TCL1 upstream neural differentiation-associated RNA), which was upregulated in β-cells of patients with T2D and promoted human β-cell proliferation via fine-tuning of the Wnt pathway.

Publisert 24.10.2022

TUNAR was upregulated following Wnt agonism by a glycogen synthase kinase-3 (GSK3) inhibitor in human β-cells.

GSK3 inhibition promotes β-cell proliferation by activating canonical Wnt signaling, in which a central event is the stabilization and nuclear translocation of β-catenin and subsequent activation of cell cycle regulators.

Reciprocally, TUNAR repressed a Wnt antagonist Dickkopf-related protein 3 (DKK3) and stimulated Wnt pathway signaling. DKK3 was aberrantly expressed in β-cells of patients with T2D and displayed a synchronized regulatory pattern with TUNAR at the single cell level. Mechanistically, DKK3 expression was suppressed by the repressive histone modifier enhancer of zeste homolog 2 (EZH2). TUNAR interacted with EZH2 in β-cells and facilitated EZH2-mediated suppression of DKK3. These findings reveal a novel cell-specific epigenetic mechanism via islet-enriched lncRNA that fine-tunes the Wnt pathway and subsequently human β-cell proliferation.

To identify potential target genes of TUNAR, we first looked into the human homologs of genes known to be regulated in mouse embryonic stem cells (mESCs) under neuronal differentiation. We selected seven TUNAR target genes identified in mESCs for investigation in EndoC-βH1 cells, but none of them were regulated by TUNAR silencing, and two of them (the TUNAR neighboring genes TCL1A and TCL1B) were not expressed in β-cells. This suggests that TUNAR does not regulate the same gene network in human β-cells as in neuron-differentiating mouse embryonic stem cells. For this reason, we employed RNA-seq as an unbiased approach to identify target genes of TUNAR in human β-cells. TUNAR fine-tuned expression of multiple genes, but failed to upregulate or downregulate any genes with statistical significance. Nevertheless, since we knew that TUNAR regulates Wnt signaling, we used the RNA-seq data to specifically examine the impact of TUNAR silencing on expression of Wnt signaling genes, instead of performing a traditional unbiased analysis. With this approach, we identified that the Wnt antagonist Dickkopf-related protein 3 (DKK3) was the most highly regulated gene among those genes with higher than median expression in β-cells.