BASIC AND TRANSLATIONAL—PANCREAS Early Requirement of Rac1 in a Mouse Model of Pancreatic Cancer IRINA HEID,* CLARA LUBESEDER–MARTELLATO,* BENCE SIPOS, ‡ PAWEL K. MAZUR,* MARINA LESINA,* ROLAND M. SCHMID,* and JENS T. SIVEKE* *II. Medizinische Klinik Klinikum rechts der Isar, Technische Universität München, Munich; and ‡ Department of Pathology, University Hospital Tübingen, Tübingen, Germany See editorial on page 427. BACKGROUND & AIMS: Pancreatic ductal adenocarci- noma (PDAC) is a fatal disease without effective chemo- preventive or therapeutic approaches. Although the role of oncogenic Kras in initiating development of PDAC is well established, downstream targets of aberrant Ras sig- naling are poorly understood. Acinar-ductal metaplasia (ADM) appears to be an important prerequisite for devel- opment of pancreatic intraepithelial neoplasia (PanIN), a common precursor to PDAC. RAS-related C3 botulinum substrate 1 (Rac1), which controls actin reorganization, can be activated by Ras, is up-regulated in several human cancers, and is required for cerulein-induced morphologic changes in acini. We investigated effects of loss of Rac1 in Kras-induced pancreatic carcinogenesis in mice. METH- ODS: Using a Cre/lox approach, we deleted Rac1 from pancreatic progenitor cells in different mouse models of PDAC and in mice with cerulein-induced acute pancreati- tis. Acinar epithelial explants of mutant mice were used to investigate the role of Rac1 in vitro. RESULTS: Rac1 expression increased in mouse and human pancreatic tu- mors, particularly in the stroma. Deletion of Rac1 in Kras G12D -induced PDAC in mice reduced formation of ADM, PanIN, and tumors and significantly prolonged survival. Pancreatic epithelial metaplasia was accompa- nied by apical-basolateral redistribution of F-actin, along with basal expression of Rac1. Acinar epithelial explants that lacked Rac1 or that were incubated with inhibitors of actin polymerization had a reduced ability to undergo ADM in 3-dimensional cultures. CONCLUSIONS: In mice, Rac1 is required for early metaplastic changes and neoplasia-associated actin rearrangements in de- velopment of pancreatic cancer. Rac1 might be devel- oped as a diagnostic marker or therapeutic target for PDAC. Keywords: Genetically Engineered Mice; Ductal Cell; Cyto- skeleton; Signaling. I nvasive pancreatic ductal adenocarcinoma (PDAC) is the most frequent and highly lethal type of pancreatic cancer. One of the reasons for the high lethality of PDAC is the very late and difficult diagnosis due to absence of early symptoms and of robust diagnostic markers. To date, 3 main forms of precursor lesions have been de- scribed: pancreatic intraepithelial neoplasia (PanIN), in- traductal papillary mucinous neoplasm, and mucinous cystic neoplasm. The progression of PanIN lesions from PanIN1 to PanIN3, the carcinoma in situ, is the most common and best-characterized model for development of PDAC (for review, see Hezel et al 1 ). Nevertheless, the initiation process, the cell of origin, as well as the signal- ing events leading to precursor occurrence are still poorly understood. Morphologic similarities suggest PanINs derive from ductal cells. 2 However, recent lineage tracing studies con- firmed that premalignant lesions can arise from differen- tiated acinar cells in part through a reprogramming mechanism named acinar-ductal metaplasia (ADM). 3 Along this process, acinar cells react to various stimuli, including Notch signaling, growth factors, and inflamma- tion or cellular damage, thereby reducing expression of exocrine markers and developing into tubular structures with ductal properties. One of the best-known pathways involved in ADM is epidermal growth factor receptor (EGFR) signaling. Exposure of acinar cells to the EGFR main ligand transforming growth factor (TGF)- strongly induces ADM in vitro and in vivo. 4–9 In addition, aberrant activation or overexpression of oncogenic Kras G12D results in occurrence of ADM. 5,10 This process is dramatically accelerated when oncogenic Kras G12D is combined with overexpression of TGF- 11 or activation of Notch signal- ing. 3 Finally, repeated injections of cerulein, a pancreati- tis-inducing cholecystokinin (CCK) analogue, have been used in several studies to investigate the role of ADM in initiation and progression of PDAC. 12–14 Thus, EGFR-, Notch-, and/or Kras-activation driven as well as inflam- mation-induced transdifferentiation of acinar cells to a Abbreviations used in this paper: ADM, acinar-ductal metaplasia; BrdU, bromodeoxyuridine; CK19, cytokeratin 19; CytD, cytochalasin D; EGFR, epidermal growth factor receptor; LatA, latrunculin A; PanIN, pancreatic intraepithelial neoplasia; PDAC, pancreatic ductal adenocar- cinoma; Rac1, RAS-related C3 botulinum substrate 1; TGF, tumor growth factor; WT, wild-type. © 2011 by the AGA Institute 0016-5085/$36.00 doi:10.1053/j.gastro.2011.04.043 BASIC AND TRANSLATIONAL PANCREAS GASTROENTEROLOGY 2011;141:719 –730