DEFINING A SIGNALING CIRCUIT INVOLVING RIBOSOME BIOGENESIS AND P53 IN REGENERATION AND METAPLASIA

Although every cell has ribosomes, organ sensitivity to dysregulation of ribosome homeostasis (ribosomopathy) varies considerably. Related clinical syndromes are similarly disparate, ranging from craniofacial anomaly to bone marrow failure. Ribosomopathy as an aggravating factor in numerous disease processes is likely common and wholly unstudied. The principal known cellular response mechanism to ribosomopathy is "nucleolar stress," which depends on p53. Here, we report the effects of disrupted ribosome biogenesis in the pancreas at homeostasis and during regenerative response to injury.
Nat10 encodes a gene critical for ribosome biogenesis, so we generated Nat10 floxed mice with mice expressing pancreatic acinar cell-specific inducible Cre-recombinase or globally-expressed inducible Cre-recombinase to delete Nat10 in multiple adult organs. Cells in Nat10^Δ/Δ mice showed an expected decrease in 18s rRNA and acetylation of 18s rRNA cytidine residues, and reduction in cell volume in various cell types in multiple organs. Significant differences did not occur until 6 weeks after recombination induction, indicating a surprising capacity for cells to maintain homeostasis based on existing, mature ribosomes.
Pancreatic acinar cells were the most sensitive to p53 induction and death, indicating that they are relatively more vulnerable to nucleolar stress. Over time, Nat10-deleted acinar cells shrank in number and size, as non-recombined cells proliferated to restore the lost pancreatic volume. To determine if the cell shrinkage and death depended on nucleolar stress, we generated Nat10^Δ/Δ;Trp53^Δ/Δ mice. When Cre recombinase caused double knockout of NAT10 and p53, the recombined acinar cells showed significantly more regenerative proliferation, and pancreas volume was rescued to near wild-type levels. Thus, p53 stabilization was required for cell death and cell cycle arrest in this model of ribosome homeostasis.
We reasoned that inducing injury that required cell reprogramming and ribosome biogenesis would show more requirement for Nat10. We studied Nat10^Δ/Δ acinar cells after we induced paligenosis, an evolutionarily conserved program that differentiated cells, including acinar cells, use to re-enter the cell cycle and regenerate injured tissue. Cerulein-induced pancreatitis causes paligenosis with cells becoming metaplastic then re-entering the cell cycle by day 5. Nat10^Δ/Δ cells were particularly deficient in entering S-phase, which, again, was rescued by additionally deleting p53.
Thus, we begin to outline how ribosomes and proteostasis play a role in homeostasis along the GI tract and in paligenosis, as mature cells are called back to the cell cycle. The findings are important for understanding acute pancreatitis and how the metaplasias that can fuel PanIN and pancreatic ductal adenocarcinoma form, potentially offering NAT10 as a new therapeutic target.