Notch signaling is required for exocrine regeneration after acute pancreatitis

Pancreatic beta-cells: from generation to regeneration

The pancreas is composed of two main compartments consisting of endocrine and exocrine tissues. The majority of the organ is exocrine and responsible for the synthesis of digestive enzymes and for their transport via an intricate ductal system into the duodenum. The endocrine tissue represents less than 2% of the organ and is organized into functional units called islets of Langerhans, comprising alpha-, beta-, delta-, epsilon- and PP-cells, producing the hormones glucagon, insulin, somatostatin, ghrelin and pancreatic polypeptide (PP), respectively. Insulin-producing beta-cells play a central role in the control of the glucose homeostasis. Accordingly, absolute or relative deficiency in beta-cells may ultimately lead to type 1 and/or type 2 diabetes, respectively. One major goal of diabetes research is therefore to understand the molecular mechanisms controlling the development of beta-cells during pancreas morphogenesis, but also those underlying the regeneration of adult injured pancreas, and assess their significance for future cell-based therapy. In this review, we will therefore present new insights into beta-cell development with focus on beta-cell regeneration.

Notch2 is required for progression of pancreatic intraepithelial neoplasia and development of pancreatic ductal adenocarcinoma

Pancreatic cancer is one of the most fatal malignancies lacking effective therapies. Notch signaling is a key regulator of cell fate specification and pancreatic cancer development; however, the role of individual Notch receptors and downstream signaling is largely unknown. Here, we show that Notch2 is predominantly expressed in ductal cells and pancreatic intraepithelial neoplasia (PanIN) lesions. Using genetically engineered mice, we demonstrate the effect of conditional Notch receptor ablation in KrasG12D-driven pancreatic carcinogenesis. Deficiency of Notch2 but not Notch1 stops PanIN progression, prolongs survival, and leads to a phenotypical switch toward anaplastic pancreatic cancer with epithelial-mesenchymal transition. By expression profiling, we identified increased Myc signaling regulated by Notch2 during tumor development, placing Notch2 as a central regulator of PanIN progression and malignant transformation. Our study supports the concept of distinctive roles of individual Notch receptors in cancer development.

Notch1 functions as a tumor suppressor in a model of K-ras-induced pancreatic ductal adenocarcinoma

K-ras is the most commonly mutated oncogene in pancreatic cancer and its activation in murine models is sufficient to recapitulate the spectrum of lesions seen in human pancreatic ductal adenocarcinoma (PDAC). Recent studies suggest that Notch receptor signaling becomes reactivated in a subset of PDACs, leading to the hypothesis that Notch1 functions as an oncogene in this setting. To determine whether Notch1 is required for K-ras-induced tumorigenesis, we used a mouse model in which an oncogenic allele of K-ras is activated and Notch1 is deleted simultaneously in the pancreas. Unexpectedly, the loss of Notch1 in this model resulted in increased tumor incidence and progression, implying that Notch1 can function as a tumor suppressor gene in PDAC.

Targeting Notch signaling in pancreatic cancer patients--rationale for new therapy

Pancreatic cancer is one of the most aggressive and devastating human malignancies. Despite new knowledge in the molecular profile of pancreatic cancer and its precursor lesions, survival rates have changed very little over the last 40 years. Therefore, a better understanding of the detailed mechanisms underlying the pathogenesis of this disease is critical if we expect to develop new and effective strategies for prevention, early diagnosis and treatment of pancreatic cancer. The review herein focuses on a distinctive signaling pathway, the Notch pathway, which has recently been associated with carcinogenesis, including pancreatic cancer. It is aimed at summarizing key results which support a role for this pathway in the initiation, progression and maintenance of pancreatic cancer as a rationale for targeting and inhibiting this pathway in pancreatic cancer patients.

Pancreatic duct glands are distinct ductal compartments that react to chronic injury and mediate Shh-induced metaplasia

BACKGROUND & AIMS

Pancreatic intraepithelial neoplasia (PanIN) are pancreatic cancer precursor lesions of unclear origin and significance. PanIN aberrantly express sonic hedgehog (Shh), an initiator of pancreatic cancer, and gastrointestinal mucins. A majority of PanIN are thought to arise from ducts. We identified a novel ductal compartment that is gathered in gland-like outpouches (pancreatic duct glands [PDG]) of major ducts and characterized its role in injury and metaplasia.

METHODS

The ductal system was analyzed in normal pancreata and chronic pancreatitis in humans and mice. Anatomy was assessed by serial hematoxylin and eosin sections and scanning electron microscopy of corrosion casts. Expression of mucins and developmental genes and proliferation were assessed by immunohistochemistry or real-time quantitative polymerase chain reaction. Effects of Shh on ductal cells were investigated by exposure to Shh in vitro and transgenic misexpression in vivo.

RESULTS

Three-dimensional analysis revealed blind-ending outpouches of ducts in murine and human pancreata. These PDG are morphologically and molecularly distinct from normal ducts; even in normal pancreata they display PanIN and metaplastic features, such as expression of Shh and gastric mucins. They express other developmental genes, such as Pdx-1 and Hes-1. In injury, Shh is up-regulated along with gastric mucins. Expansion of the PDG compartment results in a mucinous metaplasia. Shh promotes this transformation in vitro and in vivo.

CONCLUSIONS

PDG are distinct gland-like mucinous compartments with a distinct molecular signature. In response to injury, PDG undergo an Shh-mediated mucinous gastrointestinal metaplasia with PanIN-like features. PDG may provide a link between Shh, mucinous metaplasia, and neoplasia.

Beta-catenin blocks Kras-dependent reprogramming of acini into pancreatic cancer precursor lesions in mice

Cellular plasticity in adult organs is involved in both regeneration and carcinogenesis. WT mouse acinar cells rapidly regenerate following injury that mimics acute pancreatitis, a process characterized by transient reactivation of pathways involved in embryonic pancreatic development. In contrast, such injury promotes the development of pancreatic ductal adenocarcinoma (PDA) precursor lesions in mice expressing a constitutively active form of the GTPase, Kras, in the exocrine pancreas. The molecular environment that mediates acinar regeneration versus the development of PDA precursor lesions is poorly understood. Here, we used genetically engineered mice to demonstrate that mutant Kras promotes acinar-to-ductal metaplasia (ADM) and pancreatic cancer precursor lesion formation by blocking acinar regeneration following acute pancreatitis. Our results indicate that beta-catenin is required for efficient acinar regeneration. In addition, canonical beta-catenin signaling, a pathway known to regulate embryonic acinar development, is activated following acute pancreatitis. This regeneration-associated activation of beta-catenin signaling was not observed during the initiation of Kras-induced acinar-to-ductal reprogramming. Furthermore, stabilized beta-catenin signaling antagonized the ability of Kras to reprogram acini into PDA preneoplastic precursors. Therefore, these results suggest that beta-catenin signaling is a critical determinant of acinar plasticity and that it is inhibited during Kras-induced fate decisions that specify PDA precursors, highlighting the importance of temporal regulation of embryonic signaling pathways in the development of neoplastic cell fates.

Notch signaling in solid tumors

In recent years a substantial body of evidence derived from not only preclinical but also clinical studies has accumulated in support of Notch signaling playing important oncogenic roles in several types of cancer. The finding that activating Notch mutations are frequently found in patients suffering from acute lymphoblastic leukemia is one of the best examples for a critical role of Notch signaling in cancer, a fact that motivated many researchers and clinicians to study the role of Notch also in solid tumors. Hence Notch signaling has gained increasing attention as a potential therapeutic target. In this book chapter we would like to discuss our current knowledge of Notch signaling within different types of solid cancers as well as advantages and disadvantages of potential new therapies that try to target the oncogenic properties of Notch signaling.

Context-dependent transformation of adult pancreatic cells by oncogenic K-Ras

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal human malignancies. To investigate the cellular origin(s) of this cancer, we determined the effect of PDAC-relevant gene mutations in distinct cell types of the adult pancreas. We show that a subpopulation of Pdx1-expressing cells is susceptible to oncogenic K-Ras-induced transformation without tissue injury, whereas insulin-expressing endocrine cells are completely refractory to transformation under these conditions. However, chronic pancreatic injury can alter their endocrine fate and allow them to serve as the cell of origin for exocrine neoplasia. These results suggest that one mechanism by which inflammation and/or tissue damage can promote neoplasia is by altering the fate of differentiated cells that are normally refractory to oncogenic stimulation.

Effect of Notch activation on the regenerative response to acute renal failure

Episodes of acute renal failure (ARF) are not always fully reversible and may lead to chronic disease, due in part to an inadequate regenerative response. The Notch signaling pathway is involved in determining cell fate during development, and tissue maintenance and repair in adult organs. The purpose of this study was to examine the role of the Notch pathway in renal regeneration following ARF. Kidney injury, induced by ischemia-reperfusion, resulted in early activation of the Notch pathway, as evidenced by increased expression of Notch1 and Notch2 intracellular domain (cleaved Notch). The effect of exogenous administration of the Notch ligand Delta-like-4 (DLL4) on recovery from ARF was then studied. Rats were pretreated by intraperitoneal injection of DLL4 or vehicle control. Two days following the last DLL4 dose, ARF was induced by bilateral renal artery clamping for 45 min followed by reperfusion. The severity of renal injury was similar in DLL4 and control rats. Renal recovery was facilitated by DLL4 treatment, as evidenced by faster return of serum creatinine to baseline by 48 h in DLL4-treated rats as against 5 days in vehicle-treated control rats. Cell proliferation was higher in the DLL4-treated group. In conclusion, activation of the Notch pathway occurs following ARF. Pretreatment with the Notch ligand DLL4 enhanced recovery from ARF and represents a potential novel therapeutic option for regenerating the injured kidney.

Molecular and cellular regulation of pancreatic acinar cell function

PURPOSE OF REVIEW

This review focuses on studies from the past year that have greatly advanced our understanding of molecular and cellular regulation of pancreatic acinar cell function.

RECENT FINDINGS

Recent advances focus on signals dictating pancreatic development, acinar cell fate, pancreatic growth, and secretion. Regeneration of acinar cells after pancreatitis depends on expression of embryonic signals in mature acinar cells. In this setting, acinar cells can also transdifferentiate into adipose cells. With the forced induction of certain early and endocrine-driving transcription factors, acinar cells can also transdifferentiate into beta-cells. There has also been an increased understanding of acinar-to-ductal metaplasia and the subsequent formation of pancreatic intraepithelial neoplasia lesions. Multiple proteins involved in secretion have been characterized, including small guanosine triphosphate-binding proteins, soluble N-ethylmaleimide-sensitive factor attachment proteins, and ion channels.

SUMMARY

These findings demonstrate the regenerative potential of the acinar cell to mitigate injurious states such as pancreatitis. The ability of acinar cells to transdifferentiate into beta-cells could potentially provide a treatment for diabetes. Finally, the results might be helpful in preventing malignant transformation events arising from the acinar cell. Developments in proteomics and computer modeling could expand our view of proteins mediating acinar cell function.

Notch1 represses osteogenic pathways in aortic valve cells

Calcific aortic stenosis is the third leading cause of adult heart disease and the most common form of acquired valvular disease in developed countries. However, the molecular pathways leading to calcification are poorly understood. We reported two families in which heterozygous mutations in NOTCH1 caused bicuspid aortic valve and severe aortic valve calcification. NOTCH1 is part of a highly conserved signaling pathway involved in cell fate decisions, cell differentiation, and cardiac valve formation. In this study, we examined the mechanism by which NOTCH1 represses aortic valve calcification. Heterozygous Notch1-null (Notch1(+/)(-)) mice had greater than fivefold more aortic valve calcification than age- and sex-matched wildtype littermates. Inhibition of Notch signaling in cultured sheep aortic valve interstitial cells (AVICs) also increased calcification more than fivefold and resulted in gene expression typical of osteoblasts. We found that Notch1 normally represses the gene encoding bone morphogenic protein 2 (Bmp2) in murine aortic valves in vivo and in aortic valve cells in vitro. siRNA-mediated knockdown of Bmp2 blocked the calcification induced by Notch inhibition in AVICs. These findings suggest that Notch1 signaling in aortic valve cells represses osteoblast-like calcification pathways mediated by Bmp2.

Dll1 haploinsufficiency in adult mice leads to a complex phenotype affecting metabolic and immunological processes

BACKGROUND

The Notch signaling pathway is an evolutionary conserved signal transduction pathway involved in embryonic patterning and regulation of cell fates during development and self-renewal. Recent studies have demonstrated that this pathway is integral to a complex system of interactions, involving as well other signal transduction pathways, and implicated in distinct human diseases. Delta-like 1 (Dll1) is one of the known ligands of the Notch receptors. The role of the Notch ligands is less well understood. Loss-of-function of Dll1 leads to embryonic lethality, but reduction of Delta-like 1 protein levels has not been studied in adult stage.

METHODOLOGY/PRINCIPAL FINDINGS

Here we present the haploinsufficient phenotype of Dll1 and a missense mutant Dll1 allele (Dll1(C413Y)). Haploinsufficiency leads to a complex phenotype with several biological processes altered. These alterations reveal the importance of Dll1 mainly in metabolism, energy balance and in immunology. The animals are smaller, lighter, with altered fat to lean ratio and have increased blood pressure and a slight bradycardia. The animals have reduced cholesterol and triglyceride levels in blood. At the immunological level a subtle phenotype is observed due to the effect and fine-tuning of the signaling network at the different levels of differentiation, proliferation and function of lymphocytes. Moreover, the importance of the proteolytic regulation of the Notch signaling network emphasized.

CONCLUSIONS/SIGNIFICANCE

In conclusion, slight alterations in one player of Notch signaling alter the entire organism, emphasizing the fine-tuning character of this pathway in a high number of processes.

Notch signaling as gatekeeper of rat acinar-to-beta-cell conversion in vitro

BACKGROUND & AIMS

Exocrine acinar cells in the pancreas are highly differentiated cells that retain a remarkable degree of plasticity. After isolation and an initial phase of dedifferentiation in vitro, rodent acinar cells can convert to endocrine beta-cells when cultured in the presence of appropriate factors. The mechanisms regulating this phenotypic conversion are largely unknown.

METHODS

Using rat acinar cell cultures, we studied the role of Notch signaling in a model of acinar-to-beta-cell conversion.

RESULTS

We report a novel lectin-based cell labeling method to demonstrate the acinar origin of newly formed insulin-expressing beta-cells. This method allows for specific tracing of the acinar cells. We demonstrate that growth factor-induced conversion of adult acinar cells to beta-cells is negatively regulated by Notch1 signaling. Activated Notch1 signaling prevents the reexpression of the proendocrine transcription factor Neurogenin-3, the key regulator of endocrine development in the embryonic pancreas. Interfering with Notch1 signaling allows modulating the acinar cell susceptibility to the differentiation-inducing factors. Its inhibition significantly improves beta-cell neoformation with approximately 30% of acinar cells that convert to beta-cells. The newly formed beta-cells mature when transplanted ectopically and are capable of restoring normal blood glycemia in diabetic recipients.

CONCLUSIONS

We report for the first time an efficient way to reprogram one third of the acinar cells to beta-cells by adult cell type conversion. This could find application in cell replacement therapy of type 1 diabetes, provided that it can be translated from rodent to human models.

gamma-Secretase heterogeneity in the Aph1 subunit: relevance for Alzheimer's disease

The gamma-secretase complex plays a role in Alzheimer's disease and cancer progression. The development of clinically useful inhibitors, however, is complicated by the role of the gamma-secretase complex in regulated intramembrane proteolysis of Notch and other essential proteins. Different gamma-secretase complexes containing different Presenilin or Aph1 protein subunits are present in various tissues. Here we show that these complexes have heterogeneous biochemical and physiological properties. Specific inactivation of the Aph1B gamma-secretase in a mouse Alzheimer's disease model led to improvements of Alzheimer's disease-relevant phenotypic features without any Notch-related side effects. The Aph1B complex contributes to total gamma-secretase activity in the human brain, and thus specific targeting of Aph1B-containing gamma-secretase complexes may help generate less toxic therapies for Alzheimer's disease.

Ductal malformation and pancreatitis in mice caused by conditional Jag1 deletion

BACKGROUND & AIMS

Alagille syndrome is an autosomal dominant disorder caused by mutations in Notch signaling pathway genes, usually JAGGED1. Up to 40% of Alagille syndrome patients also display exocrine pancreatic insufficiency, the pathobiology of which is unknown. Additionally, no mouse model recapitulating this aspect of the disease has been reported.

METHODS

We conditionally deleted both alleles of Jagged1 in the murine pancreas using Cre-loxP technology and analyzed histologic and morphologic features in postnatal and adult pancreas such as duct structure, acinar mass, and T-lymphocyte infiltration, as well as markers of pancreatic function, including fecal fat.

RESULTS

Jagged1-deficient mice displayed malformed pancreatic ducts with resulting acinar cell death, fatty infiltration of the parenchyma, fibrosis, pancreatitis, and pancreatic insufficiency.

CONCLUSIONS

Pancreatic ductal malformation and acinar cell loss may be responsible for pancreatic insufficiency in Jagged1-deficient mice and, by corollary, in Alagille syndrome patients.

Inhibition of gamma-secretase activity inhibits tumor progression in a mouse model of pancreatic ductal adenocarcinoma

BACKGROUND & AIMS

The Notch signaling pathway is required for the expansion of undifferentiated pancreatic progenitor cells during embryonic development and has been implicated in the progression of pancreatic ductal adenocarcinoma (PDAC). The interaction of Notch ligands with their receptors promotes a gamma-secretase-dependent cleavage of the Notch receptor and release of the Notch intracellular domain, which translocates to the nucleus and activates transcription. We investigated the role of this pathway in PDAC progression.

METHODS

We tested the effects of a gamma-secretase inhibitor (GSI) that blocks Notch signaling in PDAC cell lines and a genetically engineered mouse model of PDAC (Kras p53 L/+ mice).

RESULTS

Notch signaling was activated in PDAC precursors and advanced tumors. The GSI inhibited the growth of premalignant pancreatic duct-derived cells in a Notch-dependent manner. Additionally, in a panel of over 400 human solid tumor-derived cell lines, PDAC cells, as a group, were more sensitive to the GSI than any other tumor type. Finally, the GSI completely inhibited tumor development in the genetically engineered model of invasive PDAC (P < .005, chi2 test; compared with mice exposed to vehicle).

CONCLUSIONS

These results suggest that Notch signaling is required for PDAC progression. Pharmacologic targeting of this pathway offers therapeutic potential in this treatment-refractory malignancy.

Acute pancreatitis markedly accelerates pancreatic cancer progression in mice expressing oncogenic Kras

Chronic pancreatitis increases by 16-fold the risk of developing pancreatic ductal adenocarcinoma (PDAC), one of the deadliest human cancers. It also appears to accelerate cancer progression in genetically engineered mouse models. We now report that in a mouse model where oncogenic Kras is activated in all pancreatic cell types, two brief episodes of acute pancreatitis caused rapid PanIN progression and accelerated pancreatic cancer development. Thus, a brief inflammatory insult to the pancreas, when occurring in the context of oncogenic Kras(G12D), can initiate a cascade of events that dramatically enhances the risk for pancreatic malignant transformation.

Ligand-dependent Notch signaling is involved in tumor initiation and tumor maintenance in pancreatic cancer

PURPOSE

Aberrant activation of the Notch signaling pathway is commonly observed in human pancreatic cancer, although the mechanism(s) for this activation has not been elucidated.

EXPERIMENTAL DESIGN

A panel of 20 human pancreatic cancer cell lines was profiled for the expression of Notch pathway-related ligands, receptors, and target genes. Disruption of intracellular Notch signaling, either genetically by RNA interference targeting NOTCH1 or pharmacologically by means of the gamma-secretase inhibitor GSI-18, was used for assessing requirement of Notch signaling in pancreatic cancer initiation and maintenance.

RESULTS

Striking overexpression of Notch ligand transcripts was detectable in the vast majority of pancreatic cancer cell lines, most prominently JAGGED2 (18 of 20 cases, 90%) and DLL4 (10 of 20 cases, 50%). In two cell lines, genomic amplification of the DLL3 locus was observed, mirrored by overexpression of DLL3 transcripts. In contrast, coding region mutations of NOTCH1 or NOTCH2 were not observed. Genetic and pharmacologic inhibition of Notch signaling mitigated anchorage-independent growth in pancreatic cancer cells, confirming that sustained Notch activation is a requirement for pancreatic cancer maintenance. Further, transient pretreatment of pancreatic cancer cells with GSI-18 resulted in depletion in the proportion of tumor-initiating aldehyde dehydrogenase-expressing subpopulation and was associated with inhibition of colony formation in vitro and xenograft engraftment in vivo, underscoring a requirement for the Notch-dependent aldehyde dehydrogenase-expressing cells in pancreatic cancer initiation.

CONCLUSIONS

Our studies confirm that Notch activation is almost always ligand dependent in pancreatic cancer, and inhibition of Notch signaling is a promising therapeutic strategy in this malignancy.

Notch and Kras reprogram pancreatic acinar cells to ductal intraepithelial neoplasia

Efforts to model pancreatic cancer in mice have focused on mimicking genetic changes found in the human disease, particularly the activating KRAS mutations that occur in pancreatic tumors and their putative precursors, pancreatic intraepithelial neoplasia (PanIN). Although activated mouse Kras mutations induce PanIN lesions similar to those of human, only a small minority of cells that express mutant Kras go on to form PanINs. The basis for this selective response is unknown, and it is similarly unknown what cell types in the mature pancreas actually contribute to PanINs. One clue comes from the fact that PanINs, unlike most cells in the adult pancreas, exhibit active Notch signaling. We hypothesize that Notch, which inhibits differentiation in the embryonic pancreas, contributes to PanIN formation by abrogating the normal differentiation program of tumor-initiating cells. Through conditional expression in the mouse pancreas, we find dramatic synergy between activated Notch and Kras in inducing PanIN formation. Furthermore, we find that Kras activation in mature acinar cells induces PanIN lesions identical to those seen upon ubiquitous Kras activation, and that Notch promotes both initiation and dysplastic progression of these acinar-derived PanINs, albeit short of invasive adenocarcinoma. At the cellular level, Notch/Kras coactivation promotes rapid reprogramming of acinar cells to a duct-like phenotype, providing an explanation for how a characteristically ductal tumor can arise from nonductal acinar cells.

Molecular basis for pancreatitis

PURPOSE OF REVIEW

This timely review will focus on clinical and basic science studies that have greatly advanced our knowledge of the molecular mechanisms of both acute pancreatitis and chronic pancreatitis over the last year.

RECENT FINDINGS

Animal models of both severe acute pancreatitis and chronic pancreatitis have recently been developed. Several unexpected protective mechanisms, mediated by the protease activated receptor 2 and heat shock protein 70, have been described. A genetic study suggested that polymorphisms in toll-like receptor-4 might affect the risk of developing infections in acute pancreatitis. Studies of chronic pancreatitis have shown that specific neural receptors, transient receptor potential vanilloid subtype 1, mediate pain responses in a model of chronic pancreatitis. The pancreatic zymogen, chymotrypsin C, can degrade pathologically activated trypsin in the acinar cell. Inactivating mutations in chymotrypsin C have been reported to predispose to the development of chronic pancreatitis, especially in those who are prone to alcohol abuse.

SUMMARY

The implications of the last year's findings are widespread. Improved animal models of acute pancreatitis and chronic pancreatitis will be critical for performing pilot studies of therapy. A greater understanding of genetic factors and pain responses could lead to potential treatments. This review will first discuss issues related to acute pancreatitis, and then conclude with studies most relevant to chronic disease.

Receptor-mediated signal transduction pathways and the regulation of pancreatic acinar cell function

PURPOSE OF REVIEW

Recent studies on pancreatic acinar cell function have led to a more detailed understanding of the signal transduction mechanisms regulating digestive enzyme synthesis and secretion as well as pancreatic growth. This review identifies and puts into context these recent studies, which further understanding in these areas.

RECENT FINDINGS

Receptors present on acinar cells, particularly those for cholecystokinin and secretin, have been better characterized as to the molecular nature of the ligand-receptor interaction. Other reports have described the receptors for natriuretic peptides and fibroblast growth factor on acini. Intracellular Ca(2+) signaling remains at the center of stimulus secretion coupling and its regulation by inositol 1,4,5-trisphosphate, nicotinic acid adenine dinucleotide phosphate and cyclic ADP-ribose has been further defined. Work downstream of intracellular mediators has focused on molecular mechanisms of exocytosis particularly involving small G proteins, soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and cytoskeletal proteins. Considerable progress has been made defining the complex in acinar cells and its regulation. In addition to secretion, recent studies have further defined the regulation of pancreatic growth both in adaptive regulation to diet and hormones, particularly cholecystokinin, and in the regeneration that occurs after pancreatitis or partial pancreatectomy. This regulation involves calcineurin-nuclear factor of activated T cells, mammalian target of rapamycin, mitogen-activated protein kinase, Notch signaling pathways as well as various tyrosine kinases.

SUMMARY

Understanding the mechanisms that regulate pancreatic acinar cell function is contributing to our knowledge of normal pancreatic function and alterations in diseases such as pancreatitis and pancreatic cancer.

Conditional ablation of Notch signaling in pancreatic development

The role of the Notch signaling members Notch1, Notch2 and Rbpj in exocrine pancreatic development is not well defined. We therefore analyzed conditional pancreas-specific Rbpj and combined Notch1/Notch2 knockout mice using Ptf1a(+/Cre(ex1)) mice crossed with floxed Rbpj or Notch1/Notch2 mice. Mice were analyzed at different embryonic stages for pancreatic exocrine and endocrine development. The absence of Rbpj in pancreatic progenitor cells impaired exocrine pancreas development up to embryonic day 18.5 and led to premature differentiation of pancreatic progenitors into endocrine cells. In Rbpj-deficient pancreata, amylase-expressing acini and islets formed during late embryonic and postnatal development, suggesting an essential role of Rbpj in early but not late development. Contrary to this severe phenotype, the concomitant inactivation of Notch1 and Notch2 only moderately disturbed the proliferation of pancreatic epithelial cells during early embryonic development, and did not inhibit pancreatic development. Our results show that, in contrast to Rbpj, Notch1 and Notch2 are not essential for pancreatogenesis. These data favor a Notch-independent role of Rbpj in the development of the exocrine pancreas. Furthermore, our findings suggest that in late stages of pancreatic development exocrine cell differentiation and maintenance are independent of Rbpj.