PDX1 Phosphorylation Mediates Amino Acid-mTORC1 Signaling in Pancreatic β-Cells [ChIP-seq]

PDX1 is known as the insulin transcription factor that regulates insulin expression in pancreatic β-cells in response to circulating glucose levels. Our previous study showed PDX1 also acts as a downstream effector that mediates β-cell mTORC1 signaling in the control of whole-body glucose homeostasis in response to stimulation by amino acids (AAs). Here we report that PDX1 has a broad role in AA-mTORC1-regulated transcriptome in β-cells, especially in processes related to β-cell proliferation and function. Mechanistically, mTORC1 phosphorylates PDX1 at S61 in an AA-dependent and rapamycin-sensitive manner. S61 phosphorylation promotes PDX1 protein levels and transcriptional activity. Interestingly, S61 is located within the transactivation domain targeted by monogenic diabetes mutations, which interfere with S61 phosphorylation and PDX1's transcriptional activity. To help understand the role of S61 phosphorylation in vivo, we generated S61A and S61E genomic mutations in mice, mimicking the unphosphorylated and phosphorylated states, respectively. Our results indicate that S61 phosphorylation positively regulates insulin expression and β-cell proliferation, promoting Western diet-induced hyperinsulinemia, obesity, and liver steatosis. These findings reveal the mechanism and roles of AA-mTORC1-PDX1 signaling in the control of pancreatic β-cell proliferation and function under physiological and pathological conditions.