Tumor Suppression by ARF in Carcinogen-Exposed Mice

Abstract

The <italic>ARF</italic> tumor suppressor gene, encoded along with a second tumor suppressor, <italic>p16INK4a</italic>, at the <italic>CDKN2A</italic> locus, is frequently inactivated in human cancers. However, the close proximity of <italic>ARF</italic> and <italic>p16INK4a</italic> lends itself to the concurrent loss of both genes, raising important questions about the specific contribution of ARF to tumor suppression <italic>in vivo</italic>. Multiple lines of evidence indicate that sustained signaling from oncogenes such as RAS or MYC induces the expression of ARF, which then stabilizes p53 activity to arrest cell proliferation. To address the role of ARF in RAS-driven cancers, we exposed wild-type and <italic>Arf</italic>-deficient mice to two separate carcinogens, ethyl carbamate (urethane) and 7,12-dimethylbenz[a]anthracene (DMBA). <italic>Arf</italic>-deficiency facilitated the growth and malignant progression of lung adenocarcinomas after urethane exposure. A subset of lung tumors in <italic>Arf</italic>-deficient animals presented as poorly differentiated and metastatic, with many characteristics of pulmonary sarcomatoid carcinoma, a transitional tumor type previously undocumented in mice. Unexpectedly, urethane injection also resulted in the development of hepatic vascular tumors in <italic>Arf</italic>-deficient animals. Suppression of hemangioma development by ARF was strongly influenced by mouse genetic strain and was likely p53-dependent. Subjection of mice to the DMBA/TPA two-step skin carcinogenesis protocol further revealed that ARF inhibits the metastatic spread of carcinoma cells. The pro-invasion and metastasis phenotype of <italic>Arf</italic>-deficient carcinomas was apparent at the transcriptional level, and our data provide preliminary evidence of a role for ARF in regulating the epithelial-mesenchymal transition program. Taken together, these studies demonstrate that induction of ARF is an early response in carcinogenesis that mounts an effective barrier against tumor growth, invasion, and metastasis. For this reason, <italic>ARF</italic> status may have important implications for cancer patient prognosis and clinical management.