Transcriptional Maintenance of Pancreatic Acinar Identity, Differentiation, and Homeostasis by PTF1A

Abstract

Maintenance of cell type identity is crucial for health, yet little is known of the regulation that sustains the long-term stability of differentiated phenotypes. To investigate the roles that key transcriptional regulators play in adult differentiated cells, we examined the effects of depletion of the developmental master regulator PTF1A on the specialized phenotype of the adult pancreatic acinar cell in vivo. Transcriptome sequencing and chromatin immunoprecipitation sequencing results showed that PTF1A maintains the expression of genes for all cellular processes dedicated to the production of the secretory digestive enzymes, a highly attuned surveillance of unfolded proteins, and a heightened unfolded protein response (UPR). Control by PTF1A is direct on target genes and indirect through a ten-member transcription factor network. Depletion of PTF1A causes an imbalance that overwhelms the UPR, induces cellular injury, and provokes acinar metaplasia. Compromised cellular identity occurs by derepression of characteristic stomach genes, some of which are also associated with pancreatic ductal cells. The loss of acinar cell homeostasis, differentiation, and identity is directly relevant to the pathologies of pancreatitis and pancreatic adenocarcinoma. Loss of cellular identity has long been associated with tissue injury and a first step in cancer progression (for examples, see references 1 and 2). Maintenance of a specific cellular phenotype depends on the continued transcription of cell-type-specific genes, largely through open chromatin architecture (3, 4) maintained by a small group of lineage-restricted DNA-binding transcription factors (TFs) (5, 6) that establish a unique transcriptional regulatory network (7). Many physiologic or pathophysiologic perturbations can affect the differentiated state of a cell quantitatively, but fewer affect the state of differentiation qualitatively. Qualitative changes involve the acquisition of characteristics of another cell type (or types), often defined by one or a few cell-specific markers, in addition to the diminution of the original phenotype. Despite progress with cellular reprogramming (for example, see reference 8), the molecular and genetic mechanisms that maintain cellular identity within the context of adult organs remain incompletely understood. In this report, we show that inactivation of the transcriptional regulatory gene Ptf1a in adult pancreatic acinar cells has pleiotropic effects on gene expression that cause quantitative and multigene qualitative changes of acinar differentiation.The acinar cell of the pancreas has been an informative model of terminal cellular differentiation (9). Common cellular processes are greatly exaggerated in support of the prodigious synthesis, processing, storage, and exocytosis of secretory proteins. The pancreatic acinar cell has the most ribosomes (10) and the highest rate of protein synthesis (11) of any mammalian somatic cell; it synthesizes, stores, and secretes its weight in protein daily. Specialized cellular functions ...

Pancreas regeneration

“…Mechanistic studies in animal models have identified several genes and pathways required for the exocrine regenerative response in pancreatitis. Deletion of key components of the Hedgehog, Notch, and Wnt pathways from acinar cells severely disrupts exocrine regeneration 52–54 , as does deletion of the acinar-restricted transcription factors NR5A2 and PTF1A 55,56 . The dedifferentiated state of acinar cells appears to represent a vulnerability in which environmental and genetic factors could conspire to induce neoplastic transformation towards the deadly pancreatic cancers 57,58 .…”

MIST1 and PTF1 Collaborate in Feed-Forward Regulatory Loops That Maintain the Pancreatic Acinar Phenotype in Adult Mice

“…However, knowledge of the downstream genes controlled by PTF1 in mature acinar cells is largely limited to the genes of the secretory digestive enzymes (8,13). Inactivation of Ptf1a in the adult pancreas (17) reduced Mist1 mRNA to 24% of the normal level after 6 days (0.24 Ϯ 0.13; P ϭ 3.4EϪ4). To determine whether Mist1 is a direct target of PTF1, we performed ChIP-Seq experiments with chromatin from adult pancreas using antibodies against the PTF1 subunit PTF1A, RBPJ, or RBPJL.…”

“…The raw data are available through the NCBI Gene Expression Omnibus (GEO) database under accession number GSE86262 (17). Identification of regions bound by either PTF1A or RBPJL followed the pipeline of MIST1 ChIP-Seq analysis described above.…”

“…Cufflinks (37) calculated the number of reads per kilobase per million mapped reads (RPKM), a measure of the relative level of a transcript normalized to 1 million reads. RNA-Seq results for three control and four Ptf1a conditional knockout (Ptf1a-cKO) mouse pancreases were deposited in the NCBI GEO database under accession number GSE86261 (17). Differential gene expression analysis and RPKM calculation followed the pipeline described above.…”

“…Bhlha15) is a bHLH transcription factor present selectively in serous secretory cells, including pancreatic acinar cells (14,15). In the pancreas, Ptf1a and Mist1 are expressed selectively in acinar cells, and inactivation of either gene affects the differentiated acinar phenotype (16,17). Mist1-null pancreatic acinar cells have aberrant calcium signaling, reduced content of digestive enzymes, and decreased expression of critical genes of the secretory pathway, such as Rab genes (18)(19)(20).…”