Title: Pancreatic Cancer: Basic and Clinical Aspects
Abstract: More than 30,000 people develop pancreatic adenocarcinoma each year in the United States, and almost all are expected to die from the disease.1Jemal A. Tiwari R.C. Murray T. Ghafoor A. Samuels A. Ward E. Feuer E.J. Thun M.J. Cancer statistics, 2004.CA Cancer J Clin. 2004; 54: 8-29Crossref PubMed Google Scholar The 5-year survival rate is <5%, and of the 10% of patients with resectable disease, only approximately 1 in 5 survive for 5 years. Despite tremendous scientific efforts and much gain in knowledge of the basic cellular events in pancreatic ductal adenocarcinoma (PDAC), survival rates have not changed much during the last 20 years, and our understanding of the different aspects of this devastating disease, such as initiation, progression, and metastasis, remains incomplete. In this review, we discuss different aspects of genetic alterations in precursor lesions of PDAC, mechanisms of intrinsic tumor suppression, the development of animal models, and current aspects of treatment. PDAC shows a characteristic pattern of genetic signature lesions involving mutations of K-RAS, CDKN2A, TP53, BRCA2, and MADH4/SMAD4/DPC4 at different stages,2Hruban R.H. Adsay N.V. Albores-Saavedra J. Compton C. Garrett E.S. Goodman S.N. Kern S.E. Klimstra D.S. Kloppel G. Longnecker D.S. Luttges J. Offerhaus G.J. Pancreatic intraepithelial neoplasia a new nomenclature and classification system for pancreatic duct lesions.Am J Surg Pathol. 2001; 25: 579-586Crossref PubMed Scopus (563) Google Scholar thereby supporting the paradigm of accumulation of multistep genetic alterations in the development of carcinoma.2Hruban R.H. Adsay N.V. Albores-Saavedra J. Compton C. Garrett E.S. Goodman S.N. Kern S.E. Klimstra D.S. Kloppel G. Longnecker D.S. Luttges J. Offerhaus G.J. Pancreatic intraepithelial neoplasia a new nomenclature and classification system for pancreatic duct lesions.Am J Surg Pathol. 2001; 25: 579-586Crossref PubMed Scopus (563) Google Scholar In addition to multiple studies investigating genetic alterations in developed PDAC, the precursor lesions and putative cells of origin have attracted many researchers. In classic studies by Sommers et al3Sommers S.C. Murphy S.A. Warren S. Pancreatic duct hyperplasia and cancer.Gastroenterology. 1954; 27: 629-640PubMed Google Scholar and Cubilla and Fitzgerald,4Cubilla A.L. Fitzgerald P.J. Morphological patterns of primary nonendocrine human pancreas carcinoma.Cancer Res. 1975; 35: 2234-2248PubMed Google Scholar, 5Cubilla A.L. Fitzgerald P.J. Morphological lesions associated with human primary invasive nonendocrine pancreas cancer.Cancer Res. 1976; 36: 2690-2698PubMed Google Scholar increased numbers of abnormal ductal structures (papillary hyperplasia) were observed in patients with PDAC compared with patients with noncancerous pancreata. Because transition of papillary hyperplasia to invasive PDAC was noted in some cases, these early lesions were thought to resemble precursor lesions. In 1994, these hyperplastic noninvasive lesions were proposed to be named pancreatic intraepithelial neoplasia (PanIN) by Klimstra and Longnecker,6Klimstra D.S. Longnecker D.S. K-ras mutations in pancreatic ductal proliferative lesions.Am J Pathol. 1994; 145: 1547-1550PubMed Google Scholar and in a Pancreatic Cancer Think Tank in 1999, a classification system for PanINs based on morphological features was developed.2Hruban R.H. Adsay N.V. Albores-Saavedra J. Compton C. Garrett E.S. Goodman S.N. Kern S.E. Klimstra D.S. Kloppel G. Longnecker D.S. Luttges J. Offerhaus G.J. Pancreatic intraepithelial neoplasia a new nomenclature and classification system for pancreatic duct lesions.Am J Surg Pathol. 2001; 25: 579-586Crossref PubMed Scopus (563) Google Scholar, 7Hruban R.H. Takaori K. Klimstra D.S. Adsay N.V. Albores-Saavedra J. Biankin A.V. Biankin S.A. Compton C. Fukushima N. Furukawa T. Goggins M. Kato Y. Kloppel G. Longnecker D.S. Luttges J. Maitra A. Offerhaus G.J. Shimizu M. Yonezawa S. An illustrated consensus on the classification of pancreatic intraepithelial neoplasia and intraductal papillary mucinous neoplasms.Am J Surg Pathol. 2004; 28: 977-987Crossref PubMed Scopus (445) Google Scholar The earliest precursor lesions, PanIN-1A and -1B, are characterized by elongation of ductal cells with abundant mucin production and, in the case of PanIN-1B, with papillary instead of flat architecture (Figure 1). These lesions are found in up to 40% of nonmalignant pancreata in patients older than 50 years of age.7Hruban R.H. Takaori K. Klimstra D.S. Adsay N.V. Albores-Saavedra J. Biankin A.V. Biankin S.A. Compton C. Fukushima N. Furukawa T. Goggins M. Kato Y. Kloppel G. Longnecker D.S. Luttges J. Maitra A. Offerhaus G.J. Shimizu M. Yonezawa S. An illustrated consensus on the classification of pancreatic intraepithelial neoplasia and intraductal papillary mucinous neoplasms.Am J Surg Pathol. 2004; 28: 977-987Crossref PubMed Scopus (445) Google Scholar In addition to mutations in proto-oncogenes and tumor suppressors, these lesions are characterized by autocrine epidermal growth factor (EGF) family signaling with overexpression of ligands such as transforming growth factor (TGF)–α and receptors such as EGF receptor (EGFR) and ERBB2 and ERBB3. Because activation of RAS signaling can be identified in early precursor lesions and even in morphologically normal duct cells, aberrant RAS signaling is thought to play a role in initiating pancreatic carcinogenesis. As will be described in further detail, activation of RAS signaling in the murine pancreas indeed leads to the formation of PanIN lesions and the development of metastatic PDAC, thus underscoring the fundamental role of this pathway in pancreatic carcinogenesis. As PanIN lesions progress, they acquire moderate (PanIN-2) and eventually severe nuclear abnormalities with abnormal mitoses and budding of cells into the lumen (PanIN-3, formerly known as carcinoma in situ). Whereas PanIN-3 lesions are seen in <5% of noncancerous pancreata, they are present in 30%–50% of pancreata with invasive PDAC. This suggests that high-grade PanIN lesions are precursors of invasive pancreatic cancer, and this assumption is underscored by progressive genetic alterations.8Hruban R.H. Goggins M. Parsons J. Kern S.E. Progression model for pancreatic cancer.Clin Cancer Res. 2000; 6: 2969-2972PubMed Google Scholar The PanIN classification formed the basis for molecular analyses, including the genetic alterations mentioned previously that helped to define a progression model for pancreatic neoplasia (Figure 1). Several alterations in oncogenes and tumor-suppressor genes have been detected in pancreatic cancer specimens and cell lines. Here we focus on molecular events under the scope of cell intrinsic tumor suppression. Therefore, not all genetic (such as BRCA2 and SMAD4 mutations) and epigenetic (such as the prosurvival PI3K/AKT pathway) disturbances of pancreatic cancer are mentioned. Furthermore, the familial forms of pancreatic cancer are beyond the scope of this article. Here we refer to some excellent recent reviews.9Hruban R.H. Petersen G.M. Goggins M. Tersmette A.C. Offerhaus G.J. Falatko F. Yeo C.J. Kern S.E. Familial pancreatic cancer.Ann Oncol. 1999; 10: 69-73Crossref PubMed Google Scholar, 10Bardeesy N. DePinho R.A. Pancreatic cancer biology and genetics.Nat Rev Cancer. 2002; 2: 897-909Crossref PubMed Scopus (500) Google Scholar, 11Hansel D.E. Kern S.E. Hruban R.H. Molecular pathogenesis of pancreatic cancer.Annu Rev Genomics Hum Genet. 2003; 4: 237-256Crossref PubMed Scopus (74) Google Scholar, 12Li D. Xie K. Wolff R. Abbruzzese J.L. Pancreatic cancer.Lancet. 2004; 363: 1049-1057Abstract Full Text Full Text PDF PubMed Scopus (767) Google Scholar One prerequisite of mammalian cancers are gene mutations that drive unrestrained cell-cycle progression. These oncogenic mutations are controverted by intrinsic fail-safe programs, oncogene-induced apoptosis, and senescence, inhibiting uncontrolled cellular proliferation. Hence, the second prerequisite of mammalian cancers is the deactivation of these fail-safe programs.13Schmitt C.A. Senescence, apoptosis and therapy—cutting the lifelines of cancer.Nat Rev Cancer. 2003; 3: 286-295Crossref PubMed Google Scholar, 14Lowe S.W. Cepero E. Evan G. Intrinsic tumour suppression.Nature. 2004; 432: 307-315Crossref PubMed Scopus (583) Google Scholar Analysis of PDAC and PanIN lesions and evidence from genetically defined murine pancreatic cancer models show that K-RAS is the key oncogene in pancreatic cancer. A codon 12 mutation of this oncogene is found in almost all PDAC and in up to 40% of the earliest premalignant lesions.15Almoguera C. Shibata D. Forrester K. Martin J. Arnheim N. Perucho M. Most human carcinomas of the exocrine pancreas contain mutant c-K-ras genes.Cell. 1988; 53: 549-554Abstract Full Text PDF PubMed Google Scholar, 16Caldas C. Kern S.E. K-ras mutation and pancreatic adenocarcinoma.Int J Pancreatol. 1995; 18: 1-6PubMed Google Scholar, 17Terhune P.G. Phifer D.M. Tosteson T.D. Longnecker D.S. K-ras mutation in focal proliferative lesions of human pancreas.Cancer Epidemiol Biomarkers Prev. 1998; 7: 515-521PubMed Google Scholar, 18Apple S.K. Hecht J.R. Lewin D.N. Jahromi S.A. Grody W.W. Nieberg R.K. Immunohistochemical evaluation of K-ras, p53, and HER-2/neu expression in hyperplastic, dysplastic, and carcinomatous lesions of the pancreas evidence for multistep carcinogenesis.Hum Pathol. 1999; 30: 123-129Abstract Full Text PDF PubMed Scopus (84) Google Scholar K-RAS belongs to a group of small guanosine triphosphate-binding proteins that mediate pleiotropic effects including cell proliferation, survival, and migration.19Shields J.M. Pruitt K. McFall A. Shaub A. Der C.J. Understanding Ras ‘it ain’t over ‘til it’s over’.Trends Cell Biol. 2000; 10: 147-154Abstract Full Text Full Text PDF PubMed Scopus (510) Google Scholar The fact that oncogenic K-RAS is found frequently in benign lesions of the pancreas and the low risk of progression to malignancy in the absence of cooperating genetic events suggest an properly working fail-safe program against oncogenic K-RAS in the pancreas.20Tada M. Omata M. Kawai S. Saisho H. Ohto M. Saiki R.K. Sninsky J.J. Detection of ras gene mutations in pancreatic juice and peripheral blood of patients with pancreatic adenocarcinoma.Cancer Res. 1993; 53: 2472-2474PubMed Google Scholar, 21Yanagisawa A. Ohtake K. Ohashi K. Hori M. Kitagawa T. Sugano H. Kato Y. Frequent c-Ki-ras oncogene activation in mucous cell hyperplasias of pancreas suffering from chronic inflammation.Cancer Res. 1993; 53: 953-956PubMed Google Scholar, 22Tada M. Ohashi M. Shiratori Y. Okudaira T. Komatsu Y. Kawabe T. Yoshida H. Machinami R. Kishi K. Omata M. Analysis of K-ras gene mutation in hyperplastic duct cells of the pancreas without pancreatic disease.Gastroenterology. 1996; 110: 227-231Abstract Full Text PDF PubMed Scopus (230) Google Scholar, 23Moskaluk C.A. Hruban R.H. Kern S.E. p16 and K-ras gene mutations in the intraductal precursors of human pancreatic adenocarcinoma.Cancer Res. 1997; 57: 2140-2143PubMed Google Scholar Although this has not been studied in detail in the pancreas, experimental data provide evidence that senescence is the main fail-safe program triggered by oncogenic K-RAS. Senescence, a permanent growth/cell-cycle arrest that occurs after extended periods of cell division, oxidative stress, or activated oncogenes, is clearly induced by K-RAS in nonimmortal human and mouse cells.24Ishikawa F. Cellular senescence, an unpopular yet trustworthy tumor suppressor mechanism.Cancer Sci. 2003; 94: 944-947Crossref PubMed Scopus (21) Google Scholar Senescent cells are characterized by an active metabolic state and altered morphology, physiology, and genetic signature. These cells typically show a senescence-associated β-galactosidase activity and are unable to express the genes needed for cell-cycle progression, even in a mitogenic environment.25Dimri G.P. Hara E. Campisi J. Regulation of two E2F-related genes in presenescent and senescent human fibroblasts.J Biol Chem. 1994; 269: 16180-16186Abstract Full Text PDF PubMed Google Scholar, 26Dimri G.P. Lee X. Basile G. Acosta M. Scott G. Roskelley C. Medrano E.E. Linskens M. Rubelj I. Pereira-Smith O. et al.A biomarker that identifies senescent human cells in culture and in aging skin in vivo.Proc Natl Acad Sci U S A. 1995; 92: 9363-9367Crossref PubMed Scopus (2937) Google Scholar, 27Dimri G.P. Testori A. Acosta M. Campisi J. Replicative senescence, aging and growth-regulatory transcription factors.Biol Signals. 1996; 5: 154-162Crossref PubMed Google Scholar Two cellular systems, the ARF-p53 and the p16INK4A-RB (retinoblastoma) tumor-suppressor system, are critically involved in the molecular regulation of oncogene-induced premature senescence, whereby the relative contribution of each system differs significantly among species and tissues.28Lowe S.W. Sherr C.J. Tumor suppression by Ink4a-Arf progress and puzzles.Curr Opin Genet Dev. 2003; 13: 77-83Crossref PubMed Scopus (453) Google Scholar Whereas in rodent cells an intact ARF-p53 system is required for RAS-induced senescence, human senescence relies more on the p16INK4A-RB pathway.29Serrano M. Lin A.W. McCurrach M.E. Beach D. Lowe S.W. Oncogenic ras provokes premature cell senescence associated with accumulation of p53 and p16INK4a.Cell. 1997; 88: 593-602Abstract Full Text Full Text PDF PubMed Scopus (2327) Google Scholar, 30Kamijo T. Zindy F. Roussel M.F. Quelle D.E. Downing J.R. Ashmun R.A. Grosveld G. Sherr C.J. Tumor suppression at the mouse INK4a locus mediated by the alternative reading frame product p19ARF.Cell. 1997; 91: 649-659Abstract Full Text Full Text PDF PubMed Scopus (1385) Google Scholar, 31Palmero I. Pantoja C. Serrano M. p19ARF links the tumour suppressor p53 to Ras.Nature. 1998; 395: 125-126Crossref PubMed Scopus (444) Google Scholar Both senescence-regulating cellular systems are influenced through the products of the cyclin-dependent kinase inhibitor 2A (CDKN2A) locus on chromosome 9p21. This locus codes for 2 tumor-suppressor genes, p16INK4A and p14ARF (p19Arf in mice). INK4A and ARF are generated by the use of a different first exon and an alternative reading frame of the second exon. The INK4A protein inhibits cell-cycle progression as an inhibitor of the cyclin D/cyclin-dependent kinase 4/6 complex, indirectly influencing the activation status of the RB protein. In contrast, ARF activates the tumor-suppressor p53 by inhibiting its negative regulator Mdm2.28Lowe S.W. Sherr C.J. Tumor suppression by Ink4a-Arf progress and puzzles.Curr Opin Genet Dev. 2003; 13: 77-83Crossref PubMed Scopus (453) Google Scholar Biallelic inactivation of the INK4A locus occurs in up to 95% of ductal pancreatic cancers, thus suggesting that the overcoming of the p16INK4A-RB fail-safe program is a sine qua non in human ductal pancreatic cancer.32Caldas C. Hahn S.A. da Costa L.T. Redston M.S. Schutte M. Seymour A.B. Weinstein C.L. Hruban R.H. Yeo C.J. Kern S.E. Frequent somatic mutations and homozygous deletions of the p16 (MTS1) gene in pancreatic adenocarcinoma.Nat Genet. 1994; 8: 27-32Crossref PubMed Scopus (796) Google Scholar, 33Rozenblum E. Schutte M. Goggins M. Hahn S.A. Panzer S. Zahurak M. Goodman S.N. Sohn T.A. Hruban R.H. Yeo C.J. Kern S.E. Tumor-suppressive pathways in pancreatic carcinoma.Cancer Res. 1997; 57: 1731-1734PubMed Google Scholar, 34Schutte M. Hruban R.H. Geradts J. Maynard R. Hilgers W. Rabindran S.K. Moskaluk C.A. Hahn S.A. Schwarte-Waldhoff I. Schmiegel W. Baylin S.B. Kern S.E. Herman J.G. Abrogation of the Rb/p16 tumor-suppressive pathway in virtually all pancreatic carcinomas.Cancer Res. 1997; 57: 3126-3130PubMed Google Scholar, 35Wilentz R.E. Geradts J. Maynard R. Offerhaus G.J. Kang M. Goggins M. Yeo C.J. Kern S.E. Hruban R.H. Inactivation of the p16 (INK4A) tumor-suppressor gene in pancreatic duct lesions loss of intranuclear expression.Cancer Res. 1998; 58: 4740-4744PubMed Google Scholar Sporadic tumors inactivate INK4A by biallelic deletion, promoter methylation, or, seldom, by intragenic mutation.32Caldas C. Hahn S.A. da Costa L.T. Redston M.S. Schutte M. Seymour A.B. Weinstein C.L. Hruban R.H. Yeo C.J. Kern S.E. Frequent somatic mutations and homozygous deletions of the p16 (MTS1) gene in pancreatic adenocarcinoma.Nat Genet. 1994; 8: 27-32Crossref PubMed Scopus (796) Google Scholar, 34Schutte M. Hruban R.H. Geradts J. Maynard R. Hilgers W. Rabindran S.K. Moskaluk C.A. Hahn S.A. Schwarte-Waldhoff I. Schmiegel W. Baylin S.B. Kern S.E. Herman J.G. Abrogation of the Rb/p16 tumor-suppressive pathway in virtually all pancreatic carcinomas.Cancer Res. 1997; 57: 3126-3130PubMed Google Scholar Because mutations that selectively target ARF are rare and germline or sporadic mutations in INK4A were identified that omitted ARF, INK4A is the more important tumor suppressor in humans.33Rozenblum E. Schutte M. Goggins M. Hahn S.A. Panzer S. Zahurak M. Goodman S.N. Sohn T.A. Hruban R.H. Yeo C.J. Kern S.E. Tumor-suppressive pathways in pancreatic carcinoma.Cancer Res. 1997; 57: 1731-1734PubMed Google Scholar, 36Liu L. Dilworth D. Gao L. Monzon J. Summers A. Lassam N. Hogg D. Mutation of the CDKN2A 5′ UTR creates an aberrant initiation codon and predisposes to melanoma.Nat Genet. 1999; 21: 128-132Crossref PubMed Scopus (144) Google Scholar, 37Lal G. Liu L. Hogg D. Lassam N.J. Redston M.S. Gallinger S. Patients with both pancreatic adenocarcinoma and melanoma may harbor germline CDKN2A mutations.Genes Chromosomes Cancer. 2000; 27: 358-361Crossref PubMed Scopus (34) Google Scholar, 38Randerson-Moor J.A. Harland M. Williams S. Cuthbert-Heavens D. Sheridan E. Aveyard J. Sibley K. Whitaker L. Knowles M. Bishop J.N. Bishop D.T. A germline deletion of p14(ARF) but not CDKN2A in a melanoma-neural system tumour syndrome family.Hum Mol Genet. 2001; 10: 55-62Crossref PubMed Google Scholar INK4A plays a key role in human senescence. The expression of p16INK4A is induced during cellular senescence, and overexpression of p16INK4A induces a senescence-like growth arrest.39Alcorta D.A. Xiong Y. Phelps D. Hannon G. Beach D. Barrett J.C. Involvement of the cyclin-dependent kinase inhibitor p16 (INK4a) in replicative senescence of normal human fibroblasts.Proc Natl Acad Sci U S A. 1996; 93: 13742-13747Crossref PubMed Scopus (529) Google Scholar, 40Hara E. Smith R. Parry D. Tahara H. Stone S. Peters G. Regulation of p16CDKN2 expression and its implications for cell immortalization and senescence.Mol Cell Biol. 1996; 16: 859-867Crossref PubMed Google Scholar, 41Kato D. Miyazawa K. Ruas M. Starborg M. Wada I. Oka T. Sakai T. Peters G. Hara E. Features of replicative senescence induced by direct addition of antennapedia-p16INK4A fusion protein to human diploid fibroblasts.FEBS Lett. 1998; 427: 203-208Abstract Full Text Full Text PDF PubMed Scopus (63) Google Scholar, 42McConnell B.B. Starborg M. Brookes S. Peters G. Inhibitors of cyclin-dependent kinases induce features of replicative senescence in early passage human diploid fibroblasts.Curr Biol. 1998; 8: 351-354Abstract Full Text Full Text PDF PubMed Google Scholar How does oncogenic RAS induce p16INK4A in cellular senescence? Here, one crucial pathway identified is the RAS/MEK signal transduction pathway, whereby the ability of RAS to induce senescence depends on MKK3/6-activated p38.43Lin A.W. Barradas M. Stone J.C. van Aelst L. Serrano M. Lowe S.W. Premature senescence involving p53 and p16 is activated in response to constitutive MEK/MAPK mitogenic signaling.Genes Dev. 1998; 12: 3008-3019Crossref PubMed Google Scholar, 44Zhu J. Woods D. McMahon M. Bishop J.M. Senescence of human fibroblasts induced by oncogenic Raf.Genes Dev. 1998; 12: 2997-3007Crossref PubMed Google Scholar, 45Wang W. Chen J.X. Liao R. Deng Q. Zhou J.J. Huang S. Sun P. Sequential activation of the MEK-extracellular signal-regulated kinase and MKK3/6-p38 mitogen-activated protein kinase pathways mediates oncogenic ras-induced premature senescence.Mol Cell Biol. 2002; 22: 3389-3403Crossref PubMed Scopus (185) Google Scholar Downstream of this signaling pathway are the transcription factors Ets1 and Ets2, which directly target the p16INK4A promoter.46Ohtani N. Zebedee Z. Huot T.J. Stinson J.A. Sugimoto M. Ohashi Y. Sharrocks A.D. Peters G. Hara E. Opposing effects of Ets and Id proteins on p16INK4a expression during cellular senescence.Nature. 2001; 409: 1067-1070Crossref PubMed Scopus (379) Google Scholar RB is indirectly activated by INK4A, but how RB promotes senescence is less clear. Together with the related p107 and p130 proteins, RB represses the E2F transcription factors, which are needed to activate genes important for the G1 to S phase transition. In quiescent cells, RB-family members recruit histone deacetylases to E2F-dependent promoters, and this leads to the deacetylation of nearby histones and gene repression. At the boundary to the S phase of the cell cycle, RB becomes inactivated by phosphorylation through the cyclin D- and E-dependent kinases, thus allowing E2F to recruit histone acetylases and gene activation.47Trimarchi J.M. Lees J.A. Sibling rivalry in the E2F family.Nat Rev Mol Cell Biol. 2002; 3: 11-20Crossref PubMed Scopus (635) Google Scholar In rodent cells, the targeted inactivation of all 3 RB family members prevents senescence, and this shows the importance of the RB-controlled system for the induction of senescence.48Dannenberg J.H. van Rossum A. Schuijff L. te Riele H. Ablation of the retinoblastoma gene family deregulates G(1) control causing immortalization and increased cell turnover under growth-restricting conditions.Genes Dev. 2000; 14: 3051-3064Crossref PubMed Scopus (258) Google Scholar, 49Sage J. Mulligan G.J. Attardi L.D. Miller A. Chen S. Williams B. Theodorou E. Jacks T. Targeted disruption of the three Rb-related genes leads to loss of G(1) control and immortalization.Genes Dev. 2000; 14: 3037-3050Crossref PubMed Scopus (380) Google Scholar In contrast to the triple knockout, Rb−/− mouse embryonic fibroblasts undergo senescence in culture, and this senescence seems to be mediated by the compensatory up-regulation of p107 in these cells. In an attempt to mimic acute mutation of Rb, it was recently shown that loss of Rblox/lox in senescent cells by Cre-lox-mediated recombination overcomes cell-cycle arrest induced by oncogenic RAS, thus showing the need to inactivate Rb function to overcome senescence.50Sage J. Miller A.L. Perez-Mancera P.A. Wysocki J.M. Jacks T. Acute mutation of retinoblastoma gene function is sufficient for cell cycle re-entry.Nature. 2003; 424: 223-228Crossref PubMed Scopus (306) Google Scholar Molecular RB is involved in the stable repression of E2F-responsive promoters, such as cyclin A and proliferating cell nuclear antigen, under senescent conditions. Potentially, RB interacts with histone methyltransferases and HP1, proteins involved in heterochromatin formation, thereby altering the chromatin structure at E2F target genes to a repressed state.51Narita M. Nunez S. Heard E. Lin A.W. Hearn S.A. Spector D.L. Hannon G.J. Lowe S.W. Rb-mediated heterochromatin formation and silencing of E2F target genes during cellular senescence.Cell. 2003; 113: 703-716Abstract Full Text Full Text PDF PubMed Scopus (728) Google Scholar The importance of the RB-p16INK4A pathway in ductal pancreatic cancer is underscored by the observation that every tumor investigated harbors functional inactive RB.33Rozenblum E. Schutte M. Goggins M. Hahn S.A. Panzer S. Zahurak M. Goodman S.N. Sohn T.A. Hruban R.H. Yeo C.J. Kern S.E. Tumor-suppressive pathways in pancreatic carcinoma.Cancer Res. 1997; 57: 1731-1734PubMed Google Scholar, 34Schutte M. Hruban R.H. Geradts J. Maynard R. Hilgers W. Rabindran S.K. Moskaluk C.A. Hahn S.A. Schwarte-Waldhoff I. Schmiegel W. Baylin S.B. Kern S.E. Herman J.G. Abrogation of the Rb/p16 tumor-suppressive pathway in virtually all pancreatic carcinomas.Cancer Res. 1997; 57: 3126-3130PubMed Google Scholar In addition to the RB-p16INK4A pathway, the ARF-p53 pathway influences the senescent program. The guardian of the genome—p53—is a sequence-specific transcription factor that regulates growth checkpoints protecting normal cells against genomic rearrangements or the accumulation of mutations. p53 is activated by extracellular stress, such as γ-irradiation, or intracellular stress, such as oncogene activation. Once activated, p53 is involved in checkpoint control of the cell cycle, induction of apoptosis, or the senescent program.52Vogelstein B. Lane D. Levine A.J. Surfing the p53 network.Nature. 2000; 408: 307-310Crossref PubMed Scopus (3489) Google Scholar The TP53 gene is mutated in more than 50% of ductal pancreatic adenocarcinomas. The mutations, especially missense mutations in sequences coding for the DNA-binding domain of p53, are often accompanied by loss of the wild-type allele. Loss of heterozygosity of TP53 is detected in PanIN-3 lesions; therefore, impaired p53 function occurs late in the progression to pancreatic cancer.18Apple S.K. Hecht J.R. Lewin D.N. Jahromi S.A. Grody W.W. Nieberg R.K. Immunohistochemical evaluation of K-ras, p53, and HER-2/neu expression in hyperplastic, dysplastic, and carcinomatous lesions of the pancreas evidence for multistep carcinogenesis.Hum Pathol. 1999; 30: 123-129Abstract Full Text PDF PubMed Scopus (84) Google Scholar, 53Barton C.M. Staddon S.L. Hughes C.M. Hall P.A. O’Sullivan C. Kloppel G. Theis B. Russell R.C. Neoptolemos J. Williamson R.C. et al.Abnormalities of the p53 tumour suppressor gene in human pancreatic cancer.Br J Cancer. 1991; 64: 1076-1082Crossref PubMed Google Scholar, 54Boschman C.R. Stryker S. Reddy J.K. Rao M.S. Expression of p53 protein in precursor lesions and adenocarcinoma of human pancreas.Am J Pathol. 1994; 145: 1291-1295PubMed Google Scholar, 55DiGiuseppe J.A. Hruban R.H. Goodman S.N. Polak M. van den Berg F.M. Allison D.C. Cameron J.L. Offerhaus G.J. Overexpression of p53 protein in adenocarcinoma of the pancreas.Am J Clin Pathol. 1994; 101: 684-688Crossref PubMed Google Scholar, 56Redston M.S. Caldas C. Seymour A.B. Hruban R.H. da Costa L. Yeo C.J. Kern S.E. p53 mutations in pancreatic carcinoma and evidence of common involvement of homocopolymer tracts in DNA microdeletions.Cancer Res. 1994; 54: 3025-3033PubMed Google Scholar In fibroblasts, the initiation of RAS-induced senescence clearly depends on functional p53, but the maintenance of the senescence state needs a robust INK4A response.29Serrano M. Lin A.W. McCurrach M.E. Beach D. Lowe S.W. Oncogenic ras provokes premature cell senescence associated with accumulation of p53 and p16INK4a.Cell. 1997; 88: 593-602Abstract Full Text Full Text PDF PubMed Scopus (2327) Google Scholar, 43Lin A.W. Barradas M. Stone J.C. van Aelst L. Serrano M. Lowe S.W. Premature senescence involving p53 and p16 is activated in response to constitutive MEK/MAPK mitogenic signaling.Genes Dev. 1998; 12: 3008-3019Crossref PubMed Google Scholar, 57Stein G.H. Drullinger L.F. Soulard A. Dulic V. Differential roles for cyclin-dependent kinase inhibitors p21 and p16 in the mechanisms of senescence and differentiation in human fibroblasts.Mol Cell Biol. 1999; 19: 2019-2117Google Scholar, 58Lin A.W. Lowe S.W. Oncogenic ras activates the ARF-p53 pathway to suppress epithelial cell transformation.Proc Natl Acad Sci U S A. 2001; 98: 5025-5030Crossref PubMed Scopus (142) Google Scholar, 59Ferbeyre G. de Stanchina E. Lin A.W. Querido E. McCurrach M.E. Hannon G.J. Lowe S.W. Oncogenic ras and p53 cooperate to induce cellular senescence.Mol Cell Biol. 2002; 22: 3497-3508Crossref PubMed Scopus (156) Google Scholar, 60Beausejour C.M. Krtolica A. Galimi F. Narita M. Lowe S.W. Yaswen P. Campisi J. Reversal of human cellular senescence roles of the p53 and p16 pathways.EMBO J. 2003; 22: 4212-4222Crossref PubMed Scopus (391) Google Scholar One executer of the p53 response in the senescence program is the cyclin-dependent kinase inhibitor p21Cip1, which also influences the activation status of RB.61Noda A. Ning Y. Venable S.F. Pereira-Smith O.M. Smith J.R. Cloning of senescent cell-derived inhibitors of DNA synthesis using an expression screen.Exp Cell Res. 1994; 211: 90-98Crossref PubMed Scopus (1055) Google Scholar The up-regulation of p21Cip1 in PanIN-1 lesions was recently shown, again arguing for a properly working p53-dependent fail-safe program in ductal pancreatic cancer.62Biankin A.V. Kench J.G. Morey A.L. Lee C.S. Biankin S.A. Head D.R. Hugh T.B. Henshall S.M. Sutherland R.L. Overexpression of p21(WAF1/CIP1) is an early event in the development of pancreatic intraepithelial neoplasia.Cancer Res. 2001; 61: 8830-8837PubMed Google Scholar One mechanism to induce the p53 response is sensed by ARF, which is activated by the hyperproliferative signal originating from oncogenic RAS.63Sherr C.J. The INK4a/ARF network in tumour suppression.Nat Rev Mol Cell Biol. 2001; 2: 731-737Crossref PubMed Scopus (586) Google Scholar With respect to the early carcinogenesis of ductal pancreatic cancer, an alternative mechanism to activate the senescent program exists. The earliest detectable genetic lesions in pancreatic cancer are shortened telomers.64van Heek N.T. Meeker A.K. Kern S.E. Yeo C.J. Lillemoe K.D. Cameron J.L. Offerhaus G.J. Hicks J.L. Wilentz R.E. Goggins M.G. De Marzo A.M. Hruban R.H. Maitra A. Telomere shortening is nearly universal in pancreatic intraepithelial neoplasia.Am J Pathol. 2002; 161: 1541-1547Abstract Full Text Full Text PDF PubMed Google Scholar At least in human cells, this telomere damage can be sensed and answered with a senescent-like phenotype that is mediated by p16INK4A and p53.65Shay J.W. Pereira-Smith O.M. Wright W.E. A role for both RB and p53 in the regulation of human cellular senescence.Exp Cell Res. 1991; 196: 33-39Crossref PubMed Scopus (404) Google Scholar, 66Smogorzewska A. de Lange T. Differe