Lhx1 is Required for Beta Cell Function Via Regulation of Glp1 Receptor (Glp1R)

Abstract

Diabetes mellitus is a disease by which functional insulin producing beta cells are lost. Understanding the transcription factor and co‐regulator complexes that govern beta cell development and function is essential for the generation of novel therapeutics and or cell replacement strategies to treat this disease. Our lab previously showed that the LIM‐homeodomain (LIM‐HD) transcription factor Islet‐1 (Isl1) interacts with the LIM domain binding protein 1 co‐regulator (Ldb1) to regulate key beta cell genes (e.g. Glp1R and MafA). However, recent mRNA analysis and immunofluorescence studies revealed the presence of another LIM‐HD transcription factor in the pancreas, LIM homeobox 1 (Lhx1). Through co‐immunoprecipitation experiments in beta cell nuclear extracts, we found that Lhx1 also interacts with Ldb1 and surprisingly, Isl1. This suggests that a novel Lhx1‐Ldb1‐Isl1 complex exists in beta cells. We therefore hypothesized that Lhx1 regulates similar target genes as Ldb1 and Isl1. To test this, we employed siRNA‐mediated Lhx1 knockdown along with chromatin immunoprecipitation experiments. We discovered that, Lhx1 knockdown in beta cell lines resulted in significantly reduced Glp1R transcripts. Additionally, we revealed that Lhx1 occupied a regulatory domain of Glp1R. Together, these data suggest that Lhx1 directly regulates Glp1R. Glucagon‐like peptide‐1 (Glp1) is an incretin hormone secreted from the L‐cells in the gut that acts through a receptor (Glp1R) on pancreatic beta cells to increase the production and secretion of insulin. To uncover pancreatic Lhx1 roles in vivo, we generated a mouse model of pancreas‐wide Lhx1 deficiency. Mutant mice were born in expected ratios and appeared overtly normal. However, Lhx1 loss led to hyperglycemia in aged mice, plus significantly reduced Glp1R transcripts in isolated islets. Therefore, we assessed beta cell responses to Glp1 in mutant mice by employing a Glp1R agonist during an intraperitoneal glucose tolerance test. Mutant mice treated with agonist displayed a dampened ability to clear blood glucose when compared to agonist treated controls. This suggested that Lhx1‐deficient mice lack an ability to respond to Glp1. Taken together, our data reveal for the first time that Lhx1 directly regulates the beta cell Glp1R gene. Future studies will examine the role of Lhx1 in pancreas following physiological stressors (e.g. high fat diet).Support or Funding InformationWe thank our funding sources: NIH‐NIDDK DK094842 and DK105209; ADA 1‐16‐JDF‐044, UAB Endocrinology Startup, and the UAB Comprehensive Diabetes Center