RAS is the most frequently mutated oncogene in human cancers. Despite decades of effort, anti-RAS therapies have remained elusive. Isoprenylcysteine carboxylmethyltransferase (ICMT) methylates RAS and other CaaX-containing proteins, but its potential as a target for cancer therapy has not been fully evaluated. We crossed a Pdx1-Cre;LSL-KrasG12D mouse, which is a model of pancreatic ductal adenocarcinoma (PDA), with a mouse harboring a floxed allele of Icmt. Surprisingly, we found that ICMT deficiency dramatically accelerated the development and progression of neoplasia. ICMT-deficient pancreatic ductal epithelial cells had a slight growth advantage and were resistant to premature senescence by a mechanism that involved suppression of cyclin-dependent kinase inhibitor 2A (p16INK4A) expression. ICMT deficiency precisely phenocopied Notch1 deficiency in the Pdx1-Cre;LSL-KrasG12D model by exacerbating pancreatic intraepithelial neoplasias, promoting facial papillomas, and derepressing Wnt signaling. Silencing ICMT in human osteosarcoma cells decreased Notch1 signaling in response to stimulation with cell-surface ligands. Additionally, targeted silencing of Ste14, the Drosophila homolog of Icmt, resulted in defects in wing development, consistent with Notch loss of function. Our data suggest that ICMT behaves like a tumor suppressor in PDA because it is required for Notch1 signaling.
Helen Court, Marc Amoyel, Michael Hackman, Kyoung Eun Lee, Ruliang Xu, George Miller, Dafna Bar-Sagi, Erika A. Bach, Martin O. Bergö, Mark R. Philips
ICMT deficiency in conjunction with oncogenic Kras driven by Pdx1-Cre expression in the epidermis leads to papillomas on the face that can progress to squamous cell carcinoma. (A) A 4-month-old Icmtflx/flx;Pdx1-Cre;LSL-KrasG12D mouse manifested a facial papilloma between the eye and pinna. Fifty-nine percent of animals with this genotype developed these lesions compared with 7% of Icmtflx/+;Pdx1-Cre;LSL-KrasG12D mice. (B) H&E staining showed the lesion to be consistent with a verruca vulgaris. Scale bar: 100 μm. (C) A papilloma from an Icmtflx/flx;Pdx1-Cre;LSL-KrasG12D mouse that also carried a Rosa26-LSL-LacZCre reporter allele was stained with X-gal to reveal Cre expression in the transformed (arrow) but not in the normal (arrowhead) epidermis. Scale bar: 500 mm. (D) A 3-month-old Icmtflx/flx;Pdx1-Cre;LSL-KrasG12D mouse showing a larger papilloma. (E and F) H&E staining of the lesion shown in D revealed areas of malignant transformation into well-differentiated squamous cell carcinomas. Original magnification, ×10 (E) and ×20 (F). Scale bars: 100 μm.