Deficient ER Acetyl-CoA Import in Acinar Cells Leads to Chronic Pancreatitis

Abstract

Maintaining endoplasmic reticulum (ER) proteostasis is essential for pancreatic acinar cell function. Under conditions of severe ER stress, activation of pathogenic unfolded protein response pathways play a central role in the development and progression of pancreatitis. A key event in this pathogenic response is a loss of the transcription factor spliced XBP1 (XBP1s) and activation of the PERK pathway. Less is known of the consequence of perturbing ER-associated post-translational protein modification during pancreatitis. Here we show that expression of the ER acetyl-CoA transporter AT-1, necessary for ER protein acetylation, lies downstream of XBP1s and is significantly downregulated during the onset of pancreatitis. Genetic deletion of AT-1 in acinar cells of adult pancreas induces chronic ER stress marked by activation of both the XBP1s and PERK pathways, leading to mild/moderate chronic pancreatitis evidenced by accumulation of intracellular trypsin, immune cell infiltration, and fibrosis, but little pancreatic degeneration. Two-day induction of acute on chronic pancreatitis in AT-1 acinar specific knockout mice results in a severe CP phenotype with pronounced pancreatic atrophy. These findings uncover a new layer of complexity of the pathological ER stress response and its impact on pancreatic disease. (189/200) ResultsRelationship of AT-1 expression, ER stress, and pancreatitis. It was previously reported that AT-1 expression is regulated downstream of the IRE1/XBP1s pathway (24). Consistent with this, AT-1 mRNA is significantly downregulated in acinar-specific Ela-Cre XBP1 -/pancreas ( Figure 1A), which display spontaneous pancreatic disease (unpublished results). XBP1s is upregulated in acinar cells during AP as a protective mechanism against ER stress-induced damage, then significantly decreases as the disease progresses (12). Accordingly, AT-1 mRNA is reduced by greater than 50% following cerulein induced AP (CER-AP) in WT mice ( Figure 1B, left), while AT-1 mRNA was decreased in 75% of WT mice subject to CER-CP ( Figure 1B, right). These results suggest that AT-1 loss during pancreatitis is secondary to the reduction in XBP1s activity and, based on the current results, likely exacerbates disease progression.Given the critical role of AT-1 in maintaining ER proteostasis and the well-defined relationship of elevated ER stress and pancreatitis, we examined whether dysregulation of AT-1 function itself would be sufficient to induce pancreatitis. AT-1 S113R/+ mice, which express a hypomorphic mutation that prevents AT-1 dimerization, have an approximate 50% reduction in ER acetyl-CoA transport activity (S113R homozygosity is embryonic lethal) (21).Notwithstanding the immune dysfunction reported in these mice, AT-1 S113R/+ acinar cells display widespread ER dilation and vacuolization, indicative of ER stress and cellular damage, which progresses over time ( Figure 1C). Likewise, AT-1 S113R/+ pancreas have increased collagen deposition ( Figure 1D), a marker of fibrosis.Generation of acinar-specific AT-1 KO ...

Protein Lysine Acetylation

“…Such modifications can have a range of biologic effects that include cellular targeting, biologic activity, protein stability, and protein-protein interactions. Although described more than 50 years ago, the functional and cellular effects of protein acetylation on surface exposed lysine residues of nascent proteins has been much less studied and is the topic examined by Cooley et al 1 The importance of this study is that it highlights a previously undescribed role for protein acetylation in acinar cell physiology and disease and makes several key observations. First, mRNA levels of a key modulator of protein acetylation, AT-1, which mediates the transport of acetyl-CoA into the ER lumen, are regulated (upregulated with physiologic stimulation of acinar cells, and downregulated during pancreatitis).…”