Yy1 Depletion in Pancreatic Beta Cells Leads to Energy Source Switch From Glycolysis to Oxidative Phosphorylation

Background: Gene expression is determined by structural interactions in between transcription factors, cofactors and enhancer elements, as well as enhancer-promoter interactions (1). Both YY1 and CTCF are essential, zinc finger proteins that bind hypo-methylated DNA sequences, form homodimers, and thus facilitate DNA loop formation (1). ​However, YY1 preferentially occupies interacting enhancers and promoters, whereas CTCF preferentially occupies sites distal from these regulatory elements, forming larger loops and participating in insulation (1). A sequencing study of spontaneous functional insulinomas in a Chinese cohort identified a somatic a hotspot mutation in YY1 (c.C1115G/p.T372R) in 30% of the cases, associated with increased YY1 activity (2). YY1 is a critical transcription factor involved in the regulation of proliferation and metabolism (2). Hypothesis: YY1 loss-of-function alters energy source preference in pancreatic β-cells. Methods: YY1 stable loss-of-function in mouse insulinoma cell lines was achieved by shRNA lentiviral transduction. Mitochondrial membrane potential (MMP) was measured via flow cytometry of aggregated mitochondria to monomeric mitochondria ratio. Mitostress and complex-substrate controlled respiration were measured by Seahorse analyzer. Mitochondrial copy number was assessed by mitochondrial to nuclear DNA ratio. Quantitative qPCR and Western blotting were used to assess mitochondrial gene and protein expression. Results: Our data indicated that YY1 deficient β-cells showed increased MMP and maximal respiration. No significant differences were found in basal respiration, ATP production, proton leak, non-mitochondrial oxygen consumption or coupling efficiency. We also found that YY1 deficient β-cells exhibited reduced glycolytic capacity and decreased ETC complex IV activity, with concurrent increased complex I and II activity. In addition, YY1 deficient β-cells exhibited elevated mitochondrial copy number​ and increased quantitative mRNA of mitochondrial gene expression, which could be correlated with increased PGC1-α expression. Conclusions: YY1 is critical in the metabolic regulation of β-cells, particularly in the facilitation of glycolytic metabolism. YY1 activating mutations in functional spontaneous insulinoma cells can lead to a proliferation dysregulation accompanied by a metabolic switch that favors glycolysis, while the opposite occurs in YY1 deficient β-cells. References: (1) Weintraub AS et al, Cell 2017 Dec 14; 171:1573–1588 (2) Cao Y et al, Nat Commun 2013 Dec 10; 4:2810