Mbnl1-mediated alternative splicing of circMlxipl regulates Rbbp6-involved ChREBP turnover to inhibit lipotoxicity-induced β-cell damage.

Background: Diabetes, a global epidemic, is the leading cause of mortality globally. The aim of this study is to get better understanding of pathophysiology of diabetes.

Methods: Palmitic acid (PA)-treated β-cells, db/db mice and high fat diet (HFD)-fed mouse model of type 2 diabetes were established. H&E was used to assess the histological changes of pancreas. IHC, FISH, western blot or qRT-PCR was employed to detect the expression of key molecules in primary islets or lipotoxic β-cells. Cell behaviors were detected by MTT, EdU incorporation assay, TUNEL assay and glucose-induced insulin secretion (GSIS). The associations among circMlxipl, Mbnl1 and Rbbp6 were validated by RIP and RNA pull-down assays, and the direct binding between Hdac3 and Mbnl1 promoter was examined by ChIP and luciferase assays. Co-IP was employed to assess the interaction between ChREBP and Rbbp6, as well as the ubiquitination of ChREBP.

Results: Hdac3 and ChREBP were upregulated, but Mbnl1 and circMlxipl were downregulated in islets from diabetic mice and lipotoxic β-cells. Mbnl1 overexpression protected against PA-induced impairments in lipotoxic β-cells through modulating back-splicing of circMlxipl and suppressing ChREBP. Hdac3 served as a transcriptional repressor of Mbnl1, and it was implicated in circMlxipl-mediated protection via regulating ChREBP expression in lipotoxic β-cells. Lack of circMlxipl inhibited Rbbp6-mediated ubiquitin-proteasomal degradation of ChREBP in lipotoxic β-cells. In vivo studies revealed that Hdac3 knockdown or Mbnl1 overexpression alleviated diabetes symptoms through circMlxipl-regulated ChREBP in diabetic mice.

Conclusion: Mbnl1-mediated alternative splicing of circMlxipl regulates Rbbp6-involved ChREBP turnover to inhibit lipotoxicity-induced β-cell damage.