Abstract IA25: Targeting K-Ras signaling in pancreatic cancer
Authors: Barbacid Mariano 
Published: October 2015

K-RAS oncogenes have been implicated in about one fifth of all human cancers including pancreatic ductal adenocarcinoma (PDAC). We have developed genetically engineered mouse (GEM) models that closely recapitulate the natural history of this human neoplasia and use them to validate targets of potential therapeutic value. Briefly, we have crossed a GEM strain that harbors an endogenous K-RasG12V oncogene whose expression is controlled by a doxycycline-regulated Elastase promoter (via a Cre-lox system) with mice that carry conditional knock out mutations in loci encoding potential therapeutic targets. These targets are ablated by genetic means (also via the Cre-lox system) at the time of tumor initiation. Then, we follow the fate of the tumor in the absence of the target. This genetic-based strategy has also been used to interrogate the role of the inflammatory pathways by crossing this K-Ras driven GEM model with strains of mice defective for T and B lymphocytes (Rag-1 KO), IL6 (IL6 KO), Cox-2 (Cox-2 KO) and the NF-kB pathway (Ikkbeta KOlox mice). We have observed that ablation of any of these inflammatory mediators results in various degrees of tumor delay, but none of them had a significant impact on overall survival. These observations suggest that whereas blocking inflammation has an inhibitory effect on tumor development, its ultimate therapeutic utility to treat the disease is likely to be relatively minor. We have obtained more promising results ablating signaling components directly involved in K-Ras oncogenic signaling. For instance, we have observed that when PDAC tumors are initiated by expression of the resident K-RasG12V oncogene, ablation of its downstream c-Raf kinase completely prevents tumor development. Similar results were obtained when we knocked out the locus encoding the EGF receptor, a surprising result considering that the EGF receptor signals upstream of K-Ras. However, when the PDAC tumors were initiated by expressing the K-Ras oncogene along with inactivation of the p53 tumor suppressor, a common mutation in human PDAC, mice developed tumors in the absence of either c-Raf or the EGF receptor. However, mice displayed a significant increased (as much as 90%) in survival time. More recently, we have generated mice carrying both conditional c-Raf and EGF receptor alleles. These mice (n=14) have not developed even preneoplastic PanIN lesions at one year of age even in the absence of the p53 tumor suppressor. These observations indicate that EGF receptor signals through a different pathway that synergizes with the main Raf/Mek/Erk signaling cascade. We are now exploring whether concomitant ablation of c-Raf and EGF receptors will also induce a significant therapeutic effect in already developed K-Ras driven PDAC tumors lacking the p53 tumor suppressor. To this end, we have generated a new GEM model in which the resident K-Ras oncogene and the p53 tumor suppressor are flanked by frt sequences (instead of lox) and the doxycycline-regulated Elastase promoter directs the expression of the FLpase recombinase (instead of Cre). This compound GEM model allows us to separate tumor development (based on a frt/FLpase system) from ablation of the therapeutic targets (mediated by the classical cre-lox system). We hope that these genetic approaches will define therapeutic strategies that may eradicate aggressive K-Ras driven PDAC tumors with the ultimate goal to translate these results to the clinic.

Citation Format: Mariano Barbacid. Targeting K-Ras signaling in pancreatic cancer. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Innovations in Research and Treatment; May 18-21, 2014; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2015;75(13 Suppl):Abstract nr IA25.